[time-nuts] syncronized clocks

[Attila Kinali]

Merhaba Can,

On Thu, 17 Sep 2015 03:00:47 +0300
Can Altineller <altineller at gmail.com> wrote:

> 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.

That would mean they are within 2ppm of eachother, which sounds reasonable.
 
> 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.

Yes, depening on your antenna position, this should produce a PPS with
less than 1us jitter. With a good antenna position <50ns are likely.
Please do not trust the +/-10ns of the spec, unless it offers some
form of sawtooth correction. The 100MHz clock already causes in a
+/-10ns jitter of the PPS.



> 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.
[...]
> 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.

Depends on what you mean by "better" and on what time scale you operate on.
Below measurment times of 1000s, the rubidium is usually more stable.
Above between 1000 and 100'000 it depends on the quality of the GPS
system (and its setup) and the quality of the rubidium. Above 24h it's
AFAIK always GPS that is more stable.


> 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.

My recommendation would be to first read the wikipedia entry for
the Allan Deviation, then have a look of the various measurements
on GPSDO and Rubidiums done by Tom (www.leapsecond.com) and John (www.febo.com)

Also: welcome to the rabit hole! How far will you go down? :-D


			Attila Kinali
-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson