Re: NIST NTP servers

[Mel Beckman <mel () beckman org>]

"Either method needs the specs" should read "Either method meets the specs."

 -mel beckman

> On May 13, 2016, at 1:39 PM, Mel Beckman <mel () beckman org> wrote:
>
> 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 () 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.....