[RFI] monitor interference to baseband audio

Pete Smith n4zr@contesting.com
Mon, 19 Nov 2001 13:24:30 -0500

When I pass my microphone audio through my SO2R box, which is mounted right
under my noisy monitor, and listen to the audio in my transceiver monitor,
I can hear a small amount of impulse noise that is keyed directly to my
monitor -- it comes on at low level when the monitor is switched on, rises
to full intensity when the monitor syncs to the driving signal. and cuts
off completely as soon as the monitor is shut off.

I have disconnected the microphone line coming into the box (which contains
no active devices in the audio path, only relays), which causes the noise
to become louder when it is disconnected, and have one by one eliminated
cable pickup on the various lines going into and out of the box.  In fact,
proximity of the box, its cables and the monitor don't really seem to make
a difference.

My suspicion is now focusing on the ac line -- the box is powered by a
wall-wart with a short length of RG-174 as the power cable.  Is it possible
that the monitor's ac line is feeding enough power supply chop into the ac
line that it is being relayed into the SO2R box through the power supply,
and then coupled to the mic audio line through adjoining PC board traces?
If this is the case, it doesn't sound like an AC line filter with
attenuation at HF will necessarily be much help, because the interfering
enrgy appears to be down in the audio range.  any ideas?

73, Pete N4ZR