EMC Zone

Template: EMC Zone

2012-04-02T11:06:04.085-07:00

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Measuring Cable Resonance with a Comb Generator

2016-10-28T16:31:00.001-07:00

by Kenneth Wyatt

Cables are one of the key metallic structures on a product that can act as an antenna and couple energy, causing radiated emissions and possible compliance test failures. This has been discussed extensively in the literature on product design for EMI compliance. In many cases, they can resonate at their half-wave frequency and actually amplify any harmonic currents traveling along the cable shield near that resonance (or higher-order resonances). Very often, you'll observe several broad resonant peaks in the emissions profile of a product under test.

One easy and quick way to characterize the resonant frequency of cables and wires in a product or system is to inject harmonic energy into the cable and measure the actual resonances. We can do this in a controlled manner by using a harmonic comb generator. In this case, we'll use the new Picotest Labs Model J2150A, which can produce a multitude of harmonics at several clock frequencies.

The J2150A includes five different user-switchable modes (see Figure 1):

Stepped (modes 2 through 4)
1 kHz impulse
100 kHz impulse
1 MHz impulse
10 kHz square

Figure 1 - the Picotest J2150A harmonic comb generator with Crystek CBLK-300-3 DC block connected. This produces closely-spaced harmonics out beyond 1 GHz.

I chose to use the 10 kHz square pulse, which offers a very high resolution, that is, it produces many closely-spaced harmonics. Because the output of the J2150A is DC-coupled, I used a Crystal CBLK-300-3 DC block to protect the generator output once I connected the (essentially short-circuit at DC) H-field probe.

The comb generator is powered via USB, so I merely plugged it into the Siglent SSA 3032X spectrum analyzer. One Beehive Electronics 100C H-field probe was connected to the comb generator through the DC block and the other was connected to the input of the spectrum analyzer See Figure 2 and 3 for the test setup.

Figure 2 - The test setup for measuring cable or wire resonance. In actual use, one may handhold both probes and quickly characterize the cable resonances in a product or system.

Figure 3 - A photograph of the test setup.

The spectrum analyzer should be set to a frequency range of expected cable resonance - in this case I used 10 to 500 MHz. I set the input attenuator to zero dB, turned on the preamp, vertical scale to dBuV, reference level so one of the horizontal lines read "zero", the resolution bandwidth to 120 kHz, and frequency scale to log.

I then laid the wire under test on top of a couple styrofoam blocks and taped the two H-field probes down so the tip of the probe just touched the wire. The two probes should be spaced a distance apart so the harmonic radiation from the one doesn't couple directly into the other. The exact spacing is not critical.

You should see at least one primary peak where the harmonic content is resonant. See Figure 4. The wire under test was 62 cm long. This is equal to approximately 200 MHz resonance at the half wavelength of the wire in free space.

Figure 4 - A screen capture of the resulting resonant peak in the wire.

References:

André and Wyatt, EMI Troubleshooting Cookbook for Product Designers, Appendix F ("Measuring Resonant Structures").
Picotest: http://www.picotest.com
Siglent Technologies: http://www.siglent.com
Beehive Electronics: http://www.beehive-electronics.com

Budget SDR-Based Spectrum Analyzer

2016-09-28T12:14:00.000-07:00

by Kenneth Wyatt

While there are a number of affordable spectrum analyzers available to the product designer or EMC engineer, such as the Rigol DSA800-series or Siglent SSA3000X-series, I discovered an extremely low cost analyzer useable for general-purpose EMI troubleshooting. This tiny module (Figure 1) is actually a high quality software defined radio (SDR) that can tune from 24 MHz to 1.8 GHz and is sensitive down to -130 dBm. If you need to go down further in frequency, they have the companion Spyverter that tunes from DC to 60 MHz. Both units are controlled via USB and the processing is performed by a standard PC running Windows.

A special effort was made to provide shielding and I/O connector transient protection. The cost for the AirSpy is just $199 and the Spyverter is an extra $59. Both may be ordered from http://www.airspy.com.

What's really cool is that you can brag about being able to carry a spectrum analyzer in your pocket. It will also easily fit in a briefcase, along with your PC laptop and troubleshooting probes. No more carrying around a 40-pound analyzer!

Figure 1 - The AirSpy software defined radio (on left) may be used successfully as a budget spectrum analyzer. Also shown is the Spyverter low frequency converter (on right) allowing tuning from "DC" to 60 MHz. The Beehive Electronics 100C H-field probe is shown for comparison.

When driven from the free "Spectrum Spy" PC-based software, AirSpy can display a relatively accurate spectral plot (Figure 2). Note, also, the "waterfall" display feature, that records frequency versus time. This is useful for tracking down intermittent EMI pulses or radio transmissions. In addition, the software allows a continual recording of the emission to the PC's disk drive (up to its capacity).

Figure 2 - The AirSpy measuring the emissions from an embedded processor board. The 270 MHz processor clock may easily be observed.

The AirSpy web site includes links for a variety of useful software applications. The commonly-used "SDR#" (pronounced "SDR Sharp") may be used and can display a span of 10 MHz at a time.

I also recommend downloading the SpectrumSpy application, as that more closely duplicates the controls of a normal spectrum analyzer. There are a lot of niceties, such as measurement in dBuV, max hold, markers, adjustable resolution bandwidths, etc., that are missing, but the SpectrumSpy does work nicely for general measurements. Center frequency, span, and upper/lower frequency limits may all be specified. One nice thing is that the span is not limited to just 10 MHz, but can display a wide band of frequencies - much more useful for EMI troubleshooting. Note that the amplitude is only available in dBm.

Figure 3 - Here is a screen capture of the embedded processor clock at 270 MHz, with the waterfall display underneath. In this case, we're demonstrating the use of the SDR# application, which may be used as a receiver and can demodulate AM or FM broadcast stations. It's handy as a spectrum monitor and can help pin down interfering signals.

Figure 3 shows a sample of the SDR# application. It is a more general tool for monitoring radio communications and broadcast stations. It can demodulate AM and FM signals. There are a number of built-in features and the AirSpy web site includes much more detail on the operation.

I find the frequency accuracy is excellent and the amplitude accuracy is decent, as well. I'm not sure I'd use this product for pre-compliance testing, but for general troubleshooting and educational use, it should perform well.

Other reviews:

http://www.rtl-sdr.com/review-airspy-mini/

http://www.rtl-sdr.com/review-of-the-spyverter-upconverter/

http://airspy.com/reviews/

Welcome to Mega Automotive

2016-09-19T17:04:00.001-07:00

by Joanna Hill

Today is Arnold’s first day at Mega Automotive. He is fresh out of college with an electrical engineering degree and is eager to meet his new boss, Mr. Buttsworth.

Mr. Buttsworth: Hello Arnold welcome to Mega Automotive. I’m sure you’re going to like it here. It’s a fast-paced environment with lots to learn and loads of good people to work with. We have an exciting new position for you it’s called a Component Electromagnetic Compatibility Engineer. Your job is to prevent electromagnetic compatibility failures at final vehicle testing. We design the vehicle and many of the components. Some of the components are designed and manufactured by suppliers. After all the parts have been designed they are assembled into prototype vehicles. These vehicles are then tested to make sure they can meet all sorts of requirements; one of these requirements is electromagnetic compatibility.

First, you must understand the specific EMC tests we perform on a vehicle are considered a company intellectual property and you are not allowed to disclose these requirements outside of the company.

Arnold, your first assignment will be to test an engine controller for EMC as a component. You are to develop a component level test plan to catch the EMC issues so they can be fixed early in the programs rather then at the end of a program. It is very expensive to make any last minute changes.

This is what an Engine Control Unit looks like, we call it an ECU.

Figure 1 - An Engine Control Unit (ECU) from a 1996 Chevrolet Beretta courtesy of Delco Electronics and Wikipedia.

Arnold: Cool, I love working on cars. It’ll be great fun to work on them for a living. What exactly is EMC?

Mr. Buttsworth: Our vehicles are put into a lot of different electromagnetic fields created by a lot of different devices. Many of these devices are “self-certified’. In fact, automobiles are also “self-certified”. That means that Mega Automotive is obligated to make sure a vehicle will work correctly when exposed to the fields that it may encounter and not interfere with any other device. The problem is that not all of the companies making things are as diligent as we are here at Mega Automotive.

For example, farms can have a water sprinkler called a Pivot that is driven by a three phase, 480 volt, Variable Frequency Drive. The VFD takes in 60 Hz AC and makes whatever frequency you want at 480 volts and 800 amps. The variable frequency is used to drive the pump motor of the sprinkler. The VFD does this by rectifying the three phases with a bank of diodes and capacitors. Then this DC voltage is pulse width modulated to feed a three phase motor that drives the water pump. If any of the case ground connections in the wet and dirty farm environment gets rusty, a PN junction is formed in the ground path. This splatters RF noise all over the spectrum. To make matters worse, the pump drives water into a 16 foot high quarter mile long water pipe.

The farmer thinks of this as an agricultural sized water sprinkler, we know it as a folded monopole antenna driven by lots of high frequency PWM noise. The Farmer has no idea he is creating huge electromagnetic fields that can interfere with all sorts of devices including vehicles. And to make matters worse, occasionally this quarter mile long short wave antenna with taps every 50 feet made up of an A-frame with two tractor tires to elevate the water pipe is parallel to the highway! Never the less our vehicles driving by within 100 feet of this antenna are not allowed to misbehave.

Your job is to come up with a component level EMC test to make sure that when the initial prototype is assembled and tested there will not be any issues with EMC.

Arnold: Okay, can I see the harness of wires that will be connected to the ECU?

Mr. Buttsworth: No, it has not been designed yet.

Arnold: Okay, can I know how many harnesses will be connected to the ECU?

Mr. Buttsworth: No, it has not been designed yet.

Arnold: Are there things connected to this unknown harness?

Mr. Buttsworth: Yes, we know what kind of sensors and actuators will be connected to the ECU and we have the documents that describe their function, but we don’t actually have any of then yet. They’re all in design as we speak.

Arnold: Okay, can I see the schematic of the ECU?

Mr. Buttsworth: No, it has not been designed yet.

Arnold: What can I know about this thing?

Mr. Buttsworth: We know the pinout and we have a description of the engine control system. After the harness has been designed and routed on the engine and when we find out how the engine group is going to ground the ECU, it will be assemble into a prototype vehicle and it has to pass the vehicle EMC testing.

Let me show you to your desk. We will have a computer for you shortly. In the mean time please read these documents. And here is your list of mandatory safety and company training classes.

Barney: Hello Arnold, welcome to the neighborhood. I hear you’re the new EMC component engineer. I do the vehicle level EMC testing.

Arnold: Oh boy am I glad to meet you. I am to develop an EMC test to prevent issues from showing up in your EMC testing. But I have to do this without knowing how the thing is going to be wired up. This is impossible.

Barney: Actually our job is to write test plans that specifies the EMC test that need to be run by EMC technicians. The EMC equipment itself is very expensive and getting chamber time is really hard. So we write an EMC test plan, it gets approved, and then it is performed by the technicians.

All you have to do is come up with an idealized harness with a load box to simulate the sensors and actuators at the end of the harness. Make the harness about half a wavelength long in the FM band and pass on the test equipment design to the test equipment group. Tell them to test for stuck at one and zero for all loads.

It’s not really that hard if you don’t get too picky about simulating the vehicle. We have no idea what it will look like at this point, but never the less it has to pass my vehicle level EMC test to be sold. And we cannot delay the first day of production of the vehicle if it fails the EMC testing. As a result we almost always end up working nights and weekends to get it out the door in time.

Good luck and welcome to automotive. It’s not for everyone, but I love to see my vehicles on the show room floor or on the road. You know you had a part of making it happen. We have lunch down in the cafeteria. I’ll come by around noon to show you where it is. By the way, the hardest part of the job is staying awake while you read all of the documentation. See you at noon.

Epilogue

After many years of trial and error, Arnold came up with a technique to test modules for EMC before the vehicle design was complete. At the same time others were also working to solve the same problem thoughout the industry. But as you can expect, their solutions were all different. Some demanded the load box be in plastic, where others wanted metal. All of these empirical techniques work, sort of. But still at the end of a vehicle program, it must pass an EMC test.

Disclaimer, the individuals named in this article are fictional, any reference to actual people or companies is unintended.

Joanna Hill holds a MSEE from Georgia Institute of Technology and a BSEE from Florida Institute of Technology. She turns the magic of EMC into the technology of EMC with consulting and classes globally. Her classes have demystified EMC in China, Mexico, Germany and the United States.

She is a member of the IEEE, IEEE EMC Society, SWE, SAE EMI Task Force, ISO TC22/SC3/WG3 USTAG, and CISPR/D USTAG. And a member of the IEEE EMC Society board of directors. She has worked as an engineer for 39 years always with an interest in fields and waves. Her LinkedIn page URL is:www.linkedin.com/in/JoannaEMC. She may be contacted at: joannaemc@icloud.com.

DC-DC Converter Noise Evaluation

2016-08-31T18:15:00.000-07:00

By Kenneth Wyatt, Wyatt Technical Services LLC

More of my clients are starting to use small third-party DC-DC converters to provide the multitude of voltages required for today’s processor and DDR RAM ICs. While these are convenient to drop onto a circuit board, they can be quite a source of radiated and conducted emissions – especially those that switch in the MHz range.

I recently published an article on how these converter circuits can generate harmonic noise all the way up to 1 GHz, and above, severely compromising RF receiver sensitivity in the wireless telephone bands [1]. Kevin Slattery and Harry Skinner called this “Platform Interference”, in their book, Platform Interference in Wireless Systems – Models, Measurements, and Mitigation [2].

One example of this type of “drop-in” DC-DC converter is manufactured by Murata and we’ll use their model UWE-24/3-Q12, which is an “Eighth Brick” power supply that can take 9 to 36V and convert it to 24V at 3A (Figure 1). My client was using three of these converters on a product and was measuring a high level of radiated emissions, as well as observing broadband noise throughout his system all the way through 150 MHz.

Figure 1 – The Murata UWE-24/3-Q12 DC-DC converter. The manufacturer recommends specific capacitors on the input and output and I’ve tack-soldered these on as shown.

If you read the manufacturer’s specification sheet, you’ll generally find that to “pass” EMI will require “additional filtering”, and this converter is no different. In this case, the additional filtering required to meet EMI limits was not described. I decided to bring one of these back to the lab and try some experiments to attempt to quiet the EMI.

To do this required some instrumentation. I used a Siglent Technologies SPD3303C three output power supply, a Tekbox Technologies TBOH01 5uH LISN, A Tekbox Self-Powered Active Load, and a Siglent Technologies SSA3032X spectrum analyzer. All this gear is available from the U.S. distributor, Saelig Electronics [3]. The active load was really handy, because I could dial in the exact load current I wanted…in this case 0.5 amps, to avoid cooling issues. I connected a couple of Fluke DMMs to monitor the output voltage and current (Figure 2). The spectrum analyzer picked off the conducted emissions via the LISN.

Figure 2 – The test setup for evaluating the conducted emissions from the Murata DC-DC converter.

After trying some inductors and common-mode chokes I had on hand, I determined that simply placing a 100uH inductor in series with the input terminal was enough to quiet the emissions rather drastically (Figure 3).

Figure 3 – A 100 uH inductor was all that was required to dramatically reduce the conducted emissions.

Figure 4 shows the result. The yellow trace was the ambient (baseline) signal level. The red trace was unfiltered (no inductor) and the blue trace was with the inductor added in series with the input voltage to the converter.

The addition of a single inductor nearly reduced the conducted emissions down to the noise floor of the measurement.

Figure 4 – The results with the 100 uH inductor installed (blue trace) looking from 150 kHz to 150 MHz. The red trace is the unfiltered noise and the yellow trace was the ambient baseline noise. The display line is the approximate emissions limit.

Conclusion

Many EMI tests may be conducted right at the bench top. Evaluating various vendor products, such as DC-DC power supply converters, is always wise, prior to committing to a PC board design. It’s also wise to verify EMI performance as well as reading the “fine print” within the product specification sheet. Very often claimed EMI performance will require additional components.

References

[1] Wyatt, Platform Interference, http://www.edn.com/electronics-blogs/the-emc-blog/4441086/Platform-interference

[2] Slattery and Skinner, Platform Interference in Wireless Systems – Models, Measurements, and Mitigation, https://www.amazon.com/Platform-Interference-Wireless-Systems-Measurement/dp/0323281451/ref=sr_1_2?ie=UTF8&qid=1450492921&sr=8-2&keywords=Platform+interference

[3] Saelig Electronics, http://www.saelig.com

EMC & Its Role In Reliability

2016-08-24T07:12:00.003-07:00

EMC and Its Role In Reliability

by Ken Javor, EMC Compliance

Totally anecdotal, but this just happened to me.

My wife uses a powered wheelchair. A very high end, high quality chair. We were on vacation driving north and had just crossed the 45th parallel in the state of Michigan. The 45th parallel anywhere is halfway between the equator and the North Pole. In Michigan it’s also halfway to nowhere, and it was there my wife’s chair broke down.

The chair weighs close to 250 pounds empty, and is not much fun to move around less the electric motors, especially say up and down the handicap van’s ramp. The chair has enough smarts to give you a code when something fails. Maybe not the correct code, but a code. In this case, it told us the joystick module had gone bad, so we called a repair place and got one ordered to be sent to the place we were headed. I mentioned we were in the middle of nowhere, but we had pulled over in a cell phone-serviced spot just off I-75 because I was supposed to support a telecon with a customer and I had been looking for just such an oasis.

While looking at the chair, I saw a cable had been abraded and the sheathing was damaged and so were internal wires. I repaired those, and although that cable had nothing to do with the joystick module, once I got it repaired the fault code changed to bad power module cable. We haven’t priced the joystick module, because essentially price was no object with the chair stalled at the side of the road north of the 45th parallel, but I know it will be well north of $500 US. Seeing a new code unrelated to the joystick module did not lighten my mood.

Several days later after cooling our heels at what is called the “tip of the mitt”, the northern most point of Michigan’s lower peninsula, the new joystick module arrived and I installed it and got the same cable fault code as before and no life from the chair. So I removed all the chair plastic covers and traced the cable from the joystick module back to its other end, and I ended up at two plastic connectors with various wires emanating, and ferrite cores around them.

Now we are finally getting near the end of the shaggy dog story. Or maybe not the end, but the beginning of the end. Or maybe just the end of the beginning. But I digress.

One of those connectors or a wire/pin connection within had loosened and when I pushed on it, the chair woke up and worked. So clearly an intermittent. The chair is several years old, so this was not some sort of infant mortality problem, nor something to be caught by QA before shipping the chair. The ferrite cores I mentioned earlier are not the typical snap-on type, but they are actual toroids around which one could wind a choke or transformer. But they weren't so wound; they were just encircling the cables. Anyway, pushing on one of the two cores caused the intermittent to change states, so clearly over time that heavy core bouncing around the cable had caused something to open up.

So the lesson here is that adding EMI fixes can actually decrease reliability. Stating that in some circles where EMC is held to be a reliability issue is nigh on to heresy and can literally cause a violent argument. I could tell that story, but I won’t.

But it is in fact the case that adding parts decreases reliability – it’s just probability theory. Snapping a core around a cable at the last minute may be attractive and relatively risk-free on a piece of portable electronics, but when it is a vehicle and/or people’s lives depend on its functionality, then the modification must be subjected to review.

Related to this problem, I once worked a spacecraft that had a long RS-232 cable interface, for which they had chosen some very fast logic family, and the cable was electrically long relative to the transition time. There was considerable ringing on leading and trailing edges, and there was an expressed concern that such ringing could look like an extra bit. The fix here was to slow down the rise time (the data rate was OTOO 100 kbits/sec), but no one wanted to change anything, so I suggested one of these press-in connector filters (ee-seal by Quell), and that was demonstrated to work. But the ancient program manager, confronted with this news, asked the question: “Do capacitors fail open or closed?” The answer was “closed.” The program manager, the recipient of no telling how many whippings by Murphy, responded, “Not on my spacecraft,” and that was that.

Intelligence guided by experience.

Mr. Javor has worked in the EMC industry for over 30 years. He is a consultant to government and industry, runs a pre-compliance EMI testing facility, and curates the Museum of EMC Antiquities, a collection of radios and instruments that were important in the development of the discipline, as well as a library of important documentation. He can be reached at ken.javor@emccompliance.com.

CISPR 35 Published - Multimedia Immunity

2016-08-24T07:12:00.002-07:00

CISPR 35 – How does it compare with CISPR 24?

by Ghery Pettit, Pettit EMC Consulting LLC

After 15 years of work in CISPR Subcommittee I, the IEC Central Office published CISPR 35 Edition 1.0 on Tuesday, August 16, 2016. The IEC webstore link is https://webstore.iec.ch/publication/25667. In the US, ANSI has the standard available on their website as well. Go to www.ansi.org and type in CISPR 35 in the search window for standards. You must purchase a copy of the standard to see what the specific requirements are.

Now that CISPR 35 is finally published, the questions that you want answered are: What is the same as CISPR 24? What has changed? What is new?

The full title of the standard is, "CISPR 35:2016 Electromagnetic compatibility of multimedia equipment - Immunity requirements"

The abstract, off the IEC webstore page, states,

"CISPR 35:2016 applies to multimedia equipment (MME) having a rated AC or DC supply voltage not exceeding 600 V. The objectives of this document are:

- to establish requirements which provide an adequate level of intrinsic immunity so that the MME will operate as intended in its environment in the frequency range 0 kHz to 400 GHz; and
- to specify procedures to ensure the reproducibility of tests and the repeatability of results."

Before going into any details, please note that CISPR 35, like other CISPR standards, uses dated references to other standards. Thus, the laboratory must use the version called out in the standard, even if a newer version is available. A particular example where this is necessary is in the use of IEC 61000-4-5:2005 for surge testing. A newer version is available on the IEC website, but CISPR Subcommittee I has determined that for the time being the 2005 version is the one to use.

So, what is different between CISPR 24 (ITE immunity) and CISPR 35?

A key difference is that CISPR 24 provides guidance on testing different types of devices while CISPR 35 focuses on “functions” of the EUT. For example, in the annexes of CISPR 24 there are annexes aimed at telephony terminal equipment, data processing equipment, local area networks, printers and plotters, etc. The annexes in CISPR 35, on the other hand, deal with broadcast reception function, print function, scan function, display and display output functions, musical tone generating function, networking functions, audio output function and telephony function. Testing to CISPR 35 need only be performed for the primary function(s) of the product.

The general types of immunity tests are the same in both CISPR 24 and CISPR 35. There are a few key differences, however.

For continuous RF electromagnetic field disturbances, CISPR 35 adds the option of testing in a TEM cell (IEC 61000-4-20) or a reverberation chamber (IEC 61000-4-21) to the testing method used in CISPR 24 (IEC 61000-4-3). Thus, CISPR 35 offers more choice in testing environments. CISPR 35 adds spot frequencies of 1800 MHz, 2600 MHz, 3500 MHz and 5000 MHz to be tested. 3 V/m is the required test level for all these tests (swept from 80 MHz to 1000 MHz and spot frequencies as noted above). Note that this test level is the rms value of the field prior to turning on the modulation. There are additional spot frequencies required in CISPR 35 for equipment with a primary function of telephony. Devices processing analog composite video signals (for example, PAL, NTSC or SECAM) have a relaxation from performance criteria A to performance criteria B test testing within +/- 1.5 MHz of a relevant subcarrier frequency. The key item to note in this is that laboratories that were equipped to test to CISPR 24 will now have to purchase new test equipment to allow performing radiated immunity testing above 1 GHz and manufacturers of equipment to be tested must be prepared for the addition test time and expense associated with this testing.

Table 2 providing immunity requirements for analogue/digital data ports is an expansion of the requirements contained in CISPR 24. Testing requirements unique to CPE (customer premise equipment) xDSL ports has been added. The test level for continuous induced RF disturbances has been changed. In CISPR 35 this is 3 V from 0.15 MHz to 10 MHz. From 10 MHz to 30 MHz this test level decreases with the logarithm of the frequency to 1 V and is 1 V from 30 MHz to 80 MHz. As noted above, this is the rms value of the voltage prior to turning on the modulation.

Immunity requirements in Table 3 for DC network power ports are the same in CISPR 35 as they are in CISPR 24, except for the change in test level for continuous induced RF disturbances noted above.

There are some changes in Table 4 for AC mains power ports, as well. The change noted above for test levels for continuous induced RF disturbances applies here, as well. For voltage dips and dropouts the voltage reduction is stated in terms of the residual voltage, rather than amount of reduction, although the actual requirements are the same. In addition, CISPR 35 provides requirements for either 50 Hz and 60 Hz distribution systems for the remaining reductions.

Table 5 provided requirements on EUT arrangement with respect to whether the EUT is intended for table top, floor standing, either table top or floor standing, rack mounting or “other” operational arrangement.

CISPR 35 is very much like CISPR 24, except where the two standards are different. If you are familiar with CISPR 24 you have a good start on understanding CISPR 35. However, you must obtain and read CISPR 35 carefully to ensure that you test your product correctly to show compliance with the new standard. And make sure the lab has the new test equipment needed for radiated immunity testing above 1 GHz.

A past president of the IEEE EMC Society, Ghery S. Pettit has worked in the areas of TEMPEST and EMC for the past 40 years, working at the Naval Electronic Systems Engineering Center, Vallejo, Martin Marietta Denver Aerospace, Tandem Computers, Intel Corporation and now as an independent consultant. He is presently the Vice Chair of CISPR SC I and will take the Chair’s position on November 1, 2016. He has been active in CISPR standardization work since 1998, initially as a member of the USNC IEC / CISPR G TAG and now as a member of the USNC IEC / CISPR I TAG, USNC IEC / SC77B TAG, CISPR SC I WG2 and WG4, in addition to the leadership position noted above. Mr. Pettit is also a member of ASC® C63 SC 1.

Ghery has written 8 papers and articles for publication and contributed a chapter for the 2^nd Edition of the ARRL’s Radio Frequency Interference Handbook. He is a member of the dB Society and serves as a Technical Advisor for the ARRL in the area of EMC. He holds an Amateur Extra class ham license (N6TPT) and is an instrument rated private pilot.

Mr. Pettit may be reached at Ghery@PettitEMCConsulting.comor at (360) 790-9672.

Britain after Brexit, with the accent on EMC

2016-06-25T18:19:00.000-07:00

Editor's Note (Updated June 27, 2016): Britain has voted to leave the European Union, a monumental decision that’s triggering some serious alarm bells throughout the country. As it stands, many UK laws and regulations are determined by EU legislation and it’s not immediately clear what the EMC standards ramifications in Britain will be during the two year transition and in the future after that.

There’s still a long road ahead, and many of the concerns expressed by the EMC standards community are likely to be addressed in the coming negotiations. Panic is obviously not the way to go, and to understand the issues with more clarity, I've asked John Woodgate and Keith Armstrong, product regulations/EMC consultants and standards experts in England, to provide some initial analysis. Both of these consultants are based in England, so have a unique perspective as events unfold. As the transition takes place, you can depend on Interference Technology to help inform and clarify the issues as the transition progresses.

In the meantime, we would invite your comments on the issue. See below.

K. Wyatt

Britain after Brexit, with the accent on EMC - Part 1

by John M. Woodgate, BSc(Eng) C.Eng MIET SMIEEE FAES HonFInstSCE

Email: jmw1937@btinternet.com / Web: www.jmwa.demon.co.uk

(June 24, 2016) - What is the future for Britain after the historic 'Brexit' vote to leave the European Union? It's very early days – as early as you can get, in fact – but there are some signs. Controlling factors can be described as 'Dame Luck and the Lame Duck'. Dame Luck is market sentiment, always volatile but in this case reason appears to be holding. The British stock market bubbled up before the vote, so the post-vote fall has really returned the market to where it was ten days ago. The Lame Duck is Prime Minister David Cameron, who says he will resign, but not till October, so until then the government will be hamstrung, both at home and abroad.

This may not matter if enough of British industry takes advantage of the drop in the value of the pound sterling to combat rising material costs by increasing export sales. There are always some with enough insight (and low enough debt) to do that. Every economic change allows someone to make money, and it doesn't have to be bankers. The EU leaders are pressing for a quick resolution, perhaps even shorter than the two-year period embedded in the treaties. That is likely to benefit industry, especially if British leaders use that appeal for speed to toughen their negotiations.

So what does this mean for EMC? We are in the age of world markets, so obviously British industry will continue to fulfil all significant market entry requirements, such as respecting EU Directives and applying European standards (ENs). If Britain joins EFTA (European Free Trade Association), it will continue to participate in CEN and CENELEC, but with changed voting rights. If it doesn't join, it can choose whether to apply EU Directives (or a variant thereof) and CEN/CENELEC or ISO/IEC/CISPR standards, whichever is advantageous. ETSI standards are a different matter, while ETSI is nominally and originally 'European Telecommunication Standards Institute', it is de facto an independent non-governmental body, with world-wide activity, and there seems no reason why anything should change regarding British participation.

Of course, all this could be wildly wrong, and disasters forecast by the 'Remain' faction may happen. But most events are not beyond our control. We just have to be clever enough to control them correctly.

Britain after Brexit, with the accent on EMC - Part 2

by Keith Armstrong, Consultant, Cherry Clough Consultants Ltd.

Email: keith.armstrong@cherryclough.com / Web: http://www.cherryclough.com/home

Yes it is big news, but I don't feel that I have anything very significant to say about how the issues you mention might be affected.

All I can say is that, from my point of view as a design consultant, whatever country I am working in (most recently Australia, New Zealand, Belgium and England, UK) if my customers want to sell to the EU they must declare compliance to all relevant EU Directives and most find that easiest to do by declaring that they have applied all the relevant standards that have been notified under those Directives in the OJEU.

As far as I can see, this process and its associated documentation is exactly the same for any country in the world that is outside the EU – as it is for manufacturers in Belgium and in the UK (both of whom are currently Member States within the EU).

I suppose that if the UK actually left the EU (which is by no means a certainty, and anyway would most likely take 2 or more years to happen) UK experts would no longer be involved with creating EN standards. But as EN standards are mostly based on international standards anyway, and UK experts would still be involved on those international committees, I don't see any big changes happening.

It is interesting to note that even though the UK is still a Member State within the EU, it has not yet published National Regulations corresponding to the new EMC or LV Directives which came into force on 20th April 2016 (2104/30/EU and 2014/35/EU respectively).

Normally we in the UK would have seen draft UK Regulations at least a year in advance of the date when new EU Directives required National Regulations to be enforced, but not this time.

Quite possibly this same situation applies to all the other EU Directives which came into force on that date, too.

I am told that enquiries by others were told by the responsible UK government departments that they were waiting for the outcome of the referendum.

So it is already the case that manufacturers in the UK have National Regulations that are not harmonized with the corresponding EU Directives, just like manufacturers in China, Japan, the USA, or anywhere else outside the EU!

Our Top Five Articles

2016-06-25T11:27:00.003-07:00

Hi Everyone! I wanted to introduce myself as the new senior technical editor for Interference Technology. I spent the early part of my career in the aerospace industry as a design engineer and the last 20+ years as an EMC engineer, test center manager, and EMC lead for Hewlett-Packard and later the spin-off, Agilent Technologies. In 2008, I “retired myself” early and started a consulting business helping other companies with their EMC compliance issues, as well as provide training in EMC, troubleshooting, and pre-compliance testing. I’ll be continuing as a consultant while providing editorial direction.

I started subscribing to Interference Technology in 1972, just a year after Robert Goldblum started the publication. I was still attending college at the time, but was always impressed with the technical content of what was then called Interference Technology Engineers Master (ITEM). This was also well before I had any inkling I’d spend most of my career in EMC. Now here I am honored to be editing the same publication!

Those of you who may have been following my past scribblings are probably aware I blogged for EDN.com for nearly three years on various aspects of EMC testing, product reviews, and pre-compliance testing. Since taking on the role of editor for Interference Technology, I felt I ought to turn that job over to my good friend and colleague, Arturo (Art) Mediano, professor at the University of Zaragosa (Spain) and an accomplished EMC consultant and trainer. Check out my past blogs and Art's newest postings at The EMC Blog. I'd like to thank Martin Rowe, and the other editorial staff at UBM for their past support.

The top five articles in Interference Technology

Shortly after I joined ITEM Media and started editing Interference Technology, I decided, with the help of the staff, to compile and rank all our past technical articles from 2005 to present. I was hoping to determine the most popular topics as determined by reader "clicks". I thought knowing this would help me determine what readers feel is most engaging. Some of these topics include EMC basics, product design for compliance, EMC standards updates, and military EMC.

For this initial posting, I'd like to point you to our five most popular technical articles published in Interference Technology. Please enjoy them!

1. The HF current probe: theory and application - Kenneth Wyatt

2. New EMC requirements for commercial avionics: RTCA/DO-160F - Eric Bergstrom

3. Electromagnetic interference sources and their most significant effects - Anthony DiBiase

4. Designing electronic systems for EMI: Grounding and the control of EMI - Willian Duff

5. Radiated emissions measurements at 1/3/5/10/30 meters - Daniel Hoolihan

Interference Technology has always had great technical content for the EMC professional and my vision is to continue that, as well as publish basic information on product design and pre-compliance measurement techniques for those product designers that may not have a formal education in electromagnetic compatibility. If you’re interested in contributing technical articles to Interference Technology, check out the Editorial Contributions link at the bottom of our web site and feel free to drop me an email with your proposal.

Troubleshooting EMI Seminar with Lee Hill: Interview

2015-12-22T08:40:00.001-08:00

By Senior Technical Editor, Kenneth Wyatt.

This interview was conducted on Dec 9th, 2015 in anticipation of the upcoming Troubleshooting EMI with Lee Hill: Identify, Characterize, and Prevent Interference Problems Seminar Tour hosted by Rohde & Schwarz and Interference Technology.

The tour is running from March 10-24, 2016 in five different cities - Austin, TX; Irvine, CA; Milpitas, CA; Livonia, MI; and Chelmsford, MA. Attendance for the seminar is free, and engineers can reserve a place now by signing up online HERE.

Contact emily.webster@rsa.rohde-schwarz.com with any questions.

Kenneth Wyatt: What changes in technology are driving today’s EMC issues?

Lee Hill: One observation is that it’s easier for cheaper, smaller things to run fast. The other is that more products are networked or incorporating wireless technologies. An example is home exercise equipment – in the past these devices were purely mechanical. Today, exercise equipment you bring into your home is now loaded with electronics, variable speed motor drives, networking capability, and even video and audio entertainment. Many low-end appliances are full of high-speed stuff.

There’s also such a high-level integration of silicon. Single ICs can contain high-speed video, Wi-Fi, RFID, etc., all in the silicon. Other products I’ve worked on include drones and all the remote control aspects of real-time flight. Wireless self-interference (digital noise reducing on-board receiver sensitivity) has also been more of an issue.

There’s been a revolution in silicon design and simulation for EMC outside the U.S. – especially in vehicular design. I think the silicon has gotten so much better that system-level radiated emissions, radiated immunity, and ESD have improved dramatically over the years.

KW: What do you find are the top product design issues that are experienced?

LH: The top design issues really depend on who you’re talking to. IC manufacturers might say a different set of problems is their main EMC nemesis.

In the area of EMC, the common problem is a lack of understanding of EMC engineering fundamentals – for example, identifying dipole antennas (antenna structures that can radiate) and coupling paths. Generally, engineers can identify the source and victim, but they have no clue where to go from there.

KW: About the Troubleshooting EMI Seminar with Lee Hill sponsored by Rohde & Schwarz, I think it’s great you’ll be presenting in so many cities around the U.S. It seems you’ll be hitting some of the top tech areas on both coasts and in the Midwest.

LH: Looking forward to it! (Tour dates will take place in Austin, TX; Irvine, CA; Milpitas, CA; Livonia, MI; and Chelmsford, MA.)

KW: Why is Rohde & Schwarz doing this now?

LH: That’s a great question, and I have asked Faride Akretch, Segment Marketing Manager of Rohde & Schwarz, to respond:

Faride Akretch: We want to provide value to our customers. And besides compelling products, we also deliver educational content and activities on a regular basis. Those can be application notes, white papers or customer trainings like this one. Sometimes these seminars are being held in person, or virtually like our sponsorship and participation in EMC Live hosted by Interference Technology. We’ve identified a real need for an increased focus on EMC and EMC pre-compliance. It seems that with so much integration, EMC failures have become more difficult to understand and that customers want and need to dig deeper as to where possible interferers are originating from.

KW: One more question for Faride Akretch. Are there changes in measurement technology that is driving this Troubleshooting EMI Seminar with Lee Hill?

FA: As part of the Seminar, we will be highlighting some newer real-time tools that allow visualization of the spectrum yielding a very powerful EMI diagnostic capability. We’ll also be including probing solutions and how to approach testing and EMC pre-compliance tests.

KW: And now Lee Hill, while many of the EMC compliance tests continue to specify a standard EMI receiver or swept spectrum analyzer as dictated by the standards, real time analysis has become much more affordable. Will you be incorporating some of this newer measurement technology in your presentations?

LH: Yes, we’ll be demonstrating how to use real-time spectrum analysis to help identify issues that otherwise would be completely hidden.

KW: So, why should product designers attend these seminars? Why is this subject important?

LH: Some of the important takeaways would include discovering tools they were unaware that existed, learn why these tools are so incredible and useful, learn how to use them, and to have fun watching them be used.

KW: Finally, I know your courses include lots of real-time demonstrations. Will you be able to do the same for this seminar?

LH: The seminar will contain live demonstrations (not prerecorded video) of EMC measurements and troubleshooting ideas using measurement tools and real-life products that are familiar to EMC engineers and representative of commonly encountered noise problems in industry.

Registration for this free seminar is open now. For more information and to register for a date, CLICK HERE.

Lee Hill is an industry expert in electromagnetic compatibility and founding partner of SILENT Solutions LLC, an EMC and RF design firm established in 1992. Lee provides EMC troubleshooting services, design reviews, and training to a wide variety of industries nationally and around the world and also is a member of the adjunct faculty at Worcester Polytechnic Institute (WPI) where he teaches graduate-level classes in EMC. Lee also teaches at the University of Oxford (England), and for the IEEE EMC Society’s annual Global University and EMC Fundamentals program. He earned his MSEE in electromagnetics from the Missouri University of Science and Technology EMC Laboratory.

The Simplified Method and the Near-Field Catastrophe

2015-07-09T09:35:00.002-07:00

The simplified method, and in particular the system base-RF-power determination used in the simplified method equation, only applies to far-field data. Why? Well, as an example, the radiation polar-plot provided by antenna suppliers is far-field data and in no way represents how the field-strength varies with angle close to the antenna.

So where exactly is the near-field / far-field divide? That is, where does the simplified method no longer apply? We could talk about this until the cows come home and get nowhere, because basically one man’s near-field is another man’s far-field, with both parties eager to prove how the dividing line was derived quoting various permutations of wavelength and Pi as the precise answer.

The Near-Field / Far-Field Decree

So for the purposes of this blog-thread we are going to create a dictate. As regards RF waves surrounding a radiating antenna, when a wave is an independent entity the wave is in the far-field. By independent we mean the excitation signal feeding the source can do a backward-summersault for all the wave cares, it has already escaped the influence of the source and will carry on along it’s merry way at the speed of light (in vacuo).

Conversely, if a change in the excitation signal causes a change in a surrounding RF wave, that wave is in the near-field. For example, if the excitation source to a resonating dipole was suddenly removed the voltage and current standing waves present across the resonating structure would die away almost instantaneously.

We complete the dictate by stating that the near-field / far-field dividing line is at 3 wavelengths.

Effect on the Simplified Method

The new decree means the lowest frequency at which the simplified method is valid is 300MHz. The determination is easy enough:-

Our test distance is 3 meters. Obeying the decree means that in order for the calibration plane to just be in the far-field, the 3 meter distance must represent 3 wavelengths. Therefore the limiting wavelength is 1 meter. Using c = f/Lambda, this corresponds to a lowest frequency of use of 300MHz.

The Antenna Suppliers’ Get-Around

Given antenna suppliers sell antennas starting at 80MHz (wavelength 3.75 meters) into this 3m RF immunity test market, how do they get around that the supplied gain and radiation pattern data only applies at far-field distances? (This of course is where the “yes it is, no it isn’t in the near-field” brigade kick in and try to claim that 3m distance is far-field even at 80MHz. And this of course is why we ‘cut them off at the pass’ by announcing the decree).

The supplier get-around is to provide data on how much RF power is required to produce a particular test-field at a particular test-distance. The antenna supplier is not forthcoming with (or doesn’t know) how much of the test-field is created by field type, that is how much of the measured field is contributed by the far-field and how much by the near-field. In fact you will find that the only data supplied is boresight data. That is the data represents the field we would measure with a field-probe mounted at the center of the calibration plane. No guarantee is given regarding the field achieved elsewhere across the calibration plane. Why does this matter? Answer – we are designing the test system under ideal conditions (perfect, very large fully-anechoic chamber). The premise being that if we are unable to generate a compliant field across the calibration plane under these ideal conditions, we do not stand ‘a snowflake’s chance in hell’ of achieving one under real 3 meter semi-anechoic chamber conditions. For instance, what if the measured field across the plane was to vary due to multiple peaks and nulls in the near-field radiation pattern?

To be continued...

Tom Mullineaux
Lionheart Southwest

Incorporating Antenna VSWR Loss into the Simplified Method

2015-06-05T08:17:00.002-07:00

The Simplified Method of Establishing the RF Power Required by a RF Immunity System Continued...

Previously we presented the novel idea that the RF power required by a RF immunity system is a base RF power level (determined entirely by constants), minus the gain of the antenna. The basic principle is shown pictorially in Figure 1. The picture says the system RF power requirement in dBm equals the base RF power level in dBm, minus the antenna gain in dBi. A quick sanity check says this makes sense in that the higher the antenna gain, the less power required by the system.

Figure 1

The approach is also self-correcting in that should the linear gain of the antenna drop to less than 1 as can happen in lower frequency test systems (say SANT / SISO = 0.9), the gain in dBi will become minus, resulting in the system power requirement equaling the base power level PLUS the antenna gain.

So far so good.

We then built on this by converting all phenomena requiring more system RF power into loss-blocks and adding the loss blocks to the system diagram as shown in Figure 2.

Figure 2

Later on, we will simply add the overall dB loss of the blocks to the antenna dB gain to obtain the ‘overall system gain’ such that we are back to Figure 1, with the rightmost block amended to represent the system gain.

This approach, combined with the graphical representation described in the AH Systems webinar [Link Here] provides superb understanding / visualization of the system behavior across the band of interest, and uses the power of dB notation to simplify power computation.

In this particular blog entry we will concentrate on the mismatch presented to the system by the antenna.

Generating the VSWR Loss-Block

We need to convert the antenna VSWR phenomenon into the ‘basic loss-block form’ shown in Figure 3.

Figure 3

Please Note: this section also gives the solution to the teaser question posted last time.

The VSWR loss-block must obey the form of the equation in Figure 3, that is:

We need to establish Pout / Pin. We start with the classic VSWR representation as applied to an antenna shown in Figure 4a. Note in all cases below, the antenna symbol represents any type of antenna and the measurement plane is at the antenna connector.

Figure 4a

The figure shows the incident voltage Vinc striking the measurement plane, a portion of Vinc passing through the measurement plane to the antenna (shown as Vnet, the net voltage used by the antenna to create the test field), and a portion of Vinc being reflected back along the transmission line (the reflected voltage Vref).

By inspection we can see that Vinc is the input to our loss block and Vnet is the output. Again by inspection

Here we introduce ρ, the reflection coefficient at the measurement plane. This is equal to the ratio Vref / Vinc, so Vref can be written as ρVinc. This is shown in Figure 4b.

Figure 4b

Figure 5

We still want Pout / Pin so we need to convert Figure 4a to the power based diagram of Figure 5

So the

of Figure 4a becomes the

of Figure 5

To do this we convert all voltages to power with reference to the characteristic impedance of the transmission line (cable). That is: -

For future reference that is

And

So the

Of Figure 5 becomes

Pnet (that is Vnet2 / Zo ) is the output of our VSWR loss-block in Figure 2 and Pinc (Vinc2 / Zo) is the input

Rearranging to make Vnet2 / Zo the subject of the equation

But looking again at Figure 4b, Vref = ρVinc, where ρ is the reflection coefficient presented at the measurement plane

That is

Dividing throughout by Vinc2 / Zo will give us our Pout / Pin (since Pin is Vinc2 / Zo)

The final step in the conversion to a loss-block is to take 10log10 of this

One last little trick and we are there.

And at last we have our VSWR loss-block (Figure 6)

Figure 6

As a sanity check we should test this to see if the block works for ρ = 1 (100% reflection at plane) and ρ = 0 (No reflection at all)

When ρ = 1, none of Vin gets through the measurement plane and the loss should be infinite
When ρ = 0, all of Vin gets through and the loss should be 0

So testing for ρ = 1

Correct

And testing for ρ = 0

But log10[1] = 0, so

Correct

As a further step, we can now place this VSWR Loss-Block in front of a perfect match antenna (VSWR = 1:1 over the entire antenna frequency range).

Figure 7

To be continued...

-Tom Mullineaux
Lionheart Southwest

Use of Commercial Items

2015-05-15T07:41:00.003-07:00

The use of commercial items (CI) or commercial-off-the-shelf (COTS) equipment presents a dilemma between imposing military E3 standards and the desire to take advantage of existing commercial systems, and accept the risk of unknown or undesirable electromagnetic interference (EMI) characteristics. Regardless of the pros or cons of using COTS, any procured equipment should meet the operational performance requirements, including electromagnetic compatibility (EMC) requirements, for that equipment in the proposed installation.

Integration of COTS electrical/electronic equipment on DOD platforms is an increasingly common practice for a variety of good reasons. COTS typically offer the latest technology and can be cheaper and more quickly fielded than military systems developed from scratch. Unfortunately, commercial equipment is not designed for the harsh electromagnetic environments (EME) found in military platforms and theaters of operation.

One of the biggest difficulties with integrating COTS products into complex military systems is achieving EMC. EMC is the ability of electrical and electronic equipment and systems to share the electromagnetic (EM) spectrum and to perform their desired functions without unacceptable degradation from the EME and without causing EMI to other systems. Blindly using COTS carries the risk of increasing serious EMI problems within the platform or system.

COTS equipment has typically been designed, tested and fielded to much less demanding commercial EMC standards, if tested at all, than MIL-STD-461 or MIL-STD-464. However, the simple fact that it is a commercial item should not be taken as a reason to accept lower EMC performance. Rather than forgoing robust EMC requirements, program managers (PMs), system acquisition personnel and E3 engineering professionals must first assess the EMC-related risk to full operational capability performance from the use of COTS equipment. They must impose a detailed methodology by which to assess the risk of using COTS and achieving EMC.

To mitigate the risk, an assessment should be performed to evaluate the equipment’s immunity characteristics against the planned EME and ability to meet the desired performance. Factors to be considered in evaluating the suitability of COTS for military applications include:
• Impact on mission and safety
• The operational EME
• Platform installation characteristics
• Equipment immunity/susceptibility characteristics

After determination of the intended operational environment, the risk assessment process starts with obtaining and reviewing existing design criteria (commercial specs), analysis/test data and conducting additional EMI testing (if necessary.) If, after evaluation of the EMI data, it is determined that the equipment would not operate satisfactorily in the intended EME, then the equipment needs to be modified, or it might prove to be necessary to select different COTS equipment with adequate characteristics.

On the whole, most COTS equipment has less strict EM requirements (lower immunity levels, higher allowable unintentional emissions, lax or nonexistent susceptibility limits) than military equipment and could therefore be more apt to be upset or damaged when exposed to high level radio frequency (RF) fields or could interfere with legacy systems. Therefore the use of COTS introduces additional risk of incompatibility and can result in problems, plus associated extra costs, in maintaining performance through life and for re-use in other scenarios. When considering COTS or NDI in an acquisition, it is important to include E3 requirements and obtain and review any existing EMI test and/or analytical data.

-Brian Farmer

Beamwidth Loss and Why Test Staff and dB Notation should be Friends

2015-05-13T06:28:00.001-07:00

Please Note: - This blog entry continues the thinking behind the creation of the AH Systems webinar on selecting antennas for today’s test requirements.

Supporting notes on the webinar can be found on the AH Systems website.
(Hint - click the ‘Tech Notes’ Tab and simply select the first document listed on the main page itself).

In the last entry we introduced a method that provides superior visualization of the system behavior. We continue on this theme by factoring in beamwidth loss.

Antenna Beamwidth Loss

To fully appreciate the impact of beamwidth when illuminating the test field calibration plane, we must first interpret the data on the polar chart provided as standard by all good antenna suppliers.

Make Up of the Polar Chart

The pictures above show the two main components that make up the polar chart. The first picture shows the 360 degrees surrounding the antenna divided up into 5 degree segments. The black arrow shows the antenna boresight direction. With our higher frequency EMC antennas the maximum power density created by the antenna will always be in this direction.

The second picture shows the concentric rings on the chart that indicate the number of dBs down from the maximum power density. The outer ring represents 0dB down (no reduction from the maximum power density), the first ring in is 3dB down (half the power), the second 6dB (quarter the power), etc.

The picture above shows the complete chart with the dB rings overlaid onto the 5 degree lines.

The next picture is a real polar plot provided on the AH Systems website. Note that the 0 degree marker now marks the boresight direction. The data on the chart shows the performance of the antenna at three frequencies. The red trace is the radiation pattern at 1GHz, the blue is at 9GHz, and the green is at 18GHz.

The webinar was all about better visualization of the system performance, so it explains how to mark the chart with the sector of interest, how to mark the 61000-4-3 Standard’s plus 6.0dB limit as a ‘Do Not Cross Within Sector!’ line, and finally goes on to explain how to transpose key points on the calibration plane onto the chart

Reminder: - A proviso in the webinar is that the test field is created under ideal anechoic conditions. Why? – because if we struggle to achieve a compliant field under ideal conditions, and even with the option to discount 4 of the measurement points, it will be difficult to achieve a compliant field one under practical chamber conditions, and we may need to resort to the worn out excuse called ‘room effects.’

The picture above is a snapshot of the marked up chart used in the webinar.

In the snapshot the calibration points and red crosses could have easily (and more correctly) been positioned on the outer circle, but some audience members might have run away with the idea that any reflection at the plane would all be directed back through the zone of interest. This is not so, so the plane is portrayed as a straight line. As long as the calibration points are at the correct angle, and as long as the data on the chart is read from the outer circle (0dB) along this angle, the straight line portrayal of the plane is just as visually informative as one on the outer circle. Some might argue even more so.

Why call it Beamwidth LOSS?

A generalized block diagram of a loss in a system is shown below

We are quick to relate this with cable loss where the system needs more power to overcome the loss. Let’s put a lossy cable in the block and assume that for an input power of 100W the output power is 80W. Then the loss in dB is 10log10 [80/100] = 1dB. We conclude we need 1dB more power from the amplifier to overcome the cable loss. No arguments here.

Likewise, due to the beamwidth phenomenon, we need more system amplifier power to create the greater power density required at the center of the calibration lane. Instead of putting the cable in the block, we can just as easily put the beamwidth effect in there. If we accept the need for more power to overcome cable loss, it is completely rational to accept the need for more power to overcome beamwidth loss.

Why Test Staff and dB Notation should be Friends

We are about to interpret the data on the snapshot chart, so this is a good point to make the case that dB notation is not out there just to annoy and confuse test staff, but rather it is there to aid computation and visualization.

We start with the ease of use and interpretation of the marked up chart. Irrespective of the actual maximum power density (could be a high value, could be a low one), the radiation pattern fall off in value is always with respect to this 0 degrees boresight maximum. In other words, the value at any angle on the chart as you move away from 0 degrees is automatically referenced / compared to this maximum in dBs.

Let us read the chart data to determine the beamwidth loss at 1GHz. We follow the red trace from the boresight in a clockwise direction until it intersects the brown line defining the periphery of the calibration plane. The intersection is around one third of the distance between the 0dB and minus 3 dB circles, making the beamwidth loss at 1GHz around 1dB. This says the calibration points on the periphery of the calibration plane will be 1dB down from an imaginary point at the dead-center of the plane. Conversely, the power density at the boresight is 1dB greater than that at the periphery, so we need more power from the system amplifier to create this extra boresight power density.

We can do the same thing with the blue trace (9GHz), and determine the beamwidth loss at 9GHz as 2dB or so. Our system design goes to 10 GHz, so we extrapolate this to 2.5dB.

Because we are using dBs we can now simply add this loss to the cable loss (also in dBs) to get the combined loss at any particular frequency of interest. And we can carry on doing this with all forms of loss to get the overall system loss. We then combine the overall system loss with the antenna dBi gain to obtain the resultant system wide gain GSYS. This in turn is then combined with the base system power to determine the amplifier power required by the system. And there ladies and gentlemen, is the beauty of dBs. There will be more on this later.

Teaser Question

Here is a teaser for the more experienced EMC Test Staff out there and for those that relish a challenge. For higher frequency EMC antennas beamwidth loss dominates antenna mismatch. For lower frequency EMC antennas VSWR loss caused by the poor antenna match dominates beamwidth loss. When expressing VSWR as a loss, I took the time to derive the equation:

Where LVSWR is the VSWR loss and ρ is the reflection coefficient presented by the antenna.

How did I derive the first equation, and more importantly, why I did I take the time to do it? It is all there in either the webinar, the supporting notes or these ‘thinking behind’ blog entries. Or maybe it takes all three.

To be continued...

-Tom Mullineaux
Lionheart Southwest

A Simplified Method of Calculating the Amplifier Power Required by an RF Immunity System

2015-05-05T09:35:00.002-07:00

AH Systems recently sponsored an EMC-Live webinar on selecting antennas for today’s test requirements in which I introduced a simplified method of establishing the amplifier power required by an above 1GHz RF immunity system. Existing methods, such as slotting numbers into a spreadsheet, do not provide the necessary visualization of the test system behavior, and provide little or no understanding of the system dependencies.

It will aid understanding if you watch the presentation before going into this thinking behind the webinar’s creation. A recording of the webinar can be found here.

The webinar supporting notes can be found by clicking the ‘Tech Notes’ tab on the AH Systems website www.ahsystems.com

And so, here is the thinking that led to the simplified method.

Taming the Field Strength Equation

A first glance at the beast that is ‘the field strength equation’ can create a feeling of foreboding. But studying and classifying its component parts not only tames the equation, it also gives superb visualization on how the system behaves across the band of interest. The equation is:

Where E is in v/m, d is the distance (set by the standard so a constant), G is the linear gain of the antenna, and P is the RF power injected in at the antenna connector.

Regular blog followers will know we showed how this equation was derived in a previous blog posting

www.emc-zone.com/2014/02/the-linearization-of-emc-amplifiers.html

We start the taming process by recognizing and pulling out the obvious and hidden constants within the equation. We then take a brand new step where the field strength E itself is made a constant. In order to do this we introduce a new compensatory factor called ‘Beamwidth Loss’. More on this later.

