|MIL-STD-461 evolves to version F
The military EMC standard for emissions and susceptibility has new requirements and reinstates an older one.
Steve Ferguson, Washington Laboratories
On December 10, 2007 the US DoD (Department of Defense) released MIL-STD-461F, which updates the standard for military EMC product qualification. The update--the first since 1999--addressed several changes, some subtle, others significant, including resurrection of a test deleted in 1993 (when Version D was adopted). The differences may affect product design and force a need for new EMC control measures. Design and test engineers should be aware of these changes. As part of the update, the DoD has issued three DIDs (Data Item Descriptions). These documents force changes in documentation as part of the total MIL-STD-461F package for compliance testing.
This article reviews MIL-STD-461F and the associated DIDs and provides some insight into the subtle issues regarding the test requirements and methods. In the following review, I identify the clauses that have changed and discus the differences between the “E” and the “F” versions. I also provide some commentary regarding the tests and point out items that need consideration when planning or performing MIL-STD-461F testing. Although MIL-STD-461F was released in December 2007, new government contracts are now just beginning to call for testing to the new version. (You can download a copy of MIL-STD-461F at http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=35789).
Section A.4: General Requirements
Interchangeable modular equipment (4.2.7): This new item in MIL-STD-461F requires that assemblies be qualified when new line replaceable modules (LRMs) are incorporated into devices. The qualification may be accomplished by test or by similarity. Either method requires approval of the procuring agency.
Construction and arrangement of EUT cables (188.8.131.52): “Input (primary) power leads, returns, and wire grounds shall not be shielded.” This requirement entails breaking out power leads that are part of a shielded cable bundle to conduct the test (184.108.40.206.2). This revision removes the ability to shield power cables as an EMI control measure because, in most installations, shielded power cables are not normal. An exception appears in paragraph A220.127.116.11, where filtered power is provided from another device. In addition, shielded cables for Navy surface ship applications may allow an unshielded section for radiated tests but not for conducted testing. The test configuration needs to be described in the test procedure and approved by the procuring agency, but basically for most applications the power cables should be unshielded.
Computer-controlled instrumentation (18.104.22.168): Verification of software needs to be described in the test procedure. Identification of the manufacturer, model, and revision for commercial software must be provided for locally-developed “homegrown” software, the control and methodology must be described. Procedure writers will need to become familiar with the laboratory software especially when preparing a procedure for testing at a contracted test laboratory.
Bandwidths (22.214.171.124.1) – Alternate Scanning Technique: “Multiple faster sweeps with the use of a maximum hold function may be used if the total scanning time is equal to or greater than the Minimum Measurement Time defined…,” (referring to the measurement time found in Table II). This may give an impression that the test duration can be reduced but that is not the intent. The discussion in the appendix indicates that the faster sweep allows for capture of low duty cycle or intermittent signals. However in section 126.96.36.199.3, the scanning rate must be adjusted to provide to support capture of infrequent emissions. Net result – the overall scan needs to measure at every ½ resolution bandwidth over the EUT cycle to assure that the full energy of all signals (particularly narrowband signals) is captured. Realizing that signals such as a frequency hopping modulations would require an exceptionally long sweep for all of the energy to be quantified, don’t hesitate to use the faster sweep with multiple “max hold” capture to show the envelope of the signals of this type.
Frequency Scanning (188.8.131.52.1): The susceptibility sweep rate or step size has increased for frequencies above 1GHz, which provides for a faster susceptibility test. This change of sweep rate or step size reduces the test time even with a long EUT cycle time because the cycle time affects the dwell period and not the step.
Thresholds of Susceptibility (184.108.40.206.3): MIL-STD-461F adds a statement “Susceptibilities and anomalies that are not in conformance with contractual requirements are not acceptable. However, all susceptibilities and anomalies observed during conduct of the test shall be documented.” What is the implication? It is not unusual to test “hot”—at levels above that required—and the possibility exists to observe an anomaly at the elevated test levels only to find that the anomaly is not present when the specified test level is applied to the EUT. This new statement requires documentation of the all observed (albeit compliant) anomalies. This indicates that observations be documented (e.g., the standard calls for RS101 testing at a level 10dB above the test level but if susceptibility is noted it could easily be compliant but the observation would be reported).
