[time-nuts] Portable Time Standard

[Petr Titěra]

Hello,

as I've said these are quite new, cheap RTC modules. There is DS3231SN
on them. Based on markings they were manufactured somewhere in 2008 but
I've bought them recently.

I have two those modules. Both connected using I2C.

First module is connected to Orange Pi sitting on my table. Pi is
running NTP and is controlled by NEO-6M GPS module (not timing, I know
butat this level accuracy of PPS signal should be enough).

I measure in 10 seconds intervals difference between system time (which
I consider stable in long term) and time reported by RTC chip as
reported by adjtimex in compare mode.

This is a little bit naive setup because timing from RTC is noisy but in
long run (its running for 25 days now) it should give me reasonable results.

I did modify offset register. Again using naive approach. I've let run
unmodified module for one day and from that I did compute offset required.

For second module I'm meassuring just now I don't care about time
reported and meassure only interval between rising edge of its PPS
pulses and PPS pulses from another GPS. Again for this measurement I
consider GPS PPS signal as stable.

This second one is a little bit worse (or may be wrongly calibrated) but
I still get about 0.3PPM from my measurements.

Petr Titera




Dne 17. 2. 2019 v 19:00 Joe Hobart napsal(a):
> Hello Petr,
> 
> Your accuracy results are impressive.  I have questions:
> 
>    What manufacture or brand DS3231 do you have?
> 
>    What do you use to communicate with the DS3231 and display time?
> 
>    How long has your DS3231 been running?
> 
>    Have you changed the crystal aging offset register(s) to compensate for
> crystal aging?
> 
> Thank you,
> Joe
> 
> 
> On 2/15/2019 12:51 PM, Petr Titěra wrote:
>> Hello,
>>
>> sorry to reopen this old thread but it took me some time to measure. As
>> I've said I have one RTC module sitting right on my desk in my room
>> where temperatures can go anywhere between 3C and 20C.
>>
>> Using naive method of comparison of time difference between system time
>> and RTC module I get time difference only about 4.487869e-02 seconds in
>> 22 days.
>>
>> Petr Titera
>>
>> Dne 14. 1. 2019 v 16:11 Joe Hobart napsal(a):
>>> Hello Petr,
>>>
>>> I have not found a low power RTC chip that will keep 1 second a month with
>>> changes in temperature.  This clock will be used to synchronize a portable
>>> communications system.
>>>
>>> Thank you,
>>> Joe
>>>
>>>
>>> On 1/13/2019 2:23 PM, Petr Titěra wrote:
>>>> I will probably sound dumb but did you consider cheap RTC chips? I have
>>>> one based on DS3231 currently on my table and although its is specified
>>>> to have precision +/-2PPM my seems to be in range about 0.5PPM right
>>>> from package with possibility to do some corrections.
>>>>
>>>> Petr Titera
>>>>
>>>> Dne 11. 1. 2019 v 20:57 Joe Hobart napsal(a):
>>>>> I need a relatively lightweight, self powered, portable clock accurate to 1
>>>>> second per month.  Temperatures may range from 10 to 35 C; altitude from 1000 to
>>>>> 7000 feet.  Although expensive, some of the marine quartz chronometers appear to
>>>>> meet this requirement. ~0.3 PPM.
>>>>>
>>>>> Does anyone have any experience with these devices?
>>>>>
>>>>> If so, has anyone tried to detect the probably weak magnetic field generated by
>>>>> the clock mechanism for time comparison purposes?
>>>>>
>>>>> Thanks,
>>>>> Joe, W7LUX
>>>>>
>>>>> ---
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>>>>>
>>>>>
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>>>>
>>