Obvious and Hidden Constants

The obvious constant is √30 / d. We will call this constant k1

The less obvious constant is √(PG). Note, we are not saying that P or G is a constant, we are saying for a particular fixed field strength E, that P multiplied by G is a constant.

Let’s say P = 10 and G = 10. Therefore P.G = 100 which means √(PG) = 10
If at another part of the band the antenna linear gain G was to drop from 10 to 5, then for P.G to maintain its constant value, the power P would need to compensate by increasing to 20, keeping P.G = 100 constant and √(PG) = 10 constant.

And so P and G do a sort of dance across the band, where as G changes, P changes in the opposite sense to maintain the constant. We could allocate this constant a letter, but for our purposes we will keep it as √(PG), we just need to keep in the back of our minds that this has a constant value for a fixed field strength. So we will stick with

This says that for a fixed field strength (let’s say the minimum allowed field strength stipulated in a standard such as 61000-4-3), any change in G is compensated for by a change in P. All makes sense and stands to reason.

Now we have a better feel for the equation dynamics (the dance), we turn our attention to its real use in the webinar, which is to determine the power required to obtain a particular field strength.

First we rearrange to make P the subject of the equation.

For fixed field value Ek

Taking 10 log10 of both sides puts the equation in dB notation

With logarithms, a linear division is represented by a subtraction, so we can write

What this says is that the power required from the amplifier (assuming it is connected directly to the antenna, that is with no cable loss) is a power level dictated by constants, minus the antenna gain in dBi. In other words, the better the gain of the antenna, the less power required from the amplifier.

Again, this all makes sense and stands to reason.

Note that for the case where the linear antenna gain is 1 (equates to G = 0dBi), the amplifier power level is entirely dictated by the constants. For this reason we will call this power level the ‘base power level’, and we will regard GdBi as a modifier to this level.

So what is the amplifier’s base power level for a 61000-4-3 RF immunity system where the test distance d is 3m, and the minimum permitted field level (our Ek) at distance d is 18v/m?

We will round [32(18)2 / 30] = 97.2 up to 100W

Amplifier power is more commonly stated in dBm, that is the power is relative to 1 milliwat, not 1 watt. The conversion from dBW to dBm is simplicity itself – just add the number 30.

So our base power level is 50 dBm or 100W. We now have a starting point for our system.

Why did we go the dB route? Answer – because antenna manufacturers supply gain data in dBi, cable loss per meter is given in dB, and the soon to be introduced ‘beamwidth loss’ is in dB. The beauty of everything being in dBs is we can just add these algebraically to get the overall system Gain and then simply add this algebraically to the base power level to get the power required from the amplifier.

Don’t forget that the webinar and the supporting notes offer further explanation, particularly regarding visualization of the system wide performance.

Next time we will introduce ‘beamwidth loss’, rationalize its introduction, use it to determine the overall system gain GSYS, and finally use the worst-case GSYS and the base power level to determine the power required from the amplifier.

To be continued ………

-Tom Mullineaux
Lionheart Southwest

Spectrum Supportability Risk Assessment – A Last Word….For Now….

2015-04-08T10:27:00.001-07:00

Hopefully as you’ve been following along our SSRA related posts, you realize that Spectrum Supportability is not something that can be assumed; spectrum demand is increasing and available spectrum is decreasing. The requirement to perform and submit SSRAs is part of the DoD effort to ensure that we don’t continue to field systems with spectrum use issues and/or interference problems. From the list of suggested tasks noted in DoDI 4650.01, you will also realize that producing a meaningful SSRA is a significant engineering undertaking, not a task for the faint of heart. An understanding of the entire gamut of required information, the sources and availability of that information and the technical ability to collate, analyze and present the data, requires a specialized expertise and particular experience. And as a relatively new requirement, knowledgeable, experienced help in producing and reviewing SSRAs can be hard to find. EMC Management Concepts’ staff have extensive experience in DoD spectrum and E3 requirements, we have experience analyzing potential interference and RF interactions during program lifecycles and we can help YOUR program minimize spectrum supportability risks.

To recap, an SSRA provides a formally documented spectrum supportability risk assessment, with mitigation measure(s) identified, to achieve a spectrum supportability determination from the service Spectrum Management Office or CIO (depending on ACAT and/or level of Interest). An SSRA should include the following components:

• Regulatory component: Addressing the compliance of the RF system with US national and international tables of frequency allocation as well as with regulatory agreements reached at the International Telecommunication Union.
• Technical component: Quantifying the mutual interactions between a candidate system and other co-band, adjacent band, and harmonically related RF systems, including the identification of suggested methods to mitigate the effects of possible mutual interference.
• Operational component: Identifying and quantifying the mutual interactions among the candidate system and other US military RF systems in the operational environment and identifying suggested methods to mitigate for possible instances of interference.
• Electromagnetic Environmental Effects (E3) Assessment: Determining the potential for EMC and EMI interactions between the proposed system and other systems, and with the anticipated operational EME. Identification and resolution of co-site EMI issues during system acceptance testing. Demonstration of repeatable EMC utilizing appropriate development models.

SSRA analysis results are summarized for senior leadership as “stoplight charts” where the colors of each box are an indication of the possibility of a system obtaining spectrum supportability or that the system may cause or experience EMI problems. This is presented in standard DoD risk assessment matrix language/format and anything that is not rated “green” must include mitigation information. SSRAs are presented for review and approval at the programs major milestones.

You must – MUST – apply due diligence to Spectrum Supportability considerations
– It is a critical tenet for program success:
– It requires application of resources and knowledgeable people
– You should apply Spectrum Supportability resources early and “Up-Front” in a program life cycle and coordinate with the service frequency management offices
– It will save you potentially BIG $’s in the end

-Brian Farmer

A Show Report on the 2015 Santa Clara Symposium on EMC & SI

2015-03-26T12:13:00.000-07:00

Whenever I missed an EMC show I always wished there was some sort of report I could read to get a feel for some of what I missed. To that end, here is one on Santa Clara. I have tried to make it informative and fun for non-attendees and attendees alike.

This is a major event with a lot of presentations happening in parallel, and I could only attend a short time, so I apologize in advance to those speakers / vendors I did not get around to. I took a lot of notes and have much to say, so this first excerpt will talk about the show in general. Further excerpts will follow.

I decided to attend the show Monday afternoon, the entirety of Tuesday (day and nighttime events), and the first half of Wednesday.

The Travel Experience

To me, travel is always an important part of the symposium experience. At 11am on Monday your intrepid reporter stepped off an airplane at San Jose International airport, hopped on the free airport flyer, and then for the princely sum of $2, took the modern light rail system (tram lines along main arteries and on the city streets), which in no time at all, dropped me right outside the Santa Clara Convention Center. All went without a hitch.

The picture shows the Light Rail Train that whisks you to the Santa Clara Convention Center

So getting there was simplicity itself. What about the show and the hospitality events that surround it?

The Venue

The Convention Center is a modern affair made of several interconnected pyramids all covered in tinted glass-curtain wall. The Center is attached to the Hyatt hotel, its own walls clad with the same colored glass in keeping with the Center décor.

As you walked through the main doors of the Convention Center you were greeted with bright yellow EMC&SI 2015 registration counters. A cacophony of bleeps and beeps emitted by a bunch of forklifts to the right told you where the exhibit hall was, and arrows directed you to the many meeting rooms on the left and on the second floor. Dotted throughout the lobby, and later on inside the exhibit hall, there were many easel-mounted placards announcing various events and highlights.

One placard of interest welcomed 12 new exhibitors, an indication that the show might be growing. Sometimes an apparent reduction in booth numbers is in fact just consolidation, where a company buys up smaller companies to create synergy (the total effect of the union is greater than the sum of the parts). A case in point is TESEQ buying up MILMEGA and IFI, and TESEQ in turn being bought up by AMETEK. The MILMEGA and IFI booths did not go away as such; they just joined the large island booth that is AMETEK.

The picture shows the list of new companies.

On a more somber note, one placard was dedicated to two EMC engineers no longer with us.

The links go to the respective obituaries:

http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7023194
http://www.legacy.com/obituaries/venturacountystar/obituary.aspx?pid=173653986

The Keynote Presentation

The keynote presentation took place at 8:30am on Tuesday. It was well attended and took place in a hall so large that two screens (one on the left of the stage, one on the right) were required. Dr. Lee showed how the explosion of wireless services and internet connectivity has increased our dependence and how the sudden loss of these would create a catastrophic standstill. Then came a history of Carrington’s discovery of the connection between major coronal discharges and high voltages on telegraph lines, how in the 1960s Hawaii felt the effect of an experimental high-altitude nuclear detonation, and a recap of the 1989 blackout in the Northeast due to a major corona discharge. He then reminded us that the technology of today is far more prone to these phenomena.

Rounding up, Dr Lee was of the view that the EMC industry was ideally equipped to come up with solutions to deal with corona discharge / HEMP related issues.

The speech was very interesting but did not go into the depth I hoped for as it only lasted 30 minutes or so, not the 90 minutes duration as posted.

The pictures show the keynote presentation placard and Dr. Lee speaking in the distance

The SAE-G46 EMC Subcommittee Meeting

This was a four hour meeting covering several topics but the principal reason I was there was my recent interest in HEMP, and the keynote speech by Dr. William Radasky on HEMP/IEMI/Geomagnetic storms and their effect on the Electrical Power Grid. We were greeted with a lunch courtesy of Elite Electronics Engineering, Fischer Custom Communications, CST of America, and Universal Shielding Corporation.

Dr. Radasky really knows his stuff and actually pointed out some factual inaccuracies in Dr. Lee’s speech. I suspect Dr. Lee’s speech was originally penned for a student audience and therefore has not benefited from expert feedback.

I cannot do the entire speech justice here, but in a nutshell, Dr. Radaski relayed the following:

• The practices used in EMC translate across to HEMP protection
• The susceptibility of key subsystems on the power grid should be fully characterized, that is test field levels should be increased slowly until the actual failure level is established
• Videos of the high altitude tests of the 1960s are now declassified and can be viewed on line (Post Meeting Note, I found them by searching youtube for ‘high altitude nuclear tests’)
• In the main, intentional electromagnetic interference is used by criminals to disable security systems. There is little evidence of its use outside this sphere
o Check out the Diehl Device and the JULTI12A Generator (see www.futurescience.com/emp/ferc_Meta-R-323.pdf)
• The Ethernet is prone to IEMI

The picture shows Dr. Radasky addressing the G-46 Subcommittee

There were other interesting topics at the subcommittee meeting and I will report on these in future excerpts.

Hospitality Events

Welcome Reception

As usual, the welcome reception was held on Tuesday evening when the exhibit hall closed. Finger food and hot dishes were provided in the large hall, and a soft-rock band played for the two hours we were there. It took me a while to realize that it was not actually a professional band playing, and that all the players were EMC guys. They were pretty darn good. You are furnished with two drink tickets and, given it was St Patrick’s Day, there was free dyed-green beer on tap. The atmosphere was great and I met up with Ross Carlton of NI who is toying with the idea of organizing a Texas EMC Fest, and talked with Bob Skully about me giving a talk at NASA on the Elephant in the Test Room christened ‘Underperforming 3m Chambers’. The project is moving on nicely and the intention is to present the 3D EM software analysis results obtained to date.

The St Patrick’s Day ‘Disco’ and the EMC Bowl-A-Rama

As soon as the reception event finished there were two competing follow-on events vying for your presence; the HVT St Patrick’s Day Celebration ‘Disco’ and the EMC Bowl-A-Rama hosted by Haefely Hipotronics, AE Techron and Advanced Test Equipment Rentals.

I admire companies that put their marketing dollars where their mouth is. Too many companies, large and small, are niggardly with their marketing dollars, or too entrenched in where marketing dollars are spent, and then wonder why their market share slowly evaporates or never grows.

Getting back to the subject at hand, I opted for the EMC Disco, mainly because it was on-site and this gave me full control as to when I called it a night. The bowling event sounded tremendous fun but required a shuttle bus and I had an early breakfast meeting commitment.

To get into the events you needed ‘badges’.

The Disco badge was a flashing clover leaf that you wore on your lapel, and the Bowl-A-Rama was a beer mat on a lanyard you wore around your neck.

You had only to follow your ears to find the ‘Disco’ in full swing just off the hotel’s main lobby. Lots of people were there and part of the entertainment was the opportunity to wear a costume and have your photo taken with pretty girls. I met up with a bunch of friends, the list is long but includes Donnie Gray of TDK, Jerry Meyerhoff of JDM Labs, the indefatigable Harry Hodes of BACL, Owen Wiseman of N4L, Jim Baer of Comtest DMAS, and Leo Smale and Cindy Catlin of Lionheart Northwest (soon to be wed). The host company HVT has taken a brave new direction since Jason Smith joined them as head of sales and marketing, and I see the Disco as one example of the determination to succeed in winning greater visibility and market share.

The first picture shows Jim Baer of Comtest DMAS at the ‘photo opportunity’ wearing despicable me goggles. He is pretending he is there under extreme protest. The second picture shows the soon to be married Leo Smale and Cindy Catlin of Lionheart Northwest ‘getting on down’ on the Disco floor.

As an aside, they say that before a potential customer has the confidence to place business with a new company, or even solicit a request for quotation, they must have been exposed to five impressions. Impressions are things like a product alert, company press release, advertisement in a journal, booth at a show, a placard showing your logo because you are a sponsor, etc. Well, to my mind, HVT just made one heck of an impression.

The Interference Technology Breakfast

On Wednesday morning the Interference Technology team hosted a breakfast for those interested in content marketing.

I sat with Andreas Barchanski of CST and we tucked into a hearty breakfast as we went through the progress of the 3D EM Software simulation of 3m chamber ‘hot wall’ design permutations. Graham Kilshaw of ITEM then stood up, welcomed everyone, and held up a sheet of paper with the single word CONTENT written on it. Graham went on to explain what Content is and why it matters. Content is the buzz word given to useful information. I am paraphrasing Graham here, but one key reason Content matters is that it gives a motive to someone with an equipment or service need, to visit and stay on your website. Without Content, a website, generally speaking, is simply a list of products. Content educates potential customers, helps them make informed buying decisions, and as a bonus, helps search engines find your site.

Then Graham talked about the EMC-Live series of educational webinars coming up this April, the sponsorship opportunities they create (yep, that’s one of those impressions), and introduced the content creators present at the breakfast. These were Ken Wyatt of Wyatt Technical Services, Keith Armstong of Cherry Clough Consultants, and Yours Truly, me, Tom Mullineaux of Lionheart Southwest. I already have one potential customer very interested in my creating and presenting an EMC-Live webinar for them.

To practice what I preach on the 5 impressions rule, here is a proposed website banner.

Impressed?

To be continued…………

-Tom Mullineaux
Lionheart Southwest

Spectrum Supportability Risk Assessments – Where’s the Beef?

2015-03-23T09:46:00.000-07:00

Ideally, an initial spectrum supportability risk assessment is generated in the first phase of the DoD acquisition process. Early identification of spectrum and EMC related regulatory and technical issues allows program management personnel to focus attention and resources on critical spectrum issues in the remaining acquisition phases. The SSRA’s author uses inputs from several sources:

• Technical and regulatory information is obtained from DoD data bases, specifically the:

System Certification System (SCS) data base is used to generate lists of co-band and adjacent band DoD emitters, providing an overview of other systems sharing expected electromagnetic environments. Basically, you’re looking for the J/F-12 and frequency assignment data
Host Nation Spectrum Worldwide Database Online (HNSWDO) data base is used to identify host nation comments on previous systems in the same frequency band and with similar technical parameters as the system being acquired.
US and non-US tables of allocation, which can be obtained in many cases directly from the internet.

• The latest pertinent Host Nation supportability comments are obtained by the Program Office from the Combatant Command (COCOM) spectrum managers. The COCOM spectrum managers will forward any resulting comments to the authors of the SSRA.

• The PMO defines the system’s technical parameters and intended operational deployment required for spectrum support, e.g. the frequency bands of interest and the intended worldwide development, test and operational areas and host nations. Other technical data might include:

Data Sheets from Developer
RF Modeling

Propagation
Interference Analyses
Selectivity Curves
Antenna Coupling
Inter/Intra Co-site data/modeling

Platform vs. EME at Key Locations
E3 Test Data – Program Office

EMI Test Plans/Reports
EMC Test Plans/Report

Coordination with the cognizant MILDEP Spectrum Management Office (SMO) is key to a successful SSRA. The SMO should be made aware of initial activities and be kept informed of major SSA developments. The PMO should provide the SSRA’s authors with copies of any DD Form 1494s sent to the SMO. The national and host-nation comments resulting from previous J/F 12’s submissions should be reviewed to see what comments may have been provided on earlier versions of the system.

The results from the regulatory portion of an SSA can be summarized for senior leadership as a “stoplight chart” where the colors of each box are an indication of the possibility of a system obtaining spectrum supportability in the US and selected host nations. Typically, reading the rows indicates whether or not the frequency band used by each of the program’s sub-systems will have major spectrum issues in many of the intended host nations by the color in that box. The colors result from a careful comparison of the radio service of each RF system with the technical and regulatory information contained in the databases and the host-nation tables of allocation.

Likewise, the results of the Technical and Operational analyses as previously discussed, will constitute additional input into an overall risk assessment. The technical component would focus on the RF engineering related risks associated with possible mutual interference with other systems in the same band and the operational would focus on the risks of possible mutual interference within its intended operational environment.

The major result of the SSRA may be that the PMO considers options such as: changing the system’s spectrum use or other technical parameters or beginning consultations with the cognizant SMO regarding possible courses of action. Typical courses of action include coordinating bi-lateral negotiations with individual host-nations or briefing the spectrum requirements of the system to groups such as the NATO Frequency Management Sub-Committee (FMSC), the DoD spectrum Summit or various COCOM spectrum conferences. All PMO involvement with these groups must be closely coordinated with the cognizant MILDEP frequency management office and DoD representative.

-Brian Farmer

Spectrum Supportability Risk Assessments (continued)…

2015-03-12T08:03:00.001-07:00

Just One More Thing on the List!

Available expertise and the existence of service Spectrum and E3 related organizations notwithstanding, it is widely known in the DoD Spectrum Management community that program offices, for a variety of reasons, including a lack of understanding of the requirements and their importance, frequently avoid spectrum supportability considerations early in program or take them on belatedly at the expense of cost, schedule and operational capability. The General Accounting Office has documented a variety of issues related to the implementation of spectrum management issues in DoD acquisition systems over the years.

So what are the obstacles that keep program offices and acquisition personnel from complying with federal laws and DoD directives on RF spectrum use and instituting good engineering practices on control of electromagnetic environmental effects (E3)? Volumes have been written on the need to comply with the spectrum regulations but the list of infractions continues as does the list of radio interference issues, both during acquisition and operationally. Current requirements and methods for assuring that systems have spectrum access and electromagnetic compatibility are scattered among a variety of DoD Directives, Instructions, MIL-STDs and Handbooks; and they can be poorly defined with approval processes that are hard to understand, slow, subjective and inconsistent. These volumes of requirements documents, which currently define the processes for obtaining spectrum access, acquiring authorized frequencies and controlling E3, have created complexities that can inhibit successful implementation by program managers. Some of the requirements are technically daunting on the surface, yet technical experts are available within every military department to help as necessary.

In addition to documenting the requirement for SSRAs, DoDI 4650.01 also provides a great deal of guidance, in the form of suggested tasks, for the program offices to follow. Unfortunately, it doesn’t provide a specific approach to integrating the tasks into an overall SSRA product. Subject matter experts from the three services have developed more detailed guidance and well as acceptable document format and content guidance. Some current suggested guidance for Program Offices, Acquisition Managers, and system developers to follow include:

(1) Determine the spectrum required to support the mission and define the intended EME in which the system will operate.
(2) Ensure E3 control and SS requirements are addressed in JCIDS and defense acquisition system documentation.
(3) Apply interface standards such as MIL-STD-461 and MIL-STD-464 to ensure that the system and its subsystems and equipment are built to operate compatibly in the mission EME.
(4) Define E3/SS test objectives in the Test and Evaluation Master Plan (TEMP) and allocate sufficient resources to conduct test objectives.
(5) Verify and document SS and E3 control issues during developmental and operational test and evaluation.
(6) Conduct early E3 and SS operational assessments that consider the intended mission including single Service, Joint, and international deployments.
(7) Provide E3 assessments during operational test readiness reviews. Report the operational impact, system limitations, and vulnerabilities from unresolved E3 and SS problems.

-Brian Farmer

Feedback on the Central Texas EMC Chapter Event and the Upcoming Santa Clara Show

2015-03-03T11:41:00.001-08:00

The Central Texas Presentation

The potential audience stayed away in droves with the result that the number attending the presentation was disappointing. This seems strange when you consider the Central Texas Chapter capture area has two major EMC chamber manufacturers, and that previous presentations I have given attracted a big audience. A lot of pizza was taken home – but thanks all the same to Nexio EMC Software for sponsoring this. The chapters are given a pittance to cover expenses such as providing hospitality at presentations.

Still, it is an ill wind that blows no good, and the small audience allowed deep discussions during the presentation as opposed to the normally practical requirement of holding all questions to the end.

And those few that did attend were rewarded by witnessing never before seen animations of a wave striking an angled hot-wall.

Some of what the absentees missed

After I reminded the audience about the various optical phenomena to watch out for, and gave my view on why absorber is shaped the way it is (maximizes the number of pyramids you can cut from a block of absorber material), I introduced my alternative ‘non-polarized’ layered-cone absorber design.

Foam based layered tapered loss approach

Layered approach a candidate for absorber cone design

Non-polarized layered-loss cone design

An astute observer spotted the flaw in the design before I got there, and for now it looks like the non-polarized idea won’t fly.

Any blog readers out there spot the flaw?

I have decided to create an ‘Inner Circle’ as regards early access to simulation results. The circle needs to contain a mix of ‘the right stuff’, from hands-on EMC engineers to engineers from outside the industry. The astute observer mentioned above won his spurs in the discussions and is now a member of the Circle.

The Inner Circle will plow it’s own furrow, and so has no use or time for ‘experts’ that know test standards chapter and verse, and blindly defend existing test set-ups. These guys are as much use to the Inner Circle as a chocolate coffee pot.

The expectation is that members will develop from passive observers to live participants in designing experiments that isolate various phenomena.

As I said, it is an ill wind that blows no good, and Ross Carlton of NI may be interested in my offer of on-site test-staff education on the physics behind EMC chamber tests.

If anyone is as keen as I am on using powerful 3D EM software to examine new ways to solve old issues, maybe we can meet up briefly at the Santa Clara show and I can run through the important slides in the presentation over coffee? Or if you wish to be considered for the Inner-Circle think tank, again, maybe we can meet up at the show? ‘Experts’ need not apply.

tom@lionheartsouthwest.com

Talking of Santa Clara, the show is almost upon us.

The Upcoming EMCSI Show in Santa Clara

I have an admission to make - I was under the impression that the Santa Clara show was created purely as a compensatory gesture to those who cannot make the annual August EMC event taking place in Germany this year. I have touched on the Dresden show in previous posts and, in my usual self-righteous manner, concluded that on occasion, it was good to push the international nature of the IEEE. But it still seems a little like two allied political parties ‘splitting the vote’ to neither party’s benefit.

However, now that I am checking the Santa Clara EMSCI website in preparation for attending, my eyes are opening to the fact that this is likely to be an event on par with Dresden. One key indicator is a booth space count of the two exhibition halls. A quick look shows around 196 booth spaces at Santa Clara (plus 16 table top spaces), against the currently planned space for 140 booths at Dresden (plus 50 table tops). In both totals, I took into account the number of booth spaces taken up by the ‘islands’ the bigger companies utilize.

So to all you Cinderellas out there unable to attend Dresden – “you shall go to the ball”, and on the face of it, it is likely every bit as good as the overseas event. Time alone will tell.

So, what should I go see at the show?

The keynote presentation is a must. Made all the more topical with the North Korean threats to South Korea, I have followed the HEMP topic with interest and would be interested to know how the HEMP filters placed at vital establishments guarantee to ‘catch’ the pulse coming in from the grid. Given the short rise-time of the HEMP induced wave, and given the finite time-constant that dictates the performance of all MOV based circuits, the race between the fast-rising pulse and the trigger time of the MOV is likely a race too close to call. Could be the MOV triggers 10 times faster than the time it takes the pulse to rise to a damage level. Somehow I doubt this. Or maybe giant ferrites slow the leading edge of the pulse giving the MOV time to trigger. The great thing about a show is that there is always someone there that knows.

So that is 8:30 – 10:00AM on Tuesday gone, and given my interest in absorber technology as regards chamber performance, I will definitely shoot at attending two papers on the use of absorbers at 11:00 and at 11:30AM. I already marked them in red on the program.

Then between 1:30 and 3:00PM there are panel discussions where the second panellist, and I quote, ‘.... will present aspects of absorber material and their principles of operation including various test methodologies for quantifying absorber effectiveness.’ Right up my street - yes please.

Naturally I shall be meeting with Dr Andreas Barchanski of CST at booth #412 to discuss the work on diagnosing the underperformance of 3m chambers. Add to this a bunch of scheduled meetings, and add a hunt on the exhibit floor for the newest and greatest new products out there, and I doubt there will be much time for lazing around.

To be continued .......

Tom Mullineaux
Lionheart Southwest

SSRA Components

2015-02-27T06:18:00.002-08:00

As we noted, the purpose of the spectrum supportability risk assessment is to identify and assess regulatory, technical, and operational spectrum issues with the potential to affect the required operational performance of the candidate system. For example, in addition to determining that a system’s bandwidth requirement complies with an individual nation’s frequency allocation scheme, a new or modified system must also be evaluated with respect to:

• The system’s potential to cause interference to or suffer from other military and civilian RF systems currently in use or planned for operational environments.
• The effect of the system’s proposed spectrum use on the ability of the force structure to access the RF spectrum without interference.
• How the system’s spectrum use conforms to the tables of frequency allocation of intended host nations, ensuring regulatory protection from other national co-band spectrum users.
• If individual host-nation frequency allocations include enough bandwidth to fully support the system’s operational mission, for example, required data rate.

An SSRA should include the following components:
• Regulatory component: Addressing the compliance of the RF system with US national and international tables of frequency allocation as well as with regulatory agreements reached at the International Telecommunication Union.
• Technical component: Quantifying the mutual interactions between a candidate system and other co-band, adjacent band, and harmonically related RF systems, including the identification of suggested methods to mitigate the effects of possible mutual interference.
• Operational component: Identifying and quantifying the mutual interactions among the candidate system and other US military RF systems in the operational environment and identifying suggested methods to mitigate for possible instances of interference.
• E3 Assessment: At a minimum, EMC and EMI are to be addressed to determine the potential for interactions between the proposed system and its anticipated operational EME.

When conducting an SSRA, operational restrictions, availability of frequencies, host nation approval (HNA), and known incidents of electromagnetic interference (EMI) must be considered. S-D systems and equipment cannot be operated legally until they have been granted equipment spectrum certification (ESC) by National and DoD authorities; in addition, a frequency assignment must be obtained from the appropriate area frequency manager. For systems that will operate outside the United States & Possessions, an HNA also is requested prior to operation in each foreign country designated for use.

Additionally, the program must be monitored to determine the EMC and EMI impact of any changes to such operational RF parameters such as tuning range, emission characteristics, antenna gain and height, bandwidth, or output power, etc. Changes to these parameters may require additional E3 analyses or tests. The E3 Assessment should:

• Identify and resolve co-site EMI issues during system acceptance testing.
• Demonstrate repeatable EMC utilizing appropriate development models.
• Maintain system E3 design integrity during operations.
• Implement procedures for EMI problem reporting.

Producing an SSRA isn’t a trivial exercise! Next time we’ll take a look at some of the challenges in putting an acceptable product together.

-Brian Farmer

The Diagnosis of Under-Performing Semi-Anechoic Chamber Designs #2, and a Report on the Joint Dallas EMC Chapter/Northwest EMC Event

2015-02-20T12:31:00.000-08:00

I apologize for the prolonged absence, my career has taken a new path and the preparation for this took up all my spare time. Things are freeing up once more, so I am glad to say I can now return slowly to blog writing.

I actually made the time recently to attend a technical presentation on obtaining wireless device approvals, jointly hosted by the Dallas IEEE EMC Chapter and Northwest EMC. The event took place on Wednesday 18 February at Northwest EMC’s new test facility in Plano Texas. I found the presentations full of useful information and the hospitality was simply great. If there is one of these free technical events locally, you are crazy not to go. And talking of events, I shall be making a presentation myself at the Central Texas IEEE EMC Chapter on Wednesday 25 February (see below). If you are local – you are crazy not to go.

Getting back to the Dallas event, the theme was on how to obtain wireless device approvals at home and abroad. After watching the presentations, if I was forced to summarize the current wireless device approval situation in a single clause, I would say ‘A fluid maze with many pitfalls in foreign approval processes’.

Harry Ward (of Regulatory Consultancy Approvals) kicked things off with an overview of what an approval is and the route to getting one. This was followed by Greg Kiemel (Director of Engineering at Northwest EMC) who filled in the drivers behind the explosion in the number of wireless devices and the current role of TCBs (TCB = Telecommunications Certification Body)

According to the presentations, service providers are spending billions at spectrum auctions because kids want video streaming, which translates to ever faster data rates. Apparently faster data rates became a reality when MIMO married OFDM, and the technology grew exponentially with spatial multiplexing in conjunction with OFDM achieving rates of 20Mbps.

As an indicator, it seems there is an average of 13 new tablet approval applications every month. With this number of approval applications it was a wise move by the FCC to pass the approvals process to TCBs. Without the introduction of the TCB program the FCC would have been swamped.

The Wisdom of the FCC Policy on Using TCBs

Readers should not run away with the idea that being a TCB is a license to print money. TCBs like Northwest EMC have invested a small fortune on wireless telecom instruments, many $70k to $100k each, and this despite business being very slow in the early days of the FCC program. The plot shows the hand over and pick up of applications over the years.

Getting back to market drivers, it seems it is not all about kids playing games on line. Bluetooth technology is now being used to provide better patient care by relaying monitored vital signs to care providers via MedRadio.

One pitfall that came out of the presentations was the hard to believe situation where companies actually design and build a wireless device without fully researching the approval hurdles set by each country in the World. They then find to their horror that they are not able to achieve the necessary approval for sales into that region.

Another pitfall involved not fully characterizing the emissions pattern of a device. Peaks and nulls can move around with small changes in device azimuth and measurement antenna height. The Northwest EMC solution is to gather emissions data over all permutations of the two. The first picture shows how the emissions pattern changes with antenna height (the red trace is the antenna at 1m high, the blue is at 1.5m high). The second picture shows how the device emissions are fully characterized.

Snaoshot #1 of Emissions from PDF of Presentation

Snapshot #2 of Data Providing a True Picture of EUT Emissions

Without this, there is the very real risk a competitor may inform the governing body that a device is not compliant (market surveillance is the main method of policing devices on the market).

If you want more / better pictures, I suggest you contact Alee Langford
alangford@nwemc.com to see if she will email you the PDFs of the presentations. We lucky attendees received them on a free memory stick.

The event was followed by a tour of the new Plano test facility, where I noticed that the layout was identical to the one I have seen at their California facility. That is different rooms hold different test set ups, and a EUT is simply wheeled from one room to the other. This avoids the need to tear down and re-assemble test set ups time and time again. Of course this makes for excellent measurement consistency and reduced wear and tear on equipment / connections.

Enough about the Dallas Chapter presentation, time to talk about my upcoming Austin presentation.

The full announcement with location, times, etc. can be found at

https://meetings.vtools.ieee.org/m/31729

My Upcoming Presentation on Under-Performing Chamber Designs

I will be explaining my approach on solving why today’s 3m chambers struggle to create a compliant test field over 1-6GHz. To conduct practical experiments would be costly and time consuming. But this is not the case with 3D EM simulation. With software modelling, you can change parameters at a stroke and observe the corresponding change in performance (thanks are due to CST for the loan of their CST Studio Suite® Software).

For those able to attend, it will pay to catch up on the background to these experiments by reading the article in the current issue of Interference Technology’s European Guide

http://www.interferencetechnology.com/applying-stealth-technology-to-emc-test-chamber-designs/

As explained in the article, the first target is the ‘hot wall’ behind the calibration plane, and hence the wall directly illuminated by the antenna.

There is an ever-present potential problem that constructive and destructive interference may occur at the calibration plane when the forward wave and any reflected wave combine. This is a non-issue when the absorber clad wall is doing its job and the reflected wave is small in magnitude. The contention is that the reflected magnitude is far from negligible, and when the electrical length of the return path is just right, worst-case constructive interference, or worst-case destructive interference occurs right at the calibration plane.

With the help of the software we should find be able to find out where and why. But in the olden days of software there was a saying, ‘Garbage In, Garbage Out’. This still applies today, and level heads must prevail in designing and conducting the experiments. That is, we must be able to isolate the contributions of the major phenomena causing the wave to revisit the calibration plane.

The phenomena will likely include partial reflection / scattering from the faces of the pyramid (the pyramidal shape is intended to present a mechanical impedance-taper to match the incident wave to the absorber material), refraction changing the wave direction away from the ideal on entry into the absorber, and reflection from the ferrite tiles behind the absorber. The pyramidal matrix may also look like a diffraction grating to the incident wave, causing further unintended wave redirection.

And let’s not forget that as regards a reflected wave bouncing off the ferrite tile, the mechanically matched absorber arrangement is a two way street providing as good a path back to the calibration plane as it presented to the incident wave.

Hope to see all you Austin locals at the Central Texas Chapter event.

To be continued............

Tom Mullineaux
Lionheart Southwest

SSRA’s Continued…Background on Spectrum Management

2015-02-11T07:03:00.000-08:00

To understand Spectrum Supportability Risk Assessments, we need a little background. In the DoD acquisition process, spectrum management usually begins with equipment spectrum certification, a process whereby a system is approved to operate in a particular spectral band. To actually operate the system, spectrum certification must be followed by obtaining specifically assigned frequencies. Obtaining frequencies to operate equipment in the U.S. is a two-step process which is managed by the submittal of a properly filled out DD Form 1494. The first step is Equipment Spectrum Certification. The certification process assesses equipment transmit and receive characteristics to determine if it complies with existing RF spectrum regulations. The second step, Frequency Assignment, coordinates operational use of specific frequencies within specific bands among current users so that they do not interfere with each other. The Manual of Regulations and Procedures for Radio Frequency Management, issued by the Department of Commerce’s NTIA, is the standard for both steps. The NTIA is the regulatory authority over all federal equipment and spectrum in the US&P. The Federal Communications Commission (FCC) regulates non-federal spectrum in the US&P.

The DD Form 1494, a document that captures an exhaustive variety of technical data, serves two functions:
(1) Provides a uniform method to capture the basic spectrum-dependent and operational parameters of military spectrum-dependent systems in a format that can be easily provided to US National and host nation spectrum authorities
(2) Standardizes the format of the technical data required to be inserted into DoD and national databases to generate frequency assignment approvals enabling initial EMC analyses, and checks for compliance to military, US national, and host nation spectrum standards. System developers will complete and obtain approval for a DD Form 1494 during each phase of the acquisition process for each newly developed spectrum-dependent system.

In addition to the Frequency Allocation and Frequency Assignment processes, DoDI 4650.01 now requires the conduct of a Spectrum Supportability Risk Assessment for the procurement of all spectrum dependent systems, including COTS. SSRAs will be required of programs at milestone reviews A, B and C as part of the overall balance of program success against future risks. A PM’s failure to obtain spectrum supportability for components in its systems has direct consequences to the program in meeting performance, schedule and cost objectives established by its Acquisition Review Board and to the Combatant Commander in meeting Joint Mission Area requirements.

It’s important to remember that the SSRA is about assessing risk. The Risk Management Guide (RMG) for DOD Acquisition defines Risk as a measure of the potential inability to achieve overall program objectives within defined cost, schedule, and performance/technical constraints and has two components: (1) the probability/likelihood of failing to achieve a particular outcome, and (2) the consequences/impacts of failing to achieve that outcome.

We’ll get into the specifics of the SSRA starting with the next post….

-Brian Farmer

Spectrum Supportability Risk Assessments (SSRA) – A Review

2015-01-29T06:01:00.000-08:00

The first in a series of discussions on a DoD acquisition requirement

Early consideration of spectrum supportability in spectrum dependent (S-D) system acquisitions is a fundamental criterion that must be satisfied before the DoD develops and fields communications-electronics (CE) equipment and related weapons systems. Development or acquisition of systems that meet operational requirements, but fail to obtain spectrum supportability, means those systems will not be allowed to operate in the United States or in host nations or may not operate properly give permission to operate. These systems create a potential for severe mutual interference between themselves and other spectrum users, waste valuable time and resources, and delay fielding fully functional warfighting capabilities.

Spectrum access is fundamental to all DoD missions. On-demand access to the spectrum and electromagnetically compatible operations in the EM environment cannot be assumed. The first step is to realize that you need to plan and engineer this into your system or capability. Then you must make sure you have the resources on hand to deal with this. In the end, getting the experts involved early will save you money, time, and aggravation. There are many examples of how the failure to properly address spectrum supportability during the design, test and production processes have caused program impacts in the areas of schedules, missed Milestones, significant financial issues, and/or a system that was produced with significant operational constraints on its use. A major reason why it is so important to manage spectrum resources is because it is under constant and increasing pressure from commercial service providers through reallocation and auctions.

“Spectrum Supportability” can be defined as an assessment as to whether the electromagnetic spectrum necessary to support equipment is available for use by the system. The assessment requires, at a minimum, receipt of equipment spectrum certification, reasonable assurance of the availability of sufficient frequencies for operation from host nations (including the US), and a consideration of ElectroMagnetic Compatibility (EMC). Guidance for these requirements are found primarily in DoDI 4650.01, Policy and Procedures for Management and Use of the Electromagnetic Spectrum, and to a lesser extent in DoDI 3222.03, DoD Electromagnetic Environmental Effects (E3) Program. DoDI 4650.01 requires the submission of a Spectrum Supportability Risk Assessment (SSRA) at each program milestone.

So just what is an SSRA? It is an evaluation performed by DoD Components of all S-D systems to identify and assess EM spectrum and Electromagnetic Environmental Effect (E3) issues that can affect the required operational performance of the overall system based on the mission needs defined by the combat developer and/or Joint Staff in the capabilities development process.

It’s important to remember that the SSRA is about assessing risk. The Risk Management Guide (RMG) for DOD Acquisition defines Risk as a measure of the potential inability to achieve overall program objectives within defined cost, schedule, and performance/technical constraints and has two components: (1) the probability/likelihood of failing to achieve a particular outcome, and (2) the consequences/impacts of failing to achieve that outcome.

We’ll get into the specifics starting with the next post….

-Brian Farmer

What is the future of SAR testing and relevant handset design?

2014-12-19T08:35:00.000-08:00

The requirements for SAR levels have remained largely unchanged for many years (requirements depend on the country/region). In fact, the FCC mobile/handset specific regulations are derived from levels determined in the 1960s during early research of radiation hazards.
Meanwhile, mobile phone designs advanced at light speed while the systems and methods made to test the SAR of those devices remained relatively unchanged for twenty years.
Times are changing now, though, and we’d like to look down the road to see how these changes will turn out.

More and more wearables will include cellular technologies
Currently, most wearables use WiFi or Bluetooth technologies that operate on low power transmission modes and do not require SAR testing. However, wearable devices in the near future are likely to use cellular technologies, emit more electromagnetic radiation, and require SAR testing to ensure compliance.

New technologies will mean more SAR testing
Recent and new technologies that are meant to increase datarates will also mean more SAR testing. With phones against the SAR limits, R&D stage testing is already increasing.
Fast SAR technology makes it possible to do serious line testing
Any piece of equipment that is mass produced has a margin of error in the production, resulting in physical variations from unit to unit. These variations can and do produce variations in SAR levels. With regulatory bodies testing devices across a statistical sample of units to determine the maximum SAR level, manufacturers also must perform random tests over a number of units to statistically ensure SAR compliance.
Fast SAR technologies are capable of performing instant line tests to further increase the statistical accuracy of SAR measurements.

Powerful chipsets that support more bands
The proliferation of chipsets like the Qualcomm RF 360 that support 40 bands mean that thousands of tests must be performed, rather than just hundreds. As chipsets become more powerful and band combinations to support LTE Advanced become more numerous, the amount of SAR testing required to launch a handset or mobile device will only increase.

LTE Carrier Aggregation
LTE Carrier Aggregation combines carriers at the device, using more frequency bands to get more bandwidth. This results in faster downloads and better performance. “Bursty” applications that require sudden bursts of bandwidth will perform much better.
However, all of this affects SAR and will require additional SAR testing, especially as LTE Advanced becomes more common, and more and more combinations of bands are used across the spectrum to deliver content. The exact SAR testing requirements are uncertain, at least according to the FCC.
LTE will likely make obsolete SAR testing technologies that cannot discriminate between different frequency combinations. With SAR values already on the edge of non-compliance at the moment, manufacturers are turning to Fast SAR technologies that can discriminate between frequency bands and also perform the tests faster. These next generation technologies are much faster, which is important because the number of tests to perform is greatly multiplied with LTE, due to the need to test all the band combinations.

Dynamic Antenna Tuning
Dynamic antenna tuning, for example the Qualcomm GFE15, improves antenna performance as the environment interacts with (gets in the way of!) antennas in handsets and other wireless devices. If you cover the antenna with your hand, for example, dynamic antenna matching adjusts the frequency to ensure the best possible reception.
This does, however, affect SAR. The FCC is still currently in the process of deciding exactly what needs to change in terms of taking SAR measurements to ensure that dynamic antenna tuning does not put SAR out of limits.
What it likely means is a whole new set of tests designed to account for and trigger Dynamic Antenna Tuning.

Designers will continue to come up with new tricks
Staying within the SAR certification limits is becoming more and more difficult for designers. With screens becoming larger while phones become lighter and thinner, antenna designers have less and less room, literally, to design their antennas and meet SAR regulations.
What’s more, people are increasingly dependent on their mobile phones, and their expectations are to have reception at all times, even when they are far from the nearest tower.
Thus, you have tricks of the trade like dynamic antenna tuning or proximity sensors that are meant to keep SAR within limits. MIMO is another tricky subject; check how these engineers dealt with MIMO designs. With phones consuming more and more power, we anticipate designers will innovate a few more tricks in the coming years, because they must.

SAR testing will only get faster
Fast SAR Testing technologies already exist. These technologies use a vector-based system that greatly reduces the time it takes to perform testing on handsets, as well as other innovations that can reduce the time-to-market for some handsets by a couple of weeks - a boon for competitors in the race to release new products.

-ART-Fi contributed this blog post

What is an EMC Design Review?

2014-12-05T06:24:00.000-08:00

What is an EMC Design Review?

Most hardware engineers are used to doing functional design reviews on their circuit boards and systems before pressing the 'go' button for manufacturing. In the engineering teams I've worked with, that type of design review has usually taken the form of standing around a board room table with a red pen and the largest print outs of the schematics that we could get our hands on. It can also be done by just one person at a PC with cross probing enabled between schematics and layout (if your boss was kind enough to get that license!). We sifted through the design, pin by pin and connector by connector, determined to find an accidentally reversed interface or incorrectly pinned out symbol. You knew that there was a good chance that the schematics in front of you contained a hidden mistake that had the potential to turn your high end circuit board into an expensive beer coaster, or cost you a lot of rework time at the least!

Companies that are on the ball have standardized this kind of review process into a checklist based procedure that is run through and verified on each and every design. By doing this, they increase the robustness of the review and minimize the chances of a major issue slipping through the cracks.

Surprisingly, very few companies that I've been involved with over the years have migrated this important process into the EMC world. In just the same way that a functional design review can catch many design errors, a solid EMC design review can catch lots of known poor design practices for emissions and immunity performance. It can mean the difference between your product passing or failing at an EMC test lab.

A good EMC design review should cover many aspects of an overall product design. At the top level, a design review should cover topics such as:

EMC as a Functional Specification

I like to think of EMC testing as a completely separate functional specification. There are a specific set of pass/fail criteria that your product must meet, or you can't sell it. It's that simple. Without a clear specification of the tests your product is going to be subjected to at an EMC lab, you are lacking a functional specification that should really be taken into account at the design stage of your project. You wouldn't dream of designing a product without a clear view of the feature set being provided to the end customer would you? EMC testing is a just a different kind of feature set that your design needs to include. Without an EMC functional specification, you're leaving a pass or fail at an EMC lab up to chance. Is it any coincidence then, that according to Intertek (one of the largest lab groups in the world), the global EMC first time pass rate is only 50%?

In this post I'm going to give an overview of what an EMC design review is and how to go about doing one on your next product.

Why Bother to do an EMC Design Review?
An EMC failure can very easily cost your company several thousand dollars in tangible expenses such as hiring a consultant, renting debug equipment, re-working/manufacturing designs and paying for re-tests. A larger concern for many companies is the delay to market that an EMC failure can cause. Since EMC approval is required before you can legally sell your product, a failure can introduce a delay to market of several days to several weeks. The longest delay to market that I've come across due to an EMC failure is about 4 months as the company really struggled to solve the issues through several design iterations, but I wouldn't be surprised if longer delays exist.

Accountants generally agree that a delay to market will reduce the peak sales volume of any given product by a few percent and depending on the nature of your product, may reduce the length of the overall sales window. For many medium to large hardware companies, a 1 month delay can easily lead to a reduction in overall revenue for a particular product during its' lifespan of greater than $1 million. Suddenly, considering EMC early in the design cycle doesn't look like a bad idea at all.

Lastly, doing a robust EMC design review can increase the performance of your product and therefore reduce the number of customer returns. For example, a company that has a product with relatively poor immunity protection may get lots of returns due to damage from static discharges coming from the end user's fingers when they're pushing buttons on the product. Or perhaps the product will be used in a relatively noisy electromagnetic environment and the functioning of the device is affected in some ways such as degradation of RF performance or measurement accuracy. Good EMC design can reduce these sorts of issues.

Companies who are lucky enough to have an EMC expert on staff may not have implemented a robust EMC design review (yet), but the knowledge locked away in the expert's head is probably already introducing an ad hoc review of sorts. What I've noticed is that design engineers who have significant experience with EMC testing tend to use their memory of EMC failure modes to avoid running into the same issues again in the future. For example, as they're laying out a circuit board (or directing a draftsman), they may remember a time when a previous product they designed failed radiated emissions due to excessive noise on the external cabling. The engineer then takes whatever design improvement they implemented to reduce the noise on the cabling and applies that methodology to all designs in the future. What they've essentially done is to build up a set of design rules that they follow for each successive design. As they experience more EMC issues and solve them, they add these rules to their existing mental list.

Standardizing these rules into formal design review procedure is key to repeatedly performing a robust EMC design review. Companies such as Ford and Jaguar who face some of the toughest EMC challenges (automotive) implement robust EMC design reviews. I believe everyone can benefit from taking this approach. One huge benefit of a formalized EMC design review is that it can be undertaken by a relatively junior engineer without much experience in design for EMC compliance. This is because it's much easier to check that a specific design rule has been implemented correctly than it is to do comprehensive EMC training and understand why a rule makes sense from a physics perspective. For companies without a seasoned EMC pro on staff, an EMC design review is a great way to quickly verify that your design has the best chance of passing EMC testing.

Unfortunately, most companies do not have a skilled EMC engineer on staff and do not have the in house knowledge to create an EMC design review process. EMC training is usually incredibly expensive, with typical pricing for a short 2-3 day seminar of >$1500 USD plus expenses per engineer.

I can't go into every single item covered in a comprehensive EMC design review, but in this post I'm going to detail a few of the most important aspects that I always check. The section below gives you a top level overview of how I do EMC design reviews for clients.

How to Get Started
Define your Pass/Fail Specifications

Without a clear definition of the pass/fail criteria, you don't have a good specification to work with. In some cases, your product may only be tested for radiated and conducted emissions performance. In others, your product may also be subjected to immunity phenomena such as ESD, surge and EFT. The first task is to detail the limits and levels for emissions and immunity that your product must achieve during EMC testing.

Emissions Limits
First, you should define what the emission limits are for your product. These may vary depending on the environment that your product will be used in and the regions of the world where it will be sold. One of the most important places to start is to work out whether your product falls under "Class A" or "Class B" limits. Here's how the FCC makes the distinction:

Class A digital device: A digital device that is marketed for use in a commercial, industrial or business environment, exclusive of a device which is marketed for use by the general public or is intended to be used in the home.
Class B digital device: A digital device that is marketed for use in a residential environment notwithstanding use in commercial, business and industrial environments. Examples of such devices include, but are not limited to, personal computers, calculators, and similar electronic devices that are marketed for use by the general public.

It's important to note that Class B limits are significantly lower than Class A limits.

Limits also differ between geographic region and between product types. Check the standards that apply to your product (or ask a test lab) to work out the limits that you're going to need to meet.

I've included an example below of some limits for a class B unintentional emitting device so that you can see what it looks like. The lower and upper frequencies covered by the limits vary depending on the internal clock frequencies present within your product and also vary depending on the standards that apply to your product.

Although you can get a feel for the radiated emissions from particular nets in your design using free tools like the 'Maximum Emission Calculator' tool from Clemson university, it isn't very practical to use tools like this for analyzing a full design. Emission limits typically cover radiated and conducted emissions as well as Ethernet conducted emissions (if applicable).

Finding out the emission limits that apply to your particular product gives you a good idea of how difficult it's going to be to get your product's emissions below the limits.

Immunity Levels and Pass/Fail Criteria
If immunity testing applies to your product (usually mandatory for CE Mark and global product family standards), then it's worth finding out what test levels an EMC lab is going to use, where the tests will be applied and how well your product needs to perform during and after the test (this is called performance criteria). This will help you to work out where to apply EMC suppression devices, what ratings to choose and will also give you an idea of how much care you need to take. If you have a regular test lab that you use, they should be happy to provide you with a test plan up front so that you can use it in the design stage of your product development cycle. If you have a relationship with a test lab, they should be willing to help you. If they're not, it might be time to start looking for a different test lab.

You can see an example of an immunity test plan below, which includes the levels that will be applied to your product and the criteria that your product needs to meet.

The definition of the pass/fail criteria can vary a bit from standard to standard, but here is a description of each performance criteria taken from the international generic immunity standard for residential, commercial and light-industrial environments (IEC 61000-6-1):

a) Performance criterion A: The apparatus shall continue to operate as intended during and after the test. No degradation of performance or loss of function is allowed below a performance level specified by the manufacturer, when the apparatus is used as intended. The performance level may be replaced by a permissible loss of performance. If the minimum performance level or the permissible performance loss is not specified by the manufacturer, either of these may be derived from the product description and documentation and what the user may reasonably expect from the apparatus if used as intended.

b) Performance criterion B: The apparatus shall continue to operate as intended after the test. No degradation of performance or loss of function is allowed below a performance level specified by the manufacturer, when the apparatus is used as intended. The performance level may be replaced by a permissible loss of performance. During the test, degradation of performance is however allowed. No change of actual operating state or stored data is allowed. If the minimum performance level or the permissible performance loss is not specified by the manufacturer, either of these may be derived from the product description and documentation and what the user may reasonably expect from the apparatus if used as intended.

c) Performance criterion C: Temporary loss of function is allowed, provided the function is self-recoverable or can be restored by the operation of the controls.
It's essential to find out the performance criteria that your product will have to meet at a test lab.
Emissions

1. Pick the right stack-up

Configuring the right PCB stack is fundamental to EMC performance. Without an adequate layer count for a given complexity and without the correct layer order and usage, you're going to make your job of controlling radiated and conducted emissions pretty hard. If your boss is pushing you to reduce the layer count to save on PCB costs, just remember that there is usually a trade off with EMC performance, which has the potential to hurt the company wallet even more.

The desirable features of a good PCB stack (from an EMC perspective) are:
• Signal layers adjacent to plane layers
• Signal layers tightly coupled to return power/ground plane
• Power and grounds closely coupled
• High speed signals on buried layers
• Symmetrical

With those features in mind, here are sample layer stacks for 4 and 6 layer boards:

4 Layer Example Stack

In this diagram you can see a couple of different ways that 4 layer stacks can be implemented. See if you can work out from the desirable characteristics listed above, what the trade offs are for each stack.

6 Layer Example Stack

Can you work out why the stack on the left is not recommended?

I don't have time to cover it here, but the stack geometry and PCB materials can also heavily affect EMC performance. That's because it will define the impedance of nets and planes which can vary from layer to layer. Without careful consideration of the board impedance, you can run in to significant signal integrity and power delivery network (PDN) problems.

2. Does every IC have adequate de-coupling?

This one is repeated a lot, but it's worth verifying and confirming during an EMC design review that every single IC has adequate decoupling. When I do EMC design reviews for clients, I usually find at least a couple of ICs or PWR/GND pin pairs where the designer forgot to add de-coupling caps or the caps have been implemented in a way that would make them pretty ineffective. You can see the effect of adding de-coupling to an IC below (reproduced with permission from Williamson Labs).

The upper left diagram shows the effect of de-coupling on supply noise in the frequency domain and the upper right diagram shows the effect in the time domain. You can see that the optimal performance is when the decoupling cap is place below the IC and routed as directly as possible between the PWR and GND pin pair.

It's worth noting that the important characteristics of decoupling capacitors include not only the value of the capacitor (nF, pF etc) but also ESR (Equivalent Series Resistance) and ESL (Equivalent Series Inductance). This EDN blog post does a good job of explaining the effect of those characteristics on PDN impedance.

3. Check for breaks in the return path

If you're not familiar with return paths, it just means the route current flows back to the source. Whenever you have an IC that supplies current to another device, there is an equal and opposite current that travels back to the source device.

The diagram above illustrates that the return path in DC travels the path of least resistance. The return path in AC (higher frequency signals) travels the path of least inductance, which is usually on a reference plane directly underneath the source signal. Whenever there is a cut in the return path on the reference plane, the return current has to flow around the cut to find the path of least inductance. This forms a nice current loop that has the potential to become an efficient radiator.

The diagram below shows a 0.5 GHz signal that traverses a cut in the ground plane. You can see in the right hand picture that the current distribution on the return path finds a way around the slot and therefore makes a loop. The left hand picture shows a scenario where the cuts in the return path have been bridged by capacitors which allow the high frequency return current to 'jump' over the gaps.

The effect on far field radiated emissions from this one signal might surprise you. The simulation of this scenario shows that emissions measured at 10 metres from this net alone can be more than 30 dB higher with a cut in the return plane. This can definitely mean the difference between a pass or fail at an EMC lab.

[Source unknown]

During an EMC design review, you should view your layout 2 layers at a time and carefully verify that all of your digital signals have a well defined return paths without cuts. If a cut in the return path is necessary for any reason, then ensure that the cuts have been bridged with capacitors that have low impedance at the frequencies of interest.

Immunity

1. Has ESD protection been applied to all external ports?

Once you know which ports of your product will be tested, you can add the ports to your EMC design review checklist. Now go and verify that each of the ports has adequate ESD protection on the circuit board, in the right places and of the correct rating. Here is an example of a USB port that has good ESD protection (from Texas Instruments).

Note that the protection should be placed as physically close to the port connector as possible. Many different types of ESD protection devices are available. Check out the ESD section in the "Getting EMC Design Right First Time" eBook for more details.

Closely related to this item is to verify that net to net clearances and and net to plane clearances near the ESD injection points do not allow for 'arcing'. This is to ensure that the ESD can't jump to another conductor and potentially cause damage elsewhere in the product. The suggested clearances required to ensure that arcs do not occur for given voltage levels in various circumstances are covered in the IPC-2221 standard.

2. Sensitive Circuitry Shielding

Radiated immunity is a very common test that applies to most electronic devices destined for Europe, or those that adhere to international product standards. The field strengths that your product may be subjected to can vary between 1V/m and 200 V/m depending on the standard. The field is also usually swept across the frequency range 80 MHz - 4 GHz (some standards may go higher and lower than this).

If your product contains sensitive circuitry such as analog measurement circuits or RF circuitry, then even low power radiated electric fields can introduce major issues. A common and inexpensive way to protect circuitry from radiated fields is to provide a small shield over the circuitry of interest.

A few simple checks that I do during an EMC design review are:
• Are there any circuit elements that are likely to be susceptible to radiated electric fields?
• Is shield grounded at multiple points?
• Is shield fitted tightly to PCB?
• Check solder joints are not dry
• Are apertures (e.g. for ventilation) in the shield minimized?
• If apertures are required, are they positioned at furthest point away from sensitive circuitry?

Note that shielding alone often isn't enough to immunize your sensitive circuitry from radiated electric fields. Often the field is picked up by external cabling and is coupled on to a circuit board. Once that energy is on the circuit board, it can affect the sensitive circuitry by things like voltage fluctuations on the ground or power planes. The EMC Design Review Pro software goes into much more depth regarding radiated immunity, covering items like bandwidth limiting, sensitive net considerations and internal cabling to name a few.

-Andy Eadie

Note: This blog post was edited and reproduced with permission from EMC FastPass. EMC FastPass helps manufacturers get to market cheaper and faster with design review software and online EMC/RF video training courses.

The Diagnosis of Under-Performing Semi-Anechoic Chamber Designs

2014-11-18T05:54:00.001-08:00

As mentioned in the upcoming article ‘Applying Stealth Technology to EMC Chamber Designs’, and also mentioned during the recent EMC-LIVE webinar ‘Elephants in the TestRoom,’ the intention is to use 3D EM simulation software to explore possible design improvements to today’s under- performing chambers.

The article itself is not out yet, but we can make useful progress in the meantime by exploring the performance of the present ‘hot-wall’ arrangement. The hot-wall is the absorber clad wall behind the RF immunity calibration plane, and currently test labs are forced to point the antenna at a corner of the chamber to achieve test field compliance. Later on we will explore hot-wall wave-deflection (angled faces) as a possible design improvement.

Thanks are due to CST for the loan of their CST Studio Suite® 2014 Software, and particular thanks are due to Dr. Andreas Barchanski of CST for helping with the modelling and analysis.

IN THE BEGINNING

As with any engineering project, before any engineering time is invested, it pays to be sure of the required outcome, and have a direction of attack.

Well, the first objective is to establish why it is so difficult to obtain field uniformity at the calibration plane with the present hot-wall design, so I propose our direction of attack is to gather information on the performance of a standard flat absorber clad wall. I further propose we establish the individual performance of each type of absorber (pyramidal and ferrite tile).

GETTING STARTED

We will start with an analysis of the pyramidal absorber performance. As stated above, we first need to be clear on what we want the software to do and how we want it to do it.

Figure 1 shows a side view of a single pyramidal absorber. It is made from carbon loaded dissipative material shaped to form an impedance taper. We want to examine it’s two-way wave attenuation performance, so there is a fully reflective PEC (perfect electrical conductor) sheet covering the surface of the pyramid base. The pyramid is designed to face the wave it is intended to attenuate directly, so we will get the software to strike it with a plane wave at normal incidence and make a relative measurement of the ‘echo’.

A field probe positioned to the left of the pyramid will see both the incident and reflected waves, so I suggest we use a trick of the trade. In RF engineering, if you want to establish the loss (unwanted copper and dielectric loss) in say a four-way combiner, one approach is to manufacture two of them, connect them back-to-back (connect four connectors to the other four connectors), and measure the overall loss. Half that loss and you have the loss of one combiner.

We can use a similar approach with the pyramid absorber by placing two of them back-to-back as shown in Figure 2. We then place one field probe on the left and one on the right and measure the fields in isolation. We lose the 180 degree phase change caused by the reflective sheet (we will need to remember this), but we gain data on the reduction of the magnitude of the ‘echo’ wave.

Figure 3 and Figure 4 show the same approach used for a non-normal angle of incidence.

ANTICIPATED RESULTS

It is good practice to anticipate the results so you can use them in a sanity check later.

Figure 5 shows the plane wave about to strike the absorber. The vertically polarized RF waveform is represented by colored arrows showing the direction and strength of the field along one wavelength. Starting from the right and working left, the arrows show the upward pointing field of the first half-cycle gaining in strength to its peak (red), then falling in strength for the remainder of the half cycle, and then repeating this variation in strength for the downward pointing field of the second half-cycle. The upward pointing arrow on the right will impinge on the absorber first.

Figure 6 shows the anticipated results. For clarity only the effect on the peaks of the wave are shown. The right hand arrow in the pyramid represents the first half-cycle peak as it travels though the absorber. The center of the arrow has passed furthest through the pyramid lossy material and so that segment is attenuated the most (now green). Segments above and below the center have travelled different distances and so display different levels of attenuation. The left hand arrow has not travelled as far and only the center segment (orange) has dropped in field strength up to that point in the pyramid.

EARLY RESULTS

Well, the anticipated results went straight out the window because the actual results were a complete and utter surprise.

Figure 7 shows a still shot of the animated simulation results. When the animation is run, blocks of concentric field-rings march left to right in much the same way as the satellite view of a hurricane making its way West to East over the Gulf of Mexico.

It is mesmerizing to watch. Hopefully the EMC-Zone editorial staff can arrange for readers to witness the animation on-line.

They say there is nothing new under the sun and the effect may have been predicted/observed by some great scientist (Fresnel or another), but until we know what they are, and for the purposes of this EMC-Zone discussion alone, we will christen the blocks of concentric rings ‘Eddy Field Blocks’ (EFBs).

Even if turns they only occur over a comparatively narrow frequency band, or due to slowing of the wave (dielectric constant of pyramid material holding carbon in suspension not equal to 1), they are absolutely fascinating to observe. Anyone out there know a meteorologist or someone familiar with fluid dynamics that has seen these type of circular fields before? I know vortices form in fast moving rivers etc, but these seem quite different (no spiral swirl to the center).

ROUGH DESCRIPTION

Initial observation suggests:-

An EFB begins to form inside and outside the material at the apex of the pyramid.

The EFBs are created at the apex every half-cycle of the incident wave, and dissipate at the base of the pyramid at the same rate.

EFBs are made up of concentric field rings, each ring pointing in the same direction, each ring remaining separate from the others within the eddy block.

Adjacent EFBs have field rings pointing in the opposite direction (clockwise or anti-clockwise).

The diameter of the EFBs is about ¼ wavelength of the incident wave.

The created EFB moves to the right and then pauses for the time it takes ¼ wavelength of the incident wave to pass. The EFB creation rate and dissipation rate are still the same (one every half cycle of the incident wave).

The EFB seems to be an entity in its own right, evidenced by continuing to exist while an incident wave minimum passes past it during the time the EFB pauses.

To be continued............

-Tom Mullineaux

Elephants in the Test Room Roundtable Commentary 1

2014-10-27T07:40:00.002-07:00

On Thursday, Oct. 16, Interference Technology hosted a roundtable during EMC Live - 'Elephants in the Test Room' based on our blog series. Below is additional commentary about testing issues, from our expert panel.

Panel
Tom Mullineaux - Moderator, Consultant and Author
Patrick Andre - President, André Consulting, Inc.
Fin O'Connor - Defense and Space Consultant and Contractor, Alion Science and Technology
Adiseshu Nyshadham - Senior Consultant, DVT Solutions Inc.
Steve Koster - Vice President, Washington Labs

Elephants discussion points:

Elephant Discussion #1 – Poor EMC Measurement Consistency
No one is surprised when a round robin test shows multiple EMC testhouse measurements taken under supposedly identical test conditions are up to 10dB apart. By its nature, the ISO17025 laboratory accreditation standard covers a very broad church of test situations. However, the EMC industry is a distinct, identifiable niche. The various compliance groups providing the audit-service should be able to work together to improve inter-EMC laboratory measurement accuracy.

Can the EMC industry work with ISO17025 laboratory accreditation teams to improve measurement consistency?

Elephant Discussion #2 – Underperforming EMC Chambers
When calibrating a test field to 6GHz for commercial RF immunity testing, to obtain a compliant test field many test houses are finding they are forced to point the antenna at one corner of the room. All standard 3 meter semi-anechoic test chambers are cuboids with flat walls, ceiling and floor. The four walls and the ceiling are clad in RF absorber. The ‘hot’ wall (the one behind the calibration plane) performs the same as the other three walls.

Is a flat absorber lined ‘hot’ wall truly the only possibility?

Elephant Discussion #3 – Automotive Tests that Put the Car Audio System Performance First
Until fairly recently it seemed the RF immunity tests conducted inside the car cabin space were purely to ensure good sound system performance.

What is the future of RF immunity tests that actually check for electronic sub assembly compatibility inside the cabin space of a car?

DISCUSSION:

Patrick:
Q. Can the EMC industry work with ISO17025 laboratory accreditation teams to improve measurement consistency?
A. Yes, improvements can always be made. However the challenge will be to first establish repeatability in a single test setup for each chamber. Often, changes of ±4 dB is seen with the most minor of test setup changes. The movement of a cable or the position of the support equipment can radically alter the results. Added to this the difficulty of different laboratory layouts, with different sized rooms, anechoic material differences, antenna configurations (both type and location), power line and signal line routings into and out of the room, to name a few, and soon you have so many variables between two so-called “identical setups” that getting only 10 dB differences is actually remarkably good.
Q. It might be necessary to introduce the use of a fully anechoic chamber where the floor is now covered with absorber, reducing at least one variable in the measurements.
A. All standard test chambers are cuboids with flat walls, ceiling and floor. Is this shape truly the only possibility?
Of course not. Reverberant chambers are already being made with non-parallel walls. Also, simply adding a metal or anechoic panel at a 45 degree angle in a corner can alter the fields significantly. However, I am somewhat surprised by the difficulties the labs have, since the cone style of anechoic material should work remarkably well at these frequencies.

Fin:
Boy, the #1 could go on for days!! I’m on the 461 committee and we will not have any uncertainties in the spec (ever as some say) the uncertainties are built into the limits. Now the test problem differences are very difficult to get a handle on. I travel to many independent and gov labs witnessing testing for NAVAIR and the differences in the way these test houses INTERPRET the standard is surprising. I am trying to get more description for each test into the ‘G’ version of the standard (which we are working on now) but I am getting resistance because they feel the standard has enough description for the competent engineer/tech. I was also involved, years ago, with the round robin testing for the NVLAP Mutual recognition project and this was a for the very simple open filed testing with specific parameters. And even with this test control, the differences between the test houses was 10 + db at least. I would love to see some way to get the testing differences smaller so anything that can be suggested and proven would be welcome but it’s a uphill road.

Patrick:
I think Fin and I are in violent agreement. As a fan of MIL-STD 461 and DO-160 (mainly due to my aerospace background), I love the fact that cable length and positions are defined and controlled (1 meter, 2 meters, 3.3 meters, and so forth, depending on the test and standard). Cables are placed on 5 cm standoffs, 10 cm from the front of the ground plane. And even then, even with all this control, we get these variations.
I am working on a piece of medical equipment, with five transducers, five monitors, plus peripheral and support equipment. How do I ever lay out the cables and support equipment the same so I can get the same results twice? I have seen 5 dB swings just moving one cable.
I think the only science less accurate than ours is Astronomy, where they state distances of remote objects within a couple decades, e.g. 10^6 to 10^8 light-years away. And I wanted to be an Astronomer.
Then again, I have had no success convincing the police officer of the accuracy of my speedometer. He just doesn’t buy the idea that as long as I am between 6 MPH and 600 MPH I am within the range of accuracy.

Tom:
Just to make sure you are up to speed with where the elephants are history wise, you should read the article I wrote recently for ITEM.
http://www.interferencetechnology.com/a-design-review-of-the-automotive-radiated-emissions-test-fixture/
In the article I am very critical of the design of the emissions test fixture. I have just received 3D EM software from CST so I can come up with a superior arrangement. Might have preliminary results in time for our webinar. But you should know that if you praise the 5cm high cable over a ground plane I am likely to lambast you without mercy, and have supporting data to do it with.

Fin:
Oh no, not a 5 cm debate. I’m always interested in why the specs are the way they are. I will be real interested in your research, and will pass on to the committee,if it will make the readings better. But being honest, we have been using this method for the longest and our planes fly, missiles find there mark and radios work, in theater/hostile environments and we have feedback from the fleet and we track problems and why they occur (NAVAIR has the ASEMICAP group which tracks fleet problems and finds the reason and solutions) so to get the community to change, you will have to have a really good reason. But sometimes change is good!! Just hard to get through.

Tom:
Yes, the long history and the fact that planes are not falling out of the sky speaks volumes. Hard to argue with physics though, particularly the possible mismatch between the noise source (EUT) and the test fixture.

Fin:
Tom, ‘test fixture’?? Not sure what you mean? The 5 cm is to closely resemble the common actual installation (Military environment, loop impedances), to get some consistency and to work with the LISN (below 10 MHz). Unfortunately though, as Patrick had, most of the time, the set-up is a mess and standardization is mostly impossible so these controls are the best comprise we have. But, like I’ve said before, suggestions are welcome, at least by me!

Patrick:
I agree with Fin completely.
A story I was told about 5 cm standoffs: I was doing testing at Celect, a division of Cincinnati Electronics, in the early ‘90’s with a man named John Day. When I made a comment about 5 cm standoffs, he stated that he may have had something to do with the fact they are a bit inconvenient. He told me that in the early days of military EMC emissions, someone thought that it would be a good idea make sure the cables are not laying directly on the copper bench, so they grabbed 2x4’s and put the cables onto them (which are 4 cm high). However, once things needed to be documented, and they were converting to metric, John was asked to go measure the height of the cables off the ground plane in centimeters. When he reported the measurement, he reported the height of the cable, which was a large cable bundle, and had measured to the center of the cable – which added 1 cm to the overall height.
He said the next thing he saw was a document stating the cables needed to be on standoffs 5 cm high based on the past tests. And he did not correct it.
Or this could just be a story he told me.

Fin:
FYI, some history . I knew Ken would have the lowdown. He has a pretty good museum of old equipment and books on EMI. And a pic of some of our old generators!!
Fin,
The answer involves some heavy math.
The first part of the heavy math is if one picture is worth a thousand words, how much is two pictures worth?
The first pic shows a radio room in a WW II-era bomber. On the left is the radio, on the right the unshielded antenna lead from the radio is visible held above aircraft structure via porcelain standoffs, to minimize capacitive loading of the high impedance signal.
The second pic shows the standoff dimension, in inches.
The rest of the heavy math is the conversion to MKS, which is left as an exercise for the reader...
- Ken Javor

Patrick:
Oh, of course. And I should have known Ken would know.
And I kinda figured John Day was handing me a line. Kinda.
Thanks! This is great stuff.

To be continued........

For more information on the roundtable, visit EMC Live's website here.

Continuation of the Annual IEEE EMC Show in Dresden, Plus Updates on Other Topics

2014-10-07T06:38:00.003-07:00

Welcome back. Before we get started - two updates:-

3D EM Software Analysis of the Automotive Emissions Test Fixture

The tutorials in the handbooks are completed (there are quite a few because there is an entire suite of analysis tools). Also done are the ones that are downloaded with the software. With all tutorials, a lot of time is devoted to constructing the models to be analyzed. This makes sense since an accurate 3D EM analysis of a real-life component depends entirely on an accurate 3D model.

Attention has now moved to the more comprehensive tutorials on the software house’s website (you have to sign in with the license details).

It can be hard not to try running before you can walk. One of the booklet tutorials is on the analysis of a monopole. It crossed my mind that if you lay the monopole on its side, placed a conducting sheet below it, and terminated the far end in 120 ohms, then ‘hey presto’, you have the basis of the automotive emissions test arrangement. Probably better to be more familiar with the many tools available before going down that path. For instance, maybe creating the test set-up in the cable analysis tool, and then transferring it into the microwave tool is the best way to go.

By the way, I added a new task to the EM analysis list: – Establishing the change of attenuation when an RF wave strikes a pyramidal absorber at an acute angle.

It is all to do with a proposal that could improve the performance of 3m semi-anechoic chambers. An article titled ‘Applying Stealth Technology to EMC Test Chamber Designs’ will appear in the next Interference Technology publication (2014 EMC Europe Guide).

The pictures show the first efforts at defining and placing primitives and then using the knowledge gained to create a pyramidal absorber shape.

To be continued.......

UPDATE ON EMC-LIVE ROUNDTABLE EVENT

Theme: Elephants in the Test Room

Date: October 16

Time: 1:30 pm EST

Venue: Your computer

A prize-winning academic has joined the panel, and one of the expert panellists at the round table actually has access to a study that collected and compared test-house emission measurements on a ‘golden’ test piece.

I, armed with a jaundiced eye, am at odds with the entire premise of the study and see the ‘problem’ being addressed as systemic, even before operator competence is factored in. I think due to its longevity, and hence people growing up with it, many in the industry are in awe of the OATS set up. To my mind they worship a false idol.

And that’s just one of the topics - be sure to mark your calendar, be sure to register beforehand (http://emclive2014.com/elephants-test-room/), and when the big the day arrives, be sure to set the alarm on your cell phone. And then listen in on the fireworks!

The Annual IEEE EMC Show – Why Dresden?

So far we have covered the motives of those at the show to attend presentations and those at the show to sell (vendors).

We concluded that vendors are a sideshow, and that the survival of the symposium is down to the audience count, that is the number of registrants there to learn from papers and workshops.

Obviously the number of registrants that sign up to learn is directly proportional to the relevance and quality of the papers / workshops on offer.

So what entices a person to go to the trouble of composing and then delivering a paper / workshop?

The Presenters Tale

We start with papers. We will focus on workshops next time.

Presenters of Papers

For the purposes of this discussion, academic papers will not be a factor. In my view the main reason academic staff present papers is down to the ‘publish or die’ syndrome, and as such, content is rarely a compelling reason for prospective registrants to attend. There are exceptions, but they are few.

That pretty much leaves industry professionals as the technical-paper presentation candidates. So if we accept the main body of presenters is made up of industry professionals, what do they get out of it?

Presenter Motives

These include but are not limited to: -

· The travel and subsistence costs are covered by their employer

· To raise or maintain their profile in the industry

· Straightforward self promotion (consultants)

· Have stumbled on something big in their work (hands-on professionals)

· Have a gripe about a technical issue, and it is gnawing at them

· They are selling something

Most of these are self explanatory so I will expand on only a few.

Vendor staffs often get to witness life at ‘the coal face’, so on occasion they have something useful to contribute. Their flight and hotel are already paid for, so expense is not a barrier. As much as they would like to deliver marketing dressed as technical discourse, this won’t get through the paper acceptance filter. The paper has to have relevance and a good measure of technical content, so the only reward the company backing the endeavour can expect is their company name in the presenter’s title.

Papers are a great way to raise the profile of a newcomer to the industry, and one of the quickest ways to get to know key players in the industry.

One man companies such as consultancies know papers are an excellent way to promote their services

If something is seriously new in the industry and an innovative ‘mousetrap’, with the addition of a bit of gratuitous math, these have been known to get through the symposium filter process.

Presenter De-motivators

These include but are not limited to: -

· Travel and subsistence costs not covered by a sponsor

· Pressure at work

· Employer has not budgeted for staff excursions

· No experience at writing papers

· Not good at public speaking (jitters)

· Cannot guarantee will be available in 8 months time (too far ahead, fluid situations at work and at home)

Let me pose some questions: -

1. If you were on the symposium board, what recommendations on raising the relevance and quality of papers would you suggest?

2. Do any of your suggestions tie in with the strategy behind the European venue?

To be continued......

The 3D EM tutorials are digging into to the time available for blogging, so I will pick up Elephant #4 ‘The Zip Code Lottery in Achieving Product Compliance’ next time.

-Tom Mullineaux

The Annual IEEE EMC Show in Dresden, Plus Elephant #4 ‘The Zip Code Lottery in Achieving Product Compliance'

2014-09-24T11:16:00.002-07:00

Hurrah!! The 3D EM software has arrived and I have started to familiarize myself with it. I am going to enjoy this. Reading through the features, it is extremely powerful and looks perfectly suited to simulating the automotive test fixture. It always seems to take me about 40 hours at the keyboard to get up to speed with a new software package, so it will be a while. Meantime ....,

The Annual IEEE EMC Show – Why Dresden?

Last time we covered the vendors’ motives for attending a show, and made the claim that, despite the self-declared importance of the largest vendors, technical paper attendees rule when it comes to the survival of symposiums. Here is why: -

Shows come into existence when a small number of like-minded individuals (usually members of an engineering society) have a common interest in an emerging engineering discipline. Sooner or later it makes sense to meet up to share knowledge, often through the mechanism of technical presentations. This draws vendors like bees to a honey pot, and right from the start a ‘table-top’ sideshow becomes part of the event. As attendee numbers grow, so does the size of the sideshow. Exhibition hall fees soon make up a sizable part of the show revenue, which is why the largest of the EMC exhibitors see themselves as indispensable to a show’s survival. But when did you ever see even the largest vendor actually go out on a limb and pay for their own event on a scale equivalent to the annual IEEE EMC show? Why would anyone attend? It is like asking someone to subscribe to a technical journal with nothing but advertisements in it.

Vendors need to face facts, they are a sideshow, beholding to the symposium organizers; not the other way around.

The Presentation Attendee’s Tale

Given their importance, what draws a technical presentation attendee to a show?

Obviously, the papers/ workshops being presented. Now. unless they have a thousand dollars or two burning a hole in their pocket, these attendees are likely to be employer backed and likely be responsible for the EMC compliance of that company’s products. If newly appointed, an employer with an ounce of sense will want the new appointee ‘up to speed’ as quickly as possible. If the attendee is mid-level experienced, a worldly-wise employer will want that knowledge kept up to date. If highly experienced, the employer better keep that ‘hard to replace’ employee sweet should they wish to retain him/her.

Attendee Motivators

For the new guy, being responsible for product compatibility all of a sudden is disconcerting. Often due to an ad-hoc internal transfer or promotion, it is not uncommon for say a digital-electronics engineer to be saddled with the responsibility. The employer and new appointee both know they need to fill vast knowledge gaps quickly, either directly by way of attending ‘introductory’ technical presentations and workshops, or indirectly through making the acquaintance of experienced attendees at the show (so they can tap their brains later).

For the guy with mid-level EMC experience, keeping knowledge current is the chief motivator, and they will seek out and attend papers that update EMC personnel on standards changes, new technology, etc.

For the highly experienced guy, one pertinent topic can be enough to entice. They like to keep ahead of the curve with theoretical sessions, and during question time, can give vital input to leading-edge papers.

For both the mid and high level attendee, networking can be seen as important for job security

De-Motivators

Pressure at Work and No Travel Budget

Sadly, it is harder for staff to attend shows nowadays. Sometimes due to project pressures (everything seems to be project based these days), and sometimes due to increased pressure at work due to previous payroll cuts. It can be impossible for key staff to be ‘lost’ for a week, or even for a day. And in the rare situation where they can be spared, travel is often the first victim of budget cuts.

Regarding payroll cuts, I can state from first-hand observation that that the disappearance of support staff results in highly paid design engineers being reduced to busying themselves with jammed photocopiers, when they should be pressing ahead with the design of next year’s world leading product. Corporate madness prevails sometimes.

Types of Paper Delivered

Papers seem to range from the outlandish (university staff doing the ‘publish or die’ thing), to the same-old, same-old revamping of old topics. ‘Introduction to EMC’ type papers are always of use to a newbie, but to the more experienced engineers, I think perhaps the papers are less of a draw. Possibly as a result of the maturing of the industry.

To be continued.........

Elephant in the Test Room #4 - The Zip Code Lottery

The room: Whether a manufacturer’s product passes the relevant emissions test can be a matter of where in the Country their premises are located

The culprit: The seemingly EMC industry wide acceptance that a 10dB variance in emissions measurements made on the self-same ‘golden’ emitter test-piece is OK

The consequence: This opens up the prospect of the unfair situation where one manufacturer’s design with comparatively superior emissions performance fails, while a second manufacturer’s design with inferior emissions performance (compared to the first) passes at another location

NOTE: Be sure to listen in on the EMC-LIVE webinar event October 16 as this topic will be one of the roundtable discussions ( EMClive2014 ). This is sure to be fun because in truth I am simply a bystander with a pretty good understanding of the physics behind EMC tests, and the event is a chance for the experts on the panel to put me in my place.

Continuing with the elephant at hand, in my view, the ‘root cause’ blame for its existence, and likely continuing existence, lies with the accepted design of open area test sites (OATS). The OATS test methodology involves the inclusion in the measurement of a deliberately created second field, routed via a deliberately created indirect path.

Here is the history of emissions testing as I see it. In the early days it made sense to build an open area test site (OATS) at a rural green-field site far from urban turmoil and its associated ambient RF noise. The trouble was it did not take long to realize that inter-site measurements varied enormously and there was no repeatability worthy of the name.

Figure 1 shows a key reason why these first failures of the method occurred. The figure depicts an unintentional path allowing a second field to be presented to the measurement antenna. The strength of this second field varied depending on the reflectivity of the substrate between the EUT and the antenna. Pavements and soil types at different sites would vary in reflectivity, and one site’s reflectivity could change depending on when it last rained.

Therein lay the problem, and it was decided not to attenuate or divert the second field, but rather to make it consistent in strength (site to site) by installing a fully reflective ground plane as shown in Figure 2.

This presented a problem in that the maximum combined field might not occur in direct vertical alignment with the EUT. So to rectify this, a further decision was made to add complexity by searching the vertical space at the measurement plane.

Once accepted as a method, chamber manufacturers wishing to claim equivalence had to replicate the second path (simple and cheap) and also had to increase the chamber volume to allow for the 4m height scan (simple, but far from cheap).

I wonder if the founding fathers of EMC realized the ramifications this method would have later on in the design and cost of semi-anechoic chambers.

Question - Could they have come up with another solution? Easy for me to say since I wasn’t there to face their constraints, but I wonder if they gave serious consideration to diverting the second field away from the antenna as shown in Figure 3.

If you are going to search for a rural green-field site, why not choose one with a natural slope such as a hill? The hill slope might need to be graded to the best angle and access roads and unobtrusive scaffolding for the antenna mount would be an issue, but the impact on chamber designs would have been a lot kinder.

To be continued....

-Tom Mullineaux

The Annual IEEE EMC Show in Dresden (continued) Plus Elephant #4 ‘The Zip Code Lottery in Achieving Product Compliance’

2014-09-08T11:04:00.002-07:00

Great news, borrowing 3D EM software to diagnose the performance of the automotive radiated emissions test fixture is a go! I have been given a contact at the 3D EM software house and things should pick up quickly after the contact returns from the EMC Europe show in Sweden. I am excited. Meantime ....

The Annual IEEE EMC Show - Why Dresden?

Leaving aside the European location for the time being, why does anyone, whether presenting a paper, attending the technical presentations, or exhibiting, bother to go at all?

Let’s hear it from the exhibit floor first.

The Vendor’s Tale

Motives enticing the vendor to attend are varied and complex. They include but are not limited to:

• It is a place to display application-specific wares to a highly-targeted audience

• It is an economical way of catching up with a lot of key people all in the one place, all at one time, saving the expense of flitting all around the country to meet each in turn.

• It is a low-cost opportunity to provide Rep product training, and a place to meet, interview and recruit reps. This situation is symbiotic as Reps are often touring the hall looking to add lines to their line card too.

• You can arrange to meet local customers either at the show, or visit them just before, or just after the show.

• You get a chance to meet your competitor’s customers. I have seen booth staff look on in abject horror as their best customers wander off in the direction of a competitor’s booth.

• You can check out what is new with the competition – new products, new hires, etc.

• It is a chance to get to know and recruit a competitor’s best staff (recruitment activities such as advertising positions at the booth is banned, but this does not prevent poaching).

• Strangely, it can be seen as dangerous not to exhibit – rumours of solvency issues can spread, prospects may read into it that you are reducing your market presence in this sector, and there is always the concern that a competitor choosing to attend may gain from what turns out to be a ‘good’ show after all.

The main de-motivators to signing up for a trade show include:

• Disappointment with the number and quality of sales leads at last year’s show.
Many vendors go in with unrealistic expectations. In their dreams they would like orders to be placed at the show, or very soon afterwards. More savvy companies know exhibition attendance plants the seeds of future sales by reinforcing market presence.

• It is hard to measure the return on the dollars invested
The problem of how to measure return on dollars spent is not limited to exhibiting. If that full page advertisement in a magazine resulted in a major sale 18 months later – how would you know? If not attending a show or looking small (you decided to reduce the booth size) at a show cost you the opportunity of a major sale, how would you know?

• Other marketing opportunities competing for scarce marketing dollars
With little hard measurement data to compare the return on investment types, adding an exhibition to the marketing mix will always be a dilemma for companies.

• Opportunity costs
These are all the other things the sales and marketing staff twiddling their thumbs at the booth could be devoting their time to (sales presentations, customer visits, etc). The saving grace of emailing from the booth or the hotel room is only ever fire-fighting, or trying to hold things until you get back. It is never as efficient as being at the office.

Before we leave the vendor’s tale, there is a personal reason booth-staff like exhibitions. Depending on the attractiveness of the location, it can mean a cheap vacation of sorts. Your flight and hotel room are already paid for by the company, so you only need to find the airfare for the wife and kids and ‘wham’, you have a vacation. The better shows have tours to amuse family members while you are away working the booth.

Next time we will hear from the attendee principally at the show to learn from the technical presentations. Here is a ‘heads up’. It is in the nature of vendors to complain, and some of the big ones truly believe the show exists because they fund it with their exhibition hall floor-space fees. Nothing could be further from the truth – no technical presentation attendees, no show.

The Linearization of EMC Amplifiers

This topic has not been forgotten, it has just been on hold while Elephant #2 ‘Disharmony in Harmonic Limits’ was explored to the full. The previously proposed minus 13dBc giving 75% field purity may not be good enough. However we haven’t factored in the cable loss (higher for the harmonic), so let’s do a few more calculations.

Meanwhile – as another way of looking at how the linearization is achieved, think of the noise cancelling headsets as used by airplane passengers.

The principle of operation for the headset is to sample the ambient repetitive noise (aircraft engine hum), invert it, and add it to the signal feeding the speakers. The result is cancellation of the hum. The blurb on the box says you can still hear announcements etc, as if this was a designed in feature, but this is a trick. The headsets can only cancel repetitive deterministic noise, they cannot cancel stochastic noise.

For our EMC application things are even simpler. We know the noise frequency in advance and have a PC that has the necessary cancellation signal attributes stored and ready for use.

Elephant in the Test Room #4 - The Zip Code Lottery

Elephant #4, like the other elephants we pretend not to see, is one where many already know about it, but few mention it.

With this particular elephant, whether a company’s product passes RF emission tests can depend on which test house it is taken to. Most companies prefer to use a local test house for ease of transport of the product, and for the ease of having one of their engineers at the site to try and fix problems as they arise.

Some years ago, a round robin study showed that for the self-same test-piece sent out to several test houses, the measured emissions levels varied by many dBs. The word around the water cooler was it was up to 10dB difference between the labs.

So if a company is unfortunate enough to be in the ‘capture’ area of a test house that measured high emission levels from the round robin test-piece, then their new product may fail the test. Conversely, and somewhat ironically, a second company’s product, with worse emissions than the first one’s, could pass at a test house that measured lower emissions during the round robin exercise.

You may argue that there could be myriad reasons as to why this situation exists, including operator error, the stacking of uncertainties in one direction, etc. But to my mind it is because we are using an unevenly damped reverberation chamber to try to contain and control the fields within the overall test space.

To put this in context, we will start next time by looking at the layout and equipment arrangement of open area test sites.

To be continued

-Tom Mullineaux

Ta DAH!

2014-08-28T09:35:00.001-07:00

At long last, Department of Defense Instruction 3222.01, the DoD Electromagnetic Environmental Effects Program, has been signed and released. For those of you following along, its been a long road! See the post from Dec 2013.

So, we have a new DoD E3 Instruction…what does that mean? First of all, as the title indicates, it's an Instruction, not a Directive – the difference is that directives tend to be very broad brush with a lot of “thou shalt” but not a lot of “how shalt I?” That is, the Directive has no procedures in it, while the Instruction does. While the new Instruction isn’t chock full of new procedures, it does strengthen the requirements to consider E3 in military procurements and to assign various responsibilities to DoD organizations at every level.

So, what’s new and exciting in DoD Instruction 3222.01?

The policy section is a lot more specific in the new document. There is reference to operations without degradation due to EMI, the operational EME and Joint Service control through DoD –wide techniques and procedures. Definitely trying to get the Services to play as a team, E3-wise.
It formalizes the DoD E3 Integrated Product Team, which holds monthly meeting to discuss E3-related topics and issues relevant to all military services.
It provides a more formal and expanded description of the DoE E3 Program details of which are managed by the Defense Spectrum Office under DISA. It details many of the requirements for E3 control through the life cycle of a military system.
It incorporates the criteria for the installation and operation of systems and equipment in the vicinity of designated DoD sites, which was previously the topic of a separate Directive.
It provides greater references and procedures for controlling electromagnetic hazards.
And finally, it greatly emphasizes the need for E3 related awareness and training. This includes several references in the Responsibilities section requiring DoD organizations to develop and provide such training to both operational personnel as well as the acquisition corps.

OK, maybe its not as exciting as running a full threat lightning test on an aircraft you’re sure is going to fail, but it's great news for those us of in the E3 business. The Instruction strengthens the requirements to implement E3 control and to make sure that everyone involved in military systems acquisition is aware of the importance of E3 control. And at the end of the day, it's to make sure that our soldiers, sailors, marines and airmen have equipment and systems they can count on in today’s operational electromagnetic environment.

Download the Instruction from acc.dau.mil/library….go to Policy and Instructions!

-Brian Farmer

The Annual IEEE EMC Show – Why Dresden? Plus Proof of the Arbitrary Selection of Automotive Amplifier Harmonic Limits

2014-08-26T10:05:00.001-07:00

Read other posts in the "Elephant in the Test Room" series here.

It is looking good for using 3D software to diagnose the performance of the radiated emissions test fixture. Should get the go ahead any day now. Meantime, let’s explore this year’s hot topic.

The Annual IEEE EMC Show - Why Dresden?

It is no surprise that some USA residents in the EMC industry are wondering why the 2015 symposium is being held in Germany. Here is my take.

I learned long ago in history class that the first duty of the State (Monarch, Oligarchy, elected Government, Dictator) is to protect the State’s borders. Other duties may be of the utmost importance, but are always secondary to this first duty.

Likewise, when it comes to the board presiding over an engineering society symposium, the board’s first duty is to ensure the survival of the symposium.

With this in mind, let’s examine the decision to hold the 2015 symposium in Dresden.

History on a Postage Stamp

The symposium really took off with the explosive growth in interest caused by European legislation stating that Electrical/Electronic goods must be stamped with the CE mark.

Before being stamped with the CE mark the goods had to comply with regulations on electromagnetic compatibility. No CE mark, no sales into the European Common Market.

This got the attention of US companies intent on selling into Europe, many of whom were completely ignorant of European EMC regulations. Employer backed symposium attendees hungry for knowledge went through the roof. Subsequently, so did the number of fee paying vendors buying booth space. A happy time in the history of the symposium, and a time of plenty for the vendors.

The powerhouse behind the regulation was Germany, so to me it is quite fitting that if the event is to be held outside the USA, then it should be in Germany. I have been to the annual EMV show many times in Munich and in Dresden (the cities take turns). Good shows when I used to go, but at the time not as ‘grand’ as the US show. To me, and I dare say many others, the IEEE EMC symposium was and always will be ‘the greatest show on earth’.

At the time (1980s) I saw the European regulation as protectionist, and simply a market barrier erected against low-price / low-quality products flooding in from third world countries. However, since then, EMC has become a world-wide concern in its own right, whether covertly protectionist or not.

All good things come to an end and like most industries, the EMC industry matured and saturated. The hunger for knowledge long since satisfied, symposium attendance has declined with a corresponding shrinkage in the number of vendors. Year on year the show seems to get smaller. This is the current situation faced by the IEEE EMC board.

[Somewhat off topic here, but it is widely acknowledged that the car industry is a key bellwether of looming economic recessions. The first thing people do as times harden is put off buying that new car. Well, guess what? There is another. Sales in the entire EMC industry fell by 75% overnight during the lead up to the last recession (the near financial meltdown, auto-manufacturer bankruptcies, etc). I suppose companies can put off buying that new piece of test equipment just as we can put off buying that new car.]

I heard long ago that the types of attendees at the show were basically made up of one third, one third, one third. That is, one third was made up of vendor booth-staff, one third came from the surrounding area, and one third came from ‘out of state’.

In my view, this will translate across the Atlantic Ocean, with any shortfalls due to the European location made up by local vendors and attendees.

Next time we will look at current incentives / disincentives to attend the show, and why attendees bother to attend at all. Meantime, a heads up on where this is leading - the Dresden decision was a hard-nosed business-style decision, completely in line with the first duty of the board.

Proof of the Arbitrary Nature in the Selection of Automotive Harmonic Limits

When we first questioned the automotive edict stating power amplifiers used in RF immunity testing must have -20dBc harmonics or better, we wanted to know ‘just how sensitive is the test field integrity to amplifier harmonic performance?’ Is it a lot or a little? Is -20dBc a great deal better than -19dBc?

On the way to the answer, we derived the simple equation that determines the integrity of the test field itself,

E2_dBc = P2_dBc + G2_dBc

Where E2_dBc is the harmonic field level compared to the fundamental field level in the test field itself (in dBs), P2_dBc is the harmonic power level compared to the fundamental from the amplifier (in dBs), and G2_dBc is the gain the antenna presents to the harmonic compared to the gain it presents to the fundamental (in dBs).

For our purposes, our 1-18 GHz horn antenna has a fixed worst-case G2_dBc of 4dB (fundamental gain to harmonic gain ratio).

Calculating the Percentage of the Intended Field

E2_dBc doesn’t give much of a feel for how pure a test field is, other than we know the bigger the magnitude of the number, the better the field (-11dBc is better than -10dBc). Fine, but how much better? To get a true feel for test field integrity, it is best expressed as the percentage of the total test field created by the intended test frequency.

The following example shows how the percentage is calculated: -

For no particular reason we choose E2_dBc = -10

E2dBc = 20 log₁₀ [E2/E1]
where E2/E1 is the ratio of the harmonic field to the fundamental field.

20 log₁₀ [E2/E1] = -10
E2/E1 = antilog₁₀ -10/20
E2/E1 = antilog₁₀ -0.5 = 0.316
E2 = 0.316E1

Substituting this into Total field E = E1 + E2

Total Test Field E = E1 + 0.316E1

E = E1 (1 + 0.316)

E = 1.316E1

E1 = E/1.316

E1 = 0.76*E

That is E1 is 76% of the total field

As a demonstration of the diminished return from improving the harmonic performance of the power amplifier by 1dB, look at the table showing the percentage of the intended test field making up the total test field.

As can be seen, an amplifier with worst-case harmonics of -19dBc will result in 85% test field integrity, whereas one with -20dBc results in 86%. One percentage point difference seems tiny, is barely measurable due to uncertainties, and seems unreasonable. Maybe the automotive technical committee that deemed this arbitrary number lacked the engineering grounding that would have allowed them to see just how ridiculous the edict is.

It is down to you as a blog audience to draw your own conclusions, but let me state mine - the responsible automotive technical committee was duped. Look, there is a lot to be said for having industry representatives on committees, they are a ‘calming’ influence on what can, and what cannot be done in terms of the feasibility/affordability of a proposed solution. But how do you stop those supplier influencers from skewing decisions that give the supplier company paying their salary a market advantage?

-Tom Mullineaux

Determination of Practical Harmonic Levels

2014-08-01T13:28:00.000-07:00

Well we are still waiting to see if borrowing 3D EM software is on the cards. I should know soon, one way or the other. I hope it happens, but for now let’s keep holding off on Elephant #3, ‘Fixing the Broken Automotive Test Fixture,’ In the meantime, we can continue the engineering approach on determining a practical level of harmonics in RF immunity for Elephant #2, ‘Disharmony in Harmonic Limits,’ which became a subset of our project on linearizing EMC amplifiers.

Also, I listened in on the Agilent July 24 webcast, “a day in the life of your cell phone," which was pretty good. During the webcast, I asked via the dialog box what the maximum radiated power from a cell phone was, and when did it emit this maximum level (at switch-on, when setting up an incoming call, etc)? The answer given was "when the cell phone is furthest away from a base station." This is a partial answer and I suspect only applies to when the transmission channel between the base station and the cell phone is already set up. It's not the answer we wanted, so I will ask for further clarification from the webcast organizers. Until they get back to me, we will hold off on ‘The Cell Phone Threat.’

The Linearization of EMC Amplifiers

Reminder: In previous posts on this thread, we asked the $64,000 question,"what is a practical level of harmonics in a RF immunity system?"

To help answer this question, we derived the equation for the amount of harmonic field compared to the wanted test frequency field. We christened this test field integrity indicator as E2_dBc.

We then used the equation to determine what level of amplifier harmonics would create the commercial limit of -6dBc test field harmonics. This came out at -10dBc, to which we added a safety factor of 3dB, giving our initial determination of sensible amplifier harmonics as -13dBc.

The EMC amplifier linearization exercise we are conducting is over 1-18 GHz, so previously we chose a horn antenna with characteristics very similar to the majority of 1-18 GHz horn antennas out there in the marketplace. Their performances are so close I think we can assume the various suppliers use the self-same design or rebrand the self-same antenna. For ease of reference, the amplifier data table is repeated here.

For the particular antenna in question, the relevant worst-case gain difference was at 8 GHz when the 8 GHz linear gain G1 was 12.66 and the 16 GHz linear gain G2 was 31.56. The table already states the relative difference for us in decibels as 4dB. We will need this later.

Separately, we established the amount of harmonic field permitted in commercial product testing as up to one-third of the total test field.

One-third seems on the high side, so let’s play with the E2_dBc equation a little longer to try and justify a level suited to both commercial and automotive RF immunity testing.

We want to flit easily between the variables so we will make life easy for ourselves by deriving E2_dBc in its simplest and most useful form.

E2_dBc = 10log₁₀ [G2.P2]
[G1.P1]

Expanding gives:

E2_dBc = 10log₁₀ [G2/G1] + 10log₁₀ [P2/P1]

Which, purely for convenience, we will rearrange to read:

E2_dBc = 10log₁₀ [P2/P1] + 10log₁₀ [G2/G1]

But 10log₁₀ [P2/P1] is P2_dBc and 10log₁₀ [G2/G1] is G2_dBc.

That is P2_dBc is the level of the harmonic relative to the fundamental frequency in dBs (worst-case is always stated on the amplifier data sheet) and G2_dBc is the gain the antenna presents to the harmonic relative to the gain the antenna presents to the fundamental, again in dBs (obtainable by inspection of the antenna data sheet).

Therefore, the test field integrity indication equation is:

E2_dBc = P2_dBc + G2_dBc

Now it is simplicity itself to flit between a) E2_dBc, the harmonic level compared to the fundamental in the test field itself, and b) P2_dBc, the harmonic level compared to the fundamental from the amplifier, and c) G2_dBc, the gain the antenna presents to the harmonic compared to the gain it presents to the fundamental in dBs.

Now we use the new equation to establish the field harmonic level E2_dBc if we use the proposed -13dBc amplifier harmonics.

E2_dBc = P2_dBc + G2_dBc
E2_dBc = -13 + G2_dBc

We already know from the antenna table that the worst-case G2_dBc is 4dB.

E2_dBc = -13 + 4

E2_dBc = -9

Impact of First Determination of Amplifier Harmonics on the Integrity of the Test Field

If E2_dBc is changed from -6 to -9, what change does this cause in the make-up of the commercial test field?

The Total Test Field E = E1 + E2

If E2_dBc = -9, that means 20 log₁₀ [E2/E1] = -9

E2/E1 = antilog₁₀ (-9/20)

E2/E1 = 0.355

E2 = 0.355*E1

Substituting this into Total Test Field E = E1 + E2

E = E1 + 0.355*E1

E = E1 (1 + 0.355)

E = 1.355*E1

E1 = E/1.355

E1 = 0.74*E

This means E1 is pretty close to 75% of the total field which in turn means E2 is close to 25% of the total field.

For the calibrated commercial test field of 18v/m, 13.5v/m will be at the intended frequency and 4.5v/m will be at the harmonic frequency.

For the automotive test field of 200v/m, 150v/m will be at the intended frequency and 50v/m will be at the harmonic frequency.

It is plain that this basic method could be used to see the effect of amplifier harmonic limits anywhere between -10dBc and -20dBc.

In terms of our TWT Amplifier linearization target, I believe we are there at -13dBc.

Next time we will look at the diminished return between improved amplifier harmonics and the integrity of the total test field.

-Tom Mullineaux

Avoid the Most Common Failures with These 7 Essential EMC Design Rules

2014-07-11T13:56:00.000-07:00

A reader recently asked me what the top design rules were that they could implement on their product to maximize the chances of passing FCC and CE testing. There’s no doubt that there are countless ways to fail emissions and immunity tests, but a pattern of failure modes did emerge at my EMC lab.

Regardless of the type of product I was testing, be it medical, industrial, consumer or any other industry for that matter, there were a few areas where manufacturers consistently failed EMC tests. Was it possible that EMC failures were distributed according to Pareto’s Law? i.e. 80% of the failures were due to 20% of the cause mechanisms. It seemed possible.

If you want to avoid some of the most common EMC pitfalls, and avoid costly re-tests and launch delays, read on…

1. Reduce RF Noise On The Cables

Cables act like antennas for radio noise. One of the top failure modes I saw at the lab were due partly or wholly to EM radiation from the cabling attached to the device under test (DUT).

How good they are at converting the conducted noise to radiated noise (i.e. how good an antenna it is) varies greatly depending on several key factors including the impedance match, the length of the cable and the wavelength of the noise. To avoid radiated emissions issues due to cabling, it’s really important to ensure that as little unintentional noise is coupled on to external and internal cabling as possible.

If a manufacturer runs into a radiated emissions issue at a test lab, one of the first methods of debug is usually to detach as many I/O cables as possible. If the offending emission disappears, then you can start re-connecting the cables one-by-one until you (hopefully) find the culprit. Note that if there are multiple cables, it can very often be the case that each cable contributes towards a particular issue and connecting/disconnecting them in various configurations can lead to seemingly inconsistent measurement results. This is typically due to connecting/disconnecting ground loops of various sizes. Regardless of this, the rule of thumb to keep in mind at the design stage is to keep as much unintended RF energy off every cable as possible.

There are a few ways you can help to minimize the noise emissions from cabling:

Ensure Slew Rate of Signals on the Cables are Minimized

Digital signals can contain RF energy across a high bandwidth. The upper limit of the bandwidth depends on the slew rate of the rising and falling edges of the signal. The higher the slew rate, the more energy the signal contains at higher frequencies. As you can see from the figure to the right, the RF energy contained within a 1 MHz square wave spans a large spectrum.

It’s usually possible to reduce the slew rate of digital signals coupled on to cabling by either reducing the drive strength of the driver, or passing the signals through a low-pass filter. By lowering the slew rate, you can greatly reduce the RF energy coupled on to the signals and therefore reduce the amount of radiation from the cabling.

A good ‘get-out-of-jail-free’ rule of thumb is to include a non-populated low-pass filter on every digital signal that passes over a cable. A simple and cheap R-C filter will do the job if your eye diagram can take the hit. Otherwise, more expensive integrated solutions are available. See section 4.2.2 in our EMC design guidelines eBook for more details.

Make Sure Any Power, Ground Shield GND and Static I/O Signals Are Clean

Although on the surface of it, you’d think that DC signals such as power supplies and static I/O signals should not contribute a significant amount of RF energy to cabling. However, these signals can often be the culprit with cable noise. A global reset signal for instance usually goes to several chips and areas of a PCB. If you’re not careful, this trace can pick up a significant amount of RF noise. When the reset signal traverses a cable, the noise can now be emitted as electromagnetic radiation, sometime causing issues with radiated emission testing.

You could write books on ways to minimize these contributions, and many people a lot smarter than I am, have written many. However, here are a couple of easy to implement tips that I’ve seen work very effectively at keeping RF noise off cabling.

(a) Add in-line ferrite beads on power supply and static I/O signals

Noise suppression beads are designed to absorb RF energy and convert it to heat. There are thousands to choose from and so it’s worth putting in a bit of time to choose carefully. To over-simplify a complex issue, basically you want the frequency of the peak impedance of the ferrite bead to match the frequency of perceived worst case noise in your circuit so that the energy at that frequency is turned into heat.

Even if you decide not to incorporate ferrite beads into your design, it’s a very good idea to leave space for them and put 0 Ohm resistors instead. It could save an expensive and time consuming re-spin of your circuit board if you run into problems at an EMC lab.

(b) Provide adequate power supply de-coupling at the connector

If your on-board power supply does not provide a low impedance source of RF current across the spectrum that your switching circuitry demands it, then it’s going to look for it up-stream. Practically speaking, that means the power supply signals on your cabling are going be noisy. And noisy cables tend to radiate pretty well. Although there is much more to it than this, make sure you have adequate bulk capacitance on board and distributed capacitance across your PCB to supply current locally across a broad frequency spectrum.

2. Connector Ground ESD

One of the most common issues that came up again and again was related to ESD testing. ESD testing is usually required as a general CE testing requirement and also applies to other product specific standards. In particular, electrostatic discharges applied to chassis ground connections at I/O connectors showed a disproportionate number of failures. The failures usually showed up in the form of the devices resetting, or worse – a ‘permanent degradation of performance’ i.e. It fried something important.

According to the adjacent table, taken from the root ESD testing standard (61000-4-2), ESD pulses are applied using the ‘contact discharge’ method to exposed metallic shells on connectors. These are usually connected to the PCB chassis ground through the connector. The amplitude of the discharge pulse depends on your product standard, but is typically 4kV or 8kV for most applications.

This is applied using a pointed tip as shown in the picture below.

Here is a breakdown on ways to avoid this very common issue:

Identify the ESD Testing Locations

In order to apply ESD protection to your product in the right places, you should know where the test lab is going to ‘zap’ it.

Unless stated otherwise in the generic, product-related or product-family standards, the
electrostatic discharges shall be applied only to those points and surfaces of the EUT which are accessible to persons during normal use.

What this is saying is that if you can reach it with your finger, then it should be tested. The test lab will do some exploratory testing on the housing of your product to see if they can find any susceptible locations. If the chassis is non-conductive then often there won’t be many discharge points. Even on non-conductive chassis though, there are some common discharge points:

Connectors
Screw heads
Buttons/Keypads
Seams where two parts of the chassis come together
Areas where internal PCBs come very close to the chassis
LEDs or graphic displays

At 8kV or 16kV, the arcing distance can be quite far, so it’s possible for discharges to find a way to the circuit board, even through a non-conductive housing.

Identify the ESD Levels

The ESD test levels are typically set by the product standard that applies to your product. For example, the consumer audio EMC immunity standard is EN55103-2, where the ESD test levels are defined as 8kV air discharge and 4kV contact discharge for almost all usage environments.

Select The Right Transient Suppressor

Once you know the discharge test levels and where the discharges will be applied, you can proceed to selecting some ESD protection for your circuit boards. You can refer to section 4.6.1 from our EMC design guidelines eBook which goes into detail on these selections. There are plenty of options to choose from including spark gaps, resistors, capacitors, varistors and TVS diodes to name a few. A good ESD suppressor will clamp the maximum voltage seen across the device to a known level, and dissipate the extra energy into heat or noise.

Apply in the Right Location

A critical aspect that is sometimes overlooked is the placement of the ESD protection. It must be placed as close as physically possible to area where the discharge event is going to happen. This minimizes the series inductance of any routing between the discharge location and the transient suppressor.

Design Your Discharge Path Properly

It’s important to visualize where the discharge path is for the discharges applied to a chassis ground on a connector. For many designs, the chassis ground is separated from the main board ground for safety reasons or even specifically for dealing with ESD.

A tried and tested methodology is to bring the chassis ground on your circuit board ground back to the main power input so that any discharges can dissipate through this point without affecting the rest of your circuitry.

3. Choose the Right Power Adapter

If your product has an external power supply or internal sealed power converter, it’s very important to know how noisy these devices are. This can have a critical effect on your product’s conducted emissions performance. A $4 power supply can ultimately mean the difference between a pass or a fail at an EMC lab.

At the simplest level, you should ensure that the power adapter has passed the emissions class that your product needs to pass.

Class A digital devices are ones that are marketed exclusively for use in business, industrial and commercial environments.

Class B digital devices are ones that are marketed for use anywhere, including residential environments.

Class B emissions limits are more strict than class A limits, so if your product is class B, then make sure that you’re using a class B power supply.

Unfortunately a compliant power adapter cannot guarantee that you’ll pass conducted emissions testing at an EMC lab. There are a couple of main reasons for this:

1. The emissions performance of a power adapter will likely have been tested at an EMC lab with only a DC resistive load connected to the output. It may well have passed class B emission limits with a DC load, but the emission characteristics of the adapter may be much different if it’s connected to a dynamic load with current supply requirements spread across a wide spectrum. A compliant power adapter can’t solve all of your conducted emission problems, you will also need to design your on-board power circuitry carefully to ensure that the RF current demands on the power adapter are minimized.

2. Power adapters are occasionally (ahem!) not as compliant as their labeling suggests. Giving the benefit of the doubt, some cheaper power adapter manufacturers suffer from quality control issues whereby some batches of adapters have worse emissions performance than the adapters they sent for testing at an EMC lab. A less trusting person might guess that some power supply manufacturers intentionally cost reduced their power adapter design after receiving their FCC or CE certificate. Or even worse, they may have acquired fraudulent test reports. But I’m not suggesting that.

It’s important that you request the test report from the power adapter manufacturer so that you can verify their claims. It’s also a good idea to provide your test lab with a couple of alternative samples of power adapter in case one of them is non-compliant.

4. All LCDs are Not Created Equally

In the same way that your power adapter can have a big impact on your product’s conducted emissions performance, an LCD display can have a huge effect on your product’s radiated emissions performance.

Often an LCD will have a built-in controller of some sort, with an integrated micro. Some LCDs can have much worse radiated emissions performance than a seemingly identical model from a different manufacturer.

It’s always a good idea to order some samples from several LCD manufacturers and either do an emissions pre-scan at a test lab or do some relative emissions measurements in-house if you have the right pre-compliance test gear. Usually there will be an identical drop in module from a different manufacturer available, so you can swap out an offending LCD if you run into issues. But if the LCD design is electrically or mechanically unique, this can be a really expensive and time consuming issue to fix.

As a side note, LCDs connected via a flex cable, or sometimes even via a DIP connector can exhibit significant radiated emissions due to the high slew rate digital signals. Follow the guidelines in point (1) above for implementing filters on the signals before they hit the connector/cable.

5. Make Sure Auxiliary Equipment is Compliant

Here’s how a $5 USB flash drive can cost your company $2,000 and 1 week time to market.

It’s a bit unfortunate, but lots of manufacturers fail due to no fault of their own. As a general rule for emissions testing, manufacturers are required to connect auxiliary equipment to all I/O ports in order to fully exercise the functionality of their device. The FCC want to see worst case emissions profile, which involves exercising all interfaces and functionality.

For example, if the product includes a USB port that can connect to a USB flash drive in normal usage, then a USB flash drive must be provided with your equipment to your test lab. Surf on over to EBay and you’ll find that $5 can get you a no-name brand (possibly not FCC/CE compliant) 2GB flash drive, including shipping! At USB frequencies, a non-compliant device can cause you some really expensive headaches. You may spend hours debugging what you think is an emissions issue on your circuit board when it’s actually emissions emanating from a cheap USB flash drive. It may mean a trip back to your office and re-booking the test lab for a later date. All of a sudden, a $5 USB drive can cost you well over $2000 in extra expenses.

The rule of thumb here is easy:

Make sure that all auxiliary equipment that you supply to the test lab is from a reputable manufacturer so that you know it’s tested and compliant.

6. Select the Correct Rating of Protection

It was very often the case that manufacturers had included transient protection circuitry, but had neglected to ensure that the rating of the devices were high enough to cope with the pulse that had to be applied.

I’ve included a table below of the normal ‘suite’ of transient disturbances that are applied to products during CE testing. I’ve also included some examples of test levels that may be applied. Note that this is from an old test report and the base standards and levels may not be up to date.

Selecting a transient device with a high enough rating to deal with the applied disturbances is critical to ensuring a first time pass. You need to be sure of the test levels that a test lab is going to apply to your product in order to know what type of protection you need to design in.

7. Shield Any Sensitive Analog/RF Circuitry

If your device contains any relatively sensitive analog circuitry, it’s a good idea to include a well grounded conductive shield. For CE testing, your product is more than likely going to be subjected to a radiated RF field in the 1 V/m to 10 V/m magnitude, modulated with a sine wave in the kHz region, swept over the frequency range 80 MHz – 6 GHz. It’s worth noting that some industries such as automotive, aerospace and military must design their products to a much higher specification and different types of modulation and sweep ranges are possible.

The energy in the RF field can couple to your circuit board in a number of ways, but in terms of pass/fail criteria, what you should be interested in is whether the field can negatively affect the performance of your device. Although traces on your circuit board may be electrically ‘short’ compared to the wavelength of the applied RF field, the field can still induce minuscule voltages/currents that can adversely affect the operation of your circuit. Sensors that provide current source outputs in the pA/nA can especially be affected. This typically shows up as a measurement error outside of the general specifications of the product.

Even non-analog parts of your circuit can be adversely affected by an applied RF field. For example, a very weak pull up resistor in the 1 M Ohm region on a static reset signal can be pulled below the ‘On’ threshold by an external RF field disturbance inducing a very small current on the signal. In terms of RF susceptibility, if design constraints allow you to avoid weak pull ups, it’s best to avoid them.

Wrap Up

In this post I’ve outlined the 7 most common causes of EMC failures I saw at my EMC lab over a 3 year period. Hopefully these tips can help you to avoid some seriously expensive failures.

-Andy Eadie

Republished with permission from EMC FastPass.

Linearization of EMC Amplifiers Continued – Determining Practical Harmonic Levels

2014-07-01T07:14:00.000-07:00

Read other posts in the "Elephant in the Test Room" series here.

At the time of writing, I am waiting to see if borrowing 3D EM software is on the cards, so let’s postpone progress on Elephant #3 for the time being. I'm quite excited at the prospect of modeling the present automotive emissions test fixture, and equally important, modeling the proposed improvements. In the meantime, we can continue with the current live threads.

The Cell-Phone Threat

The 3G (WCDMA) Transmitted Waveform

As previously explained, unique allocated orthogonal codes running at high speed (3.84 MHz) are used to spread each user’s narrowband data. This is shown in Figure 1. The horizontal colored lines at the output of the mixers are the spread-data ready for combination, frequency translation, amplification and finally, transmission. Again, as previously explained, each user’s data can be recovered from the composite transmitted signal by use of the user’s self-same unique orthogonal code.

A screenshot of a real transmit signal amplified and ready to be fed to an antenna is shown in Figure 2 (the yellow trace). Clearly, this is an Agilent instrument, as is the waveform generator that created the test signal (WCDMA FDD Downlink Release 8, Test Model 1 + 16 DPCH). So, credit is due to Agilent for the screenshot.

The yellow trace doesn't tell the entire story in that occasionally the codes phase align, creating very large peaks. A different plot (not shown) shows the percentage of time peaks of various heights occur.

When I was first acquainted with the linear amplification of WCDMA signals (I’m guessing around the year 2000), these occasional peaks were 13dB (20 times) above the average composite signal power, a tough requirement for the amplifier if information was not to be lost. If you think about it, for the information to reach the other end undistorted, the amplifier must be able to amplify the peaks, so you need an amplifier rated at the peak power. If the average power is 10W and the peak is 20 times this, you need a 200W amplifier. Strangely, web research today says that the peak to average is far lower. Maybe software clipping is employed as well as amplifier linearization.

Topical and timely for our purposes is the fact that Agilent is presenting a webinar on how the cell phone communicates with the base station when first setting up a call. This is when the cell phone is emitting at its worst in terms of the likelihood of interfering with nearby electronic equipment.

The title is “A day in the life of your cell phone” and the presenter is Darcy Smith, a field application engineer at Agilent.

Well, wouldn’t you know it? This clashes with an event given by Rohde & Schwartz that I am registered to attend. The R&S tutorial on “RF Back to Basics” takes place in Austin, TX and sounds pretty good. Guess I need to choose which one to attend, as I really wanted to ask Darcy questions on the latest cell phone peak and average powers when they are polling for the nearest base station. My guess is it is somewhere between 1.0 and 2.0 watts, but it would be good to hear it ‘from the horse’s mouth.”

The Linearization of EMC Amplifiers

Reminder: In the previous post on this thread, we asked the $64,000 question, "what is a practical level of harmonics in an RF immunity system?"

With commercial testing, the amount of the test field created by harmonics is limited by stipulating that the harmonic field must be at least 6 dB down from the field created by the fundamental. Applying limits on the make-up of the test field to maintain the integrity of the test field has to be the way to go, since it covers all the imperfections in all of the equipment used in generating the test field. To do otherwise is simply conjecture based on likely worst-case performance of all the components in the line-up, plus perhaps an arbitrary safety margin. Bear in mind that cables will attenuate the harmonic, while the antenna will likely favor it to some extent or other. But, some antennas are better than others. What if you invest in components that give performance way better than the supposed worst cases? What need of the automotive 1 percent maximum amplifier harmonics then?

So, we will stick with the eminently superior approach that frames the integrity of the test field itself. Right then, if the harmonic field is 6dB down from the fundamental field (i.e. minus 6dBc), what portion of the total test field is allowed to be created by the harmonic?

Easy enough to deduce, but long winded in terms of step-by-step. The total field created during commercial calibration is 18v/m (10v/m test level plus 8v/m to allow for the peak field created by the AM modulation). We put the harmonic content at its allowed limit of -6dBc.

We are talking v/m, so

-6 = 20 log₁₀ [harmonic field / fundamental field]

-6/20 = log₁₀ [harmonic field / fundamental field]

-0.3 = log₁₀ [harmonic field / fundamental field]

antilog₁₀ (-0.3) = [harmonic field / fundamental field]

[harmonic field / fundamental field] = 0.5

That is the fundamental field is twice the level of the harmonic field.

The total test field is 18 v/m = fundamental field + harmonic field.

But, we just stated the fundamental field is twice the harmonic field.

So, 18v/m = 2 x harmonic field + harmonic field,

which means that 18v/m = 3 x harmonic field.

The harmonic field is 6v/m, leaving 12v/m for the fundamental field.

So, the maximum portion created by the harmonic is one-third of the test field (I misstated this in a previous post, sorry).

Just because we can, let’s apply this limit to automotive RF immunity testing. This means the prescribed 200v/m test field would be allowed to include up to 67v/m of harmonic field.

We established the dependence of the harmonic in the field E2 previously as:

To reinforce the fact that this is the level of the harmonic compared to the fundamental (the carrier, i.e. dBc), we will rename E2 as E2_dBc

To find out the harmonic level from the amplifier when the harmonic field is -6dBc, we can make P2/P1 the subject of the equation.

For E2_dBc = -6, the antilog₁₀ [E2_dBc/10] part is antilog₁₀ -0.6 = 0.251

If we only knew the linear gain of the antenna at the fundamental and harmonic frequencies, we could complete the unknowns and establish the harmonic performance required from the amplifier, that is 10log₁₀ [P2/P1].

A web search gives data on a typical 1-18 GHz antenna. I extended the data to create Table 1. It shows the harmonic gain next to the fundamental gain, and the delta between the two in dB.

A glance shows the worst case delta between a fundamental and its harmonic is at 8 GHz where the antenna gain at 8 GHz is 11 dBi and its gain at 16 GHz is 15 dBi.

The table shows the respective linear gains. Slotting these into the equation gives:

So, the harmonic performance from the amplifier is required to be -10dBc, a far, far cry from the automotive -20dBc.

I suppose if I wanted to alarm the automotive technical committee and insist amplifiers need to meet -20 dBc, I could say something like this:

“Data shows irrefutably that the low frequency antenna gain and the high frequency antenna gain can differ by more than 8dB, so harmonics must be kept extremely low.”

This is true in that the worst case between 1 GHz and 16 GHz is 15dBi minus 6.8dBi equaling 8.2dB. Pretty frightening but not representative of the actual worst case delta between any real life test frequency and its harmonic. Who cares that at 1 GHz, the gain is 6.8dBi and at 16 GHz, the gain is 15dBi. The two frequencies will not co-exist during testing. In fact, when the test frequency is 1 GHz, the delta between it and its harmonic (2 GHz) is only 1dB.

Personally, I’d double this minimum harmonic performance to -13dBc, partly as a safety margin, but in truth mainly because this is the performance of TWT amplifiers once they are past the first 20 percent of the band. Also, when it comes to our linearization exercise on improving the harmonics in the first 20 percent of the TWT amplifier band (can be as bad as minus 3dBc), an improvement of 10dB is not to be sniffed at. This is 10 times better and after all is said and done, when using one instrument to calibrate another, we are happy to insist the accuracy should be 10 times better than the instrument being calibrated.

-Tom Mullineaux

A Report from the Southeastern Michigan EMC Fest

2014-05-27T11:01:00.002-07:00

I recently attended the Southeastern Michigan EMC Fest, so this posting is dedicated to the show.

I have a lot of admiration for Scott Lytle (Chapter Chair) and the Southeastern Michigan chapter he runs, so I decided to attend this year’s EMC Fest. The key attraction was Doug Smith, who gave attendees the benefit of his wisdom on diagnosing automotive EMC issues.

The presentation room, filled to the brim.

Here is a show report based on my scribbled notes.

The EMC Fest took place May 8th at the conference center of Canton Summit on the Park, in Canton, MI.

The place was packed with attendees and vendors. Vendor tables lined the walls of the large break room and overspill tables lined both sides of the long hallway leading to the presentation room, which in itself was filled to the brim with attendees.

Scott keeping the well oiled wheels oiled.

Doug delivers his talks in a pleasing conversational style, and the sections I observed were fascinating in that he had real-life test set-ups on camera, which allowed the audience to see on the presentation screen what Doug was doing, and the corresponding change in the test instrument displays.

One story Doug gave was on the success of his website, why it outranks IBM, ITT and all the other giants in web searches, and how this gave him the power to take life options closed to many engineers. And he turned this story into advice to the young engineers in the audience – start a website, because by starting his website many moons ago, he became so well known that it gave him the option to strike out on his own, and so free himself of the grip of employers.

There were a lot of vendor displays.

The Vendor Displays

I specifically asked each vendor what was new – that is ‘what is here today that was not for sale at this show last year?’. I took them at their word as to whether the new products were in fact new. There were a lot of vendors to go around in a short time, and I scribbled notes as I went, but I cannot guarantee I got it all down. I apologize in advance for omissions / errors.

Vectawave

Doug sets up the next demo.

Vectawave (who I know of old) is expanding its sales channels and now has a network of sales Reps in the USA. Vectawave designs and manufactures Class A amplifiers for RF Immunity testing. New, is the 2,500Watt model VBA 250-2500, suited to automotive testing over 10kHz - 250MHz. By chance, Doug Smith, the presenter, has one of Vectawave’s amplifiers. It is a 10MHz – 2.5GHz 3W convection cooled amplifier. Doug uses it for close-in susceptibility testing.

CST

CST is a software house offering 3D electromagnetic computation software. The news here is that CST2014 is now out. The sales guy claims CST2014 is now faster, more accurate, and more user friendly. Turns out he had already done some work on crosstalk between cable arrangements on a set-up that looks a lot like the automotive FEF (Elephant in the Test Room #3). He will check with head office if it is OK to work with me on analyzing the fields created by the FEF, but I warned him that the topic is somewhat controversial, so maybe not.

ETS-Lindgren

Three pieces of news here: The new version of TILE EMC software is out, ETS has a new line of absorber material called FlexSorb, and ETS now offers turnkey solutions.

Elite Electronics

News here is that Elite is celebrating 60 years of EMC testing. Other news from the Elite booth is that there is a big impact coming for test labs as regards testing WLAN-Bluetooth devices. The new version 1.8.1 of EN300328 requires testing at 1 Megasamples per second, which means the lab will need a new receiver with a price tag of around $200k. Ouch. Also, labs must now measure the ability of equipment to only transmit on a free channel (wifi has eleven channels) as equipment cannot transmit more than 10mW total.

FEKO

News is FEKO is to be acquired by Altair which will lead to the addition of FEKO to the Hyperworks suite of engineering software. Also FEKO Suite 7.0 is scheduled for release at the end of this quarter. FEKO had the demo version of FEKO Suite 6.3 on a CD at their booth, so I will check this out and maybe get back to you regarding modeling the automotive emissions FEF.

Busy break room.

Fair-Rite

News is that Fair-Rite has new flexible ferrite sheets that block at low frequencies and absorb at higher frequencies – the booth staff claim this is not officially launched, but is ready to be launched now.

AR

After the success of utilizing spare amplifier power-overhead in commercial testing, AR is now focusing on multi-tone testing for automotive RF immunity testing. This is smart thinking and as long as the new composite power is still far away from the amplifier saturation power, and as long as the system chooses the test frequencies wisely (no doubt it does), this appears to be an excellent way to increase test-house productivity.

EM Scan

News is EM Scan has an all-new high resolution scanner. EM Scan claims that ‘very near field’ data is first class at predicting far field antenna performance. Apparently the antenna labs out there said results within 1.5dB were not all that impressive, but EM Scan did a round-robin and the inter-lab comparison was way worse than 1.5dB.

TDK Corporation

News is that TDK now sells complete turnkey RF immunity systems. These comprise the TDK EMS software, a signal generator, an amplifier, antenna, and all the interconnecting cables. TDK says buying the complete system from them takes the worry away. Also new is a color high definition video camera that can withstand the high electric fields in a test chamber. I got a demonstration of the camera reading the small print on a poster on the opposite side of the room. It was impressive.

Rohde & Schwarz

I was shown the new ESR receiver. It holds and displays the data from previous sweeps so the history can be scrutinized for spectrum changes and for sporadic signals.

Haefely

The just released AXOS 8 expandable immunity test system was being promoted. This produces several immunity waveforms covering many standards in one test system. The big brother to AXOS 5, the AXOS 8 can produce 7kV and 3.5kA surge combination waves.

TESEQ

What is new is that TESEQ is now owned by AMETEK. AMETEK already owned EM Test and by buying TESEQ, it now has MILMEGA and IFI. Will be interesting to see how the new AMETEK EMC group changes the dynamics of the EMC Industry over the next several years. Other news from the booth was that the NSG 5500 (capacitive discharge interference generator) is now beefed up to meet Ford tests.

EM Test

Check out the TESEQ entry above for the bigger picture. One item of news here is that Dan Odum (was ETS-Lindgren) now heads up USA sales for EM Test.

Gauss

New for a year now but of interest to me is the TDEMI X digital receiver, a multi-functional instrument used to measure emission levels. This instrument is specific to EMC, something I always like. The new instrument acts as a receiver, a spectrum analyzer and oscilloscope all in one.

Why of interest to me? Well, another feature is that it can demodulate AM, which could prove very useful for an upcoming change to 61000-4-3, where the purity of the 80% AM test signal is checked against limits. The proposed check is to be made in the frequency domain which on the face of it makes a lot of sense, as it is hard to see slight distortion in the time domain (on an oscilloscope), but it is easy to see the change of sideband levels on a spectrum analyzer. However I think they might be leaning too much on the assumption of linearity (no side products) of the spectrum analyzer. I have seen data where a true 80% modulation from an Agilent PSG signal generator came out as 88% modulation when the sidebands were measured on a spectrum analyzer. When the analyzer was run at zero span and the peak and trough of the demodulated wave was measured it came out at 80.13%. Could be the makings of an elephant in the test room.

Getting back to the new Gauss multi- instrument, in the little I managed to gather at the booth, it could be a world beater. But a real test (for my purposes) of the new instrument would be to measure an 80% AM waveform from an Agilent PSG signal generator using all the instrument functionalities, Oscilloscope, Receiver, Spectrum Analyzer , Spectrum Analyzer set for zero span, and AM demodulation, and see how close the results are to each other when measuring the 80% modulation.

And speaking of Agilent, I see from their website they are running ‘RF back to basics’ seminars on July 22 in Richardson, TX and July 24 in Austin, TX.

ADD2 / Spes Development

Biggest, latest and greatest thing at ADD2 / Spes Development is Yellow Board Testing via their Dynamic Test Board (DTB-VS). The test board simulator connects to a laid out wiring harness with all its peripherals connected. The simulator mimics various scenarios / sequences allowing early testing in the design cycle. The simulator uses a CAN bus for communication which to me is very clever since engineers in the auto industry are already very familiar with this inter-microprocessor communication system.

AE TECHRON

On display at an EMC show for the first time are two new power amplifiers used in creating high magnetic fields, or injecting noise onto aviation 400Hz AC power systems. Designed for MRI but suited to EMC, Models 7548 and 7796 cover DC – 100kHz at 100A / 200A respectively.
Free memory sticks containing specification sheets and even operation manuals were available at the booth. I am of an age where I much prefer to flick through a brochure to get an overall impression and home in on what I want to know in seconds. You can’t do that with a PDF reader, but in this instance I was grateful as it was better than lugging all their literature on the plane.

Pearson

Not particularly new, but Pearson’s latest and greatest is the Power Line Ripple Detector model PRD-120. This is noteworthy in that it allows measurement of the ripple injected onto the AC mains during MIL-STD testing at a far lower cost compared to purchasing a dynamic signal analyzer. The ripple detector box allows you to simply use a regular spectrum analyzer as found at all EMC test-houses.

DLS

The news at DLS Electronic Systems is that they just expanded their MIL-STD-810 and DO-160 environmental testing capability to include sand and dust testing plus vibration and shock testing. Although not at the show in this instance, Jack Black is with this company. Jack is the chapter chair from Chicago ‘across the lake’. Jack is always in friendly competition with Scott on who gets the most attendees at their EMC chapter presentation events.

TEST EQUITY

These guys have a wonderful marketing idea where they visit your company and deliver on-site, hands-on training in CAN bus decoding and triggering using Agilent oscilloscopes. CAN is so good that it is now used in non-automotive systems to allow communication between system microprocessors, and to use the built-in-test and black-box recording features. I know first-hand that it really helps when you have a CAN history history of the system vital signs (device voltages and currents etc) and how they changed leading up to a fault alarm.

Michigan Scientific

This is a local company whose new product is a fiber optic CAN link called FO-CAN. This allows test and monitoring of CAN controlled systems during RF immunity testing. The units are battery powered and can withstand 200v/m (600v/m at higher frequencies).

The Greatest Show on Earth – Aug 2-8, 2014

The Greatest Show on Earth!

Don’t blink, the next thing you know it will be time for the EMC2014 international symposium. As you likely know, it is going to be held during the first week of August in Raleigh, North Carolina.

As regards visiting the exhibition floor while you are there, I will see if I can tease what will be new at the show and try to create a list of ‘must sees’ for attendees.

Well, that is the end of my show report.

This event was, as ever, a highly successful one, and the IEEE EMC Society could do worse than to arrange for new chapter chairs to attend one of Scott’s events as part of the induction process.

The chief protagonists.

-Tom Mullineaux

Linearization of EMC Amplifiers and Fixing the Broken Automotive Emissions Test Fixture

2014-05-05T06:59:00.003-07:00

Read other posts in the "Elephant in the Test Room" series here.

Linearization of EMC Amplifiers

This section of the posting got a little long-winded but it at least serves two purposes. It is a step closer to closing Elephant in the Test Room #2 ‘Disharmony in Harmonic Limits,’ and it also helps determine the linearization target on the TWT amplifier we are working with.

The $64,000 question - What is a practical level of harmonics in a RF immunity system?

In order to be in a position to give an answer, we need a better feel for how sensitive the relative level of harmonics is to the variations / variables in the components that create the RF test field. So for instance we know that the antenna will likely favour the higher frequency harmonic when it is creating the field. OK, but by a great amount for a small change in antenna gain, or by a small amount for a big change in antenna gain? And is the change linear? In the case of an amplifier approaching saturation, we know the change in harmonics is not linear with power.

The commercial standard focuses on the harmonic content in the test field itself, which makes eminent sense, so we shall do the same thing. Note, I am well aware that readers are at different places, some very familiar with dB notation, some fairly new to it. If familiar, I recommend you skip to the bottom. The rest of you – hang on in there, the math is not too heavy, and by the end of it you will be in the same position as those more familiar.

Let’s try to pull together what we need. We want to follow the commercial sector approach so we start with the equation for calculating the field strength.

Where E (v/m) is the field strength, G is the linear gain of the antenna at the frequency in question, P (watts) is the RF input power at the antenna connector, and d (meters) is distance from the antenna.

If you want to know how the equation is derived we already did so in a previous posting under the title ‘The Magic Number 30’ here.

The diagram shows the antenna treating the fundamental (wanted) signal and the harmonic (unwanted) signal differently; where the gain the harmonic sees (G2) is generally higher than the gain the fundamental sees (G1). However the fundamental power is normally far greater than the harmonic power and so the wanted test field is usually greater than the harmonic field. Both signals actually follow the same physical path of course, the separation is purely for ease of explanation.

Applying the equation to the shown field strengths E1 and E2,

We want the ratio of one to the other so,

And cancelling

We would like the level of E2 relative to E1 in dBc, so we take 20 log10 of E2/E1. (We multiply by 20 because the field is in volts per meter, not watts per square meter. Also, the c in dBc stands for carrier, another name for our fundamental signal F1).

And of course to keep the equation valid we have to do the same to the right hand side.

[An important thing to remember here is that E1 is bigger than E2, so the ratio of E2 to E1 will be less than 1, so 20 log10 (E2/E1) will have a negative value.]

In dBc,

So,

Now we are in a position to establish how sensitive the left hand side is to changes to the variables on the right hand side. Technically we should couch the sensitivity in terms of ratios, but for ease of understanding we will determine the sensitivity by numerical example.

Let us assume that the gains and powers are such that E2 is -10dBc.That is the harmonic field level is 10dB down from the fundamental field level.

What would happen if we increased one of the variables by the linear equivalent of 1dB?

Aside: I know in my head that adding 1dB is approximately the same as adding 27% in the linear world (or equivalently multiplying by 1.27). For no other reason than it comes first, we will pick G2. So we will add the equivalent of 1dB to G2, that is multiply by it 1.27.

Here we go:

Let the original E2 be E2a and let the new E2 be E2b. All in dBc of course.

The harmonic got bigger and the fundamental stayed the same, so things got worse.

Therefore, an increase in G2 by the equivalent of 1dB worsens the relative harmonic level by 1dB. That all makes sense because if the antenna favored the harmonic even more, then the harmonic level would worsen. It is easy to see that if we added the equivalent of 1dB to P2 the same thing would happen.

However if we increased either G1 or P1 by the equivalent of 1dB, the situation would get better by 1dB as shown:

There is now a wider gap between the fundamental (the carrier c) and the unwanted harmonic. A good thing.

OK, we are a step closer to figuring out the thinking behind the automotive amplifier harmonic limit, and simultaneously deciding on the level of harmonic improvement we want from the TWT amplifier we are linearizing.

To be continued......

Elephant #3 – Fixing the Broken Automotive Emissions Test Fixture

The Room: Automotive RF Emissions Testing

The Elephant: Hard to justify systemic uncertainties

The Culprit: An over-reliance on weakly related historical methods compounded by the turning of a blind eye to known serious flaws

In this post I would like to plant the seed of an idea, that although unable to be used quite as elegantly as will be described, might direct our thinking to a way of nullifying some of the variables in the current emissions test system.

Walk into any RF design house and you will find various stations used in the development of new designs. Let’s home in on one where the development engineer is just setting up to measure the RF performance of a newly designed RF power module. There will be an analyzer of sorts that provides the swept input signal to the module (EUT) and monitors its output signal. To conduct the test the engineer will need cables and a high power attenuator. Both the attenuator and the cables will have different loss at different frequencies, but the engineer only wants to see what the EUT is doing, and so he first measures the performance of the cables / attenuator combo.

This is shown in the first picture. He tells the analyzer to remember the power / frequency trace, and then tells it to subtract this trace, leaving a flat line as shown in the second picture. This is known as ‘normalizing’ the test set up. The EUT is then introduced and any trace on the screen is due to the EUT alone (third picture).

How could we use this general idea to nullify the imperfections of the E-Field transducer measurement?

I believe any test house already has everything at hand to do so. Modern spectrum analyzers and receivers have tracking generators, and if we want to do the calibration at a sensible test field level, we have the RF amplification train used in RF immunity testing. Perhaps we can put our minds to capturing a trace showing imperfections for fixed input power, or better still, for input power for fixed RF field strength at 1m (make this the independent variable). Won’t be as elegant, but....

To be continued ...

Other Live Topics

Sorry, but as previously mentioned, the linearization section got long winded and so I will pick up and concentrate on the other topics next time.

-Tom Mullineaux

A Crossword Just for EMC

2014-04-17T06:13:00.002-07:00

ACROSS

3) and 6) down. If the walls have ears in this judge’s room, don’t expect a reply
7) and 8) across. EHT can cause this to light up the dark
8) See 7) across
10) A heck of a lot of power, apparently
12) The theme of this crossword
13) Germany’s own version of 12) across
14) A few billion of these involved in 7) across
16) With 15 down the bible of EMC since 1970
18) Abbreviated unit of measurement for a French natural philosopher
20) This antenna sounds like it has plane section curves with a focus
21) 1 million Coulombs per second
22) and 23) across. Samuel Plimsoll was way ahead of the game on these constraints
23) See 22) across
25) With 28 across, checking the horizon for the tallest emitter?
28) See 25 across
30) Military purchasers will buy these items if they suit, but expect them to be in the stock room

DOWN

1) Sounds like this piece of lumber only turns up now and then
2) When measuring, dBms. Watts, Amps, and Volts are all one of these
4) This is the mark for Europe
5) German scientist George involved in the resistance?
6) See 3) across
9) Electric guitar control used in compliance testing?
11) This doesn’t look good in the test report
13) Energy per unit charge produced by a dynamo
15) See 16 across
17) Never direct and undecided on polarity
19) A horse should never go hungry in this field set up
24) Times are a changing to disseminate
26) The center of attention initially
27) and 29) down. This container contains fields, not food
29) See 27 down

Answers below.

ANSWERS

-Tom Mullineaux

Elephant in the Test Room #3 – Fixing the Broken Automotive Emissions Test Fixture

2014-04-16T07:57:00.000-07:00

Read other posts in the "Elephant in the Test Room" series here.

Please note, regarding the fate of Elephant #2 – ‘Disharmony in Harmonic Limits’, it has now fallen under the umbrella of ‘Linearization of EMC Amplifiers’ and will be alluded to again during the part of the design exercise that determines practical harmonic levels.

And so, with no further ado, onto a whole new elephant…………

Elephant #3 – Fixing the Broken Automotive Emissions Test Fixture

The Room: Automotive RF Emissions Testing

The Elephant: Hard to justify systemic uncertainties

The Culprit: An over-reliance on weakly related historical methods compounded by the turning of a blind eye to known serious flaws

Note: It is important that we are all at the same place when we discuss this topic or you may be left behind, or find yourself pondering needlessly over points already covered in the recently published article ‘A Design Review of the Automotive Radiated Emissions Test Fixture’, so it will pay to read this first.

We start by describing a means of getting a feel for the present level of error when measuring the fields emitted by the wiring harness. The test currently relies on the 1m calibration data supplied with the biconical antenna.

Very simply, ten single frequency fields are emitted from the harness one at a time over 20-200MHz and a field probe is used to measure the field strength 1m away. The article already describes how the test fixture can be characterized with a signal generator providing a known spot frequency at a known injected power level.

The measurements are then repeated with the biconical antenna in place of the field probe. It is easy to convert the dBuV reading on the receiver back to the field the antenna ‘sees’ using the supplied calibration data.

Anyone out there have the facilities, time, and inclination to try this? For the purposes of this experiment we can probably devise a way of initially avoiding the need for a 50-300 Ohm transformer.

Before signing off until next time, I would estimate the figure for getting an independent RF house to review and fix the system as $30k-$50k. Even if it turned out to be twice or even three times that estimate, surely this pales into insignificance when compared to the cost, (financial and injury to reputation), of just one vehicle recall. You might think that the automotive manufacturers’ EMC departments would get their heads together and fund the project jointly. In my view, the various groups that presided over this debacle of a test set-up to date have demonstrated their ability already, and should be excluded from the project until the new fixture design has been proven.

To be continued…

Linearization of EMC Amplifiers

Homing in on the Phase Control

For optimum control of the harmonic cancellation process we need to be able to adjust the phase and amplitude of the cancelling signal. The amplitude control is comparatively simple as we can get the PC to change the output power of the signal generator as required. So for now we will concentrate on the phase control, but should bear in mind that any phase shifter we choose will be frequency dependent. That is the insertion loss of the shifter will increase with frequency, causing a reduction in signal amplitude. However this is of no great matter as we are designing a self monitoring / correcting system.

To make the objective we are seeking here perfectly clear, we will divorce our thinking from the job in hand and simply look at the output of two signal generators.

The generators are locked together by connecting the ‘sync’ connectors on the back panels and so will start in phase as shown in the first figure. The amplitudes are different only so we can see both signals when they coincide.

The second figure shows what happens when we introduce a phase shift (lengthen the electrical length of the second generator cable connection).

The phase shifter can only add electrical length to the output connection from the second signal generator and so, as shown, the black trace can only move to the right. This is a nuisance when to phase align the two signals we only need to move the black trace slightly to the left. So we will deliberately put a phase lag into the red trace. This way we can always get the two to align when we phase shift the black trace to the right. This is shown in the next figure.

The final figure shows the resultant when we press the ‘invert’ button on the second signal generator and use the phase shifter to align the two signals.

The resultant is the only signal that will appear when the two outputs are combined.
Getting back to the job in hand, we must remember that the two signals we are talking about are signals at the output of the TWT amplifier. And so we must bear in mind that we do not as yet know the insertion phase of the TWT amplifier, so the offset lag (read the length of the line stretcher we will install) and which arm it needs to go in will need to be determined when we characterize the TWT amplifier.

To be continued….

The Cellphone Threat

In this post we complete the description on the 4G orthogonality concept, then in the next post we will move onto the 4G and 3G transmitted waveform characteristics.

4G (OFDMA) Orthogonal Codes Continued……

It was explained previously that OFDMA splits the very fast serial data to be sent into many parallel, far slower, frequency separated channels sent as a tightly packed spectrum. At the receive end the slow parallel data channels are converted back into the original fast serial data stream. The separation of each parallel channel from the ‘pack’ is achieved through the fact that a signal frequency multiplied by itself and integrated produces a non-zero result, whereas when this frequency is multiplied by any other frequency in the pack a zero result is produced. The figure below shows the concept.

The variable signal generator presents each ‘bin’ frequency in turn (f1 then f2 etc) and this is multiplied with the ‘pack’. To see why only the wanted signal produces a result when mixed with the correct bin frequency we need to revisit the high school trigonometric identities we were all forced to learn by rote.

Proof of a zero result when sinusoidal waveforms of different frequency are multiplied and integrated

From the trigonometric identity cos(u)cos(v) = ½[ cos(u-v) + cos(u+v)]

∫cos(u)cos(v)dt = ½ [∫cos(u-v)dt + ∫ cos(u+v)dt]

But the algebraic sum of the area under the curve of the cosine function over one cycle is 0, so

½[ ∫ cos(u-v)dt + ∫ cos(u+v)dt] = ½ [(0) + (0)] = 0

That was easy. Now for the next.

Proof of a non-zero result when a sinusoidal waveform is multiplied by itself and integrated

From the trigonometric identity cos(u)cos(v) = ½[ cos(u-v) + cos(u+v)]

And letting v = u

cos(u)cos(u) = ½[cos(u-u) + cos(u+u)]

cos²(u) = ½[cos(0) + cos(2u)]

cos²(u) = ½[1 + cos(2u)]

Again, the algebraic sum of the area of the cosine function over one cycle is 0, so

∫ cos²(u)dt = ½ ∫ [1dt + cos(2u)dt] = ½T + ½ (0)

= ½T, where T is the time to complete one cycle

And that completes the basic concept behind 4G OFDMA.

Next time we will look at the transmitted waveform characteristic.

To be continued ……

-Tom Mullineaux

Previous posts in the Elephant in the Test Room Series:

1. Elephants in the Test Room
2. Elephants in the Test Room #1 Continued - Further Expansion on the Culprit
3. Elephant in the Test Room #1 Continued – How Other EMC Sectors Dealt with the Elephant
4. Elephant in the Test Room #1 Continued – The "Money No Object" Approach to Fixing the Under-Test Situation
5. Elephant in the Test Room #1 Continued – The List of Practical Solutions
6. Elephant in the Test Room #1 Continued - The 'Get Around' Explained
7. Elephant in the Test Room #1 Continued - The Solid State Approach
8. Elephant in the Test Room #1 Continued – The Final Piece in the Conventional Approach
9. Elephant in the Test Room #1 Continued - Linearization of EMC Amplifiers
10.The Linearization of EMC Amplifiers Continued and Introduction to Elephant in the Test Room #2
11. Training the Intern and Elephant in the Test Room #2 Continued
12. Elephant in the Test Room #2 Continued and the Design Guide Article

Elephant in the Test Room #2 Continued and The Design Guide Article

2014-04-03T10:18:00.000-07:00

Read other posts in the "Elephant in the Test Room" series here.

Elephant #2 - Disharmony in Harmonic Limits

The Room: RF immunity testing

The Elephant: The curbing of the contribution made by harmonics to a calibrated test-field varies wildly from standard to standard, and within standards

The Culprit: Harmonic limits that are seemingly not well thought through, and / or are open to interpretation

The consequence: A customer’s product sent to two different test houses for the same RF immunity test can be subjected to test fields of very different harmonic content

Continuing…

The dilemma faced by test houses when trying to abide by the automotive -20dBc harmonic limit is that it is hard to figure out how to interpret this. As mentioned previously a test house may have been bitten when buying a 1kW amplifier after it is realized that the amplifier -20dBc specification is at less than the amplifier rated power. What if the amplifier needs to be run at rated power to achieve 200v/m? Is the test house in compliance with the automotive limit or not?

And it only gets worse. Sellers of amplifiers provide data on the amplifier performance under ideal conditions. Is the data still valid under real test conditions? You will be hard pressed to find an amplifier supplier willing to guarantee the same performance as that specified on the data sheet.

I do not envy the procurement guy that has to buy an amplifier costing in excess of $100k who through due diligence discovers he needs to buy one at double the power (and double the price) in order to guarantee -20dBc harmonics when generating 200v/m under all test conditions. That will make for a fun conversation when he talks to the company accountant.

In my view the commercial sector approach to harmonic limits is eminently more sensible where you can work backwards to the amplifier and deal with the two issues mentioned above.

And here is an entirely realistic situation that may impact on a customer’s product going through a test house. The test house offers automotive and MIL-STD testing. One of the TWT amplifiers in the MIL-STD test suite goes down right in the middle of the test run. The customer’s delivery deadline is looming and the MIL-STD chamber is booked for another customer next week. However the commercial/automotive chamber is free at present. There is every chance that the test manager will borrow a filtered TWT amplifier from the automotive suite to replace the failed amplifier. There is nothing ‘illegal’ about this but now the test applied has changed substantially. If corrected, instead of 120v/m at the intended frequency and 80v/m at the harmonic, the product is now exposed to almost 200v/m at the intended frequency. If the EUT fails, would it have passed if the original amplifier was available? And if left uncorrected, is the 200v/m achieved?

To be continued……

The Linearization of EMC Amplifiers

Now we have the intern up to speed in terms of what to expect when cancelling the harmonic noise we can start to create a proof of concept instruction set for single test frequency 1GHz.

INSTRUCTION SET

Set F2 to 2GHz
Set F2 amplitude to look-up table entry
Set F2 phase to look-up table entry
Set F1 to 1GHz
Set F1 amplitude to look-up table entry
Set F2 to ON
Set F1 to ON
Check harmonic level is as in look-up table
If not then run subroutine 1
Check Pi is as in look-up table
If not then run subroutine 2
Loop through subroutine 1 and 2 until both requirements are met
Dwell
Set F1 to OFF
Set F2 to OFF

The subroutines are required to cater for the TWT amplifier quirks we educated the intern about (non-monotonic behavior, possible changes in the power of the fundamental as the harmonic is cancelled), but more than this, we want the system to be self-correcting when under the control of the PC. Too many RF immunity tests are completed ‘flying blind’, where the GO button is pressed and everyone hopes for the best. We will get the PC to ramp up the power and fine tune the cancelling as it does so, before it dwells.

Looking ahead it occurred to me that we could use some of these low priced USB controlled ‘half brick’ instruments that rely on being connected to a PC (Sig Gens, spectrum analyzers, power meters, etc). Would be great fun, but for now let’s get the intern up to speed and get the system automated using ‘real’ instruments.

Sanity Check

Before we spend any further time and effort we should do a sanity check regarding the practicalities of the exercise.

Let’s play devil’s advocate and list two possible objections to the implementation of the new system:

1. The test time will be doubled compared to the present system arrangement.

Only the first 20% (possibly less) of the band is involved
No time is wasted investigating whether the intended test frequency or its harmonic caused an EUT failure

2. The amplifier behavior may change under real test conditions.

This question is a bit rich given the present ‘flying blind’ mentality
Once the concept is proven, the self-correction capability can be expanded to cater for further scenarios

Another feature is that the amplifier/linearizer harmonic level can be changed to suit different standards

To be continued…..

The Cellphone Threat

In this post we complete the description on the 3G orthogonality concept. It is fairly long so we will continue with the 4G orthogonality explanation next time.

3G (WCDMA) Orthogonal Codes Continued…..

If you had a shot at auto-correlating one of the special orthogonal codes (EXNORed it with itself) then you established the output of the EXNOR gate was permanently at +1 volts. It was also easy to establish that when two of the special codes are cross-correlated the average output is zero volts.

I completed the other exercises needed to complete the picture in this concept.

The first diagram ( The effect of ‘1’ and ‘0’ …) shows how a user’s data modulates the special code allocated to the user, where basically a ‘1’ leaves the code as it is and a ‘0’ inverts the code.

The next diagram (Recovering the data …..) shows how demodulation is accomplished by simply EXNORing it with the user’s special code again (in actual fact the decoding code is the self-same allocated special code).

The last diagram (Data not recovered…) shows how another special code does not recover the data (sum of output = zero volts).

In summary, we can state the following:

On the transmit end:

When the user’s data is at 1, the special code is unaltered
When the user’s data is at 0, the special code is inverted
These are the only two circumstances as regards the sent modulated data

On the receive end:

When the unaltered code is EXNORed with itself the gate produces all ‘1’s
When the inverted code is EXNORed with the special code the gate produces all ‘0’s
When the unaltered code is EXNORed with a different special code the average output is zero volts
When the inverted special code is EXNORed with a different special code the average output is still zero volts (this is because inverting a special code creates another orthogonal code)

Next time we will look at the characteristics of the transmitted WCDMA waveform spectrum.

EMC Directory and Design Guide Article

I recently wrote an article that questioned the operation of the automotive RF emissions test set-up. The article, titled, "A Design Review of the Automotive Radiated Emissions Test Fixture," appeared in Interference Technology’s EMC Directory and Design Guide magazine. I could put only a fraction of what I wanted to say within an article of reasonable length, so I will expand on this here. We will start formally next week by christening it ‘Elephant in the Test Room #3’, but for now, here is one of my thoughts on ‘testing’ this test set-up. If we were to inject known single frequencies of known power level into the test set-up wiring harness, we could use a field probe to measure the field at 1m and compare the measurement with the field strength picked up by the nearby antenna. Under normal circumstances we cannot use a field probe as it sums all the noise fields into one single output power level. However, when the noise field is one single frequency the field probe works perfectly in measuring the single emitted field.

To be continued…..

-Tom Mullineaux

Training the Intern and Elephant in the Test Room #2 Continued

2014-03-17T11:23:00.000-07:00

Read other posts in the "Elephant in the Test Room" series here.

Elephant #2 - Disharmony in Harmonic Limits

The Room: RF immunity testing

The Elephant: The curbing of the contribution made by harmonics to a calibrated test-field varies wildly from standard to standard, and within standards

The Culprit: Harmonic limits that are seemingly not well thought through, and / or are open to interpretation

The consequence: A customer’s product sent to two different test houses for the same RF immunity test can be subjected to test fields of very different harmonic content

Continuing…

Reminder – commercial RF immunity testing says no more than 25% of the test field can be created by harmonics, automotive stipulates minus 20dBc harmonics from the amplifier, and military has nothing set in stone.

So on what grounds did the automotive industry determine the 1% harmonic limit? For instance why is 1% harmonics acceptable yet 2% is not?

I see the need for limiting the proportion of the test field created by harmonics, but 1% seems absurdly tiny. Maybe the limit setting got caught up in the ‘unintended acceleration’ maelstrom and the industry wanted to be seen as taking strong action regarding car safety, and as a result overcompensated for other weaknesses in the suite of automotive tests.

To my mind, all the 20dBc limit did was cost test houses a small fortune and create a mini-bonanza for amplifier sellers.

So what harmonic limit would be required if automotive had gone for the commercial limit?

Easy enough to calculate. Let's go for a 1GHz test with the harmonic at 2GHz. So the 200v/m total test field would be made from 150v/m at 1GHz and 50v/m at 2GHz (25% of total or minus 6dB)).

We assume we are using an antenna with 10dBi (reasonable) at 1GHz and 16dBi (excessively cruel but let’s go with it) at 2GHz.

Let the power required to generate the 150v/m with antenna linear gain G₁ = 10 be P₁, and let the power required to generate 50v/m with antenna linear gain G₂ = 40 be P₂. The powers are the power required at the antenna connector.

That is a difference of 4dB compared to the automotive limit. This may not look much on paper but it means the automotive harmonics have to be way less than half those of an amplifier used in commercial testing. Put alternatively, you can use an amplifier with 16dBc harmonics for commercial testing, but need one with 20dBc for automotive.

To be continued…….

The Linearization of EMC Amplifiers

Training the Intern

If we are going to use an intern for mundane tasks, it is only fair that in exchange we provide them with knowledge, combined with hands-on practice in the use of that knowledge.

We start by providing knowledge on the characteristics of RF power amplifiers likely to be met in the course of the intern’s compliance testing career.

Here we go--the input-output characteristic of power amplifiers is monotonic. Monotonic is a flash way of saying that as the input power is increased, the output increases too, and the situation never arises where an increase in input power results in a decrease in output power. All amplifiers exhibit gain compression as the maximum output power is approached, and ultimately the amplifier reaches saturation, where an increase in input power produces no discernible increase in output power. However the output power does not drop, it just no longer increases with increased input power.

Sounds like we are stating the obvious, but TWT amplifiers are non-monotonic. As the input power is increased the gain compresses as maximum power is approached, then the amplifier saturates, then as the input power is increased further, the output power drops.

So what is the big deal? Well, here is the big deal. We want to use cancellation to reduce the harmonic power of a TWT amplifier (see picture). We are using an intern to apply active pre-distortion via the second sig gen level and the phase changer, and if unaware of this characteristic, the intern could find themselves in a declining spiral.

To be fully cognizant, the intern needs to know one more thing about power amplifiers. Again it seems obvious, but we will spell it out for them all the same, and in this case the snippet of knowledge applies to solid-state and TWT amplifiers.

It refers to the most you can expect from an amplifier. Let’s say you have an amplifier that at its best can output 450 watts. So you can reasonably expect the amplifier to produce 450W at one spot frequency, 225W (each) at two frequencies, 150W (each) for three frequencies, etc. What you cannot expect the amplifier to do is produce two frequencies at 450W. That would be crazy, the total power cannot exceed 450W.

So when the intern is adjusting the phase and amplitude of the cancelling signal, and during the circumstances where to get 200v/m the amplifier is running at or close to full power, the intern should anticipate weird and wonderful things happening to the fundamental signal power as the cancelling signal is aligned with the harmonic.

It all sounds very frightening, but we will get the intern to fully characterize the amplifier by recording its behavior under different power situations. They can then refer to these ‘look-up’ tables when conducting actual testing. That of course, is what the PC will to do when it takes over this mundane task.

To be explained further when we continue ……

The Cellphone Threat

In this post, we dig a little deeper into the 3G and 4G air interfaces.

More on 3G Orthogonal Codes

Reminder - Each user is allocated a special code that ‘chips’ the user’s data, and this self-same code is used at the receive end to recover the data. Cyclical, and pseudo random in nature (so noise like when viewed on a spectrum analyzer), any two of the special codes have low cross-correlation. That is they have little similarity. Happily for us this is digital electronics, and there is a logic gate that provides the level of similarity between any two codes – the EXNOR gate. The output of an EXNOR gate is only true when the bits of the two codes presented to it are the same.

To make progress, we just need to do one more thing, and then we will be all set to use the EXNOR gate to determine level of similarity. For logic 1 we will allocate +1 volts (could be +5V or +3.3V or whatever), and for logic 0 we will allocate minus the logic 1, that is -1 volts (again, could be whatever).

The new truth table:

Now we are all set to use the EXNOR gate to determine the level of similarity between any two of the special chipping codes.

As a preliminary test, we will compare the similarity of a code with itself (should be all TRUEs, right?), and then we will compare two special codes. I will stop here to allow you to take a shot at this yourself. Here are two sample codes:

01101001

00110101

Can you establish the average voltage when auto (=self) cross-correlated? And then when the two codes are cross-correlated?

More On the 4G Air Interface

A key feature of 4G OFDMA is the splitting of very fast serial data into many slow parallel channels (see picture), and then reversing the process at the receive end. Seems like a lot of messing about, but each slow parallel channel will have a superior bit error rate (from the simple fact that the data rate is slower) and moreover there is scope to add guard periods and so forth, making the bit error rate better still. The parallel channels are separated by frequency and spread across a wide band, with the result that 4G is less prone to multi-path fading (a serious issue with 3G).

To be continued...

-Tom Mullineaux

What’s in a Word?

2014-03-03T06:43:00.001-08:00

I learned a new term the other day, and it actually seemed to catch a lot folks by surprise, myself included. I am very used to hearing and am very familiar with the term “electromagnetic environment” or EME. For the US DoD, the EME is defined as (from Joint Publication 1-02):

“The resulting product of the power and time distribution, in various frequency ranges, of the radiated or conducted electromagnetic emission levels encountered by a military force, system, or platform when performing its assigned mission in its intended operational environment. Also called EME.”

Makes perfect sense to a long time EMC guy.

So what’s the “electromagnetic operational environment”? That’s the new term, by the way. Also known as the EMOE. Googling the acronym doesn’t help…unless you like pictures of a rather odd looking, ostrich like bird from Australia.

Googling the exact phrase was a little better and it turns out there are a variety of publications that reference that term. So I found myself looking at my old friend, Joint Pub 1-02, thinking that there’s no way that term is in there but sure enough, there it was:

“electromagnetic operational environment — The background electromagnetic environment and the friendly, neutral, and adversarial electromagnetic order of battle within the electromagnetic area of influence associated with a given operational area. Also called EMOE.”

Now, I’m confused! Sounds a lot like the original definition of the EME, one’s just a little more technical sounding than that other. “The resulting product of the power and time distribution, in various frequency ranges...yada, yada” versus “The background electromagnetic environment and the friendly, neutral, and adversarial electromagnetic order of battle…etc.” Doesn’t that sort of mean the same thing? Well yes…and no. A very big deal in the DoD EM Spectrum business today is Joint Electromagnetic Spectrum Operations, or JEMSO, which is essentially an integration of joint operational spectrum management AND electronic warfare (EW). Historically, particularly in recent actions, the lack of coordination between spectrum managers and EW operators has resulted in an unacceptable level of electronic fratricide between jammers and communications systems. NOT a good thing operationally!

So, how does the EMOE differ from the garden variety EME? The key words are at the end of the definition “within the electromagnetic area of influence associated with a given operational area.” Without going into a thesis (which I’m not qualified for anyway), the EMOE is actually a subset of the overall EME for a particular operational area and includes both friendly and enemy frequency use considerations. The EMOE is composed of the background EME and the friendly, neutral, and adversarial electronic order of battle (EOB) within the EM area of influence (a term for another day) associated with a given operational area. The EOB details all known combinations of emitters and platforms for both friendly and adversary force data. Confused? Me too…but basically it’s the EME we all know and love PLUS the intended use of the EM spectrum by both our side and theirs, whether for comms, radar, jamming or what have you.

So, what’s in a word? Quite a lot once you start peeling the onion…..but we’ve just scratched the surface here.

-Brian Farmer

The Linearization of EMC Amplifiers Continued & Introduction to Elephant in the Test Room #2

2014-02-18T08:42:00.001-08:00

Read other posts in the "Elephant in the Test Room" series here.

As promised in the last post, here is the start of the next elephant!

Elephant #2 - Disharmony in Harmonic Limits

The Room: RF immunity testing

The Elephant: The curbing of the contribution made by harmonics to a calibrated test-field varies wildly from standard to standard, and within standards

The Culprit: Harmonic limits that are seemingly not well thought through, and / or are open to interpretation, and / or pay no regard to how other standards bodies determine harmonic limits

The Consequence: A customer’s product sent to two different test houses for the same RF immunity test can be subjected to test fields of very different harmonic content

Introduction

As a test house, the amount of notice you take of harmonic noise depends on the type of RF immunity test being conducted. Commercial testing says the portion of the test field created by harmonics should not exceed 25%. Automotive says the harmonic levels from the amplifier must be minus 20dBc or better. Military (last time I looked) is not specific on either of these type of limits.

Let’s start with the automotive 20dBc limit, as I can see no RF engineering rationale behind this seemingly arbitrary and overcautious limit (20dBc means only 1% or less can be harmonic power), and even if you could justify the limit, and I don‘t see how you can, a coach and horses can be driven through the way the minus 20dBc is specified.

Example: It is a given that an amplifier tends to have to work hardest at the lowest frequency point in its band. This is because the antenna it is driving generally has lower gain with lower frequency. Yet, almost without exception, amplifier harmonic levels are worse at the start of the amplifier band.

So let’s say a test house puts a significant dollar investment into an amplifier rated at 1kW, and (not too carefully looked at at the time of purchase) the data sheet states the harmonics are minus 20dBc at a power level of 700W. Should it turn out that the amplifier needs to produce 800W to get the required field strength, does the stipulated 20dBc limit still apply?

To be continued…….

The Magic Number 30

During a reply to a blog reader, the question was posed “Where does the magic number ‘30’ in field strength E = sqrt(30P.G)/d come from?”

Well done those who worked it out for themselves. In order of clicking the ‘publish‘ button, they are 1. Bruce Curry 2. Jim Pollock and 3. Hans Joubert

Here is how, long ago, I worked it out: -

E/H = Zo so H = E/Zo

Power density S = ExH

But H = E/Zo

So S = ExE/Zo

That is power density S = E^2/Zo

(by the way, intuition tells us that since this is watts per square meter, this has to be ERMS, but again, let’s just stick with finding where the magic number 30 comes from)

Power density is also S = P.G/ 4Pi.r^2 (shown in the picture where a point source of power emanates a wavefront spherically, followed by one from an antenna emanating in a particular direction with linear gain over isotropic G. Note: The arrows are for illustration only and simply show the direction of the emanating wavefront through the square meter drawn for each case)

Given that S = E^2/Zo and S = P.G/4Pi.r^2

Then E^2/Zo = P.G/4Pi.r^2

But Zo = 120Pi

Therefore E^2/120Pi = P.G/4Pi.r^2

E^2 = 120Pi.P.G/4Pi.r^2

E^2 = 30P.G/r^2 (ping! - out pops the magic number)

E = sqrt (30P.G)/r

Incidentally. Somewhere around 2008, while investigating testing to 6GHz for the new 61000-4-3, in some instances the calculation was amazingly close to actual measurement for bore-sight field strength (the strength right in the middle of the measurement plane). OK, it needed cladding on the floor around the antenna and between the antenna and the measurement plane, and the antenna was pointing to one corner of the room, but impressive all the same.

The Linearization of EMC Amplifiers

Linearizing TWT Amplifiers

The first approach we could try is simple cancellation of the high power harmonic by adding a phase inverted harmonic frequency at the input to the amplifier. We inject the anti-phase signal at the input and monitor the level of the output harmonic while adjusting the level/phase of the injected signal (ultimately we will get the PC running the software to do this). The concept diagram below shows the key parts of this idea. We will be forced to add many more components later such as circulators, line stretchers etc., and we will replace the second signal generator with a frequency doubler, but let’s stick at the concept level for now (so please let’s ignore easily fixed foibles.

I recommend we press a long-suffering intern* into adjusting the level and phase of the injected signal for now, and then once he/she has got the hang of it, write what they actually do in pseudo code in readiness for writing instructions for the dumbest intern of all, the PC running the EMC software.

For now we want the intern to familiarize him/herself with the working of the system. We ask them to get the system up and running with the upper signal generator set for say 50W of fundamental (at 1GHz) output power from the amplifier. He/she should be able to see the fundamental and the harmonic at 2GHz on the spectrum analyzer as shown. Now the second signal is introduced at say at 1.8GHz so it can be seen on the screen, and the level is adjusted to something similar to the level of the harmonic. The frequency is then changed to 2GHz so it coincides with the harmonic, and this increases or reduces the harmonic level depending on the relative phase. The intern now plays with the phase and fine tunes the level of the signal until the harmonic level on the screen is down by 10dB.

I can see at least one juggernaut that is going to hit us hard later, but let’s see how far we get first.

* No interns were harmed in the writing of this post

To be continued ……

The Cellphone Threat

As mentioned in the last posting, I am of the view that those involved in EMC testing should be familiar with the most prevalent radio frequency interferer out there - the cellphone. A cellphone in close proximity to a victim is the threat we should be concerned about, not the signals from the base station, even though at their source these are of far higher transmit power. Nonetheless, to get a grasp of the working of the 3G and 4G air interfaces as presently in use, we really need to cover base station transmissions. Once covered, we can then look at what is transmitted by the real threat, the cellphone.

In this particular post we will start with one fundamental principle, key to all. That is orthogonality, and how it applies to both 3G and 4G air interfaces. Once we have this down, we will start to fill in the details (frequency bands, transmitted waveform signature, etc.).

Orthogonality

Simply put, orthogonal signals allow users to share a composite transmitted band, and yet the signals can still be extracted/separated from each other at the other end. Both 3G and 4G rely on orthogonality. With 3G’s WCDMA, the codes used to modulate data are orthogonal to each other. With 4G’s OFDMA, the channel frequencies are orthogonal to each other.

3G’s WCDMA Codes

Each user’s data is modulated by a special code allocated to the user and the user alone. The special code runs at high speed, ‘chipping’ the data as shown in panel [a] below. At the receive end, the self-same allocated code is used to extract this one user’s data [b]. Panel [c] shows how another user’s allocated code does not extract the data. This is because all the special codes are orthogonal to one another.

4G’s OFDMA Pulsed RF Spectrums

In regular frequency division schemes, channels are separated by frequency and guard bands are placed between the channels. OFDMA takes advantage of the fact that a repetitive RF pulse in the time domain produces a sin(x)/x shaped spectrum in the frequency domain as shown below (Panel [d]). Note the frequency domain characteristic central peak with well-defined nulls either side. Unlike regular frequency division, the sin(x)/x spectrums of adjacent channels can be placed right next to each other with the peak of one corresponding with the null of the other (i.e., orthogonal to one another), see [e] and [f]. This allows very tight packing of the channels without cross-channel interference, and without the need for guard bands.

Orthogonality concepts will be continued ………

Unintended Acceleration

No doubt you have noticed that the subject of unintended acceleration has reared its head again as reported recently in the Interference Technology News section.

I have often wondered about possible reasons for unintended acceleration and my mind tends to go back to early days and discrete digital electronic design, where often, to save putting in another multiple JK flip-flop IC, it was common to use spare NAND gates on a hex NAND gate chip to create a flip-flop. In those days medium scale integration was in it‘s infancy, and there was a phenomenon called ’latch-up’, where for no apparent reason a flip-flop (this applied to both NAND gate based and the JK flip-flop ICs) would latch-up in the ‘set’ state, and only removing the dc power from the chip could release it from this state. It has crossed my mind that if the most significant bit of the flip-flop bank that is the memory address holding the accelerator position, was to latch-up, then even when the disturbance that caused the latch-up went away, the engine management system would continue to provide maximum acceleration as seemingly requested by the accelerator memory address.

And my musing got me to wondering about how the current EMC standards check potential RF interferers attached to the wiring harness before the new car design is released. I recently wrote on this in the guise of a mock design review, where the existing test set-up is put through the rigors of a real design review as required of RF systems destined for the commercial marketplace. In the real world of RF product design there are mandatory reviews written into the design cycle, where the first review is often of the internal design specification written by the design team itself. As well as ensuring designed inputs and outputs interface with other parts of the system being designed by others, this also ensures the design team knows the particular performance requirements that the proposed design will be measured against. The article will appear in the 2014 Interference Technology Design Guide.

-Tom Mullineaux

To EMP or Not to EMP… Is That the Question?

2014-02-11T07:49:00.001-08:00

So what are our thoughts on the threat of an electromagnetic pulse event (God forbid) causing some sort of catastrophic damage to our telecommunications, power, and whatever other kind of electronic infrastructure that might be out there? I know, I know…it depends who you talk to! Being in the EMC business, I’d like to believe that I understand the fundamental issue (danger) to our infrastructure and systems but at the same time I’d also like to believe that it’ll never happen. At the end of the day, I don’t think we can afford to be that naïve. While we’ll probably never suffer an actual nuclear EMP from a burst over the US (he says knocking wood), I can certainly imagine terrorist produced, electronically created EMPs on a smaller scale, say, taking out a city, for example. And the possibility has certainly gotten the attention of both Congress and the Pentagon, as there have been EMP programs popping up in all the Services in recent years. It’s a bit like Back to the Future…a lot of you will remember the robust EMP related programs (can you say EMPRESS II?) back in the day that died out with the end of the Cold War. Like previous efforts, some of the current efforts are to assess the current protection levels (EMP “hardness”) of existing infrastructure, some are to develop and promulgate better design standards and some are to examine improved protection methods.

I’m all for EMP-hardened infrastructure, platforms and systems. A lot of that is just good EMC design and manufacturing practices (grounding, bonding and shielding). But at what cost? Retrofitting existing facilities, platforms and systems, even those considered critical to our defense or strategic in nature is simply beyond the governments financial means these days. From a purely selfish (business) perspective, the development and publication of new EMP design standards as well as the imposition of existing hardness requirements, is a great start. But it really all starts with making EMP survivability a system performance requirement and then following that through design, testing/verification, production, etc. to ensure that the fielded product is actually EMP survivable. The requirement shouldn’t be watered down, pushed out to the right (schedule lingo!) or heaven forbid waived! We can’t just try to field hardened systems, we should simply do it or not do it. The design techniques are pretty robust at this point in technology and frankly, pretty simple for the most part.

In the famous words of Yoda: Do or do not, there is no try. That is why you failed!

-Brian Farmer

Elephant in the Test Room #1 Continued - Linearization of EMC Amplifiers

2014-02-04T13:16:00.001-08:00

Read other posts in the "Elephant in the Test Room" series here.

Recap On this Thread

The room: MIL-STD RF immunity testing – 1-18GHz 200v/m

The elephant: everyone in the room is aware that a significant fraction of the ‘calibrated’ 200v/m test field is actually created at the wrong test frequency.

The culprit: notoriously high ‘start of band’ harmonics produced by all octave band TWT amplifiers.

The consequence: equipments purportedly demonstrated to be resilient to 200v/m have in many instances been exposed to only around half this field strength, the rest of the test field created by an unintended frequency.

Welcome back readers, I hope 2014 is turning out well for you all.

I have spent more time on Elephant #1 than I intended, but I really want to plant the seed of the linearization of EMC amplifiers before leaving it. However, I will start introducing Elephant #2 “Lack of Harmony in Harmonic Limits” at the next posting regardless of where we are with Elephant #1. Also, I am a great believer in EMC engineers/technicians being conversant with the most common threat out there – cell phone signals, so in parallel with prodding elephants, I shall do a 101 on 3G and 4G wireless telecom signals (again, starting in the next post).

So where were we last year? – ah yes, Elephant #1, we had a fix for the system harmonics up to 7.5GHz, and were checking the power margin available if we were to place a filter to remove the start of band harmonics emitted by the 7.5-18GHz amplifier.

A back of the envelope calculation showed there was insufficient power if we go with the guaranteed minimum power available. If 250W is truly all that is available at the start of the band, then something fundamental needs to change (higher power amplifier $$$, or higher gain antenna $, or….).

However there is one saving grace. As opposed to the situation where you are buying an amplifier, and do not yet know its performance (risky to assume the available power due to manufacturing spread in the amplifier manufacturing process) you already own the amplifier and can establish the available power yourself.

The picture shows one way of doing this. Note we want to establish the power in the fundamental peak only, the intention is to remove the harmonic to the right will by filtering.

So if we are lucky enough to confirm we have sufficient power overhead, how much will the filter (diplexer) cost? As ever, it depends. If the design already exists I would hazard a guess at $2k-$3k. If a design does not exist, then non-recurring engineering charges will apply, and this could add $5-$7k, making a price of $7k-$10k. So it is worth doing a thorough search of all vendors to see if they already have a product, or at least one close enough that only needs tweaking.

The Linearization of EMC Amplifiers

Linearization is common enough in wireless telecoms, where the push for the technology came from economic necessity. Strangely it has not touched EMC amplifiers, perhaps because the number of amplifiers is relatively small and the market is too small to chase.

In my view, EMC amplifiers are ripe for the technology since compared to the very complex wireless telecom modulation schemes:

1. The signal to be linearized occurs at one spot frequency at a time
2. The EMC modulation schemes are the simplest there are, AND
3. The PC running the EMC software is idle for 99.9% of the time. This means a very powerful tool is available for any linearization scheme we choose to deploy

There are several linearization schemes, some of which I will mention here. Note: I use passive to mean the distortion element behaves the same irrespective of the amplifier output, and active to mean feedback is involved in the pre-distortion.

Passive Pre-Distortion Concept

A non-linear element displaying gain-expansion where the amplifier has gain-compression distorts the input signal such that the resultant output is a straight line.

Active Pre-Distortion

With active pre-distortion the input signal is compared to the output signal and any difference is used to control an attenuator. This is actually only feasible with an AM modulated waveform, where the AM envelopes are compared and the attenuator is used to ensure the output envelope follows the input envelope.

Feed-Forward

This is best explained by following the signal through the concept diagram. The input signal in this case is a two tone signal.

The input signal [1] splits at the input, some going to the input of the amplifier, the rest going to a phase inverter. The amplifier amplifies the input signal as intended [2] but also introduces unwanted signals (red arrows). The sampled wanted and unwanted signal s [3] and the inverted input signal [4] are summed leaving only the distortion products [5]. These are inverted [6], amplified [7] and injected into the output. This causes cancellation of the unwanted output signals [8].

We will tackle the linearization of TWT amplifiers first, and at the next posting I will be proposing a form of active pre-distortion using the PC as the ‘brains’ of the error amplifier. But a different form of linearization could also be used to great advantage in lower frequency solid-state amplifiers since this allows ‘forbidden’ compressed power to be accessed. We will cover this in a later post.

To be continued……

-Tom Mullineaux

In response to the comment on the previous post, "The Final Piece in the Conventional Approach," that stated the calculation for required power, P = d^2 x E^2 / 30G watts, is not correct, Tom says,

"It is true that at a plane the power density is E^2/Zo (equivalent to power density = P/4Pixr^2 for a point source of power emanating spherically at a distance r from the point). But we are looking for the power required from an amplifier to create a certain field level at a certain distance using an antenna of certain gain G. I will elaborate on this in the next post. By the way, does anyone know where the magic number 30 comes from in the equation E = sqrt (30xPxG)/d^2 ? Years ago I looked for and couldn't find the explanation in any textbook, and always thought it was a practical 'fudge' factor to make the v/m level come out right (or nearly right). Maybe the explanation is out there somewhere but I could not find it. Eventually, in the absence of any reference, I worked it out myself. Anyone know where it comes from before I tell you at the next post? You have a week."

They're Everywhere, They're Everywhere - Part II

2014-01-29T07:02:00.001-08:00

In part one of this post, I noted that there are three primary EMC coupling problems that are associated with the green wire safety ground:

1. The green wire conducted noise current modulates the signal and shows up as a differential voltage change across the load.

2. The green wire forms a pick-up loop and acts like a magnetic field antenna.

3. The green wire loop acts as a radiator. If the wire or cable that makes up the loop is a large linear structure it can be modeled as a dipole antenna. But if it is a distinctly loop configuration, the model is different. Generally the only known current is the differential current and the models presented below apply rigorously only for the differential mode. The common mode radiation could be estimated based on the default CM to DM conversion values given in item 1 above but as the loop size increases there would be significant error associated with the estimate.

Nevertheless, the simple method Kraus uses to estimate loop radiation makes it easy to understand the issue and provides an approach to solving the radiation problem. For small non-resonant loops, oriented at angle (θ), plane-wave emission levels resulting from loop currents are calculated from:

Assuming the loop is oriented for maximum emission, sin (θ) = 1; converting to frequency instead of wavelength, F (MHz) x λ(m) = 300 m/µs; and introducing the attenuation (as a ratio, not as dB) provided by any shielding, S; the equation can be rewritten as:

Where:
ε(max) = Microvolts/meter
A = Radiating loop area (sq cm), 0.1<(l/w)<10 br="">I = Drive current (amps)
F = Emission frequency (MHz)
r = Separation between the radiating loop and the Measurement point (meters)
S = Shielding effectiveness ratio

A review of the above equation shows that to minimize the radiated emission levels at any given frequency we need to do the following:

1. Keep the loop areas as small as possible
2. Reduce the current
3. Separate the emitter and receptor
4. Add shielding.

I could turn this into a quiz and ask you, “What should I do to solve the interference problem in my office?”, but I won’t. I’ll just tell you.

But first, here’s what I couldn’t do.

1. I couldn’t add shielded cables, because unless the entire CM volume is enclosed, shielding doesn’t work, and I didn’t want to make my office into a shielded enclosure.
2. I couldn’t separate the emitter and receptor any more than I already had without moving the TV out into the yard.
3. I couldn’t reduce the current without modifying the signaling scheme between the computer and the printer. Although I did clamp a lot of ferrite on the power and signal cables and that helped but did not eliminate the problem.
4. The loop area was defined by the location of the printer relative to the computer and I didn’t want to change the layout again.

What I did do was remove the interconnecting cable between the printer and the computer by going wireless. This effectively eliminated the common mode loop. I could have done it by converting to fiber optics at considerable expense and that part stopped me.

I really didn’t want to go to a wireless interconnect scheme because I’m now immersed in a radiated EF environment which-can’t-be-good and the radiated RF energy may contain sensitive personal information . . . like credit card info. I’m not into the conspiracy idiocy but as a TEMPEST design engineer I do have some concern about the hackers. And you should too.

For a good spoof about this whole issue watch: Prime Risk, a pseudo-entertaining film from 1985 about a female EMC/TEMPEST test engineer who discovers a way to rip off ATM machines. She also discovers a group trying to destroy the U.S. monetary system. Some of the stuff is factual some rather hokey.

-Ron Brewer

They’re Everywhere, They’re Everywhere - Part I

2014-01-22T12:26:00.002-08:00

Read Part II here.

I just finished rearranging my office. It all started with buying a new MS Windows 8 computer (that’s a story all on its own) followed closely by the failure of my printer to feed paper. So I bought a new printer, which is about two times bigger than my old one, and it wouldn’t fit in the previous location. So I moved my little flat screen TV into the old printer location and decided to change everything around so I could locate the printers on the other side out of the way. This of course required longer Ethernet cables. I finally finished my office and discovered that when I sent data to the printers, I had noise on the TV and on the AM radio band. This prompted my wife to remind me that the plumber’s faucets leak, the roofer’s roof leaks (but only when it rains), and the HVAC’s service tech’s air conditioning only works during the wintertime.

We just can’t escape from ground loops! They’re everywhere! Except for battery powered or doubly isolated electrical equipment almost every other electrical device has a three prong power connector and one of the pins is the green wire (or green/yellow) safety ground. We end up with something that looks like the following figure.

It's not blatantly obvious, but there are three primary EMC coupling problems being illustrated in this figure that are associated with the green wire safety ground:

1. The green wire conducted noise current modulates the signal and shows up as a differential voltage change across the load. If lead impedances were perfectly balanced, only common mode voltage would appear at the input to the line termination circuitry. There would be 0-Volts differential mode voltage. Unfortunately, the balance is not perfect and line unbalance will convert a portion of the common mode voltage to a differential mode voltage on the wire pair. The system must be able to operate with this noise.

Fortunately for most balanced signal transmission the conversion from common mode to differential mode is not very efficient and at the lower frequencies is on the order of -20 to -40 dB. Circuits using differential signaling and high grade cabling will generally have a balance value of 60dB (0.1%) or greater. For example, typical commercial CAT-5 cable (when new) will provide balance greater than 70 dB at 1MHz (60dB up to 10 MHz). For worst case analysis, the following conservative values can be used for CM to DM conversion:

• DC – 30 MHz: 40 dB
• 30 – 300 MHz: 20 dB
• 300 MHz and higher: 0 dB

2. The green wire forms a pick-up loop. This loop can behave as a low impedance magnetic field antenna and is sensitive to lower frequency magnetic fields being created by high level AC current loops, especially nearby switched mode power supplies. This statement is not meant to imply that loop antennas can’t pick up high frequency electric fields. They can and they do . . . and their sensitivity increases as a function of both loop area and frequency. The energy picked up by the loop results in common mode conducted currents like the first problem described above and can be mitigated using the same techniques.

3. The green wire loop acts as a radiator.

This topic requires more words in order to do it justice so I plan to cover it in more detail in the second part of this post along with the solution to my interference problem.

-Ron Brewer

Elephant in the Test Room #1 Continued – The Final Piece in the Conventional Approach

2013-12-20T08:17:00.005-08:00

So if we are lucky enough to confirm we have sufficient power overhead, how much will the filter (diplexer) cost? As ever, it depends. If the design already exists I would hazard a guess at $2k-$3k. If a design does not exist, then non-recurring engineering charges will apply, and this could add $5-$7k, making a price of $7k-$10k. So it is worth doing a thorough search of all vendors to see if they already have a product, or at least one close enough that only needs tweaking.

The Linearization of EMC Amplifiers

Linearization is common enough in wireless telecoms, where the push for the technology came from economic necessity. Strangely it has not touched EMC amplifiers, perhaps because the number of amplifiers is relatively small and the market is too small to chase.

In my view, EMC amplifiers are ripe for the technology since compared to the very complex wireless telecom modulation schemes:

1. The signal to be linearized occurs at one spot frequency at a time
2. The EMC modulation schemes are the simplest there are, AND
3. The PC running the EMC software is idle for 99.9% of the time. This means a very powerful tool is available for any linearization scheme we choose to deploy

There are several linearization schemes, some of which I will mention here. Note: I use passive to mean the distortion element behaves the same irrespective of the amplifier output, and active to mean feedback is involved in the pre-distortion.

Passive Pre-Distortion Concept

A non-linear element displaying gain-expansion where the amplifier has gain-compression distorts the input signal such that the resultant output is a straight line.

Active Pre-Distortion

With active pre-distortion the input signal is compared to the output signal and any difference is used to control an attenuator. This is actually only feasible with an AM modulated waveform, where the AM envelopes are compared and the attenuator is used to ensure the output envelope follows the input envelope.

Feed-Forward

This is best explained by following the signal through the concept diagram. The input signal in this case is a two tone signal.

The input signal [1] splits at the input, some going to the input of the amplifier, the rest going to a phase inverter. The amplifier amplifies the input signal as intended [2] but also introduces unwanted signals (red arrows). The sampled wanted and unwanted signal s [3] and the inverted input signal [4] are summed leaving only the distortion products [5]. These are inverted [6], amplified [7] and injected into the output. This causes cancellation of the unwanted output signals [8].

Well THAT Didn’t Take Long ...

2013-12-06T12:12:00.000-08:00

At long last, the US Department of Defense Chief Information Officer has released the updated version of DoD Instruction 3222.01, the DoD Electromagnetic Environmental Effects (E3) Program, into the DoD Portal for formal stage 3 coordination, or in English, for official review and comment by interested DoD organizations. It’s actually been several years on the making, constantly pushed to the right (schedule-wise speaking) due to higher priorities, reorganizations, etc. It’s been so long in fact, that I’m not sure if I have seen the last draft that was under review. Suffice it to say that the original “final” draft has been tweaked (not twerked!) several times over the last few years. Overall, the updated Instruction will represent a strengthening of the requirement to impose and enforce E3 control and protection requirements into military system procurement. I’ll try to summarize what I believe are some of the important changes:

1. It's an Instruction, not a Directive – both are “directive” in nature, that is, the DoD must follow the direction given in the document (not optional) but an Instruction is longer and provides more specifics on the requirements included. Directives tend to be very broad brush with a lot of “thou shalts” but not a lot of “how shalt I’s?”. The current version is 6 pages and the draft Instruction is 29…that’s like going from the Cliff Notes version to War and Peace.

2. The Policy list, i.e., the overall requirements imposed by the Instruction, specifically calls out the following (which weren’t included in the previous version):
           a. Built-in design and mitigation measures to preclude EMI, rather than relying on “after-the-fact remedial measures”
           b. Great detail on specifying E3 control requirements at various points in the acquisition process and in various acquisition and T&E documents
           c. Validation and verification requirements in both developmental and operational T&E phases
           d. Mandatory E3 awareness and training throughout DoD

3. A variety of Procedures are included such as E3 in Acquisitions (milestone reporting requirements), T&E, Standards and Training.

4. Specifying the Spectrum Supportability Risk Assessment (SSRA) as part of the acquisition reporting requirements (the subject of a lot of my posts). That shows another level of seriousness about SSRAs and a strengthening of the connection between E3 and Spectrum Supportability.

5. Incorporation of requirements for the installation of transmitting system on DoD sites, which used to be spelled out in a separate Instruction.

6. Greater detail on protection from the electromagnetic radiation hazards (HERO, HERP, HERF).

And there’s more, but I don’t want to ruin all the fun! It’s all that the E3 (and Spectrum) community could hope for and I think it validates the DoD/Contractor E3 engineering community that has been pushing for its release for these many years. Hopefully enforcement won’t be a big issue because it just reflects the way we’ve been trying to do business already.

-Brian Farmer

Elephant in the Test Room #1 Continued – The Solid-State Approach

2013-12-02T08:52:00.000-08:00

Read other posts in the "Elephant in the Test Room" series here.

Recap On this Thread

The room: MIL-STD RF immunity testing – 1-18GHz 200v/m

The elephant: everyone in the room is aware that a significant fraction of the ‘calibrated’ 200v/m test field is actually created at the wrong test frequency.

The culprit: notoriously high ‘start of band’ harmonics produced by all octave band TWT amplifiers.

The consequence: equipments purportedly demonstrated to be resilient to 200v/m have in many instances been exposed to only around half this field strength, the rest of the test field created by an unintended frequency.

Hi again. In the last posting we had a look at filtering combined with band overlap. We have yet to determine the losses introduced by the filters, but it makes sense to postpone this exercise until we have fully explored the use of solid-state amplifiers, so solid-state is the main thrust of this post.

First, let’s finish the review of the ‘get around’ solution suggested by a blog reader. The reader suggested that to get around the TWT amplifier high harmonics issue, the test should be run backwards – that is start at the highest frequency and step downwards to the lowest frequency.

Here is the ‘hole’ in this ‘get around’. Let’s say the test operator follows this advice and the EUT fails at 1GHz but did not fail at 2GHz. The operator can conclude that the EUT is susceptible to the 1GHz field and not susceptible to the unintended 2GHz harmonic field hidden in the 1GHz test field.

Yet here is how a conversation might go between a savvy customer (represented by the EUT design engineer) and the test house (represented by the test house manager).

-----------------------------------------------------------------

Design engineer: “Your preliminary test report says my design failed at 1GHz. What was the level of the applied test?”

Test house manager: “The test system applies a field strength of 200v/m at a frequency of 1GHz.”

Design engineer: “I hear you are using TWT amplifiers. These have high harmonic levels at the start of the band. How much of the 1GHz test field is in fact at 2GHz?”

Test house manager: “We have not actually measured the level of the components making up the field, but we can calculate from the amplifier harmonic power level and the antenna gain plot that the 2GHz component is about 70v/m.”

Design engineer: “So my design fails at 1GHz with a field strength of around 130 v/m?”

Test house manager: “Correct.”

Design engineer: “OK, let me revisit my design and resubmit the DUT for test. To be sure of the fix, I want the 1GHz field to be 200v/m at 1GHz, but I guess that by extrapolation, we will be stuck with 108v/m at 2GHz too.”

Test house manager: “The system can probably just about make the 200v/m at 1GHz. However we still need to see how we make an accurate calibration for this (since we can’t use the field probe), and how we justify the change in the test report required by your customer. The trouble is that for consistency, we really should do this for the first 20% of the band for all three TWT amplifiers. I guess we can say in the report that you requested extra testing. Will your customer be willing to pay for the extra testing?”

Design engineer: “Oh boy, unlikely, I guess we are stuck with the existing ‘under-test’ situation. I am not impressed with the service you are selling to me, your system has a serious problem - how much did you say you guys paid for it?”

Test house manager: “I really don’t want to be reminded of that right now.”

-----------------------------------------------------------------

I am sure you would agree this situation is far from a satisfactory for either party in the conversation. So let’s continue with our efforts to dislodge the elephant.

The Solid-State Approach

Compared to TWT amplifiers solid-state amplifiers provide the following advantages/disadvantages:

Vastly superior harmonic performance
Less prone to catastrophic failure

Upper frequency limitation of currently available broadband models
Only economically viable for newly installed systems or when the 1-2.5GHz 250W TWT amplifier in the existing suite has failed or reached the end of it’s useful life.

Solid state broadband amplifiers typically display harmonic power levels 20dBc (one one-hundredth) below the power level of the intended test frequency. This means that if the intended frequency power level is 200W, the unintended power is 2W. Perfect.

Also, if a failure does occur, it will likely involve replacing one or two transistors, not the replacement of the tube. Replacing the tube in a TWT amplifier is no small affair and often costs about the same as buying a new TWT amplifier.

However for broadband power levels of 250W, the solid-state upper frequency stops around 4GHz (more recently 6GHz), so we are still going to need TWT amplifiers to get to 18GHz.

And as ever there is the issue of price. If you already have $200k or so tied up in your existing suite of TWT amplifiers you will be reluctant to invest even more. The approximate investment price for today and 3-5 years ago (what was paid for existing TWT amplifier suite) is:

1-2.5GHz 250W New $90k (existing suite price was probably around $60k)
2.5-7.5GHz 250W New $90k (existing suite price was probably around $60k)
7.5-18GHz 250W New $100k (existing suite price was probably around $70k)

So we are looking at around $280k new, $190k existing (ignoring depreciation).

Reminder - A rule of thumb with TWT amplifiers is that once you are 20% into the band of operation, the harmonics have improved by around 10dB.

So to solve the high harmonic problem in a suite of TWT amplifiers (that is the bands of 1-2.5GHz / 2.5-7.5GHz / 7.5-18GHz) we need solid-state to cover the wanted test signal over 1-1.3GHz, 2.5-3.5GHz and 7.5-9.6GHz. The 7.5-9.6GHz won’t happen any time soon, but we can cover the first problem area with a 1-2.5GHz 250W amplifier, and the first and second problem area with a 1-4GHz 250W amplifier.

A quick search on-line shows the following solid-state models:

1-2.5GHz 250W $90k (solves one third of the issue)
1-4GHz 250W $110k (solves two thirds of the issue)
1-6GHz 200W $200k (still only solves two thirds of the issue, but also suitable for 1-6GHz medical testing).
Note these are fairly recent arrivals so the price is an educated guess.

The first two are so similarly priced that for the purposes of this exercise we will go with the 1-4GHz model. Models providing this power and frequency band are available in dual-band and single-band versions. These have equal merit since this application has no need of instantaneous bandwidth (the EMC software simply switches bands at the appropriate time in the test). We will discount the third amplifier as prohibitively expensive. The two figures below show the all TWT amplifier and the proposed Solid-State / TWT amplifier arrangements with today’s prices. Easy for me to say I know, but at only 7% higher in price and yet with two thirds of the harmonic issue solved, I am going with the solid-state / TWT amplifier proposal. To be continued………… Next time we will look at the filter solution for the 7.5-9.6GHz problem area. And then – spoiler alert, we will introduce what this topic was always leading to: using telecom industry linearization techniques to remove the elephant once and for all.

-Tom Mullineaux

E3 Risk Gap Analysis or Once More into the Breech!

2013-11-04T09:35:00.000-08:00

Assessing the EMC-related risk of using commercial items (CI or COTS, as they are known) has long been a pet project of mine. So, I received a question lately that went something like this (paraphrasing the main points):

I am aware the risk analysis or gap analysis is part of the process for analyzing the risk severity depending on test methods, level, frequency coverage etc.
However, this is hard especially for laymen or those that are lesser-experienced to figure out whether is a low risk, medium or even high risk before comparing to the platform or mission criticality.
How am I supposed to gauge the level of risk given some of the tests in terms of either commercially or military that I did not touch on before?

So, this person understands the crux of the commercial item EMC issue, that there is a “gap” between commercial EMC standards and military EMC requirements. And that gap represents a risk that the system will operate without interference in its electromagnetic environment. He also brings up another very important item: it takes some expertise to do such a risk analysis. I will add a conclusion that is reached after consideration of the three items, that it takes the entire system engineering team to “gauge the level of risk.” Maybe that’s the part that’s not so well understood, that it’s not all up to the EMC folks. The definition and acceptance of risk is largely out of their hands. The EMC engineers can help determine the likelihood and severity of potential EMI problems that may arise from the use of COTS items that are qualified to particular commercial EMC standards. But, that’s not an exercise for a beginner or the faint of heart! After that, it’s up to the systems engineering team to determine how big the risk is to fielding an effective system. That is, these are the guys that build the big “risk cube” of red, yellow and green squares and determine the overall risk “number,” hopefully in close coordination with the EMC engineers. And finally, it’s up to the program management team to accept the risks or approve the implementation of some kind of fix.

Bottom line: it’s a team effort! Unfortunately, a lot of time today, the EMC engineer is like the poor kid that’s picked last and stuck out in right field (bad youth baseball metaphor!). He’s out there to be kept out of the way. Likewise, there is a tendency among uneducated (unaware would be a kinder word) program managers that since a commercial item has passed some sort of commercial EMC standard that it is ready to deploy. Sadly, that’s not usually the case. And just like how that kid in right field sometimes can actually make a catch, sometimes the EMC engineer will catch a potentially dangerous EMI problem with a commercial item before a system is fielded.

-Brian Farmer

Wi-Fi Anyone?

2013-10-07T12:21:00.001-07:00

So I found another item of EMI interest to whine - I mean write - about: EMI in legacy equipment. You can find the article in question here.

In a nutshell, the FAA wants certain older cockpit displays (made by Honeywell) replaced because they believe them to be susceptible to interference from Wi-Fi systems that are going on board many aircraft today. It’s an interesting exercise for me to contrast my experience with the US DoD against the commercial flight/FAA world, and all I seem to have are questions:

1. Assuming that the equipment met the original EMI requirements, what has changed? Have the requirements changed? It certainly seems that the EM environment has changed. But this is a totally different paradigm than I’m used to with DoD procurements. The DoD specifies the technical requirements, including EMI, of systems that they purchase and once tested and accepted, the developer is done. If the device fails acceptance tests, including EMI, it’s incumbent on the developer to correct the deficiencies. How does that work in the commercial flight world? Are the requirements imposed by the FAA? Or by the airlines? Who certifies that the aircraft meets the specs and how? I’m sure there’s a process but I’m not familiar with it.

2. The potential issue was discovered two years ago in testing to certify the Wi-Fi system. So who gets the blame (i.e. gets to pay for the fix)? The source? Or the victim? This is a regular argument in DoD procurements when different program offices procure individual systems that are then integrated into a whole. Put a new radio on an aircraft, find out it doesn’t work because of EMI and who pays for the fix? The Radio PM (who may or may not have imposed and verified adequate requirements) or the aircraft PM (who presumable wants the latest and greatest capability on his plane and may have even been part of the specification process)?

3. And this is my favorite (thanks, Ken Javor!): If the original display units were qualified to DO-160D or later (as that model aircraft presumable was) level of 200 V/m, how could it be susceptible the Wi-Fi emissions, which would be more in the < 1 V/m range. There are additional interesting technical comments related to this question on the article in general in various EM related technical forums on LinkedIn; check them out.

Just so we’re clear, I certainly want the aircraft I fly on to be interference free and above all, safe! I’m not suggesting that the problem should not be corrected. Just that maybe there are options other than total replacement of systems. I bet some of the experts out there could find a fix that runs a bit less than total system replacement. But I’ve written many times in the past about correcting EMI problems after the fact, so don’t get me started! That’s not really the case here; it’s more about introducing a new transmitting system into the equation.

In the future, maybe everyone can connect to the in-flight Wi-Fi, log in to their favorite navigational app, and help the pilots fly to the correct destination….

-Brian Farmer

Zap – You’re Gone! A Few Truths About Lightning Safety

2013-10-03T07:20:00.003-07:00

The last several posts have been about EM radiation hazards (RADHAZ), but there are other electromagnetic hazards. If you’ve ever been shocked by an ESD jumping from your finger to a doorknob after walking across the carpet on a cold dry winters day just imagine that incident being magnified by multiple orders of magnitude. I live in Florida and we are now winding up the season the natives call the 90-90-90 period. That’s 90 days at 90°F, and 90 percent humidity. It rains a lot down here during that time. In fact, in June and July, from about 3:00 p.m., to 5:00 p.m., there is a greater than 20 percent chance that we are having a thunderstorm, whereas prior to 11:00 a.m. any day of the year there will be less than a 1 percent chance of rain. September and October bring the most hurricanes.

The mornings may start out beautiful, but the weather can change quickly. It can go from golf weather to a thunderstorm in 15 – 20 minutes . . . and let me tell you, when we are having a thunderstorm, we are having lightning! There are places around the world with more lightning than Florida, but we are the lightning capital of the US, with more than 100 thunderstorm days per year.

The problem is: the people enjoying the theme parks, and the anglers, the boaters, the roofers, the golfers, the gardeners, and everyone else enjoying the beautiful weather just don’t want to quit. They wait until the very last minute to give up what they are doing and seek shelter. Often that’s too late. Florida has the most lightning injuries and deaths in the US and it can sneak up on you. The storm doesn’t have to be overhead, hammering you with hail, and drowning you with rain at two or three inches per hour. Lightning can strike 25 miles away from the leading edge of the storm.

The good thing about hearing the thunder is that means you are still alive, since the lightning flash creates the thunder. Plus, the thunder produced by the strike generally provides adequate warning to prompt us to seek shelter, if we follow the NOAA 30/30 flash-bang rule. This rule is based on the fact that sound travels about 1100 feet/second and, if the time between seeing a flash of lightning and hearing the associated thunder is less than 30 seconds, you are within about a six mile strike zone and need to find shelter.

If you are outside and the flash-bang is nearly simultaneous, run for shelter while the cloud is recharging. You don’t have much time. If there’s no shelter, find a low spot or depression, keep your feet together, keep your head lowered, hunker down as low as possible, and don’t touch the ground with your hands. This position reduces the chance of being struck and the severity of lightning injury if you are. With luck, you may live through being struck, but if at all possible do plan for and find shelter fast. Don't lie down on the ground! Lightning ground current spreading away from the strike location can kill you. Once in a sheltered location, remain there for at least 30 minutes after the last sound of thunder, just to be on the safe side.

Your car—or anyone else’s for that matter—makes a reasonably good lightning shelter provided the car body is metal and the windows are closed. Just be sure you aren’t in contact with any metal surfaces. The metal envelope of the car acts as a leaky RF shielded enclosure and also provides a lightning path around you to ground. There is a mistaken belief that the rubber tires protect the occupants. Not so! Lightning just jumped thousands of feet from the cloud to the car. A few inches of rubber won’t even slow it down.

If you are in Florida, or anywhere there is lightning, here’s a brief reminder list of safety recommendations.

If you are outside:

•    Seek immediate shelter
•    Get out of—or off of—the water. This also includes indoor pools.
•    Get off the beach
•    Don’t stand near isolated tall structures such as light poles or trees
•    Keep away from metal objects such as bicycles, golf carts, umbrellas, fences, etc.
•    Don’t use open pavilions or huts for shelter. Get in a car, or go inside a substantial structure. Open pavilions or other small open shelters work OK for rain but not for lightning.

If you are inside:

•    Stay away from doors, windows, and fireplaces. Most fireplaces have chimneys that extend above the roof.
•    Shut down and unplug your computer and other sensitive electronic devices well ahead of the storm. Don’t touch the electric cords during the storm. Consider installing transient suppressors even though they may not always protect the devices.
•    Stay away from bathroom/kitchen sinks and fixtures. These things often have roof vents and metal plumbing.
•    If you need to make a phone call, use your cell or wireless phone, not a land line.

Although lightning often strikes the same place multiple times, especially if it’s tall/high, I want to end this post with the following lightning quote by Willie Tyler. He indicates that lightning makes major changes in the things it strikes and we don’t want it to be us.

The reason lightning doesn't strike twice in the same place is that the same place isn't there the second time. ~Willie Tyler, comedian

-Ron Brewer

Elephant in the Test Room #1 Continued – The ’Get Around’ Explained

2013-09-24T06:56:00.001-07:00

Read other posts in the "Elephant in the Test Room" series here.

Recap on this Thread

The room: MIL-STD RF immunity testing – 1-18GHz 200v/m

The elephant: everyone in the room is aware that a significant fraction of the ‘calibrated’ 200v/m test field is actually created at the wrong test frequency.

The culprit: Notoriously high ‘start of band’ harmonics produced by all octave band TWT amplifiers

The consequence: Equipments purportedly demonstrated to be resilient to 200v/m have in many instances been exposed to only around half this field strength, the rest of the test field created by an unintended frequency.

Hello again readers. We are still looking at using a mix of filters and band-overlap as a way of fixing the compromised test field, but before continuing, let me keep my promise to expand on the test house ‘get around’ alluded to by an EMC Zone reader.

“Hi Tom, This is why some amp manufacturers are looking towards solid state, with the decreasing cost of solid state devices it's getting such that there is very little difference between the two technologies. There is actually a way around this problem. We have advised that it is always possible to conduct your sweep from HF down to LF, TWT's have very little radiation outside their operating bandwidth so the potential for misleading harmonic interference goes away.

You do always have the potential problem in 2 signal mixing products within the EUT but that's a different story!”

Here is the situation faced by the test house that the suggestion addresses:

When subjecting an EUT to two test fields at once (the elephant), if the EUT malfunctions, did it do so because is it susceptible to the first field or the second?

For ease of explanation, let’s assume a 1-2GHz TWT amplifier providing an intentional test signal at 1GHz. This is at the start of the band so there will also be a substantial signal at 2GHz as shown in Figure 1. The marked 2GHz signal creates the unwanted second field.

The ‘get around’ suggests that if an EUT failure takes place, the test engineer should apply an intentional test signal at 2GHz and see if the EUT still fails. The thinking here is that harmonics created by wanted signals at the top of the TWT amplifier band are relatively small as shown in Figure 2. If the EUT still fails then it is susceptible to 2GHz at that field strength.

In fact, as the reader suggests, the test could be run backwards from the very beginning, that is start at the uppermost frequency in the relevant test band, and step down to the lowest.

Can anyone see the ‘hole’ in this solution?
To be continued …………………..
Right – Lets get back to exploring the use of filters and band overlap combined:

Figure 3 is a conceptual block diagram showing how the original three amplifiers (1-2.5GHz, 2.5-7.5GHz, 7.5-18GHz) and the introduced 2-4GHz amplifier are switched to the antenna. The switches are 50 ohm with coaxial connectors. With all switches normal (un-operated) the 1-2.5GHz amplifier is connected through. Any amplifier can be switched to the antenna under the control of the EMC software.

The introduction of the 2-4GHz amplifier creates a band overlap such that the harmonic issue at the start of the 2.5-7.5GHz amplifier band is alleviated.
That is the software uses the 2-4GHz over 2.5 – 4.0GHz and then connects the 7.5-18GHz amplifier to finish the test over 4.0-7.5GHz.

Figure 4 shows the four amplifier arrangement with low pass filters added to alleviate the remaining harmonic issues. That is at the start of the 1-2.5GHz amplifier band and at the start of the 7.5-18GHz amplifier band. The software switches in low pass filter LPF1 at the start of the 1-2.5GHz amplifier band and LPF2 at the start of the 7.5-18GHz amplifier band. The low pass filters are by-passed where the harmonic performance is acceptable.

Some of you may be wondering why I did not simply use a low pass filter for the 2.5-7.5GHz amplifier. Answer – I wanted to demonstrate the use of both approaches.

To be continued ………

Next time we will look at the losses associated with the filters and determine whether the system can still create a lean 200v/m with a healthy safety margin.

-Tom Mullineaux

Which Came First? The Chicken or the Egg?

2013-09-23T08:33:00.000-07:00

I came across an interesting article in ITEM this past week about a Canadian helicopter that apparently has EMI problems. Like most such articles that I come across, I dutifully posted it in a variety of EMC related groups on LinkedIn. I do that for a couple of reasons. First, I believe its always a good idea to circulate relevant information to the EMC community. How can we not benefit as a community by sharing information of EMI problems, their causes, and (hopefully) their solutions. The second reason is that I greatly enjoy reading the comments of bona fide experts in the field. I think I learn more from some of the back and forth from them than I ever did actually working in the EMC field!

We certainly don’t get all the technical information from the article that we’d like as EMC engineers, but it appears that the big question is: was the aircraft EME properly considered in the development of the EMI requirements? As properly pointed out in comments on the article, it makes all the difference in the world as to who’s responsible for fixing this EMI problem. If the system was underspecified, that is, the EMI requirements were insufficient to preclude interference problems in the operational EM environment, and the contractor met those requirements, then the contractor has fulfilled his obligation and the customer would be on the hook to fund fixes for the EMI problems. If the system was properly specified but failed the required tests, then the contractor would be required to fix the problems (most likely, depending on the specifics of the contract). And someone else rightly pointed out that its ALWAYS cheaper to find and fix problems in the design stage than after a system is in production or even fielded and in use.

So which is it? Well, we may never know all the details we’d like to, but the article says that the Canadian government refused to accept the aircraft as being non-compliant. That would lead us to believe that systems failed specified tests. I agree with comments that I read regarding Sikorsky’s experience with military aircraft and EMI requirements, going back to the US Army Blackhawk issues. It would surprise me that Sikorsky would 1) be unaware of what would be “good” military EMI requirements based on a military EM operating environment and 2) try to somehow pass non-compliant systems off for acceptance. I hope we’ll get the full story details in the not too distant future. And all you EMC experts out there (you know who you are), please keep commenting because I’m going to keep posting. You’ll make a decent EMC engineer out of me yet!

- Brian Farmer

Elephant in the Test Room #1 Continued – The List of Practical Solutions

2013-08-26T06:27:00.000-07:00

Read other posts in the "Elephant in the Test Room" series here.

Recap on this thread

The room: - MIL-STD RF immunity testing – 1-18GHz 200v/m

The elephant: - everyone in the room is aware that a significant fraction of the ‘calibrated’ 200v/m test field is actually created at the wrong test frequency.

The culprit: - Notoriously high ‘start of band’ harmonics produced by all octave band TWT amplifiers

The consequence: - Equipments purportedly demonstrated to be resilient to 200v/m have in many instances been exposed to only around half this field strength, the rest of the test field created by an unintended frequency.

Hello again. Now we have returned from our fantasy trip we need to look at realistic methods of removing the elephant. That is with a practical budget and with a practical time for implementation. Previously we came up with a list of practical means of dislodging the elephant. Let’s look at each in turn.

a) The Installation of Low Pass Filters

A low pass filter will allow the wanted test signal through (e.g. a 1GHz signal from a 1-2GHz TWTA) and reduce the level of the unwanted harmonic (e.g. at 2GHz). The harmonic is a high power one (at around half the power of the wanted signal) so the filter must be absorptive, that is it must dissipate the harmonic power). A reflective type filter will bounce the harmonic power back at the amplifier, not a good thing as TWT amplifiers are prone to damage from high reflected power.

The low pass filter is switched in only when the test frequency is at the start section of the TWT amplifier’s band. Once the test frequency gets to the point where amplifier produces more reasonable harmonic levels the filter is switched out (bypassed).

A back of the envelope calculation using sensible assumptions says the achievable test field strength is around 275v/m:

That is:

Assumptions:
Amplifier power P at start of band is 250W
Antenna gain G at 1GHz is 10dB

The test distance d is 1m

Test field E = sqrt (30.P.G) / d

E = sqrt (30. 250.10) / 1

E ≈ 275 v/m

We need 200v/m so we do not have a lot of margin to play with. The back of the envelope estimate does not include current system losses never mind the additional losses due the filter / associated switches / cabling. It looks like we will need to source a very high gain 1-2GHz antenna or alternatively use a higher power amplifier if we are to salvage the filter solution.

To be continued…..

Please note, this is all straight from the top of my head so if the thinking contains fundamental errors, now would be a good time to point them out so we don’t get too far down the road before realizing the mistake.
Oh, and if anyone wants to know how the ‘magic factor’ 30 gets into the formula, just let me know and I will explain how it arises.

b) The Use of Band Overlap

This is where the low-harmonic tail-end of one amplifier band overlaps the start-of-band of the next amplifier in the suite. So for instance if part of the suite comprised a 1-2.5GHz TWT amplifier followed by a 2-4GHz TWT amplifier, the test would use the 1-2.5GHz amplifier to 2.5GHz and then switch in the 2-4GHz amplifier over 2.5-4.0GHz.

The approach taken here is to start with what we would like to see using ‘virtual’ amplifiers, followed by an attempt to match our wish list with amplifiers that actually exist in the marketplace.

What we would like to see (including the sub 1GHz solid-state amplifier used in 200-1000MHz testing), is the following overlapping suite:

200-1400MHz 1kW / 1-2.5GHz 250W / 2.0-4GHz 250W / 2.5-7.5GHz 250W / 7.5-18GHz 250W

Note we will be forced to resort to the using a low-pass filter for the start of the final amplifier band, however this is feasible as the antenna gain at 8GHz is superb creating a healthy power margin to overcome additional system losses.

Also, a sub-1GHz power amplifier going to 1400MHz is simply not available (ALL available amplifiers stop at 1GHz), so we must use a low-pass-filter here too, but with a 400W amplifier to cover the additional losses introduced by the filter arrangement.

So the practical ‘overlapping’ suite becomes 200-1000MHz 1kW / 1-2.5GHz 400W (filtered) / 2.0-4GHz 250W / 2.5-7.5GHz 250W / 7.5-18GHz 250W (filtered)

To be continued – comments welcome

-Tom Mullineaux

That’s a Wrap! The 2013 IEEE EMC Symposium

2013-08-14T07:22:00.001-07:00

BLUF—that’s "Bottom Line Up Front" for those that don’t speak acronym-ese—the 2013 IEEE International Symposium on EMC was another smashing success! Denver is a great town for a convention, the facilities were excellent and as always, the Symposium committee pulled it all together and put on an outstanding show. If there were any hitches or glitches, it sure didn’t show. I’ve been involved in putting on a lot of smaller events and meetings, and I can’t imagine trying to put on a symposium of several thousand, with all the sessions, papers, committee meetings, social outings, meals (the food was terrific!), etc. But, once again, they pulled it off and it was fantastic.

I’ve written in the past about the need for participation in technical societies like the IEEE (in general) and the EMC Society (in particular), and I took what I consider to be a couple of large steps toward putting up or shutting up (as the saying goes). I’ve been participating in Technical Committee 1 (EMC Management…seems fitting somehow, seeing who I work for) but I can’t claim to have contributed much. This time, I volunteered to be the committee secretary for the next two years. May not seem like much, and its not the Chairmanship, but it will force me to really participate rather than just being another guy in the room or listening on the phone conference. We’ll see where this goes, but I really do hope that it’s a stepping stone to more participation in the society committees to actually help the society as a whole move forward.

The other step was to join TC6, Spectrum Engineering (previously Spectrum Management), which is a relatively new technical committee, but one that is working in an area that is of greater and greater interest to the community as a whole. EMI Control and management of spectral use go—hand—in—glove these days when literally everything we do depends on wireless communications in some form or another. So it’s a natural extension of EMC to consider spectrum engineering and management considerations (and vice versa for that matter). While I didn’t take any committee positions (they weren’t looking to fill any), I did participate heavily and took several actions. May not seem like much, but it’s a start and I have high hopes of making significant contributions to this group.

The only thing missing from the conference was government participation from the EMC and/or spectrum communities that exist in so many agencies, and the DoD (with which I’m most familiar) in particular. Seems sequestration and tight travel budgets, not to mention that “conference” is now a dirty word to the DoD, have reduced government participation to nearly zero in all technical conferences and symposia. And that’s a shame, because government requirements drive a great deal of what the EMC community does, so their technical experts should be there to contribute. I look forward to their return to their rightful place(s) in the IEEE EMC Symposia in the future.

So, the word for this day is: participate. If I can do it, so can you. The next might be to participate in your local EMC Society chapter. Maybe if YOU will, so will I!

-Brian Farmer

Elephant in the Test Room #1 Continued – The "Money No Object" Approach to Fixing the Under-Test Situation

2013-08-14T07:18:00.002-07:00

Read other posts in the "Elephant in the Test Room" series here.

Recap on this thread

The room: MIL-STD RF immunity testing – 1-18GHz 200v/m

The elephant: Everyone in the room is aware that a significant fraction of the ‘calibrated’ 200v/m test field is actually created at the wrong test frequency.

The culprit: Notoriously high ‘start of band’ harmonics produced by all octave band TWT amplifiers

The consequence: Equipments purportedly demonstrated to be resilient to 200v/m have in many instances been exposed to only around half this field strength, the rest of the test field created by an unintended frequency.

Things are getting a little messy so to maintain some semblance of order I will tackle the open topics as follows:

1. A restatement of my general stance regarding this particular elephant

2. The money no object approach

3. A reminder on the list of practical solutions

4. Replies to the latest readers’ comments

In the next post I will:

5. Discuss the ins and outs of each practical solution and

6. Keep my promise to provide a description of the ‘get around’ test houses are currently forced to adopt.

So here we go.

1. Restatement of my general stance

To ensure this is not lost in the course of the discussion, let me repeat that:

From my standpoint MIL-STD testing works as evidenced for instance by no planes dropping out of sky. However, in my view, this sector of the EMC industry should start to address this under-test situation

2. The "money and time no object" approach

This is where we have fun by letting our imaginations run wild.

We have an infinite amount of money and all the time in the world so we simply:

• Re-commission one of the space shuttles with a payload of 200 v/m remote controlled test equipment and the EUT (equipment under test). We won’t need the test chamber where we are going.

• Head for the quiet zone halfway to Alpha Centauri. At 4.37 light years away, Alpha Centauri is the closest system to us.

• Come to a halt halfway (Einstein would argue ‘at a halt’ relative to which coordinate system?), but we will go with our understanding of ‘come to a halt.’

• Unload the equipment to be tested along with a self powered radiator providing a perfect 200v/m over 1-18GHz at a distance of 1000m.

• By some miracle of science (we have all the time we need) arrange for the volume immediately surrounding the EUT to be filled with air at 25C.

• Employ the straightforward substitution method (as opposed to the questionable net-power method used in some automotive tests).

• Take the space shuttle a distance of one light year back towards Earth so that it does not disturb the test and so we are closer to home.

• Conduct the tests remotely

• Return to find everyone that waved us off died thousands of generations ago and no one cares about us, our tests, or the test results anymore.

Note we went to all this time and trouble to ensure no nearby bodies disturb the test field. We declined to use the ground plane table as we suspect this is there to try and give a measure of repeatability to a test that has notoriously high uncertainties. Talking of uncertainties we can now state categorically that if the EUT passed the test then it is immune to illumination by a field strength of 200v/m over 1-18GHz. Here on Earth imperfections in the chamber, the proximity of the antenna, and the presence of the ground planes (table and floor) give grounds (pun intended) for doubt in such a claim.

3. List of practical solutions

Getting back to Earth, repeated here is the list of solutions we proposed earlier (some of which readers guessed before the list was published)

• Switch in low-pass filters at the output of the amplifier

• Use frequency band overlap (tail-end of one amplifier overlaps start-of-band of the next amplifier in the suite)

• Replace TWT amplifiers with solid-state amplifiers

• A mix of solid state amplifiers and band overlap

These will be tackled one by one in the next post.

-Tom Mullineaux

4. Check out Tom's responses to comments on his last post, Elephants in the Test Room #1 Continued - How Other EMC Sectors Dealt with the Elephant

Another IEEE International Symposium on EMC

2013-08-02T09:37:00.000-07:00

By the time you read this, you SHOULD be at the annual IEEE EMC Society’s Symposium in Denver, CO. At least I hope you are! I know that the fiscal environment is tight these days, and if you happen to work for the US DoD and/or are a contractor supporting the US DoD, funding travel is almost impossible. I suspect that numbers will be down, but hopefully not down too much. We’ll have to wait and see what the organizers say. I’m actually going and paying my own way, which I’ve done for the last several years (full disclosure – I’m a small business owner and it’s both an allowable government overhead expense and a tax deductible item). Regardless, if I didn’t go, theoretically I’d have more profit and so, in a very real sense, its still coming out of my pocket. But I consider it worthwhile and here’s why:

1. I get to catch up, in person, with many professionals in the EMC business that I know I won’t see or hear from for the whole year (most likely). Some of these people are friends and some are only acquaintances, but they are all bright, have forgotten more about EMC than I’ll ever know, and fun to talk to and hang around with. And I always meet new people….tagged along at a sponsored dinner in Pittsburgh and had a blast! Who might YOU meet in Denver?

2. I’ll probably learn something! At least I hope I will….most of the technical papers are a little (OK, WAY) over my head, but I’ve been in the business long enough to understand some of the basic principles. I pretty much cherry pick the sessions I go to, but there’s always plenty to keep my attention. And its not unusual to have some technical presentation click with an idea you’ve had or with something you’re working on. What might YOU learn in Denver?

3. I’m trying to participate more in society activities. This might be the most important reason; after all, what good is a technical community unless the community members participate in the activities of the community, attempt to further the objectives of the community and in general help to keep the community moving forward. I have mad respect for the Society leadership and the leaders of the various technical committees. They have chosen to ignore the “I don’t have the time” excuse and simply chosen to do something to help move the community forward. I like to think I’m on the next level down – I want to help implement their plans. I can’t claim to have been much help so far, but I am going, and I am participating in some of the committee meetings. I’m even giving a presentation on the status of some of the DoD’s E3 and Spectrum related instructions. It’s a start….what are YOU doing to help in Denver?

4. Denver happens to be one of my favorite cities….Pittsburgh was great last year and I’m very much looking forward to Raleigh! I hope to see you there too…

-Brian Farmer

Do We Only Believe Things that Match Our Opinion?

2013-07-31T06:09:00.000-07:00

Whoopee! I finally got some feedback that people actually read some of the stuff I write. I received several great comments about a previous post titled: Electromagnetic Fields Don’t Exist. I’m not sure they liked what I had to say, but most were nice about it. I apparently excited some nerves. That happens when they’re exposed to electromagnetic radiation.

