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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