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For low noise performance of SIS junctions as heterodyne mixing elements a common requirement is an coRC of the junction of around 3-4. For R=505), co=27t•230 GHz, and a specific capacitance of 60 fF/µ,m2, this means a junction area of 0.69 ptm2, i.e. submicron area junctions. Junctions of these dimensions are difficult to fabricate. Using arrays of junctions to reduce the effective capacitance results in the necessity of high current densities and requires good junction uniformity. Using an integrated superconducting impedance matching structure, we were able to get good performance at 230 GHz with single junctions as large as 2.5 IA m2 (Nb-A10x-Nb junctions fabricated at KOSMA, University of Cologne). An additional advantage of the large size is the low magnetic field necessary to suppress the Josephson currents. The matching structure is described in a companion paper [1]. It consists of a loaded stub compensating for the junction capacitance and two ',za /4 transformer sections which transform the real part of the resulting impedance. The waveguide mixer uses a backshort and an E-plane tuner in full height waveguide and was optimized together with the RF filter by scale modeling to give a maximum impedance tuning range [2]. A sketch of the test receiver is shown in Fig. 1. The IF HEMT amplifier has a noise temperature of 8K and was developed in house.
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