I’m using this post to respond to those comments and I think everyone will learn from it. First, let me start out by saying that this is a blog, not an IEEE-peer reviewed technical debate, so I normally limit the number of words in a post. Otherwise, I end up with something like this one (1200 words). The post about RF causing asthma in kids was intended to get reader attention, but it was not intended to be alarmist. I wanted to flag the asthma issue and get people thinking about it. I can’t help it if that’s not the way they think or what they want to believe.

Everybody’s different. Just because I choose to err on the side of caution and exercise RF prudent avoidance doesn’t mean that the rest of the people have to. People don’t need to worry about me attempting to get some law passed that prevents them from carrying and using their hand-held transceiver or cellphone, or prevents them from smoking, or from drinking the large cups of sugar-laden soft drinks. I believe in personal responsibility ... and as far as I’m concerned, everyone is on their own.

I tend to apply Socrates Test of Three to the things that I write, i.e. Truth, Goodness, and Usefulness. I don’t know why I was accused of being an alarmist or a fear monger. The commenters could have simply said he’s got to be kidding, do a search, and find this stuff is true all on their own . . . and they should have! The research is out there. Just keep in mind that sometimes opinions and theories get presented as fact. Even though I do my best not to lead my readers astray, it is still a good practice to verify statements that seem contrary to what has been learned. It could be that things have changed since the learning process occurred.

If I wanted to be an alarmist, I could have cited a number of alarming studies such as "Long-Term Exposure to Microwave Radiation Provokes Cancer Growth: Evidences from Radars and Mobile Communication Systems." A lot of scary new RADHAZ information is now coming out from the National Institutes of Health.

The asthma study was published in the Archives of Pediatric and Adolescent Medicine. Here’s a link to the study, along with two other articles about it.

Maternal Exposure to Magnetic Fields During Pregnancy in Relation to the Risk of Asthma in Offspring (Archives of Pediatric & Adolescent Medicine)

Can Exposure to Electromagnetic Fields Cause Asthma? (All Children's Hospital, St. Petersburg, Fla.)

Electromagnetic Fields Linked to Asthma in Kids (WebMD)

The fact that some cells can have an athermal response to electrical current has been known since about 1771, when Galvani discovered that a frog’s legs would jerk under the influence of static electricity. It wasn’t until about the end of WWII that the RF community started researching the effects of electromagnetic energy and since then, there have been lots of significant information that associates electromagnetic energy with health problems, many of which are caused by very low level RF sources. Recently, i.e. within the last ten years or so, there have been a large number of papers that indicate that EM energy can damage cells in a manner similar to ionizing radiation, but at levels well below the present government safety standard. See the post titled: I Can’t Believe I Just Cooked My Pinna.

In spite of the big gaps in our knowledge base, we now know that some cells such as neurons and muscles are more sensitive than some of the others; we know that cell communication is to-some-extent non-linear, which makes them able to detect various forms of modulation; and we also know that some cells experience retarded growth and some, more rapid growth (especially plant cells), when exposed to electromagnetic energy. The effects such as altering circadian rhythms, hearing microwave pulses, seeing light flashes from optic nerve stimulation, cataracts, and sterility may or may not be athermal, but they are a function of the amplitude, waveform, modulation and frequency, so the effects are more than just energy density.

I have personally witnessed migrating flocks of geese being scattered during flight when the AN/FSS-7 microwave radar system I was evaluating illuminated a flock during scanning. After the beam passed, the geese would regroup, only to scatter again when the antenna returned to their bearing position. I believe that their time of exposure was so brief that it was not a thermal effect.

Here are several other observations about electromagnetic energy that pertain to the comments. A lot can be learned by researching these.

I use one heaping teaspoon of Coffee-Mate to achieve the creamy color and taste I like in my coffee, and if I reheat it in the pot, the color, taste and amount of Coffee-Mate doesn’t change. But, if I reheat it using the microwave, the taste is different, it takes more Coffee-Mate to flavor it, and I am not able to achieve the same color. This is an easy enough experiment. Try it at home.

Light from the sun is electromagnetic energy. In a manner similar to the relationship between the electric and magnetic fields from a dipole, there is an inverse relationship between wavelength and photon energy. UV light which is primarily responsible for sunburn is a high energy electromagnetic wave with some really interesting particle-like properties based on its extremely short wavelength. The UV wavelengths are roughly the same as the dimensions of atoms and molecules.

Instead of experiencing whole body displacement current, which happens at the lower frequencies where the body acts as an antenna, the UV interaction with the skin is more like being immersed in a steady stream of energy particles. Each molecule acts like an independent antenna and so the coupling effect is entirely different than if the whole body acts like a single dipole. The distinction is somewhat analogous to the difference between AC and DC.

Readers that are into signal analysis will appreciate the fact that biologists at Tufts University have discovered that potential cancer cells have a unique bioelectric signature, and they have shown that by changing the bioelectric code and hyperpolarizing tumor cells they can suppress abnormal cell growth. Since Murphy is the evil one in the EMC community, I would suspect that an inverse could happen at any time and cause abnormal cell growth.

Regarding blood warming problems associated with microwaves see: Rapid In-Line Blood Warming Using Microwave Energy: Preliminary Studies from the Department of Surgery, New England Medical Center, Boston, MA.

Also, check out this Penn State University Department of Electrical Engineering paper discussing microwave DNA covalent bond breakage without invoking the energy levels of ionizing radiation on exhibit.

Ah, those electromagnetic fields ... we can’t live with them and we can’t live without them. But, I can guarantee that most of us would not live 120 years without them because we need the energy from the sun to warm the planet, create greenhouse gases and support the growth of plant life.

Thanks for the comments! Don’t get caught up in wishful thinking!

-Ron Brewer

I Can't Believe I Just Cooked My Pinna

2013-07-30T09:30:00.001-07:00

In the previous post, I was discussing cellphone RF safety and the fact that we really don’t know what level is safe and what’s not.

Those individuals that go around with a cell phone or Bluetooth device glued to the side of their head will be happy to know that the FDA and the FCC are concerned about cell phones cooking their outer ear (the pinna). They have apparently noticed after all these years that ears stick out from the head kind of like an appendage and they have changed the SAR measurement procedure slightly to account for this. They didn’t, however, change the requirement levels.

Fortunately, the RF levels produced by the current 100 mW families of cell phones are not nearly as much of an issue as the 3 Watt bag phones of yesteryear. The new cell phones are also a lot more convenient to carry and the really fancy ones have a calendar, will calculate your take home pay, and play music. However just because the power is 15 dB lower doesn’t necessarily make them safe.

Dr. Henry Lai, a bioengineering professor at the University of Washington has done considerable research on the effects of low level microwave energy on brain cells. He and his colleagues reported that brain cells were clearly damaged by low level MW energy at levels significantly below those considered safe by U.S. government standards. You might want to do an Internet search about the impact of Dr. Lai’s test results. The information is very enlightening!

After watching what happens to food placed in a microwave oven, no one would deliberately stick their head in one while it’s operating. Yet almost everyone puts their cell phone next to their head, presses it tightly against their ear, and thinks nothing about it. My feeling is that since we really don’t know if the cell phone RF levels closely coupled to the brain are safe, we should at least practice prudent avoidance. There is no reason to take unnecessary risks. It’s possible to reduce our RF exposure without making drastic changes to our overall lifestyle, so long as we are aware of the need to do so.

My kids seem to have it right, even though their texting annoys me. That’s because I can talk a lot faster than I can text, but sending text messages, using the speaker phone mode, or wearing a wired headset does reduce RF exposure. Try not to use the cell phone in the car. Not only is it a driving distraction, but there is the possibility of internal RF reflections and cavity resonance which increases the field strength levels in the car. Pregnant women or women with small children should not carry the phone near the child. Maybe children should not have cell phones or if they do, have limited use times. Also, please don’t sleep with it. A cell phone does not make a good bedfellow. It is always generating low-level signals, and keeping your cell phone on the nightstand or under your pillow results in continuous RF exposure.

If you are really paranoid about cell phone radiation, users have found that carrying the phone in a small metal box or wrapping it thoroughly with several layers of aluminum foil will solve the RADHAZ problem. It also takes care of any annoying calls that you may not want. So will turning it OFF – and that doesn’t run down the batteries. More about shielding later.

I’ll be at the 2013 IEEE EMC Symposium. Stop by and say hello.

-Ron Brewer

Electromagnetic Fields Don’t Exist!

2013-07-23T07:58:00.000-07:00

Some time ago, I made a big to-do about smart electric meters and the fact that they might be hazardous to our health. The general response I got was: you got to be kidding. Sometimes from the more heavily invested the response is: there is insufficient evidence to make that conclusion. The World Health Organization concluded that electromagnetic fields from cell phones and wireless devices could possibly cause cancer. The general response was: yeah right, you got to be kidding. Researchers found that babies born to women who are exposed to strong electromagnetic fields during pregnancy had three times the risk of developing asthma compared those whose mothers were not exposed to strong electromagnetic fields. I haven’t heard yet but I suspect that the general response will be: you got to be kidding.

It’s not a joking matter. When 33 percent of children born to women with the highest electromagnetic field exposure develop asthma this is serious. It’s even serious for those with lower levels of exposure where the percentage was only 13 percent. Only 13 percent? I admit it’s difficult to understand how that could happen because electromagnetic fields are very illusive. You can’t hear them, see them, smell them, or taste them. You certainly can’t touch them. Based on the number of people denying their effects, they obviously don’t exist. They must be like the ghosts that come out around Halloween. Yet late at night when all the lights are off I am amazed at how many little red, blue, and green lights that are ON in my house. Some blinking – some not – but all of them trying to tell me something and the ones associated with cell phones and wi-fi stuff are telling me that I’m being irradiated.

What if electromagnetic fields really do exist and they cause all of these health problems? We will be in big doo-doo if that’s the case. Electrical/electronic devices are such a necessary part of our lives that shutting even some of these OFF will be like going back to the Stone Age. Never the less we know that low-level electromagnetic fields effect cell functions and we know that high level electromagnetic fields can warm up your coffee, cook your hot dog, and do-in gremlins if they happen to take refuge in your microwave. We also know that they can cause cataracts. Dr. Ken Atkinson even has a patent on a whole body microwave to destroy cancer cells.

Even with all that we know ... we still don’t know what levels are safe, or even if there is a level that is safe. We don’t know if some individuals are more sensitive to RF than others and we don’t know if the type and frequency of modulation has any affect. We only seem to know or at least acknowledge the thermal effects of RF energy. But the thermal effects of RF energy are different than simply applying heat by conduction or convection. The material properties are changed by radiation. For example, microwaved coffee doesn’t taste the same and blood warmed by a microwave is destroyed and cannot be used for patient transfusions.

For a short article with some very interesting reading about cell phone and RF safety check out this wikipedia write-up: http://en.wikipedia.org/wiki/Mobile_phone_radiation_and_health ... and may the RF not be with you!

-Ron Brewer

OMB Circular A-11: Preparation, Submission and Execution of the Budget

2013-07-19T10:44:00.000-07:00

What’s that you say? What does OMB Circular A-11 have to do with anything EMC related? That’s a pretty good question, and one that I would have asked myself. Until I read (because it was pointed out to me) section 31-12 of the latest edition of the Circular. It starts off:

“The value of radio spectrum required for telecommunications, radars, and related systems should be considered, to the extent practical, in economic analyses of alternative systems/solutions.”

And it continues with some “guidance” on how to estimate the value of spectrum. But this rant isn’t really about RF spectrum valuations, its more about how we expect DoD program offices who already struggle with technical areas like EMC and spectrum supportability as well as developing and meeting budgets, especially for major, multibillion dollar programs. Now we are asking program management offices to throw a wildcard into the budgeting process that is a kind of black magic combination of technical expertise in RF spectrum use and fortune telling. Considering that we auction off spectrum these days, I’m not sure that there’s any good way to put a value on spectrum much less factor spectrum value into military program procurements. But I’ve been wrong before…

Reading a little more about the guidance provided, it turns out to be a little thin (but in fairness, you really can’t expect a lot of detailed technical guidance in a budget policy document):

“One method for determining spectrum efficiency when assessing procurement of Federal systems is to develop a baseline that measures: 1) the technical characteristics of the frequency used by the system, 2) the population of an area where spectrum is utilized and 3) the amount of bandwidth utilized.”

I suppose #2 and #3 are fairly straight forward, although the population of the area with US DoD system are used is in the billions, worldwide! Its item #1 that is going to require a lot of clarification to be a meaningful measure of any sort. Just what are “the technical characteristics of the frequency used by the system” and how do you measure it? I don’t know either! There a variety of directions one could take with that statement but there’s no guarantee it would provide the information that management is looking for.

This reminds me a little of the implementation of the Spectrum Supportability Risk Assessment requirement. There was very little specific guidance provided and many of the initial submissions just didn’t provide what was required to actually assess the desired risks. We really shouldn’t have been surprised. I think we’re in for the same type of response in the implementation of this requirement. That the folks on the working end won’t really know what’s expected or how to actually produce the required data. But they’ll do the best they can to check on the latest item on the list while they work to get systems out to the users that need them.

-Brian Farmer

Elephant in the Test Room #1 Continued – How Other EMC Sectors Dealt with the Elephant

2013-07-02T11:21:00.001-07:00

Read other posts in the "Elephant in the Test Room" series here.

Recap on this thread

The room: - MIL-STD RF immunity testing – 1-18GHz 200v/m

The elephant: - everyone in the room is aware that a significant fraction of the ‘calibrated’ 200v/m test field is actually created at the wrong test frequency.

The culprit: - Notoriously high ‘start of band’ harmonics produced by all octave band TWT amplifiers

The consequence: - Equipments purportedly demonstrated to be resilient to 200v/m have in many instances been exposed to only around half this field strength, the rest of the test field created by an unintended frequency.

How other EMC sectors control test field fidelity

Commercial Sector

Test field integrity was an issue with commercial RF immunity testing due to amplifiers (note not TWT amplifiers) being run into hard saturation and thereby creating high harmonic power levels. As previously explained high harmonic power levels combined with antenna gain characteristics can make a significant unintended contribution to the measured test field.

The elephant has been removed from the room in that the commercial standard now restricts rogue test field components to one quarter of the total test field. This stipulation is checked at all test frequencies during system calibration.

Automotive Sector

To address what once was poor test field integrity, current automotive standards limit amplifier harmonics to 20dBc maximum (Ford and GM) or 13dBc maximum (Chrysler). 20dBc seems arbitrary and will be a topic in an upcoming ‘Automotive elephant in the test room’ thread.

Addressing MIL-STD Test-Field Integrity

So what are our options in trying to dislodge the elephant? We will take a ‘money no object’ stance later, but first let’s examine the pros and cons of the more commercially minded approaches. A non-exhaustive list is: -

• Switch in low-pass filters at the output of the amplifier
• Use frequency band overlap (tail-end of one amplifier overlaps start-of-band of the next amplifier in the suite)
• Replace TWT amplifiers with solid-state amplifiers
• A mix of solid state amplifiers and band overlap

In actual fact readers outguessed me by suggesting the first and third solutions too (see below).

----------------------------------------------

Anonymous May 31, 2013 at 12:08 AM

I beg to differ. I agree that there are elephants in the EMC world but harmonics from a TWT should be removed by band pass (or low pass)filters. As in your example a 1-2GHz amplifier should be fitted with a 2 GHz low pass filter to reduce the level of the harmonics. We 1-6 GHz TWT that has high harmonics but a 2,4,6 GHz low pass filter to connect to the same amplifier. It is just a matter of doing it right

Tom's answer: Thank you for your comment. I note you say 'should' be fitted with ... I have visited many many test houses and only saw this once. In actual fact filters appear in the list of possible solutions to this issue as you will see in upcoming posts

Anonymous May 31, 2013 at 2:49 AM

Hi Tom, This is why some amp manufacturers are looking towards solid state, with the decreasing cost of solid state devices it's getting such that there is very little difference between the two technologies. There is actually a way round this problem. We have advised that it is always possible to conduct your sweep from HF down to LF, TWT's have very little radiation outside their operating bandwidth so the potential for misleading harmonic interference goes away.
You do always have the potential problem in 2 signal mixing products within the EUT but that's a different story!

Best Regards
Tim Hague
European Technical Manager
AR Europe.
Reply

Tom's answer: - Thanks Tim, good comments. Solid state with band overlap is one of the possible solutions I will be discussing in upcoming posts. I am familiar with the 'get around' where the test is run backwards to try and establish which frequency the equipment under test failed at (the intended or the harmonic, I will expand on this for others later), but see this as a fix for an embarrassing situation (for the provider of a very expensive system)

To be continued ...

-Tom Mullineaux

Commercial EMI Standards vs. MIL-STD-461 … Now What?

2013-07-01T11:31:00.001-07:00

I seem to hear a never ending litany of complaints about how the US military acquisition community has to use more and more commercial equipment (COTS or CI) and that said equipment tends to be highly problematic from an EMC perspective. I tend to dwell on this subject in this blog space, hopefully from different angles, but at the end of the day, I don’t see much being done to help ensure that commercial items integrated into military systems are electromagnetically compatible. Hopefully I’m wrong! I’m sure there a some programs that levy good EMI requirements and avoid commercial equipment that doesn’t meet those requirements. But there are also programs that don’t impose good EMI (and other military specific) requirements, hiding behind the mantra of “costs less, gets fielded quicker.” There are many EMC engineers working in the civil service that are passionate about potential interference problems that can be passed on to front line troops, both from the lack of robust EMC requirements in general and the use of EMI-unproven commercial items specifically. So what can we do?

I’ve written before about the process of conducting a COTS EMI-related risk assessment and how a comparison of commercial EMI standards to MIL-STD-461 is a tool that is central to the ability to conduct such a risk assessment. I still believe that is a legitimate and valuable tool in the EMC engineers arsenal. Unfortunately there is no good comparison of commercial EMI standards to MIL-STD-461. An attempt to compare was made by the DoD’s Joint Spectrum Center in the late 1990’s with a document published in 2001 titled: “Results Of Detailed Comparisons Of Individual EMC Requirements And Test Procedures Delineated In Major National And International Commercial Standards With Military Standard MIL-STD-461E” or more affectionately known as Engineering Practices Study (EPS) 0178. The table shown below is the overall result of the study. For the different military platforms down the left side, an attempt was made to compare three primary categories of commercial standards (across the top) to the individual test methods of MIL-STD-461E. (that’s our first issue: we’re currently at Revision F of MIL-STD-461 and the TriService Working Group is working on Revision G as we speak!). You will notice from the legend that there is a small table corresponding to the individual MIL-STD-461E tests that is inserted into each intersection of a Platform and a Commercial Standards Group. You’d have to go into the details of the text of the document for specifics, but you will notice a great deal of red (high risk) and black (medium risk) and very little green (low risk) in the results of the comparisons. I invite you to make your own assessment of the document…its available online at acc.dau.mil/library.

So, we have a risk assessment process that needs a better tool (standards comparison) to be of any real use to EMC engineers and DoD programs. It's not a trivial exercise but I believe it's time that the DoD EMC community figured out some way to update this comparison in such a way as to enable the risk assessment process to actually work. Who’s with me?

- Brian Farmer

Growling in Hi-fi and Stereo

2013-06-26T11:46:00.002-07:00

In the last couple of posts I’ve been discussing ground loops and bonding and I made a comment that at the higher frequencies, i.e. VHF/UHF and higher multiple point grounding is used. It occurs at these higher frequencies because of capacitive coupling whether we planned it that way or not. The so-called ground loop exists whenever there are multiple ground connections between equipment. I created a firsthand example last week end when I decided to extend my sound system from the corner of the living room to the far end of the house.

It seemed simple enough and would have probably worked OK if I hadn’t put in the extra amplifier to enhance the Bass. It enhanced it all-right but it was also putting out a wonderfully distorted 60Hz tone. If you could call that growl a tone. The growl was highlighted by a cute little whine whenever the refrigerator turned on. In the good old days turning the AC plug 180 degrees often solved this problem. Now days, the plugs are polarized, so with the help of an old non polarized extension cord I reversed the amplifier plug and made it a lot worse! However I noted that running a long extension cord from the source outlet to the distant amplifier almost solved the problem.

Ground loops aren’t a problem unless there is unwanted current flowing in the loop. The current is developed by differences in grounding potential or by magnetic fields being coupled into the loop. In this case I suspected that there might be a wiring error in the house which splits the return current loop increasing its coupling area or possibly a neutral to green wire short. This is illustrated in the figure.

When doing magnetic field surveys a lot of the problems are caused by ground loops created by incorrectly wired feeds and branch circuits, but sometimes the problem is an equipment problem. For example an RF filter. It’s just like any other EMC problem. It takes a Level III Wizard (or higher) to find the culprit!

I spent most of the weekend looking for the problem and finally found it – in the laundry. When our new water softener was installed the installer who was obviously not an electrician pushed a knock-out into a metal conduit box and fed his wire through the hole. He did not twist off the knock-out plug and when the duplex outlet was reinstalled the neutral wire made contact with the knock-out plug. (Yes, it was a he.) This shorted the neutral to the safety wire. The refrigerator in the kitchen is on that same branch circuit. Murphy strikes again!

Anytime there is unbalanced circuit current we will have EMC emission problems. That why EMC engineers insist that through any wire pair through any connector there must be a net zero current flow and the outgoing and return leads should be kept as close together as possible to minimize loop areas. When/if you have a similar problem to the one I just described associated with power wiring look for:
• Interconnected neutrals from separate power circuits. This causes major return current sharing. The further the interconnect occurs from the source the more devices are involved and the larger the magnetic loop area
• Neutral-safety ground shorts. Can occur anywhere but do check subpanel neutral bus wiring for errors.
• Check three-way switch wiring. Neutral and Line may be fed to different points in the circuit increasing loop area. The further the switches are from the associated load the more likely this is to be true.
Since all of these problems create magnetic fields, it’s amazing how much troubleshooting you can do with an oscilloscope and a clamp on current probe . . . even a DIY one. See The HF Current Probe: Theory and Application by Ken Wyatt in the 2012 Interference Technology Directory and Design Guide for more info on how to do it yourself.

-Ron Brewer

Elephants in the Test Room #1 Continued – Further Expansion on the Culprit

2013-06-17T13:28:00.001-07:00

Read other posts in the "Elephant in the Test Room" series here.

Recap on this thread:

The room: - MIL-STD RF immunity testing – 1-18GHz 200v/m

The elephant: everyone in the room is aware that a significant fraction of the ‘calibrated’ 200v/m test field is actually created at the wrong test frequency.

The culprit: notoriously high ‘start of band’ harmonics produced by all octave band TWT amplifiers.

The consequence: equipments purportedly demonstrated to be resilient to 200v/m have in many instances been exposed to only around half this field strength, with the rest of the test field created by an unintended frequency.

Further expansion on the culprit:

TWT amplifiers create high levels of harmonic noise at the start of their operational band (lower band edge). These high levels subside steadily with increasing fundamental signal frequency until around 20 percent into the band, by which point the level has dropped from an unacceptably high harmonic level at half the power of the fundamental (3dB down) to an acceptable twentieth of the power (13 dB down).

The significance of this can be seen when the 1-18GHz suite of amplifiers is described.

First, a little history. In days of old, the suite was comprised of a 1-2GHz 200W amplifier, a 2-4GHz 200W amplifier, a 4-8GHz 200W amplifier and an 8-18GHz amplifier. As explained above, high harmonic content compromised the test field at the start of each amplifier band. As you can imagine, this suite represented a significant financial outlay (of the order of $240k since the amplifiers are around $60k each).

Where there is a will there is a way, and around two decades ago some bright spark came up with a way to win business on price by reducing the amplifier count to three. This required ‘detuning’ three of the existing amplifiers. The suite now comprised a 1-2.5GHz 200W amplifier, a 2.5-7.5GHz 200W amplifier and a 7.5-18GHz amplifier.

On the face of it, this was an attractive move (system buyer pays less, system provider wins the bid), but was a bad move from a test-field fidelity viewpoint, as a trade-off in stretching the band beyond the original amplifier design remit was worse harmonic performance. In short, the elephant got bigger.

In the next post, we will look at how other EMC sectors deal with test-field fidelity, and we will begin to look at possible engineering approaches to the issue of MIL-STD test-field integrity.

-Tom Mullineaux

Apples and Oranges … If Only!

2013-06-07T10:33:00.002-07:00

Military use of commercial equipment has grown exponentially over the last decade; and not just in the area of whole equipment, which is easy to see.

The use of commercial components and subassemblies within systems has grown as well, so that parts of what look like a full military development effort are actually commercial items. What’s an DoD EMC engineer to do? I wish there was better news, and we’ve been down this road before, but it bears repeating. There just isn’t a good, up to date comparison of MIL-STD-461 (US military system level EMI compliance requirements) and commercial EMI standards. The Joint Spectrum Center (part of DISA) undertook a comparison effort back in the late 1990s and in March 2001 published Engineering Practices Study 0178, “Results of Detailed Comparison of Individual EMC Requirements and Test Procedures Delineated in Major National and International Commercial Standards with Military Standard MIL-STD-461E.” THAT’S a mouthful! So there’s essentially one useful tool to help EMI engineers attempt to correlate commercial EMI test results with MIL-STD-461 requirements in the development of military systems. Unfortunately its woefully out of date now (although the information in it is still good). I invite you to check it out in the E3 library at acc.dau.mil/library.

Another tool at our disposal is an informal but effective Commercial Items EMC Risk Assessment Process, developed under the auspices of the DoD E3 IPT several years ago. Based on some initial work by the UK EMC community, the process attempts to walk the EMC engineer through a step-by-step procedure from an assessment of the system initial Military operational EMC requirements, through the actual EMI requirements imposed on the commercial item, assigning a risk category for each requirement based on comparison of the actual commercial test results to the desired military test requirements. Not an easy task under ideal circumstances and exceptionally difficult without a good comparison tool such as the EPS 0178 document discussed above. This document is available at acc.dau.mil/e3, do a search for “DoD COTS E3 Risk Assessment Process.” I invite you to check this one out also.

So what’s a good EMI engineer to do when assessing COTS for military applications? First and most importantly, set good requirements and stick to them. Obviously none of us get our way 100 percent of the time and you may be overruled. But that’s why the PMs get the big bucks – to make the trade-offs between performance, schedule and money. And any EMI engineer who’s been around for a while will understand that performance, especially a black magic art like EMI compliance, can take a back seat to money and schedule pressure. Make sure to document any EM-related requirements changes or waivers. It's very possible that it could come back to haunt the system once it gets out in the operational electromagnetic environment.

- Brian Farmer

Elephants in the Test Room

2013-05-29T13:25:00.000-07:00

Read other posts in the "Elephant in the Test Room" series here.

I apologize for the prolonged absence, my attention was drawn elsewhere for a while – back in the saddle now for the foreseeable future at a rate of at least one blog post per month.

For this new raft of posts, I decided to highlight open EMC controversies that everyone seems to know about, yet few talk about. I shall christen this new series of controversies ‘Elephants in the Test Room’.

Elephant in the Test Room #1 – Compromised MIL-STD Test Fields

Aside: Before launching into this – let me state that from my standpoint MIL-STD testing works as evidenced for instance by no planes dropping out of sky. However, in my view, this sector of the EMC industry should start to address this under-test situation

So let’s begin the scrutiny : -

The room: - MIL-STD RF immunity testing – 1-18GHz 200v/m

The elephant: - everyone in the room is aware that a significant fraction of the ‘calibrated’ 200v/m test field is actually created at the wrong test frequency

The culprit: - Notoriously high ‘start of band’ harmonics produced by all octave band TWT amplifiers

The consequence: - Equipments purportedly demonstrated to be resilient to 200v/m have in many instances been exposed to only around half this field strength, the rest of the test field created by an unintended frequency.

Let’s expand on the culprit first – The culprit is the characteristically high level of harmonic power created by TWT amplifiers at the start of the amplifier band. Look at any TWT amplifier data sheet and you will see worst-case harmonic levels are just 3dB below the intended frequency power. This means that a 1-2GHz 200W TWT amplifier creating an intended 1GHz signal at 200W, is simultaneously creating a 2GHz signal at 100W.

The 200W at 1GHz and the 100W at 2GHz are presented to the antenna. The antenna dutifully creates two fields, generally favouring the higher frequency as antenna gain tends to improve with frequency.

A field probe is used to measure the field strength one meter away from the antenna. The field probe is unable to differentiate between the two fields and simply sums the contribution of both fields. This is the 200v/m presented to the equipment under test.

To be continued…

- Tom Mullineaux

Togetherness is Another Way of Saying Well Bonded

2013-05-23T07:16:00.004-07:00

Electromagnetic compatibility (EMC) is often described in terms of RF coupling phenomena. Usually the big four: Radiated and Conducted Emission and Radiated and Conducted Susceptibility/Immunity. These four elements cover all of the EMC issues in a very broad sense. Occasionally lightning and ESD will be divorced from susceptibility and described as if these two items were something other than just a high level transient susceptibility condition. That’s very understandable if you’ve witnessed a nearby lightning strike or been zapped on a cold winters day by a seemingly harmless doorknob. Sometimes in the clandestine world of communications security even the low level emissions are carefully examined for their content. It’s all a matter of interest ... the reality is that EMC simply means that electronic widgets can live together in peace and in harmony with one another and with the electromagnetic environment, and we get to define peace, harmony, and environment. It’s all a part of togetherness.

As I mentioned in the last post on MPG systems, as wavelengths (λ) approach the dimensions of boxes, cables, and grounding connections parasitic capacitive coupling tends to interconnect circuits having changing voltage potentials to other circuits and to ground references whether that coupling is desired or not. There are two primary ways of handling parasitic capacitance depending on whether a circuit to circuit connection is permitted. If the circuits must be kept isolated then shielding is used. The shielding can be a simple grounded fence between the cir-cuits or the circuits (one, both, or all) can be completely enclosed. For a peek at how this is done with a small system take a look inside a cell phone. Enclosure can be accomplished using PCB layers, small postage stamp sized boxes, or entire multistory buildings. Shielding will be covered later in the EMC-Zone.

The parasitic capacitance can be shorted if the circuits can be interconnected or connected directly to the ground reference. The intent is to keep the circuits at the same voltage potential. Unfortunately any conductor has inductance which results in voltages being developed from any changing currents through the inductance: V = L di/dt. In addition, the combination of the parasitic capacitance with an inductance may create either a parallel or series resonant circuit (only Murphy knows for sure) with the first resonance occurring at a frequency (F) = 1/ (2 π √(L C)). At this frequency the inductive reactance cancels the capacitive reactance leaving only the resistance of the conductor. Above and below the resonant frequency either the capacitance or the inductance will dominate. Consequently as the frequency increases, long individual wires cannot be used for grounding and bonding because their higher inductance (L) lowers the frequency where the grounding/bonding system fails..

The figure shows the migration from a wire to direct bonding as the frequency increases. The symbol L is used to honor Heinrich Lenz, even though Joseph Henry actually discovered both self and mutual inductance. However, L is also used by most people (especially the mechanical types) as the symbol for length (L). This turns out to be really appropriate because inductance (L) is primarily related to conductor length (L)!

Here’s a memory hook just to illustrate the point (Click to enlarge). Imagine that the figure shows a solid-flat bonding conductor with length (L), width (W), and thickness (t) where L >> W >> t. If the thickness (t) is increased by 10x the inductance is decreased approximately 10%. If the width (W) is increased by 10x the inductance is decreased approximately 30%. If the Length (L) is decreased by 10x (to 1/10th the original length) the inductance is decreased approximately 93%. That’s ninety-three percent! So keep conductors as short as possible, make them as wide as possible, and don’t worry too much about their thickness.

Before there is true togetherness, there are some other factors to be considered such as material types, surface finishes, and methods of connection. Stay tuned to EMC-Zone.com for the next piece on togetherness.

- Ron Brewer

And Now, a Shameless Plug….

2013-05-22T06:17:00.005-07:00

The Joint Spectrum Center (part of DISA for those of you who support the US DoD) sponsors and populates the premier electromagnetic environmental effects and spectrum supportability web site on the World Wide Web.

We in the business know that compliance with Spectrum Supportability (SS) requirements and control of electromagnetic environmental effects (E3) can be critical performance parameters for platforms, systems and electronic equipment acquired by the DoD. If these technical areas are not given the proper level of attention during the acquisition process, you can be sure that fielded systems will suffer from some level of operational degradation up to, and including, catastrophic failures or unacceptable operating restrictions. As part of their charter, the Joint Spectrum Center manages the DoD E3 Program, which includes a mandate to provide outreach and training services to the DoD acquisition community so that they understand E3 and Spectrum requirements and how to implement them in their specific programs. Thus was borne the Spectrum and E3 requirements Community of Practice (previously Special Interest Area) as part of the Defense Acquisition University Acquisition Community Connection (that’s a mouthful). The site is available at acc.dau.mil/e3 or acc.dau.mil/spectrum (no www!). Let me give you a quick tour….

The figure above is from the front page of the site and the icons provide and entry into the various sections of the website. Basically it starts with the Spectrum and Electromagnetics 101 icon at the 11 o’clock position and goes around counter clockwise, providing information on basic technical concepts, policy requirements and various DoD process and procedure requirements to ensure that E3 control and spectrum supportability is fully considered in the design, development, testing and fielding of military spectrum dependent systems.

Beyond these areas of technical and procedural guidance, there are various areas that provide links and information to relevant organizations, technical articles, and website, both commercial and military, that provide users with all the contacts and additional information they need to ensure EMC in their systems.

The site has a feature where you can become a member…don’t worry, it doesn’t cost anything and takes only a minute to join. Benefits of community membership include the ability to:

Ask and/or answer questions;
Connect with members/experts with a shared specific interest;
Receive community-wide emails;
Access restricted community knowledge.

Future plans include the development of a compendium page of EM-related blogs so that you could find all your favorite blogs (like this one!) in one spot! The Joint Spectrum Center is very interested in constructive feedback from the EMC and Spectrum communities on the site content (primarily) and construction. While the site layout and functionality is dictated by the DAU, we do have some control over the look. So dive in, join up and let the JSC know what you think!

-Brian Farmer

The Ground Loop

2013-05-01T07:42:00.002-07:00

Several readers have made comments regarding multipoint grounding (MPG) systems. The concern is that this arrangement creates multiple ground loops. As a result, we need to address that. As some of you know, I make a lot of presentations and teach a lot of EMC seminars. A long, long time ago, I was discussing ground loops in one of my training classes when a student interrupted me and told me that I obviously didn’t know what a ground loop was, so I let him tell me.

As it turned out, he was a member of a Civil Air Patrol (CAP) squadron flying an antique Piper J-3. This is a little wooden and metal framed tail-dragger covered with canvas, which looks a lot like a large version of a model airplane that we—with the help of our kids— might build at the kitchen table. That said, in the world of aviation, a ground loop is an on-the-ground spin-out of the airplane, where the pilot is suddenly looking back at where he just came from. This usually only happens with conventional landing gear (not tricycle), where the center of gravity is behind the wings. I suspect that this kind of ground loop is very scary . . . especially if going fast.

We all had a good laugh at the airplane definition of ground loop, but ever since then I have tried to use the more technical term: common impedance coupling. However, that term is hard to visualize. Regardless, the MPG creates a lot of ground loops and they are just as scary to the EMC engineer as they are to the airplane pilot.

Figure 1 illustrates the issue of loop creation within a MPG system. This figure shows five cases of grounding connections, i.e. none (0), 1, 2, 3 and 4.

None (0) and 1 (which is the single point ground) do not create a loop. Two ground connections (one loop) might be all right if there are no ground currents and no radiated electric or magnetic fields. Yeah right! With three or more intentional or unintentional ground connections, there are progressively more loops than ground connections, and the loop complexity gets worse and worse. This is a graph theory problem, with the number of loops (L) related to the number of grounding (G) connections as follows:

If G = n, then L = n (n-1) / 2.

So, four interconnected cabinets might have 6 loops, but a PCB with 20 devices might have 190 loops. A similar situation occurs when the number of interconnecting cables is considered, but that is another problem for another day.

Unfortunately, as frequencies increase to where wavelengths approach the dimensions of boxes, cables and grounding connections, single point ground systems fail and capacitive coupling creates MPGs whether they are wanted or not. Since many of the high frequency ground connections (if we want to call them that) are the result of capacitive coupling, they are not visually identifiable. We can’t see where they are or determine what loops were formed. Consequently, the grounding design for high speed systems must be adapted to work at these higher frequencies. Individual grounding wires cannot be used because their resistance and inductance is too high. Instead, ground planes are used. This may be a dedicated PCB layer, the chassis or a grid underlying a complete installation. It all depends on the frequencies of the device.

Although there are many loops created by MPGs, that doesn’t mean that they should be avoided. MPG ground systems have connections that are shorter and more direct. In addition, they are easier to build, their loop areas are smaller and they can operate to higher frequencies before standing waves become a problem. Nevertheless, like anything else (even SPGs), MPGs can’t be ignored, especially the large-scale grounding systems. There needs to be routine maintenance to counter the effects of corrosion, shock and vibration, and mechanical damage to prevent the introduction of unwanted high impedances.

- Ron Brewer

Do You Feel Lucky?

2013-04-29T12:37:00.000-07:00

Do You Feel Lucky? Well do ya…..PUNK?

Classic line from a classic movie, Dirty Harry, who was also in the Enforcer. Frankly I forget which one came first. So who is our enforcer of electromagnetic environmental effects (E3) requirements, specifically for US DoD program?

The requirement to field electromagnetically compatible systems is mandated in DoD Directive 3222.2, “DoD Electromagnetic Environmental Effects (E3) Program.” It’s pretty straightforward about the need for E3 control and defines a wide variety of roles and responsibilities for the implementation of E3 control, but at this point it’s a bit out of date. The current version is dated 8 September 2004, so its going on 10 years old. That can be a lifetime in technical discipline like E3 as many things can and obviously have changed over the last decade in the E3 business. Priorities change, definitions change, organizations change (including people), technology changes causing the electromagnetic environment to change (that’s a biggie!), all contributing to the need for policy to change or at least to be updated. This version of DoDD 3222.3 has seen changes to MIL-STD-461, MIL-STD-464, the creation of MIL-HDBK-240 (HERO Test Methodology), DoD Instruction 4650.01 (DoD Spectrum Use), the disbanding of the Office of the Secretary of Defense Networks and Information Integration (office responsible for E3 and spectrum directives) and a wide variety of other changes.

There is some good news – there is an updated version of the directive waiting for signature. DoDD 3222.3 will become DoD Instruction 3222.03, which is a significant change. Its actually an improvement as an Instruction is allowed to include more information (its physically allowed to be longer, more pages). This will allow the E3 community to ensure that more guidance is provided on HOW to implement the E3 program, not just the fact that an E3 program is required. Better guidance allows program offices to plan more effectively, with a better understanding of what is expected of them. The final coordination draft of DoDI 3222.03 contains significantly more information on specific actions that must be taken by acquisition programs to effectively implement E3 controls with the acquisition process. In particular, there is an entire enclosure that details the procedures that should be followed during the acquisition process to achieve electromagnetic compatibility. Just as importantly it requires that the DoD provide awareness and training to the acquisition and operational communities, so there is a better understanding of the importance of E3 and related technical disciplines and the processes associated with them.

The bad news is that the draft has been stuck for quite some time in the final signature cycle, hostage to various recent reorganizations and never quite getting high enough on “the list” to actually get promulgated. But there are good people pushing for it and we’re all confident that it’ll be out soon. Then hopefully fielding electromagnetically compatible systems won’t just be a matter of luck ….

- Brian Farmer

Can an Ordinary Human Stop RF Noise?

2013-04-24T12:47:00.003-07:00

When my son was working on his EE degree he found an article titled: Stop That Noise in the IEEE Potentials, October - November 1995. Being a dutiful son, he brought it home to his EMC-dad and I read it. It is a great little article giving an overview of the cause and effect of EMC problems. The articles primary thrust is to introduce EMC to college students and encourage them to become EMC engineers. It does a superb job with that, which is really good; because the field needs some newbie's to replace the retirees

The article contains a list of valuable attributes for becoming a successful EMC engineer. It says, to solve electromagnetic compatibility problems, a person needs:

• Common sense,

• The right education,

• Some experience,

• A logical diagnostic procedure,

• Good observation skills,

• A sense of humor,

• Persistence, and an

• Occasional bit of luck.

Doesn’t that sound like the attributes of all successful people?

The article goes on to say that the right education for EMC means a good understanding of:

• Basic circuit theory,

• Electromagnetic theory,

• System concepts,

• Electrical modeling,

• Electronic circuits,

• Transmission lines, and a

• Little antenna theory.

The article was written by the gang from U of Missouri, Rolla: Van Doren, Hubing, Drewniak, Fei Sha, and Hockanson. I don’t know all these guys, but the ones I know certainly live up to the published list of requirements and they are all very willing to share their experiences and expertise.

I might add to their list of attributes, a comment I made to my boss when he asked me how I knew that the solution I proposed would work. I told him all that was needed to develop a good EMC solution was to think like an electron, i.e. what would I do if I were one.

EMC is one of those fields (if you pardon the pun) that subtends all disciplines so to the above education list could also be added thermodynamics/heat transfer, shock and vibration, and electrochemistry. The reasoning behind adding these three design areas is that they affect the EMC behavior of a system.

Heat affects the operation of semiconductor materials -- some more than others. Adding shielding to solve an RF problem creates thermal barriers which then require some method of getting the heat out. If the heat removal is convective and depends on the movement of ambient air then ventilation openings are required. Vent openings are not RF tight so we still have a problem.

Vibration changes the separation distance between components, PCB’s and subassemblies at the vibration frequency – or in the case of shock at their natural mechanical resonance frequency. The movement changes the capacitive and inductive coupling resulting in vibration induced modulation. In the vacuum tube days this was called microphonics.

Corrosion is an electrochemical effect in which the more reactive material is consumed creating non-conductive and/or nonlinear junctions where conductors ought to be. If mixing occurs, a non-linear junction can generate frequencies that aren’t part of the systems operation. Non-conductive junctions destroy the bonding between shield panels. Given enough time, corrosion can eat through the panels. Corrosion is a major EMC problem, especially for systems located in harsh environments or requiring long term reliability.

All of these things create potential EMC problems which should be considered during the systems/equipment design. These design efforts can be worked into the schedule during the off times when the EMC engineer is not trying to outrun speeding bullets, stopping powerful locomotives, and leaping over tall buildings. But, they absolutely have to be done before PDR, i.e. the preliminary design review.

- Ron Brewer

Where are the Resumes?

2013-04-05T06:01:00.003-07:00

Sometimes it’s a bit of a struggle to come up with something relevant to write. I try to write about topical subjects in EMC or spectrum supportability, but sometimes things are just slow. Right now we’re trying to hire a RF Spectrum engineers for some Navy work and it's just hard to find quality resumes. I started wondering why, and I’ve concluded it’s a combination of high employment in this particular niche business and a lack of training in that particular area.

In the DoD spectrum management (and EMC, by extension) worlds, demand for work is far exceeding supply of capable people to do it. This can be directly attributed to the ongoing spectrum reallocation and repurposing (I LOVE that term!) from a variety of spectral auctions in past years to the current 500MHz Presidential Initiative. When these actions affect government spectrum users (with the DoD being the largest user by far), it spawns an endless supply of analysis work to determine the possible impact to DoD operations. I’m certainly not knocking this work; it needs to be done and done well because billions of dollars are at stake. It also tends to be high paying work done by senior engineers that have certain expertise in spectrum engineering. In addition, mandates in the form of DoD Directives and Instructions related to spectrum use and EMC help to ensure that those technical requirement areas are included when procuring spectrum dependent equipment and other electronic systems. Suffice it to say, there’s no lack of work, both civil service and contractor, in the military communications-electronics and platform integration world. There isn’t always enough funding to do the work properly, but that’s another story!

The rosy employment scenario is one reason it’s hard to fill positions. The other big reason, at least from what I’m seeing right now in resumes, is that a lot of what the military needs regarding spectrum engineering work, just isn’t taught anywhere. Sure, the actual uniformed military trains its spectrum managers, and they do it well, but these guys aren’t the RF engineers needed for all the technical analysis, testing, etc. The current crop of DoD contract spectrum engineers seem to be very senior, started with EEs, maybe working in the non-DoD manufacturing sector and transitioned at some point to this nebulous engineering support world. From there on, there is an awful lot of on-the-job training that turns into experience that is only available in that one person.I could easily name several very senior EM/Spectrum engineers that have spent the majority of careers working on a single program until they are literally irreplaceable. Until they retire, that is! So its nigh on impossible to find a junior or even mid-level spectrum engineer for DoD work with any significant experience in DoD spectrum certification processes. Most of the experienced ones have a lot of telecomm companies on their resumes doing cell phone and wireless internet work. Close, but no cigar! That leaves a pretty big learning curve for the client to have to consider for most candidates we’ve come up with.

- Brian Farmer

Let's Twist Again

2013-04-03T12:23:00.001-07:00

Some time ago I introduced the equation for DM radiation from a loop. I also let Howard Johnson make fun of the single ended crowd that do not believe in loops by quoting from one of his articles. (Refer to “Why Digital Engineers Don’t Believe in EMC”, IEEE EMC Society Newsletter, Spring 1998.) The whole article is available on the internet.

Anyway, to continue:

E = 1.32 A I F2 / r

Where:
E(max) = µV/meter
A = radiating loop area (sq cm), 0.1<(l/w)<10 br="">I = drive current (amps)
F = emissions frequency (MHz)
r = measurement distance (meters)

All of the parameters in this equation directly affect the emission levels. However, the loop area is one that the designers have a lot of control over. It affects PCB layout, component placement, sub assembly location in a rack, rack configuration and cable design. Since cables tend to be the biggest emission/pickup antennas, that’s what gets discussed next.

Loop antennas are low impedance coupling transducers that are sensitive to magnetic fields. Magnetic fields are created by sources with impedances less than 377 ohms. Most semiconductor devices have low impedances, which are significatly lower than 377 ohms. This means they generate magnetic fields and will be a good match to a loop antenna for both magnetic field emission and pickup. It is very difficult to shield for magnetic fields, so the most effective hardning techniques are to reduce the drive and/or the coupling loop area.

The DM loop area for a cable is the product of the length (L) of the cable multiplied by the separation (S) between the signal and return wires (i.e. A = L * S). Because it is probably impossible to bring the wires closer together, the area is reduced by making the cable shorter. I can hear the response now – What’s a matter with you? I wouldn’t have made it that long if I could have made it shorter! How about field cancellation instead?

Okay, we're going to do the twist and it goes like this: Take the signal and return lead and twist them together so that the lay is uniform and the twisted pair is homogeneous, and make sure that there is an even number of half-twists between the ends of the cable. A twist is one complete 360 degree rotation, and there needs to be a minimum of one twist per half wavelength (λ/2) of the highest offending frequency. More is better.

If each half-twist is considered as an individual current loop, the loop current reverses in the adjacent half-twist and the magnetic fields are partially cancelled. The reason for requiring an even number of half-twists is to make sure each half-twist is provided cancellation by its neighbor. The tighter the twist, the greater the reduction in magnetic field coupling and the higher in frequency the technique works. Balanced twisted pair transmission line may have earned its reputation in the analog telephone industry at 300 – 3000 Hz, but today we are using the Cat 7A cable over short distances (10-15 meters) at 100 Gbps data rates.

Twisted pair reduces the magnetic coupling, so it works best in low impedance circuits. The lower the impedance, the better the technique works. To help as a memory hook, twisted pair provides cancellation for DM currents because they are out of phase. Common mode (CM) currents just get dizzy as they spiral around the twists, but they won’t be cancelled.

- Ron Brewer

The Hybrid Ground System

2013-03-27T13:07:00.000-07:00

We’ve already concluded that for low frequencies the Star or SPG ground works best and at the high frequencies, the so-called equipotential ground plane with a MPG system works best. No one grounding system is perfect. On top of that, most of the equipment these days operates at frequencies from DC to daylight, so the grounding design has to cover all frequencies including the mid-range.

There is a grounding configuration we have yet to discuss—which is not really a grounding system at all—called the Floating Point Ground (FPG), in which all electrical/electronic circuits are deliberately isolated from everything at least by conductors. Believe it or not, there are some advantages to doing this, particularly if some of the equipment circuits are sensitive to circulating ground currents or low level magnetic fields. It gets used from time to time in spacecraft and on shipboard, it is often used for power systems to maintain weapons operation in the event of a short to ground and to minimize electrogalvanic corrosion.

Implementation of a FPG is very difficult and at the circuit level requires extensive use of optical couplers, fiber optics, and isolation transformers. At the box level, it requires plastic boxes, insulated spacers, isolated panels, etc. A disadvantage of the FPG is that all isolated conductors become one plate of a capacitor with respect to ground and in a high static field environment, floating objects (whether insulators or conductors) can accumulate charge. When charge levels become high enough corona/streamer discharges can occur and/or arc over to nearby conductors that are at a different charge potential or that are grounded.

For a system that has a large range of operating frequencies, all three forms of ground systems are used. An example of such a combination might be a platform monitoring system where data from various analog sensors such as pressure, temperature, strain, vibration, etc is being multiplexed to a digitizer, which then sends its output to a microwave telemetry transmitter. The construction of some sensors requires them to be grounded. Other sensors are floated. The digitizer may have both SPG and MPG internally and the microwave telemetry transmitter probably only uses a MPG system.

The choice of grounding system is determined by frequency principally because of the inductance of the grounding conductor. This is directly related to conductor length. Many EMC guidelines suggest that the length should be held to less than λ/20. This is based on the assumption that the ground system will be a disaster at λ/2. More likely it will be a disaster long before the length reaches λ/4 because this simple guideline does not consider the capacitance of the device in conjunction with the inductance of the ground conductor.

The first parallel resonance occurs at F = 1/ (2π (L * C)0.5) with F(Hz), L(H) and C(F). When resonance occurs, the impedance becomes infinite. Not really of course, but it does get very high and the ground doesn’t work at that frequency. Murphy’s Law says that is the frequency at which the widget generates the most RF noise. To minimize the problem and provide a better rule of thumb for those concrete thinkers, plan on the SPG being usable to around λ/50. In addition, a MPG is absolutely required after λ/10 and a combination of the two (Hybrid) is used when the frequencies fall in the middle. There are always exceptions, but that’s EMC!

- Ron Brewer

Personnel Certification – What’s in it for Me?

2013-03-19T09:14:00.000-07:00

I have recently heard, as part of some of my activity with the IEEE EMC Society (which really should be a lot more than it is (my activity, not the EMCs!), but that’s another story), questions about what iNARTE should do for us, as certified EMC Engineers. The gist of the question is: what is the overall benefit to the EMC Society and its members from iNARTE and having one or more of their EM related certifications?

It’s a good question but I think the answer is going to be that there is very little tangible evidence of any direct benefit to the EMC Society or its individual members from any iNARTE relationship. Don’t get me wrong, I’m not against the certifications themselves. I believe they provide a valuable initial measure of someone’s ability, particularly someone you don’t know that you might be considering for a position or to do business with. After all, don’t you want your doctor to be AMA board certified? In general certifications should provide:
• Third party credentials in specific areas of expertise based on recognized standards.
• Recognition of proof of professional competence in a particular technical area
• Assurance to employers a measurable level of expertise and quality of work.
• A demonstrable benchmark to which organizations can train and evaluate their staff.
• Certification fosters industry growth by demanding standards of technical excellence.

They are basically an acknowledgement from industry peers that someone has met a certain minimum standard of competence in that industry. Beyond that it doesn’t tell you much. And while the goal of certification might be the above listed bullets, but they certainly can’t take the place of personal interviews, reference checks, etc. And I don’t think NARTE or any other certification group “owes” us anything beyond that pretty certificate.

I can remember when NARTE started the EMC technician and engineer certification programs (something like 20 years ago – I don’t like to remember that part!) with a lot of push from the Navy, the Naval Air Systems Command in particular. The idea was that NARTE certification would be a personnel requirement in service contracts related to EMC work so that companies bidding on this work that had NARTE certified people would be at a distinct advantage. Unfortunately, that didn’t come to pass; there were very few RFPs released with these requirements and for the life of me I can’t remember why. There were a some in the first few years after the certifications were started but they quickly died out. Maybe if Russ Carstensen is out there, he can tell us the story (at the risk of having him point out the errors in mine!).

So why do we continue to get these certifications? I’m not sure. Professionally, I don’t believe anything in my career would be different if I didn’t get that certification. Being part of the IEEE and the EMC Society has far greater professional potential. Maybe its an insecurity thing. After all, I was grandfathered in as a NARTE EMC Engineer at the beginning! You actually need to know your stuff to get one now and I have great admiration for those that have earned the certifications. So I’m not sure why I re-up every year (or three years at a time) at this point but I do.

-Brian Farmer

The Equipotential Plane That Isn’t

2013-03-14T07:55:00.000-07:00

The success of the multipoint ground (MPG) system requires having an extremely-very-low RF impedance grounding connection to an extremely-very-low RF impedance equipotential reference plane. It also helps if the conducting materials are homogeneous and the bad things are kept separate from the good ones. Does that sound easy? You’re right – it’s not!

A couple of posts ago, I noted that short-wide-flat conductors have lower impedance than wires (even with the same resistance) because they have lower inductance, not because they have no inductance. Plus, I have stressed that inductance (L) is directly related to length (L). Still, by extension, a large highly conductive ground plane represents the lowest impedance conductor arrangement. Thickness does affect resistance and inductance somewhat, but it is not a major factor at high RF frequencies because of skin effect.

That being said, when the systems speeds/frequencies increase to the point where a multipoint ground system is required, it is implemented by attaching components and subsystems directly to an underlying ground plane using the shortest-widest possible connection. Component placement is important because of inductance and resonance.

The impedance between two points on a ground plane as a function of frequency is given by:

Z = R_RF (1 + | tan 2 π d / λ |)

Where:

R_RF = 0.26x10^-6 (μf / σ)^0.5

f = Frequency (Hz)

μ_r= Relative permeability

σ_r = Relative conductivity

d = Distance between points (m)

λ = Wavelength at frequency of interest (m)

Obviously, at high frequencies it is not an equipotential plane!

In the past, we used to tell PCB designers to configure their boards the way microwave aerospace systems are configured. Now, it’s easier to tell large scale microwave systems designers to configure their systems like PCB designers do with high speed multilayer boards. Both approaches are similar, even though one is steady sate and the other is transient; one uses bond straps and the other vias; and one is big and the other is small.

Most aerospace widgets are built in rectangular hogged-out aluminum boxes. They may not be as cute as the plastic ones with a lot of curves and they may be more expensive, but they are more robust and they also provide better shielding. We’ll cover shielding sometime in the future.

Ideally, RF grounding should be done by placing the widget side with the largest area against the equipotential ground reference plane and pulling the surfaces tightly together to assure close intimate contact. If surface-to-surface contact is not possible, weld the widget completely around its perimeter to the surface. If that’s not possible because grounding wasn’t considered until the last minute, then a bonding strap can be used. This is a poor quality extension of the ground system from the plane to the widget, but it is way better than a wire.

