Introduction
Recently I noted that my series on MIL-STD-461 test methods was closing since I had accomplished a review on each of the test methods. It seems appropriate to continue with a review of General Requirements – those things common to the various test methods.
These requirements have been around since the first release of MIL-STD-461 and MIL-STD-462 dealing with common elements associated with device design concerns and test concerns. With the release of MIL-STD-461E, the general requirements merged into one document categorized as Interface Requirements and Verification Requirements. This categorization remains today in MIL-STD-461G evolving with each consecutive revision release.
This review goes through each topic with some discussion to highlight things that are expected but are frequently neglected. It seems that facility procedures that are passed on through training on the job tend to become the norm and this causes the procedure outlined in the standard to become different between facilities instead of following the “standard”.
As we go through the various requirements, I will identify the location in the standard and try to comment on the goal of the requirement. I will also discuss observations I have noted over the years, where the requirement is misunderstood or more commonly omitted. Details about the requirements are part of the standard, so I will not be repeating many of the specifics.
4.1 General
This paragraph provides an introductory statement requiring that compliance with Interface and Verification requirements are mandatory in addition to the detailed emission and susceptibility requirements.
4.2 Interface Requirements
4.2.1 Joint procurement – requires that multi-agency procurements meet all user agency requirements. When selecting limits and test levels, the most restrictive requirements must be used to assure acceptance by the various agencies but above that various applications should be considered. For example, a ground-based system could be applicable to shipboard use, so both applications would be considered.
4.2.2 Filtering – Specific maximum values for line-to-ground capacitance are listed. This is required to limit the leakage current flowing on the structure that could interfere with other systems or become a shock hazard if the structure connection bonding is disrupted.
4.2.3 Self-compatibility – requires that the operational performance not be degraded when all the devices in the subsystem are operating together. A qualifying remark is present regarding designated levels of efficiency or design capability indicating that degradation of performance may be accepted if the procurement specification performance is met.
4.2.4 Non-developmental items (NDI) refers to items developed exclusively for governmental purposes, indicating that items developed for an application be procured for other applications so long as it meets the needs or requires only minor modification to meet the needs avoiding a research and development type of procurement.
Commercial items are also mentioned where an item on the public market be considered if it meets the needs. Minor modifications to meets the agency requirements are allowed or integration of commercial items to create a unique system fall under this category.
Responsibilities for compliance of commercial items depend on the selecting body – if the contractor selected the item then the contractor is responsible and if the government selected the item then the government is responsible. If vendor A’s model 123 is integrated into a system, it is the responsibility of the selecting body to establish a means to assure that vendor A does not make changes that impact compliance on an ongoing basis.
Equipment that has been previously procured must continue to meet the earlier procurement requirements – for instance, a new procurement of an item used on older aircraft must meet the requirements for that aircraft. Requirements to meet the current standard may be imposed by contract.
4.2.5 Government-furnished equipment (GFE) – if GFE is the cause of EMI non-compliance the failure is reported. Modification to resolve a GFE non-compliance must be authorized by the procuring activity – normally requiring a separate or modified contract for that type of change. If the procuring activity allows the non-compliance, then it would be unlikely that the non-GFE components could be validated as compliant – a matter to resolve with the procuring activity.
4.2.6 Switching transients – a switching transient emission at the “moment” of a manually activated switching function is exempt from emission compliance limits. Switching transients from automatic sequencing must comply with the emission requirement. The capture of switching transient emissions has been somewhat difficult, but MIL-STD 461G supports the use of FFT receivers that simplifies these measurements.
4.2.7 interchangeable modular equipment – requires that modular equipment being replaced be verified for compliance by testing or analysis. This is not to imply that repairs by exchanging of one module with another identical module would require verification but if a module is replaced by a different model that performs the same function, verification is required. Sometimes I wonder if this is adequate because I have encountered emission measurements that varied by as much as 70 dB from a single integrated circuit chip.
4.3 Verification Requirements
This section relates to test procedures, test facilities, and equipment used in conjunction with the detailed test procedures in section 5 of MIL-STD-461G. We are reminded here that approval of the test procedure is required, and we should examine the overall device test and evaluation to avoid unnecessary duplication.
