The myth: Electromagnetic Compatibility (EMC) performance was initially intended for the realization of system-product compliance with the requirements of regulatory agencies through standards applications.
The reality: EMC performance criteria were conceived to assure system-product functionality.
In history and in reality of practice, the essential and fundamental first axiom of systems-product design criteria is literally electromagnetic compatibility (EMC); system-product circuits, signals, modules, circuit architecture and sub-systems must be functionally compatible with themselves! In recent years, understanding and application of the term “EMC” increasingly seems to have become narrowly and inappropriately limited to defining the comparative performance between regulatory agency-mandated EMI immunity and emission requirements, and the profiles exhibited by system-products. Historically, the intention of EMC as both a term and a process was to assure that the integrated system or system-product will perform as intended within defined functionality requirements with regard to: a) sources of fields and currents within the design itself (for signal integrity and signal/noise ratios at the unit level), b) sources of fields, potentials and currents produced to and from other modules/units of a system or among multiple systems in a common “platform” environment, and c) sources of fields, potentials and currents to and from the external operating environment.
Item “b” can be carried forward in military or space implementations to define the first increment of “environment” as a sphere of functionality within a common-platform, such as a warship, spacecraft or air form (war bird). The concept in these applications is sufficiently compartmentalized to conceptualize the ‘envelope’ of the common-platform (often a weapon, surveillance or communication package) first as an environment within itself, and secondly with that envelope as a platform to determine EMC with the intended operating environment external to the platform. These concepts are typically conveyed through environmental specifications such as MIL-E-6051. For this application, the measurement processes are intended to assure that no system component or sub-system interferes with the needed operation of any other component or sub-system, and that fields to or from the platform as a whole are compatible with a rigorous (e.g. battlefield) operating environment. In commercial parallels, the platform could be any individual package, such as a desktop computer or a medical device. Within these descriptions, it is observed that specific EMI emissions or susceptibility (immunity) specifications are only interim tools to increase confidence that the goals of the ultimate functional environmental demand will be achieved.
Given this viewpoint, signal integrity, signal timing (propagation time), signal perturbation, cross-talk and coupling, signal-to-noise ratios, common-mode containment and partitioning, common-mode architectural derivations, power delivery/quality, and electromagnetic field captures and exclusions are all mutually inclusive to the term “EMC”. In impact, great performance profiles with regard to all of these sub-set characteristics of EMC will combine to produce a thoroughly integrated and compatible system-product with reference to factors of compatibility within itself, other systems, and the external environment.
While the limited definition of EMC described by system-product compliance with the requirements of regulatory agencies is a fact of contemporary electrical engineering, it does not adequately relate the whole significance of what the term “EMC” represents, in intention or reality.
|W. Michael King
is a systems design advisor who has been active in the development of over 1,000 system-product designs in a 50 year career. He serves an international client base as an independent design advisor. Many terms used for PC Board Layout, such as the “3-W Rule”, the “V-plane Undercut Rule”, and “ground stitching nulls”, were all originated by himself. His full biography may be seen through his web site: www.SystemsEMC.com. Significantly, he is the author of EMCT: High Speed Design Tutorial (ISBN 0-7381-3340-X) which is the source of some of the graphics used in this presentation. EMCT is available through Elliott Laboratories/NTS, co-branded with the IEEE Standards Information Network.