On 03/12/2018 20:25, Ed Deichler wrote:
I noticed several stations during the FT8Roundup contest that
exhibited one or more decoded traces on the waterfall, usually when
calling CQ. The stations are mostly 0 dB or stronger on their main
trace when the multiple decodes appear. Each additional trace is 120
Hz below the previous trace, i.e. the second trace is 120 Hz below the
main trace; the third trace is 120 Hz below the second trace, etc. I
have also seen the same multiple decode behavior when the station is
in QSO although not as often.
Hi Ed,
this is due to line noise on the audio path. E.g. a poor earth return to
a full wave rectified PSU will inject 2 times the line frequency hum.
This noise mixes with the signals creating side-bands at multiples of
twice the line frequency.
It can happen at either the Tx or the Rx end. It normally happens at the
Tx end of a few offending stations but not exclusively. A sure fire
check is if you see 120Hz side-bands on a station from a country with
50Hz line frequency like here in the UK, in that case the problem is
definitely at your end of the link. OTOH if you see 100Hz spaced
side-bands on any station then it is surely at the Tx end. A 'scope on
the Rx audio connection would be worthwhile.
The repeatable level change along the side-bands and that you are seeing
it a lot might imply the problem is at your end. You should be seeing
side-bands on both sides of the received signals.
If you narrow the IF filter right down on an offending signal and that
attenuates the side-bands relative to the main signal then the problem
is almost certainly at the Tx end (requires a very narrow IF or DSP filter).
73
Bill
G4WJS.
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