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
---
http://www.elecraft.com
_______________________________________________
Elecraft mailing list
Post to: [email protected]
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
