K. Balasubramanian,
47 CFR 17 (December 8, 2003 version) paragraph 15.31(a)(3) specifies
that the test procedures in ANSI 63.4-2001 are to followed for most
intentional and unintentional radiators.
ANSI C63.4-2001 section 5.3 says that a ground plane is optional for
radiated emission tests below 30MHz, while section 5.4 almost always
requires a ground plane for radiated emission tests from 30MHz to 1GHz
(some exceptions in paragraph 5.4.2). CISPR 22 section 10.3 also
requires a ground plane for radiated emission measurements from 30MHz to
1GHz.
The ground plane can have a huge effect on vertical-polarized radiated
emissions, making electric fields vary between 0 and 200% of their
free-space values as would be measured in a fully anechoic chamber (FAC)
at the same relative positions. In my previous job we saw up to a
+/-9dB variation at 240MHz from the expected 1/r falloff of electric
fields, between radiated emission tests in a 3m semianechoic chamber
(SAC) and a 10m open-air test site (OATS) or 10m SAC. This Vertical
Correction Factor (VCF) for the measurement distance falls off to
+/-1.5dB at 30MHz, and is essentially 0dB above 450MHz.
If I was using a FAC for pre-qualification tests of a product, I
personally would aim for at least 9dB margin in my radiated emissions
measurements before going out for official measurements.
You asked earlier about using different test setups for radiated
emissions and conducted emissions testing. I don't have a problem with
slightly different physical arrangements of the equipment/cables between
tests. In fact, we often have to change the physical arrangement
slightly to get the linecord of the equipment under test (EUT) to reach
the line impedance stabilization network (LISN) for conducted emissions
testing. I *might* be okay with a different monitor/keyboard/mouse on
the host computer driving the EUT. But even there I would expect a note
on the test report explaining the reason for the difference(s).
But a significantly-different test configuration, such as changing the
power source, host computer, or EUT cables between radiated emissions
and conducted emissions tests, is very suspicious to me. If I saw this
kind of hocus pocus in a test report, I'd wonder what else they were
lying about in their product literature, and whether I even wanted to
deal with that company... In my previous job as a product developer, my
support equipment sometimes got lost or broken in between official EMC
tests. After discussions with the company's EMC engineers, I almost
always found it easier to rerun one or both sets of emission tests than
to try to explain away the differences in the test setups.
I don't have a problem with using different support equipment for
emissions and immunity tests, especially if the support equipment is at
the far end of a local area network (LAN) cable or phone line that
connects to the EUT. We used to keep some old "clunker" host computers
around for electrostatic discharge (ESD), surge, and lightning surge
susceptibility tests, because these tests have a fairly-high probability
of degrading/damaging the support equipment.
John Barnes KS4GL, PE, NCE, ESDC Eng, SM IEEE
dBi Corporation
http://www.dbicorporation.com/
@scmmicro.co.in wrote:
>
> Dear Experts,
> I am conducting pre-scans in a 3-meter fully anechoic chamber.
> It is noticed that even when no equipment is present inside the
> chamber the field strength is about 20 to 25 db micro volts per meter
> in the frequency range 30 to 350 MHz, about 27 to 32 db micro volts
> per meter in the frequency range 350 to 600 MHz and about 33 to 39 db
> micro volts per meter in the frequency range 600 to 1000 MHz. What is
> the level of ambient noise permissible in fully anechoic chambers?
> Which section of FCC deals with measurement site qualifications?
>
> Sincerely
>
> K.Balasubramanian
> Sr.Engineer, Hardware.
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