How do you centre your plot at '0' on the X-axis after performing a Fourier transform?

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Matt
Matt on 16 Jul 2012
I have a generated signal which I perform an fft on as well as zero-padding it with 5000 zeros. The problem I have is that the resulting plot is centered at 2500 (i.e. half of 5000).
Is it possible for me to shift my plot 2500 spaces to left on my X-axis so that the plot is centered on 0?

Answers (1)

Dr. Seis
Dr. Seis on 16 Jul 2012
Edited: Dr. Seis on 16 Jul 2012
fftshift might be what you are looking for. Does this example help?
dt = 0.1; % Fs = 1/dt;
N = 16;
t = (0:N-1)*dt;
g = randn(size(t));
Nyq = 1/2/dt;
df = 1/N/dt;
f = -Nyq : df : Nyq-df;
G = fftshift(fft(g));
figure; plot(f,G);
[EDIT]
f1 = -3; % minimum frequency
f2 = 3; % maximum frequency
idx = f1 < f & f < f2;
figure; plot(f(idx), G(idx));
  2 Comments
Dr. Seis
Dr. Seis on 16 Jul 2012
Why not specify a range using logicals... continuing my example above:
f1 = -3; % minimum frequency
f2 = 3; % maximum frequency
idx = f1 < f & f < f2;
figure; plot(f(idx), G(idx));
Note: Your "N" will be based on the number you pad up to (i.e., 200000) for determining what "f" is for your data.

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