MIL-STD-461G Requirements and Tests

Data Item Descriptions (DIDs) provide guidance for preparation of documents associated with procurement specifications. MIL-STD-461 lists three specific DIDs that are normally required as deliverable items during the design to delivery phase of a procurement contract.

DI-EMCS-80199C – Electromagnetic Interference Control Procedure (EMICP) supports a review of a product design as related to EMC. This document is prepared early in the product development process to provide a design guide to incorporate EMC control measures. If associated with a development procurement, the EMICP is required within 60-days of contract award. This gives the agency an opportunity to confirm that the contractor understands the EMC requirements and has a feasible approach to meeting those requirements. The content includes:

• Management – Document responsibilities for design, test and overall program oversight with names and contact information for key personnel. List sub-contractors and how the EMC requirements are imposed on the sub-contractor. Describe the role for Government Furnished Equipment (GFE), for example, if your item needs to be installed in an F-35 to verify compatibility, then the use of an F-35 may be a needed GFE item. Of course, the device needs to be fully described including the installation. Discuss the plans for identifying and mitigating potential problems. This is also an opportunity to identify conflicting requirements and seek resolution via a contractual review – if contract has resolved conflicts then be sure to document the final agreement. Note that the EMICP may be revised as more information becomes available and issues are resolved.
• Design techniques and procedures – Describe how to incorporate control measures to address: 1) spectrum management, if applicable; 2) mechanical design (shielding, compartments, openings, bonding, corrosion control, etc.); 3) electrical design (wiring, grounding, filtering, component selection, isolation, etc.).
• Analysis – provide a thorough analysis on how the design techniques will yield control for each of the identified requirements. If filtering is to be used as a control measure, discuss how a filter insert with selective capacitance can reduce emissions and limit susceptibility without degrading the signal integrity.
• Developmental testing – describe how models will be used to assess the EMC performance through testing and how the test results will be used to make changes in the design.

DI-EMCS-80201C – Electromagnetic Interference Test Procedures (EMITP) describes how the testing will be conducted to qualify the item for EMC. This document provides the details for testing and the instructions for the test technical staff. Content includes:

• Introduction contains a complete listing of the tests to be performed along with a detailed product description that includes operation, installation data, and power usage. List approved deviations to contract requirements.
• Applicable documents – list military, government, industry, and company standards that support the testing program.
• Test site – describe the facility (shielded enclosure or open area site), the grounding scheme and precautions listed in MIL-STD-461.
• Test instrumentation – describe what test equipment is planned for use and its characteristics (e.g., antenna factors, filter performance, etc.). Describe operations by software and the verification process to assure software is functioning properly. Provide a listing of the scan rates and dwell times and confirm that the cycle time of the test item is within the dwell period.
• EUT setup – describe the test configuration management with details on cable layout and bonding/grounding of the test article. Document how the interfaces (electrical and mechanical) are used to support the functionality and monitoring of test article performance.
• EUT operation – describe how the test article is operated during testing and the rationale for selecting the operational modes. Define the acceptance criteria objectively, with clear guidance on what performance element constitutes non-compliance.
• Measurements – provide step-by-step instructions for performing each of the tests with the applicable instrumentation for the individual tests. List the information to be recorded during the test and provide samples of the data sheets, logs and graphs.

DI-EMCS-80200C – Electromagnetic Interference Test Report (EMITR) describes the results of the testing for evaluation by the approval authority. Understanding the EMITR content requirements is necessary to assure that all pertinent information is collected during the testing.

• Administrative data – this area should provide the overview of the results along with the administrative details related to the contract. Describing the actual test article is specified with detailed information on the build status (e.g., circuit board revision level, wiring, bonding/grounding and current flowing on each of the power lines. Information should be adequately detailed to support re-creating the test if necessary. The details are frequently associated with individual tests, so locating with the detailed results section may be appropriate.
• Detailed results – calls for documenting a lot of information about the EUT design, test setup, test instrumentation, measurements and data reduction calculations. If the EUT is found to be susceptible, threshold measurements are to be part of the report.
• Conclusions and recommendations should be included, and if non-compliance issues are identified the conclusions should include actions to be taken to assure compliance.