There are a number of EMC design guides that call out conductor length to width ratios. The required 4:1 or 5:1 L/W ratios are an attempt to maintain somewhat of a plane configuration between the connection points instead of just a wire. The truth is a grounding/bonding conductor should have zero (0) length and be infinitely (∞) wide—or at least as wide as the widget being grounded.

The contact surfaces should be bare but coated with something conductive to minimize corrosion, such as iridite or alodine—but not anodize, because it is nonconductive. MIL-C-5541F, Chemical Films, covers these types of materials. Contact surfaces should always be measured to assure conductivity. This prevents Murphy from painting the surfaces with non-removable, nonconductive epoxy paint.

- Ron Brewer

Spectrum Soapbox

2013-03-06T11:50:00.000-08:00

“Soapbox: Also, soap box; an improvised platform, as one on a street, from which a speaker delivers an informal speech, an appeal, or political harangue” (dictionary.com).

Well, it may feel like a harangue, but the fact is that the U.S. Department of Defense remains on its soapbox about the potential impacts of continuing changes in the arena of spectrum use. Rightfully so, IMHO! Access to the electromagnetic spectrum (EMS) is critical to the success of all military operations; it seems that wired systems are quickly becoming a thing of the past. Spectrum is the thread that ties all operations together. Military operations rely on spectrum for their mobility, information superiority, accelerated operations tempo, targeting and accuracy. Spectrum has, in fact, become another warfare domain that must be controlled along with land, air, sea and space for communications, sensors, radars, navigation, electronic warfare, identification, and guidance systems.

There remains considerable pressure on the DoD’s spectrum access from many fronts:
    -Radio Spectrum Inventory Act (RSIA) Legislation
    -Spectrum Relocation Improvement Act
    -Executive Order
    -FCC Broadband Plan
    -NTIA assessment request on 11 “bands of interest”

However, the DoD’s spectrum requirements are actually increasing due to bandwidth, sensor and information transfer requirements, while spectrum resources have declined due to transfer of federal bands. Some examples of both increasing use and declining access, courtesy of the Defense Spectrum Organization:
    -Operations in Afghanistan/Iraq require ten times more bandwidth than Persian Gulf War.
    -DoD had only one Unmanned Aerial System (UAS) in 1998, but had over 5000 in 2008.
    -Federal government lost exclusive access to ~ 5 GHz of spectrum since 1980.
    -Omnibus Budget Reconciliation Act of 1993 and Balanced Budget Act of 1997 repurposed 237 MHz (under 5 GHz) of Federal spectrum – all used by DoD
    -Relocation from 1710-1755 MHz required 4 years -- DoD cost of $355M, Federal Gov’t cost ~$1B (reimbursed).
    -FCC’s National Broadband Plan, NTIA’s 10-Year Plan & Presidential Memorandum (June 2010) call for 500 MHz (both Federal and non-Federal) to be reallocated to commercial broadband use this decade

While a variety of technical working groups and organizations are analyzing the impacts of repurposing spectrum and the possibilities of sharing bands, it pays to bear in mind that the DoD uses spectrum differently than commercial wireless, in that mission effectiveness comes first. Some general differences include:
    -DoD systems use spectrum based on physics, performance requirements, and platform limitations.
    -Mobility, reliability, redundancy and security are issues for both commercial and military spectrum systems, but these operational factors increase by orders of magnitude—and are paramount—for DoD systems.
    -DoD sensors often require extreme resolution translating to unique spectrum demands.
    -DoD mission requirements are less comparable to commercial systems and more like public safety systems (which have unique requirements for network coverage with robust, reliable, secure communications).

So, the spectrum community is in high gear developing a DoD Spectrum Strategy in support of the National Military Strategy that addresses the aforementioned issues. They are investing in spectrum-efficient technologies and more capable spectrum management tools to address challenges while balancing national defense and economic interests. Hopefully all parties, not just the DoD, are putting a greater emphasis on efficiency and technological solutions to solve the increase in spectrum demand—not just robbing Peter of spectrum to pay Paul.

-Brian Farmer

Talking Smack….

2013-02-27T12:54:00.002-08:00

So, I attend regular US DoD E3 meetings—aptly titled the DoD E3 Integrated Program Team (IPT) and chaired by the Defense Spectrum Office—held monthly and attended by representatives of the four services, the DoD CIO’s office, the Joint Staff and others. A majority of the discussion these days centers on spectrum activity, rather than purely E3 discussion (those days are long gone!)—but that’s not surprising considering the level of attention on RF spectrum use. One of the things that is discussed at length is the Commerce Spectrum Management Advisory Committee (CSMAC); in particular, the activities associated with various working groups thereunder. And, unless you’re up to your eyeballs in the actual work, it’s hard to understand what the Committee is and what the working groups are up to. Fortunately, it’s very well defined on the NTIA website (thank you, Bing!):

“The Commerce Spectrum Management Advisory Committee (“CSMAC”) advises the Assistant Secretary of Commerce for Communications and Information at NTIA on a broad range of issues regarding spectrum policy and on needed reforms to domestic spectrum policies and management to enable timely implementation of evolving spectrum-dependent technologies and services to benefit the public.”

The CSMAC charter was revised in 2011 to focus specifically on the implementation of the President’s Spectrum Initiative and how to repurpose up to 500 MHz of spectrum in the next decade. And since the you-know-what flows downhill, pretty much the entire DoD spectrum community and half the E3 community are busy analyzing the impacts that the loss of primary use of several major spectrum bands will have on their services’ systems. Here are the bands under analysis by the various working groups (working on just two bands of interest):

WG1: 1695-1710 MHz Meteorological-Satellite
WG2: 1755-1850 MHz Law Enforcement Surveillance, EOD, and other short distance links
WG3: 1755-1850 MHz Satellite Control and Electronic Warfare
WG4: 1755-1850 MHz Tactical Radio Relay and Fixed Microwave
WG5: 1755-1850 MHz Airborne Operations (Air Combat Trainings System, Unmanned Aerial Vehicles, Precision-Guided Munitions, Aeronautical Telemetry)

Their meeting minutes and the reports are published on the NTIA website. It makes for some interesting, if rather dry, reading, but that’s not the point. The point is that this information, including the concerns of various agencies, impacts to capabilities and potential options going forward, is all out there for whoever’s interested in reading it. It’s a pretty good example of “transparency” (where other parts of the US government fail miserably!) and an opportunity for anyone with a dog in the hunt, as they say, to better understand what’s happening in the spectrum world today. Specifically, with respect to spectrum use and repurposing (I love that word!). That will eventually lead us to better technology and hopefully more efficient processes and procedures to take advantage of the technology. It’s a very complicated big picture, one that I wish I understood better, and one which I hope the very smart people at the top of the spectrum food chain have a good grasp of.

-Brian Farmer

Twinkle, twinkle, little star . . .

2013-02-13T11:22:00.000-08:00

We often wonder how good you are. Want to start a heated argument in the EMC community? Just start talking about grounding systems and why the Star or single point ground (SPG) system is so-o-o-o much better than those ridiculous multipoint ones being used by all those other people. Really? Are they that good?

Before I answer that, let me first mention Scott Roleson. Scott is an EMC expert with HP. He and I served overlapping tours as Distinguished Lecturers for the IEEE EMC Society. This is where you go unto all the world, make EMC technical presentations and eat rubber chickens.

Scott had an outstanding presentation in which he stated that there are three answers that every EMC engineer should commit to memory and be able to voice instantly when necessary:

Answer number 1: Yes, we have to have that!

Answer number 2. No, you can’t take that out!

Answer number 3: It all depends!

The reply to the question about which is better, the Star or SPG ground system, is: Answer number 3!

It all depends on the signal frequency range and if there are noise currents flowing in the ground system. For example, the blue electron current (I) in Figure 1A is created by a source external to the system and, because of grounding system impedance between points a and b, a CM voltage Vab = I * Zab is developed between the two ground points. This voltage (Vab) drives current around the loops formed by our system. Some take the low road, while some take the high road and excite both CM radiation from the loops and any cabling associated with them.

In addition, the low road (a.k.a. the return path) has lower impedance than the signal path because the blue return path electrons do not have to pass through the source and load impedances. This unbalanced CM current creates a DM voltage across both the source and load, which contaminates the desired DM signal.

Moving points a and b closer together reduces Vab. Changing the ground plane to a lower impedance material also reduces Vab. Connecting all the grounds to either a or b eliminates Vab. In a really small system that might work, but in a large system the conductor lengths have to increase—so does their inductance. Those who read the last post know that this is a bad thing.

One way around this shortcoming is to use a SPG system. This is illustrated in Figure 1B. Remove the inductive conductor from c to b and replace it with distributed parasitic capacitance from all system conducting surfaces to the underlying ground plane. This is illustrated as a lumped capacitor from c to b. The grounding conductor that was removed has a reactance X_L = 2π f L, and the parasitic capacitance that replaced it has a reactance X_C= 1 / 2π f C. The long Star conductors work okay at low frequencies, but fail as frequency increases because the conductors no longer have a low impedance. The SPG works great at low frequencies, but fails as the frequency increases because the capacitance no longer has a high impedance.

What can we conclude? The Star and SPG work great at the lower frequencies, but parasitic capacitance interconnects all conducting surfaces as the frequencies increase. At high frequencies, this forces the system into a multipoint cross-coupling and grounding configuration no matter what.

While these choices are being considered, think about what happens in the mid range of frequencies.

- Ron Brewer

Grounding Conductor Limitations!

2013-02-06T11:00:00.002-08:00

I briefly mentioned some of the elements of grounding in the last post. Technically, there are three types of grounding systems that work: (1) the single point (a.k.a. star) ground (SPG), which uses wires; (2) the multipoint ground (MPG), which requires a ground plane or mesh; and (3) the floating (no) point ground (FPG). All three utilize some variety of RF conductor—a wire, a plane or a capacitor. If conductors had no resistance and no inductance, implementation would be easy. Unfortunately they do have these properties and that forces the grounding design to change based on frequency.

Even though there are three grounding configurations, there’s about six different flavors depending on which industry and what problem is being considered. Each system has a name— power, lightning, safety, technical, signal, maintenance or maybe static, equipment, systems, lightning, electronic and maintenance. The one that may not be familiar to everyone is the maintenance ground. This is the installation of an intentional (but temporary) ground or short across non-energized circuits to protect technicians and maintenance personnel from inadvertent TURN-ON. Sometimes a lock-out tag just doesn’t give enough warm fuzzies—like when you’re working in a 15 MW RADAR transmitter cabinet. Since we don’t want to fry anyone, safety is extremely important and we do what we must in order to be safe.

Grounding is used to minimize voltages that are being developed between two or more locations and are caused by errant currents. One location is generally an active part of a system, while the other is a reference. The power industry, NFPA, and NEC people have standards and procedures in place to cover things like power, fault, lightning and maintenance grounding. These are primarily low frequency systems in which lengths are long and impedance is dominated by conductor resistance. For a conductor Zo = R + jωL and at really low frequencies for large wire sizes R < jωL, wiring sizes that are used are based on current carrying ability and can be quite large for systems with large fault current.

Generally, at the signal frequencies (F > 1 MHz), inductive reactance is greater than the conductor resistance (jωL > R), and it dominates the impedance. Because of that, a lot of design time is spent trying to reduce the inductance of electronic/signal grounding systems. For insight on what’s needed to do that, consider a flat conductor/trace with thickness (T), width (W), and length (L). Making the conductor 10x thicker reduces the inductance by 10 percent. Making the conductor 10x wider reduces the inductance by 30 percent, but reducing its length to 1/10^thof the original length reduces the inductance by 93 percent. That’s 93 percent! Whoopee! Yahoo! Eureka!

We just learned to keep conductors as short as possible. How short should it be? Short enough to minimize resonance effects. At a quarter wavelength (λ/4), the impedance is very high, so most design guides have a rule of thumb limiting grounding/bonding conductor lengths (L) to no more than λ/20 at the highest frequency that causes concern. If the highest frequency of concern is 3 MHz, then L < (100m * 40 in/m) / 20 = 200 inches = 16.7 feet. Unless we are in a really big system, this is not a problem. If it’s 100 MHz, then L < (3m * 40 in/m) / 20 = 6 inches and it’s getting to be a problem.. At 3 GHz, L < (0.1 * 40) / 20 = 0.1 inches and unless we are working at the chip level, this is most definitely a problem. Even at the chip level this isn’t easy.

The goal is to design an RF short circuit, not an antenna. Stay tuned! There’s more to it than this.

-Ron Brewer

Common Mode Emission. Can It Get Any Worse?

2013-01-21T10:57:00.000-08:00

You betcha! Once upon a time I was in charge of EMC design for a large scale military digital telecommunications system. This system had to handle both red and black analog and digital data with the proviso that never the twain shall meet. In the lab where all the engineering tests were taking place everything went great until the cables were hung on the system with care. Then all through the lab radiated RF noise got much-much worse, so a special set of shielded cables were procured in hopes of solving the curse. Read this as: we spent a lot of money . . . management was unhappy! And sure enough things got worse. There are times when you just wish Murphy would stay home.

With some experimentation we were able to determine that the presence of the cable’s shields was the culprit that made things worse. High frequency common mode loop current from unbalanced differential mode signal harmonics were driving the cable shields making them behave like the antennas that they were. This is illustrated in Figure 1.

Briefly, the ground plane has impedance because of material resistivity and inductance, i.e. Z = R + X_L. CM current flowing through the ground plane impedance produces a voltage drop Vab between the points a and b where the cable shields are connected to the grounding system. This voltage appears across the cable shields forming an antenna. This is illustrated in Figure 2.

This is not an easy problem to solve but there are some things that can be done to dramatically reduce the emissions.

1. Don’t generate any CM currents. . . yeah right! We already know how difficult it is to reduce these let alone eliminate them.

2. Reduce the amplitude of the CM current. Use lower power circuit designs, low power logic, LVDS, etc, and try hard not to have the CM source in the middle of a cable arrangement. In the configuration shown in Figure 1 the connection from c to b is in parallel with any unbalanced parasitic capacitance from the DM signal and its return. Eliminating this connection will reduce the common mode currents at the lower frequencies. If that’s not enough, the entire system can be floated but this must be designed in from the beginning.

3. Reduce the impedance of the ground plane. If the rack, frame, chassis, or enclosure forming the ground plane is made from steel, using higher conductivity materials such as aluminum and copper will certainly reduce the resistance. This will reduce the voltage drop at the lower frequencies. Unfortunately as the frequency increases the inductance becomes the dominant factor. Even a ground plane made from aluminum or copper will have increasing impedance with frequency but nowhere near that of steel or other materials with high permeability.

4. Move points a and b closer together. This will reduce both ground system R and X_L by creating a single point ground (SPG) system and band limiting the CM currents at the lower frequencies.

5. At the higher frequencies conductor impedance can be higher than parasitic capacitance and the CM is reduced using low pass filters, generally in the form of series ferrites or CM chokes. The filter configuration depends on where the filters are placed.

Keep in mind that in the normal course of performing their intended function, power, control, and signal leads can and do act as transmit or receiving antennas for both CM and DM and they unintentionally couple signals through radiation to other collocated systems and equipment.

- Ron Brewer

What’s this about CM and DM radiation?

2013-01-09T07:00:00.000-08:00

We’ve been discussing loops all along and we are still at it. The last post looked at DM, this time it’s back to CM. These loops are the worst -- big – bad – ugly! Finding the loop may be a problem . . . but where there’s current, there’s a loop. The DM loop is easy, typically a differential pair over a ground plane in a high-speed system, the CM is some synchronous current from unbalanced impedances associated with both the signal and its associated return lead that is conducted via a common reference surface.

All conductors have series inductance (L) and are coupled to the underlying ground plane or other closely coupled conductors via parasitic shunt capacitance (C_P). The physical characteristics are not constant. The width or diameter of the conductors change along the length, the insulation or PCB thickness changes, the conductors have uneven lengths, different numbers of turns, and their spacing changes. These varying conductor characteristics unbalance the signal and its return, create the CM loop, and result in differential signal skew. Consequently, small CM currents, called longitudinal mode by telecommunications designers, arise from the unbalanced LC characteristics of the signal and associated signal return leads with respect to any closely coupled underlying return path, usually the chassis or grounding system.

These CM currents travel simultaneously in the same direction on each signal and return conductor and can be visualized as returning via a capacitively coupled return path in the nearby conducting surfaces. At the lower frequencies, this forms an antenna structure very similar to a monopole antenna oriented parallel to the cable runs.

At the higher frequencies where the cable lengths are a wavelength or longer, the structure becomes a traveling-wave antenna. In both of these antenna structures, the loop current is returned via the ground reference.

For an electrically short monopole antenna of length l, the electric field E(θ) radiation created by the CM current on the cable as denoted by Henry Ott is:

E(max) = 4 π ( F * I * l ) sin (θ) / 10 r

By orienting the antenna for maximum emissions, sin (θ) = 1 and simplifying, the equation can be rewritten as:

E(max) = 1.256x10⁴ * F * I * l / r

Where:

            E(max) = µV/meter
            I = drive current (amps)
            l = cable length in centimeters
            F= emissions frequency (Hz)
            r= measurement distance (meters)

The CM-generated EMI levels predicted from this antenna model correspond well with measured data.

Speaking of antennas, all conductors can be antennas. Good conductors can be good antennas, but even poor conductors can be poor antennas. A poor antenna may be all it takes for a product to fail its EMC qualification test.

The amount of RF energy collected by or radiated from an antenna is related to its capture area which is related to its physical area. If the antenna is a microwave horn or a defined loop the physical area can be determined by inspection. With a wire it’s not so obvious, but since area is the product of length times width (A = L x W) keeping wire lengths short reduces the capture area. When designing a high speed system, think small.

- Ron Brewer

Tell Me Again Ron – What’s This About CM & DM Radiation?

2013-01-02T11:45:00.002-08:00

Apparently, I didn’t throw enough words at the CM/DM problem in the last several posts so I’m going to add to it, but from a slightly different perspective. It’s either that or hide my email address. I’ll do DM this time, CM next time.

We’ve been discussing loops all along. If there’s current, there is a loop somewhere. Finding it may be a problem. DM loops are bad, but CM loops are worse. Sometimes the loop is so poorly defined or obscure that it is difficult to comprehend that there really is a loop and that it is returning current to a source. This is especially true with logic traces, wires, and cables because they all appear to be single ended.

Dr. Howard Johnson said in the article, “Why Digital Engineers Don’t Believe in EMC”, IEEE EMC Society Newsletter, Spring 1998, that many digital engineers don’t believe current flows in loops and, because of this, think signal return paths are irrelevant.

The return paths are very relevant! If the circuits that are being driven do not have a stable, well-defined return path, the current will return to its source the best way it can. Unfortunately, this may be the worst way for EMC. Generally, these strange return paths are unacceptable. They increase radiated emissions from the circuit, capacitive and inductive cross-coupling (AKA crosstalk) between circuits, and the susceptibility of circuits that are closely coupled to the undefined path . . . most of the time by wires and cables.

Controlling the circuit loop area (A = conductor length L x spacing S) is probably the most important element in meeting EMC requirements. As the loop area increases, the levels of generated or intercepted energy will increase. Therefore, keep the loop areas as small as possible, especially those operating at a high frequency/speed. Unfortunately, since the conductors are the principle determinant of loop area, there is a limit on how small we can make the radiated emissions or susceptibility loop because of physical constraints. Loops can’t be any smaller than the area required by component placement. On a PCB, the trace loops will be a lot smaller than the loops formed by wiring and cable placement between racks located on separate floors within a building.

Unless the circuit loop is completely isolated from all other conductive surfaces, each loop will have two coupling modes: the green differential mode (DM) and the red common mode (CM). DM, typically a differential pair over a ground plane in a high-speed system, and CM, where some synchronous current from both the signal and its associated return lead, are conducted via a common reference surface, are illustrated in Figure 1.

The electric field (E) radiation created by DM current circulating around a small, nonresonant loop oriented at angle (θ) is given by John Kraus as:

E = 120 π2 A I sin (θ) / (λ2 r)

By orienting the loop for maximum emissions, something that is done during the EMC measurements by rotating the unit, raising and lowering the antenna, and changing the antenna polarization angle, we maximize the signal making sin (θ) = 1. Converting to frequency instead of wavelength, F(MHz) * λ (m) = 300 m/μs the equation can be simplified as:

E = 1.32 A I F2 / r
Where:
E (max) = µV/meter
A = radiating loop area (sq cm), 0.1<(l/w)<10 br="br">I = drive current (amps)
F = emissions frequency (MHz)
r = measurement distance (meters)
Kraus indicates that for frequencies below resonance this equation is accurate to within 1% for loops up to 10 x 10 inches. This makes it ideal for PCB design.
However, it certainly is usable for any loop current at frequencies below loop resonance where the length to width ratio is <10 font="font" size="3">.

- Ron Brewer

EMC Engineers Come Bearing Gifts

2012-12-20T05:41:00.001-08:00

It’s the holiday season again, when we celebrate the serious things in life and have some fun too.

When EMC engineers come bearing gifts...tis NOT the season to be jolly. Fa-la-la, le-la-la-laa. In fact, it’s time to be worried. For to the EMC engineer, GIFTS is an acronym for: Grounding, Isolation, Filtering, Transient suppression, and Shielding. Sometimes it takes a lot of GIFTS before a system achieves electronic happiness.

In keeping with the spirit of things, this blog write-up is going to take a fast look at the GIFTS. We can’t do much else with a limited number of words, so don’t expect this to be a detailed how-to-do-it piece.

GROUNDING

There are several different grounding schemes (i.e. chassis, signal and power to mention three). The choice is based on the purpose of the ground. In general, a hybrid single-point/multi-point grounding system should do the trick. This will provide a short multi-point ground reference for all equipment within a system while simultaneously reducing common mode currents.

Chassis ground serves as the reference for filters and transient suppressors, and is directly connected to the equipment/device frame. Signal and power grounds can be routed through the cable back to the single point ground.

Component bonding should be bare metal to metal with some form of conductive corrosion protection, and have less than 2.5 mohms resistance with an impedance of no more than 20 ohms at 30 MHz (≈100 nH inductance). The impedance requirement is to keep people from using long wires for grounding.

ISOLATION

Whenever possible, wiring should be categorized and segregated to isolate noisy circuits from sensitive circuits. Place like-function wires together and isolate from others (i.e., analog, digital, or power). This also holds true for PCBs. In that case, analog PCB sections should be physically separated from digital sections to prevent image-current from digital signals from wandering into the analog areas of the PCB.

If the physical layout allows, wires, cables, and traces should be routed over a continuous ground or power plane along their full length. This keeps the common mode loop area to a minimum. Long parallel runs should be avoided and incompatible categories should be run perpendicular . . . yeah right!

FILTERING

Filters are used to provide bandwidth limiting of interference. If needed, the filters should be placed as close as practical to the source of interference or to the susceptible component. If an enclosure is being used, filters should at least be used at the shield entry points to protect the integrity of the shield.

TRANSIENT SUPPRESSION

Transient suppression is used to provide amplitude limiting of interference. The transient should be suppressed at the source with voltage limiters to prevent spreading of the generated RF noise. Lightning, ESD, and EMP are all exceptions to this rule. For these we have to protect the sensitive circuits. Suppression can be done with silicon diodes, RC snubbers, or Pi filters. The choice is determined by the transient’s frequency and current.

SHIELDING

Shielding works! It can be used as a standalone solution BUT to be cost effective, good EMC design practices should be done first. The goal should be a minimum of 40 dB at 1 GHz unless it’s a military device. In this case, 80 dB would be better. The limiting factor on shield integrity is the number and size of required penetrations, including cables. Using shielded twisted wire pairs (STP) will significantly reduce coupling onto harnesses.

Apertures, including seams, should be made as small as possible, and all mating surfaces should be conductive. RF gaskets can help in this regard but they are often unnecessary.

Back in one of the early 1970s IEEE EMC Symposiums, EMC Engineer Bob Cowdell presented a paper that demonstrated that a conductive plated paint can lid embedded in the side of an enclosure was capable of providing >120dB SE with no fasteners and no RF gasket material! And, it’s watertight! An analysis of this configuration shows its action to be similar to that of a knife edge arrangement with a single row of fingers.

Connector shells are considered to be a part of the cable shielding and are electrically connected to the equipment/device enclosure. In general, cable shields are grounded at both ends with 360 degree shield terminations. In cases where a lot of high frequency noise exists on the cable shield, first try to eliminate the source, then use ferrites on the cable. If it’s a low frequency problem, an additional shield may have to be used or one end of the cable may have to be electrically floated using a capacitor. In all cases, avoid Pigtails.

Well, it was a lot longer than I planned but hopefully, this piece will help you in the new year. Have a Merry Christmas and may all the other holidays that you celebrate at this time of the year be great too.

- Ron Brewer

CM Feeling Bad, Check its Pulse

2012-12-12T06:14:00.001-08:00

The last post examined CM radiation. Wow, what a problem! Over 100,000 microvolts/meter from a 3.5 ma signal. If the DM is normally the intended signal, where does the big common mode signal come from? Let’s take a look at that. The whole world seems to be enamored with high speeds. I know I could hardly wait to get a 1 GHz notebook. Did it help me any? Probably not. I couldn’t type any faster on it than I could on the 300 MHz system I had previously. Now, I have a 2.4 GHz quad core system and it still doesn’t type any faster than I do.

In order to process the data at high speeds without jamming the local TV stations, most designers are using high speed balanced differential signaling. This technique uses complementary signals (ie +signal / -signal) where the transmitted pulses are equal but opposite in polarity. These pulses run on a wire/trace pair and terminate into a signal receiver that responds to the difference in the transmitted signals. It takes more time to change a signal from a low voltage level to a high level; hence, the voltage swing is kept to a minimum. By maintaining the balance and keeping the signal level and loop area small, the electromagnetic fields from the complementary signals cancel each other out. This keeps the radiated emissions levels down. Plus, by using a balanced configuration, any external fields will couple equally into both signal leads and will appear as common mode and thereby be rejected by the receivers. Unfortunately nothing is perfect (just ask Murphy) and thus, signal rise and fall time variations, timing skew, differences in amplitude and pulse width all result in creating common mode currents in the ground and power systems.

Although the effects of dissimilar rise and fall times are not included in Figure 1, it does show some of the CM issues. From left to right: (1) perfect with no CM created, (2) dissimilar pulse width, (3) dissimilar amplitudes, (4) skew, (5) dissimilar amplitude and skew and (6) dissimilar amplitude and pulse width.

Figure 1 – Creation of CM noise.

To paraphrase Thomas Paine: these are the things that try men’s souls. At least the ones doing intra-systems EMC design, also known as signal integrity (by the digital crowd), where timing, timing, and timing are the three most important elements.

Low voltage differential signaling (LVDS) is good . . . way better than singled ended where everything, especially the noise, is referenced to ground. However, it’s not perfect. The great thing about LVDS signaling is that it reduces both the radiated emission as well as radiated susceptibility. Much of the radiated RF coupling into a system from an external source affects all conductors simultaneously, creating a CM signal with respect to ground. A lightning strike is a good example. An LVDS receiver is non-responsive to CM noise because it is expecting a complementary differential signal pair; with CM, the signals are not complementary.

If your common mode emission is bad, check balance, check termination impedance (more on that later), and check complementary pulse symmetry. They all go hand in hand.

And The CM Currents Go Round and Round

2012-12-10T06:15:00.000-08:00

When we were discussing green electron current (differential mode) in the last post, I mentioned that common mode (CM) currents (red electrons) are typically caused by unbalanced differential mode (DM) currents. It’s pretty easy to visualize that in an unbalanced differential transmission system, one of the traces is longer, has an extra via, needs a branch, the spacing between +signal and -signal had to be changed to get around a component or something else. This also happens in a single ended system. The signal leaves the source, goes to the load and, from the load, returns back to the source. The impedances of the signal path and the return path are not completely equal because of differences in path length and capacitive coupling to ground.

If both the source and load are hard grounded (such as to the safety ground) as shown in Figure 1, then an alternative return path is established through some mysterious parasitic coupling inductance. If they are not hard grounded, then the alternative return path is through some equally mysterious parasitic coupling capacitance. With the inductance, the CM impedance is increasing with frequency while with capacitance, the CM impedance is decreasing with frequency. This small amount of CM current flowing through a very large loop area creates a potential difference between the source and load.

In the post titled, “Radiated emission from green electron current”, I indicated that a simplified radiated emission model for the DM case could be developed by assuming that each wire of length L behaves as a small dipole, the two dipoles are out of phase (green current) and the two dipoles are separated by spacing (S). Because they are out of phase, the closer the spacing, the more field cancellation results. A similar configuration exists for the CM (red) current, except that the CM currents in the two dipoles are in phase. Because they are in phase, the closer the spacing, the more field enhancement occurs. It just looks like a single bigger wire.

The following relationship for the CM gives the electric field (E) at a distance (d).

E = 1.257x10⁴ L I F / d

Where:

E = Field strength in microvolts/meter

L = Length of wire in cm

I = Current in Amps

F = Frequency in MHz

d = Distance to observation point in meters; usually the measurement distance.

Using the same example that was used in the previous post for the DM, let’s see what happens now with the CM. Assume that a replaceable electronic module for a space application has a 0.5 meter (20 inch) jumper cable with a 1 mm wire pair spacing that interconnects two subsystems. This cable is carrying a 3.5 ma data signal running at a frequency of 50 MHz. Will it meet the MIL-STD-461F RE102 requirement?

Calculating the electric field E at 1 meter gives:

E = 1.257x10⁴ * 50 * 3.5 x 10^-3 * 50 / 1 = 109,988 microvolts/meter

Converting to dBuV/m gives:

dBuV/m = 20 log 109,988 = 101 dBuV/m. The MIL-STD-461F spec limit at 50 MHz is 24 dBuV/m!

With the DM the level was 36 dBuV/m, which was out of spec by approx 11 dB. The CM from the same signal is out of spec by 77 dB . . . 66 dB higher.

With the DM, we only thought we had a problem. Now, with CM, we know we do.

- Ron Brewer

Mom! Dad! Do Open & Short Circuits Really Exist?

2012-10-17T06:38:00.001-07:00

I debated whether I should introduce radiated pickup by the DM loop area next and then dismissed it so that I could continue with radiated emission from common mode (CM) currents. These tend to be very mysterious because they are typically caused by unbalanced differential mode (DM) currents. Often these are caused by a few extra grounds floating around somewhere in the circuit. As a result I decided to do neither and discuss, instead, how the enigmic grounds get there. It was going to be a brief discussion, but there are a couple of things that we take for granted that at RF do not exist. These are (1) the open circuit and (2) the short(ed) circuit.

A short across the ends of two conductors or from one conductor to ground simply connects them together increasing the total conductor length. Also, the short may cause DC and low frequency resistance to suddenly decrease, resulting in a significant increase in current. At RF, it’s a completely different story. There may not have been any change at all. Conductors have inductance. which means that any changing current in the conductor will induce a voltage in the conductor that is related to how fast the current changes.

We know the law as V= L di/dt. The symbol L for inductance was chosen to honor an early physicist named Heinrich Lenz, but if you ask any budding young science student what L stands for, they will probably answer L stands for length. Good answer, because inductance is so dependent on conductor length that almost all other dimensional characteristics pall by comparison.

Whatever inductance the conductors had before the short, they still have after the short; so as the frequency increases (di/dt increasing), so does the voltage developed across the conductor. Shorting the circuit to ground to eliminate a high frequency RF problem probably will not work, especially with long conductor lengths because the conductor impedance is Z = R + jωL and L (inductance) is highly dependent on L (length). Additionally, at any given frequency when the length L = λ/4, the conductor looks like an open circuit, NOT a short. This is the primary reason that the length of bonding straps and grounding systems is restricted to L = λ/20 at the highest frequency of interest. If it’s “gonna” work it’s “gotta” be short!

The open circuit is the inverse of the short circuit. In this case, two conductors are separated by a dielectric, probably air; but it could be the wiring insulation, the PCB material, or any other non-conductor . . . so instead of having an inductor, we have a capacitor. At DC the open circuit is truly open; but as the frequency increases, the impedance Z= 1/ jωC becomes smaller and smaller until the two conductors may as well be connected together.

How bad is it? Suppose we have two 0.1 inches wide flat conductors crossing each other at 90 degrees with a 10 mil (0.00025 m) spacing. For reference, 0.1 inches is the diameter of number 10 AWG wire. An inch is approximately 1/40th of a meter, so the projected capacitive coupling area A = (1/400)²= 0.00000625 sq meters. It’s not a very big area, but it still forms a measurable air dielectric capacitor:

C = ε A /d = (8.84pF/m) (0.00000625)/(0.00025) = 0.221 pF

- Ron Brewer

Upcoming Revision to the Medical Standard

2012-10-15T08:50:00.001-07:00

We asserted last time that the existing amplifiers in the current test set up might be used to generate the higher stress levels if the test distance was reduced from 3 meters to 1 meter. As a reminder, we argued that testing at 1 meter means testing takes place in the near field as is the case for the threats faced by medical equipment (nearby cellphones etc.). Using the same amplifiers would significantly lower the investment (by about half a million dollars) required by the test house

Note that the medical manufacturer still has to face the fact that his equipment needs to pass at the higher stress levels, so no gain for him other than he does not have to indirectly pay for the test manager’s half a million dollar investment in new amplifiers.

Now, it is true that there are issues with 1 meter testing that offset the power savings in military and automotive testing. The key issue with these two types of test is cross-coupling between the antenna and the ground plane. The ground plane is a raised metal sheet grounded through a low impedance path. The cross-coupling reduces the effective gain of the antenna, so you need more amplifier power. However, we are not advocating the use of a ground plane, so this particular issue goes away.

In the next blog I will make the case that the test waveforms should truly represent the waveforms emitted by nearby threats. This will, of course, require special test instruments and this will offset the savings made by not replacing the amplifiers

-Thomas Mullineaux

Radiated Emission from Green Electron Current

2012-10-04T07:35:00.001-07:00

Well . . . I’m back. I hadn’t planned on being out of print so long and it will take a few paragraphs to bring everyone up to date.

A number of you learned at the IEEE EMC Symposium in Pittsburgh that my family and I were taking our little travel trailer on an extended camping trip immediately after the symposium.

There were several National Parks that we have wanted to visit but they have always been too far out of the way. Our goal on this trip was Mount Rushmore - South Dakota, Glacier National Park - Montana, and Mesa Verde – Colorado . . . filling in with others (some repeats) such as Henry Ford Museum/Greenfield Village, Yellow Stone, Bryce Canyon, the National Boy Scout Museum, etc., along the way.

We also ate in a number of out-of-the-way places listed in the Road Food Guides. Many of these are on historic US-66 in little towns that look a lot like Radiator Springs from the movie Cars staring Lightning McQueen. This move is based on Route 66 (where you get your kicks) and an internet search will reveal where the various places in the movie are located along the route.

This was a fun filled 6 week trip that that started August 12^th right after the EMC Symposium and ended on September 23^rd. During that time we traveled 12,289 miles through 33 states. Our biggest expense category was $5,100 for gasoline – such a shame! BTW: If any of you are thinking about doing your own Great American Road Trip and want to send me an email for an opinion feel free to do so. Note however, if there turns out to be a lot of emails I may not be able to answer each of them individually.

When we started we learned almost immediately that in the National Parks the radiated emissions are low and that the World Wide Web really isn’t worldwide. It certainly hasn’t reached most of the National Park System. This disappointed my 11 year old grandson who thought he would play games on the internet instead of hiking. I was surprised to learn that in some park areas we had to travel as much as 30 miles outside the park just to be able to use our cell phones. From this you could almost conclude that it must have been really great not having to worry about EMC problems. That was not the case. Sometimes there’s a problem with conducted emission and susceptibility.

We learned the hard way that if someone else with a big 220 volt RV system having much greater electrical demand makes a mistake and tries to share your wimpy little electrical box they can create a DM voltage transient that may destroy your DC electrical converter, your radio, and your computer. Fortunately I have a second computer. Unfortunately it was at home in Florida and I was in North Dakota. .

I’d like to continue on my soap box about dastardly DM currents and their conducted issues. We need them but oh what a pain they can be. Often the differential mode (DM) currents are functional currents and our widget won’t work without them -- we just don’t want the associated conducted or radiated emissions.

The good news is that conducted emissions are confined to the conductor so that even if we are unable to reduce the driving source levels during design we can get a big hammer and beat them into submission. Brute force filtering is certainly not elegant but it does work. Probably if I had had a great big filter on the power input to my travel trailer I would have already published this material. I can tell you that I will have transient suppression before I take it out again!

The primary problem with DM current is RF radiation. In Figure 1, the green electrons flowing around the loop with area (A) = L * S represent the DM current. If we make the assumption that the current is the same in all four segments of the loop, the greatest levels of radiation in this figure will be from the horizontal segments because they are the longest. The constant current assumption is reasonable so long as the loops perimeter is electrically small. Once the loop size versus frequency allows standing waves to develop this assumption no longer holds.

The vertical segments are cross polarized and thus do not contribute to the radiation from the horizontal segments and vice versa. The horizontal green electrons are flowing in opposite directions and if they could be superimposed their radiated fields would cancel. But that can’t happen. The closest that we can bring the wires together (smallest S value) is twice the insulation thickness. This results in creating a loop antenna with area A = S * L having a null along a plane passed through the centerline of the broadside of the loop and maximums on either side in the plane of the loop itself. As the spacing S increases there is less cancellation and the radiated field strength increases. Restating this more bluntly: Keep high frequency, high speed, high current loops as small as possible!!!

A simplified radiated emission model for the DM case can be developed by assuming that each wire of length L behaves as a small dipole, the two dipoles are out of phase, and they are separated by spacing (S). This gives the following relationship for the electric field (E) at a distance (d).

E = 1.316 A I F²/ d

Where:

E = Field strength in microvolts/meter

A = L * S = loop area in sq cm

I = Current in Amps

F = Frequency in MHz

d = Distance to observation point in meters. Usually the measurement distance.

Let’s try it out for size, just to see how it works. Assume that a replaceable electronic module for a space application has a 0.5 meter (20 inch) jumper cable with a 1 mm wire pair spacing that interconnects two subsystems. This cable is carrying a 3.5 ma data signal running at a frequency of 50 MHz. Will it meet the MIL-STD-461F RE102 requirement?

Calculating the electric field E at 1 meter gives:

E = 1.316 * 0.5 * 100 * 0.1 * 3.5 x10^-3 * 50²/ 1 = 57.6 microvolts/meter

Converting to dBuV/m gives:

dBuV/m = 20 log 57.6 = 35 dBuV/m. The MIL-STD-461F spec limit at 50 MHz is 24 dBuV/m!

Houston -- we have a problem!

- Ron Brewer

Comment from Blog Reader Plus Test Field Calculations

2012-08-14T11:26:00.001-07:00

Before launching into calculating the power required to create the test fields proposed by the upcoming medical standard, we have a comment from a blog reader on the realistic threats faced by electronic equipment….

Comment from blog reader:

I think the real question here is valid for all testing. Do we test according a repeatable verifiable and implementable and above all safe and secure way or do we test an EUT as in real life conditions? f.i. all signals can be present at one time blue tooth, GSM, WiFi radio communication and emitting at various levels and intervals creating different harmonics in time and even combined harmonics.And this all within a proximity of less than 10cm of the EUT. How can you test this kind of situation?

Reply:

Dear reader, thank you for your interesting comments.

I believe the power levels of bluetooth and WiFi are limited by the FCC and equivalent authorities to a few hundred milliwatts, so I don’t see these as a significant threat. Also, the drive in medical equipment today is wireless connectivity, so modern equipment will be immune to signal levels from its integrated antenna. The power level of a cell phone is 2000 milliwatts, significantly higher than WLAN levels. However I see your point on multiple simultaneous threats. My thoughts on this are that though not ideal, testing one threat at a time seems to work. For instance, military aircraft are subjected to high test fields a single frequency at a time, and these aircraft tend not to fall from the sky. An argument I shall be making later on, is that the tests should check susceptibility to real cell phone signals, not susceptibility to simple amplitude modulated signals that in no way represent the real threat waveform.

Aug 12, 2012 Post

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The basic formula for calculating the field strength at a distance d from the antenna is:

Field strength in volts per meter

E = sqrt (30*G*P) / d

Where:
G is the linear gain of the antenna at the frequency of interest
P (in watts) is the RF power entering the antenna connector
30 is the constant of proportionality (actually 120pi/4pi, let me know if you want to know why)
d is the distance from the antenna in meters

Rearranging so we can establish the power required at the antenna connector to create a specific field strength gives:

P = (d.E)^2 / 30.G

A reasonable assumption for say a frequency of 80MHz is an antenna gain of 5dBi (linear gain of 3.2), so for a field strength of 20 v/m (36v/m modulation peak) at 3m, the power required at the antenna connector is:

P = (3*36)^2 / 30*3.2

P = 11,664/96

P = 121.5 Watts

This is optimistic as it does not include overhead for systems losses, including cable / switch losses, coupling between the antenna and the room, an allowance for field uniformity, etc. A sensible overhead is 6dB, giving the power required at the amplifier output connector at approximately 500W.

If the test distance is reduced to 1m, the power required drops to:

P = (1*36)^2 / 30*3.2

P = 1,296/96

P = 13.5 Watts

Adding the system loss overhead of 6dB the power required from the amplifier is 54W.That is the required amplifier power is reduced nine-fold, raising the possibility that the existing amplifiers could be used to generate 20v/m (36v/m modulation peak) at 1m test distance

More of the same in the next blog

- Tom Mullineaux

What is Differential and Common Mode Current?

2012-08-02T07:41:00.001-07:00

Answering the title question will be a hot topic at the IEEE 2012 EMC Symposium in Pittsburgh. Hopefully many of you will have the chance to attend it this year. There are always some really great technical papers presented at this show – some more practical than others. If you’re there look me up. I’ll be spending some time at the Interference Technology booth, #817.

The unwanted conductive and radiative emissions can be caused by differential or common mode currents. Both of these are illustrated in Figure 1.

To have continuous direct current (DC) flow requires a conducting loop. Otherwise with charge separation on a open ended conductor, current flow will stop when the potential developed across the separated charge reaches equilibrium with the DC voltage that’s causing charge separation. Since this takes place at the speed-of-light in the conductor (modified by reactance), it does not take long to reach equilibrium! What happens is a transient occurs at turn on with equal but opposite current flowing in the outgoing and return lead; and when equilibrium is reached, it just sits there. If the source is alternating current (AC) and the isolated path is open ended, capacitive coupling between the outgoing and return conductors completes the loop and displacement current flows through the capacitance and returns to the source. In a similar fashion, if the AC source is terminated into a DC conductive load, the outgoing and return current will be equal but flowing in opposite directions. These three cases illustrate Differential Mode (DM) current which is designated in Green above.. The telecommunications industry calls this normal mode because this is what normally happens – Ha! Normal to the wire.

If the conducting loop is not isolated but is located in conjunction with other circuits on a PCB located within a cable bundle or run over a ground plane, differential voltages developed between the circuits and alternative ground (typically because of radiation coupling or unbalanced differential circuits) will return to their source. This may not be the circuits intended reference and it may involve a number of simultaneous conductors. In-phase currents flowing in the same direction on multiple conductors with respect to another reference is known as Common Mode (CM). This is designated in Red. Because the current is traveling in the same direction along the wires, the telecommunications industry calls this longitudinal mode.

When capacitive coupling completes either the CM or DM loops, the loop current will primarily be a function of frequency. At low frequencies the capacitive reactance (Xc = 1/ j ω C) and associated loop impedance will be so high that very little displacement current will flow. However, as the frequency increases or the conductor lengths increase, the capacitive reactance decreases with a corresponding increase in current. For example, at 10 kHz a 1000pF capacitance has an Xc=15, 923 ohms, but at 1 GHz it has only 159 milliohms. The higher current brings with it an increase in RF radiation. Even though that is true, until the circuit dimensions (d) approach the resonant lengths at the radiation frequencies (λo/10 As a result, the radiation is small compared to the energy in the circuit. A really great discussion of CM and DM is found in Introduction to EMC by Dr. Clayton Paul.

Even though the common mode current is generally much smaller than the differential mode, its loop area is so much larger than the DM that the CM often dominates. Since the CM may be simultaneously on multiple circuits and on the ground reference, it is difficult to cure. When doing design, remember Ben Franklin’s old saying: “An ounce of prevention is worth a pound of cure!”

- Ron Brewer

Ron Brewer will be at the Interference Technology booth #817 on August 7, at 2:00.

Unwanted Conductive and Radiative Emissions

2012-07-27T09:52:00.000-07:00

The last post indicated that RF signal energy coupling from the emitter to the receptor can be by conduction or by radiation and that we would discuss that later. Well it’s later!

The two coupling processes are very closely linked. Radiative coupling always starts out as a conductive signal and ends up as a conductive signal. In between and especially at the higher frequencies, it manages to get launched as an electromagnetic wave. To some extent that’s too bad, because we then have to switch from circuit theory where we can model the behavior of currents in a conductor using Ohms Law and PSPICE over to Maxwell’s Equations and Field Solvers. Conceptually it’s not too difficult but mathematically it becomes a complex problem. For those who are worried that I may suddenly turn vicious and snow everyone with a lot of vector calculus, don’t worry, be happy! I’m not going to do that, but I will pass along a suggestion for those that are thinking about a vector calculus tune up.

Between Antennas, EM Fields and Waves, and Fluid Dynamics, I learned a lot of vector calculus, but if it isn’t used from time to time, brain rust forms, and the corrosion needs to be removed. In the mid 70’s I ran into a little book titled: Div, Grad, Curl, and All That: An Informal Text on Vector Calculus, by Dr. Harry M. Schey. It’s a great rust remover for vector calculus, and Dr. Schey uses Maxwell’s equations as the thread to hold it together. It’s paperback and I’ve seen it on the WWW priced anywhere from $0.99 to as high as $240.00. List at Amazon is $35.00. Try it – you may like it and, if not, it can always be used to treat insomnia.

Back to coupling . . . not much coupling takes place in a fixed geometry unless the electron current is changing. Hook up a DC supply to a resistor (or any other passive device)—turn it on, and once the initial turn-on transients have died out, any magnetic or electric field created by the steady electron flow never changes. To get change an active device is required, such as a vacuum tube, transistor, logic gate, or other widget capable of controlling electron flow. Depending on the device, electron flow (current) control can be accomplished by a small variable voltage or current. As a result, active devices can provide gain.

A transistor amplifier is an active device configuration that is the basis for many other configurations. Depending on its bias, it can be an amplifier; or over drive it and make a switch. Hook two together and make a multivibrator. Use multiple inputs and get a gate. Provide in-phase feedback and make an oscillator. All of these operations require controlling currents!

The operation of active devices causes changes in the demand current of their power supply, which creates conducted emissions. Since the power supply has a non-zero source impedance (Zs), the changing demand current (I) produces a voltage (V) on the power supply leads. This voltage is equal to V = I * Zs, and the voltage varies with frequency because Zs = R + j ω L is not a constant.

Every circuit that carries alternating RF current will radiate some RF energy. If the circuit dimensions are small with respect to a wavelength, the energy level will be low; but as the relative dimensions change either by increasing the conductor length or increasing the frequency, the radiation efficiency increases.

Before I sign off I want to mention that August 5 –10 is the IEEE 2012 EMC Symposium (Pittsburgh) show time. I’ve been attending the EMC Symposium since the infamous Asbury Park, NJ show in1969 and I’ve missed 4. One can accumulate a lot of Blog material during that period of time. I’ll be spending some time at the Interference Technology booth and also wandering around in the dark. EMC engineers are used to doing that! To paraphrase Mae West: “come [over] and see me sometime.”

-- Ron Brewer

Ron Brewer will be at the Interference Technology booth #817 on August 7, at 2:00.

The EMC Community Perspective

2012-07-23T06:46:00.000-07:00

As indicated in a previous blog post, it seems hard to justify the rationale behind testing medical equipment for RF immunity at 3m distance. It is this greater test distance (as opposed to the 1m test distance accepted in automotive and MIL-STD testing) that is driving the high price of the replacement amplifiers. A change from the current test field strength of 10v/m @ 3m to 20v/m @3m requires four times the power (basically equates to four times the amplifier price).

It is hard to imagine a culprit radiator that emits fields of 20v/m (or even 30v/m) into a hospital ward or a local clinic from 3m or more away. It is true enough that the airwaves are full of signals from WLAN, wifi, bluetooth, etc. But these are all low power emitters.

The real culprit and hence the real threat is the ubiquitous cell phone in close proximity (placed on top of the medical equipment). Being at such a short distance away, this means by definition the medical equipment is within the near field of the threat emitter. So why test against this threat with a source 3m away?

By the way, when does a cell phone emit the greatest field?

Answer – at switch on. The power is at maximum while the phone identifies all available base stations in its vicinity. The power is then turned down to a level conducive with the distance to the selected base station.

In the next posting we will do the engineering calculations permitting a valid comparison between testing at 3m and 1m.

-Thomas Mullineaux

Do I really need that signal?

2012-07-17T09:36:00.000-07:00

We’ve been discussing signals from the perspective of their bandwidth relative to the bandwidth of the receptor and have not considered them from the EMC point of view. RF signal energy can get from the emitter to the receptor by conduction or by radiation. We’ll discuss that later. Regardless of the energy transfer process, using the FCC classification scheme we can divide RF signals into three categories: (1) intentional, (2) unintentional, and (3) incidental.

Intentional emitters deliberately generate RF energy with the specific intent of transferring it to a receptor located some distance from the emitter. Looking at radiated emissions, which are easy to visualize and are also the most EMC problematic, Bluetooth, 802.11 Wi-Fi, cellular telephones, and garage door openers are common examples of intentional RF signals. The devices that produce these signals must be FCC certified and they are required to operate within specific frequency ranges and RF power levels. In general, the signals from these devices are narrowband.

Unintentional emitters deliberately generate RF energy, but only for internal use. That doesn’t mean that it always stays bottled up inside the box, but it’s supposed to. The fact that it doesn’t mind well is why we often hear that EMC means Easy Money for Consultants. Anyway computer clocks and receiver local oscillators are good examples of unintentional RF signals. The clocks are impulsive broadband signals and the local oscillators are narrowband signals.

Incidental emitters do not deliberately generate RF energy. Unfortunately every time there is a rapid voltage, current, or impedance change, electrons are accelerated and RF energy is created. Examples of these emitters are fluorescent lights, electric razors, ignition systems, thermostats, etc. Most of the time incidental emitters generate broadband emissions.

Overall it is a lot easier to design for and suppress narrow band signals. However, for intentional and unintentional emitters, suppression of the signals may prevent the system from operating properly. With incidental emitters, those unwanted signals aren’t required for the operation of the device so we can work diligently to eliminate them. Not needing the signals makes a big difference in the level of attenuation that can be applied. Always ask, do I need that signal for the operation of my device? If yes, then find out if it needs to be that strong, switch that fast, or operate at such a high frequency?

The system must meet its appropriate EMC requirements, or it can’t be sold. In fact, there are special restrictions placed on unapproved new units that must be met or they can’t even be shown as soon to be available. Even an approved device has to be able to accept interference from intentional, unintentional, and incidental emitters and cannot cause interference to licensed broadcast services.

This may sound funny, but unless the device is being used by the DOD or being shipped out of the country, it can interfere with other things as long as it’s not interfering with broadcasts or is a health/safety hazard. Even then, I would not recommend ignoring an interference problem, because the user won’t like being maligned by their neighbors. Check out the FCC rules in Title 47, CFR Section 15.5 General Conditions of Operation for more information about interference from incidental emissions.

- Ron Brewer

Abracadabra the Broadband Signals Disappeared

2012-07-02T10:33:00.001-07:00

Almost all EMC engineers these days are familiar with MIL-STD-461 and its various revisions because it has influenced the procedures in virtually all present day EMC standards. The original document and the A, B and C revisions all had requirements for measuring both broadband (BB) and narrowband (NB) emissions. When MIL-STD-461 was first released (July 31, 1967), the Empire Devices NF-105 receiver was the workhorse of the RFI/EMI/EMC industry, and the early standard releases show line-art diagrams of the receiver setting on the shielded enclosure ground plane. Yes, folks, we actually sat in the shielded enclosure when we did the measurements – turned the knob, recorded three frequencies per octave, repeated on and on until we were finally finished.

The NF-105 had a number of manually tuned plug-in units to enable coverage of the frequency range from 14kHz to 1GHz. Each of these tuning units had a single bandwidth. It might seem as if making a narrowband/broadband determination with only one bandwidth would be an impossible task. As I indicated in the previous post, a narrowband signal has zero bandwidth (or nearly so) whereas a broadband signal’s spectral occupancy extends beyond the bandwidth of the receiver. Armed with this knowledge all that needs to be done is to apply the Tuning Test. Just tune up and down by one receiver impulse bandwidth. If there is essentially no change in the measured amplitude, the signal is BB; otherwise it is NB. This is great if the tuning is being done manually but does not lend itself to being easily automated.

If the measurement receiver has multiple bandwidths, the BB/NB determination is really easy. Tune in the signal, then change the bandwidth, and if there is no change in measured amplitude or it’s less than or equal to the NB bandwidth criteria (usually either 3 or 6 dB), the signal is NB; otherwise it’s BB. Select the appropriate bandwidth and abracadabra all broadband signals disappear. They can then all be called narrowband. The same thing holds for coherent broadband. If the bandwidth is made small enough to encompass a single harmonic, then the signals are narrowband.

If you are still using a specification that requires measurement of broadband signals, there is a problem determining whether an intermittent signal that appears to be CW is really a NB signal or a transient, which is a BB signal. The only way to know for sure with a single measurement is to simultaneously measure its peak and average levels or analyze its modulation characteristics if it has any. The peak vs. average test can also be used for those weird signals that seem to defy categorization, provided they exist long enough to apply the test. A BB signal will have a significant change in amplitude when switching from Peak to Average while a NB signal will not.

Those that live in front of a spectrum analyzer will be annoyed that I didn’t discuss the sweep time test. It’s great if you have repetitive pulses, but it isn’t worth a hoot for determining randomly occurring NB emissions or pulses. That holds for most of the tests. Besides that, the displayed amplitude and frequency on many spectrum analyzers changes with a change in sweep time.

That’s one of the many reasons why MIL-STD-461D gave up measuring broadband signals! All current revisions now specify the receiver response characteristics and demand that the sweeps be slow enough to capture any signals that may repeat. If measurements are made from DC to daylight in a nanosecond or two, all EUT’s will pass! That’s also why many of the spectrum analyzers (or the way they are used) aren’t fully compliant with MIL-STD-461.

These problems and the opportunity for signal detection manipulation were first described by a pioneer EMC engineer named William (Bill) Cipperly in an article titled “EMC Engineer, Scientist or Witch Doctor?” that appeared in Frequency Technology magazine circa 1968.

