[time-nuts] TAPR Thunderbolt Temperature Reporting Question

[Arnold Tibus]

On Sun, 26 Jul 2009 01:04:04 +0000, Mark Sims wrote:


>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.

Thank you Tom!

How can one verify the rev. of the chip? Is there a code on? 
How can I be sure to get from a distributor the revision I want? 
What in fact is the difference in the design of both versions? 

All what I found are some programming hints to read a internal 
register with intermediate count status. This way it is (was?) 
possible to get a resolution downt to 0.01 deg. C or so with 
a real 'precision' still of 0.5 deg. C. 

But a question does remain,
how will this temp sensing on the top of the board get the chance 
to correctly compensate some (fast) temp. variations when the oscillator 
is sitting away, well encapsulated and 'stabilized' inside via it's own heater 
system (time shift!)? Outside fluctuations will not correspond 
at a certain moment with the inside temp. variations of the OCXO I think. 
Can such a system not work well only when having very slow temp. 
variations?
If that is the case it is still of big importance to have a kind of temperature 
stabilizing housing around the box.

Here one will find some description of the principle of the high 
resolution readout with the DS1620:
OWL2 to DS1620 temperature sensor
http://www.owlogic.com/OL2d1620.htm

I add a picture showing the DS1620 position as U9 on the board close to 
the DB9 RS232 connector.
(very much zoomed in and a compressed, I tried to get the size as 
small as possible)

regards
Arnold




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