NIST NTP servers

Lamar Owen lowen at pari.edu
Fri May 13 16:29:54 UTC 2016


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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