Scott,
There is no requirement for a minimum passing margin built into FCC Part
15, CISPR 22 or EN 55022.
Minimum passing margin of 2 dB was mentioned in old, now obsolete VDE
EMC specifications. There is 2 dB rule in CISPR procedures to determine
"representative configuration" in a case of multiple units, ports,
cables, etc.
Accredited test laboratory must have uncertainty calculations for each
test method, but it should be ignored when they compare measured value
with limit.
It is a good practice to pass specification with a margin in order to
compensate unavoidable manufacturing mechanical and electrical
tolerances, correlation, repeatability, and uncertainty issues, etc.
Sale opportunity could be lost due to a low passing margin. It would be
a good idea to "fix" the product and get better margin, keep tighter
manufacturing control and have more often manufacturing audits until
then.
You can always challenge test house and if not satisfied change them,
but you'll probably be better off to "fix" marginal product.
Mirko Matejic
-----Original Message-----
From: s_doug...@ecrm.com [SMTP:s_doug...@ecrm.com]
Sent: Monday, October 26, 1998 9:50 AM
To: emc-p...@majordomo.ieee.org
Subject: Required Margin for EMI?
Hello all,
When doing EMI scans of ITE, I have always worked with a
margin of 2 dB
less than the actual limit. No test house I have ever
been to will give a
"meets the requirements..." without having this margin.
Can anyone point
out if this is an actual requirement built into any of
the standards, i.e.
EN 55022 or FCC Part 15, etc. and if so, where is it in
writing?
I understand about measurement errors, product to
product differences and
repeatability of testing issues. Just want to know if
the 2dB margin is
actually written into law.
Thanks for your comments.
Scott
s_doug...@ecrm.com
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