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Topic: frequency to time domain using IFFT and FFT for a real-valued function
Replies: 2   Last Post: Nov 24, 2012 6:41 PM

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 nerdynerd Posts: 2 Registered: 11/24/12
frequency to time domain using IFFT and FFT for a real-valued function
Posted: Nov 24, 2012 4:03 PM

Hi, I have a real-valued function in frequency domain and I'm trying to IFFT to get the signal in time domain and then FFT back to frequency domain. However, I'm unable to get back the original signal. The first step IFFT gives me the correct function but FFT'ing it does not give the original signal. Any ideas what I'm doing wrong?
here's my code:

fsam=-10:0.1:10; % frequency vector
test_func=exp(-fsam.^2); % test-function

fsamp=fsam(1)-fsam(2); % \Delta_frequency
nfft=2^12;
r0=length(test_func);
dt=1/fsamp;

x = ifft(ifftshift(test_func),nfft); % DFT of signal
g = (-nfft/2:nfft/2-1)*(1/(fsamp*nfft)); % Time range
q = ifftshift(x);

dg=g(1)-g(2);
y = fft(fftshift((q)),nfft);
w = (-nfft/2:nfft/2-1)*(2*dg); %Frequency range; also tried "fsamp" instead of "2*dg"
s = fftshift(y);

figure
subplot(2,1,1)
plot(fsam,testfunc) %original signal
hold on
plot(w,real(s),'r') %signal after IFFT and FFT
hold off

subplot(2,1,2)
plot(g,real(q)) %real part of the signal after IFFT
hold on
plot(g,imag(q),'r') %imaginary part of the signal after IFFT
hold off

Date Subject Author
11/24/12 nerdynerd
11/24/12 nerdynerd
11/24/12 Greg Heath