[Elecraft] Re: K1 AGC time constant [and how to radically improveK1 AGC]

[Rick Dettinger]

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" <n6kr at elecraft.com>
To: "Elecraft Reflector" <Elecraft at 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
>
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