I must confess to some bewilderment about the seemingly endless discussion about
the adjustment, or mis-adjustment, of AGC slope, threshold or "RF" gain.
Claims are made that one's favorite settings cause signals within the passband
to retain their relative amplitudes thus allowing the discrimination between
them, while less favorable settings compress them into "mush."
I will confess that my experience with the design of AGC systems is limited to
analog receivers and perhaps there is some digital magic that makes DSP radios
act differently from analog ones in this case. But in my experience, AGC
control is derived from the stronger signal received.
After the SNR is adequate (delayed AGC in 1960s terms, above threshold today),
the overall gain is reduced by some amount to maintain a desired output or to
prevent overload, and any other signals present suffer the same gain reduction.
Hence a signal 30 dB stronger than another is still 30 dB stronger even after
the application of AGC. If it isn't then we have a very nonlinear receiver,
which is desirable if we're receiving FM but highly undesirable otherwise.
My reading between the lines suggests that the "mush" proponents think that
after achieving threshold, changing the slope somehow changes the ratio between
signals, i.e. there is less gain for strong signals than there is for weaker ones.
Frankly, after 60 years of listening to shortwave noise and in my youth working
in a machine shop and hanging around too many alcohol and nitro burning race
cars, my tinnitus practically drives me nuts at times; I welcome a flat AGC slope.
If I'm all wet with this, I'd like to be enlightened.
Wes N7WS
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