[time-nuts] syncronized clocks

[Tim Shoppa]

Wow, you're all over the place here :-)

The "1-2 microsecond drift every second" you observe for the two RTC's
relative to each other, is entirely consistent with the few ppm spec for
that device. If you start tweaking the trim frequency you should be able to
get it trimmed to better than a ppm at room temperature.

The 10ms second-to-second variation you see in the serial GPS message, is
entirely consistent with expected serial port jitter. If you want to use
your current i2c module to trim the RTC frequency, 10ms serial message
jitter will get you to 0.1ppm if you wait a day.

It's likely but not guaranteed that somewhere in the i2c module you already
have, there is a PPS signal, it just doesn't come out on a header (or isn't
quite documented on a header).

Tim N3QE

On Wed, Sep 16, 2015 at 8:00 PM, Can Altineller <altineller at gmail.com>
wrote:

> Hello,
>
> I started putting a test setup together when I suspected the DS3231 RTC's I
> got from dx.com were fake. So I put 2 mcus, each measuring microseconds
> from each 1PPS output with an interrupt, for the unit itself, and another
> interrupt to measure 1PPS time from another unit and display them on a
> nokia LCD.
>
> I have found out the RTC's differ by 1-2microseconds each second, on of
> them was falling behind 1microseconds each second, relative to other one.
>
> Then I put a i2c gps module, (which does not have 1pps output
> unfortunately) and parse out the time string, and generate a pulse for
> another interrupt.
>
> I found out that time from gps (arriving time of timestamp) varies within
> 10 milli seconds.
>
> So I already ordered some gps modules with 1PPS output,
> http://navspark.mybigcommerce.com/navspark-mini-6pcs-pack/ and I am
> thinking they should be good enough.
>
> Here is picture of my test setup:
>
>
> ​
> So here is the reason I am writing to the list: I am also out to buy a
> rubidium frequency standard, or a trimble gps disciplined clock.
>
> Like:
>
> http://www.ebay.com/itm/Trimble-GPS-Receiver-GPSDO-10MHz-1PPS-GPS-Disciplined-Clock-Antenna-power-/181810679481?hash=item2a54c2ceb9
> or
>
> http://www.ebay.com/itm/Trimble-GPS-Receiver-GPSDO-10MHz-1PPS-GPS-Disciplined-Clock-with-rs232-port-/261997391557?hash=item3d0042eec5
>
> I have read somewhere that these newer GPS disciplined clocks are much
> better than an rubidium based atomic standard, like datum modules, or some
> other modules we can get on ebay, since they are based on much more
> advanced atomic clocks that are on board gps satellites, and correct its
> oscillator continously.
>
> Is this true? What would be your recomendation? A GPS disciplined unit, or
> a rubidium standard? What are the differences between them. For right now I
> am interested in making two clocks beat (having the same 1pps output, in
> sync) but later on I might be interested in other measurements. It seems to
> me getting an exact measurement of time is really challenging and
> interesting as a hobby.
>
> Any ideas/help/recomendations appreciated
>
> Best Regards,
> C.A.
>
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