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 <xda...@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 > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@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....@gmail.com > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@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 arch...@mail-archive.com
