We have made an all-refractory SIS mixer with four Nb/Al-AlOx/Nb junctions connected in series. The mixer is designed to operate at 80-120 GHz. The area of each junction is 2.5 × 2.5 μm2. Both these SIS junctions and a mm-wave choke filter are fabricated on a crystalline quartz substrate. These Nb/Al-AlOx/Nb junctions have shown ideal dc-characteristics and clear photon-assisted tunneling steps. RF measurements were made with a mixer mount which had two mechanical tuners. The conversion loss was less than 9 dB for 75-95 GHz. The minimum value was 8 dB. This value will be drastically improved by optimizing the normal resistance of the SIS junctions.
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Inatani et al. (1987) studied this question.
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