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

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?

[John BG]

where is the time reference?

is the time delay between adjacent samples known?

please spcify

John BG

[e-mail]

Answer 1

[John Chilleri]

Hello,

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]

unfortunately not, when I attempt to do this on my values I get results along the lines of

NaN + 0.0000i
NaN - 0.0067i
NaN - 0.0056i
NaN - 0.0146i

and then that goes on for quite a bit. To be clear this happens with both my in_i and in_q values.

[John Chilleri]

Would you mind posting your values?

I may not have time to experiment with your values, but this question seems to be similar to yours, and perhaps it can be of use to you:

https://www.mathworks.com/matlabcentral/answers/251061-how-to-plot-frequency-spectrum-of-a-signal-in-matlab

[Steven Seppala]

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]

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');