NIST NTP servers

Mel Beckman mel at beckman.org
Fri May 13 20:38:41 UTC 2016


Lamar,

Because you need microsecond-level time accuracy (which is beyond NTP's capabilities) you'll requires an adjunct protocol, such as PPS, to get that.  For continued NTP delivery despite periodic GPS signal loss, then you need an OCXO internal clock. 

But anyone satisfied with NTP's millisecond time accuracy at worst needs a $1 temperature-compensated internal clock. Either method needs the specs for a Stratum 1 time source on a local network. 

As you point out, the hobbyist SBCs can't deliver even basic clock accuracy.  

But another key consideration beyond accuracy is the reliability of a server's GPS constellation view. If you can lose GPS sync for an hour or more (not uncommon in terrain-locked locations), the NTP time will go free-running and could drift quite a bit. You need an OCXO to minimize that drift to acceptable levels. 

But I see that the price premium for an OCXO clock is only $100 to $200 on low-cost (I.e., ~$500) commercial NTP servers. So buyers need only make a minor cost adjustment to have very good, and inexpensive, COTS NTP performance and reliability. 

 -mel beckman

> On May 13, 2016, at 9:30 AM, Lamar Owen <lowen at pari.edu> wrote:
> 
>> On 05/13/2016 10:38 AM, Mel Beckman wrote:
>> You make it sound like TXCOs are rare, but they're actually quite common in most single board computers. True, you're probably not gonna find them in the $35 cellular-based SBCs, but since these temperature compensated oscillators cost less than a dollar each in quantity, they're quite common in most industrial species for well under $100.
> 
> Correct, they're not rare in the industrial line (for that matter you can get TCXO-equipped RTL-SDR dongles, but that's not NTP-related).  Something like a Transko TFC or TX-P or similar is enough for reasonable timing for basic purposes, and they're not expensive.  They're also not a stock item on the consumer-level SBC's either.  I looked at one of our half-dozen ODroid C2's, and the main processor clock, X3, is under the heatsink, so I can't see what part is being used.  X1 and X2 are outside, and it doesn't appear that they are TCXO modules, although I didn't use a magnifier to check the part number and might have made an error.
> 
> The Nicegear DS3231 RTC has a TCXO, and might be the best low-cost choice at $12 (need to have an RPi, ODroid, or similar on which to mount it).  It's not that TCXO's are rare or expensive, it's that they're not often considered to be important to accuracy in many circles.
> 
>> An Ovenized XCO is absolutely not required for IT-grade NTP servers.
> 
> No, but it is for my purposes here.  But, as I said in my post:
> 
> 
>> You really have to have at least a temperature compensated quartz crystal oscillator (TCXO) to even begin to think about an NTP server, for anything but the most rudimentary of timing.  Ovenized quartz oscillators (OCXO) and rubidium standards are the next step up, ...
> 
> I was just saying that OCXO and Rb are just the next step up if you would like more stability, that's all.  For 'within a second' on a GPS-disciplined clock (even without the 1PPS signal) you wouldn't necessarily need TXCO.  But that's what I meant by 'anything but the most rudimentary of timing.'  Rudimentary is down to the millisecond in my environment.  PTP takes you to the next level, and beyond that you don't use network timing but put a dedicated time distribution network running IRIG-B or similar.  But that is beyond the scope of a typical IT NTP server's needs.....
> 


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