VK3YE's 'non-invasive' AGC circuit for BiTx40 or similar QRP rigs

[@harrymagooslum5770]

That is pretty clever. I like it! I do have one additional thought (improvement?): if the signal coming in is really strong and there is clipping, noise spikes, or other distortions in the earlier stages, wouldn't it make more sense to attenuate an earlier stage of the receiver to prevent those from potentially generating "unwanted products"? It might keep the signal in the linear range and keep the signal cleaner as it goes to the later stages. Once distorted, reducing the audio gain wouldn't repair the damage - it would just make it less loud. I think that is how it is done in most commercial radios. In fact, looking at the schematic of the venerable Kenwood TS-830S, known for having an superb receiver, they reduce the gain at multiple early stages with AGC derived from late stage IF right after a diode ring detector. Perhaps I don't have the whole picture and when you say that AGC circuit drives the "top end of the audio gain control" you mean something else.? In direct conversion (DC) receivers the audio chain is most of the gain. But I think you are working with a rig similar to the Bitx40 which appears to be a heterodyne type. Controlling the amplitude of the signal before the mixer or at least before the crystal filter would ensure that any energy that might overload is cut down to size before it can result in offensive and unwanted products. So you might aim toward the base of Q1 and Q2 (bitx40) if you wanted an AGC more like what is done in rigs like the TS-830s (and many others!). :-) Cheers!

[@Paul_VK3HN]

Harry Magooslum Good comments Harry. Of course you are correct, a proper AGC reduces the gain of the stages ahead of the detector, specifically the IF stages. My receiver is indeed a heterodyne type, but a simple one without a gain controlled RF stage, so the compromise is to reduce the audio stage gain automatically in the presence of a strong signal. It works for fixed gain superhets like the BiTX. Because it is a compromise circuit it doesn't have the dynamic range of a proper AGC circuit. But it's better than nothing!

[@Frisky0563]

do they make a opto coupler with the LDR photo cell built in it? that's pretty cool Paul

[@Paul_VK3HN]

My AGC uses discrete LDR and white LED. As per the VK3YE video.

[@andrewferg8737]

@@Paul_VK3HN I'm wondering if an IR LED setup might work as well, and would not need any shielding.

[@Paul_VK3HN]

@@andrewferg8737 Can't see why it wouldn't work. Try it and let us know!

[@andrewferg8737]

@@Paul_VK3HN Just wondering how to set it up in the split power push-pull amplifier I'm working on...Any suggestions regarding polarity or grounding of the agc transistors?

[@iceberg789]

@@Paul_VK3HN why not use an optocoupler anyways.