[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

[Dr Bruce Griffiths]

SAIDJACK at aol.com wrote:
>  
>  
> Hi Ulrich,
>  
> good details on how to set the time constant for best GPSDO performance  etc!
>  
> Some issues you did not mention are but that are essential to get a  good 
> GPSDO are:
>  
>    * Aging compensation
>  
>    * Temperature compensation
>  
>    * fault recovery, such as mechanical shock to crystal,  power surge, 
> acceleration, thermal shock, atomic particle impact (pop  noise) etc. 
>  
> => Aging is mostly constant over periods of several days, except after  first 
> cold power-on. I just received a plot from an OCXO factory showing 1E-07  
> aging the first day, asymptotically slowing down to <1E-010 after 10 days!  The 
> oscillator is rated at 1E-010 per day (which is quite good) but clearly does  
> not meet this spec in the first couple of days.
>  
> Aging will show up as a constant offset as the GPS and the DAC is  constantly 
> "chasing" the crystal. The slope of the change in DAC voltage over  time can 
> be measured (but only after it has been temperature corrected!!) and  then the 
> software can compensate for aging so the TIC/GPS/Loop filter doesen't  have 
> to do it. This helps tremendously for maintaining stability during  hold-over 
> as well. I have seen some cheaper OCXO's age more than 1E-013 per  second after 
> being on for weeks and that's not a bad spec either!
>  
> => Temperature compensation is helpful even on a near perfect oscillator  (Rb 
> or even Cesium!) because as long as the unit is being corrected by a voltage  
> from an external DAC, the DAC, and Dac reference will have temperature  
> dependencies that can be corrected. The problem becomes measuring temperature  
> changes on the order of 0.001 Degrees Celsius resolution...
>  
> Temperature compensation should be done for Aging prediction/compensation  to 
> work well. Otherwise one would not know if the drift is caused by aging or  
> temp changes.
>  
> Electronic temp compensation works great to make a good OCXO out of a  
> mediocre one, but the temperature sensor's thermal response must match the  OCXO's 
> (and DAC/Reference) response as well as possible, otherwise the unit  will be 
> over/under-compensated.
>  
> => Some non-linear processing can be done to speed up fault recovery.  The 
> problem then is: is it a GPS induced error, or an OCXO induced problem?  Fuzzy 
> logic helps to get the OCXO locked quickly when a big jump happens by for  
> example momentarily changing the loop time constant for fast locking, then  slowly 
> going back to the intercept point on the ADEV curve you mentioned.
>  
> bye,
> Said
>  
>  
>  
>  
>
>
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>   
Said

With only one integrator in the loop a constant frequency drift due to 
ageing produces a constant phase offset at the phase detector.
To correct for ageing you need to add another integrator to the loop.
Of course this makes it more difficult, but not impossible, to stabilise 
the loop.

Detecting/measuring temperature changes of a millidegree isn't 
particularly difficult even when using a PRT sensor.
A thermistor sensor allows detection of temperature changes of a few 
tens of microKelvin without great difficulty, however the long term 
stability is perhaps not that good.

Rather than relying on a calibration of the DAC gain which will 
inevitably change as the components age, surely its better to measure 
the DAC output with an ADC that has a lower tempco and drift. Suitable 
ADCs are available. Ideally a lower tempco DAC would be nice but they 
are not always available at an affordable price or resolution.

If the OCXO and or DAC are both drifting linearly then a loop with 2 
integrators will compensate for such drift. Such a loop will also make 
appropriate corrections during holdover.

You can always enclose the DAC and/or OCXO in an oven, however to ensure 
that the heat generated by the OCXO doesn't fry everything it may be 
advisable to use a Peltier heat pump to control the temperature of this 
oven.

It may be more practical to just increase passively the thermal time 
constant of the response of the DAC to ambient temperature fluctuations. 
The feedback loop will correct any very slow drifts in the DAC.

Bruce