Section A.5: Detailed Requirements
Emission and susceptibility requirements, limits, and test procedures (5.3): MIL-STD-461F resurrects this requirement. CS106 (formerly “CS06” from MIL-STD-461C) has been added to 461F in Table IV, dealing with voltage spikes on power input lines. The applicability list (Table V) includes a few changes in applications associated with some of the test methods (CE101, CS109, CS115, CS116, and RS101).
CE101, conducted emissions, power leads, 30 Hz to 10 kHz (5.4): Applicability added to surface ships. In addition, the appendix provides some tailoring guidance for high current loads or certain wiring considerations. Suggestions are made for the use of a 5 H LISN (line impedance stabilization network) and limit adjustments with frequency range changes. The MIL-STD-461 has long-supported tailoring for several revisions but this represents one of the few specific suggestions for tailoring the test approach instead of applying the test approach as written. Note that the tailoring suggestion falls on the procuring party, but a test plan could suggest the tailoring for approval by the procuring party. Calibration verification with the test equipment (all equipment – cables, probes, attenuators, amplifiers, and receivers) are accomplished prior to test. If multiple limits for different power inputs are specified, use the most restrictive limit to demonstrate.
If alternate LISNs need to be used for particular applications, then customizing (tailoring) the CE102 testing will need to be accomplished. The MIL-STD-461F appendix provides some guidance that will help the test procedure development for this special circumstance.
CE106, conducted emissions, antenna terminal, 10 kHz to 40 GHz (5.6): Testing both receive and standby modes are unchanged. CE106 transmitter limit for the 2nd and 3rd harmonics was redefined to a level of minus 20dBm (87 dBV) or 80dB below the fundamental, whichever requires the least suppression; all other harmonic and spurious emissions are to be suppressed by 80dB. Assuming a 100 W (50 dBm – 157 dBV) transmitter the suppression would be 70 dB to achieve the -20 dBm level for the second and third harmonic and 80 dB for all other frequencies. Some of the practicalities of making this measurement are getting dynamic range necessary to show compliance to the limit. Other issues include determining the frequency span associated with the harmonic emissions.
You’ll have to address several questions prior to testing– typically during the test procedure development—so both the right equipment and test approach are in place to support the test. Be prepared for several hardware configurations and the associated calibration verification. Questions include:
How is the transmitter output power verified since in-band testing is not required?
How is the fundamental suppressed without sacrificing sensitivity at out-of-band frequencies?
Is the power in the harmonics sufficient to cause a non-linearity in the detection system?
How do you handle connection to a transmit port with a type N connector during testing up to 40GHz?
CS101, conducted susceptibility, power leads, 30 Hz to 150 kHz (5.7): No changes in the requirements, but don’t forget the capacitors in the test setup – the higher frequency losses in the LISN without the capacitors is fairly dramatic and results in a significant under-test. Make all personnel aware of the potential for shock hazards from the isolated “floating” oscilloscope configuration.
CS106, conducted susceptibility, transients, power l (5.11): This is a new requirement that brings power line voltage transient testing back into the requirements for some applications. This new requirement restores CS06 testing from MIL-STD-461C (super-ceded in 1993) but with only one pulse duration. The details are spelled out in the standard but basically the waveform (5S pulse at 400V) is pre-calibrated into a non-inductive 5 resistor and that generator setting is used as the maximum applied if the 400V pulse is not generated during the test with a lesser generator setting. The waveform characteristics are very well defined and some of the older spike generators are not adequate for the specification. Once the generator level is calibrated, the transients (both positive and negative transients) are applied to all ungrounded power inputs between phases or between the phase/positive and the neutral/return (application between the chassis and phase/positive is not applicable. The test duration is 5-minutes with a 5-10 pulse/second repetition rate for each polarity. A difference from the old CS06 is that phase synchronization is not applicable. Again watch for the shock hazard with the ungrounded oscilloscope.
