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I have I and Q data in time but I need to plot them in frequency

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I have I & Q data in the time domain (all numbers are between +/- 1) And I can plot the time domain representation with.
figure ; plot(1:length(in_i),in_i) ; hold on ; plot(1:length(in_q), in_q); hold off;
which gives me the following :
However I need to look at the frequency domain of these values to see if it is displaying the correct frequencies that I belvie it should be. Is there a way to do this?
  1 Comment
John BG
John BG on 27 Jan 2017
Edited: John BG on 27 Jan 2017
where is the time reference?
is the time delay between adjacent samples known?
please spcify
John BG

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Answers (1)

John Chilleri
John Chilleri on 26 Jan 2017
Your solution lies within your tags. The Fourier transform is a way to change a signal from its original domain to a representation in the frequency domain [ WIKI ].
MATLAB has an implementation of the fast Fourier transform named fft.
To change your data to the frequency domain, you can:
Q2 = fft(Q);
I2 = fft(I);
Hope this helps!
Steven Seppala
Steven Seppala on 26 Jan 2017
This is the file I am working with. It's a standard CSV with the in_q and in_i in the C & D col respectively.
Thank you !
Walter Roberson
Walter Roberson on 27 Jan 2017
data = dlmread('data_capture.txt', '\t', 2);
in_q = data(:,3);
in_i = data(:,4);
fin_q = fft(in_q);
fin_i = fft(in_i);
nnz(isnan(fin_q)) %gives 0 -- none of the values are nan
nnz(isnan(fin_i)) %gives 0 -- none of the values are nan
subplot(1,2,1); plot(abs(fin_q)); title('abs fft of in_q');
subplot(1,2,2); plot(abs(fin_i)); title('abs fft of in_i');

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