AN SIS MIXER USING 2 JUNCTIONS CONNECTED IN PARALLEL

We have fabricated a Superconductor-Insulator-Superconductor (SIS) mixer using a couple of junctions connected in parallel through a stripline inductor, The junctions have significantly larger area, i.e. larger capacitance, and smaller normal resistance than conventional ones, In order to obtain a good impedance matching between the source and the junctions, an impedance transformer made of a superconducting stripline was integrated with the junctions, The capacitance of the junctions was tuned out by the inductor to obtain a broadband operation without mechanical tuning elements, It was shown that the double sideband noise temperature of the receiver employing this type of mixer device was less than 40 K over the bandwidth of 90-115 GHz. The lowest receiver noise temperature of similar to 20 K, which is only 4 times as large as the quantum limited photon noise of h nu/k(B), was obtained around 105 GHz.