Finding the main frequency after fft

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Pawel Burzynski
Pawel Burzynski 2011-9-17
Hi,
I've got these blocks in simulink:
Sine Wave -> Buffer -> Matrix sum -> Matlab function.
I have to find the frequency of the input, so I wrote some code in Matlab function (by the http://www.mathworks.com/help/techdoc/ref/fft.html) :
{
function [freq, amp] = fcn(u)
%#codegen
Fs=2000;
T=1/Fs;
L=length(u);
t = (0:L-1)*T;
NFFT = 2^nextpow2(L); % Next power of 2 from length of y
Y = fft(u,NFFT);
f = Fs/2*linspace(0,1,NFFT/2+1);
amp = abs(max(Y));
}
It's working properly, I'm getting the maximum value, but I can't get from code frequency of maxium value :/ how Can I get it? Thank You for response.
Best regards, Pawel

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采纳的回答

Wayne King
Wayne King 2011-9-17
Hi, You have to find the corresponding index in a frequency vector. For example:
Fs = 1e3;
t = 0:.001:1-0.001;
x = cos(2*pi*100*t)+randn(size(t));
xdft = fft(x);
[~,index] = max(abs(xdft(1:length(x)/2+1)));
freq = 0:(Fs/length(x)):Fs/2;
% freq(index)
fprintf('Maximum occurs at %2.3f Hz\n',freq(index))
Wayne

更多回答（1 个）

Pawel Burzynski
Pawel Burzynski 2011-9-17
It's excatly what I was looking for :) I had problems with those way of notation. I'm getting some mismatch, but I can handle this.
Thank You Wayne very much! :)
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Pawel Burzynski
Pawel Burzynski 2011-9-23
I've got the same problem, mainly i can't get the real frequency of the input event if it is pure sinusoidal source... There is always +/- 15% error. Can someone can help me?
Best regards
Wayne King
Wayne King 2011-9-23
Can you please provide code? It is hard to say the source of the error without a coded example.

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