Help with Fourier transform fft

Question

aurc89 2014-9-18

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https://ww2.mathworks.cn/matlabcentral/answers/155349-help-with-fourier-transform-fft

编辑： Rick Rosson 2014-9-20

I need to use the matlab function 'fft' to perform a Fourier transform, but the problem is that it always gives me a double-sided signal, while I would like to have a one-side signal. I sometimes use this function :

function four=FourierDir(t,s,nu)
% number of points in the time domain
N=length(t);
Nf=length(nu);
% sampling step in the time domain
Dt=diff(t);
Dt(N)=Dt(N-1);
four=zeros(1,Nf);
for ii=1:Nf
      four(ii)=sum(Dt.*s.*exp(-1i*2*pi.*t.*nu(ii)));
end;

Can you suggest me an equivalent one made with fft , given as input t, s and nu ? Thanks so much.

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Mikhail 2014-9-18

Please explain, what do you mean by double-sided signal and one-side signal

Rick Rosson 2014-9-19

编辑：Rick Rosson 2014-9-20

The Discrete Fourier Transform of a real-valued signal is conjugate symmetric. The double-sided spectrum shows the spectrum as a function of both positive and negative frequencies on the range [-Fs/2,+Fs/2), whereas the single-sided spectrum shows the spectrum for positive frequencies only, on [0,+Fs/2).

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Answer 1

Rick Rosson 2014-9-19

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/155349-help-with-fourier-transform-fft#answer_152198

编辑：Rick Rosson 2014-9-19

在 MATLAB Online 中打开

function [ X,f,t ] = mydft(x,Fs)
     % Assumes x is an N x 1 column vector
     % and Fs is the sampling rate.
     N = size(x,1);
     dt = 1/Fs;
     t = dt*(0:N-1)';
     dF = Fs/N;
     f = dF*(0:N/2-1)';
     X = fft(x)/N;
     X = X(1:N/2);
     X(2:end) = 2*X(2:end);
     figure;
     plot(f,abs(X));
end