4545-Paper by Steve Ferguson

7/31/2019 4545-Paper by Steve Ferguson

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MIL-STD-461 evolves to version FThe 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 mayaffect 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 aspart 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 incorporatedinto devices. The qualification may be accomplished by test or by similarity. Eithermethod requires approval of the procuring agency.

Construction and arrangement of EUT cables (4.3.8.6): 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 (4.3.8.6.2). Thisrevision 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 A4.3.8.6, 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 mostapplications the power cables should be unshielded.

Computer-controlled instrumentation (4.3.10.2): 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

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familiar with the laboratory software especially when preparing a procedure for testing at a

contracted test laboratory.

Bandwidths (4.3.10.3.1) Alternate Scanning Technique: Multiple faster sweeps withthe 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 reducedbut that is not the intent. The discussion in the appendix indicates that the faster sweepallows for capture of low duty cycle or intermittent signals. However in section 4.3.10.3.3,

the scanning rate must be adjusted to provide to support capture of infrequent emissions.

Net result the overall scan needs to measure at every resolution bandwidth over theEUT 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, dont hesitate to use thefaster sweep with multiple max hold capture to show the envelope of the signals of this

type.

Frequency Scanning (4.3.10.4.1): The susceptibility sweep rate or step size has

increased for frequencies above 1GHz, which provides for a faster susceptibility test. Thischange 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 (4.3.10.4.3): MIL-STD-461F adds a statement

Susceptibilities and anomalies that are not in conformance with contractual requirementsare 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 hotat

levels above that requiredand 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 isapplied to the EUT. This new statement requires documentation of the all observed (albeit

compliant) anomalies. This indicates that observations be documented (e.g., the standardcalls for RS101 testing at a level 10dB above the test level but if susceptibility is noted itcould 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. Theapplicability 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 highcurrent 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 rangechanges. 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 procuringparty. Calibration verification with the test equipment (all equipment cables, probes,

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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 secondand 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 harmonicemissions.

Youll have to address several questions prior to testing typically during the testprocedure developmentso both the right equipment and test approach are in place to

support the test. Be prepared for several hardware configurations and the associatedcalibration 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 detectionsystem?

How do you handle connection to a transmit port with a type N connector duringtesting up to 40GHz?

CS101, conducted susceptibility, power leads, 30 Hz to 150 kHz (5.7): No changes in

the requirements, but dont forget the capacitors in the test setup the higher frequencylosses 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 requirementthat 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 notgenerated 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

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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 loopimpedance 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 leveland 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 hasissued 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 thepulse 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 dampeningthe 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 forcomparison. 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 distancefrom 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. Applicabilityand 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 testfrequency 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 theenclosure 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

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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:

1. Verify calibration of measurement system (include transmit frequency rejectionnetwork)

2. Establish far-field test location and position equipment

3. 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.

4. Establish limit based on measured ERP

5. 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 rejectionnetwork 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 andlow-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 willcause 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 avery 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 thedetection 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 thestandard 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 alternateHelmholtz 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

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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 ofthe 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 wasadded 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 specifiedfield 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 receiversare 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 andsubmarines. 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 isnt

provided in the standard. The prior revision provided a limit of RE102 limit plus 20dB butwith 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 180dBabove 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 changes

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 idealdocument to analyze the requirements and to identify tailoring for the planning effort and

to support contract modifications to approve tailored requirements. Organizations thattruly 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. Changesplace more emphasis on documenting the software for automated testing and the

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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/failcriteria. 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 testresults. 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 beenignored 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 theemissions 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 thresholdmeasurements. 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.

Conclusion:

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 externalapplications.

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 thegoal. 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.

BIO

Steven G. Fergusonis 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 performingtests. He presents a hands-on course in testing to MIL-STD-461 at the Washington Labs

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facility in Maryland and on-site for multiple government and industrial clients. E-mail:

[email protected].

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4545-Paper by Steve Ferguson