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

Rob Kimberley robkimberley at btinternet.com
Mon Feb 20 19:39:36 UTC 2012

I've looked at the posts on this topic, and from what I've seen so far I
think is going to be a tough if not impossible call. 

30 minutes to get GPS is OK, but what about the oscillator? It will need a
lot longer than that to stabilise. You also then have the vagaries of the
network. NTP won't hack the spec he is quoting and PTP won't hack the
network. And, the budget is way too low.

Food for a lot of thought methinks....

Rob K

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jim Lux
Sent: 20 February 2012 19:07
To: time-nuts at febo.com
Subject: Re: [time-nuts] Low-long-term-drift clock for board level

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
> 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

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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