Calibration/Correction in MATLAB I/Q Data

The 32-bit floating point I/Q data has the corrections applied. It is scaled to dBm. You can read more about this in the API manual in the I/Q data types section. If you are doing an FFT on the I/Q data, ensure your are normalizing your window function and properly scaling the output of your FFT. You should be able to simply do a log power conversion of the FFT results to get dBm.

Andrew

Can you share the script you are using to calculate I/Q power? How are you measuring power in Spike? With the standard sweep mode and markers? Are you using the default Spike configuration (full 20GHz span on bootup)? Or have you modified any settings in Spike?

• This reply was modified 3 years, 2 months ago by Andrew.

Consider doing your own FFT and windowing with something like the flattop window. The pwelch function uses the Hamming window which has ~1.8dB of scalloping loss and could easily account for most of the error you are seeing. The flattop windows has ~.1 dB of scalloping loss, and is much more appropriate for amplitude accuracy, and is likely what Spike is using in your measurements.

Here’s some example code for generating your own spectrum

function [spectrum] = calculatespectrum(t)
%CALCULATESPECTRUM
% Return the log spectrum of the input time domain signal
% The input is a real or complex column vector of floating point values
window = flattopwin(length(t));
if(~isreal(t))
window = complex(window);
end

% Normalize window
scalar = length(window) / sum(window);
window = window.*scalar;

spectrum = fftshift(fft(t.*window)./length(t));
spectrum = 10 * log10(abs(spectrum).^2);
end

If you want to compare with Spike, ensure you use the same reference level.

• This reply was modified 3 years, 2 months ago by Andrew.