Modules gain EMC standard
By Richard A. Quinnell, Contributing Editor- March 1, 2009
PAR-1688 W.G. Chair
Electromagnetic environmental effects lead for JSF
For the past four years, an IEEE working group has been developing an EMC (electromagnetic compatibility) standard that addresses the design and test needs of plug-in cards for modular systems such as PXI. While the initial efforts of the PAR-1688 Replaceable Electronic Modules working group have targeted EMC qualification for military systems, the methodology is applicable in the commercial sector as well. I discussed the standard with the working group’s chair, Fred Heather, who is the electromagnetic environmental effects lead for the Joint Strike Fighter program.
Q: What was the motivation for developing PAR-1688?
A: Integration has been turning systems that used to be large boxes into small cards in a cage that offer modularity for repair or upgrade replacement. But most current EMC compliance standards require retesting of an entire system even if only one module changes. A full system qualification test is expensive, so we wanted a standard that could qualify just the module. There will still need to be an initial system-level qualification, but by having standards for individual modules, we can simplify the maintenance of compliance when modules change. Module-level standards also help assure system designers that their final result will pass qualification testing
Q: How would system designers apply the standard?
A: We envision a multiple choice based on market conditions. One way is for the system designer to tell the module provider what qualifications level the module is to achieve. Another way is for the module developer to target a qualification level based on expected customer applications. In either case, the standard addresses only module performance, not backplanes or cables. The test fixture we developed eliminates all cabling issues common to system-level EMI (electromagnetic interference) testing.
Q: What is the timeline for release?
A: We have completed the design limits and test requirements portion of the standard and are working on the appendix, which covers applications guidance. Our goal is to have a review draft to the EMCS Standards Committee by the time the IEEE International Symposium on EMC is held in August. Comment and review followed by approval and release could come by the end of the year.
Q: Any surprises during development?
A: We did have to develop some new test methods. Current standards address near-field magnetic coupling but not electrical coupling. We had to create a way of measuring high-impedance electric fields at distances of a centimeter or so. Also, when we started reducing field-strength test levels (to account for the effects of shielding), we got below the level that standard probes could sense, and we had to solve that problem.
Q: What is the applicability of the PAR-1688 standard?
A: The initial standard addresses the needs of systems that will be tested to MIL-STD-461, including air, ship, and ground-based systems. Our plan is to follow up by adapting the standard to address commercial applications such as networking and industrial systems. We will adopt the same test approach and methods and adjust the compliance levels, bandwidths, and such to match various application needs.
Q: Is there a way for PXI users to benefit from the standards effort now?
A: Become part of the committee. The committee is open to the industry and welcomes new members. There is a lot to be learned as we share understanding and insights.