MIL-STD-461G added a requirement to document the verification results of the EUT bonding in the test report. The bonding is required to use the design provisions for connecting the equipment case to mounts or to the ground plane and to use methods as specified in the installation drawings.

This addition effectively removed the tendency to make special ground connections and bonds of 2.5 milliohms or less for the complete grounding path for the test configuration and ignoring the installation practices. Note that a value is not specified but conformance to the installation is required.

Verification of the bonding should be accomplished prior to test and corrections made if found to be deficient before testing proceeds. Using bonding points identified in the EUT documents should be measured and the test points and measurements are recorded for the test report.

MIL-STD-464C provides some measurement guidance on the DC resistance of the bonding:

• 10-milliohms from equipment enclosure to the system structure
• 15-milliohms from cable shields to the equipment enclosure
• 2.5-milliohms across individual faying interfaces within the equipment

This guidance would place a requirement of 25-milliohms between a cable shield and the system structure. This could be significant for rack mounted enclosures if the rack provisions are not carefully considered.

The bonding also has an impact on the performance of filter connectors, filter inserts or transient suppressers that rely on connection to the equipment ground point or chassis. The bonding resistance is placed in series with the reactive component of the filter component that decreases the effectiveness of the filter.

When verifying the bonding, examine the assembly for incidental bonds that may not be present in the installation. For example, a rack mounted unit could electrically connect at the equipment mounting tabs in the test configuration and then be placed on a powder coated rack mount in the actual installation negating the bond.

For safety purposes if hazardous voltages are present, MIL-HDBK-2036 reminds us to achieve 100-milliohms or less to avoid shock hazards presented by fault conditions.

CE102 is applicable to all applications that have power connections to mains or sources serving as a common power source. The goal is to limit emissions conducted into other devices sharing the power source and, as a by-product, help reduce radiated emissions from the power cables.

The limit is based on the voltage used by the EUT with a basic curve shown in the standard and a table listing the adjustment level for common voltages. For voltages not listed, one should use the more restricted voltage limit (i.e., if the voltage is 240Vac then use the 220Vac limit relaxation – avoid extrapolation). Many devices operate over a large voltage range. In this case, testing should use the nominal voltage for the application, but for wide-spread usage, consider testing at the low and high nominal levels.

Voltage measurements are made at the LISN measurement port on each line. The LISN measurement port on each line should be terminated with 50-? (the receiver provides the termination when making measurements).

As with most of the MIL-STD-461 test methods, a pre-test signal integrity check is accomplished to verify the measurement system accuracy. Figure 3 provides a basic sketch of the signal integrity check configuration. The verification process measures a known signal voltage normally set to be 6 dB below the limit for the various check frequencies. In addition, MIL-STD-461G adds a step that checks the LISN impedance compensating for the reduced calibration requirements implemented in the “G” revision. This sounds like a very straight-forward check – set a voltage level and then measure it. The problem is that the LISN impedance at low frequencies is low (~5-? @ 10 kHz) loading the signal generator 50-? output. When connected to the LISN, the signal level is less than the signal generator readout. The signal generator true output is adjusted to the correct level as measured by the oscilloscope with the LISN connected. This method is used for the two lower signal integrity check frequencies. The oscilloscope measurements are re-accomplished with the LISN disconnected from the signal generator and the difference between the loaded and unloaded measurements should meet the specification in MIL-STD-461. If this step fails to comply, repair of the LISN is indicated.

CS101 is applicable to all applications that connect to an external primary power source that is not dedicated to a single device. The test goal is to determine if power line conducted interference over the frequency range of 30 Hz to 150 kHz causes the device performance degradation.

