[time-nuts] Thunderbolt performance vs temperature sensor

Mark Sims holrum at hotmail.com
Fri Mar 6 17:55:35 UTC 2009

I did some testing on a Thunderbolt with the new revision E2 (low resolution, flat line) and old revision D1 (high resolution,  curvy line) DS1620 temperature sensor chips.  The only thing that changed was the temperature sensor chip.

Basically,  I put a unit that had the new revision E2 temperature sensor into manual holdover mode and ran Lady Heather over several 24 hour (approximately) periods with the log enabled.  Between each period,  I put the unit back into GPS discipline mode and let it recover.  

Next I swapped out the temperature sensor chip with an old revision D1 chip and let the unit run for a week so that it had a chance to relearn any filter coefficients.  Then I repeated the holdover log runs.

I processed the logs to calculate the spread in the PPS error reading over 1 hour intervals.  With the new revision E2, low res temp sensor the Thunderbolt averaged 1.73 uS of PPS change per hour.  With the old revision D1,  high res temp sensor the unit averaged 0.82 uS of PPS change per hour.

So,  it appears that the Thunderbolt does indeed use the temperature sensor readings in its disciplining of the oscillator (which is also obvious from the plots of DAC voltage vs temperature) and that the units performance (at least the holdover performance) was adversely affected when the DS1620 temperature sensor chip was changed going from rev D to rev E.

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