In the commercial world, EMI testing includes emissions testing and what is termed “immunity” testing. As an example, for radiated immunity, the test requirement is performed at a given level, usually 3 V/m or 10 V/m for most devices. If the equipment to be tested becomes upset or responds in any way to the radiated field, the equipment has failed the test.[1] The only options are redesigning the equipment or claiming a lower category for the standard. There is no relaxation or allowance for not meeting the test level.
In the case of military and aerospace testing, the tests are called “susceptibility” and not immunity tests. They tend to be performed to a much higher level than commercial tests, and over a wider frequency range. Most test levels vary over frequency. In the case of DO-160, the test levels for radiated susceptibility may be much higher at higher frequencies than at lower frequencies. One example is Category R, which is run at a maximum of 20 V/m from 100-400 MHz but requires pulse modulation testing of 150 V/m at 400 MHz and higher.
Since equipment can be subjected to higher test levels, upset and disruption of the operation can be an issue. However, we try to avoid the term “failure” when referring to these tests. A device might be susceptible at a given frequency and amplitude – say 3.75 GHz and 200 V/m. However, that frequency band is dedicated to sources such as satellite transmitters and is not an aeronautical frequency.
In these cases, there may be reasons to allow the equipment to be delivered as is without modification. Reasons for accepting the unit may include whether the fix for this problem will add size or weight to the unit. The airframe manufacturer should test the system to ensure that all known transmitters and potential fields the equipment may encounter will be neither at this frequency nor anywhere near these field strengths. If cleared, the equipment can be considered acceptable as it is.
But, as the equipment manufacturer, it is best to understand why this susceptibility exists and why it will always be at this frequency. What is not allowed is that one unit is susceptible at 3.75 GHz, and the next at 2.75 GHz, for example. This is because 2.75 GHz is in the Airport Surveillance Radar band, with transmitter peak power up to 25 kW. The whole manufacturing run of equipment may need to meet 2.75 GHz without susceptibility, and having one unit susceptible may indicate other issues.
Another issue is the type of effects that might be seen from EMI susceptibility testing. A unit with a display that glows slightly brighter in intense fields may not be an issue in the galley but may be on the flight deck. On the flight deck at night, night vision may be interfered with by a display becoming too bright. Or a warning light may improperly ignite, creating confusion for the crew. Data lost on the flight deck is a critical issue, but a display that clears immediately after a short duration exposure may or may not be an issue, depending on the function and location of the display.
For these reasons, knowing the susceptibility criteria for the equipment under test is important. It is important to have the criteria well-defined and agreed to in the test procedure for the equipment. The procuring activity, along with the manufacturer, must know and understand what is considered “susceptible”. There may be information that can be provided for guidance and tailoring to the standards used. It is common to modify emission limits to allow for certain transmitter emissions, or very low frequency noise issues, e.g., rectification of AC power. There is often no need to have equipment with excessively large filters to quiet rectifier noise at 10 kHz for MIL-STD 461, when there may be nothing on the power bus that will be affected by that type of noise. Such EMI energy should be validated by CS101 and CS114 testing for all equipment on the power bus.
The other aspect is that during testing, if the susceptibility criteria are well defined, the testing laboratory will have a better understanding of when to halt testing. Supporting technicians and engineers from the manufacturer’s company will also know when to stop testing and perform troubleshooting. In each case, the testing becomes more efficient and effective.
Engineering testing early in the design can provide foresight into what these issues may be, and if certain tests and limits should be tailored. Remember, these standardized tests are designed to validate a wide range of equipment. Significant efforts are made to fit the equipment into the right test categories, but nothing is perfect and nothing is universally applicable. If possible, there needs to be a level of reason brought into the test requirements. But often, it also requires education on the part of the purchasing group to know what is reasonable.
For some, it may be easier to redesign the equipment.
[1] Other immunity tests have different criteria, which may allow for some upset, as long as it recovers on its own, without user intervention.
