Low IF ?

I was taking advantage of the ARRL Periodical Archive / Search, as mentioned before:

https://wb9kzy.blogspot.com/2024/08/woo-hoo-good-news-from-arrl.html

to look at old QEX articles.  This one by W6IOJ caught my eye:

It is not so much that I am interested in PSK as a mode but I have a hardcopy of that QEX issue and remember that W6IOJ upconverted an audio signal to a low IF (think of those frequencies just above audible) to accomplish the PSK demodulation.  Now I know that working at frequencies roughly 10 times normal audio can have a lot of advantages.  If nothing else it can provide much faster response to changes in a signal.  This might be quite useful for Automatic Gain Control where a normal audio AGC may take multiple periods of audio before it can tame a sudden karge signal - hard on the ears!  Another application might be for tone decoding, again a PLL may need multiple audio cycles before a tone detection (lock) occurs.  The low IF idea could be a springboard to useful projects.

Here is the upconverter part of the W6IOJ circuit:

It has a diode ring mixer but instead of toroidal trifilar wound transformers there are two audio output transformers.  Also an isolation transformer on the input.  The local oscillator is an op-amp based unit.  The filter is four op-amp active filter sections cascaded.  Easy to build circuits is another benefit of working at a low IF rather than at RF.

Also note that in the early days of SSB, the low IF technique was used to convert AM gear to SSB.  The sideband filters were usually LC but this of course predated monolithic IC opamps.

Anyway, I will have to explore this further in the "lab"  :)

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm