So back to time. If the goal is keeping good time then it is best
not to use Microsoft Windows. There are good technical reasons having
to do with the way MS Win. keeps and adjusts time. The bottom line is
that you will never be able to do better then about the millisecond
level even with a directly attached GPS.
I agree that Windows is not the best OS for timekeeping, but with a
directly attached GPS/PPS device I consistently see better than the
millisecond level with Windows. Please see:
http://www.satsignal.eu/mrtg/performance_ntp.php
PCs Feenix (XP), Stamsund (Win-7/32), Alta (Win-7/64) and Bacchus (Win
2000) demonstrate this. All are connected to either GPS 18, GPS18x, or
Sure GPS devices. There is also interpolation code available allowing you
to read time more precisely than 1 ms.
http://www.lochan.org/2005/keith-cl/useful/win32time.html
Using another OS, BSD or
Linux you can do almost three orders of magnitude better. (Three
orders is huge.) The OS and software is free and all you need is any
computer that has a physical serial port, not USB but a real DB9
connector. This is a good use for a 10 year old notebook PC. (The
computational load is trivial so even a 486 class computer is OK)
If you run BSD or a newer Linux then Pin-1 on the DB9 can be used for
time interval measurement at almost 1 uSec precision. You can use
this for many things, NTP being just one of them. If you have
multiple DB9 type serial ports then you can time tag multiple channels
to about a uSec or two. (certainly 3E-6 level)
Then if the Windows PCs need correct time they could use the server
running on the old BSD based notebook PC. Zero added cost assuming
you can find and old computer. Then you also have a tool that can
time tag any pulse at the uSec level.
Chris Albertson
Redondo Beach, California
My own experience suggests that you need rather more than a PC running a
good timekeeping OS - such as FreeBSD. Take a look at PC Pixie:
http://www.satsignal.eu/mrtg/performance_ntp.php
This is an Intel Atom box running FreeBSD 8.0, and yet in an open domestic
environment there are reported excursions of approaching 10 microseconds
when the heating is switched on in the morning. So put that PC in some
sort of thermally-controlled enclosure if you want better than the 10
microsecond level. The GPS puck is outside, by the way, unaffected by the
heating switch-on.
Comparing PCs Pixie and Feenix suggests that the FreeBSD system is only
two orders of magnitude better than Windows - still a great improvement,
of course!
Cheers,
David
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