MIL-STD-461G Requirements and Tests
Table of Contents
What is MIL-STD-461?
MIL-STD-461 has been the DoD standard for Electromagnetic Compatibility (EMC) qualification testing for many years, and has become the basis for many industrial and commercial applications. The standard has evolved since the initial issue was released more than 50 years ago, with many changes to the specific requirements and tests. Technology advances and issues with compatibility have spurred updates to the standard. The main goal, however, has been to provide a reasonable assurance that equipment’s unintentional emission limits are managed, and that the devices are not vulnerable to interference from natural and machine-made electronic signals and noise.
MIL-STD-464 provides the system level requirements. This system document points to MIL-STD-461 as the standard for qualifying individual subsystems and equipment. The system requirements may impose tailored MIL-STD-461 requirements for the subsystem qualification to support a specific application. This indicates that MIL-STD-461 requirements support generic usage for equipment that serve most cases, and tailoring applies to unique cases. Even as a generic type of standard, many variances to support wide-spread applications are included.
The evolvement of MIL-STD-461 includes some significant milestones that the product developers have faced and how changes have affected their approach to designing control measures. Although MIL-STD-461 is a test standard, the requirements drive many design parameters to obtain compatibility.
Documentation has always been a key element of EMC test and evaluation programs. Three specific documents are identified in MIL-STD-461G under the umbrella of Data Item Descriptions (DIDs) as applicable to the EMC qualification.
Configuration Management is a key to standardization, as described in MIL-STD-461 for various applications. However, tailoring may be preferred where the test configuration conforms to the actual installation if the device is used for a specific purpose. Documentation of the test configuration should support being able to re-create the initial test.
The standard includes 19 test and evaluation methods that document the various test parameters, plus general requirements to document many of the test parameters.
These methods are:
- CE101 – Conducted Emissions, Audio Frequency Currents, Power Leads
- CE102 – Conducted Emissions, Radio Frequency Potentials, Power Leads
- CE106 – Conducted Emissions, Antenna Port
- CS101 – Conducted Susceptibility, Power Leads
- CS103 – Conducted Susceptibility, Antenna Port, Intermodulation
- CS104 – Conducted Susceptibility, Antenna Port, Rejection of Undesired Signals
- CS105 – Conducted Susceptibility, Antenna Port, Cross-Modulation
- CS109 – Conducted Susceptibility, Structure Current
- CS114 – Conducted Susceptibility, Bulk Cable Injection
- CS115 – Conducted Susceptibility, Bulk Cable Injection, Impulse Excitation
- CS116 – Conducted Susceptibility, Damped Sinusoidal Transients, Cables and Power Leads
- CS117 – Conducted Susceptibility, Lightning Induced Transients, Cables and Power Leads
- CS118 – Conducted Susceptibility, Personnel Borne Electrostatic Discharge
- RE101 – Radiated Emissions, Magnetic Field
- RE102 – Radiated Emissions, Electric Field
- RE103 – Radiated Emissions, Antenna Spurious and Harmonic Outputs
- RS101 – Radiated Susceptibility, Magnetic Field
- RS103 – Radiated Susceptibility, Electric Field
- RS105 – Radiated Susceptibility, Transient Electromagnetic Field
How has MIL-STD-461 Evolved?
MIL-STD-461 and MIL-STD-462 were first published in 1967 to support EMC qualification of electrical, electronic and electromechanical devices. MIL-STD-461 established the tests and limits/test levels for various types of equipment and applications. MIL-STD-462 provided the “how to” test standard describing the techniques for measuring emissions and inducing susceptibility to assess the EMC performance. In the early days, the focus was interference with radio communications that evolved into communications system. The goal was to evaluate products and build EMC into the design process, instead of dealing with the issue when interference was observed after deployment.
MIL-STD-461A was published in 1968 to correct and clarify some aspects of the requirements. MIL-STD-462 remained without revision. Revisions “B” in 1980 and “C” in 1986 were normal review cycle updates that added some tests and modified the limits based on data that had been collected and lessons learned from the years of monitoring the EMC program. MIL-STD-462 was updated through a series of notices (1-6), which expanded the scope of the testing with methods to support the evolving changes of MIL-STD-461 and to correct errors identified over the years.
