Elephant in the Test Room #2 Continued and The Design Guide Article

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

Previous posts in the Elephant in the Test Room series:

About the Author

Tom Mullineaux

Industry Sponsors