[ntp:questions] Installing more stable oscillator?

Jason Rabel jason at extremeoverclocking.com
Fri Jul 13 15:55:37 UTC 2007


I think Paul hit it right on the head with his post...

A higher-grade oscillator in your PC won't *really* matter (for computing
purposes), and even NTP over the internet you just aren't going to get that
resolution because of all the delays and interrupts. Your drift would be
lower, but since NTP can compensate for that once it had enough data then
you really haven't netted much. 

At the end of your post, it sounds more like you are wanting a high-end
clock, but to somehow build it out of your PC. A PC just wasn't meant to be
a highly-accurate time keeper (which is what I think a lot of people want
them to be), it was meant to be a calculator on steroids. A better approach
would be to build the clock as a stand-alone dedicated device, then use one
of the outputs to feed into your PC to discipline it.

If you watch eBay, they have some old HP 5065A rubidium standards that come
up for sale from time to time at reasonable prices. Whether they are in
working order or not is another story. Some have the front clock display
option as a little bonus, but the big thing is you can sync the PPS output
to an external PPS source (like GPS) and / or adjust it via some thumbwheel
dials. Once you have it set the long-term drift is 1 x 10-11/mo (10 ps?)
assuming you have a stable room temp and all that jazz.

Another option, if you check out some products made by Symmetricom / Datum /
TrueTime (and many others), their "network time servers" can be installed
with a variety of crystal options (some even have a rubidium option), which
are disciplined via a GPS, PPS, Time Code, or ACTS. If the signal is ever
lost, then the system can 'flywheel' on that oscillator and keep time
accurate to whatever the specification might be. There are other GPSDO
systems that do similar, though don't have a built-in NTP server (which is
something I think you want).

If you look at the following page:

http://www.febo.com/pages/soekris/

You can see that he replaced his crystal with an external source. Once NTP
has synchronized to a proper GPS/PPS source, I suppose you could technically
disconnect it and let NTP use the local clock (and fudge the stratum) for a
similar flywheel effect. The effects of the OS and interrupts though might
throw your clock off regardless? And since you are using a high-end
oscillator to act as the PC's oscillator, it would make more sense to
discipline that via other means to keep it on-time instead of trying to
fudge it through NTP.




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