How to do FFT on I,Q data

I am Working on a climate orbiter satellite data, from there i have extracted the In_Phase and Quadrature_Phase in decimal form now wanted to do further processing such as FFT , PSD etc.

Can Someone help how to do FFT on that type of data?

Note: I have Very large dataset which is about 144crores but i have divided it into chuncks of files which is about 8 lakh each.

Answer 1

[William Rose]

@Ankit,

Let's assume you have read in data from a two-column file. Column 1 is in-phase, column 2 is quadrature.

N=256;
fs=100e6; % sampling rate (Hz)
t=(0:N-1)/fs;
data=randn(N,2);

Combine the ccolumns to make a single vector of complex numbers

The second column is multiplied by 1i, which is pre-defined as sqrt(-1).

x=data(:,1)+1i*data(:,2);

Now do FFTs or compute power spectra in the usual way. The only difference is that, since x(t) is complex, the FFTs and spectra will not be conjugate-symmetric around the Nyquist frequency. (If you prefer to think of positive and negative frequencies, then the spectra will not be conjugate-symmetric around f=0.)

X=fft(x);
f=(0:N-1)*fs/N;

Plot results

figure; subplot(311)
plot(t,data(:,1),'-r.',t,data(:,2),'-b.'); grid on; xlabel('Time (s)')
legend('In-phase','Quadrature'); title('Time domain');
subplot(312);
plot(f,abs(X),'-g.'); grid on; xlabel('Frequency (Hz)')
ylabel('|X(f)|'); title('FFT')
subplot(313);
plot(f,unwrap(phase(X))*180/pi,'-g.'); grid on; xlabel('Frequency (Hz)')
ylabel('phase(X) (deg)'); title('FFT')

Good luck.

[Ankit]

@William Rose

So if we want to find the peak how to do that , if we don't know anything about the signal?

[William Rose]

@Ankit,

findpeaks() is a very useful function with many nice options.

Are you seeking a peak in the spectrum or in the time-domain signal?

Provide a sample file if you wish to see an example of using findpeaks() to find a peak in a spectrum.

[silvia cano]

hi

i have same problemi have In_Phase and Quadrature_Phase i on a file so i try the code above but in my case if i put N=256 , i get the error x and y must have same fdimension. so i change N= length(dataI). I want to know if is possible or i'm doing something wrong

thanks

[William Rose]

@silvia cano,

Please provide an example of your data. I would not expect your InPhase and Quadrature vectors have different lengths, but if they are different lengths, then you will have to align them correctly and discard non-overlapping elements.

[silvia cano]

i think my data have the same length but if i put N=256 , i get the error x and y must have same fdimension. my doubt if N is the problem or my data

[William Rose]

@silvia cano,

I used N=256 because I needed to generate simulated data as an example.

Since you already have data, use N=length of your data.

load('data.txt');
x=data(:,1)-mean(data(:,1))+1i*(data(:,2)-mean(data(:,2)));
N=length(x);
fs=1e6;         % assume sampling rate=1e6 Hz
t=(0:N-1)/fs;
% Compute DFT
X=fft(x);
X=fftshift(X);  % shift so freq=0 is in the middle
f=(-N/2:N/2-1)*fs/N;
% Plot results
figure; 
subplot(411), plot(t,data(:,1),'-r'); 
grid on; xlabel('Time (s)'); ylabel('Real'); title('x(t)');
subplot(412), plot(t,data(:,2),'-r');
grid on; xlabel('Time (s)'); ylabel('Imag');
subplot(413); plot(f,abs(X),'-b'); 
grid on; xlabel('Frequency (Hz)'); ylabel('|X(f)|'); title('FFT')
subplot(414); plot(f,unwrap(phase(X))*180/pi,'-b'); 
grid on; xlabel('Frequency (Hz)'); ylabel('phase(X)');

The code above runs without errors, using your data.

[silvia cano]

What do I do to not have negative frequencies on the graph and

move freq=0 is in the middle?

[William Rose]

@silvia cano,

Replace

% Compute DFT
X=fft(x);
X=fftshift(X);  % shift so freq=0 is in the middle
f=(-N/2:N/2-1)*fs/N;

with

% Compute DFT
X=fft(x);
f=(0:N-1)*fs/N;

Good luck.

[silvia cano]

thank you so much , another question , if i nedd to plot FFt on axis y and number of samples on x axis ?

[William Rose]

@silvia cano,

You're welcome.

Replace

f=(0:N-1)*fs/N;

above with

f=1:N;

[silvia cano]

hi i have a csv data , with magnitude and number of samples , but if i want to plot magnitude on y axis and distance on x axis , what is the relation between samples and distance

[William Rose]

@silvia cano, I have no idea.

[William Rose]

@silvia cano, since you are asking a new question,

I recommend that you post your question as a new quesiton on Matlab Answers. Include the data that you have that relates to range.