This is a simple circuit to mix the output of two xtal oscillators to produce an output that is the sum or difference. I originally did this to get a desired freq using some xtals in the junk box.
The active device is a 74HC86 CMOS quad exclusive or gate. Exclusive or gates are interesting as you can configure than to be a buffer , an inverter, or use them as a mixer. In this case two gates ( A1 and A2 )are set up as inverters and are used for a standard single cmos inverter oscillator. The third gate (A3) acts as a mixer and the forth gate (A4) is an inverting buffer.
Between the mixer gate and the buffer is a parrallel tuned cct to pick off
the desired sum or difference of the two crystal oscillators. Best results are
to be had where the desired output frequency is not close too either F1 or F2 or
harmonics of these or other sum or difference resultants.