# Measuring Phase Difference at DC bin aliased frequency using FFT

I have a code in which two complex signals (I/Q) are created having same frequency but with phase difference. FFT of both signals is taken and atan2 is used to compute phase after which the difference between both phases is calculated.

While changing the input frequency it was observed that for the case where the signal frequency is integer multiple of Fs such that its FFT peak lies on DC bin, the correct phase difference is still obtained. How can phase calculated from DC bins give the correct value. Can anyone explain the underlying logic?

• It's alias to DC bin but it is not really DC only! It contains phase information for frequencies between 0Hz and $f_s/(2N)$ . – Max Apr 2 at 11:05