Dear LF Group,
I received G4JNT's Jason beacon again last night, and messed around further
with the gain levels, etc. I found the best results were achieved with the
AGC set to manual, the RF gain cranked up to maximum, and the audio gain
set so that the sound card ADC had a fairly high input level applied, but
not enough to overload it. Under these conditions, the IF signal was quite
severely clipped all the time (see the attachment). The audio sounded very
distorted of course. Backing off the RF gain so that little or no clipping
occurred made the signal unreadable by Jason, and almost invisible on a
waterfall display - whilst with the "Jimi Hendrix style" gain settings, the
waterfall was 'O', and Jason could copy perfectly - the difference was very
striking and counter-intuitive.
I'm now fairly sure it is not the sound card ADC that is involved in the
effect, but rather that clipping the signal under QRN conditions improves
the SNR after further processing. This is the opposite to the optimum
setting required for receiving QRSS when there are multiple signals in the
passband; in this case the gain must be reduced until no clipping occurs,
otherwise "blocking" affects the weaker signals.
I will look out for SM6LKM's 7MHz beacon on the new frequency later today
when I get a chance.
Cheers, Jim Moritz
73 de M0BMU 
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