It is true that the AGC pertains to the radio and not the
band, but the optimal AGC threshold setting does vary by
band. On 80m, for example, I set the AGC threshold on my
Ten-Tec Orion at about 20uV. On 10m, where the noise level
is very low, I set it at 0.5uV. The Orion, incidentally, has
fast, medium, and slow AGC settings, plus a custom "prog"
setting, and one can set the threshold, decay rate and hang
time independently for each speed. It is possible to spend
hours fiddling with the Orion's AGC settings.>>
One of the reasons you have to change it so much is any AGC
that has no effect at all until a threshold and then tips in
all at once is always critical to adjust. A good AGC system
has a slope rather than a hard threshold with no AGC at all
below a point, and then hard AGC after that point that
clamps the volume to one level.
At quiet locations the dominant noise is propagated via
skywave just like the desired signals. This means the noise
floor varies greatly with direction depending on propagation
or even the time of day. On the same band in different
directions the noise floor can vary 10 or 20 dB. This
requires constant adjustment of AGC threshold with any AGC
that tips in fully at a threshold.
AGC systems with a slope don't exaggerate the problem of
noise level changes and they don't "muddy" the signals into
one constant level or no AGC at all.
When I solid stated my R4C's I built a new AGC circuit. I
put a lot of gain in the AGC, there was very little audio
level change with input signal level change. On the bench it
was nearly perfect, once the AGC started working everything
stayed at nearly the same audio level. I worked to make it
have flat audio level with varying signal levels, textbook
perfect AGC.
Everyone hated it. It required constant riding of RF gain.
It made weak DX stations near noise floor muddy up or
vanish, and it was impossible to sort signals close in pitch
in pileups.
Decreasing the gain in the AGC circuit cured it, and it was
no longer necessary to ride the RF gain (same as adjusting
AGC threshold).
The K3 has an AGC SLP adjustment that cures this problem. I
was delighted to see the people at Elecraft included an AGC
slope adjustment, and that it works to make the AGC have
that "analog" sound operators here like for handling pileups
and for weak signals in rough noise. Unless you have an old
analog AGC system the slope adjustment is a "must have"
requirement. Especially for people who work pileups or work
DX that is in and out of rough noise like static crashes.
Although I just started using the K3, I really like the AGC
system so far. That's in contrast to other digital AGC
systems I quickly learned to *dislike*. Low band DXers or
contesting people will really like the "slope adjustment"
feature.
73 Tom
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