My last posting on this subject fell on deaf ears so here is a direct appeal
:-)
Would it be possible for one of the highly expereinced Soundcard / Win95+
programmers outhy there to produce a SB compatible vectorscope prog ? This
whould then allow anyone to view low data rate coherent signal coding in
real time, adjust frequency very accurately etc. etc etc. AS well as
providing a very useful piece of software to use as test equipment for
tuning, propagation monitoring etc.
What is needed is a Numerically Tuned Oscillator under operator control that
can be set in real time, by mouse or keyboard. This mixes down the input
from one channel of the soundcard to zero frequency I/Q components. These
are low pass filtered and decimated to lower rates, under operator control.
The I/Q pairs are then simply displayed on a polar plot. Gain control, or
scaling, will be required. Alternatively, a slightly more complicated
plotting routines that plots radius on a log scale (dB) but preserves X/Y
phase information might give a nicer looking display.
In addition a graphical display of phase and amplitude would add a finishing
touch
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.
I suggest an NCO tuning range of 300 - 2800 Hz in as fine a resolution as
possible, decimation and filtering by factors of 4 / 8/ 16 / 32 / 64 / 128 /
256 and a suitable gain / amplitude scaling control range. In my
vectorscope using the separate DSP card, the X/Y display shows the previous
N samples, where N is under operator control of 32/64/128/256/512. The
optimum number depends on the noisiness of the trace and residual frequency
error.
One can play endlessly with such a tool !
Andy G4JNT
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