I think that there might be an "ooops" in your analysis, of substracting 2 dB
from the limit, Richard.
A 6 dB variation in site attenuation could mean that a unit could be as much as
6 dB off the true reading. Of course, how can one measure the true reading?
Only by testing the unit on a site with complies with C63.4 (i.e. meets the 4
dB variation) and comparing the results to the other site. But the C63.4 site
could be 4 dB off in the other direction from the 6 dB site, so the readings
could be 10 dB apart. This is true even if the absolute value of the unit's
emissions is right on the passing line.
Here is an example.
A unit is built and its true emissions are 4 dB over the limit at a certain
frequency.
It is tested on a C63.4 site which, at that frequency, has a negative 4 dB
factor. The unit is judged to pass (true emissions of 4 dB over - 4 dB
variation).
It is then tested in a chamber which claims a +/- 6 dB variation, and at that
frequency the chamber has a plus 6 dB factor. The value read in the chamber is
10 dB over the limit (true emissions of 4 dB over + the 6 dB variation).
The delta between the two is 10 dB.
Well, my thoughts anyway.
Gabriel Roy
Hughes Network Systems
MD
(46)
To: emc-pstc @ ieee.org (EMC) @ SMTPGW
cc: (bcc: Gabriel Roy/HNS)
From: WOODS @ Sensormatic.com ("WOODS, RICHARD") @ SMTPGW
List-Post: [email protected]
Date: 05/09/97 10:25:00 AM
Subject: Alternate Test Sites per ANSI C63.4
SMTP Headers: Headers
Section 5.4.2 of ANSI C63.4 says, "Measurements can also be made where
the requirements of 5.4.6 [NSA] are not met but it can be demonstrated
to the relevant authority that the results achieved are equivalent to
those obtained at an open area test site over the frequency range at
which measurements are made."
Several supplies of compact semi-anechoic chambers claim a 6 dB
variation in site attenuation vs the required 4 dB. If the claims are
true, it would appear that one could successfully test products in a
compact chamber if 2 dB of margin were subtracted from the test limits.
I would like to hear from anyone who has been successful in convincing
the FCC to accept test data taken in a non-compliant compact chamber
that is corelated with the NSA requirements.
