Topband: AGC

[Tom Rauch]

> I can add just a few comments
> > Above threshold, the AGC clamps the audio output at a constant level
with
> no
> > measureable change in audio output for increasing signal input.  There
is
> > definitely no gain "slope" and no other obvious magic.
>
> I tryed an AGC with flat response above threshold (and my AGC does so when
> receiving
> the digital modes), but I found a more pleasant sound when a stronger
signal
> are a bit louder,
> so I prefer some gain slope above the threshold (10dB:1db ... 20dB:1dB).

A hardware AGC system must have something unusual inside if it has a
threshold and then clamps perfectly. Perhaps very high gain and a threshold.

In my homebrew AGC circuits, I rectify IF voltage before the last detector.
I can vary the gain of AGC amplifiers, or set bias on the first amplifier
transistor (making it non-linear) to control a fast tip-in point. Varying
gain changes the audio output vs. RF input level slope. It is very difficult
to make it "all or nothing" at one certain level.

I find by far the best sound on weak signals occurs when there is some AGC
as noise floor is approached, but not nearly as much AGC as when the signal
is well up out of noise.

With very high gain in the AGC loop every noise pulse and every signal
regardless of strength comes out the same exact audio level. While that is
OK with a smooth steady hissing noise, if anything rough at all or any other
signals are added to the mix all of the dynamics are lost. Absolutely
everything from the audio has the same level.

What I found while listening last night and this morning is listening with
hard-leveling AGC is like running volume up and clipping everything hard in
a diode limiter at the receiver output. When it gets a little noisy I have
to add long hang time to prevent "mush", and then that masks weak signals
between noise pulses.

I'm strongly coming to believe the last thing we want (unless we like to
constantly have knobs to adjust) is a very stiff AGC that all tips in at a
knee, with absolutely no AGC below that point. What that would do is require
we set the tip in point for the noise or QRM floor at that particular
instant in time.

Paying attention to what I do, I move the volume up a bit when trying to
copy a very weak signal that is just ESP and back down to a "normal setting"
for everything else, yet at ALL times I can plainly hear a background hiss
or noise. I just need to have that noise or hiss loud to dig out weak
signals, and I can't tolerate any clipping or limiting on the noise or
signal unless it is well out of noise. I find I never adjust AGC speed,
except going between SSB and CW.

That's just how I like AGC.

 I also find the less I have to adjust stuff the easier life is. Finding the
correct antenna and tuning the VFO are time consuming enough. Even logging
the QSO is a distraction from copying weak signals.

73 Tom