The susceptibility testing pass/fail is now related to the EUT performance with the test limit describing an interference level. In the CS101 case, two test levels are established – 1) a test voltage applied to the EUT or 2) a pre-calibrated drive level based on a 0.5-? load of the interference source.

Figure 5 provides the basic CS101 configuration with the calibration setup used instead of the test setup. The calibration level is a curve over the test frequency range identifying the power dissipation into a 0.5-? resistor for the frequency range. The interfering power source settings are recorded during calibration to be used as the maximum drive if the EUT impedance prevents attaining the test voltage.

During the test, the interfering signal source is adjusted to produce the test voltage measured at the EUT power terminals. Current flows in a loop from the coupling transformer secondary through the EUT and is returned via the power supply mains. As the test frequency increases the LISN impedance increases creating a voltage drop reducing the voltage drop at the EUT. The capacitor across the LISN terminals reduces the LISN voltage by providing a low impedance path for the current loop.

The EUT characteristic impedance controls the voltage drop. If the impedance is low (less than 0.5-?) the test voltage will not be attained with the calibration settings, so the drive level limiting function satisfies the required test level.

The test configuration circuit has a potential to become resonant with significant voltage drops other than across the EUT. If the test voltage is not attained at frequencies below 10 kHz, an investigation is needed to determine where the voltage drop occurs and to implement methods to minimize the effects of non-EUT voltage drops.

Notice that the measurement oscilloscope uses an isolation transformer to prevent accidental grounding of the power line under test. This allows the oscilloscope to become a potential shock hazard if connected to an ungrounded neutral or phase lead. Use caution!

MIL-STD-461G introduced the use of a Power Ripple Detector (PRD) as an alternative to the standard test approach to allow measurement in the frequency domain by separating the power frequency from the interference frequency. This is a convenient means to measure the test voltage, especially at the lower frequencies.

If issues are identified, solutions tend to be somewhat difficult at lower frequencies for AC powered devices reacting to power frequency harmonics. At higher frequencies, filtered connectors or filter inserts can be very effective.

CS104 is another receiver front-end susceptibility evaluation to quantify the ability of a receiver to reject unintended signal reception while maintaining sensitivity to receiving desired signals. The goal is to determine if out-of-band signals appearing at the antenna port cause spurious responses or desensitization of the receiver.

The test uses a 3-port network to input 2-signals simultaneously into the receiver. One of the signals is set to the receiver tuned frequency with the EUT modulation applied. The amplitude is set to be just above the threshold of the receiver sensitivity to establish receiver operation. The second frequency signal is set to the test frequency with the amplitude set to the rejection requirement level above the receiver sensitivity. The second frequency signal is tuned through the test frequency range while the receiver is monitored for indications of susceptibility. If susceptibility is observed, lower the second signal amplitude until the receiver recovers to determine the rejection capability.

The requirements and test frequency range are determined for the specific technology associated with the receiver. Therefore, the procurement specification provides the details on testing and the required rejection. It is common for the rejection level requirement to be somewhat less restrictive for frequencies within the tuning range of the receiver but outside the tuned frequency bandwidth.

Band-pass filtering is typically used to reject out-of-band frequencies in the receiver design and additional antenna port filtering tends to be avoided because of excess capacitance affecting the RF signals intentionally received.

CS109 has limited applicability dealing with structure current on surface ships and submarines that may flow on the surface of equipment operating at 100 kHz or less and a sensitivity of 1 µV or less such as tuned receivers operating in the test frequency range of 60 Hz to 100 kHz. The potential threat is to equipment with metal chassis attached to the metal structure of the ship. Current from power generation and distribution, power frequency harmonics and low frequency transmitters that are coupled to the ship structure may flow through the equipment metal chassis and induce voltages into the equipment. The induced voltage may interfere with the performance of the equipment.

Testing is accomplished by isolating the EUT from any ground forcing current applied to the EUT chassis surface to flow on the equipment surface. The current loop attachment points are at diagonal extremes of each surface that could be the path for current flow. Armored cables, shielded cables and conduit terminated at the EUT are also classified as test points. The test of an individual surface presents a worst-case current. In the actual installation, the current may be divided by various paths lowering the individual surface exposure.

