[time-nuts] Thunderbolt oven / non-stable operating temperature

WarrenS warrensjmail-one at yahoo.com
Tue Dec 11 20:31:15 UTC 2012


So much speculation on how the Tbolt uses it's temperature sensor data.
Having spending hundreds of man hrs and thousands of Tbolt running hrs,  testing all kinds of things to find ways to improve my Tbolt's performance. 
This is what I've found happens on My Non "E" TBolt with version#3 firmware, when in an indoor environment concerning it's temperature sensor.
During normal operation my Tbolt uses the temperature and ADC data to in its Kalman filter that then can predict a simple linear temperature constant, and simple linear ageing rate.
Starting from a factory reset, it has something it will use in under 1/2 day. 
If the temperature has not been thru a few cycles and /or the Ageing is still at a high initial cold start rate and still changing, 
the Kalman filter can give very poor results and actually make things worse.
It gets better as time goes on, and after a few days with a predicable Osc, the Klaman filter will improve enough to help.

But the **Only** time the Kalman filter is used is during Holdover. 
It does this by adjusting the EFC voltage in small steps making a simple linear ramp as a function of time,  
Plus a simple linear output as function of delta temperature.

I've also found that if the Temperature chip is the new one that gives only about 1 deg of resolution, 
All still works the same, But during hold over instead of seeing small continuous DAC changes as temperature changes, you see Big EFC steps.

It is a shame the Kalman filter is not also used during normal non holdover run time. 
With a more advanced PID control loop it could be made to work much better.
As it is, because the known systematic error information is not used as a  feed-forward term to help the PID loop, 
any temperature change or ageing that does take place during control has to be totally corrected by an error signal. 
In short this means there will be unnecessary errors  caused by both changing temperature or time if the Oscillator is not perfect.
The bottom line is, these errors then limits how good of control you get, 
and why the Tbolt should be in a stable, less than 0.1 deg /hr environment to get the best performance from it.
(or can use LH's temperature controller or use an external double oven Oscillator) 

So what does this mean for the average Nut's Tbolt?   Mostly nothing. 
The only time the temperature sensor has any effect is during holdover.
If the Tbolt is going into holdover long enough for any of this to have an effect there are many worse things to be concerned about.
If the Tbolt does not go into holdover, none of the aging rate or temperature data is used (except for an over temperature alarm). 

There is one exception that I have tested for:
If the Tbolt has a high resolution temperature sensor and a good Osc where both the aging rate and the temperature TC 
are constant enough to be correctly modeled by the simple linear 1st order predictor used in its Kalman filter,  
then the open loop Kalman filter correction can improve the frequency accuracy over time by 3 to 1 or better during very long holdover periods like days.


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