How to determine frequency components in a signal

How to determine the different dominant frequency components in a signal (for the whole signal duration and for specific periods)? Signal is of dim (400 x 1)

Answer 1

[Walter Roberson]

See spectrogram()

[Elysi Cochin]

sir how to do for specific time periods?

[Walter Roberson]

Extract that portion of the signal and use fft() and work out from there which components were "dominant".

I recommend reading the fft plot example to see how to calculate the frequency associated with each fft bin.

[Elysi Cochin]

any example you can show me... i havent used it....

spectrogram(signal) and fft(signal), gives me a complex value? what does it mean? how can i relate it to time period

[Walter Roberson]

mask = time >= beginning_of_period & time <= end_of_period;
selected_samples = samples(mask);
fft_of_selected = fft(selected_samples);

"spectrogram(signal), gives me a complex value?"

Yes. spectrogram does a series of discrete fourier transforms (fft). fft() of a real-valued signal is going to be complex except in the case that the signal is made up of pure sine tones that all started at the first sample. The complex portion of the entries express phase information.