From: Roy Lewallen on May 27, 6:49 pm

wrote:

I apologize for not being more precise in my nomenclature.

No problem to me...I fear I got off on a "lecture mode" again,
but was speaking in generalities to other readers about
receiver back-ends.

By "BFO" I mean the oscillator used for product detection when receiving
SSB and CW signals. No AM detector is involved. The AGC pickoff is of
course done from the IF preceding the product detector, and doesn't
intentionally use the BFO or product detector in any way. The problem I
was alluding to is that the BFO produces a large signal which is very
near the IF, and therefore can get into the AGC circuitry unless some
care is taken to prevent it. This produces a DC bias among other
problems, which can interfere with AGC circuit operation. I found it
necessary to completely shield the BFO, use a good doubly balanced
detector, and use differential amplifiers in the AGC chain in order to
reduce the BFO crosstalk to a tolerable level.

Sounds good to me. Separated, isolated detectors allow one to
concentrate on the particulars of each, makes it a lot easier to
work with.

For what it's worth on the audio-output part, I'm more fond of
rather high levels of IF into the detector to get around the
"square-law" response...looking for a better AM envelope
reproduction. While that results in better audio, it also makes
decoupling more difficult to avoid feeding the strong IF back
to the input. Different problem, same cuss-words on the bench,
though. :-)

I strongly suspect that a number of the complicated AGC circuits evolved
because a simpler AGC circuit was poorly designed and/or subject to
problems like crosstalk from the BFO. Instead of solving the fundamental
problems, increasingly complex circuits are developed until one
accidentally works correctly, then the improvement is credited to the
complex circuit rather than its accidental relative immunity to the
results of poor fundamental design. This isn't of course universally
true, but it happens pretty often.

I agree with you there. At least for voice-band detection
receivers (of which I've only built two in a half century from
my own design). Discounting copies of "All-American Five"
table-model cheapies using a single diode for both audio
rectification and (low-pass filtered) for AGC voltage to a
single controlled variable-mu amplifier. Ultimate simplicity
for reasons of price over the counter. One CAN put a BFO on
those (Hallicrafters did back in the late 40s) but the
performance is not the best.

Separating the "detectors" by function is best. The audio
"detector" (I still think of them as 'rectifiers') can be
optimized for best sound. The AGC detector can be optimized
for its action separately...and its response versus IF input
and overall receive chain amplification tailored for the
AGC control-loop "gain." Filtering-decoupling that follows
can be figured out to keep the low-frequency phase response
from upsetting the closed-loop AGC control.

Separate AGC and voice detectors lets one play around with
"attack" and "decay" time-constants with no more than a
single dual op-amp shaping circuit...multiple time-constants
under manual control if desired, that won't interfere with
the audio detection part. AGC detector input would have to
be the fastest-responding (to desired time-constant) with
a relatively simple op-amp doing the time-stretching.

Some folks might consider that op-amp addition "complicated."
Won't blame them if they do. From my experience, a
"complicated" AGC subsystem is having to AGC on a
1 uSec pulse with a time gate in the presence of other
assynchronous 1 uSec pulse sidebands located on 1 MHz
intervals (up to 3) on either side...with a decay to attack
time ratio of about 1000:1. :-) Did that for an R&D
airborne system at RCA...was somewhat too much but that
allowed a greater simplification for a following generation
of airborne equipment. A lesson there can be to "cover all
bases possible" the first time around, then investigate to
see what can be simplified for something less complicated.

AGC, in the basic consideration, should begin as a control
loop. From there on its a matter of choice of circuits.