Page 66 - Basics of MATLAB and Beyond

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We combine fftshift and fftfreq to plot the two-sided FFT:

plot(fftfreq(.5,8),fftshift(abs(Y)))
axis([-.5 .5 0 7])
zeroaxes

Let us do a slightly more realistic example. We simulate some data
recorded at a sampling frequency of 1 kHz, corresponding to a time step
dt = 1/1000 of a second. The Nyquist frequency is, therefore, 500 Hz.
Suppose there is a 100 Hz sinusoid contaminated by noise. We simulate
the data, calculate the FFT, and plot the results as follows:
dt = 1/1000;
t = dt:dt:200*dt;
sine = sin(2*pi*100*t);
y = sine + randn(size(t));
Y = fft(y);
f = fftfreq(500,length(Y));

clf
subplot(211)
stairs(t,y)
hold on
stairs(t,sine-4)
box
xlabel(’Time (seconds)’)

subplot(212)
stairs(f,fftshift(abs(Y)))
box
xlabel(’Frequency (Hz)’)

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