On Tue, 01 Sep 2009 10:01:19 -0700, Michał Słomkowski wrote:

I'm designing a HF transceiver and I'm going to use SA612/NE612 as a
product detector. I'm not sure what is the optimal signal level, which
should be delivered to the inputs from the IF amplifier to achieve the
best performance?

I did this a long time ago with an NE602, in a receiver with audio-
derived AGC (see http://www.wescottdesign.com/articles/MSK/
page_79.html). With IF input levels the mixer would suffer diminishing
output. This would cause the AGC to get "stuck" in a positive feedback
mode and push the thing to ever-higher gain and ever-lower audio levels.
I fixed it with a pair of diodes, one in each direction, at the IF input
to the mixer (they're not in the schematic). They'd distort the signal
at those levels, but at that point it was much higher than desired anyway
-- it just needed to keep it low enough to let the AGC grab hold.

I _think_ they were germanium diodes -- IIRC silicon diodes let too much
signal through. You could use germanium, Schottky, or plain old silicon
diodes plus a 6dB pad, all to equal effect.

I'd tell you how good it sounded, but it was a data-link receiver and I
don't recall ever listening to it!

--
www.wescottdesign.com