[Racal] RA6793A Microprocessor Noise

Mon, 24 Nov 2003 08:27:48 -0500

Dallas,
I've been using metal BD25 covers with the shield connected to the cover.
Recently I switched to a plastic one on my test set up. I hope I didn't
shoot my foot. The 6830 runs that ground back to the middle of the
motherboard. GOOD FIND TNX!   fc

-----Original Message-----
From: Dallas Lankford [mailto:[email protected]]
Sent: Sunday, November 23, 2003 6:27 PM
To: Racal Reflector
Subject: [Racal] RA6793A Microprocessor Noise

My RA6793A has suffered from uP crud, mainly above 20 MHz, from the day that
I received it.  Another 6793A that I examined had the same problem, which
indicates it is a common problem.  At some point I noticed that my phased
antenna array could null the crud, indicating that the noise was being
radiated by the receiver and picked up by the antenna.  I tried shielding
the speaker wire (though it was already shielded) by wrapping aluminum foil
around it.  No luck.  But then the aluminum foil slipped and accidentally
grounded itself to the 6793 chassis at about its mid point without being
grounded at any other point.  The noise was reduced.  So I used some
shielded twinax, longer than I would normally use for a speaker connection,
with it grounded only at its mid point to the 6793 on its rear panel.  This
reduced the uP noise considerably.  My phased antenna array again determined
that the receiver itself was the source of the remaining noise.  I attempted
to determine if the front end was involved, but got negative results.  To
take a break from my efforts, I decided to do some weak signal listening
with headphones.  To my surprise the remaining microprocessor noise went
away.  I didn't like the idea of leaving my headphones plugged into the 6793
as part of my uP noise elimination solution, so I wired a 10 ohm resistor to
a headphone plug.  That worked just as well; no noise.  I also noticed that
with just headphones connected and no speaker connected there is no uP
noise.  But I often like to use a speaker with my receivers.

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