When using an SDR (software defined radio), there is generally an error
between the receiver frequency and the transmitter frequency.  This results
in a frequency beat that is not necessarily audible, but it will be quite
apparent when viewing the IQ data on a Lissajous display.

I noticed this when I added a Lissajous scope to my signal analyzer.  It
never dawned on me until I mentioned this on IRC (##rtlsdr), and tejeez
indicated that the big circle is the result of a frequency error.

This prompted me to come up with a frequency error estimator so that I can
use the result (the frequency error) to drive an NCO plus phase corrector, so
that I could derotate the phase velocity of the signal.

And so it begins...

Chris G. 08/21/2023

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09/01/2023
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We now have a working phase-locked loop.  It stablizes the
Lissajous display nicely so that signal constellations can
be viewed.