Accurate noise measurements of superconducting quasiparticle array mixers

A 30-40 GHz test apparatus which makes it possible to measure the noise temperatures of SIS mixers with an accuracy of better than + or - 1 K has been constructed. This is a factor of six improvement over earlier measurements. The most accurate measurement made thus far of a mixer which uses a single Pb-alloy junction yielded TM = 9.2 + or - 0.9 K; and mixer gain: GM = 0.240 + or - 0.005. In addition, SIS mixers employing arrays of N = 1, 5, 10, 25, and 50 tunnel junctions in series have been tested. The input power required to saturate the array mixers was found to increase as N-squared and the gain and noise temperature of the array mixers were independent of N.