<p>Wow. TIL about Dual Mixer Time Difference phase comparison. The idea blew my mind.</p><p>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.</p><p>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.</p><p>!!</p>