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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