[time-nuts] syncronized clocks

Can Altineller altineller at gmail.com
Wed Sep 16 20:00:47 EDT 2015


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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