I was once sent on a interference complaint a previous troubleshooter
had attempted to fix with ferrites. *Forty-two* of them. Taking them
off didn't change this more than 1 or 2 dB but a series termination
resistor in the clock line substantially cut emissions in the lab. The
real problem was a two layer board with no power or ground planes.
FWIW department: If it was a few more than 15 years ago, Manny, I was
involved with finding and fixing a similar problem. Two words: Nickel paint.
Cortland Richmond
On 3/30/2012 1623, Barron, Manny (IS) wrote:
I used to manage an outside EMI test lab. About 15 years ago I remember one
well known client who needed to pass FCC Class B but his product utilized a
plastic case (not metalized nor conductive finish, but fully plastic). When
his product failed he started adding [many] ferrite beads to the culprit
internal I/O cable until his product was 10 dB below the limit. Then he
started taking them off one by one until the product was about 2 dB below the
limit. When I inquired why remove the beads and reduce margin his response was
cost and that his management would not be pleased with him because he had added
cost to the product.
Just an interesting story.
Manny Barron
EMC/EMI Engineer
Northrop Grumman Corp.
San Jose, California
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