FFT of a selected range of points

I have a waveform as the one attached and I need to perform FFT over a small section of this waveform (like the one highlighted by a rectangle). How can I do that? I don't have any prior experience of working in frequency domain. Any help would be greatly appreciated.

Answer 1

[Rushikesh Tade]

if you know the time where it started you can simply do it by using:

fft(signal(initial_time:final_time))

[Aqua22]

It worked. Thanks Rushikesh.

Answer 2

[Salaheddin Hosseinzadeh]

Dear Aqua,

Please be aware. You probably going to need a window! If you only take the fft of a section without properly separate it from the rest of the data you will end of with some unwanted artifacts in the frequency domain.

You should either use spectrogram or apply an appropriate smoothing window to your time domain signal. Basically spectrogram does the same thing. Otherwise the result may be inaccurate.