CS114, conducted susceptibility, bulk cable injection, 10 kHz to 200 MHz (5.13): The testing is basically the same with an additional common mode test for power leads in the 4kHz-1MHz frequency range for some applications. Note that it is a “common” mode test for power lines. Just a refresher on the process because it is often accomplished incorrectly:
Pre-calibrate the applicable calibration curve levels to establish a maximum forward power level for the test frequency range. The standard also reminds us to use the same hardware as used for the calibration.
Select a cable for test and apply the lesser of the test current (note that “test current” is the calibration current plus 6dB) or the maximum forward power. Why are they different? During calibration the actual loop impedance is 100 so the current would be reduced by 6dB compared to a 50 circuit.
The test signal modulation is specified to be pulse modulation (PM) with a 1kHz square wave. Most signal generators in the test frequency range do not support PM so AM is frequently used which would add 6dB if 100% AM was used to attain the on/off ratio specified. Two issues with using AM – does the amplifier have the necessary drive in its linear region and can the test article tolerate the over-test? Assuming that both issues are satisfactory, test with AM but remember that noted susceptibility may be because of the excessive test level from AM so measure the threshold considering this factor.
Measuring the applied current can also be tricky, if not impossible, with the modulation applied. The drive level is determined with the test signal unmodulated CW.
Previously there were some issues with testing the power leads or phase lead in shielded power cables so often the test engineer would cut the shield to access the leads. This was NOT the intent – the phase or power group should not be tested separately if the cable is shielded. MIL-STD-461F resolved the problem by declaring that shielded power cables are not allowed. For systems that allow shielded power cables, maintain the philosophy of keeping the shield intact and omitting the phase lead testing. Also note that phase leads are tested as a group (common mode) and not tested individually.
CS116, conducted susceptibility, damped sinusoidal transients, cables and power leads, 10 kHz to 100 MHz (5.15): The test requirements have a couple of changes that are easily over-looked.
The requirement to test with the power off has been removed.
Paragraph 220.127.116.11.c(3) instructs us to apply the calibrated test signal with a note to reduce the signal if necessary. Following that, we are cautioned that, for shielded cables or low impedance circuits, it may be preferable to gradually increase the drive to the lesser of the pre-calibrated level or the test current (in this case the test current is the defined as the current not plus 6dB even though the calibration loop impedance is similar to CS114). Since we seldom know the characteristic impedance of the cable loop, I think that all tests should be performed as if all circuits are low impedance. The reasoning is if we apply the full calibrated level and allowed the smoke to exit the circuit, all parties would be unhappy
(except for my guy Adam who always wants to let the smoke out).
One notable difference in CS116 versus CS114 and CS115 is that the phase leads are tested individually (differential mode) instead of as a group.
MIL-STD-461F specifies a frequency tolerance of 2%. However a DoD department has issued a deviation to accept 10% tolerance for the CS116 waveform. This tolerance is generally accepted but a change notice to the standard has not been issued so get acceptance for this in the planning stage if possible.
Finally, note that the limit is “peak current”. I point this out because at Washington Labs, we have witnessed tests where the rms current was used as the calibration level and the pulse subsequently applied. Setting the drive level with an rms voltmeter and allowing the pulse to rise to peak usually results in a fairly severe over-test. Obviously with dampening the rms level would have to be calculated and that would require decisions on how many cycles would be used to arrive at the rms. Normally, rms measurements are used throughout the standard but the measurement should be in the same terms as the limit for comparison. A rms level is inappropriate for CS116 testing.
RE101, radiated emissions, magnetic field, 30 Hz to 100 kHz (5.16): The change in this test method involves dealing with over-limit emissions. If over-limit emissions are detected at the 7-cm antenna location, MIL-STD-461F calls for determining the distance from the EUT where the emissions meet the limit. This data is used to help determine if the emissions need to be suppressed or some reasonable “set back” can be prescribed.
