Back in the 90s, I designed a receiver that used a pair of 1350s in the I.F.
strip. I could not find a suitable AGC circuit, so I designed my own.
A 160 millisecond time constant is low enough for your low-pass filter. This
will prevent bass guitar frequencies from modulating the AGC bus. Next, make
sure that you use filtering ahead of your first I.F. amplifier....if you
don't, you will not only have broadband noise from your I.F. strip, but
strong out of bandpass signals will capture you AGC system.
The manifestation will occur by having strong stations as far as 100kHz away
causing desense of your receiver.
I actually perfected this complicated design........then, I discovered the
Philips TDA1572. This chip offers a much better solution, since it has
on-board AGC, low distortion demodulator, etc. The AOR7030 uses this chip as
the 2nd I.F./AGC/demodulator. If you have ever listened to one of these
receivers, you will have an idea of the capability of this chip.
The only difficult thing about the 1572 is that it is almost impossible to
get a fundamental crystal to oscillate on the proper frequency when using
the on-board oscillator. The oscillator is great for a conventional LC
oscillator, since this chip was really designed to be used as an AM radio on
a chip. Another nice thing is the buffered 50 Ohm I.F. output, which you can
use to drive a sync detector, product detector, etc.
E-mail me directly, and I will be glad to send you design information for
both of these chips. I've been meaning to get my website up again, and
repost these designs, but that won't be for awhile. Have fun..........that
is the best part of homebrewing!

Pete

"N9NEO" wrote in message
...
Greetings to my friends at RRS.

I am working on a new pc board. I actually have gone out for boards
and got them working, but I've had to rethink one of the circuits.

In a nutshell the board is a result of the work published by Dallas
Lankford on his investigations into audio elliptic filters. Mr
Lankford uses these filters to reduce the distortion resulting from
fading - fading distortion. Terry had brought up the paper here a
while back and it's taken me a long while to do anything with it.

The board has a MC1350 Gain Controlled IF Amplifier followed by a
precision detector, tone control, audio amplifier, elliptic filter,
and a final amplifier to drive a speaker. There is also a dedicated
amplifier to provide some type of AGC action. I'm not quite sure of
the requirements necessary in order to provide a decent AGC. I'm just
trying to smooth out the audio fading some. My plan was to just put a
low pass filter with a cut-off frequency of a couple of hertz to
modulate the MC1350 amplifier. I intend to drive this thing with my
Red Sun RP2100 radio.

My friend pointed out that the noise would be amplified as well so
that I would need to devise a circuit that would not respond to
anything under a few uV. My answer is to have a switch and switch in
the agc when I got the station half-ass tuned in - HATI. Circuit has a
lot of hooks and empty board space so I can jam a little bit of stuff
in there to accomplish a basic agc. I've run through the rest of the
board and all of the other circuits are good to go.

So the point here is that I'm in a little bit over my head. I would
like to have some discussion with you brainiacs about AGC and maybe
some links to circuits or comments on radios that have had good agc
action. If you think I'm ****ing in the wind with an audio controlled
AGC circuit then I would like to hear that as well.

regards,
Bob
N9NEO
(just say 'NEO')