Monday, January 10, 2011

IQ Correction Algorithm - beginning

Greeting everyone,

Attached is the IQ Correction MATLAB model written by Bob McGwier, and the
output figures it produced after I got it working in Octave.

Two signals, x1 and y1, are created in this model. They represent I and Q
signals. Both have noise added to them (random, normal distribution, mean zero,
0.01 standard deviation).

Whenever we implement a circuit that handles I and Q, we expect to get
differences between I and Q that are due to the implementation. In this model,
y1 has a fixed gain error and a fixed phase error. The IQ correction algorithm
determines the gain error and phase error. Neat!

Running the model under Octave, I got the following when the model attempted to
draw figure 3:

octave-3.2.3:3> IQCorrectGain_Phase
error: `kaiser' undefined near line 92 column 4
error: called from:
error: /Users/w5nyv/Dropbox/MEP/IQCorrectGain_Phase.m at line 92, column 3

That particular section of the model is:


% z=zeros(1,1024);
% for kk=1:1024
% z(kk)=(x1(tt(kk))+1i*y1(tt(kk)))*ww(kk);
% end
% plot(-0.5:1/1024:0.5-1/1024,fftshift(20*log10(abs(fft(z)))))
grid on
axis([-0.5 0.5 -140 10])
title('Output Spectrum with Gain and Phase Offset')

The kaiser reference is to a windowing function. It looked like I needed to
install an extra windowing package for Octave! Octave is a free and open source
version of MATLAB.

On the Mac,
sudo port install octave-windows
worked just fine on the desktop, and the model ran with the original MATLAB
kaiser window function.

However, a dependency wouldn't easily install on the laptop. I tried to build
from source, but running the port command again (port is somewhat similar to
opkg, git, svn etc) it still hung up on a particular dependency.

Through the power of Twitter, Bob recommended changing to another windowing
function (from kaiser to blackman-harris, which appears to be called just
blackman in Octave) and that seemed to do the trick. Since this particular
function is just to present the data, it's a bit less critical than having to
use an alternate function in the filter itself.

This is a good example of what is sometimes necessary when using Octave instead
of MATLAB. While Octave shadows MATLAB very closely, it's not an identical twin.

The very next step is to write a tutorial about the filter so that the math pops
right out at you.

The next step after that is to write the filter in VHDL. All are welcome to
participate, critique, troubleshoot! If you have experience (or want some) in
writing VHDL, here is a chance. :+)

More soon,
-Michelle W5NYV

Potestatem obscuri lateris nescis.

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