Thanks so much for the answer.

**carlgrace wrote on Oct 2**^{nd}, 2017, 8:50am:The noise floor of the FFT itself declines when you use a longer FFT. If the SNR is improving as you increase the length of the FFT, that means that you are being limited by the FFT and not the ADC. When you get to the point the ADC is limiting SNR using a longer record won't improve SNR.

I understand that noise floor declines when FFT size increases because of the formula below (-10log(n/2)):

But what I don’t understand is that the noise power must be distributed between noise bins (either 62 or 510) and the total power of the noise bins should be constant hence the snr must remain constant. I don’t find any theoretical reasons for total noise power variations and it is confusing me. What do you exactly mean by SNR being limited to FFT?

**carlgrace wrote on Oct 2**^{nd}, 2017, 8:50am:You should consider using a prime number of cycles rather than just an integer. An integer is OK, but you may get some samples landing on top of each other which will make it appear like you have less data.

Yes I totally understand what you mean and I have used prime/integer number of cycles, “integer number of cycle” is a term used in Stanford lecture notes meaning the same thing you just explained:

**carlgrace wrote on Oct 2**^{nd}, 2017, 8:50am:Also, if you are using coherent sampling correctly (integer/prime number of cycles) using a Hann window should be needed (all your input power should fall into a single FFT bin).

Fine, but according to Stanford lecture notes, windowing will distribute the sigbin power between some bins: