From: Roy Lewallen on May 26, 5:39 pm

Let me add one more general note about AGC design. The BFO frequency is
very close to the IF, and it typically puts out volts of signal while
the AGC circuit is trying to operate with millivolts. Unless you're very
careful with layout, shielding, and balance, a lot of BFO signal can get
into the AGC circuit and cause disturbances and malfunctions of various
kinds.

I agree on the need for isolation of various circuits but fail
to see the relevance. A "BFO" is on for OOK CW reception and
normally a manual RF/IF amplification control is used to set a
comfortable listening level. Yes, AGC could be used on OOK CW
but it would be a mistake to derive the AGC control from an AM
detector getting "BFO" input...that would be the same as
introducing a DC bias into the AGC control loop...which would
change the AGC servo-action control...perhaps severely so.

Note: A "BFO" source is steady-state. The detector mixes the
incoming signal (usually at the IF) with the "BFO" to derive
the audio. If the AGC control line is picked off this same
detector, the DC component is akin to having a nearly fixed
DC bias inserted. To use AGC on an OOK CW signal, the audio
tone would have to be used...and that necessaitates a different
sort of AGC control source derivation. A peak-riding, perhaps
selective audio circuit could do that, but the complexity of
that part of the receiving chain is growing. It might be
easier all-around to just pick off the IF to a separate AM
detector as the AGC control line source. The "original"
detector could remain as the OOK CW output with isolated BFO
feeding it.

For SSB voice reception, a "BFO" is still present but a single
diode detector all-purpose sort of detector is far from
optimum as a combined audio source and AGC control line source.
It WILL work, but it is non-linear for both audio and AGC
purposes and that alone could be the source of AGC instability.
It depends on the IF signal level at the detector diode (or
"product detector" which is really a mixer stage).

A single diode with large time-constant on its voltage output
is a peak-riding source for the AGC control line. Whether or
not it follows fast "attack" conditions depends on the source
impedance capabilities of the final IF stage. If that is too
high then the "attack" time is slowed from the necessity to
build up a charge on the diode's load capacitance; that can
be seen on examining an ordinary AC rectifier circuit in
response to a step transient of AC input through various values
of AC source resistors. The peak-riding capability is usually
distorted on the leading edge...which then reflects on the AGC
control characteristics (when loop is closed) in trying to hold
the received signal constant at the detector.

Thought of as a servo-control loop, the AGC subsystem can get
rather involved and complex, affected by a number of different
factors, ALL of which are important insofar as AGC instability
is concerned. "BFO" level is just one item and I will disagree
that it is a very important. It is no more important than
anything else in that loop in my experience.

As a suggestion to anyone else, I would recommend first either
measuring or calculating the AGC control line versus both the
antenna input level and the IF level at the AGC detector input.
That yields a DC baseline datum on the controllable level of
the receiving chain. From that, one can "back-track" calculate
how well the closed-loop AGC action behaves; i.e., the antenna
input RF level versus the peak audio output with AGC on. If
that is using old-style "variable-mu" pentode tubes, then the
control characteristics will show whatever non-linearity it
has steady-state. That can be used as a special controlled-
gain model baseline for a Spice analysis of the AGC loop.
Differing time-constants IN the AGC control feedback can be
set to observe closed-loop response with transient signal
input to the antenna.

The last AGC circuit I did was very conventional, and it's the sweetest
operating one I've ever used. But I went to great pains to keep the BFO
out of it, and feel that was one of the essential ingredients in getting
it to operate so well.

Having had a National NC-57 receiver since 1948, I decided to
"play" with it in 1959 and "improve" its performance, such as
increasing IF gain. The first IF stage as well as the RF stage
were AGC-controlled. Not knowing enough about Control Theory
then, nor considering the low-frequency characteristics of the
AGC control voltage line R-C decoupling, that modification
became a disaster for anything but manual RF gain control. The
motorboating (very low-frequency oscillation) extended to having
the VR-150 screen supply regulator (gaseous shunt regulator to
those of solid-state era times) going on and off. It was
restored to its original components and not played with for over
a decade. Much later, on having had to get into Control Theory
and servo control loops, I could analyze how bad it was and see
what I SHOULD have done. The control was too "tight" in trying
to hold the audio output too constant over a wide signal input
range. There was low-frequency phase shift in the AGC voltage
control decoupling that was responsible for most of the motor-
boating; the VR-150 shunt regulator control range was a bit
too narrow so naturally it had dropped out of regulation and
added the final insult to the original "mod." [forty somethings
and younger may not be familiar with such relaxation oscillator
circuits :-) ]

National Radio Company had made an acceptible product in the
NC-57 but it was a low-end item in their product line. It
worked well enough as a single-conversion HF receiver but it
wasn't optimum in design and no doubt stock logistics at the
factory probably accounted for some of the parts values.
Several passive components seemed to be rather arbitrary in
value choices. I had learned (or should say re-learned) that
NO product is an example is "what something should be" as a
design example.

There just aren't any "easy" answers for some things in
electronics. But, they can be WONDERFUL, challenging
"cross-word puzzle" kinds of thing to solve! :-)