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From: Monica Rainey on 26 Mar 2010 22:58
Hello,
I'm trying to take the fourier transform of a shifted rect function. It appears that Matlab is connecting my samples in straight lines, and I want them smoothed out to be a curve. I know you can use the sinc function to do this, I'm just not sure how.
Here's what I have so far:

x = linspace (-5,5,5001); x =x(1:end-1);
dx = x(2)-x(1);
xi = linspace(0,1/dx,length(x)+1); xi = xi(1:end-1);
n = 0:(length(xi)-1); %indices in the frequency domain
k0 = x(1)/dx; %finds the offset index in the space domain
C = exp(-1i*2*pi*k0*n/length(x)); %correction factor

f = rect(x-3);

F = fft(f)*dx; %corrected for phase using correction factor

%Plot transform:

fftplot(xi,F.*sinc(k*f), 'b'); %plot real part of function
hold on
fftplot(xi,imag(F).*sinc(k*f),'r'); %plot imaginary part of function
hold off


%Format plot
set(gca,'xlim',[-5 5])
set(gca,'ylim',[-1 1])
title('Fourier transform of gaus(x)*sin(5*pi*x)')
legend('Real','Imaginary');
xlabel('\xi[cycles/unit]')
ylabel('F(\xi)')

Thanks,
Monica
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