How to read IQ data I matlab

Kefei,

To convert I/Q data to spectrum, you will need to perform overlapping windowed FFTs. Typically what this looks like, if you are doing a 1024-point complex FFT, is every N samples (for your application this would be every 1 ms), you will take the last 1024 I/Q samples, window them (multiply by a window function, like a flat top or hamming window), and then FFT them to generate a spectrum.

Kefei,

Unfortunately we do not have any examples of performing this type of processing in MATLAB. I would start by learning about FFTs and the role of window functions for the FFT. There is a lot of literature available on the internet on this topic using MATLAB as the programming language. MATLAB has several functions to assist in this type of processing.

Overlapped FFTs as described in Justin’s response are how Spike converts I/Q data into a series of closely spaced spectrums.

Regards,
Andrew

Hey, Kefei Hei, I may be able to help you (assuming you’ve not already figured it out). I’m using Gnu Octave, not Matlab, but they should be fairly similar. On my computer (an Intel-based system), the Signalhound IQ data shows up as “shorts”.

I created an IQ file called “myFile.iq”. Here’s a short script I created to read in a short bit of data from that IQ file, then generate a simple FFT display (no windowing).

f=fopen('myFile.iq','rb'); % Opens the file to read as binary data.
s=fread(f,8192,'short'); % Reads in 8192 samples.
% Signalhound IQ data is a stream of I-Q-I-Q... points.
% It must be parsed out to properly represent the data.
sReal=s(1:2:end); % Set the real points
sImag=s(2:2:end); % Set the imaginary points
sc=sReal+j*sImag; % Create the actual complex samples
frq=20*log10(abs(fft(sc/8192))); % Create FFT
frqShift=fftshift(frq); % Shift the horizontal axis so that it displays properly
plot(frq)

Note that, if I left out the “8192” from the “fread” statement, it would have read all of the data at once.

I tested this, and the spectrum I got was similar to what I got from Spike, so I believe I have the set-up correct.

Hope this helps!