On Mon, 05 Nov 2007 21:01:09 +0100, Antonio Vernucci wrote:

I would like to build an old-style receiver using a BC-453 stuck at 500-kHz as
IF chain. A possibility would be to put a pentode in front of the BC-453 mixing
the incoming RF signal with the signal generated by a variable local oscillator
(VFO). To improve selectivity, one could adopt a regenerative arrangement
whereby part of the pentode plate signal is fed back into the grid (by inductive
coupling), and the cathode resistor is then adjusted just before the tube starts
oscillating. In other words, a kind of Q-multiplying converter (I think this is
called "Q-dyne" receiver). What I am not fully sure about is if the increased
selectivity I so obtain turns into a higher rejection of the image frequency, or
just into a narrowing of the received bandwidth (which is already narrowed down
by the tight BC-453 85-kHz IF transformers). My feeling is that said
regenerative scheme would offer no advantage in terms of image rejection, but I
would value very much your opinion on that subject.

Your feeling is correct -- regeneration in the IF will narrow the IF
response, but won't correct for images before the IF. If the following
receiver suffered from images you'd correct them, of course.

I've seen designs in old handbooks that used regeneration in the RF stage
to sharpen up the response there -- that would, indeed, help the image
rejection at the expense of intermodulation performance.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html