In 1993 MIL-STD-461D and MIL-STD-462D were released with some significant changes in the EMC qualification test program. A few of the changes that had a major impact were:
- Manually actuated switching transients occurring at the moment of actuation are exempt from MIL-STD-461 requirements. Although the manual transient was removed from MIL-STD-461, the applicable power quality standard still contained testing for these events. Also, the system specification could include the requirement to control these transient emissions.
- Narrowband and Broadband emission categorization was deleted and test receiver bandwidth to be used for various frequency range was defined. This move brought more realization that EMC was more than dealing with radio noise, with equipment upset recognized as a major EMC need.
- Shielded enclosure test facilities were required to include RF absorbers to reduce reflected energy, improving the repeatability of the test measurements. The RF absorber layout was provided in figures and the performance requirements for absorption were specified.
- The test configuration requirements were defined in more detail, as was a requirement to use cables that conformed to the installation drawings. This effectively closed a loophole wherein many equipment qualification tests ignored the contribution of cables to the emission and susceptibility of the device. Some equipment manufacturers tended to avoid cables when qualifying equipment, so this change removed the ability to use unreasonable measures to avoid the cable contribution to compliance.
MIL-STD-461E was released in 1999, combining the content on MIL-STD-462 into the single standard. The requirements and the “how to” overview were now presented in a single document. A few minor updates were part of the revision changes.
MIL-STD-461F was released in 2007 as part of the normal review cycle for standards with a few changes notably:
- CS106 was added to the requirements table with applicability to ships. This test basically re-instated (with modifications) the older CS06 from MIL-STD-461C which was removed by MIL-STD-461D that had the alternative CS115 test method.
- CS116 testing with the test article powered off was deleted based on data collected demonstrating that issues identified with power off showed like issues during power on testing.
- The use of shielded power cables was not permitted. This included a mandate to extract the power leads from cable bundle (including shielded bundles) if power was part of the interconnecting bundle. The appendix provided some exceptions when the power was isolated from the mains.
MIL-STD-461G, released in 2015, is the current standard and with all revisions some changes were introduced that impacted EMC qualification test programs.
- Measurements of the test article bonding to the ground was added as information to be included in the test report. Proper grounding has been an integral part of EMC control and many test configurations installed low impedance ground connections without regard to the installation. The standard does not establish a specific value but calls for checking that the grounding conforms to the installation.
- CS106 was deleted although it had been placed into the requirements by the previous revision.
- CS117 testing was added to the requirements to evaluate lightning-induced transients for selected applications where the platform may not provide adequate protection for such events.
- CS118 testing was added to the requirements to evaluate Electrostatic Discharge (ESD) from personnel. This addition brings the system level requirement to the equipment level. ESD evaluations were previously a separate issue that was managed under various static control programs that were limited in establishing a test requirement.
- Periodic calibration of passive test equipment items was required only if the item was repaired. This helped mitigate some of lab operation costs, but the signal verification checks associated with the various tests were expanded for a more thorough check. One should realize that the checks are a necessary function to prevent errors, and the associated cost of upgrading or over-design to compensate for the error.
- Another change supported emission testing with a Fast Fourier Transform (FFT) receiver. This aided in reducing test time and made capture of the transient spectral content measurements a relatively simple process.
Over the years many changes have been made, with the list above just mentioning a few. The more recent revisions have included appendices that work toward rationalizing the test objectives and documenting how the program has evolved.
What documents are associated with MIL-STD-461 – DIDs?
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.
How is the test configuration managed?
MIL-STD-461G provides a lot of detail in the general and interface requirements section on establishing and managing the test configuration. Configuration management is critical in the standardization process where each test laboratory should obtain the same results within the margin of uncertainty. We must realize that the configuration will not follow exactly in many cases where specific parameters prevent configuration conformance. This inability to follow the prescribed layout reminds us to look at ways to get reasonably close and to document the layout thoroughly to support repeating the testing. If a known installation applies to a device in all cases, then duplicating the installation as the test configuration would be preferred.
