# Best window before FFT ? (for a signal consisting of 2 tones used for phase measurement)

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?

• Are you restricted to using the FFT for determining what would be best for measuring rhe phase of the two tones? Apr 20, 2021 at 14:50

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$$A\cos(\theta)$$.
• @Kurtul Also note due to small angle approximation ($\sin(\theta) \approx \theta$) the SNR required to have less than 0.01 degree rms accuracy is (convert degrees to radians and take -20 log of that) : $-20 \log10(0.01 (2\pi/360))$ = 75 dB. From this you can specify your sampling rate, quantization, filtering requirements etc. Apr 21, 2021 at 2:22