The test involves making sure that the isolation provides only a single point ground and then connecting the current source at selected attachments. The unmodulated signal source amplitude is adjusted to provide the test current through the surface under test and the frequency is swept through the required frequency range maintaining the specified current.

Susceptibility issues are typically resolved by placement of the sensitive circuit or shielding to divert the magnetic field generated from the surface current. Chassis isolators may be incorporated to prevent current flow across the sensitive surface if safety grounding concerns allow isolation.

CS115 testing applies to most installations except ships and submarines, unless that applicability is specified in the procurement contract. The purpose is to evaluate the EUT’s ability to withstand impulsive signals coupled onto associated cables. Transients normally have a broad spectral energy content and parts of the spectrum capacitively and inductively couple to adjacent cables, inducing transient current in the victim cable.

Testing is very similar to CS114 with the differences being:

• A pulse generator is used as the interference source instead of an RF signal source. The generator waveform is shown in Figure 8 with the waveform on the left appearing as the target waveform during the calibration. The right-side waveform during calibration is considered acceptable. The transition time is less than 2 nS, so high frequency probes and measurement bandwidths are needed to prevent expanding the response time.
• An oscilloscope is used instead of the measurement receiver for time domain measurements of the test waveform.
• The monitor probe is not required for the calibration configuration, so verification of the monitor probe is not included. The correct operation of the monitor probe should be checked to confirm that the probe factors are correct. If the probe was used for CS114, then that check will support CS115 testing.
• Testing is accomplished at the pulse generator setting established during calibration and the current applied is measured and recorded as the test current – it is NOT adjusted to the 5-amp limit. The value at 175 MHz should be used as the monitor probe factor for data reduction based on the waveform shape requiring integration of a sine wave up to that frequency to be part of the shaping function.

Once the waveform calibration is complete, the injection probe is placed 5 cm from the monitor probe, which is 5 cm from the EUT connector. The pulse generator is enabled, and the amplitude is set to the calibration level. The impulses are applied at a 30 Hz rate for 1-minute while the EUT is monitored for susceptibility. The applied current is recorded, and photographs of the applied waveform are captured for the report. Note that the waveform appearance may differ from the calibration because of the cable effects. If the waveform appears to be upside down, turning the injection probe over will tend to present a positive going pulse to be more like the expected pulse.

Susceptibility issues often use filter connectors or filter inserts as the primary means to mitigate the problem. Transient suppressor inserts can be beneficial in reducing the energy applied to the affected circuitry.

CS117 was introduced as a new method with the release of revision “G”. It has limited applicability, usually related to safety-critical equipment cables or non-safety critical equipment connected to safety-critical equipment. The purpose is to evaluate the EUT’s ability to withstand lightning induced transients coupled onto the cables.

The test approach is similar to CS116, with a variety of waveforms selected by the coupling method allowed by the installation and includes multiple stroke and multiple burst lightning events. The test levels are significantly higher than CS116.

The test waveforms are based on current or voltage with one being the test target value and the other providing a limiting function. The various waveforms and applicability of each are discussed in MIL-STD-461G. In the example (see Figure 10), waveform WF1 provides the test current (IT) and WF2 provides the limit voltage (VL) parameters.

In the calibration configuration (see Figure 11), adjust the WF1 to the designated test current (IT) with the shorted loop setting and verify the waveform parameters. Using the open loop setting, adjust the WF2 to the designated test voltage (VT) and verify the waveform parameters. It is not required that the current or voltage limit (IL or VL) be verified, but if the generator can attain those levels, record and verify the waveforms.

After the waveform levels and parameters are verified, configure the EUT cable under test through the current monitor probe and injection transformer and adjust the generator output to the test level or until the limit level is attained. If the limit is reached before the test level, the acceptability is evaluated as follows:

• If a compliant limit waveform was attained during calibration, the test is acceptable
• If the limit waveform within the waveform tolerances was attained during test, the test is acceptable
• If neither of the above criteria is met, then testing with a generator that can meet the limit waveform requirements is to be accomplished.

