OPTIMUM SOURCE CONDUCTANCE FOR HIGH-FREQUENCY SUPERCONDUCTING QUASI-PARTICLE RECEIVERS

We have used the quantum theory of mixing for extensive numerical calc ulations to determine the mixer source conductance G(s) required to op timize a superconductor-insulator-superconductor (SIS) quasiparticle h eterodyne receiver. The optimum G(s) matches an empirical formula whic h can be understood by a simple derivation. Previous work indicated th at G(s) should vary inversely with frequency, and this implies that th e critical current density of SIS junctions used for mixing should inc rease as frequency squared, a stringent constraint on the design of su bmillimeter SIS mixers. On the contrary, we find that G(s) is more wea kly dependent upon frequency, and we discuss the implications for the design of submillimeter SIS mixers.