I once read an article (IEEE, 1992), entitled: "Measurement Comparisons
of Radiated Test Facilities". The Abstract sez:
". . .A total of 44 sites worldwide were tested
involving both Open Area Test Sites (OATS) and Semi Anechoic Chambers
(SAC). Preliminary results of the study were presented at the
International Zurich EMC Symposium in March of 1991 [1]. This paper
presents the final results of the study which took two and a half years
to complete. . ."
Later on in the article, the authors state: . . ."over 60 percent of
the sites had one or more frequencies with differences of over 5 db from
the mean." If one was to examine the data closely, perhaps it would
indicate that over 5% of the sites had differences much greater than 5
db, for instance. So, in the real world it would obviously be difficult
to establish any sort of objective confidence level that could be
correlated to a particular emissions level.
Max Kelson
Evans & Sutherland
-----Original Message-----
From: WOODS, RICHARD [SMTP:wo...@sensormatic.com]
Sent: Monday, October 26, 1998 1:40 PM
To: emc-p...@majordomo.ieee.org
Subject: RE: Required Margin for EMI?
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: s_doug...@ecrm.com [SMTP:s_doug...@ecrm.com]
Sent: Monday, October 26, 1998 12:50 PM
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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