Figure 1 shows a floor-standing test configuration modeled after the drawing in MIL-STD-461 to point out a few items that are misread in the standard.
This illustration shows that a table-top arrangement for the cable layout has been brought into the standard elevating the cables 80-90 cm above the enclosure floor. The cable elevation increases the loop area formed by the cable to the ground plane even with the table-top maintaining the 5 cm elevation. The cable routing between the EUT and table-top opens the area providing a significant risk of increased radiated emission levels and reception of interference. An EUT compliance with revision “F” could become non-compliant with a revision “G” configuration. A quick solution could be to add a filter insert without having to re-design the device to control EMI from the circuitry.
As always, the cable arrangement influences the test results, so pay attention to the cable routing and document the arrangement in detail to allow re-creating the test. The conducted tests are affected by the layout because of parasitic components and mutual reactance formed by placement.
Using the RF absorber requires that the table-top ground plane use long straps to attach it to the enclosure, usually going through the absorber at the floor junction. The length-to-width ratio should be at least 5:1 to reduce the inductive reactance of the bond.
Note that a EUT bonding connection is located near the lower right corner. This should only be present if used in the installation. Many test personnel automatically install the EUT connection because it appears in the drawing, but the standard points out that the bonding/grounding should conform to the installation.
Are bonding and grounding requirements specified?
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.
CE101 Audio Frequency Currents, Power Leads
CE101 has limited applicability with a focus on applications that operate at frequencies below 10 kHz with sensitive signal levels. Several emission limits are defined with adjustment of the limit permitted based on the current demand of the device except for the CE101-4 limit for Navy ASW aircraft and Army aircraft. This inability to adjust the limit may place a significant design restriction for equipment that demands high current for operation. If this is anticipated, consider the implications in the control procedure or apply for a deviation to the overly restrictive limit.
Prior to test, a signal integrity check is made at selected frequencies by creating a known current and then using the measurement system verity that the measured current is equal to the current created. Normally the created current is set to be 6 dB blow the applicable limit to make sure that signals below the limit can be detected and measured properly.
Testing is accomplished by placing the current probe around the line selected for test and measuring emissions throughout the test frequency range repeating the frequency sweep for each applicable test lead. Emissions are often related to harmonics of the power frequency or emissions from sudden demands for current to operate various functions of the device. Non-linear components in the power supply circuit may also produce emissions in this frequency range.
If over-limit emissions are detected, you may want to determine if the emissions are common or differential mode to aid in filter design. Place the current probe around both the phase and neutral lead simultaneously and if the emission level is lower, differential mode emissions are indicated. At the lower end of the test frequency range, filter solutions can be somewhat challenging, but filter connectors or filter inserts can be effective if the emission issues are at higher frequencies.
Pay attention to the test result – does the data make sense? Figure 2 shows graphs of the phase and neutral lead measurements with a similar profile on each lead. They clearly show a compliant test article, so all is well. Looking a little closer we can see that the power frequency harmonics are at different levels. This indicates that the current on the phase is not being returned via the neutral. Where is the missing current going? This difference could present a safety issue and should be resolved before continuing the test, since the solution may affect the design.
CE102 Radio Frequency Potential, Power Leads
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.
After a successful signal integrity check, the EUT is powered through the LISNs and each line is tested over the test frequency range using the measurement system components that were just verified. Note that the configuration includes an attenuator (20 dB listed in the standard) between the LISN and measurement receiver. This component is used as a means of protection during power transients such as turn-on. The LISN has a capacitor in series with the receiver to attenuate the power line frequency. However, initial application of power the capacitor tends to act as a short circuit allowing the full power to be applied to the receiver often resulting in damage. The attenuator reduces the voltage, but it also reduces signal to be measured resulting in a measurement system sensitivity reduction. Excessive attenuation can prevent measuring signals that are lower than the limit.
Solving over-limit emission issues are typically addressed by power filters, filtered connectors or filter inserts. Performing this test early in the overall test sequence tends to identify problems that may affect the radiated emission and radiated susceptibility results. The frequency sweep readily demonstrates the effectiveness of adding the filter.
