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

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Matt
Matt 2012-7-16
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?

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回答（1 个）

Dr. Seis
Dr. Seis 2012-7-16
编辑：Dr. Seis 2012-7-16
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 个评论
Matt
Matt 2012-7-16
I have used abs(fftshift(fft(x))) but that just gives me my plot at the middle of all my values including the zero's. In your example you do not zero pad. I need to use many zero's in my case...
Lamda1 = .003;
N = pi./Lamda1;
Step1 = .03;
A4 = [-N/2:Step1:(N/2)-Step1];
R4 = zeros(size(A4));
R4 = (0.5 + (0.5)*((1 - (A4.^2)).^2));
R4 = 10*log10(R4);
A4 = asin(A4);
figure (10); plot(A4,R4)
FD4 = fft(R4, 200000) / N;
%figure (11); plot(abs(FD4));
figure (12); plot(abs(fftshift(FD4)))
Dr. Seis
Dr. Seis 2012-7-16
编辑：Dr. Seis 2012-7-16
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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