Help Me Solve FFT Sinc Function Problem

[Jmas]

Hello, I am a physics engineering student and i need a little help with fft function. I wrote the following code everything works, but when i get the fft. I was supose to obtain a sinc function and if you take a close look, it is a sinc, but in the wrong points, can anyone help me with this?

if you don't know/remember what a sinc function looks like:
https://www.google.pt/search?q=3d+s...%2Fwiki%2FFile%3AMatlab_surf_sinc.svg;561;420

x0=0;
y0=0;
dx=5;
dy=5;
px=0.05;
py=0.05;
dxx=1;
dyy=1;
n=dx/px+1;
m=dy/py+1;

xx=-dx/2:px:dx/2;
yy=-dy/2:py:dy/2;

[X,Y]=meshgrid(xx,yy);

Z=zeros(n,m);

%w=X.^2+Y.^2;

for i=1:n
for j=1:m
if X(i,j)>=x0-dxx/2 && X(i,j)<=x0+dxx/2 && Y(i,j)>=y0-dyy/2 && Y(i,j)<=y0+dyy/2
Z(i,j)=1;
end
end
end

A=fft(Z);
B=A.*conj(A);

figure(1)
surf(X,Y,Z)
colormap jet
colorbar

figure(2)
surf(X,Y,B)
colormap jet
colorbarAlso, how i make sure that Z, has always the size of the matrixs X and Y?

 

[vela]

It would help if you explained what your code is supposed to be doing.

 

[Jmas]

it is suppose to plot the fft of a square function which is the sinc function. if you take a look a the photo I've just uploaded, the second one should be a sinc function.

 

[vela]

I don't understand the second plot. You're doing a two-dimensional FFT, right? Why do you have a one-dimensional power spectrum?

 

[Jmas]

its 2d too...

 

[vela]

Really? I don't see depth in the second plot, unlike the first one.

 

[Jmas]

and now?

 

[vela]

I'm assuming this is MATLAB code. According to the documentation, the routine fft doesn't do a two-dimensional DFT. It only acts on the first non-singleton dimension.

EDIT: It looks like the function you want is fft2.

 

[Jmas]

humm ok, and how do i center it?

 

[vela]

Probably easiest just to rotate the rows and columns by 50 positions.