[time-nuts] GPS discipline oscillator vs phase lock

Attila Kinali attila at kinali.ch
Fri Jun 16 15:26:58 EDT 2017


On Fri, 16 Jun 2017 18:40:26 +0000 (UTC)
life speed via time-nuts <time-nuts at febo.com> wrote:

> I suspect the result of a GPSDO is not the same as phase-locking two
> oscillators together.  Perhaps it is frequency locking?  Which, if the phase 
> difference were held constant to within 0.1 degree, would be acceptable.  Not 
> sure this is the result either.

Nope, GPSDOs do phase lock on the "true" GPS time. The offset between
GPS time and the GPSDO output is mainly dependent on the antenna,
the antenna cable and the receiver. The antenna delay is mostly
the temperature dependence of the antenna itself and its filter.
The cable is temperature dependence of length and dielectric constant.
The receiver is mostly temperature dependence of filters.

With a bit care, you can keep the stability of these to better than 1ns.
The offset stability between two GPSDO setups of the same kind should
be an order of magnitude smaller (given similar temperatures), maybe
even two.

The analysis above is under the assumption that you have good sky view,
with little multi-path and a well surveyed GPSDO. If you have bad sky
view or lots of multi-path or are off with the survey coordinates,
you can easily get a jitter in the order of 100's of ns.

What you have not talked about is, how well you want to keep the
phase between the two GPSDO stable.

If you look at ADEV numbers of GPSDOs, then you will see that the stability
easily goes down to 1e-10, some even reach a few parts of 1e-12 at taus < 1000s.
This should give you an indication what's possible.


			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

More information about the time-nuts mailing list