RE: Noise

["Fulmer, Philip" <philip.fulmer@cplc.com>]

There may not be a quick answer to this one.  Where in the circuit are
you observing this signal?  Between the preamp and amp, or between the
amp and the MCA?  Do you have any other modules hooked into the circuit
(e.g., coincidence circuitry)?  Is it observable when there is no input
to the amplifier (i.e., preamp disconnected)?  Are you using NIM modules
or an internal card on the computer that drives everything?

Assuming that you're using a NIM system for signal processing, if you're
observing this signal between the amp and the MCA, then you may have a
ringing circuit that needs a terminator resistor.  The output impedance
of the amp is less than the input impedance of the MCA, and this can set
up a resonance which will vary depending on the degree of mismatch.  So
you may want to try a 93-ohm terminator resistor.  You'll also want to
check to make sure that all your coaxial cables that you're using for
connections are the right impedance (should be 93 ohms...it's easy to
get 50-ohm cables mixed in with the batch sometimes).  If you've
replaced any cables recently, they could be the culprit, particularly if
you've put a relatively long cable between the amp and MCA.  If you have
to use a long cable to connect components, it's better for the long
cable to be between the preamp and amp because of better impedance
matching.

Hope I haven't muddied everything further.

Good luck,

Philip

__________________________
Philip C. Fulmer, PhD, CHP 
Carolina Power & Light Company
Harris Energy and Environmental Center
3932 New Hill-Holleman Road
New Hill, NC 27562
philip.fulmer@cplc.com
(919) 362-3363      

>
>While trying to trace the source of noise in our gamma spectroscopy system,
>we observed a transient sinusoidal signal on our oscilloscope. We estimate
>the frequency to be 0.9-1.0 MHz.
>It appears to be a repeatable but not periodic radiofrequency signal.
>
>Have you observed a similar phenomenon? Any ideas on the source of this
>noise (we are in the basement of UCLA medical center)?      Any RF
>shielding solutions?
>
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