[time-nuts] Designing an embedded precision GPS time

Bob kb8tq kb8tq at n1k.org
Tue Oct 31 20:33:11 EDT 2017


> On Oct 31, 2017, at 8:19 PM, MLewis <mlewis000 at rogers.com> wrote:
> If one is building a GPS disciplined NTP Stratum 1 server from a Pi or Beaglebone, the "better" quality RTCs seem to be
> DS3231 based (DallasSemi/Maxim), Accuracy ±2ppm from 0°C to +40°C, ±3.5ppm from -40°C to +85°C
> or
> NXP:
>    PCF2127AT: ±3 ppm from -15 °C to +60 °C
>    PCF2127T: ±3 ppm from -30 °C to +80 °C
>    PCF2129AT: ±3 ppm from -15 °C to +60 °C
>    PCF2129T: ±3 ppm from -30 °C to +80 °C
> How does one translate that into an expected 24 hour holdover?

The simple answer is that you really don’t have enough information. 

The useless answer (which is easy to come up with)  is that you would be off by a bit under 0.3 seconds per day if you clock is 3 ppm off.
Since that’s just the TC error, there would have to be a zeroing process.  If you “zero out” the error at -3 ppm and then temperature
moves you to +3 ppm, you could be off by a bit under 0.6 seconds in a day. Yes you could carry this out to many more digits, it’s really
an accurate guess beyond the first digit. 


> And, are there better choices for a low cost RTC?
> Thanks,
> Michael
> On 31/10/2017 4:47 PM, Bob kb8tq wrote:
>> HI
>> TCXO is a very loosely defined term. A part that does +/- 5 ppm -40 to +85C
>> is a TCXO. A part that does +/- 5x10^-9 over 0 to 50C may also be a TCXO.
>> Dividing the total deviation of either one by the temperature range to come
>> up with a “delta frequency per degree” number would be a mistake. You
>> would get a number that is much better than the real part exhibits.
>> Working all this back into a holdover spec in an unknown temperature
>> environment is not at all easy.
>> Bob
>>> On Oct 31, 2017, at 4:03 PM, Attila Kinali <attila at kinali.ch> wrote:
>>> Hoi Leo,
>>> On Sat, 28 Oct 2017 11:14:08 +0100
>>> Leo Bodnar <leo at leobodnar.com> wrote:
>>> True. An NTP server does not need to measure time better than 100ns or so.
>>> 10ns is probably more than good enough. But then, this raises the question
>>> what your performance metric is that you optimize for?
>>> If it is hold-over, then this will be limited by the TCXO and how well
>>> you can measure its frequency, which in turn depends on how well you
>>> can measure the PPS pulse. You say that your device is typically within
>>> 4-5ms in 24h of hold-over. That translates to frequency uncertainty
>>> of approximately 5e-8. That's not that good.
>> To summarize: If you want to improve your ntp devices hold over performance
>> you have to improve the frequency measurement and use a better clock modeling.
>> Ie, use a timing GPS receiver and its sawtooth correction, and model the
>> clocks frequency change over time.
>> 			Attila Kinali
>> -- 
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