CE106 Antenna Port Emissions
CE106 applies to antenna ports of transmitters, receivers and amplifiers over the frequency range of 10 kHz to 40 GHz, with the test frequency range defined by the operating frequency of the device. The goal is to prevent unintentional signals from being radiated by the antenna.
A limit of 34 dBµV is applicable through the frequency range for receivers and transmitters or amplifiers in standby mode. Testing is accomplished by a direct connection of the measurement system to the antenna port. In cases with an embedded antenna or the characteristics prevent testing by direct connection, the RE103 test method is used as the alternative. The signal integrity check simply injects a known signal into the measurement system and verifies that the measurement matches the signal level being injected – the injected signal should be 6 dB below the limit.
For transmitters, the carrier power serves as the limit basis with an attenuation level specified for the unintentional emissions including harmonics of the carrier. Testing is normally accomplished at the maximum power output with the limit adjusted to equal the carrier peak (note: for some technology the peak is the sideband, so that level is used). If the carrier measurement is not equal to the established limit, the transmitter is not operating at maximum power and should be corrected prior to test.
For transmitters, the process is a little more complicated since the transmitter power tends to over-drive the measurement receiver. Attenuating the transmit power also attenuates the unintentional signals we are measuring unless the attenuation is selective by frequency range. To selectively attenuate the incorporate low-pass, high-pass and band-reject filters are placed in the measurement system as indicated in Figure 4.
Referring to Figure 4 the EUT antenna is connected to an attenuator if needed to reduce the power into the directional coupler. The directional coupler provides a through path without attenuation to a load substituting for an antenna to allow the transmitter to operate properly. The forward power port of the directional coupler outputs the antenna port signals at a reduced level of several dB. At this point the intentional signal is reduced along with the unintentional signals, but the intentional signal is still high enough to saturate the receiver, preventing measurement of the unintentional signals. The rejection network is configured with the appropriate filter to attenuate the intentional signal without attenuating the signal at frequency range where the filter provides negligible attenuation. The signals have been attenuated so a pre-amplifier is typically used to restore the amplitude to a level measurable above the receiver noise. Each of the configurations needed to test the complete test frequency range will need to have signal integrity checks to ensure that appropriate correction factors are added for a true measurement of the emissions.
CS101 Power Leads
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.
CS103 Antenna Port, Intermodulation
CS103 applies to receiver input to determine if intermodulation products are caused by out-of-band signals appearing at the EUT antenna port. These out-of-band signals may be combined by a non-linear device in the receiver front-end to create harmonics a plus sum and difference frequencies and multiples. If the amplitude of these out-of-band signal products is high and create an in-band signal, the receiver performance can be degraded.
Testing is accomplished by injecting out-of-band signals at frequencies to produce an in-band product at the required test level to determine if the receiver reacts to these signals. Test anomalies need to be examined closely to prevent declaring an intermodulation failure that is really caused by failure of the receiver filtering or the test equipment producing an in-band harmonic causing the receiver reaction.
The technology of the receiving equipment is used to determine the test method equipment and applicable test equipment required to evaluate that technology. The standard points out that the requirements and procedures are established on a case-by-case basis with only general guidance provided in MIL-STD-461. A test frequency range is stated, but technology advances require greater frequency range coverage. In most cases, a receiver module is qualified independently and is installed in the device without additional qualification unless the installation incorporates additional front-end circuitry.
Generally, a 3-port network allows two signal inputs to be routed to the receiver while isolating the two signal sources. The 3-port outputs the two signals at the specified test level while the receiver under test is operated with a signal near the sensitivity threshold of the receiver. The signal sources are tuned to products equal to the receiver under test tuned frequency if they are combined. If the receiver is susceptible, the interfering signal amplitude is reduced until the receiver is no longer susceptible. The difference between the receiver sensitivity amplitude and the interference amplitude is the value for intermodulation rejection.
CS104 Antenna Port, Rejection of Undesired Signals
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.