Multiple stroke and multiple burst testing are applied to the testing for each of the defined waveforms.

• Multiple stroke testing applies an initial transient at the designated level followed by thirteen subsequent transients at a decreased level over a period of up to 1.5-seconds. Ten multiple stroke applications are applied with up to 5-minutes between each application.
• Multiple burst testing applies a group of twenty transients with 50 to 1000 µS between transients with 3-sets of bursts spaced between 30 and 300 mS. Burst groups are applied every 3-seconds for a minimum of 5-minutes.

Transient suppressors are the primary means to protect sensitive circuity from the effects of the induced lightning. Cable shields and installation wire routing to prevent the coupling are potential solutions, but many applications do not support these approaches.

RE101 has limited applicability with the focus on systems that have devices sensitive to magnetic fields that operate at low frequencies. The purpose is to measure magnetic fields for compliance with the applicable limit reducing the risk of interference to other devices. Two limits are included in the standard, but as with any emissions limit the system compatibility requirements could prompt tailoring the limit.

Prior to test a signal integrity check is made as with most MIL-STD-461 test methods. In this case a known signal level is applied to the antenna cable planned for use during test and that signal is measured to confirm proper measurements. Note that the antenna is not present for the injected signal testing, but the measurement receiver should apply the antenna conversion factors, so the injected signal level should be 6 dB below the limit and less the antenna factor. The measurement system software will add the antenna factors so the resultant measurement should be 6 dB below the limit. A second step of the signal integrity check involves measuring the antenna DC resistance to confirm that the coils of the loop antenna are not open or shorted.

Testing involves placing the loop antenna 7-cm from the EUT face and measuring the field with the antenna oriented parallel to the EUT face and perpendicular to the ground plane base. The coverage area of the loop antenna is relatively small, so the antenna is slowly moved about the EUT face maintaining the 7-cm spacing while operating the receiver in a “max hold” mode. At the location of maximum emissions, the plane of the antenna is varied to obtain the maximum reading. The standard prompts the test engineer to measure at least two frequencies per octave below 200 Hz and three frequencies per octave above 200 Hz. This process is a hold-over from the days of manually making measurements – with the “max-hold” continuous sweeps, all frequencies are measured throughout the test frequency range.

If over-limit emissions are presented, the antenna is moved away from the EUT to determine the distance where the limit is met. This information may support acceptance of the over-limit condition for applications that do not have magnetically sensitive components near the EUT.

Resolving magnetic field emission issues may use filter connectors or filter inserts if the cable current is the source of the emissions. Reducing the current may aid in emission reduction, but in many cases the current is necessary to perform the function. However, if the switching speed can be slowed, the transient current will help reduce emissions. Shielding is another means of containing the emissions and the use of ferrous metals as the shield material may divert the flux lines through the shield and reduce the distance of the field.

RE103 may be used as an alternative for CE106 when testing transmitters with their intended antennas or if the antenna impedance curve is non-standard. The test frequency range is based on the operating frequency range of the EUT with test start frequencies specified. The test upper frequency for EUTs that operate at less than 1 GHz is the greater of 20-times the operating frequency or 18 GHz. For EUT that operate above 1 GHz, the upper test frequency is the lesser of 10-times the highest frequency or 40 GHz.

The limit is suppression of 80 dB from the transmit fundamental frequency except for the 2nd and 3rd harmonics. The 2nd and 3rd shall be suppressed to -20 dBm or 80 dB from the fundamental whichever requires less suppression. Special applications may vary these suppression levels.

The signal integrity check involves measurement of a known signal through the rejection network and amplifier/attenuator circuits like used for CE106 with the measurement receiver antenna disconnected. If different measurement system hardware is used for various frequency ranges, signal integrity checks for those configurations are also necessary.

