[hpsdr] HUM on piHPSDR SSB TX signal (Mike Schwendeman)

[Philip Collier]

Hello Mike,

That appears to be quite a pesky ground loop you have in your station, but
you're on the way to eliminating it.

<grin> I cooked a laptop once, battling with a bad AC power cable, so
please use caution and not cook your pi or yourself.

Here are  just a few tips that helped me in the past and may help clean up
the hum in your station:

1) Connect all of the station chassis directly to one central ground with
heavy gauge wire.

2) Signal carrying cable shields should ground to the chassis, but if
running between devices, use a capacitor to block DC on one end.

3) Use balanced audio lines when possible.

4) Use isolation transformers for preventing hum in balanced or single
ended connections.

Sometimes it is impractical to connect to a good earthy ground system, but
centralising the chassis and negative lead connections will noticeably
diminish your hum troubles.

It can get complex, but here's a good article on hum, grounding, and loops
(re the pin 1 problem).

http://www.prosoundweb.com/topics/studio/preventing_hum_
and_rfi_in_your_studio/

Regards,
Phil AB9IL
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