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

[Darwin, Keith]

Hmmm, well, I may be confused but I *think* you just told us how to
convert the K1 from AF derived AGC to RF derived.

I realize making the AGC respond faster to a large signal would take
that sort of mod, but isn't it just a cap change to speed up the decay
after the large signal is gone?

Of course with no RF gain, I'm not sure I really want to speed up the
AGC recovery.  In my TenTec rigs that were audio-derived, I'd speed up
the AGC recovery then I'd control the leading edge overdrive by backing
down on the RF gain manually.  Can't do that on the K1.

I think I'd be inclined to leave the K1's AGC just the way it is ... for
now :-)

- Keith KD1E -

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of wayne burdick
Sent: Wednesday, April 05, 2006 4:06 PM
To: Elecraft Reflector
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


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[wayne burdick]

Keith wrote:

> Hmmm, well, I may be confused but I *think* you just told us how to
> convert the K1 from AF derived AGC to RF derived.

That's correct, Keith, assuming you think 150 kHz is "RF"  :)


> I realize making the AGC respond faster to a large signal would take
> that sort of mod, but isn't it just a cap change to speed up the decay
> after the large signal is gone?

No, and this is at the heart of the tradeoffs with AF-derived AGC. No
matter how small you make the detector's capacitor, it still takes time
for it to charge, and it must do so at the rate imposed by the
bandwidth of the crystal filter. With a narrow-band filter, the
resulting slew rate is long enough to result in an unpleasant few
milliseconds of high-amplitude "pop" getting through the AF amp. The
second problem is that a small detector capacitor won't filter out AF
ripple from the signal applied to the detector, and you'll end up with
AF modulation riding on the AGC signal. This creates objectionable AF
IMD.

So-called "Hang AGC" and a very high-impedance detector can be used to
minimize this effect, but you can't completely eliminate the "pop" this
way, at least in my experience. An AGC signal derived from 150 kHz will
be far superior, as anyone who has a K2 will attest to.


> Of course with no RF gain, I'm not sure I really want to speed up the
> AGC recovery.  In my TenTec rigs that were audio-derived, I'd speed up
> the AGC recovery then I'd control the leading edge overdrive by backing
> down on the RF gain manually.  Can't do that on the K1.

In practice, RF gain is rarely backed down by K2 users -- the AGC
system has a very pleasant sound and very wide dynamic range. This
could easily be achieved with the K1, too. But you could incorporate a
manual RF gain control if you wanted to. Finding a place on the K1's
front panel is another story.

Wayne


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