New equipment is being developed for a new or an updated aircraft. The concept looks promising until the EMC requirements are given. Then it appears the laws of physics must be broken to achieve the requirements. What do you do?
This is not as uncommon as you may think. Two requirements conflict, making it impossible to meet both at the same time. A promise is made that ensures a sale, but may be unrealistic and unnecessary for the aircraft. An overcautious manager may redraw a limit line to ensure the margin will always be adequate, with notches so deep that they cannot be measured at room temperature because the thermal noise floor exceeds the limit even using a 1 Hz bandwidth.[1]
Beyond having a no-win scenario, or maybe just an unreasonable one, certain test limits may be unnecessary. A low emission requirement may exist for a satellite so it can receive low-level signals from a remote station and not mask the signal with its own generated energy. The emission limit for the satellite may have a deep notch at the receiver frequency as a result. But that limit is not intended for use on the equipment in the galley of a regional jet. To ensure such a limit will add both unnecessary weight and cost to the equipment.
Before a design is finalized, performing an analysis and preliminary engineering testing can highlight the need for tailoring. For example, a power supply on an aircraft using 400 Hz power will rectify the power to create the desired DC output power. The process of rectification can produce harmonics of 400 Hz above 100 kHz. The control of these emissions may require significant inductance and line-to-line capacitance to meet the intended limit, adding significant weight and volume to the design. However, conducted emissions below 100 kHz may not create a problem on the aircraft, either to other equipment, or generate radiated emissions.
This is a good reason to discuss tailoring with the purchasing agent. In his book, Ken Javor says, “The requirement might be relaxed over an unimportant frequency range.”[2] He points out that tailoring can carry a negative connotation, but can be employed in a positive way. MIL-STD 461G is very specific about tailoring, saying on page 1:
1.2.2 Tailoring of requirements.
Application-specific environmental criteria may be derived from operational and engineering analyses on equipment or subsystems being procured for use in specific systems or platforms. When analyses reveal that the requirements in this standard are not appropriate for that procurement, the requirements may be tailored and incorporated into the request-for-proposal, specification, contract, order, and so forth, prior to the start of the test program. The test procedures contained in this document are generic test methods and should be adapted as necessary for each application, while maintaining the intent of the test, and should be approved by the procuring activity. The adapted test procedures should be documented in the Electromagnetic Interference Test Procedures (EMITP) (see 6.3).
“Margins” are used to assure a degree of safety between where a subsystem may emit energy and where it might be susceptible. MIL-STD 464D requires margins which are greater than 6 dB for most cases, and up to 16.5 dB for critical items, such as ordinance and the like. But notice that the MIL-STD 461G emission limits for RE102 may be as low as 24 dBµV/m in the 2-100 MHz range or as high as 56-70 dBµV/m for below-deck surface ship applications. But in this range, a low-level radiated susceptibility test (RS103) may be 10 V/m, but typically much higher. 10 V/m = 140 dBµV/m, a margin of no less than 70 dB between these test levels.
Margins are used in system-level applications where many pieces of equipment and subsystems are assembled. These subsystems may increase the overall background noise level of the system to decrease margins. But you cannot add and subtract dBµV/m levels directly – they have to be converted to linear values first, and then addition may be employed. For example, ten items all radiate at 40 dBµV/m, and if they add perfectly, in phase, they do not generate 400 dBµV/m. Instead, 40 dBµV/m = 100 µV/m, and 10 x 100 µV/m = 1000 µV/m = 60 dBµV/m. And ten items will have 80 dB of margin to a 10 V/m susceptibility level.
In the commercial world, if new equipment on an existing aircraft needs to have a deviation, this may require a supplemental type certification (STC). The aircraft is operated on the ground with the new equipment operating. All critical aircraft systems are checked for proper function and lack of interference. If none exists, the aircraft can be certified as airworthy with use of the equipment. If desired, the equipment can apply for a Technical Standard Orders Authorization (TSOA).[3] Guidance around these issues should be given by a Designated Engineering Representative (DER).
[1] Those of you who know what I am talking about have likely experienced this. If you don’t, then a review of Johnson-Nyquist Noise is recommended.
[2] Ken Javor, Introduction to the Control of Electromagnetic Interference (Huntsville: EMC Compliance, 1993), 6.
[3] See: https://www.faa.gov/aircraft/air_cert/design_approvals/tso
