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My product failed EMC testing, now what do I do?

Date Posted: 10/03/14
Author: Marty Best
Marty Best

What do you do now that you've failed EMC tests?After taking your product in for EMC testing, you are told that the product exceeds radiated emission limits for FCC Part 15 and cannot be legally shipped. In order that the product passes, it will have to be modified so that it does not exceed the radiation limits. But how? The key to solving this kind of a problem is to “look for antennas”. But you might say, my product has only one antenna and it is not even turned on. Any electronic product that has an oscillator in the circuit also has antennas connected to that oscillator. They may not look like antennas but they definitely act like antennas. The key to getting a product to pass radiated emissions is to find the antennas, and convince them to look less like antennas. However, the antennas may not be in obvious places. It may not be directly connected to anything except for a DC signal. But if it runs close to another signal trace, close enough to inductively couple a signal from one to another, it may create its own antenna nearby.

Once you find all the antennas, you have to figure out how to get them not to behave as an antenna. This might be as simple as adding a capacitor or an inductor to make the antenna behave a little better, or maybe place a ferrite bead on a conductor coming into the chassis. Or it may be shielding the antenna so that it doesn’t radiate energy outside of the enclosure even though it is still radiating. Knowing which of these techniques to apply may take time to allow for trial and error. This can get expensive because you can’t afford to sit in an expensive chamber for days experimenting. Your best option would be to call an expert or company, like Percept, who can probably help you pin it down right away and follow up with an EMC scan to ensure passage on your next EMC formal testing.

Percept Technology - Boulder, Colorado
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