CS105 Antenna Port, Cross Modulation
CS105 is another receiver front-end susceptibility evaluation applicable to technologies that use Amplitude Modulation (AM) of the RF carrier. High level unintentional signals may cause an over-driven operation in the receiver creating a non-linear condition resulting in a transfer of the undesired AM to the intentionally received signal. The risk of cross modulation is brought forth when the high-level carrier is in the same band as the intentional signal.
As with other antenna port susceptibility tests, a 3-port network is used to provide two signals to the receiver input simultaneously. Typically, one of the signals is set to the receiver operating frequency with modulation applied at the sensitivity amplitude plus 10 dB. The second signal frequency is set to the start frequency specified for test with the amplitude set to the sensitivity plus the specified test level. The second signal generator frequency is modulated as specified and scanned over the test frequency range maintaining the test amplitude while the EUT is monitored for susceptibility. The test frequency range is normally limited to the operating frequency ±IF (Intermediate Frequency) of the receiver.
Cross-modulation issues are resolved in the design through gain control functions that limit the received level of strong signals. Receive modules are often qualified separately and do not require additional qualification testing of the final integrated configuration.
CS109 Structure Current
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.
CS114 Bulk Cable Injection
CS114 is applicable to all applications with cables external to the EUT. Although the name implies that the test is associated with conducted interference, the test is performed to evaluate radiated susceptibility coupled into the equipment via the exposed wiring. The conducted test method compensates for the difficulty involved in exposing long cables in the controlled shielded enclosure environment to levels encountered in an actual installation. The conducted test injects current levels over the test frequency range of 10 kHz to 200 MHz at levels associated with a resonant cable to simulate worst-case conditions.
This CS114 test method requires that the test configuration management be observed to obtain a standardized test. At the frequencies involved, parasitic reactance is a significant variable if cable positions vary in elevation above the ground plane or in proximity to other cables. Figure 1 shows that wiring distributed inductance and cable-to-plane distributed capacitance provide a return current loop path for signals injected on the cable. If the distance between the plane and cable changes, the distributed reactance changes, resulting in a change in the current flowing. These changes can be significant and difficult to model without having control of the cable positioning.
The testing phase applies the test current to each of the interface cables and to the power cable, including and excluding the power return and ground. Polyphase power leads are classified for test as a group. A monitor current probe is placed 5-cm from the interface connector and the injection current probe is 5-cm from the monitor probe. The positioning prevents the induced current from being returned by the parasitic elements without flowing in the equipment circuitry.
Prior to test, a calibration sweep is accomplished to record the forward power required to produce a calibration current defined by the applicable curve. Five curves are defined in MIL-STD-461G that correspond to the five test levels specified for RS103 evaluations. Figure 7 shows the calibration configuration with the interfering signal source drives the injection current probe to induce current in the calibration fixture with a measurement receiver used to measure the induced current. The coaxial load closes the loop to allow induced current to flow. The signal source is tuned over the test frequency range and the forward power is recorded to be used as the maximum power to be applied during test.
After the calibration, the configuration is changed to attach the measurement receiver to the monitor probe and the load is placed at the measurement receiver location to maintain a closed loop. The forward power recorded during the calibration is now used as the level to inject the calibration current again and now the monitor probe should measure the calibration current. This step verifies that the monitor current probe is operating correctly (recall that periodic probe calibration is not required by MIL-STD-461G, so this step provides a thorough check of the monitor probe).
After the calibration process, the probes are placed on the cable selected for test. The signal source is adjusted to the start frequency and the amplitude to the lesser of the test current or calibration setting for that frequency. Modulation is applied and the test frequency range is scanned with frequency steps and dwell times meeting the standard with consideration for EUT parameters requiring longer dwell times.
If susceptibility is observed, the threshold of susceptibility should be determined to quantify the non-compliance. It is instinctive to attack problem resolution through filter connectors, filter inserts or cable shielding to reduce current in the wiring. However, before applying cable control measures, verify that the issue is not from radiation. Since the test is simulating radiated interference inducing current, it is logical that inducing a current would create radiation from the cables. To eliminate radiation from the cables as the culprit, placing a temporary shield over the EUT and if the threshold of susceptibility sees a significant change consider that a chassis aperture may be the path for the susceptibility signal.
