I agree with your comments. Thanks for the clarifications!
And I did indeed forget to mention the attack/decay speed influences. I
even posted comments here about that myself back shortly after I bought
my K3 ... that the time rate of change in gain is itself a
non-linearity. I think my settings are similar to yours (too lazy to
check right now).
73,
Dave AB7E
On 3/3/2017 11:28 AM, Guy Olinger K2AV wrote:
There is some conflation of two quite different concepts going on here.
The first thing you need to know about an AGC response graph is the
speed that the incoming signal was varied to produce the curve. In
many cases, the input signal was steady state from a signal generator,
set to a list of input levels, observing output levels, both recorded
in Excel, and the resultant data pairs used to create a graph line. In
this case the input variation speed is zero. This is a static analysis.
If the input signal was **amplitude**-swept at audio rates, and
together with the output signal, used to provide the X, Y values to
drive an oscilloscope, then you have a dynamic analysis.
At this point it is good to make a note of what test equipment you are
familiar with that will provide an **amplitude**-swept, steady
frequency signal.
Inferences from a static AGC analysis and AGC induced IMD are apples
and oranges.
The second thing that bears heavily is the attack and decay speeds.
Attack speeds are usually quick. If the attack and decay are **BOTH**
quick, and that actual attack/decay is at an audio rate, then there is
a case for distortion, because the variable gain can actually work at
an audio rate.
The question is whether the attack/decay cycle can continuously recur
because the decay goes down as fast as the attack goes up, then
intermod is indeed possible on a grand scale. However if the decay
effectively holds the AGC gain level at a point set by the attack,
delaying even as little as 100 milliseconds, then the AGC cannot
create audio distortion products except very short low frequency
distortion products only at AGC attacks.
Since well before the significant AGC changes in firmware 4.7x (or
whatever that one was), I have been running my slow AGC (CONFIG:
AGC-S) at maximum fast, and my fast AGC (CONFIG: AGC-F) at maximum
slow. In retrospect, that was probably why I never heard the stuff
that a lot of people were complaining about.
In contests I always use my max fast setting slow AGC, and back off
the RF gain when I have primarily very loud signals in pile-ups to get
the signals out of hardware AGC range, which has zero intelligent
tweaks available.
[And yes I have just about guaranteed pile-ups in contests with
for-credit USA to USA QSO's, because of RBN spots which pick up
everyone. Those are "spotting pile-ups" and assisted or unlimited
class folks using point and click on the band map or control
characters to move to the next unworked station.]
My exception to using max fast setting slow AGC is when I'm trying to
copy through lightning static, and need to hear weaker stations down
in between the crashes. Then I use my max slow setting fast AGC.
To summarize, in order for AGC to create audio distortion products
strictly from the AGC, the AGC must be responding at an audio rate.
Frankly, why would anyone want to set it that way escapes me.
To Wayne, I would like to be able to set a minimum hold for fast AGC
as well. That with a fast decay, would be better than what we have.
Decay rate is something left over from analog days, when the way you
decayed AGC was letting a capacitor discharge.
73, Guy K2AV
On Fri, Mar 3, 2017 at 3:02 AM, David Gilbert
<[email protected] <mailto:[email protected]>> wrote:
I've had my K3 since 2008 or so, and over the years I've seen
people describe different forms of "mush". One set of comments
indeed involved complaints about the hard limit at the upper end
that has nothing to do with AGC. It is, as you say, simply a hard
limit ... pretty much a clipper to protect the ears (and maybe
also to help protect the output stage in the speaker driver before
that issue got addressed). That creates a distortion, but it's
not really what I would describe as "mush."
The nonlinearity I described in my earlier post was at the
opposite end of the curve ... down where the AGC just begins to
kick in. As W6LX says, it's a nonlinearity in the curve, and no
matter what you call it that contributes to the generation of
mixing products from multiple signals that happen to be at roughly
the same level within the passband. The low end of Jack Smith's
plots showed that pretty clearly. During some of my contest runs,
individual signals were perfectly clear and distinguishable, two
not terrible, but even three signals could generate enough mixing
products to cause problems if they were low enough in volume and
close enough in frequency. Since I typically operate with a very
narrow passband (about 150 HZ on CW), the mixing products end up
very close to the real signals. For example, 2x500Hz - 510 Hz
gives another phantom signal at 490 Hz. Things get really messy
with three or more signals.
It is also, possible, of course, to get mixing anywhere there is a
knee in the AGC curve, but if you put the knee up higher there is
less likelihood that multiple signals will be of the same
amplitude to cause a problem (one will dominate), and their
amplitude swings will range further afield of the knee ... meaning
that a lower percentage of the energy will be mixed. At the low
end, you're pretty much screwed ... any signal you hear will be at
that nonlinearity and the amplitude swings will be small enough
that they spend all their time in the nonlinearity. As I said
before, reputedly the new synths greatly improve this.
The bottom line is that if you have two or more signals within a
passband that traverse a nonlinearity, you get mixing products
within the same passband that blur the individual signals ...
i.e., "mush." And since the mixing products on CW only occur when
both (or more) of the signals are keyed, the mixing products
aren't even intelligible. ;)
At least this is how I understand the situation. I'd be happy to
get corrected if my comments are flawed.
73,
Dave AB7E
On 3/2/2017 3:19 PM, Don Wilhelm wrote:
Now that you mention hard limiting, there is a limiter in the
K3 that if turned on will protect your ears. I am wondering
if some instances of reported receiver mush did have limiting
set on - that would be particularly true for those who chose
to ride the RF Gain and/or run with AGC off.
73,
Don W3FPR
On 3/2/2017 3:37 PM, ab2tc wrote:
Hi,
Where in Smith's article does it say that AGC with the
slope set for 15 acts
as a hard limiter? There is a huge difference between AGC
action (which is
simply a reduction in gain with linearity retained) and
hard limiting.
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