2 dB was the required margin for initial acceptance to the VDE standard for
ITE. Of course, the VDE standards are no longer mandatory since the EMC
Directive came into effect. The "new" thinking today is to take measurement
uncertainty into consideration when determining a pass/fail criteria. For
example, assume that a lab has 95% confidence that their measurement
uncertainty is no more than 3 dB. Also assume that the measured margin or
your device is 2.5 dB. The measured value indicates that you pass; however,
based upon the measurement uncertainty, the lab cannot say that you really
do pass. For them to be 95% certain that you do pass, your margin would have
to be no less than their measurement uncertainty - 3 dB in this example.
----------
From: [email protected] [SMTP:[email protected]]
Sent: Monday, October 26, 1998 12:50 PM
To: [email protected]
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
[email protected]
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