Dear Bill, LF Group,
----- Original Message -----
From: "Bill de Carle" <[email protected]>
I tried turning off the AGC for a while but had to manually cut the RF
gain so much to avoid clipping it didn't seem like a good idea. With the
AGC on at least there is some possibility of weak-signal energy getting
through between crashes if there are QRN-lulls here and there.
I think you may find that you get better results under these conditions if
you crank up the RF gain until clipping occurs all the time. This results in
some loss of signal level, but a greater reduction in QRN level on the
spectrogram, leading to several dB SNR improvement overall. I suspect how
well this works depends on how quickly the RX gain stages recover from a
short-term overload; if the amplifiers clip in a well-behaved way, then the
QRN transients during which the signal is lost will have a duration
determined by the impulse response of the IF filter. But the overdriving of
the amplifiers that occurs during the transient might upset the DC biasing,
leading to a long recovery "tail" due to the time constants of coupling and
decoupling Rs, Cs, etc., and greater loss of signal energy. Operating the
receiver with the IF amplifier in this kind of limiting mode was quite
successful with the old RA1792 receiver, which has gain-block IC IF
amplifiers which probably work quite well as limiting amplifiers.
This kind of clipping does not work well if there are strong unwanted
signals present in the bandwidth being clipped - either the clipping
threshold has to be increased to above the level of the unwanted signal,
making it less effective at reducing the QRN, or the strong signal
"captures" the bandwidth as in an FM receiver limiting IF, reducing the
level of the wanted signal as well as the QRN. so in Europe the bandwidth
around 136kHz has to be restricted to 1kHz or so to exclude DCF39 et al.
Cheers, Jim Moritz
73 de M0BMU
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