MATLAB: How to get frequency of a discrete signal by using fft in matlab

coderfft

i want to find the frequency of a discrete signal using matlab and also want to apply a bandpass filter to that spectrum how can i do that pls help me out guys!!!

Best Answer

You identify the peak in the Fourier transform, find the index in DFT vector corresponding to that peak and relate that to frequency.

For example:

   % sampling frequency is 1 kHz
   Fs = 1000;
   % I'll create a signal consisting of a 100 Hz sine wave in noise
   t = 0:0.001:1-0.001;
   x = cos(2*pi*100*t)+randn(size(t));
   xdft = fft(x);
  % I only need to search 1/2 of xdft for the max because x is real-valued
   xdft(1:length(x)/2+1);
   [Y,I] = max(abs(xdft));
   freq = 0:Fs/length(x):Fs/2;
   fprintf('Maximum occurs at %3.2f Hz\n.',freq(I))

Note that the frequency bins in the DFT are spaced at Fs/N where Fs is the sampling frequency and N is the length of the signal. The first DFT "bin" corresponds to zero frequency.

Your next step is to construct a bandpass filter around 1 kHz, you can do that a number of ways in MATLAB. See fdesign.bandpass for example.

Related Solutions

MATLAB: Errors appear when doing FFT on data imported from text file

Your sampling interval is 1 month, so your DFT bins will be in cycles/month. So you can just set Fs = 1, but then keep in mind that the frequencies will be in cycles per month, so your highest frequency of 1/2 is 1 cycle/2 months.

Also, the code I had given you earlier was assuming you have an even number of samples. If your signal length is odd, you have to make a small adjustment.

   Fs = 1;
   xdft = fft(x);
   xdft = xdft(1:length(x)/2+1);
   freq = 0:Fs/length(x):Fs/2;
   [maxval,idx] = max(abs(xdft));
   freq(idx)

MATLAB: Get frequencies out of data with an FFT

t = 0:.001:1-0.001;
Fs = 1e3;
x = 2+sin(2*pi*60*t);
x = detrend(x,0);
xdft = fft(x);
freq = 0:Fs/length(x):Fs/2;
xdft = xdft(1:length(x)/2+1);
plot(freq,abs(xdft));
[~,I] = max(abs(xdft));
fprintf('Maximum occurs at %d Hz.\n',freq(I));

If you have the Signal Processing Toolbox, you can use spectrum.periodogram

psdest = psd(spectrum.periodogram,x,'Fs',Fs,'NFFT',length(x));
[~,I] = max(psdest.Data);
fprintf('Maximum occurs at %d Hz.\n',psdest.Frequencies(I));

If there are multiple peaks, you can use findpeaks() in SPT, or just look at the plot

    plot(psdest)

Best Answer

Related Solutions

MATLAB: Errors appear when doing FFT on data imported from text file

MATLAB: Get frequencies out of data with an FFT

Related Question