MATLAB: Use FFT in matlab and get phase plot

fft phase frequency excel

i have some polymers characterized by transmission terahertz time domain spectroscopy (THz-TDS).
ive now the time domain data in Excel, with column A being (Delay ps), column B (Reference), column C (Amplitube nA) as follows.
i would like to have "FFT phase of column C" minus "FFT phase of column B", and then plot the phase (after deduction) as a function of frequency.
i am told this could be done by adding
 plot(freq,angle(fft(X(:,3)))-angle(fft(X(:,2))))
after
   [X,TXT,RAW] = xlsread('yourfile.xls');
   xdft = fft(X(:,2));   xdft = fft(X(:,2));
   % sampling interval -- assuming equal sampling
   DT = X(2,1)-X(1,1);
   % sampling frequency
   Fs = 1/DT;
   DF = Fs/size(X,1);
   freq = 0:DF:Fs/2;
   xdft = xdft(1:length(xdft)/2+1);   phi = angle(xdft);
anyway it does not work
the command window shows
??? Error using ==> plot Vectors must be the same lengths.
Error in ==> Untitled at 12 plot(freq,angle(fft(X(:,3)))-angle(fft(X(:,2))))

Best Answer

Here are the frequencies associated with the output from FFT:
Nt =              % Number of time samples
Fs =              % sample frequency (= 1/dt)
df = Fs/Nt;       % frequency increment
Nyq = Fs/2;       % Nyquist frequency
if mod(Nt,2) == 0 % Nt is even
   freq = ifftshift(-Nyq : df : Nyq-df);
else              % Nt is odd
   freq = ifftshift([sort(-1*(df:df:Nyq)) (0:df:Nyq)]);
end
Then you can just do a xlim on the plot to zoom in on a specific frequency range.
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