[Elecraft] APF and audio limiting

[Joe Subich, W4TV]

Mike,

Don's explanation is good.  Remember, AGC does not "bring up" the level
of any signal ... only the audio gain control will do that.  AGC simply
*reduces the IF gain for signals above the AGC threshold*.

Although it is intended as an analysis of the K3's AGC behavior, the
study by Jack Smith, W8OZA is one of the best explanations of the way
an AGC system works:
    http://cliftonlaboratories.com/elecraft_k3_agc_and_s-meter.htm

Notice that the audio output shows a 1:1 slope for increasing RF input
from the receiver's noise floor (MDS) to the AGC threshold at which
time the *rate of increase* is reduced to some value set by the AGC
SLOPE parameter.  Setting the threshold to 2 (minimum) and slope to
15 (maximum flatness) will result in the least change in audio output
from the weakest to strongest signal but that lack of change is due
to *DECREASING* IF/RF gain as the input signal becomes stronger.
Based on Jack's measurements, the absolute differences in audio
output from noise floor to S9+40 dB can be as little as 10 dB
(AGC THR=2, AGC SLP=15) or as much as 45 dB! (AGC THR=8, AGC SLP=0)
- of course the input range is 100+ dB (-140 dBm to -33 dBm).

The goal is to find a set of parameters that allow the output audio
to reflect changes in signal strength - so the operator can separate
signals - without providing so much dynamic range that the appearance
of a strong signal on top of a weak one causes pain <G>.

The "problem" with APF (or when operating with AGC OFF) is that the
natural tendency of most operators is to "turn up the audio" in order
to hear a signal at or near the noise level.  Without APF, the noise
tends to set a maximum comfortable AF gain level but with APF, not
only is the noise reduced but the level is enhanced.  When a strong
signal shows up in the peak of the APF response, even if the AGC
reduces the IF gain, the "extra" audio gain allows the full difference
between a signal "at the noise" and one that may be "only" 60 dB
stronger to be reflected in the audio.

73,

    ... Joe, W4TV


On 11/7/2010 6:33 AM, Mike wrote:
> Thanks Don.
>
> No, I wouldn't want all sigs to have the same audio level. I'll play with AGC off,
> and the limiter, and the AGC settings.
>
> Mike
>