It depends on the accuracy you need
For 1e-13 accuracy the two input frequencies (could be harmonics) have to be within 0.5 Hz of each other.
If 1e-10 is sufficient accuracy you can first calibrate the internal TCXO of the tinyPFA with a GPSDO or other reference and than measure a source at any frequency between 100kHz and 300 MHz against the calibrated internal TCXO. It should even be possible to measure at a harmonic of this range.
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NanoVNA Wiki:
/g/nanovna-users/wiki/homeNanoVNA Files:
/g/nanovna-users/filesErik, PD0EK
