2 Read - out Electronics and Experimen - Tal Setup at Room Temperature

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 300 K using industrial platinium resistance thermometers, has a noise level of ∼ 25− 30 μKeff for a 3 hours measuring time and in a 1 Hz bandwidth. Using YBCO superconducting thermometers, the noise level goes down to 2.5 μKeff , which is strongly limited by excess 1/f noise in the YBCO film at the superconducting transition. The sample holder used in the 90 K 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. PACS numbers : 07.20.Dt, 07.50.Hp, 07.57.-c