Obtaining Enhanced Temperature Stability

It is sometimes necessary to improve the temperature stability of a device under test (DUT). In the case outlined here the DUT was installed it in an oven with a very sensitive and stable thermal detector and high gains in the oven heater feedback loop. The instability of the oven temperature was expected to be lower than 0.001 degree / degree change in room temperature. However, the DUT exhibited a temperature change of roughly ten times worse. What was wrong and how to fix it?

Solution

An analysis of the configuration showed that the cables connecting the DUT to the outside were providing a thermal path that caused a temperature difference between the oven sensor and the DUT. This temperature difference was just the ratio of the thermal impedance between the oven sensor and DUT, to the thermal impedance between the DUT and the outside room temperature multiplied by the change in room temperature. This ratio corresponded to the thermal gain of 100 observed. The fix was to use another sensor to measure the outside temperature and use this information to correct the temperature set point of the oven. This meant that when the outside temperature changed, the oven set point changed a very small amount (1 / 100) in the opposite direction to maintain the DUT at the correct temperature. The implementation of the additional feed-forward correction loop required only a few parts and improved the thermal stability of the DUT by about a factor of 1000 yielding a thermal instability of only 10 micro degrees for every degree change in the room temperature. Moreover, this was accomplished without the use of a second oven and the attendant restrictions on temperature range. It did not increase the power requirements or add significantly to weight or size.

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