Military Standard 461g is the EMI control requirements standard for subsystems and equipment used by the Department of Defense (DoD).
The intent of the standard is to ensure that the equipment is electromagnetically compatible with an installation site. To gain official compliance, proposed designs must be taken to specialized facilities to receive a battery of tests. This post provides a brief introduction to the process.
What is an EMI Test Chamber?
An EMI test chamber is a large metal room designed to provide electrical isolation between the outside environment to the inside environment. This type of structure is commonly referred to as a Faraday cage. To work effectively, the outer surface of the room must be one continuous conductive layer with little to no electrical differences across its seams, joints, or cable penetrations. Openings or apertures in this room shield will allow radiated electrical noise to enter the chamber or leave the chamber, causing a degradation of the test conditions.
EMI chambers are also lined with an anechoic radiofrequency (RF) absorbent material. This is used to attenuate the natural signal reflections that can occur off the chamber’s inner walls, ceiling, and floor. The anechoic material is typically a pyramid shape two feet in depth to provide an impedance match to the wall and reduce those reflection properties across a wide frequency range. Anechoic absorber performance increases with frequency.
Though typical testing follows certain protocols within a radio-frequency-controlled chamber, each setup of test equipment is customized to simulate field conditions.
Emissions and Susceptibility
Compatibility with these conditions is validated in two ways.
First, the equipment must not interfere with other equipment, in other words, the electromagnetic interference produced by the equipment must be controlled. This is validated through the emissions measurements type test.
Second, the equipment must operate reliably without degraded performance or damage as it was designed and as it would be installed on that platform. This is validated through susceptibility type tests.
An installation site can be any location where equipment can be used. This could be a platform such as a ship, an aircraft, or even a specific area on a platform, such as external to an aircraft fuselage or below deck on a metallic ship. Each site will have a unique electromagnetic environment with consistent electromagnetic conditions at any point in time.
The electromagnetic sources can be constant, periodic, or random in nature. They can be generated by equipment located in that platform sharing power bus utilities or interconnected through cabling or simply located nearby. These sources can also be produced by natural events such as lightning strikes.
The type and the number as well as the magnitude of these sources will vary from site to site, which can impact the frequency ranges and test levels of the EMI thread at that location. Therefore, the installation platform dictates the types of tests and the test conditions that need to be met for product qualification.
Additionally, the type of equipment being installed or qualified may also determine the applicability of certain test conditions.
The intended installation platform, in particular the service branch (of Army, Navy, Air Force, or special procurement) should be identified in cases where the equipment will be used on multiple platforms or in several locations within a platform. Identifying each scenario will help justify the required qualification requirements or any tailoring activity needed.
Specifying the installation setting dictates the setup of the testing procedure. Otherwise, the general approach would be to combine the most stringent emissions limits with the most severe susceptibility test criteria for each applicable platform or qualification requirement.
Bonding and Grounding
The bonding and grounding requirements for the equipment used during testing is another aspect to consider. This describes how the equipment will be electrically grounded during field installations and how that bond is made. The bonding and grounding will have a significant impact on EMI test results and will be generally used as part of the equipment’s overall EMI suppression and protection scheme.
As with cabling, MIL-STD-461 requires that the actual bonding and grounding methods used during testing must be representative in the installation and must be used during the actual test. It will be documented in the EMI test procedure, meaning that if the equipment uses a ground strap in the actual installation, then one should be used during the testing, and should be constructed of the correct material length and width that will be used in field operation.
MIL-STD-461 also requires that the tested equipment should be placed in a mode (or operating state) that will produce the greatest electromagnetic emissions as well as a mode that is likely to become the most susceptible to EMI.
The intent is to quickly identify compliance issues that could be missed during less critical operating states. This depends on the type of equipment under test, but if the equipment has multiple modes of operation and one specific cannot be determined as the worst case, then all these modes would be evaluated for emissions and susceptibility.
Another requirement is that the EUT be oriented to produce its maximum rated emissions and where it is most susceptible to radiated signals. This orientation is selected generally but not always in the same orientation to meet both goals. Determination of the appropriate orientation is usually straightforward. Not a lot of guidance is provided by the standard but prime candidates are sides that include large seams or apertures such as a video screen, ventilation ducting, or cable penetrations. In some cases, you may need to orient the unit in various positions to make sure that all worst-case conditions are met.
Note: A second 10nF capacitor can be added in parallel to the EESeal in order to target emission spikes.
Equipment cabling is also an especially important aspect of EMI testing and arguably the primary cause for EMI test failures during qualification testing. Therefore, it is critical that the equipment cabling used during testing is representative of the platform installation and this includes make, model as well as the actual or anticipated installation length of that cabling.
MIL-STD-461 specifies how the cabling will be routed during the actual test setup. This standardizes the test method that could be repeated at various test labs in the future.
In power cabling testing, for example, the cable will be routed along the front edge of the bench top for two meters, not to exceed a total length of two and a half meters between the connection to the EUT in the line impedance stabilization network (LISN) terminal.
For all other interconnecting cables, the actual installation length should be used if the installation cable length is greater than 10 meters. In this case, at least 10 meters needs to be provided in the test setup. The first two meters of that cable will be routed to the front edge of the test setup boundary, then it will zigzag to the back side of the test setup before it leaves the chamber. If the installation length is unknown for a particular cable, then at least two meters of cable will need to be presented in the setup.
Given that interference from cables can cause projects to fail testing, simple and low-cost modifications to a wide range of off-the-shelf interconnects can significantly reduce EMI.