On 09/28/2010 09:38 AM, Hal Murray wrote:
What is the purpose of the temperature sensor chip on the PCB, anyway? Isn't
the temperature inside the OCXO much more important?
The OCXO isn't perfcet. The frequency will shift slightly with environmental
temperature changes.
Let's consider a thermal gain of 1000, such that 1 K of external
temperature difference becomes 1 mK internal. If we want to compensate
the remaining error, putting a temperature sensor inside the OCXO, it
needs 1 mK resolution for 1 K external temperature shift. Putting the
temperature sensor externally will the OCXO thermal gain improvement in
resolution. So a 1/32 K resolution external becomes a 1/32 mK resolution
internally.
This model is oversimplified as the thermal response-time and that of
the oven will smooth out temperature shifts, and an external sensor does
not experience the same transient response.
It may have been more important without the high-quality OCXO option that
many recent TBolts seem to have.
Here is some data:
http://www.megapathdsl.net/~hmurray/time-nuts/TBolt-volt-temp-1.gif
http://www.megapathdsl.net/~hmurray/time-nuts/TBolt-volt-temp-2.gif
Looks like thermal lag is the reason for the opening between the heat-up
and chill-down periods.
Cheers,
Magnus
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