RE102, radiated emissions, electric field, 10 kHz to 18 GHz (5.17): Not much change in the requirements but a couple of procedure changes are present for RE102. Applicability and frequency ranges were slightly changed. For example, the exemption for testing at the transmitter fundamental frequency added the following phrase “and the necessary occupied bandwidth of the signal” to the procedure; common sense, naturally. The upper test frequency based on ten times the highest intentionally generated frequency of the EUT still applies and may offer some test time relief instead of automatically measuring out to 18GHz; the upper frequency level, then, is predicated on the EUT frequencies, which should be determined in advance of the test.
Also recall that specific antennae are called out in the standard. This brings about a significant change regarding configuration of the low frequency rod antenna. MIL-STD-461F no longer requires connection of the counterpoise to the ground plane via an elevated plate sized larger than the counterpoise. Instead, you connect the rod antenna cable to the enclosure floor as soon as the cable allows plus placing a ferrite on the cable. This procedure change has been discussed at several EMC meetings with many indicating that the previous method was more repeatable. I am unaware of movement to rescind the new procedure as of this writing.
RE103, radiated emissions, antenna spurious and harmonic outputs, 10 kHz to 40 GHz (5.18): RE103 testing presents a lot of challenges. This test is an alternative to CE106 and should be used only when CE106 is not a viable option. The changes from the previous revision are not significant but, because of the issues, a discussion here is merited.
The test process:
Verify calibration of measurement system (include transmit frequency rejection network)
Establish far-field test location and position equipment
Measure Effective Radiated Power (ERP) (assume power monitor is not available). Verify that the ERP compares with the expected ERP based on the operating parameters of EUT.
Establish limit based on measured ERP
Scan measurement receiver over test frequency range to locate harmonics and spurious emissions. Measure and compare to limit (note that the measurement system antenna may needs to be positioned to maximize detected the emissions).
Verify calibration including the rejection network. For most applications a rejection network is needed to prevent over-loading of the detection system. Even if the transmit power is low enough not to damage the receiver input, the high level signal may cause spurious signal in the receiver system that could mistakenly be observed as valid signals. The rejection network normally is a notch filter but could be a variety of high-pass and low-pass filters preventing the fundamental frequency from over-loading the receiver. Since a tuned transmitter is tested at various frequencies, provisions are required for the rejection network to be tuned to achieve the proper rejection. A simple attenuator will cause sensitivity loss throughout the test frequency range. In short, consider the RF conditions of sensitivity, receiver spurious response and dynamic range to get an accurate and valid measurement.
One issue is radiated testing in the far field, which for low frequency transmitters can be a very large distance. As an example a 300MHz transmitter may be installed in a vehicle and connected to a 1.5-meter whip antenna. In this case, the far field would be 4.5-meters using the RE103 calculation method.
Another issue is with testing low power transmitters is the limit may be lees that the detection system sensitivity. Even a 1-watt transmit power can present issues depending on the system antenna and operating frequency. Lots of planning is needed for this test.
RS101, radiated susceptibility, magnetic field, 30 Hz to 100 kHz (5.19): Changes are minimal with one significant procedural difference: The scan rate of “three times the standard” has been slowed to the “standard” scan rate. Because of the number of test locations for the 30cm X 30cm placement, this seemingly minor change results in a significant test time increase – nearly 3-times the prior test time. The use of an alternate Helmholtz coil method may need a closer examination, although for most labs, building a Helmholtz coil large enough to accommodate the variety of equipment sizes is one big drawback not only in handling the physical size but the wire length needed for construction requires a lot of power to overcome the impedance of the coils.
RS103, radiated susceptibility, electric field, 2 MHz to 40 GHz (5.20): RS103 saw some changes that may be considered significant or minor, depending on the perspective of the responsible party for the EUT or the test facility.
Sensor placement clarification was added to position the sensor at the EUT location and to position the sensor vertically at the point of the EUT illumination. In addition, a test was added to verify that the sensor is responding to the fundamental frequency, as opposed to the harmonics of the test amplifier.
Positioning of the radiating antenna at 1-meter or more from the EUT was incorporated - eliminating the capability of moving the antenna closer to achieve some of the specified field strengths at all test frequencies. This will cause many labs to lower the test levels they can support or create a demand for higher power amplifiers. Either way, the cost of testing will increase.
