If the signal of interest and a larger signal are within the preselector
bandpass the preselector doesn't do any good. In this case you can
increase the spurious-free dynamic range of the receiver by adding
attenuation. The first thing to do is to see if you can do without the
pre amp. For every 1dB you cut the signals you cut the T.O. mixing
products by 3dB.
Dave Cuthbert
Micron Technology
From: [email protected]
[mailto:[email protected]] On Behalf Of
[email protected]
Sent: Friday, March 26, 2004 2:38 PM
To: [email protected]
Subject: RE: Semi-anechoic chamber & a preselector
A preselector is necessary for two cases. You already covered one
(strong
ambients, though they could be in-band as well). The other is in the
case
where you have noise with a high peak-to-average ratio (usually some
sort
of impulse noise), and the peak-to-peak voltage exceeds the dynamic
range
of the mixer in the receiver. This is not uncommon with conducted
emissions
>from power supplies.
You could take your cable off the receiver input and connect it to a
wide-band oscilloscope set to 50 ohms input impedance. The 'scope needs
to
have enough bandwidth to take in most of the energy of the signal.
Measure
the peak-to-peak voltage of the signal. Adjusting the trigger level may
help you determine the true peaks for a waveform that is not repetitive,
or
for which the peaks are infrequent. The peak-to-peak voltage should be
equal to or less than the peak-to-peak voltage of a sine wave that the
receiver allows.
For example, you might set the receiver for 10 dBm maximum input power.
This power is 0.7 Vrms, or 2 V peak-to-peak. If your signal as seen on
the
oscilloscope is no more than 2 V peak-to-peak, you can pretty much get
by
without a preselector. If you are in doubt, increase the maximum power
input setting on the receiver to see if the emissions levels change.
Don Borowski
Schweitzer Engineering Labs
Pullman, WA
Ralph McDiarmid wrote on 03/26/2004 11:44:32 AM:
> Does anyone in the group have experience with the need for a RF
preselector
> used in a 3m chamber? I suspect it is really necessary more for an
OATS
> where out-of-band strong ambients could present a problem for the
> attenuator/mixer in a SA.
>
> Thanks,
>
> Ralph McDiarmid, AScT
> Compliance Engineering Group
> Xantrex Technology Inc.
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