[ntp:questions] Installing more stable oscillator?

Pete Stephenson pete+usenet at heypete.com
Sat Jul 14 01:43:42 UTC 2007


In article <1184319747.579985.256640 at k79g2000hse.googlegroups.com>,
 Paul.Croome at softwareag.com wrote:

> NTP's mission in life is to discipline a cheap, unstabilized computer 
> system clock (quartz oscillator) to one or more better quality, more 
> stable time sources. If your computer has a Grade A system clock, you 
> would have to consider carefully whether you would make matters 
> better or worse by trying to discipline it from time sources derived 
> via the Internet.

Indeed.

I guess my unasked question was, "How can one build a better quality, 
more stable time source?"

I could use NTP to sync my computer's cheap oscillator to an internet 
source (what I currently do), or to a radio clock (GPS, CDMA, WWVB, etc. 
-- I'd like to do this, but budget and lack of knowledge is currently 
preventing this.)...that's not too hard. But how would one build a more 
stable source of time? If the external source is interrupted, even with 
NTP adjusting for the system's drift, it will still drift further and 
further away from the actual time in relatively short order. I'd like to 
have a system here that can avoid that, mostly "because I can", not for 
any particular reason.

Surely many of the stratum 1 servers (say, time.nist.gov) that get and 
distribute time directly from atomic clocks aren't just off-the-shelf 
servers with cheap, unstabilized system clocks, right? I know that many 
of the public stratum 1 servers deployed by individuals and 
organizations get their time from GPS, and are probably ordinary 
computers, but I have this (again, perhaps incorrect) assumption that 
the servers that supply the time /to/ the GPS system are not ordinary 
computers.

> The computer's crystal oscillator (system clock) is an essential part 
> of an NTP server; it's the entity that NTP is controlling. Don't 
> confuse the external oscillator(s), which NTP uses as its input, and 
> the system clock, which is the end result. Using a better-quality, 
> e.g. ovenized, quartz oscilllator will give NTP an easier job; but 
> NTP has been designed and engineered to cope with the vagaries of 
> typical computers with typical system clocks.

Perhaps I misunderstand, but are you saying that it's possible to 
replace the computer's crystal oscillator with an ovenized quartz 
oscillator? Or are you saying that one can use the ovenized quartz 
oscillator (or rubidium, cesium, hydrogen maser, etc.) as an external 
oscillator, and it would provide PPS input to the computer running NTP?

> Yes, it should be possible to use a GPS to number the seconds and 
> provide long-term disciplining of a rubidium oscillator, which in 
> turn provides the PPS input to NTP.

Interesting. If one were to have such a setup, and GPS service was 
unavailable for whatever reason[1], the rubidium oscillator would 
continue to provide PPS input to NTP, and while the rubidium source 
would not be ticking at exactly the same speed as UTC, the drift would 
be extremely minimal, even with extended unavailability?

I apologize for the rather simple questions, but over the last few years 
I've gone from being a minor time enthusiast to wanting to get more 
involved. Syncing from internet servers is certainly adequate for my 
needs here, but I'm looking at doing bigger and better things. 
Unfortunately, doing so requires lots of rather silly questions. :/

[1] Reception at ground level is spotty due to buildings, and I don't 
have roof access.

-- 
Pete Stephenson
HeyPete.com




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