Changes in the receiver limit may have the largest impact in those cases where receivers are included in the test article – which is ever-increasing with wireless technology being incorporated into more and more products. The standard states that there is no requirement at the tuned frequency of antenna connected receivers except for surface ships and submarines. Does this mean that the frequency range is exempt from test? Review of the appendix for guidance indicates that there is no relaxation for platforms other than surface ships and submarines. What is the limit for surface ships and submarines? It isn’t provided in the standard. The prior revision provided a limit of RE102 limit plus 20dB but with the removal of a reduced limit, are receivers with embedded antennae supposed to tolerate and operate in the presence of a 200V/m interfering signal? Can we expect that a receiver with a sensitivity of -120dBm to function with a signal approximately 180dB above that sensitivity? Is the receiver front end subject to damage? The standard appears to have some gaps in this area that will need to be addressed by procedure development or a change notice. Looks like some test procedure tailoring will be warranted.
DID EMICP (DI-EMCS-80199C): This EMI Control Procedure DID (data item deiscription) documenting the contractors design procedures and techniques is unchanged from the previous revision. Often this document is prepared to satisfy the contract requirements and often goes unused during the development which is unfortunate because a well-prepared control procedure will direct the design for compliance. It is also an ideal document to analyze the requirements and to identify tailoring for the planning effort and to support contract modifications to approve tailored requirements. Organizations that truly use the EMICP are far more successful in attaining compliance with little or no changes to the product as a result of the testing.
DID EMITP (DI-EMCS-80201C): This EMI Test Procedure DID documenting the test procedures to be used to evaluate the product for compliance to the standard. Changes place more emphasis on documenting the software for automated testing and the incorporation of correction factors and presentation of the results. Like the EMICP, this document is under-used. The details provide an understanding of the exact hardware including support equipment, the test configuration, and really helps define the pass/fail criteria. This is the location where the tailoring of the testing is fully brought to maturity and upon approval reduces the urinary battle over issues that manifest themselves during the test.
DID EMITR (DI-EMCS-80200C): This EMI Test Report DID documents the test results. Changes involve presentation and documenting susceptibilities. The test standard has called for presentation of charts that support a minimum frequency resolution of 1% or twice the measurement bandwidth and amplitude resolution of 1dB. This has largely been ignored especially when the results show compliance with “significant” margin where knowing the exact values is relatively un-important. This DID states that the resolution will be required and more specifically, a single chart cannot be used to present the emissions data, rather, multiple charts may be necessary to prove that the necessary resolution/accuracy is presented. Presenting a sample in the test procedure for approval prior to testing is recommended.
In addition, the DID change specifies that susceptibility shall be noted with threshold measurements. In reviewing past reports, susceptibility is often noted, but threshold measurements often omitted. This poses a question – how many frequency points should the threshold be measured over the frequency range where susceptibility is noted? This should be documented in the test procedure.
Declaring compliance with MIL-STD-461F has limited meaning without a description of the methods used for evaluation and the test levels and limits. For example, the test parameters for a submarine are significantly different than a test for aircraft external applications.
Finally, the goal of all testing is to evaluate the product correctly, making a sensible test and getting valid results. Blind application of the standard without consideration of unique aspects associated with the product under test results in a situation that may fall short of the goal. The MIL-STD-461F states in paragraph 6.4 “When analyses reveal that a requirement in this standard is not appropriate or adequate for that procurement, the requirement should be tailored and incorporated into the appropriate documentation, prior to contract award or through contractual modification early in the developmental phase.” Hence, although there are specifics that must be applied, the standard allows judicious tailoring to prepare a viable test procedure and properly do the test.
This article has addressed some of the specific details that must be considered when developing a MIL-STD-461F test program.
Steven G. Ferguson is VP of operations at Washington Laboratories and has been working in the compliance test arena for over 35 years at test laboratories and manufacturing companies designing products, developing procedures and performing tests. He presents a hands-on course in testing to MIL-STD-461 at the Washington Labs facility in Maryland and on-site for multiple government and industrial clients. E-mail: firstname.lastname@example.org.