In the second field you put the sampling frequency. In the third field you specify the sampling period. In the fourth field you select the fft size. Spectral resolution gets better when fft size increases. In the fifth field you select the algorithm.
Current choice is rectangle.
I will add Bartlett, Hamming, Hanning and Blackman windows later. In the seventh field you can see the time elapsed in milliseconds for the FFT or DFT calculations only.