Optimum source conductance for high frequency superconducting quasiparticle receivers

The authors have used the quantum theory of mixing for extensive numerical calculations to determine the mixer source conductance G_s required to optimize a superconductor-insulator-superconductor (SIS) quasiparticle heterodyne receiver. The optimum G_s matches an empirical formula which can be understood by a simple derivation. Previous work indicated that G_s should vary inversely with frequency, and this implies that the critical current density of SIS junctions used for mixing should increase as frequency squared, a stringent constraint on the design of submillimeter SIS mixers. On the contrary, it was found that G_s is more weakly dependent upon frequency, and the implications for the design of submillimeter SIS mixers are discussed