EXTENSIVE TEST OF THE 3-PORT QUANTUM MIXER THEORY ON 345 GHZ SUPERCONDUCTOR-INSULATOR-SUPERCONDUCTOR MIXERS

Predictions of the three-port model of the quantum theory of mixing ar e compared with measured results on 345 GHz superconductor-insulator-s uperconductor waveguide mixers. Single Nb-Al2O3-Nb tunnel junctions or two or four identical junctions in series are used as mixing elements . Two different waveguide mixerblocks, one with two tuners and another with one tuner, are used. In addition a single junction with integrat ed tuning stub is analyzed. Embedding impedances are obtained from fit s to the pumped I-V curves for all three types of mixing elements. In all cases the dependence of mixer conversion and mixer noise on bias v oltage, pump power, and embedding impedance is well described by the t hree-port model. The measured mixer gain is lower than the calculated gain by a factor of 0.35-0.65, independent of the type of mixer. The u se of an additional integrated tuning element does not change this fac tor. It is concluded that an excess noise power equivalent with a blac kbody source of 40-65 K must be added to the mixer noise to account fo r the absolute value of the observed noise power.