Yes, a Windoze vector scope would be a great tool/toy!

Andy G4JNT wrote:

Some sort of calibration routine would be needed for use, either feeding in
a tone of exact known frequency and adjusting the NCO for a stationary
trace.

The soundcard clocks can be quite unstable. Therefore I would suggest a
second mode where both left and right inputs are used. The received signal
is fed into one of the channels and a reference frequency is fed into the other.
If the reference is a clean sine wave, a Hilbert transformer can be used to
split it into the quadrature L.O. needed for the half-complex input mixer
(real in - I/Q out).

Another approach could be to keep the NCO and use a software PLL to lock
the NCO "software clock" to the reference frequency. The reference doesn't
have to be a pure sine wave in this case, I think.

One can play endlessly with such a tool !

I couldn't agree more!

73
Johan SM6LKM