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?
The "DC bin" does not represent DC exclusively, but has also some low frequency energy. How much depends on the fft size and sampling rate. Simple example: if your fft size is 1000 and your sampling rate is 1000Hz, then the dc bin represents frequencies from 0 to 0.5Hz, as the distance between fft bins is 1Hz. So, a phase can be calculated for the DC bin.