Finding a specific amplitude-frequency in the time domain

[Behrouz]

Hello,

I have a signal and got the FFT result of that. I have shown them both below along with the MATLAB code.

May I ask if there is any method to find the time zone(s) in the signal that a specific frequency has(have) happened?

The reason I'm asking this is that I want to specify the time that I had a (specific) low frequency with a (specific) high amplitude.

%% apply fft on the csv and plot the result

nfft = length(M);

nfft2 = 2^nextpow2(nfft);

ff = fft(M, nfft2);

fff = ff(1:nfft2/2);

stem(abs(fff));

Any advice would be greatly appreciatd.

Regards,
Behrouz

 

[DrClaude]

https://en.wikipedia.org/wiki/Short-time_Fourier_transform