I
think the soundcard stability is too poor to be able to rely on for phase
measurements. Instead, how about a phase meter that works by
comparing the phase of a signal on the left channel input with a supplied
reference tone on the right channel. Then, all that anyone need do is
construct a stable oscillator and divider to generate, say, a 1kHz,
tone.
I
regularly use a phase meter based on the 56002EVM. The DSP card performs
the digitisation, downconversion and decimation, outputting I/Q samples at
user selectable data rates from 800 Hz right down to 1.95 Hz (8000 /
4096) via the COM port to a PC for further analysis. In my
case the EVM sampling clock is stabilised so there is no need for a comparison
tone to measure phase.
A
waterfall plot of phase would be a wonderful and easy way to get a few more dB
of sensitivity out of slow signalling over power detection methods..
Even if the frequency stability is insufficient to maintain phase over a
symbol period, it should still be easy to spot the data transitions by
eye. The only problem then would be resolving the 0/180 degree
ambiguity.
Andy G4JNT
Alan - in fact I have been playing with a "phase sensitive waterfall
display" which Markus DF6NM suggested over half a year ago. Indeed, the result
of the FFT calculation carries the amplitude and phase, but it's not easy to
interpret the phase values and putting subsequent FFT results together (all I
got up to now is a very colorfull display which looks like "moiré effect" on a
TV screen. But with a bit of inspiration, there will be more soon... :-)