I am doing phase measurments by transmitting and receiving tones (100 kHz and 16 kHz simultaneously). I am transmitting the tones and receiving them, applying Blackman Harris window and doing FFT for phase measurements. I feel the Blackman Harris window is not optimal for a signal that has only 2 tones but don't know which windowing function is better for phase measurement for such a signal. Any Ideas?
Using an FFT to measure phase for just two tones results in a lot more processing that doing the following alternate approach that can be either streamed or processed in blocks. No windowing is needed:
Apply the received signal as the input to two multipliers. Apply a normalized local copy of one tone as the second input to one of the multipliers and a normalized local copy of the other tone to the other. The low pass filter of each output will be an estimate of the phase given by$A\cos(\theta)$.
For full 360 degree phase resolution, use a complex local tone and two complex multipliers with two outputs (I, Q) with the phase determined using atan2(Q, I). (This is essentially the operation in two bins of the DFT).