Hi Matt,
I’ll try to describe my user case for measuring the phase shift.
I am testing a D-class amplifier.
I took 1kHz as the reference frequency for the phase shift of 0°. QA Analyzer shows delay 120μs, and I confirmed that value using method you described in topic https://forum.quantasylum.com/t/delay-measurements/35.
After that easy part, I measured the delay for the whole frequency range, stepping 1/3 octave. The larger frequency, the harder is to determine the right moment to mark, but it helps that I don’t expect the mark to jump more than half period between two steps (“reasonably flat passband”).
It would be easier, BTW, if the QA Analyzer would mark time where curves passes x-axis.
The delays I measured where from -3.97μs@31.7Hz to 129μs@17758Hz. To be sure to avoid transients, I took time points near to the end of measuring period, about 0.68ms, carefully counting number of periods before the output and input amplitudes start to fall at the end of measuring period (hence zoom bug I reported two days ago).
The Excel table did the rest: calculates phases from delays, I got from -46.6°@31.7Hz to 60.7°@17758Hz.
Now, I believe that the whole process can be automatized in QA Analyser, no need for the second channel as the reference, at least for the described case.
For the other cases, involving huge jitter and delays, and also when using external generator, where few ns differences in sampling period between QA401 and DAC can accumulate to several μs delay, the second channel would be the solution.