- Ron Brewer

Another Look at Broadband Bandwidth Bandwidths

2012-06-20T09:02:00.001-07:00

The last comment I made was regarding those perfect rectangular filters that we always describe with such reverence. But as I said, they don’t exist. Taking a receiver as an example, the filter amplitude versus frequency characteristics are measured by sweeping a narrowband signal through the passband frequency range of the circuit. Typically the passband width is given at the – 6 dB and – 60 dB points, and a prefect rectangular filer would have the same bandwidth at both points. This filter would define a 1:1 shape factor. A close examination of the amplitude versus frequency data on typical filters will show that different circuit types have different shape factors.

Even though the 6 dB (or 3 dB) narrowband bandwidths of various circuits could all be equal, their circuit response shape might be square, trapezoidal, triangular, cosine, Gaussian, or whatever; and the shape determines the broadband response. This makes the broadband bandwidth (also known as the impulse bandwidth) wider than the narrowband bandwidth. A lot of filters are Gaussian-like and a good rule of thumb for a Gaussian filter is BB BW ≈ 1.5 * NB BW. Expressed in dB this is approximately the – 7dB bandwidth. There are a number of ways to measure the impulse bandwidth, and either ANSI/IEEE Std 376-1975 or SAE ARP 1267 can be used. I feel that the procedure from ARP 1267 is easier to read. As an alternative, just plug your trusty calibrated impulse generator into the front end—through an attenuator, of course—and measure it directly. This provides the factor necessary to correct for the broadband signal energy filling the entire passband of the filter.

While I’m on my high horse about bandwidths, let me tell you about one of my peeves. I have many. This one is not one of my pets, but it does annoy me never the less. The EMC community frequently uses graphical Fourier analysis to convert signals from the time domain to the frequency domain. The approach is fast and easy. First, we make a simplifying assumption that the pulse is trapezoidal with amplitude A, pulse width D, and rise-time tr. Then, we translate that to an amplitude versus frequency envelope in the frequency domain, stating that the amplitude will be no greater than 2AD from DC to F1= 1/(pi D), decrease at a 20 dB/decade rate from F1= 1/(pi D) to F2= 1/(pi tr), and then decrease at a 40 dB/decade rate above F2= 1/(pi tr).

The frequency F2 defines the bandwidth (BW) of the trapezoidal pulse which is related to the rise-time giving BW = 0.318 / tr. I have often made that statement and, then, had someone tell me: “Oh no! That’s not the BW! The BW = 0.35 / tr.” I admit that this relationship is a very popular definition of pulse bandwidth; but in a real circuit, the broadband BW is a function of the filter shape (read that as Q) and the 0.35 / tr relationship only applies to a 1^st order RC low pass filter. Here’s a table that provides a comparison of the step rise-time bandwidth of different circuit types.

Pure trapezoidal wave* BW = 1 / (π * t_r) ≈ 0.318 / t_r

Pure Gaussian BW ≈ 0.332 / tr

10- 90% Gaussian BW ≈ 0.34 / tr

RC LPF (1^st order) BW ≈ 0.35 / tr

Bessel-Thompson BW ≈ 0.36 / tr

Butterworth BW ≈ 0.49 / tr

Chebychev BW ≈ 0.60 / tr

* This is a mathematical construct and not related to circuit characteristics

Part II: The Impact of the Upcoming Medical RF Immunity Revision

2012-06-13T10:52:00.004-07:00

The test-house manager’s perspective continued:

At well over a quarter of a million dollars just to replace the existing 26-1000MHz amplifier and the same again to cover 1-6GHz at 200W (will take two amplifiers), the test manager will need to justify half a million dollars for the amplifiers alone. And the total will likely rise to around $600,000 or more with other equipment purchases plus the fee to the system integrator.

The $600,000 will need to be paid for through increased revenue. If we assume a target break-even time of 36 months, the test manager will need to see additional revenue of approximately $17,000 per month.

The medical equipment manufacturer’s perspective:

The medical equipment manufacturer (and ultimately medical patients) will have to foot this bill. So let’s turn our attention to the medical equipment manufacturer’s point of view. A little thought on the matter highlights that it is not just a case of a potential increase in rates per test-day, but more likely, the greatest increased cost will come from the fact that it will be harder to get the product through immunity testing. For instance, products that comply to the current revision of the standard are extremely unlikely to pass the higher stress levels and possible extended frequency. The increased costs include the repeat tests after failure and the extra engineering involved in the fixes. It is quite probable that new products will have launch delays that could stretch out to months of expensive delays.

Click here to read part I.

-Tom Mullineaux

RF Engineer

Presenter

Freelance Author

Random or Coherent Signals

2012-06-06T10:30:00.000-07:00

In the previous post, I indicated that coupled Broadband energy levels are determined by whether the signals are random or coherent. Random/non-coherent signals originate from sources that are not time related. A couple of good examples would be receiver thermal noise and DC motor brush noise. Another would be the RF environment taken as a whole. Because the individual energy components are not on at the same time, they do not combine directly and the total increase in energy as a function of bandwidth is Δe dB = 10 log BW2 / BW1.

Coherent signals on the other hand are time related. This type of noise is frequently called impulsive noise and can be a single impulse like ESD, lightning, or repetitive like a computer clock. The primary difference between single and repetitive signals is that a single transient signal has a continuous RF spectrum. No matter what frequency the receptor circuit responds to, RF energy from the single transient exists at that frequency. The spectral amplitude may be so small that the circuit does not react, but the energy was there for the duration of the transient. For coherent energy, the increase in captured energy levels as a function of bandwidth is Δe dB = 20 log BW2 / BW1. This makes the receptor more susceptible to transients as its bandwidth increases.

For the repetitive transients, energy exists at discrete narrowband frequencies determined by the turn on repetition rate. For example, if a circuit is turned on at a 100 kHz rate, it will produce narrowband harmonic frequency components at 100 kHz intervals starting at 100 kHz. If turned on at a 1 GHz rate, the circuit will produce narrowband harmonic frequency components at 1 GHz intervals starting at 1 GHz. For a 50% duty cycle trapezoidal waveform, the odd harmonics will have the greatest amplitude. If it were possible to generate a perfect square-wave, there would be no even harmonics. That’s not possible because the rise-time must always be greater than zero.

Yes, the signals are coherent, but are they broadband? That all depends on the bandwidth of the receptor. For our 100 kHz example above, if the measurement were made with a 10 kHz receiver (with a perfect rectangular passband) and the bandwidth was increased to 1 MHz, more harmonics would be captured and the increase would be: Δe dB = 20 log 1MHz /10 kHz, or 40 dB. For the 1 GHz example if we were measuring with a 10 kHz band width and changed to 1 MHz or even 1 GHz, no additional harmonics would be captured.

The significance of the comment above regarding a perfect rectangular passband is that no perfectly rectangular filters exist . . . although mechanical, ceramic, and especially the newest digital ones are getting close. Next time we will take another look at broadband bandwidth.

-Ron Brewer

Narrowband and Broadband Signals

2012-05-31T06:14:00.001-07:00

As I indicated in the previous post, this discussion begins examining signal characteristics from an electromagnetic compatibility (EMC) perspective. But first, a few words to describe EMC. This is a relationship issue. We want all the equipment items to get along in peace and in harmony, but if one of them is a big source and the other is a wimpy receiver, they may not play together very well. The EMC community describes this arrangement as a source and a sink or an emitter (E) and a receptor (R), and this E-R pair occurs in any system or collection of equipments.

The emitter energy can be either intentional or unintentional depending on whether the emitter needs the energy for its operation or not. Here are examples of the three cases:

(1) If the E-R pair comprises an RF communications link, then the emitter energy being coupled would be intentional and desired.

(2) If the emissions from the above communications link were causing upset to some unrelated equipment, then the intentional energy being coupled would be undesired. Now, the intentional RF source has become RF interference.

(3) If the emitter was a computer and the receptor was part of the above communications link, then the coupled energy would be unintentional and undesired. Another source of interference! There are no ordinary cases where the coupling of unintentional energy is desired, but there are some in which it is exploited.

Signals come in two flavors, broadband (BB) and narrowband (NB), and, as the name implies, their spectral occupancy is significantly different. Theoretically a narrowband signal exists at only one frequency and thus has zero bandwidth. As a result, any circuit with a non-zero bandwidth will capture all of the NB energy, and increasing its bandwidth does not increase the energy that is captured. The closest we can come to that is a pure sine wave (also called CW). Unfortunately, in the real world it’s difficult to find a pure sine wave. Plus, it’s also difficult to convey a lot of information with it (other than On or Off), so it’s usually modulated, which increases its spectral occupancy.

In the early days of radio the CW (carrier) was modulated by switching it on and off using a telegraph key and radio telegraphy became known as CW. This is not the same as a continuous sine wave. Regardless, modulating a 30 MHz carrier with Morse code at 50 words per minute or even modulating a 150 MHz signal with speech (300 – 3300 Hz) does not spread the signal very much, so most non-digital radio equipment is considered narrowband.

Broadband signals occupy lots of spectrum. This also makes the interference from a broadband signal much more difficult to control. Theoretically a BB signal has a flat amplitude in the frequency range from DC to daylight. In our case a BB signal only needs to be reasonably flat over the bandwidth of the circuit that detects it. Unlike the narrowband signal, increasing the BB bandwidth will capture more of the broadband energy. How much more is determined by whether the BB signal is random or coherent?

-Ron Brewer

Impact of the Upcoming Medical RF Immunity Revision

2012-05-30T08:47:00.001-07:00

As indicated last time, there are two key parties with a vested interest in the outcome of the upcoming Medical RF immunity revision:

The test house manager who has to justify an upgrade to his test system to the people holding the purse strings.
His customer, the company wishing to sell medical equipment compliant with the revised standard.

I shall add a third party to this discussion:

The EMC community conversant with current test practices. This third party (you and I) will question the rationale behind the current upgrade route versus the actual threats faced by medical equipment.

But first, the test-house manager’s perspective:

The test-house manager needs to justify the expense involved in the upgrade before being granted the necessary funds. The argument justifying the outlay will be laid out in the form of a business plan. As a minimum, the business plan will comprise of:

Cost of the upgrade
Current and future demand for the new test capability
Predicted break-even point
Disruption to current test programs
Risk analysis

Ballpark Price of the Upgrade

The greatest expenditure by far (~90%) will be the cost of higher power RF amplifiers. So to get a rough idea of the size of the problem faced, let us see if we can estimate the cost of replacing the existing 26-1000MHz 200W amplifier. Note, we can add the cost of extending to 2.5GHz and 6GHz capability later, and it will also be fun to look at the more complex factors involved with a real life upgrade (e.g., antennas cannot handle the increased power, etc).

Let’s assume that a typical test set-up provides 10v/m@3m from 26MHz – 2.5GHz and uses a 200W amplifier for the 26-1000MHz portion of the band and a 50W amplifier for the 1000-2500MHz portion. Let us also assume that a new product standard based on the newly revised generic standard calls for 20v/m to 380MHz and 30v/m at select frequencies over 380MHz to 2.5GHz. What amplifier power level will be required for the 26-1000MHz part of the test band, and at what price?

It is a cruel fact in physics that doubling the field strength requires a fourfold increase in power, and worse still, tripling the field strength to 30v/m requires a ninefold increase in the power. So if the current power amplifier covering 26-1000MHz is rated at 200W, the new power amplifier will need to be of the order of 800W for 20v/m and 1800W for 30v/m.

A web search shows that a single amplifier covering 26-1000MHz at 800W is not available. This is bad news in that two amplifiers will be required to replace the single 200W amplifier. Compounding this is the fact that the amplifier covering the higher end of the band must be capable of creating 30v/m. However, this will be offset by the higher gain available from the antenna at frequencies above 380MHz.

A ballpark price for a 1000W amplifier covering 26-250MHz is $100,000.

Let us assume that compared to the gain at 26MHz, the antenna gain is twice as good at 380MHz – 1000MHz. This still means we need 4.5 times the existing power over 380MHz to 1000MHz, so basically we need 1000W.

A 1000W amplifier covering 250-1000MHz is $180,000.

At over a quarter of a million dollars just to replace the existing 200W amplifier covering 26-1000MHz, the test manager will need to come up with a very convincing case before being awarded the funds for the upgrade.

To me (and I daresay to you), this is commercial madness, so now is a good time to kick in with the point of view of the EMC community. The current test distance is 3 meters. A strong case can be made that the real threats to medical equipment will likely be from electronic equipment placed closer than this (for instance a visitor to the hospital room placing a smart phone on the equipment, etc). If the test distance is reduced to 1 meter, the 800W/1000W requirement above would be reduced to around 300W. Any detractors out there?

-Tom Mullineaux

RF Engineer

Presenter

Freelance Author

Are they out to get us?

2012-05-22T08:00:00.001-07:00

I didn’t realize when I addressed the smart meter RF situation that it was turning into such an uproar. Now, on top of the RADHAZ comments, people are expressing concern about protection of privacy. It seems that a lot of people have a difficult time believing in RF hazards because they can’t smell, feel, touch, taste, etc. the lower RF energy levels and, therefore, a hazard can’t possibly exist… but they are concerned about their privacy.

Not everyone could possibly have a grow room hidden in their basement. I know we don’t here in this part of Florida, because a basement would be an indoor pool with a very low ceiling. I’m at an elevation of 28 feet and the water table during the rainy season is about 5 feet below the grass.

Never the less, the power companies have failed to make it clear as to how they are going to protect the user’s privacy – let alone keep them safe.

People think that the power companies will be able to determine when they get up and turn on their coffee maker, when they take a shower (because the water heater element turns on), when they are cooking or baking with their electric range, watching TV, running the AC, etc. and, more importantly, when they are not at home. Could they do that? Most certainly! It all depends on how sensitive the meter and how often the residence gets interrogated. It’s a matter of granularity. Will they do that? I hope not, after all they only need to read the meter once a month to be able to send out a bill.

Can the hackers outside in their instrumented van also find out that information? Maybe. We know it’s possible, just read "Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?" Wim Van Eck, Computers & Security (1985).Whether the hackers can read it all depends on the signals that are being processed by the power company, the modulation characteristics, type of handshaking, and whether encryption is being used, etc. But we won’t discuss that last part because if we do, this blog will self-destruct in 15 seconds.

However, even if they could, why would they? Plus, is there any reason to be paranoid about it? Are you really that afraid someone will find out often you take a bath? For some interesting and entertaining discussions on the subject, do a Yahoo search on: “Are they out to get us?” From the results, it almost seems that everyone thinks something is out to get us.

I really want to start a discussion on signal characteristics from an EMC perspective and that’s where we are headed with the next post. However, I will comment on anything anyone wants to discuss that is RFI, EMI, EMC, ESD, etc. related. Just keep it unclassified!

-Ron Brewer

Just when you thought it was safe to turn off the light…

2012-05-16T07:54:00.001-07:00

Well, I’ve used my quota of words for the month of May so now I’m going to have to make the next several posts somewhat shorter. That’s what happens when you get on a really passionate topic. The last post was about smart electric meters and the un-smart way in which the power companies have tried to make their case. We all know that it falls under the heading of WIIFM . . . short for what’s-in-it-for-me, and that goes both ways. All kinds of rhetoric has been tossed around about how good smart meters are for the user (they allow you to make a 4-color graph of power usage one electron at a time) but a lot of people who are about to get smart meters are really worried about their safety.

Should we be concerned about RF safety? It’s pretty obvious that when you are lucky enough to trap a mean little gremlin in your microwave oven and press the GO button, you can create a big mess. Microwave 1, Gremlin 0. We can do the same thing by trying to heat a hardboiled egg. It’s possible to heat coffee, cook chicken, or warm up leftovers. It’s been known since at least the early 1960s that there are thermal effects associated with RF energy and that the higher the power, the greater the effect. It’s also known that RF energy changes the characteristics of the material being heated.

There are some athermal effects as well, and these can occur at low RF levels. Sunlight is a form of electromagnetic energy so it is not surprising that at some RF frequencies - other than light - plant growth rate increases. Wow! Cells multiply at an increased rate. Isn’t that somewhat like cancer?

Depending on the amplitude, frequency, and pulse repetition rate, RF energy can stimulate the optic nerve creating light flashes or humming/buzzing sounds when auditory nerves are stimulated. At around 400 V/m one begins to experience a sour lemony taste sensation. Been there, done that! We didn’t know any better. In the 1960s and early 1970s EMC engineers often performed RF radiated susceptibility tests with the test generators, amplifiers, antennas, and personnel in the enclosures along with the test sample.

The US DOD has a crowd dispersal weapon that causes a burning sensation on all exposed skin. A couple of weeks ago an article came out about a new Russian RF weapon that destroys parts of the central nervous system creating zombie like victims. This is really interesting because since the 1960s the Russians have had RF safety requirements that are three orders of magnitude less than the ones we use. Their list of RF reactions includes headaches, listlessness, memory loss, and indecisiveness.

Now, let me ask the question again. Should we be concerned about RF safety?

-Ron Brewer

Comments on RF Radiation Hazards

2012-05-09T13:02:00.000-07:00

This is only the second time around so you already know this blog is new, and I won’t have to remind you. Previously I made some comments about crowding and protecting the RF spectrum. It’s really difficult to protect it because there are organizations that feel a need to utilize RF communications to eliminate jobs and increase profitability. I once had a boss that told me that in the workplace of the future there would be two employees . . . a human and a dog. The dog is there to make sure that no one touches any of the automated instruments or equipment and the human is there to feed the dog. I don’t have a problem with profitability, but right now we really need companies creating jobs!

Some of you know that I live in Brevard County, FL, the home of the Kennedy Space Center. At the moment the residents are being dragged onto the smart power grid kicking and screaming. Like everything, there are both good and bad points to this endeavor; but for sure every smart electric meter contains an RF transmitter. A single family home will probably only have one meter; an apartment building will have one for each apartment; and these are usually collocated because power typically enters all apartments from the same location, consequently the fields will add.

If one of these electric meter’s RF transmitters creates a small signal with power level (P) and there are a number (N) of meters close together, the power levels for each will add together. If they are independent and therefore uncorrelated (which we can’t be sure of) the total power is: PT = P1 + P2 + . . . + PN. If all the meters are alike (i.e., P1 = P2 = . . . = PN), then the total power is PT = N x P1. If the power is correlated, the levels are even higher. Regardless, the residents of an eight (8) family apartment building that live near the power entrance panels will be receiving a minimum of 8 times the RF power that one meter would produce.

The multitudes that converge on the county commissioners meetings are armed with data indicating that RF levels from one smart meter exceeds the safe levels (RADHAZ) for continuous RF exposure. The power companies counter argument is that’s no problem because the levels from smart meters are no worse than cell phones. What does that mean? Cell phones have the ability to adjust their output power and we don’t use them continuously. We can turn them off! Also, who said the RF energy from cell phones is good for our health? Plus, how many of us simultaneously use multiple cell phones? Most of us only have two hands, two ears, and some can’t multitask so they can only use one phone at a time!

There are any numbers of studies/reports dating all the way back to the 1960’s, indicating that RF/MW energy is hazardous and exposure should be limited. Most of the studies have been performed with short-term high level RF energy exposures. Most radiation hazard standards even call out a 6-minute exposure time limit; see ANSI/IEEE Std C95.3 or FCC OET Bulletin 65. Not much is known about long-term RF exposure. Maybe in three or four years there will be some good data available on RF health hazards from the impaired residents whose bedrooms are located near their smart meters. All of this worry could have been avoided if they had used broadband data communication over their power lines instead of RF radiation.

If you are looking for an interesting book on the subject, check out Zapping of America: Microwaves, Their Deadly Risk and Cover-Up by Paul Brodeur, Norton 1977. How quickly we forget when there’s something to be gained.

- Ron Brewer

Protecting the Earth's Limited Resources

2012-05-01T13:23:00.000-07:00

May marks the dawn of a new blog created by Interference Technology with emphasis on Electromagnetic Compatibility. I am honored to have been asked to serve as the blogger in this newest venture to reach out to members of the community. I have written many articles over the years and have always strived to make them purely technical, easily readable, and non commercial. This venture is really different and I am excited about giving it a whirl . . . here, I get to express my personal opinions, make various comments, and discuss my experiences. I’ve been in the RFI, EMI, EMC, E-Cubed, E3 field full-time since 1967 so I have had lots of experiences. Some of which are only known by 007 and other special 3 letter agencies.

I’m also opinionated just like the rest of the members of the EMC community and I’m not afraid to make comments. That said, I am soliciting your help. It will be a lot more fun to make comments about things that you (the reader) are interested in. We can always discuss grounding, bonding, shielding, filtering, testing, PCB layout, signal integrity, and/or anything else of interest. Just make a comment or ask! I may not know the answer, but someone out there does.

Although it just passed and won’t be back until next year, April 22 was a very important day. It was Earth Day! A celebration of the environment. This celebration has been going on since 1970 and emphasizes concern over soil, air, and water pollution. After all, these are limited resources and when we run out – we run out! Then what do we do? Mark Twain was very perceptive when he said, “Buy land, they’re not making it anymore.” I for one would like to have all these things remain available. I’ve become quite used to breathing and would like to keep on doing that.

The same thing holds with the RF spectrum. Apparently the billions that celebrate Earth Day don’t realize that there is a least one other environmental resource that needs protecting -- that is the RF spectrum. Although we won’t all die if we run out of spectrum, they aren’t making anymore of that either. Protecting spectrum is one of the things we do as EMC engineers. It was not nearly as big an issue with wired systems, but now with the perceived need for everyone to be tethered to their RF linked electronic widget, for convenience and mobility, we are consuming spectrum at a ridiculous rate and generating EMC problems right along with it.

At a recent department of defense review, I learned that the government wants to sell a spectrum segment being used by the DOD and the equipment redesign/replacement cost to move the DOD to a new RF spectrum location will greatly exceed the sales price of the spectrum. Does that make good business sense? Should the government be taking the spectrum from the haves and selling it to the have-nots? How can the government own it in the first place? It’s a lot like saying that I own the color blue. And if the government can justify ownership why are they selling it? Why aren’t they leasing it instead?

-Ron Brewer

Welcome!

2012-04-11T09:20:00.002-07:00

Welcome to the EMC Zone, an avenue for discussion on the current issues affecting today’s EMC industry.

This inaugural posting concerns a hot topic in the industry - the current state of revision to Medical Standard 60101, specifically as regards proposed changes to the RF immunity stress levels / test-bands.

At last year’s Long Beach symposium, the word around the water cooler was that the committee could not decide on the proposed changes to ‘Electromagnetic disturbances – requirements and tests’ (Part 1-2 of the standard). We will focus on the changes involving increased RF immunity stress levels (30v/m field strength proposed at some frequencies), and on the extension of the upper test frequency (6GHz proposed).

We can hazard a guess at the main bone of contention in this revision from Edition 3 to Edition 4, – the cost of implementation, both for the manufacturer whose products must comply, and for the test house that needs to upgrade its test facility.

Standards sub-committees are commercially aware, usually helped by one or more of the team being industry representatives. As a result, where justified, you will see steps to ameliorate the total test-time implemented in the revision.

This does not help a test house where a significant investment in more powerful test equipment is required regardless of test-time.

To put things into perspective, next week’s post will do the math on the upgrade costs faced, and will suggest ways of reducing those costs.

Do you have a point of view on this? If so, please comment below or email info@interferencetechnology.com

-Thomas Mullineaux

EMC Buyers' Guide App "How to" Guide

2012-04-06T09:50:00.002-07:00

Our Bloggers

2016-06-27T07:50:00.000-07:00

Ron Brewer

Ron Brewer is an EMC/ESD expert and is currently providing EMC/RF engineering analysis for the NASA ELVIS program at Kennedy Space Center. He is a NARTE/KEC certified EMC/ESD engineer and has worked full-time in the EMC field for more than 35 years.

Ron was named Distinguished Lecturer by the IEEE EMC Society and has taught over 400 EMC technical short-courses in 31 countries. Ron has published numerous articles and papers on EMC, ESD, lightning, and shielding/packaging design. He provides EMC/ESD training/consulting and is a specialist in COTS systems design/redesign to meet military specifications. Ron holds FCC Amateur Extra Class license KE3TH and completed undergraduate and graduate work in engineering science and physics at the University of Michigan. He may be contacted at ronbrewer@ieee.org.

Brian Farmer

Brian Farmer has a long career providing E3 and Spectrum Supportability systems engineering and program management services to the DoD, including the Naval Air Systems Command (NAVAIR), the Joint Spectrum Center (JSC) and the Naval Surface Warfare Center Dahlgren Division.

After working for several companies in the E3 engineering business, Brian started EMC Management Concepts in 2002 with one goal in mind: to provide world class E3 and Spectrum related engineering management services to the DoD. Besides being the Chief Executive Officer, primary business developer, and lead program/contract manager for this growing small business, Brian still provides direct E3 program management support to several Navy offices and the JSC. He leads contract efforts to develop and deliver E3 and Spectrum Supportability training to the acquisition community. NARTE Certified EMC Engineer (EMC 000895-N). He may be contacted at farbri@gmail.com.

Thomas Mullineaux

An author and RF Engineer, Tom has been in the EMC industry for 20 years, both as a supplier to the industry, and as a hands on program manager, achieving EMC compliance for new products (starting long ago with VDE 0871, and culminating in getting a munitions pod mounted amplifier through the environmental tests applicable to a fighter aircraft).

Tom received his degree in electrical and electronic engineering from Portsmouth University, England, and is a prolific writer of EMC related articles, with all articles having a strong slant towards the engineering basics behind the tests. Tom has given many IEEE society presentations, most looking at the physics behind today’s commercial, automotive and MIL-STD tests. He may be contacted at tommullineaux@ymail.com.

Kenneth Wyatt

Kenneth Wyatt is President of Wyatt Technical Services LLC, and is the senior technical editor for Interference Technology. He holds degrees in EE and Biology from the University of California. For over 20 years, he worked as a senior EMC engineer for Hewlett-Packard and Agilent Technologies in Colorado Springs where he provided comprehensive EMC design and troubleshooting services. During that time, he managed the product compliance and environmental test lab and provided advanced EMC training and corporate leadership for EMC.

His specialty is EMI troubleshooting pre-compliance testing and is a co-author of the popular EMC Pocket Guide and RFI Radio Frequency Interference Pocket Guide. He also coauthored the book with Patrick André, EMI Troubleshooting Cookbook for Product Designers, with forward by Henry Ott. He is widely published and authored The EMC Blog hosted by EDN.com for nearly three years. Kenneth is a senior member of the IEEE and a long time member of the EMC Society. He may be contacted at ken@emc-seminars.com or kwyatt@interferencetechnology.com.

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2016-06-25T17:00:00.000-07:00

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2012-04-11T07:26:00.003-07:00

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2012-05-01T18:24:01.339-07:00

This comment has been removed by a blog administrator.

Real estate points the way to efficient sopectrum ...

2012-05-02T05:52:56.664-07:00

Real estate points the way to efficient sopectrum allocation: transferable property rights. A spectrum market would allow new players in the wireless market to displace old users. And those old users, e.g. TV broadcasters, would walk away happily because the new users could compensate them directly for their spectrum.

Funny, I was reminded of the whole 1970's &quo...

2012-05-10T09:51:11.216-07:00

Funny, I was reminded of the whole 1970's "Microwaves are bad for us" conversation just recently. I stopped for a minute and tried to picture my life with 3 kids without microwavable popcorn.

Ron, You took me waaaay back...1966 and 67, doing...

2012-07-05T11:58:02.290-07:00

Ron,
You took me waaaay back...1966 and 67, doing those very measurements at Tinker AFB, OK...an then including some in Vietnam. Singer machines, did you ever find a spare thread spool in the back of one!!

How does in home PLT fit into this argument, it cl...

2012-07-30T07:23:58.790-07:00

How does in home PLT fit into this argument, it claims to be compliant to international standards, is an intentional emitter and interferes with a large swage of the spectrum...

I think the real question here is valid for al tes...

2012-08-02T23:29:54.079-07:00

I think the real question here is valid for al testing. Do we test according a repeatable verfiable and implementable and above all safe an secure way or do we test an EUT as in real life conditions? f.i. all signals can be present at one time blue tooth, GSM, Wifi radio communcation and emitting at various levels and intervals creating different harmonics in time and even combined hamonics.And this all within a proximity of less than 10cm of the EUT. How can you test this kind of sitation?

Reply from EMC Zone (Tom). Dear blog reader, thank...

2012-08-12T07:57:36.596-07:00

Reply from EMC Zone (Tom). Dear blog reader, thank you for your interesting comments.
I believe the power levels of bluetooth and wifi are limited by the FCC and equivalent authorities to a few hundred milliwatts, so I don’t see these as a significant threat. Also, the drive in medical equipment today is wireless connectivity, so modern equipment will be immune to signal levels from its integrated antenna. The power level of a cell phone is 2000 milliwatts, significantly higher than WLAN levels. However I see your point on multiple simultaneous threats. My thoughts on this are that though not ideal, testing one threat at a time seems to work. For instance, military aircraft are subjected to high test fields a single frequency at a time, and these aircraft tend not to fall from the sky. An argument I shall be making later on is that the tests should check susceptibility to real cell phone signals, not susceptibility to simple amplitude modulated signals that in no way represent the real threat waveform.

Thanks for explaining our query in such a nice way...

2012-08-31T02:31:23.961-07:00

Thanks for explaining our query in such a nice way. I think like me many people have same question in their mind but now I get answer. Definitely EMC testing of every device is necessary as it assures us that it is totally safe to use.

So, this was a mil-spec RV?

2012-11-01T21:47:39.013-07:00

So, this was a mil-spec RV?

Hello Emscan, Big thanks for conveying the questio...

2012-11-07T22:35:24.022-08:00

Hello Emscan,
Big thanks for conveying the question which bothered me for a long time. Now I have a deep understanding that EMC testing of every device is very essential.

I think it's really important to check all the...

2012-12-12T14:26:07.512-08:00

I think it's really important to check all the tyco fuses, switches, circuits and conductors you've got cause just a simple faulty wiring on these will really cause trouble and worse it'll ruin the whole system of the mechanism you're running.

Great post, i like a lot the representation for DM...

2012-12-28T00:30:21.287-08:00

Great post, i like a lot the representation for DM-CM interaction.

It depends, as 25 years plus experience I have mad...

2013-02-21T11:11:00.392-08:00

It depends, as 25 years plus experience I have made SPG work better in some instances, MPG work in others, I've floated grounds, Pi'd them, isolated them, and made everything in the system a single ground (no inductive losses from point-point) in the ground mesh. I assure you that there is no one correct answer, ever.

I have a PE license (for what reason I’m not sure ...

2013-03-21T18:27:49.126-07:00

I have a PE license (for what reason I’m not sure except it looks good on my resume) and I wouldn’t mind having an iNARTE EMC Design Engineer certification. I have thought about doing EMC contract or consulting work and this should be a good credential but their web site says little about what’s on the exams except study every book you can lay your hands on. They have a study guide and other materials for the EMC Engineer certification but not for the EMC Design Engineer certification.

2013-05-24T07:38:03.497-07:00

This comment has been removed by a blog administrator.

I beg to differ. I agree that there are elephants ...

2013-05-31T00:08:16.692-07:00

I beg to differ. I agree that there are elephants in the EMC world but harmonics from a TWT should be removed by band pass (or low pass)filters. As in your example a 1-2GHz amplifier should be fitted with a 2 GHz low pass filter to reduce the level of the harmonics. We 1-6 GHz TWT that has high harmonics but a 2,4,6 GHz low pass filter to connect to the same amplifier. It is just a matter of doing it right

Hi Tom, This is why some amp manufacturers are loo...

2013-05-31T02:49:11.864-07:00

Hi Tom, This is why some amp manufacturers are looking towards solid state, with the decreasing cost of solid state devices it's getting such that there is very little difference between the two technologies. There is actually a way round this problem. We have advised that it is always possible to conduct your sweep from HF down to LF, TWT's have very little radiation outside their operating bandwidth so the potential for misleading harmonic interference goes away.
You do always have the potential problem in 2 signal mixing products within the EUT but that's a different story!

Best Regards
Tim Hague
European Technical Manager
AR Europe.

Hmmm so if I understand correctly you would prefer...

2013-05-31T04:32:24.508-07:00

Hmmm so if I understand correctly you would prefer an antenna system coupled to an , lets say Spectrum analyzer, and validate , calibrate the field this way iso of with a field probe. I have one comment only on the whole subject of immunity testing. The real world environtmental RF conditions that cause real problems with Objects be it equipment or otherwise can't be replicated or used for testing due to cost and time consuming reasons. So our testing is already compromised by the real world why bother with the harmonics (just joking)

thank you for an excellent article

2013-06-13T13:07:44.710-07:00

thank you for an excellent article

Tom Mullineaux :- There were three comments posted...

2013-06-18T09:20:16.236-07:00

Tom Mullineaux :- There were three comments posted about the first post on the MIL-STD Elephant. I will give brief answers on each for now and weave fuller answers into the next post

----------------------------------------------

Anonymous May 31, 2013 at 12:08 AM

I beg to differ. I agree that there are elephants in the EMC world but harmonics from a TWT should be removed by band pass (or low pass)filters. As in your example a 1-2GHz amplifier should be fitted with a 2 GHz low pass filter to reduce the level of the harmonics. We 1-6 GHz TWT that has high harmonics but a 2,4,6 GHz low pass filter to connect to the same amplifier. It is just a matter of doing it right

Tom's answer: Thank you for your comment. I note you say 'should' be fitted with ... I have visited many many test houses and only saw this once. In actual fact filters appear in the list of possible solutions to this issue as you will see in upcoming posts

---------------------------------------------

Anonymous May 31, 2013 at 2:49 AM

Hi Tom, This is why some amp manufacturers are looking towards solid state, with the decreasing cost of solid state devices it's getting such that there is very little difference between the two technologies. There is actually a way round this problem. We have advised that it is always possible to conduct your sweep from HF down to LF, TWT's have very little radiation outside their operating bandwidth so the potential for misleading harmonic interference goes away.
You do always have the potential problem in 2 signal mixing products within the EUT but that's a different story!

Best Regards
Tim Hague
European Technical Manager
AR Europe.
Reply

Tom's answer: - Thanks Tim, good comments. Solid state with band overlap is one of the possible solutions I will be discussing in upcoming posts. I am familiar with the 'get around' where the test is run backwards to try and establish which frequency the equipment under test failed at (the intended or the harmonic, I will expand on this for others later), but see this as a fix for an embarrassing situation (for the provider of a very expensive system)

--------------------------------------------

Anonymous May 31, 2013 at 4:32 AM

Hmmm so if I understand correctly you would prefer an antenna system coupled to an , lets say Spectrum analyzer, and validate , calibrate the field this way iso of with a field probe. I have one comment only on the whole subject of immunity testing. The real world environtmental RF conditions that cause real problems with Objects be it equipment or otherwise can't be replicated or used for testing due to cost and time consuming reasons. So our testing is already compromised by the real world why bother with the harmonics (just joking)

Tom's answer: - Thank you for your comments. Actually you will find that RF immunity is starting to lean towards ascertaining the level of unwanted fields in exactly the manner you describe. I agree with you that the test fields do not represent real situations but I think there is a case for replicating the real threats faced. For instance, regarding the ubiquitous cell phone, this threat can be created fairly inexpensively. Modern comms waveform generator, amplifier, antenna. Thanks again, good points. Keep them coming!!

As an EMC engineer retired over thirteen years I ...

2013-07-11T19:59:36.322-07:00

As an EMC engineer retired over thirteen years I never tested to the F version of 461. In my day there were elephants in the room but not the one mentioned. I did test to 200 v/m up to eighteen gigahertz but we measured the field strength using an antenna and a tuned receiver so if anything we over tested the test sample. The type of equipment my company manufactured was very low frequency so in twenty five years of testing no test sample ever failed in the above one gigahertz region, below one gigahertz is another matter.

Your information is dated; the current Chrysler Sp...

2013-07-12T03:36:43.718-07:00

Your information is dated; the current Chrysler Specification makes no mention of 13 dBc and simply references ISO 11452-2 for test methodology of RI ALSE Testing. The current Chrysler Specification was published in 2010.

You need to do a better job of referencing prior to publishing such statements.

Having worked at a defense contractor in the EM ar...

2013-07-18T12:03:24.261-07:00

Having worked at a defense contractor in the EM arena for 25+ years I was expecting a total of 8 mil-related questions in each 4-hour portion of the exam (as described). I was not pleased that the first portion contained 5 and the second contained 3 MIL questions. Study the IEC requirements, the NAVSEA documents and the books in the reference list (i.e. Clayton Paul) and your college calculus materials. The IEE symposium also has prep exams each year that are good for reference. If you want to focus on MIL (like the original intent) they now offer a 4-hour MIL only exam. Good luck!

I apologize for the delay in replying, I only just...

2013-07-23T08:50:28.067-07:00

I apologize for the delay in replying, I only just saw this. Yes, it is good that you illuminated the equipment at 200v/m at the intended test frequency. You were still left of course with a substantial field at the harmonic. With no failures observed you probably had no need to 'step down' in frequency to establish if a failure was due to the intended field or the harmonic field. For those unfamiliar I shall weave the 'step down' technique into the next posting. Best regards, Tom

Sorry for the delay in replying - I only just saw ...

2013-07-23T08:59:41.851-07:00

Sorry for the delay in replying - I only just saw this. Regarding the 13dBc claim, this (and the two lots of 20dBc) came from direct conversations with people with inputs to the standards ( this was around 2009). However you are absolutely right, I should go through the documents before stating numbers. Do you happen to have access to 11452-2? I don't. Does it say 20dBc harmonic level is mandatory? Best regards, Tom

You don't need to be close either. When will ...

2013-07-25T11:46:29.360-07:00

You don't need to be close either. When will we pay attention to the large EM source 149,600,000 km away from us! There is direct evidence that without shielding this source causes burns and cancer in humans! 100% of humans with asthma are exposed to this source!

I am truly sorry I took the time to click this link!

"The material properties are changed by radia...

2013-07-25T12:11:00.396-07:00

"The material properties are changed by radiation. For example, microwaved coffee doesn't taste the same and blood warmed by a microwave is destroyed and cannot be used for patient transfusions."

Microwave ovens cause the water molecule to rotate generating friction heat. Only once the heat is sufficient does it begin to damage cellular organisms. In the case of a Microwave the amplitude of the signal is about 60 dBm or more.

The only way RF can cause damage to us is by creating enough friction heat to destroy cells inside us.

Even in the case of a cell phone being held to ones head for exaggerated periods of time, the actual energy absorbed is ridiculously low. The additional heat generated would be unnoticeable.

A basic understanding of the signal levels in which we are exposed is enough to understand how insignificant the risk is to the average person.

Seriously, Fear mongering is all this is and it is irresponsible to repeat unsubstantiated claims such as these.

WOW! If it wasn't for EM fields we would all l...

2013-07-25T13:10:24.494-07:00

WOW! If it wasn't for EM fields we would all live to 120 and the increase in human longevity in the last 100 years is an illusion. I would like to see the study you cited about asthma. I started in the EMC field in 1963 and have such scare stories almost from the time I started. I am aware of no studies about non thermal effects of EM radiation on human health the have been validated by the scientific community. Give me a break! and no I don't have skin in the game.

I read through the wiki page cited and found it to...

2013-07-25T13:19:11.196-07:00

I read through the wiki page cited and found it to be nowhere near as alarmist as this blog. Contrary to your intimation, I suspect that those who are directly involved in work in the field (IYW "more heavily invested") are concerned about understanding the effects of EM radiation on the human body, and choose to continue to work in the field based on their reasonable assessment of the data.

Where is the citation to the reference of the asthma study? At this point I'm tempted to file it with the connection between immunizations and autism.

I expect to read about the pushback from the dangers of smart meters from the local news station; I would hope that the people involved in reporting issues related to the industry would adopt a more reasoned response. There is no question, exposure to high power EM radiation is very dangerous, but to date the studies of long term exposure to radiation levels below the defined safe limits tend to show that there is no risk.

Read Ron Brewer's response to the comments abo...

2013-08-01T06:08:51.709-07:00

Read Ron Brewer's response to the comments above in his latest post, "Do We Only Believe Things that Match Our Opinion?"

I have often wondered what the effects are on the ...

2013-08-01T12:37:23.234-07:00

I have often wondered what the effects are on the human body, for exposure to the near-field output of an antenna. Years ago a company designed a position/location system using a 'poorly designed' antenna, that deliberately kept the E & H fields spacially separated for as long a distance as possible, in order to measure how far the receiver was from the antenna. In the distance limit, the EM wave has fully balanced/phased electric and magnetic components. I can't help but wonder what happens on exposure for those individuals that are routinely in the near-field zone.

From Paragraph 1: "Researchers found that bab...

2013-08-01T13:46:31.525-07:00

From Paragraph 1: "Researchers found that babies born to women who are exposed to strong electromagnetic fields during pregnancy had three times the risk of developing asthma compared those whose mothers were not exposed to strong electromagnetic fields."

From Paragraph 2: "When 33 percent of children born to women with the highest electromagnetic field exposure develop asthma this is serious."

It may be serious, but the conclusion is clearly incorrect!! The first paragraph says the RISK increases by a factor of 3, not the actual incidence of asthma. Does not compute!! You go from a RISK of developing asthma in a normal population to a prediction of 33% incidence of asthma in the EM exposure. This conclusion is complete nonsense!!!

"...they have shown that by changing the bioe...

2013-08-01T18:21:10.775-07:00

"...they have shown that by changing the bioelectric code and hyperpolarizing tumor cells..."

Uhh, what? Hyperpolarizing? Any explanation as to what this means? The same with bioelectric code?

Sounds like a collection of National Enquirer pseudoscientific made-up buzzwords to me.

Having read the noted articles I am unimpressed. T...

2013-08-02T18:42:20.516-07:00

Having read the noted articles I am unimpressed. The asthma study had a small sample size and even the authors stated that their results would have to be validated by further studies. In the past many individual studies have raised the alarm about the dangers of EM fields only to be debunked in larger studies. I recall the panic that cell phones caused brain cancer only to be debunked on further investigation. The authors of the asthma study say that the women were subjected to high level magnetic fields. I could not find what those levels were but they stated that a milligauss field would cause a significant increase in asthma rates, since the earth's field is 500 times higher it is hard to believe that increasing the frequency from 0 to 40 to 800 Hz would have such a drastic effect. MRI machines produce fields that are ten million times greater than one milliguass. If magnetic fields are so dangerous people should be dropping like flies when they enter an MRI machine. Perhaps EM fields are really good for us. Transcranial magnetic stimulation (TMS) produces 10 to 100 gauss pulsed magnetic fields deep inside the brain and is used to treat depression. Maybe if we all are exposed to much higher EM fields we all will live to 120.

Maybe a study of Radio Hams should be taken. They ...

2013-08-09T01:05:21.386-07:00

Maybe a study of Radio Hams should be taken. They tend to operate in the near field and at levels above the ICNRP guidelines for exposure.

Having been to Dayton this year they seem to be aging well..

Tim M0aFJ

Re Asthma, how about looking at what these people ...

2013-08-09T01:07:39.632-07:00

Re Asthma, how about looking at what these people are eating as well, it's proven that E numbers in food cause problems, especially for children but the food manufacturers still ram them down out throats..

Doesn't the antenna usually play a role of LP ...

2013-09-06T14:39:29.784-07:00

Doesn't the antenna usually play a role of LP filter?

From Tom Mullineaux - Oops, sorry guys, just spott...

2013-10-03T10:28:31.583-07:00

From Tom Mullineaux - Oops, sorry guys, just spotted a typo:-
The sentence "That is the software uses the 2-4GHz over 2.5 – 4.0GHz and then connects the 7.5-18GHz amplifier to finish the test over 4.0-7.5GHz." should of course read "That is the software uses the 2-4GHz over 2.5 – 4.0GHz and then connects the 2.5-7.5GHz amplifier to finish the test over 4.0-7.5GHz.

Hi Brian, You pose several interesting questions....

2013-10-15T08:20:53.385-07:00

Hi Brian,

You pose several interesting questions.

Let me start witht he last one. The displays in question did comply with regulatory and contractual (DO-160 and Boeing) requirements. Without stating the levels and modulations, the displays were tested for CW, AM and pulsed but were not required to be tested for the specific modulation scheme employed by WiFi.

Number 2, my guess is that it will not be the source, this assumes that the source is operating within its specified parameters and limitations.

Number 1, what has changed is that the airlines now want to put WiFi on their planes (many commercial planes already have WiFi) and want the equipment to comply, retroactively, with the new 'requirements'. Who gets to pay should be a very interesting discussion.

Steve Deppen

Tom - fun article, thanks. My understanding is tha...

2013-12-05T11:45:24.923-08:00

Tom - fun article, thanks. My understanding is that TWTAs are rated for a power level that accounts for the relatively high harmonics. So a 1.0-2.5 GHz 200W TWTA with harmonics of -6 dBc puts out 200W at 1.0 GHz while also putting out 50W at 2.0 GHz. Your hypothetical conversation above indicates that this TWTA rated at 200W means it puts out 150W at 1.0 GHz and the other 50W at 2.0 GHz, resulting in a field strength less than advertised. I don't think this is how it works, but I could be wrong. Can you please confirm? Thanks!

What would be the results oh using Standard Gain H...

2013-12-05T13:54:54.350-08:00

What would be the results oh using Standard Gain Horns??
We use them during 200v/m testing. We have not measured the out of band responses but have assumed that the Standard Gain horns normal frequncey roll off would help with this issue.
Also have not located a vendor that carries the needed high power filters.
Example:
EMCO 3160-01 (0.96-1.45 GHz)
EMCO 3160-02 (1.12 - 1.7 GHz)
EMCO 3160-03 (1.7 - 2.6 GHz)
EMCO 3160-04 (2.6 - 3.95 GHz)
EMCO 3160-05 (3.95 - 5.85 GHz)
EMCO 3160-06 (5.85 - 8.2 GHz)
EMCO 3160-07 (8.2 - 12.4 GHz)
EMCO 3160-08 (12.4 - 18.0 GHz)

KMB

Glad you are enjoying the thread. I think your que...

2013-12-10T05:41:12.177-08:00

Glad you are enjoying the thread. I think your question is about how the power of a TWT amplifier is specified, though you may also be referring to the field strength produced by the amplifier/antenna combo. So just in case I am misreading the question, I will answer both. Let’s start with how the power of a TWT amplifier is specified. The specified minimum output power available across the band of operation is the power at the intended frequency. So if the amplifier specification sheets states 250W minimum, this is the minimum power you will get at the wanted frequency. Regarding the power of the unwanted harmonic, the specification sheet will give the worst-case harmonic level, and it is up to you to deduce the harmonic power level. For instance, with the amplifier producing 250W of wanted signal and with the worst -case harmonic level specified on the data sheet at minus 3dBc, then the worst-case harmonic power is 125W. So basically the specified output power on the datasheet (and the rating of the amplifier) takes no account of harmonic power.

Regarding the field strength produced by the amplifier/antenna combo, lets look at my made up example of 200v/m formed from 130v/m at 1GHz and 70v/m at 2GHz. For a typical test set-up we will need around 60W at 1GHz at the antenna connector to create the 130v/m. And we will need around 20W at 2GHz (again at the connector) to create the 70v/m. This equates to the harmonics being at around minus 5 dBc. All back of the envelope stuff, but we will touch on this further in the next posting (as part of the filter installation exercise).

-Tom

Hi KMB, thank you for the comments. I haven’t meas...

2013-12-10T05:41:59.345-08:00

Hi KMB, thank you for the comments. I haven’t measured the out of band performance either, but I think you will be disappointed at the harmonic suppression of your standard gain horn antennas. These antennas are very predictable and give optimum parameter performance over the stated band. So for instance the antenna factor is very flat and can be regarded as constant over the band. Above the band of operation (where the bad guy harmonics reside) the constancy of the parameters suffers, and although the harmonic is an octave away, I suspect efficient radiation still takes place. In fact it is quite possible that the harmonic is favored over the fundamental in terms of gain.

Regarding the filters, I will be suggesting the use of a diplexer for the 7.9-9.6GHz problem area. A diplexer is a very common component in wireless comms. It splits by frequency and we will use this feature to route the harmonic to a high power termination. I haven’t researched the availability of existing suitable products yet, mainly because I have been going along in the hope that a blog reader or two would do that for us once we have nailed the required performance.

-Tom

Thank you, Brian, for keeping us abreast of develo...

2013-12-12T11:28:08.872-08:00

Thank you, Brian, for keeping us abreast of developments.

So... what is the URL for this document. We canno...

2013-12-12T17:50:56.281-08:00

So... what is the URL for this document. We cannot review or comment on something we cannot find and have not read!

Its not out for public comment; only internal DoD ...

2013-12-30T13:57:02.682-08:00

Its not out for public comment; only internal DoD comments through official channels.

Hi Harry, Unfortunately, the document is not out ...

2013-12-31T05:09:44.478-08:00

Hi Harry,

Unfortunately, the document is not out for public review (DoD Instructions never are); it only being reviewed internally through official DoD channels.

The calculation for required power, P = d^2 x E^2 ...

2014-01-30T10:24:44.311-08:00

The calculation for required power, P = d^2 x E^2 / 30G watts, is not correct. The power of 200 V/m is [(V/m)^2/(free space impedance)], 200^2/120/pi = 106 W. Power density in MKS units is normalized to power per unit area, watts per meter^2. This only works if the amplifier really produces its power at free space impedance; the point of your discussion is that amplifiers don't always produce the power that the manufacturers specifications tell a person that they do at the load of interest, whatever that might be. Thank you for the article. Manufacturers are quite aware of power rating competition and will publish what makes their product look good, so the upshot of your article is to test the amplifier for power output for the load of operation.

Hi Alan, thanks for the comment. Feedback is alway...

2014-02-05T12:48:47.298-08:00

Hi Alan, thanks for the comment. Feedback is always welcome. You are right and you are wrong. It is true that at a plane the power density is E^2/Zo (equivalent to power density = P/4Pixr^2 for a point source of power emanating spherically at a distance r from the point). But we are looking for the power required from an amplifier to create a certain field level at a certain distance using an antenna of certain gain G. I will elaborate on this in the next post. By the way, does anyone know where the magic number 30 comes from in the equation E = sqrt (30xPxG)/d^2 ? Years ago I looked for and couldn't find the explanation in any textbook, and always thought it was a practical 'fudge' factor to make the v/m level come out right (or nearly right). Maybe the explanation is out there somewhere but I could not find it. Eventually, in the absence of any reference, I worked it out myself. Anyone know where it comes from before I tell you at the next post? You have a week.
Thanks again Alan
-Tom

Power Density: Pd=P/(4pi^2) P=ExH Free space imped...

2014-02-06T11:30:59.841-08:00

Power Density: Pd=P/(4pi^2)
P=ExH
Free space impedance is 120pi (377Ohms)
Therefore:
E/H=120pi
Therefore:
E/120pi=P/4pi^2

Sove for E

E=sqrt(30P)/r V/m

Adding an antenna with gain

E=(sqrt(30P*P*G))/r

Combining two equations for power density: Pd= E^2...

2014-02-06T13:46:12.481-08:00

Combining two equations for power density:
Pd= E^2/120pi = PG/4pi*d^2
solve for E^2 and 120pi/4pi = 30 leaving E^2=30*P*G/d^2

Jim Pollock at Aclara Technologies LLC

Well done guys. Bruce won the race (pressed the &#...

2014-02-08T12:24:43.563-08:00

Well done guys. Bruce won the race (pressed the 'publish button' first) and in his second method used the self same approach I used. It is clear Bruce, Jim and Hans are well versed in EM theory. Not all you readers are at this same level, so for those less familiar, I shall put the answer in a step by step solution and explain it pictorially in the next post.
Keep those comments coming!!
-Tom

Apologies from Tom, I didn't put in 26 down, T...

2014-04-17T23:11:46.131-07:00

Apologies from Tom, I didn't put in 26 down, The clue is:
DOWN
26) The center of attention initially

The instruction number should be 3222.03 vice 3222...

2014-09-05T09:55:06.590-07:00

The instruction number should be 3222.03 vice 3222.01. Minor point. --mike--

Very nice post. I just stumbled upon your weblog a...

2014-11-09T07:15:45.614-08:00

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Nicely presented information in this post, I prefe...

2014-11-24T00:06:43.956-08:00

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emc testing lab

The horizontal colored lines at the output of the ...

2014-12-14T23:09:18.775-08:00

The horizontal colored lines at the output of the mixers are the spread-data ready for combination, frequency translation, amplification and finally, transmission. Satılık hoparlör

where can I find the complete version of this arti...

2015-01-07T07:31:55.967-08:00

where can I find the complete version of this article?

Now, have a glance at the fins on the coils. just ...

2015-01-30T11:38:29.465-08:00

Now, have a glance at the fins on the coils. just in case of any planate or bent fins, run a fin comb to straighten the fins. The fin comb is bought from any appliance store. ensure to be terribly mild with the fins, because the fins ar terribly delicate, for more information click here Air Conditioning Repair North Palm Beach.

Great highlights for the national emc symposium in...

2015-03-03T15:18:59.543-08:00

Great highlights for the national emc symposium in Santa Clara. Note the it is free to attend the keynote speech with the exhibit hall pass and the exhibits open right after the presentation so don't run away. If you are still booking your flight, don't miss the united airlines discount listed under travel on the website.

Excellent post and it also comprises a lot of usef...

2015-03-14T11:35:52.937-07:00

Excellent post and it also comprises a lot of useful facts- Thanks for valuable information.Really enjoyed reading it. Please visit my MIL-STD 461 page and please leave comments.

Some of the common types of corporate events are releasing a brand, road show, galas and press event with the necessary and organized corporate support. The activities should be well handled according to their specific characteristics.

I really appreciate your professional approach. Th...

2015-03-25T05:30:05.461-07:00

I really appreciate your professional approach. These are pieces of very useful information that will be of great use for me in future.

This article was very usefull to me. Great job Tom...

2015-05-08T01:34:46.633-07:00

This article was very usefull to me. Great job Tom.

Wll writen! Really enjoyed about MIL-STD 461 great posting from first to last and it also comprises a lot of useful facts- Thanks for valuable informative information.I am interest for your next post.

Very nice post. I just stumbled upon your weblog a...

2015-06-28T01:21:15.763-07:00

2015-07-17T01:56:41.395-07:00

This comment has been removed by a blog administrator.

Great article, Ken!

2016-09-01T06:43:27.948-07:00

Great article, Ken!

Contracted for a problem at a former employer, I s...

2016-09-02T06:33:35.507-07:00

Contracted for a problem at a former employer, I saw EMC problems that had cropped up when a third party vendor didn't use module makers' recommended value capacitors to supply switching current. Layout, of course, also played a part. ka5s at earthlink dot net

Cortland
ka5s@earthlink.net

Great story and lesson, Ken. Wish I had a dollar f...

2016-10-28T14:09:48.164-07:00

Great story and lesson, Ken. Wish I had a dollar for every intermittent connection Ive seen over the years (often looking like an EMI problem.)

But wait, I DO have a dollar for each of those "problems." Ain't it great being a consultant?

All the best -- Daryl

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