On Thu, 26 May 2005 14:39:43 -0700, Roy Lewallen
wrote:

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.

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.

Roy Lewallen, W7EL

Roy, you have that right. Simple AGC works well but it's the details.
A 6m SSB design I did required a BFO and balanced mixer (DBM prod det)
to be in a sealed box with bypassed feed throughs. Once that was done
the AGC behaved far better.


Allison