4.3.1 Measurement tolerances – various measurement tolerances are provided but a tolerance for voltage or current is omitted. I default to the tolerance for resistors for voltage or current since the relationship is tightly coupled. It would be a good idea to include this in the test procedure to avoid a urinary battle during a report review.
4.3.2 Shielded enclosures – usually necessary to meet the ambient noise requirements and prevent interference with nearby devices. RF absorber is required (reverberation chamber excluded) to control reflected energy that significantly affects measurement accuracy and repeatability. Figure 1 provides a generic test layout. The absorption is specified and can be checked by checking a radiated field at several locations at the EUT test boundary. IEC 61000-4-3 includes a method for reflected variance checks.
4.3.3 Other test sites – must meet the ambient requirements. This is not realistic for most outdoor facilities and emission limits. Intentional transmitters in a very crowded spectrum are typically present and combined with other man-made and natural noise sources some parts of the test frequency range will show over-limit conditions. When encountering the need for tests outside a shielded enclosure, plan for the inability to control ambient noise and implement methods to separate ambient noise from EUT emissions.
4.3.4 Ambient electromagnetic level – the target is at least 6 dB below the specified limits. This is the test configuration ambient where all support equipment is operating and the EUT power is connected to a resistive load drawing the rated EUT current. Although the standard states rated current, I believe that the intention is to use the maximum current for any of the test modes. The rated current is more aligned with the safety and power feed requirements and is often much greater than the actual current demand. Note that the resistive load is based on the rated current which should not be confused with fundamental current used for CE101 limit adjustments and the fundamental current is to be reported per DI-EMCS-80200. If the EUT complies with the requirements, the report does not require including the ambient profile.
4.3.5 Ground plane – using a metallic ground plane is typical unless the installation calls for no ground plane or a ground plane meeting the parameters of a composite material if applicable. Bonding of the ground plane to the shielded enclosure is to provide a DC resistance of 2.5 mΩ or less.
4.3.6 Power source impedance – provides a standardized impedance for connection to power sources. The Line Impedance Stabilization Network (LISN) uses a 50 μH inductor in series with each power lead and is associated with long distribution lines such as found in a building or ship. Shorter distribution paths may support the use of a 5 μH LISN, especially for 400 Hz power. If this alternate LISN is used, then the conducted emission limits need adjustment.
4.3.7 General test precautions – several items to consider are listed that affect measurements and safety. I would like to add cables – specifically coaxial cables. In the laboratory environment, these cables are continually connected, disconnected, moved and abused leading to inaccurate measurements. A pinched cable from a door closing can cause a high VSWR at a single frequency and check well at other frequencies. Part of the signal integrity checks include cables, but these are accomplished at spot frequencies and can omit minor damage. Routine checks should be part of the lab operating procedures and having a network analyzer readily available to do a quick sweep can be invaluable.
4.3.8 EUT test configurations – discusses using lots of detail in this section with a requirement to configure the test layout as shown in the diagrams. However, the EUT orientation may not follow the diagram because 4.3.8.5 states to orient the EUT face with maximum radiated emissions and most vulnerable to radiated susceptibility toward the antenna.
4.3.8.1 EUT design status – qualification test articles should represent production. A simple circuit board trace can support significant changes in emission or immunity levels. This is especially when the board layout is used to control EMI.
4.3.8.2 Bonding of EUT – provisions in the EUT design are to be used for bonding. Historically this has been incorrectly applied for years and some still think that the EUT to the ground plane must be bonded with a 2.5 mΩ or less DC resistance. This misconception has plagued test configurations causing extreme differences between the test and the installation. It is commonly understood that a low impedance ground connection supports improved EMI control measure performance, but the object of the test is to evaluate reality. For example, a rack-mounted EUT should be rack mounted for the test with the rack bonded to the ground plane if that is the installation method. Bonding of the EUT to the rack should be limited to the installation and accidental bonding should be avoided. The rack mounting rails should not bond the EUT chassis unless all mounting racks will provide for a clean, unpainted mounting surface. If a green/yellow 12 AWG wire is provided for grounding, then that should be the bonding mechanism. MIL-STD-461G has added a requirement to verify that the bonding conforms to the installation without specifying a numeric measurement requirement. It should be noted that bonding of a metallic chassis that uses hazardous voltage levels, should be bonded with 100 mΩ or less to meet safety requirements as described in MIL-HDBK-2036. Making an artificially low bonding resistance may force extreme installation practices or non-compliance when fielded.
