That's a good point. In the AM broadcast receiver world, the ideal AGC
action, I believe, is to maintain the IF at a constant level into the
detector, so (1) the detector does not overload if you are close to the
transmitter; and (2) so that the demodulated audio is always the same
level, regardless of the signal strength, for a given modulation waveform.
In that regard, the classic AGC curves found in the Racal and Drake
receivers, as well as others that I've looked at match the ideal action.
No AGC action up to a threshold and then very aggressive AGC action
above that threshold, so that 100 dB of signal level change produces
essentially zero change in audio output.
Whether the behavior desired in the AM broadcast world should be carried
over to amateur radio is an interesting question. The fact that most
receivers are designed one way does not mean that is necessarily the
best way. Of course, the reason most receivers are designed that way may
be because it is the easiest from an engineering prospective, or it may
be a case where it also provides the optimum user experience.
Jack K8ZOA
Sverre Holm wrote:
I've finished additional K2 receiver measurements and have posted them to
my web site,
www.cliftonlaboratories.com.
Very interesting results, in particular the AGC performance curve.
I wonder about the statement "In an ideal receiver, once the threshold point
is passed, there will be little or no change in audio output with changes in
signal level." The way people speak so highly of the AGC action of their K2s
seems to contradict this. But then maybe this is called the ideal curve just
because it is easy to comprehend and draw for engineers.
I have worked enough with acoustics and psychoacoustics to have great
respect for the ear and our hearing. It is a very subtle instrument that is
not so easy to quantify in a simple curve. Maybe design of AGC
characteristics could learn something from the psychoacoustics that is built
into all modern sound coders, like mp3 and its descendants?
73
Sverre
LA3ZA
Unofficial Guide to K2 Modifications
http://www.qslnet.de/member/la3za/K2/mod.html
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