[time-nuts] Thunderbolt temperature spikes

Mark Sims holrum at hotmail.com
Mon Jul 14 14:02:42 EDT 2008


Yes indeed, those are spikes... and rather big ones at that.  They may seem small,  but they occur over a one second period.  Normally I do not see more than 1 millidegree of change over that time interval.  These spikes are 20 to 100 times that.  After the spike,  the temperature reading decays back to normal over 10-30 seconds.  Whatever they are,  they should not be there.  

The oven only affects the crystal stability.  All the frequency comparison,  EFC DAC voltage,  etc logic are outside the oven and are affected by temperature.  The Thunderbolt uses the ambient temperature readings to help compensate for these effects,  discipline the oscillator,  and to train the holdover compensation Kalman filter.  

The ThunderBolt A/D has at least a 20 bit resolution (probably 24 bit).  It reports temperature with microdegree resolution.   They would not be using such a high resolution A/D if ambient temperature was not a major factor in performance.   Although they are fairly infrequent,  the spikes do have the potential to affect the disciplining and holdover performance.   

My plots of ThunderBolt  temperature usually show a 200 millidegree or so peak-peak sinewave shape with a 30 minute or so period (probably corresponding to air conditioning cycles).  Over a full day,  the reported temperature is 40C +/- 0.5C.  The unit I am monitoring right now is sitting on the floor, next to the back door, and next to a floor mounted AC vent.  It is covered by a corrugated cardboard box.  I go though that door rather frequently (it is 100 F outside and 74F inside) but have not seen any influence on the temperature plots.  
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This is a "spike"?  Surely this kind of tiny temperature variation on 
the unit's board somewhere outside the oven does not have a lot of 
relevance or effect on anything inside the oven where it is all 
happening.  And what is the tolerance and resolution of the temperature 
measuring device anyway? Or am I missing something fundamental here?


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