IEEE to develop EMI standards for modular electronics
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 3/23/2006 8:29:00 AM
EMI standards such as MIL-STD-461, CISPR 22, and others address stand-alone electronic products such as computers. Unfortunately, these standards don't address EMI limits or testing for modular electronic systems that use replaceable electronic modules (REMs). Systems such as aerospace controls, computers, automotive systems, and telecom networks use REMs.
To address these issues, EMC engineers have formed an IEEE-EMC Society sponsored group to develop test methods and limits for REMs under the proposed IEEE Standard 1688. The group is headed by Fred Heather, the government's Electromagnetic Environmental Effects Lead for the Joint Strike Fighter Program (www.jsf.mil). The group plans to draft a standard for performing susceptibility and emissions tests for radiated and conducted EMI on REMs. To learn more about the project, I spoke with Fred by telephone.
The need for EMI test standards for REMs arose because engineers are concerned about adding or replacing modules in a backplane without completing current EMI equipment level test. "If you qualify a piece of equipment that has modules, what happens if you change a module?" asked Heather, as he explained the reason for the project. "We want to install any card and have it work in its system." Many of these systems are very complex, and the prequalification testing that is needed whenever one REM is changed can be more expensive and have a greater impact on a schedule than the change itself. The new standard will allow qualification of the REM without further system-level EMI testing.
The group's goal is to produce a draft standard in time for the 2006 IEEE EMC Symposium, which is scheduled for August 14-18 in Portland, OR. Once complete, the standard will provide a consistent test method for evaluating a card's or module's conducted and radiated EMI characteristics. The proposed 1688 standard should bring the risk that a card or module won't interfere with system operation to an acceptable level.
The group is using the test methods specified in MIL-STD-461 as a basis for their own test methods. Participants have applied the MIL-STD-461 test methods to various systems, looking at how a system's functionality changes as they change or add modules. From test results, the group plans to propose changes to the test methods and incorporate them into the draft standard.
Participants are looking at cards and modules, not backplanes or motherboards. To test a card, you need it to operate in its chassis connector. But the engineers want to test the EMI characteristics of cards or modules only. This poses a problem for EMI testing--keeping the card or module under test inside a shielded room while keeping the interface outside.
To solve the problem, the group's engineers are developing fixtures that mount in the walls of a shielded room, on an EMI bench, or in a TEM cell. Shielded cables connect the REM fixture to other interfaces. The interface equipment would be outside the room and would include the normal test equipment used to check out the REM in the factory. "We plan to use the standard EMI test equipment to measure emissions and to inject interference signals into interface lines," said Heather. Grounding, bonding, and heat sinking of the text fixture is important because you can't compromise signal integrity or thermal characteristics of a chassis.
The IEEE 1688 group, which has about 30 participants, meets every other month through a conference call. The group is open to new participants. "We want to keep a balance of equipment manufacturers, users, and test labs," noted Heather. For more information, contact him at mailto:heather@ieee.org.
