I agree absolutely with Brent. Also, even if ALL the other variables
are STRICTLY controlled you could conciveably get as much
as 8 db variation between sites.
I suggest ( as Brent has already done) getting Lowell
Kolbs paper on site comparisons.
From: "Brent DeWitt" <bdew...@ix.netcom.com>
Reply-To: "Brent DeWitt" <bdew...@ix.netcom.com>
To: "Rao, Praveen" <praveen....@fujitsu.com.au>,
<emc-p...@majordomo.ieee.org>
Subject: RE: Comparing EMI test results
Date: Sun, 6 Jan 2002 22:46:22 -0700
Rao,
The comb generator is not a bad choice, but a few things should be checked
(yes, I've done this).
First, establish the base variation in the source. No antenna, just a
direct connection to the receiver/SA that you will use for the evaluation
under an expected, reasonable variation in temperature and battery
condition. Take enough samples over enough days to make yourself
statistically comfortable with the level of uncertainty you're targeting.
Second, use the volumetric calibration procedure (as required for
"alternative" test facilities) on both the SAC _and_ the OATS. Having done
volumetric NSAs on high quality SACs and low quality OATS, I can say there
is _no_ reason the OATS can be considered a "gold standard" in all
instances.
Lowell Kolb of Hewlett-Packard did an excellent paper on comparison of OATS
sites a few years back. I would suggest looking up his paper for an idea
of
OATS site to site variation.
Lastly, everything I've just said will, most likely, be totally swamped by
variations in the EUT setup under "real" EUT testing unless you can
_strictly_ control them. Don't go there. The more "real" the simulated
EUT
becomes, the more variables you introduce into the site evaluation. As
more
variables are introduced, the greater the number of data points will be
required to resolve the result. While it is obvious to the most casual
observer that the simple comb generator does not represent a "real" EUT, it
does represent a single frequency component at a single phase component for
each of the volumetric measurement points. The best work done in this area
has used a VNA for evaluation of both quantities, but from the literature,
this would appear to be most useful in diagnostic applications.
Empirically, if the magnitude of the seven single planar volumetric points
looks good, the site is _probably_ pretty good. Of course, if the SA is
systematically off at all points, check the antenna calibration!
Best regards,
Brent DeWitt
-----Original Message-----
From: owner-emc-p...@majordomo.ieee.org
[mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Rao, Praveen
Sent: Sunday, January 06, 2002 6:16 PM
To: emc-p...@majordomo.ieee.org
Subject: Comparing EMI test results
I'm trying to setup a programme to compare test results (for
Radiated Emissions) between test labs. The plan is to circulate a
"Artifact" around the labs and compare the results. The test sites being
compared are Semi-anechoic Chambers v/s OATS. The source used is a
wideband "RF comb generator". I would like to understand the concept
fully before I initiate this programme. Is there any information readily
available on this topic? Any help will be greatly appreciated. Thanks.
Praveen
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