[time-nuts] True Position GPSDP + Rb X72

Attila Kinali attila at kinali.ch
Wed Sep 20 08:39:42 EDT 2017

On Tue, 19 Sep 2017 18:09:16 +0000 (UTC)
Stephan Flor via time-nuts <time-nuts at febo.com> wrote:

> Hi everyone,I'm new to this list, and I wanted to get the opinion of some 
> mere experienced people on my project idea.I just got a Symmetricom X72 Rb 
> oscillator, and I have a True position GPSDO on the way.  I wan't to have a 
> self calibrating 10 MHz signal to synchronize my hobby rf lab equipment.
> My idea is to make a GPS disciplined Rubidium oscillator, by removing the 
> crystal oscillator ans substituting the X72 in the GPSDO circuit.I would just 
> have to make a op amp circuit to change the 0 to 4v correction signal from 
> the True Position, to a 0 to 5v signal for the X72.

Unfortunately, this doesn't work as easily as you thing.
The control loop inside the GPSDO is designed for an OCXO. This means
that its loop constant is in the 10s to 100s of seconds, while for
an GPSDRb you would need 10'000s to 100'000s of seconds. The other,
more criticial issue is the sensitivity of the tuning input. Most
OCXOs have something like 1-10ppm of tuning range. The tuning range
of an Rb is usually 2 to 3 orders of magnitude smaller. In a normal
PI loop, this shouldn't be an issue. But you don't know what the
internal logic does. So at least be careful with that.

The more common way to build such a system is to do a phase comparison
between the 10MHz output of the GPSDO and the Rb, then steer the Rb
using a seperate control loop electronics. One easy way to do this
would be using a uC with a capture compare unit, at least one with 16bit,
better with 24bits and more (e.g. STM32F4xx). Divide both 10MHz outputs
down to roughly 1kHz-10kHz using 74LVC161. With this you can measure
the phase differnce between the two with about 12ns precision (limited
by the uC, not the dividers). Average over 2-3 days and you are good.

For additional precision, you can use an TDC7200 to measure the time
difference with around 50-100ps precision, at which point the GPSDO
noise and the stability of the Rb will be the limiting elements.

			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

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