[time-nuts] TAPR Thunderbolt Temperature Reporting Question

[Mark Sims]

The Tbolt uses the temperature reading to fine tune the oscillator disciplining.  It has a major effect on holdover performance (when the GPS signal is not available) and some effect on normal performance.   You can quite clearly see the effect of the temperature reading on the DAC setting in plots.

If the sensor produced a temperature reading that was directly proportional to the true temperature,  any error should be minor.  I don't think absolute accuracy is as important as relative accuracy since the Tbolt learns how the oscillator responds to temperature readings,  but your readings are WAY off.

The fault is almost certainly in the small eight pin DS1620 chip near the RS-232 connector.  There are two versions of this chip.  The ones before Rev E work much better than the later ones because they report the temperature in a way that can be resolved by the Tbolt firmware quite finely (around 0.01 C raw values).  The later chips report the temperature with a very crude 1 degree C granularity.  Search the archives for the details.

I found that the chips from:
    http://www.rentron.com/Products/Electronic-Components.htm
are Rev C chips and work well in the Tbolt.   The chip itself is cheap at $3.50...  shipping is not... it ran me around $11.  A group purchase/reshipment program might be useful.  I know of several Tbolts that had bad DS1620 chips.   I had several,  but wound up sending them out.


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Will this incorrect temperature be a performance issue ??  Is it 
fixable ?? Any suggestions as to where to look ?? 
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