How to choose FFT size when we measure phase difference? For constant differences in phase FFT size is bigger, for changing over time phase differences FFT seems must be smaller size. But it is unclear how it will affect phase difference measurement. Maybe it does not depend on FFT size at all, and in case of big FFT's we only get bigger "time lag" for changing phases, and no improvement on constant phase signals? For example we have one frequency signal, and two channel phase difference goes from 90deg to 0deg for time period of 100ms. How it affect error in measurements if we take FFT's each 1ms, 2ms or say 10ms bins. FFT's can be taken from sliding window each 1 ms.