[time-nuts] Low-long-term-drift clock for board level integration?

Jim Lux jimlux at earthlink.net
Mon Feb 20 19:07:15 UTC 2012

On 2/20/12 10:31 AM, Bob Camp wrote:
> Hi
> Simple answer:
> A rack mount cesium standard is as good as you can get. Figure on 15 ns per day of drift. That gets you to 15 us in 1000 days. You may or may not get one that drifts that little, a lot depends on  little details. For>  3 years, consider an ensemble of cesiums. At $50K each cost can go up pretty fast.
> There is nothing out there that will do better stand alone for less money. Either you get a sky view or you change the budget....
> Bob

But he *does* have the ability to have an external network connection.. 
So the real question is whether one could come up with a better-than-NTP 
scheme to get the 100 (or 10) microseconds.

I suspect that with sufficiently controlled network paths (which might 
be doable in a widescale deployment) and a tweaking of the NTP stuff, 
you might be able to get it to work.

100 microseconds out of a day is 1E-10, which is actually a fairly 
liberal drift spec for an OCXO or even a TCXO. (which are things like 

Maybe there's a particular time of day (or day of week) when network 
uncertainties are minimal, or can be bounded.  So what you really need 
is a onboard oscillator that is good at "carry over" between periodic 
calibration checks.

One question about the 100 microsecond spec.. is that a worst case at 
any instant, or is it an average spec over a day (so a consistent 
diurnal variation would cancel out)

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