4.3.8.3 Shock and vibration isolators – if used in installation then the test configuration should conform.
4.3.8.4 Safety grounds – connect as used in the installation.
4.3.8.5 Orientation of EUTs – as mentioned above orient the face of maximum emissions or maximum susceptibility toward the antenna.
4.3.8.6 Construction and arrangement of EUT cables – required to simulate actual installation – shielded cables, shield terminations or twisted pairs only if specified for the installation. Cables should be verified to conform to the installation – include cable details in the test procedure and describe actual in the test report.
Figure 1: General Table-top Test Configuration
4.3.8.6.1 – Interconnecting leads and cables – use as installed up to 10-meters long if actual exceeds 10-meters. Figure 1 shows a general cable layout where the cables are elevated 5 cm above the ground plane, 2-meters parallel to ground plane front and recessed 10 cm from the front. The excess cable is arranged in a zig-zagged layout at the rear of the ground plane maintaining the 5 cm elevation. The zig-zag pattern is used to prevent making coils that affect radiation and parasitic reactance in an uncontrolled manner. Multiple cables are separated from each other by 2 cm. Floor standing test item cables are routed to a table-top layout. Dealing with many cables presents some issues such as providing 2-cm separation without placing cables 2-meters from the front of the ground plane. The test procedure should address the issue and describe a sensible layout that represents installed cable management.
4.3.8.6.2 – Input (primary) power leads – uses 2.5-meters of cable length with 2-meters along the ground plane front like the interconnecting cables. Maintain the elevation and separation. With floor standing equipment the power cable length can be extended to provide routing from the device egress to the table-top cable layout, but the extension should be minimal.
4.3.8.7 – Electrical and mechanical interfaces – terminated with actual equipment or loads simulating the installation. The grounding, impedance and other elements should be considered in selecting the terminations. Mechanical loads should be appropriate – a water pump operating without water is not loaded properly. Note that the support equipment should not create ambient conditions that exceed the limit nor be susceptible.
4.3.9 – Operation of the EUT – operating modes should represent worst-case emissions and represent the most susceptible modes. The number of modes should ensure that all circuitry is evaluated. The test procedure should describe how the test modes will meet the requirements.
4.3.9.1 – Operating frequencies for tunable RF equipment – at least three frequencies within each tuning band. It is common for equipment to operate in several bands so the selection of frequencies should consider functions as well as frequencies. For example, a military transceiver could operate at VHF and UHF tuning ranges, but the channel bandwidth would be different indicating two ranges. If the VHF range provides for AM or FM selection, then two modes are presented for that range. In this case, 18-tests would be required (3-frequencies for VHF-AM, 3-frequencies for VHF-FM, 3-frequencies for UHF-AM with all of these tested in transmit and receive modes). In some tests, this may be excessive if the test is evaluating circuitry not affected by the frequency selection. Consider tests at 3-frequencies if the test includes frequencies within the operating range of the EUT.
4.3.9.2 – Operating frequencies for spread spectrum equipment – FHSS at least 30% of all frequencies, DSSS highest possible data transfer rate.
4.3.9.3 – Susceptibility monitoring – must have the ability to detect susceptibility and to determine if the susceptibility is acceptable. The EUT performance specification is the key to identifying unacceptable conditions and this acceptance criteria should be clearly defined and measurable.
4.3.10 – Use of measurement equipment – frequency selective measurement receiver meeting the parameters required for the various test methods. Specifically mentioned parameters include sensitivity, bandwidth selection, detector functions, dynamic range and frequency of operation. Revision “G” of the standard added the use of Fast Fourier Transform (FFT) receivers which are very useful for detection and measurement of transient emissions.
4.3.10.1 – Detector – peak detector is used with calibration in equivalent RMS terms where the sine wave peak produces the same level as the emission peak amplitude, but the calibration shows the sine wave RMS amplitude. Other measurement devices that measure peak amplitudes require correction factors to adjust readings to equivalent RMS values.
4.3.10.2 – Computer-controlled instrumentation – description of software and verification of proper operation is to be included in the test procedure. Verification should be part of an accreditation process and revision control is necessary.
