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Wow. TIL about Dual Mixer Time Difference phase comparison. The idea blew my mind.

To measure phase between 10 MHz signals, you mix them with a slightly lower frequency. This produces a beat note, which can be low pass filtered. The two downconverted signals will have the same PHASE difference as the input signals.

However, this phase difference on the beat frequency is a much longer DELAY with is very easy to measure. You can get sub-picosecond measurements of the 10 MHz phase difference.

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