MATLAB: How to perform a zoom FFT on certain portion of a signal

fft

I have a signal with a sample rate of 200HZ, see attached. I am interested in a certain portion of the signal, so I zoomed in that section from the original signal (see figure below). I want to compute FFT (maybe zoom FFT) on that portion to see the frequencies within that region. Please I need help, thank you.

Best Answer

First, see if the pspectrum funciton using the 'spectrogram' option will do what you want. It will segment the Fourier transform as a function of time as well as frequency. You can then easily see how the frequency content changes over time.

If that does not do what you want, please be a bit more specific.

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MATLAB: How to convert the units of the y-axis into miliseconds^2/Hz in power spectral density analysis of an ECG signal

The y-axis on a PSD estimate will not be in units of the time squared, it will be in amplitude squared per Hz, so for an ECG signal I assume that will be in millivolts, not milliseconds. The recorded values of the waveform are not in time.

Since you are using dspdata.psd, why not just use spectrum.periodogram?

    psdest = psd(spectrum.periodogram,ecg,'NFFT',length(ecg),'Fs',fs);
    plot(psdest)

If you want to plot without using dB (that is preferred by the way)

   plot(psdest.Frequencies,psdest.Data)

MATLAB: Amplitude spectrum

There are a couple things you can do. Let x be your speech signal and fs your sampling frequency. The following two are power estimates.

    plot(psd(spectrum.periodogram,x,'Fs',fs));
If you do not want it in dB:
    psdest = psd(spectrum.periodogram,x,'Fs',fs);
    plot(psdest.Frequencies,psdest.Data);
    xlabel('Hz'); grid on;

Finally, just using fft()

      xdft = fft(x);
      % assume x is even length
      xdft = xdft(1:length(x)/2+1);
      freq = 0:fs/length(x):fs/2;
      plot(freq,abs(xdft));
      xlabel('Hz');

Best Answer

Related Solutions

MATLAB: How to convert the units of the y-axis into miliseconds^2/Hz in power spectral density analysis of an ECG signal

MATLAB: Amplitude spectrum

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