CS115 Bulk Cable Injection, Impulse Excitation
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.
CS116 Damped Sinusoidal Transients, Cables and Power Leads
CS116 is applicable to all services and equipment with electrical cables egressing the chassis (exiting the pressure hull for submarines). The purpose is to evaluate the EUT’s ability to withstand the damped sine wave transients often associated with lightning or electrical switching operations. The dampening results from cable resonances or to other voltage or current resonances coupled to the cable. Various test frequencies are used to simulate a wide range of potential resonances, and if a known resonant frequency is critical to the platform, that frequency is added to the list of test frequencies.
Testing can be accomplished with a sine or cosine waveform as indicated in Figure 9 with the peak current (Ip) meeting the test level. The dampening factor (Q) is set to be 15 with the calibration configuration.
Testing is very similar to CS114 with the differences being:
- A damped sine wave generator is used as the interference source instead of a RF signal source. The generator waveform is shown in Figure 9 using a sine or cosine wave.
- An oscilloscope is used instead of the measurement receiver for time domain measurements of the test waveform. The oscilloscope is terminated with 50 ?. Using an external terminator rated for the power produced by the waveform generator is recommended to prevent oscilloscope damage. If externally terminated set the oscilloscope for high impedance.
- The monitor probe is not required for the calibration configuration, so verification of the monitor probe is not included.
- Testing is accomplished at the specified test current without exceeding the calibration drive level as with CS114.
- Individual phase leads are tested for polyphase power systems.
The calibration process involves applying the test signal to the injection probe and adjusting the peak amplitude to the calibration current. The frequency and Q are verified, the waveform generator settings are recorded, and the waveform photos taken for the report. This process is repeated for each test frequency.
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 waveform generator is enabled, and the amplitude adjusted to produce the test current without exceeding the calibration level. The damped sine transients are applied at a 0.5 to 1 Hz rate for 5-minutes 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.
The standard indicates that the calibration setting be applied at the start of testing and then lower if the current is excessive. If the characteristic impedance of the circuit is low or unknown, the application of the calibration setting could damage the EUT so use caution to prevent over-stressing the EUT.
If susceptibility is observed, resolution typically uses transient suppressor or suppressor inserts to limit the high voltage or current applied to the sensitive circuitry. If susceptibility is noted only after the test has been running for several seconds or minutes, consider that the suppression device may be over-heated and is not limiting the voltage or current properly. This could indicate that the suppressor rating may be inadequate.
CS117 Lightning Induced Transients, Cables and Power Leads
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.
CS118 Personnel Borne Electrostatic Discharge (ESD)
CS118 compliance was added with the release of MIL-STD-461G for electronic equipment with a human-machine interface (ordnance is evaluated by MIL-STD-331). The purpose is to verify the ability of the EUT to withstand ESD from personnel discharges.
Two methods of discharges are used for the evaluation. Contact discharges are applied to conductive surfaces and air discharges is required only if the contact discharge cannot be applied.
The ESD simulator follows the human-body model established in the IEC (International Electrotechnical Commission) 61000-4-2 with 150 pF capacitor and 330 ? resistance.
Prior to test the signal integrity checks (refer to Figure 12) are accomplished to verity the ESD simulator performance.
- The simulator tip voltage is checked with an electrostatic voltmeter to verify that the voltage is at the correct level for the setting. Checks are made at each voltage setting to be used during test.
- The discharge current waveform characteristics are verified by applying a contact discharge of 8 kV to a test target and measuring the timing and current levels associated with the waveform.
After the signal integrity checks are complete, EUT testing can proceed while maintaining the standard test configuration Test points are selected based on accessibility with a minimum of each face being a test point. Points to consider for testing include operator controls and indicators, ventilation openings, connector shells, and seams/slots where a gap could be present. At conductive test points, use the contact discharge method at 8 kV by placing the simulator tip in contact with the test point then firing the discharge while monitoring the EUT for susceptibility. Apply five positive and five negative discharges to each test point. If the simulator fails to discharge, use air discharge testing for that test point.
