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May 27th 05, 11:49 PM

wrote:
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.
. . .

I apologize for not being more precise in my nomenclature.

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.

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.

Roy Lewallen, W7EL

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