Testing is accomplished in the far field with the distance calculated from EUT and measurement system antenna physical parameters and the transmit frequency wavelength. The operating frequency of the EUT determines which far field formula is used to calculate the distance.

Measurements start with confirmation of the transmitter effective radiated power (ERP) by using a power monitor, if feasible to insert a power monitor, adding the antenna gain and converting the power measurement to dBW. With the EUT in transmit mode, tune the measurement receiver to the transmit frequency for the maximum measurement. Align the transmit and receive antenna to maximize the measurements. Record the measurements and the measurement receiver bandwidth (note that the standard bandwidth setting in MIL-STD-461G is replaced by the optimum bandwidth to maximize the transmit signal and the signal to noise ratio). Calculate the ERP and convert to dBW using:

ERP = V + 20logR + AF – 135

Where:

• V = measurement receiver amplitude in dBµV
• R = distance between transmit and receive antennas in meters
• AF = antenna factor of receive antenna in dB/m

If the ERP measured and ERP from the power monitor differ by more than 3 dB, check the test configuration for errors. If power monitor measurements are not feasible, then determine the ERP from other methods for the comparison. Assuming the ERP agrees, then the ERP becomes the reference for assessment of the suppression.

The tuning of the EUT is often the cause of ERP disagreement, so make sure that the transmitter is operating at the rated power.

Make sure that the compliance limit is aligned with the ERP and adjust the limit if necessary. Scan the measurement receiver over the test frequency range using the optimized bandwidth setting and the antenna positioning and compare the test results with the limit.

RS103 is applicable to all services and application with a variety of test levels defined based on the exposure threat for the environment. The test frequency range is normally 2 MHz to 18 GHz with a defined option up to 40 GHz if specified in the procurement contract. The purpose is to determine if the device is susceptible to radiated electric fields.

For receivers having permanently connected antennas, reduced performance is normally allowed over the frequency range of the receiver operation. If the antenna can be removed for test the shielding effectiveness of the antenna may present challenges in preventing interference from entering the antenna port. The test procedure should identify these challenges and how the test requirements adjusted to compensate for this potential issue.

Depending on the test approach to be used, a pre-test calibration may not be required if real-time monitoring of the field is included in the testing process.

The testing places the EUT in the shielded enclosure with a field monitoring probe located at the test boundary at least 30-cm above the ground plane. The field probe location should avoid EUT shadows or locations where signal reflections can influence the field measurements. With the EUT operating scan the test frequency range maintaining the required test level while monitoring the EUT for indications of susceptibility. During test the radiated signal is pulse modulated by a 1 kHz square wave using a 3-second minimum dwell time at each frequency step.

The radiating antenna is placed 1-meter or more from the test setup boundary with the horizontal location determined by the frequency range and antenna beamwidth. Refer to the RE102 discussion above for more detail on the EUT and cable exposure requirements but note that the antenna can be more than 1-meter from the boundary so the coverage area maybe larger.
The field generating system has the potential to create a harmonic frequency that is higher than the fundamental test frequency depending on the signal generator performance and amplifier power curve. The standard requires that the field be checked for this risk. Since field probes do not typically provide a frequency indication, many laboratories verify the system performance with a periodic room/system calibration check.

The use of a reverberation chamber is listed in the standard as an alternate means of RS103 testing with a mode-tuned approach. This method uses an unlined chamber with highly conductive walls and mode tuning paddles to distribute the field within the test volume. The reverberation chamber provides an advantage in exposing all EUT faces simultaneously and creating very high-level fields with relatively low power amplifiers. The test time may be extended because of the physical movement of the tuning paddles and if susceptibility is noted, determining the entry location is a little more difficult. Loading effects of the EUT are needed to manage control of the test levels or a maximum loading is applied for the calibration and the resulting over-test is allowed.

Radiated field susceptibility issues take on a variety of solutions, from filter connectors or filter inserts for cable coupled interference, to shielding and aperture closure if the chassis is the path for signals to enter.