Thanks Wayne - I am ready to place my order for a fast AGC option module. Without the battery option, I think I have room. Probably enough room for a TUF 1, K2 type mixer module also (just kidding, sorta). Rick - K7MW ----- Original Message ----- From: "wayne burdick" <[EMAIL PROTECTED]> To: "Elecraft Reflector" <Elecraft@mailman.qth.net> Sent: Wednesday, April 05, 2006 1:05 PM Subject: [Elecraft] Re: K1 AGC time constant [and how to radically improveK1 AGC]
> As the principle designer of the K1, I just love these AGC circuitry > discussions, and thought I'd better put in my 2.5 cents. > > The K1's present AGC circuit, being AF-derived, is a compromise between > attack time and recovery time. This topic has been the subject of > scholarly efforts by Haward, DeMaw, and others, so I'll just summarize > by saying that the slow rise time of an AF-derived signal limits how > quickly you can respond to a large signal using a simple diode > detector. Nearly all low-cost QRP transceivers that have AGC at all use > this technique, including all of the ones I've designed (KX1, Sierra, > NC40, SST). > > DSP can be used to emulate a faster response using various techniques, > including post-processing of the signal as it propagates through the > DSP's pipeline. But assuming one wants faster AGC without having to > write DSP code, a relatively simple hardware-based improvement is > possible. Actually implementing it is left to the reader, and here's > what you need to know. > > If you look closely at the K2 Control board schematic, you'll see that > the K2's fast AGC is obtained using an "auxiliary" I.F. of around 150 > kHz. This is 100 times higher in frequency than the audio signal that > the K1 uses to drive its AGC detector, eliminating the rise-time > problem. The same technique could be used in the K1. You could start > with the K2's AGC circuit (mixer, amplifier, and detector), perhaps > breadboarding it on a proto board. You could pick off the 4.915 MHz > I.F. signal from the output of the K1's crystal filter, routing this to > another '602/'612 that has its oscillator running at 5.068 MHz (a > common crystal frequency). As in the K2, you'd then need to amplify and > detect just the 150-kHz difference product coming out of the mixer. > Various circuit simplifications may be possible relative to the K2's > circuit, which also includes manual RF gain control, T/R swiching, and > AGC on/off control. > > An important subtlety: optimal results might require gain-controlling > the auxiliary I.F. mixer at pin 2, using the same derived AGC signal > that drives pin 2 of the K1's RX mixer and product detector. This would > ensure that the aux I.F. mixer's gain is scaled downward at the same > rate as the product detector as signal voltage goes up. This was not > necessary in the K2 case, because the I.F. amp (MC1350) is the only > gain-controlled stage, and it is ahead of the auxiliary I.F. mixer. > > If anyone tries this and succeeds in creating very fast AGC for the K1, > we'd be happy to publish it as an application note on the web site. Who > knows? It might even make a nice little option module. The entire > circuit, if done using SMD parts, could fit on a board placed > underneath the K1's RF board. > > I'd do it myself, but I'm a bit busy :) > > 73, > Wayne > N6KR > > > --- > > http://www.elecraft.com > > _______________________________________________ > Elecraft mailing list > Post to: Elecraft@mailman.qth.net > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
