[Elecraft] [K3] AGC White Paper

[Richard Ferch]

> AGC adjusts the receiver gain, compared to the audio bits, rather slowly
and DOES NOT change the "dynamic range."

Exactly. AGC changes the receiver's gain between two points in time, not
between two signals arriving at the same time.

I believe changing the AGC settings does improve the "mush" situation in
CW, but the reasoning is more subtle than a simple picture based on dynamic
range alone.

Consider a situation where there are two CW signals, one at S8 sending
mostly dits, and another weaker one at S7 sending mostly dahs. If the AGC
threshold is well above the strength of either signal (or AGC is off), you
will be able to tell when a dit from the stronger signal ends even if a
simultaneous weaker dah is continuing, because of the change in signal
level when the dit stops. In other words, you will hear the dits from the
stronger signal riding in above the lower-volume-level dahs.

Now suppose the AGC threshold is below both signals, say at S6, and the
slope of the gain curve is flat (i.e. the SLP is set to its maximum value).
For now, assume a perfectly instantaneous AGC decay time (super-fast AGC).
Then regardless of whether one signal or both is/are currently "on", the
AGC will immediately adjust the gain to clamp the signal strength to the S6
level. In other words, even after the stronger dit finishes, the weaker but
longer-lasting dah will still be at the same perceived volume, and you
won't be able to tell when the dit ended. Result: you just hear the
combination of two signals at a constant volume, i.e. "mush". If there are
only the two signals, when both are "off" the gain will increase, raising
the perceived noise level, but that's not the "mush" problem, it's the
"noisy receiver" problem. If there are a lot of signals at or above the
threshold, then the "mush" would be more or less continuous.

In the real world, the AGC time constant will affect this. If the decay
time is long enough (as in slow AGC settings as used for SSB), you might
hear the audio volume drop immediately after the end of a strong dit and
then rise during a continuing weaker dah, whereas if the decay time is very
short, you might hear the brief drop in volume only as a blip in an
otherwise constant-level sound. But at first blush I would think this would
be a secondary effect compared to the main effect of adjusting the AGC
threshold to be above or below the level of the signals.

As far as I can see, none of the above applies to RTTY, which is nominally
a constant-amplitude signal. Whether the signal is above or below the AGC
threshold, the ratio of the instantaneous mark and space signals will be
the same (unless one of them is on the skirts of the filter bandpass). Even
if there are two signals on the same frequency, the differences in relative
strengths of mark and space from the two signals would not be affected by
AGC, although the overall amplitude might be.

I don't know enough about how RTTY decoders work to guess at whether a
constant-level signal (aggressive AGC) vs. a time-varying signal level (no
or weaker AGC) would have an effect on decoding, but the same reasoning
that applies to CW "mush" does not appear to me to be relevant.

73,
Rich VE3KI