Air discharges are applied with a charged tip moved perpendicular toward the test point no faster than 0.3 meters/second until the discharge occurs or contact with the test point is made. Between discharges, remove the residual charge by grounding the point through a resistor or waiting until the charge dissipates. Testing is to be accomplished at each of the defined voltages with five positive and five negative discharges at each test level.
ESD issues typically are managed through positioning of sensitive circuits in a manner that separates the circuit from the discharge path, including the radiated field associated with the discharge. Filtered connectors or filter inserts reduce the potential for cable coupled discharges.
RE101 Magnetic Field
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.
RE102 Electric Field
RE102 is applicable to all services and applications with various limits covering the frequency range of 10 kHz to 18 GHz based on the application. The test purpose is to measure the electric field for compliance to the applicable limit, reducing the risk of interference to other devices.
The test begins with the usual signal integrity check. In this case a known signal level is applied to the antenna cable planned for use during test though any amplifiers, filters or attenuators 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 a radiated field from a stub radiator. The stub radiator is typically a coaxial cable with a short length of the cable shield removed so it acts like a monopole antenna. The level of the radiated signal is not defined but detection of the signal via the antenna is required. Some laboratories construct a fixed radiator and always use the same drive level to obtain consistent measurements during this part of the signal integrity check.
Once the signal integrity checks are complete, testing can begin by placing the antenna 1-meter from the test boundary at the specified elevation. Note that the 1-meter is from the test boundary, not from the ground plane. The test boundary is the area with the EUT and the associated 2-meters of exposed cables, normally 10-cm from the ground plane front. With the antenna positioned, the receiver is swept over the test frequency range measuring the peak emissions in equivalent rms terms.
Specific antennas are called out for various frequency ranges with positioning requirements described in the standard.
- Monopole antenna for the 10 kHz to 30 MHz frequency range is positioned with the center of the rod at an elevation of 120-cm above the enclosure floor. The horizontal position is near the center of the test boundary with a vertical polarization only.
- Biconical antenna for the 30 MHz to 200 MHz frequency range is positioned with the center of the balun at an elevation of 120-cm above the enclosure floor. The horizontal position is near the center of the test boundary with vertical and horizontal polarizations used for testing.
- Double ridge horn antenna for the 200 MHz to 1 GHz and 1 GHz to 18 GHz frequency ranges are positioned with the center of the horn at an elevation of 120-cm above the enclosure floor. The elevation may be varied for testing as needed to make sure that the EUT plus 35-cm of cable (200 MHz-1GHz range) or EUT plus 7-cm of cable (1-18 GHz range) are within the antenna 3 dB beamwidth using multiple positions to meet the beamwidth criteria. Vertical and horizontal antenna polarizations are used for testing. Figure 15 provides a conceptual arrangement for multiple antenna positions. Calculating the distance and beamwidth allows one to determine the EUT coverage area based on the individual antenna polarization beamwidth characteristics.
As mentioned earlier, several limits are provided in the standard with limit tailoring employed if system integration plans identify a need for special treatment. Many of the limits shown have a lower frequency of 2 MHz which supports the test frequency range for that application. When working a multi-service application that calls out a 10 kHz lower frequency range, avoid the inclination to extend the limit to 10 kHz at the 2 MHz level. The 10 kHz to 2 MHz range should meet the application limit for that range, even if it is dis-continuous.
A couple of RE102 changes were made with MIL-STD-461G release that tend to add a little complexity to the testing.
- The test frequency from 1 GHz to 18 GHz is applicable to all devices no matter what the operation or clock frequencies. Prior revisions allowed the upper frequency to be 1 GHz or 10-times the highest intentionally generated frequency within the EUT not exceeding 18 GHz. The impact added a lot of test time, especially for large EUTs that required several antenna positions. The additional time could be reduced if the FFT receiver was used.
- Prior to MIL-STD-461G, radio frequency transmitters were tested in the standby mode and transmit mode was covered by antenna port emission testing. Now testing with transmit active is specified with exemption of the transmit frequency and occupied bandwidth of the signal. Since the active transmit signal is present at the measurement system antenna the measurement receiver tends to be saturated when trying to reach the RE102 limit sensitivity. To allow sensitivity of the measurement receiver filtering is used to reject the transmit signal preventing saturation.
