"Either method needs the specs" should read "Either method meets the specs."
-mel beckman > On May 13, 2016, at 1:39 PM, Mel Beckman <m...@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 <lo...@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..... >>
