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Electromagnetic Compatibility

NOTE: SITE UNDER CONSTRUCTION. HAVING TO HIRE HELP TO FIGURE IT ALL OUT. EVENTUALLY WILL HAVE ALL THE ABOVE BUTTONS POPULATED WITH INTERESTING AND USEFUL INFORMATION. OR THAT'S THE PLAN... The electrical engineering sub-discipline of Electromagnetic Compatibility (EMC) is over eight decades old as of this writing (2020), and some of the problems are close to one hundred years old. Installing the first AM broadcast band (BCB) radio receiver in an automobile initiated the design of radio frequency interference suppression of ignition sytems and brushed generators and motors. EMC is first and foremost about protecting radio reception from radio frequency interference (rfi). Various folks with various axes to grind will say it ain't so, but it is. There are certainly other problems, but rfi was first and remains so. Many people around the periphery of the discipline consider it to be arcane or black magic. It isn't, but it is often practiced that way. Two common attitudes are well-expressed by the comic strip Dilbert (what isn't?). The first is to ignore it until you can't anymore, and the other is to use whatever is lying around (copy and paste from a previous program) because it if it was good enough for them it's good enough for us. Both attitudes stem from ignorance. Ignorance is not stupidity. The antidote for ignorance is the acquiring of knowledge. Hence this website. The purpose of this website and the museum is to act as a Rosetta Stone, translating what appears to be arcane practice and unintelligible nomenclature into plain electrical and mechanical engineering issues. The type of information found herein is different than in most websites and books on the general topic of EMC. The vast majority of such address design issues - design to compliance. That is not the topic here. The topic here is how to achieve EMC on an integrated system, typically a vehicle of some type, using programmatic requirements. It is about requirements flowing down from top-level EMC goals to individual equipment-level requirements. With such equipment-level EMI requirements the means to an end, and overall system-level compatibility the end to which all efforts are aimed.

A major distinction is important. Many people distinguish between commercial and military EMC. That is a distinction without a difference. The real differentiator is in the kind of installation. The design of vehicles (planes, trains, automobiles, ships, spacecraft) proceeds along the lines of a great deal of electronics and electrical gear integrated into a small volume, and all bonded to metallic structure. The design of equipment destined for use in homes, offices, medical facilities, and industrial plants proceeds along a different path. Here the equipment is more spread out, and there is no structural ground. The only ground available is a wired safety ground system. And there is no single home, office, hospital or plant layout. Every individual such item sold will be installed in a unique environment surrounded by other equipments unknown at the time of manufacture or sale. So MIL-STD-461 and similar (DEF STAN 59-411), RTCA/DO-160 sections 16 - 25 for aircraft and CISPR 25 and ISO 11452 for automobiles all bear striking resemblance to each other, which is not at all surprising, since they address the same sorts of concerns. And they all are quite different from CISPR 22 (EN 55022, or CISPR 35) and the EN 61000-4-X series which address similar sorts of problems, but in quite different ways, because these address the needs of equipment installed in homes, offices, hospitals and plants. Another key difference between the practices of vehicle EMC and EMC for the residence, office and plant is required measurement accuracy and precision. Equipment designed for home, office or plant can be EMI tested, but not EMC tested. This is not a distinction without a difference. Quite the opposite: EMI and EMC testing is a difference which in common parlance is a distinction ignored. To the point that one sees advertising copy for "EMC receivers." One cannot receive EMC, but one can sure enough receive EMI, or better – rfi. These latter two are the origin of the discipline. An EMI test measures quantitatively the level of emission, or the degree of an equipment's immunity from or susceptibility to external electrical stimuli. An EMI test is a means to an end. That end is electromagnetic compatibility. When a vehicle is integrated, system-level EMC is verified by a number of both quantitative and qualitative measurements and evaluations. The purpose of the equipment-level EMI testing is to allocate some of the EMC design effort to the individual equipments and subsystems, so that all efforts are not concentrated on "band-aid" fix's installed when found only at system-level integration. EMC specifications came first. When it was realized that it was more cost- and schedule-effective to put part of the EMC design effort on individual equipments, rather than all on the equipment integrator, equipment-level EMI specifcations were born. But equipment designed for home, office, hospital and plant are not part of any planned integration, and therefore the only test effort is the EMI test. This is why such equipment always comes with a disclaimer that although the equipment meets all applicable standards, if it causes disruption of licensed wireless services, its use must be discontinued. This is why there is confusion in many circles about EMI vs. EMC. For such folks, the EMI test is the only verification, and as far as they are concerned, successful completion of an EMI qualification test is all the evidence necessary to claim EMC. This is also why the EMI standards levied on such equipment are burdened with onerous accuracy and precision requirements. This EMI verification is the first and last line of defense against EMI problems, and - even more importantly - such requirements are a barrier to marketplace entry, and as such must be levied precisely in order to ensure a level marketplace. Whereas in a typical vehicle integration, with the EMC test as the final arbiter of acceptability, the equipment-level EMI requirements – while contractually obligatory – function to a larger degree as engineering data, with acceptability of EMI performance often decided by the results of the integrated EMC test.