If over-limit emissions are detected, resolution is usually required. Several methods are used once the source and point of radiation are determined. If cables are the escape point, adding filtering is often the initial approach with filter inserts providing a quick answer. If the chassis is the escape point, either suppressing the signal source or improving the shielding are typical resolution approaches.
RE103 Antenna Spurious and Harmonic Outputs
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
- 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.
RS101 Magnetic Field
RS101 has limited applicability, mainly dealing with sensitive equipment operating at low frequencies, but system parameters may add this susceptibility test as technology advances demand. The test frequency range is 30 Hz to 100 kHz with two limits defined in the standard.
Testing begins with the normal calibration process where, in this case, the check verifies the relationship of current flowing in the radiation loop to the magnetic field being produced. This step is necessary because during test monitoring of the field is not practical. The single frequency calibration check is adequate to confirm that the loop operation is correct.
The radiating loop is provided with a field sensor that physically attaches to the radiating loop assembly at the 5-cm location (refer to Figure 16). This sensor provides a connection for the measurement receiver so the signal generator can be adjusted to produce the correct field during calibration. Once the field is reached the current flowing in the radiating loop is measured. The current to field ratio is then used to establish the required current necessary to produce the test field in dBpT units over the entire frequency range.
Once the calibration is complete, the field sensor is removed so the radiating loop can be placed at the EUT face for testing. The signal generator is tuned through the test frequency range applying the required current to meet the test field flux density while monitoring the EUT for indications of susceptibility.
The coverage area for the standard radiating loop is 30-cm X 30-cm so several positions may be required for exposure of the complete EUT. Using a Helmholtz coil is an alternative method where the entire EUT is immersed in the radiating field with test. If the Helmholtz method is used, remember to rotate the EUT for exposure in three orthogonal planes.
Susceptibility issue resolution often takes the form of shielding or physical separation if feasible. Filter connectors or filter inserts may provide attenuation if the issue is at a frequency suitable for the filter operation and is related to cable current being induced.
RS103 Electric Field
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.
RS105 Transient Electromagnetic Field
RS105 verifies the ability of a device to withstand a transient electromagnetic field. Applicability is limited to equipment in exposed locations and normally only safety critical applications. The test level is 50 kV/m with a tolerance of +6 dB/-0 dB presenting a potential level of up to 100 kV/m.
Figure 17 shows a basic R105 test configuration where the EUT is placed between parallel plates that are charged by the transient pulse voltage creating a vertically polarized field between the plates.
Prior to test the calibration process is used to verify the pulse waveform and test level at five points defining the EUT boundary (4-corners + center). The pulse generator settings that satisfy all five test points simultaneously are recorded for use during the testing. Measurements of the field are made with D-dot (electric field) or B-dot (magnetic field) sensors to capture the level and rate of change of the transient pulse at each of the five points.
After calibration, install the EUT in the test location orienting the wiring normally to the vertically polarized field to limit exposure to the EUT. Establish operation of the EUT and apply the transient at 10% of the peak amplitude, and increase the amplitude in step sizes of 2 or 3 until the required amplitude is reached. Five pulses are applied at a rate not exceeding 1-pulse per minute. The test is repeated for the two remaining orthogonal plans of the EUT.
Observe caution during the calibration and testing, since very high voltages are used and the radiation from the transient can interfere with equipment in the vicinity. TPDs (transient protective devices) are used to prevent the pulse conduction to other equipment sharing the mains power. During test, if indications of susceptibility are observed during the amplitude adjustment phase, consider that the full test level could damage the test article requiring repair to continue testing.
The pulse generator may be contained in an inert dry gas pressurized vessel to prevent electrode arcing.
Provide photographs of the applied waveforms and details about the test parameters and information about the EUT responses for the test report.
MIL-STD 461 remains one of the strictest set of requirements to meet. Should a product face difficulty in complying with the standard, an EMI filter insert is a quick and permanent solution.