how to use FFT in matlab using imported data in time domain excel file

i have some polymers characterized by transmission terahertz time domain spectroscopy (THz-TDS).

[Gagan Jain]

Can you please tell where did you extract that data from? I’m doing a similar project

Answer 1

[Wayne King]

Import the data into MATLAB using xlsread()

Your data will be a Nx2 matrix in MATLAB with the first column the time data and the second column the data you want to Fourier transform.

So for example:

[X,TXT,RAW] = xlsread('yourfile.xls');
xdft = fft(X(:,2));

Answer 2

[Wayne King]

There is a slight variation depending on whether you have an even or odd number of samples in your data.

Even length:

   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);
   plot(freq,abs(xdft))

Odd length

     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:round(length(x)/2));
     plot(freq,abs(xdft))

[Ryzx7]

       xdft = xdft(1:round(length(x)/2));

For that line the x needs to be an X right?

[Walter Roberson]

Yes.

It looks to me as if the versions could be merged by using

     xdft = xdft(1:ceil(length(X)/2));

[Mathias]

This doesn't seem to work for me. But why not just use the length of freq?

    xdft = xdft(1:length(freq));

[Sahaphol Hamanee]

Hi Wayne King, thank you for your guidance. May I ask what DF = Fs/size(X,1); is for? What is the meaning of it? It is the only thing I didn´t understand.

Best Regards

Answer 3

[Wayne King]

To get the phase, use angle()

   phi = angle(xdft);

To export the frequencies and magnitudes back to Excel, place them in a matrix.

Xdftmatrix = [freq' abs(xdft)];

then use xlswrite

[Wayne King]

phi = angle(xdft);

plot(freq,phi)

You want to plot the phase as a function of frequency