MATLAB: How to get the values of different frequencies from the fft of a signal with multiple frequencies

fftfrequency

I have a signal with noise having three frequencies and I have to extract these three frequency values from the fft of the signal.

Plot the fft (link) with the code between the first (top) two plot figures.

To determine the frequencies and amplitudes of the peaks, use the Signal Processing Toolbox findpeaks function. See the documentation for details.

If you have the Signal Processing Toolbox, it is easiest to use periodogram().

That handles all the scaling for you.

   Fs = 1000;
    t = 0:1/Fs:1-1/Fs;
    x = cos(2*pi*100*t)+randn(size(t));
    [Pxx,F] = periodogram(x,[],length(x),Fs);
    plot(F,10*log10(Pxx))

If you do not have the Signal Processing Toolbox, the PSD is proportional to the absolute value squared of the DFT (calculated by fft())

For more information on that scaling, please see:

One option would be instead to use 'MinPeakProminence' (link). See the discussion on Prominence (link) for a description of it.

If you want to increase the frequency resolution of your fft, specify a significantly longer Transform length (link).