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 <att...@kinali.ch> wrote:
Hoi Leo,
On Sat, 28 Oct 2017 11:14:08 +0100
Leo Bodnar <l...@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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