fft_plot.m

%
% function file to perform a Fast Fourier Transform
% of a data vector and plot both the time and frequency domain trace
%
function fftplot = fft_plot(y,dt)
%
% input:   y - data vector
%         dt - spacing of signal in time domain
%
% output: plot of signal in time and frequency domain

% Fourier transform calculation

ffty = fft(y)/length(y)*2;  % perform FFT and scale
ffty(1) = ffty(1)/2;        % correct DC value

% time domain plot

subplot(2,1,1);

t = 0:dt:(length(y)-1)*dt;  % create time vector for plotting
plot(t,y);
xlabel('time (sec)');
ylabel('amplitude');
title('Time and Frequency Domain Representation of a Signal');

% for the FFT plot, only plot length(ffty)/2 + 1 points
% and scale x-axis as frequency...

subplot(2,1,2);

numfft = length(ffty)/2 + 1;
f = linspace(0,1/dt/2,numfft);   %                o
stem(f,abs(ffty(1:numfft)));     % 'stem' plot    | 
xlabel('frequency (Hz)');
ylabel('amplitude');