4.3.10.3 – Emission testing
4.310.3.1 – Bandwidths – specified 6 dB bandwidths are listed in the standard along with a requirement to avoid video filtering to limit the receiver response. Using a larger bandwidth is permitted but correction factors are not permitted to compensate for the larger bandwidth. Figure 2 portrays the bandwidth curve showing that if the receiver bandwidth is calibrated for 3 dB (common for spectrum analyzers without ad EMI option) a larger bandwidth would be selected. The ratio of the 6 dB bandwidth to a 3 dB bandwidth is approximately 1.8:1 which allows approximately 5.1 dB higher amplitude for a broadband signal. Noise is elevated by about 2.5 dB and a narrowband signal is not changed by the larger bandwidth. This results in higher measurements for broadband signals that may be bandwidth limited, but since we are unsure of the signal’s optimum bandwidth, we cannot make a correction.
Figure 2: Bandwidth curve
4.3.10.3.2 – Emission identification – no requirement to determine if an emission is narrowband or broadband – just use the specified bandwidth and record measurements.
4.3.10.3.3 – Frequency scanning – the bandwidth table in the standard includes a minimum dwell time (tie per bandwidth for FFT receivers) – note that the tie should be increased based on the EUT cycle time – the time required by the test article to complete an operation that could produce emissions.
4.3.10.3.4 – Emission data presentation – amplitude versus frequency plots are to be used to present the emission measurements with correction factors applied. Resolution for frequency is 1% (or 2 * BW) and amplitude is 1 dB so the data collection needs to allow replots once the data is available to determine the plot settings appropriate for each emissions test.
4.3.10.4 – Susceptibility testing
4.3.10.4.1 – Frequency scanning – frequency step sizes are specified with a minimum dwell tie of 3-seconds at each frequency step – the dwell time is increased if the EUT cycle time is longer.
4.3.10.4.2 – Modulation of susceptibility signals – CS114 and RS103 tests require modulation with a 1 kHz square wave using pulse modulation. Other susceptibility tests use an unmodulated signal unless the specific test requires modulation to assess EUT performance during test. If amplitude modulation (AM) is used instead of pulse modulation the amplitude of the interfering signal is doubled and can cause amplifiers to be saturated or an over-test by 6 dB. If AM is used, compensation for the over-test and related issues is necessary.
4.3.10.4.3 – Thresholds of susceptibility – if a test article is susceptible it is classified as unacceptable indicating that a solution be incorporated. The standard goes on to require documentation of the susceptibility including measurements of the susceptibility amplitude. Normally a manual operation is necessary to determine the threshold so it may become a time-consuming process to make a full susceptibility assessment. The revision “G” noted that as a minimum the frequency range of susceptibility and the worst-case failure be recorded. I prefer that 2 or 3 frequencies per octave be used as threshold measurement points to develop an overall profile of the susceptibility range. I have seen many reports with susceptibility noted but no threshold measurements. This prevents an assessment of the vulnerability and the lack of data does not support the resolution. The step-by-step threshold determination is well documented in the standard.
4.3.11 – Calibration of measuring equipment – measuring devices require calibration according to ISO/IED 17025 or ISO 10012 or another program traceable to NIST standards. Revision “G” relieved the periodic calibration requirement of passive devices unless they are repaired. However, signal integrity checks associated with each test were changed to provide a more thorough check that includes passive devices. Note that passive antennas are included in the “no periodic calibration” umbrella and they are often not present for signal integrity checks. In my opinion, this presents a risk that can be mitigated by doing the stub radiator test with a fixed source and fixed antenna position to verify that the signal measurement is correct. Also, cables, although not specifically mentioned, are passive devices but they need a lot of attention because of the continual handling with various tests leads to minor damage that can go unnoticed.
4.3.11.1 – Measurement system test – receiver systems (includes cables, couplers, attenuators, and other associated equipment) are to be checked at the start of each emission test. These are spot frequency checks make sure the measuring system correctly measures a known signal. Because only spot frequencies are checked cable pinches may go undetected, so pay attention to cables.
4.3.11.2 – Antenna factors – the requirement to determine antenna factors according to SAE ARP958.
Summary
A lot of interface and verification requirements are present, and these requirements supplement the detailed requirements. Using the detailed test instructions in section 5 is inadequate without understanding section 4 and often leads to inaccurate testing.
Note that tailoring of the testing is supported and that could include tailoring of the test configuration if the installation is established.
Questions – I always welcome questions – keeps me learning things every day!
