[time-nuts] Designing an embedded precision GPS time

MLewis mlewis000 at rogers.com
Tue Oct 31 20:19:22 EDT 2017


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?

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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