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

Tom Van Baak tvb at LeapSecond.com
Wed Dec 12 01:59:02 UTC 2012

Hi Warren,

> Starting from a factory reset, it has something it will use in under 1/2 day. 

And that would explain my and Charles' null results. Nice.

> 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.

Were you able to test how quickly, or how well, the filter learned the tempco of the OCXO?

> 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.

Yes (although please define "much"; are you talking 1% better or 10% better or 2x better). Would it be possible to implement this advanced PID in LH? Or as a first step, and to veryify your conclusions, would it be possible for LH to control the DAC during TBolt hold-over mode?

> 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.

I agree this is true in theory (where epsilon != zero), but it's hard for me to believe true in practice. I would guess the error term is totally dominated by short-term GPS noise, and anything else, like tempco or frequency drift, is secondary. That's why it makes sense to apply these 2nd or 3rd order corrections during hold-over mode (where there is no GPS noise) but not for normal operation.

> 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.

Thanks for stating both these facts so clearly. I cringe every time I hear someone replacing a 1C DS1620 sensor in their TBolt. The TAPR TBolts I tested years ago worked equally well regardless if they were 1C TBolts or ten milli-C TBolts.


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