It is interesting the number of times this comes up. Each time it does I point the poster to a series of articles by Keith Armstrong that are published in the compliance club. Find them at
http://www.compliance-club.com/] [url]http://www.compliance-club.com/[/url]. I have found the explanations simple to understand and not overly heavy on maths.
The single biggest price of advice is think about the EMC at the start of the project. Before any work starts, including housing, prototype circuits and the like. It is at this point you can explain to your boss that spending a little more money on a PCB will save money in the longer term. (KM did an article on this about 2/3 years ago)
On your points;
- Plastic enclosures with "EMC coatings" DO NOT WORK!
Plastic boxes with "EMC coatings" can work, but the coating has to be sprayed on correctly and correctly connected to a reference. I have used this in the past when I had a board that was a bit noisy but the coating was very expensive and an absolute pig to ensure it was connected to 0V.
- Metal enclosures with even the slightest gap between panels don't work either
See the definition of a slot antenna. (also a metal enclosure with holes covered in sprayed plastic doesn't work either)
- A badly designed box may actually cause WORSE EMC performance than the naked electronics (the lighthouse effect)
- Ground all your connectors securely to the metal enclosure - otherwise they are a path for RFI run amok inside your design.
Oh yes
- Ferrites are your friends
Yes, but preferably only as a last resort. If you can install them on a PCB rather than clipping them on leads then so much the better.
- Conductive gaskets are vital
It will close the gaps, especially if you are having problems at very high frequencies.
- Spectrum analysers are expensive, but in the long run it's worth it because it saves you running off to the expensive EMC lab all the time.
These are generally only any good in the near field, some standards will actually say that these can't be used to make the measurement. They are invaluable for finding what areas are radiating however these might not be the cause of the far field problems.
Yours
Neil
[/list]