As compared to what I understand you want I think the answer you got from SCMenasian ?is overkill raised to the Nth power.? The phase/frequency detector in the 4046 CMOS phase lock loop will do exactly what you want.? It produces an output voltage that goes from 0 to Vcc but you can return the other side of the meter to a voltage divider producing ? Vcc.? You connect the two inputs to the two sources not after the mixer.
A gadget that I built a couple of years ago.? The purpose is to compare an unknown freq to the output of a GPSDO, so that the unknown can be tweaked to match. It's only useful when the unknown's freq is rather close to a known value. The circuit does an AND of the GPSDO output with the unknown, thus "mixing" the two signals, then a small MCU measures the difference freq.
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Please don't laugh.
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?
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I could use this to drive a zero center pointer meter to display error.
