Computing and Ploting Fourier transform

Question

omarahmed.m 2016-2-21

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https://ww2.mathworks.cn/matlabcentral/answers/269206-computing-and-ploting-fourier-transform

评论： omarahmed.m 2016-2-21

How can i use the fft command to transform a rectangular pulse to sinc function and plot the sinc function, i'm using a very traditional way to compute the fourier transform and plot it, but this way is too slow, when i use the fft command and try to plot, the ploting of the magnitude is only the pins or a spike, i need the plot to be a sinc function as the picture that i attached here, also i'm using the same method to compute the inverse fourier transform so i also need to use the ifft command, can anyone help?

if true
p = heaviside(t+1)-heaviside(t-1);
  k = 0;
F=zeros(1,8001);
for f= -4:0.001:4
  k=k+1;
F(k) = trapz(t,p.*exp(-1i*2*pi*f*t));
end
f = -4:0.001:4;
figure(1);
plot(f,F);
grid
end

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Answer 1

Rick Rosson 2016-2-21

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https://ww2.mathworks.cn/matlabcentral/answers/269206-computing-and-ploting-fourier-transform#answer_210611

编辑：Rick Rosson 2016-2-21

在 MATLAB Online 中打开

Fs = 8;
dt = 1/Fs;
N = 8000;
t = dt*(-N/2:N/2-1)';
dF = Fs/N;
f = -Fs/2:dF:Fs/2-dF;
p = heaviside(t+1)-heaviside(t-1);
P = dt*fftshift(fft(ifftshift(p)));
figure;
plot(f,real(P));
grid on;

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omarahmed.m 2016-2-21

Thank you for your response, Could you just specify this command "P = dt*fftshift(fft(ifftshift(p)));", why it consist of fftshift of fft of ifftshift, and why multiplying by "dt". thank you very much.

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