Scott:
Further to my last re the PhaseScope instructions, I think the instructions
relating to the PPS polarity might be faulty. If the PPS is inverted by the
soundcard, the program will think the negative-going edge is the one it
should lock to, and there will be no way to tell that it's wrong. In other
words, regardless of which polarity you select, the displayed active edge
will always appear right. What will change, if your PPS is very short, is
that in the correct case it will have the inactive edge trailing it, and if
the polarity is wrong the inactive edge (i.e. the REAL 1-sec event) will
precede the edge to which the program has locked.
If the user knows that his PPS is short (i.e. he can see both edges on the
display), there IS therefore a way to describe to him how to recognise which
is the correct polarity for his soundcard. If the users PPS is not short,
for example 50:50 like mine, there is no way to resolve the problem by
looking at the screen. The ONLY way to be sure is to see if the program
responds to the detected edge at the 1-second time mark, identifying this by
some means external to the program itself. This would have to involve the
display blinking (or some similar event like a pip of sound) as it processed
the detected edge, and the user verifying that this occured exactly on a utc
mark, either by watching the PPS waveform itself (on a scope or meter with
known polarity, not the unknown soundcard in question!), or by audible
comparison with a WWV pip.
I don't think you can ignore this and tell the user that it doesn't matter
which edge he uses. If a given GPS specifies that the positive-going edge is
on the utc mark, we cannot assume anything about the stability of the
negative-going edge at all.
73
Peter
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