EMI standard planned for replaceable modules

Martin Rowe, Senior Technical Editor -- Test & Measurement World, 5/1/2006

EMI standards such as MIL-STD-461, CISPR 22 address stand-alone electronic products such as computers. Unfortunately, they don't address EMI limits or testing for systems that use replaceable electronic modules (REMs).

To remedy this, the IEEE-SA Standards Board has approved Project 1688, Standard for Module Electromagnetic Interference (EMI) Testing. Led by Fred Heather, 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.

The need for EMI test standards for REMs arose because engineers want to replace modules in a backplane without performing a complete EMI equipment level test. In complex systems, the cost and schedule impacts of prequalification testing because of one REM change can be more significant than the change itself. The new standard will allow qualification of the REM without further system-level EMI testing.

The group plans to produce a draft standard in time for the IEEE EMC Symposium (August 14-18, Portland, OR). The final standard should provide a test method for evaluating a card's or module's conducted and radiated EMI characteristics. While not intended as a compliance standard, the proposed 1688 standard should bring the risk that a card or module will interfere with system operation to an acceptable level.

The group is looking at cards and modules, not backplanes or motherboards. To thoroughly test a card, you need it to operate in its chassis connector, but the group wants to test the EMI characteristics of cards or modules only, separate from the chassis.

A test fixture provides an interface between the equipment under test inside a shielded room and an outside chassis and test equipment.

Group members want to keep the card or module under test inside a shielded room while keeping the interface and chassis backplane outside, so they are developing fixtures (figure) 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 will reside outside the room and will include the test equipment used to test 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 P-1688 group is open to new participants. For more information, send an e-mail to heatherf@ieee.org.

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