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 µK ef f for a 3 hours measuring time and in a 1 Hz bandwidth. Using YBCO superconducting thermometers, the noise level goes down to 2.5 µK ef f , 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.