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

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?

Answer 1

[Dr. Seis]

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));

[Matt]

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]

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.