Fourier transform and FFT for an arbitrary plot using MATLAB

Question

I have a simple problem but since I have not used MATLAB Fourier Transform tools I need some help. I have a plot obtained from n excel file. The plot is in time domain. Time range for the plot is 0 to 50 ps. And I have data for the y component of the plot every 0.5 fs. Basically the plot contains 100000 data printed every 0.5fs. Now I want to get the Fourier transform of this plot. What should i do? The following is a simple format for my excel file that includes the data I needed to have the time-domain plot.

0       116.0080214
0.0005  116.051128
0.001   116.0939229
0.0015  116.1362197
0.002   116.1776665
0.0025  116.2178118
0.003   116.256182
.
.
.
.
50.0    123.000

The first column is time in ps. Thank you so much in advance for you helps. Best, HRJ

you can use the fft() function. Also if you have it there is a Digital Signals Processing toolbox that comes with a nice psd function — willpower2727

Delyle Delyle · Accepted Answer · 2015-06-23T18:07:44

I have adapted this page for the solution.

    Fs = 100000/50;               % Sampling frequency (in 1/ps)
    T = 1/Fs;                     % Sample time (in ps)
    L = 100000;                   % Length of signal
    t = (0:L-1)*T;                % Time vector; your first column should replace this

    % Sum of a 50 1/ps sinusoid and a 120 1/ps sinusoid
    % Your second column would replace y 
    x = 0.7*sin(2*pi*50*t) + sin(2*pi*120*t); 
    y = x + 2*randn(size(t));     % Sinusoids plus noise

    NFFT = 2^nextpow2(L);         % Next power of 2 from length of y
    Y = fft(y,NFFT)/L;
    f = Fs/2*linspace(0,1,NFFT/2+1);

    close all
    subplot(2,1,1)
    % Plot your original signal
    plot(Fs*t(1:100),y(1:100))
    title('Signal Corrupted with Noise')
    xlabel('time (fs)')

    % Plot single-sided amplitude spectrum.
    subplot(2,1,2)
    plot(f,2*abs(Y(1:NFFT/2+1))) 
    title('Single-Sided Amplitude Spectrum of y(t)')
    xlabel('Frequency (1/ps)')
    ylabel('|Y(f)|')

Output of the FFT

Fourier transform and FFT for an arbitrary plot using MATLAB

1 Answers