matlab beginner: use FFT in matlab and get phase plot

Sorry for asking this question again, but i have to (so urgent)

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 Amplitude nA), column C (Sample 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.

the following are the codes some nice people suggested.

   [X,TXT,RAW] = xlsread('C:\Users\in Me u trust\Desktop\test\PP_1.CSV');
   xdft = fft(X(:,2));  

   Nt = size(X,1);             % Number of time samples
   DT = X(2,1)-X(1,1);
   Fs = 1/DT             % 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

   xdft = xdft(1:length(xdft)/2+1);  
   phi = angle(xdft);

   plot(freq,angle(fft(X(:,3)))-angle(fft(X(:,2))))

Anyway, these codes are collected from different people, and i as a beginner of Matlab just simply put them together so that they might not be coherent or what?? no idea and the question raised as follows.

so can anyone so nice to help perfect the code? thz in advance

Answer 1

[Ilham Hardy]

I just skimmed over your code. I think the problem lies in

plot(freq,angle(fft(X(:,3)))-angle(fft(X(:,2))))

it should looks

substr = angle(fft(x(:,3)))-angle(fft(x(:,2)));
plot(freq,substract);

will you give it a try?

HTH,

IH

Answer 2

[jingming cao]

good