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On Fri, Sep 9, 2022 at 01:26 PM, Jeff Green wrote:

Is there any easy way to convert the difference frequency to a voltage with positive local TXCO frequency producing a positive voltage and lower local TXCO frequency producing a negative voltage?

Hi, Max described a good solution. But FWIW, another way to do it more discretely is like this:
- GIVEN that the signal is a relatively low-frequency square wave.
- Use a diode and cap in series to create narrow pulses on the rising edge of the square (but not on the falling edge). The idea is that the pulses will all be the same width, whereas the pulses from the original square wave will vary in width. The series cap is acting like a high-pass filter, creating narrow pulses from wide pulses. The diode is so that you DON'T get negative pulses on the falling edges.
- Use the equal-width pulses to charge a larger cap. The charge on this cap varies with the frequency of the pulses, and it is acting like a low-pass filter or integrator. So the voltage across the cap is related to the frequency of the pulses.
- Connect a mechanical meter across the cap. Preferably a sensitive meter, say 50uA.
- Note that you don't necessarily need the 2nd cap at all - the mechanical inertia of the meter also acts as a LP filter! So in theory, the entire circuit could be nothing but a diode and cap in series with a meter.

It's not a precise method, but very simple. The scheme described is how old-school Geiger counters drove the meter - the faster the pulses from the Geiger tube, the higher the meter indication.
Note that getting a "zero-center" meter indication is a problem with a simple setup like mine, because the difference frequency does NOT tell you if it's high or low.

Pete