  We have designed low noise temperature sensing and control units with the
objective of using them for the fabrication of far infrared active cavity
radiometers. The sensing unit, first characterized at 300K using industrial
platinium resistance thermometers, has a noise level of
  25E-6 to 30E-6 Keff for a 3 hours measuring time and in a 1Hz bandwidth.
Using YBCO superconducting thermometers, the noise level goes down to 2.5E-6
Keff, which is strongly limited by excess 1/f noise in the YBCO film at the
superconducting transition. The sample holder used in the 90K experiments is
built with an auxiliary heating resistor, which enables an easy and accurate
identification of the electrothermal model, even in the closed loop operation.
Based on a design previously published by NIST, we estimate from these
experimental results that the overall noise limitations of radiometers could be
lowered by one order of magnitude.
