[Elecraft] RX Mush, why the fuss?
David Gilbert
xdavid at cis-broadband.com
Fri Mar 3 13:59:16 EST 2017
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
> <xdavid at cis-broadband.com <mailto:xdavid at cis-broadband.com>> 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.
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> <http://mailman.qth.net/mailman/listinfo/elecraft>
> Help: http://mailman.qth.net/mmfaq.htm
> <http://mailman.qth.net/mmfaq.htm>
> Post: mailto:Elecraft at mailman.qth.net
> <mailto:Elecraft at mailman.qth.net>
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to k2av.guy at gmail.com <mailto:k2av.guy at gmail.com>
>
>
More information about the Elecraft
mailing list