A 75–110 GHz Sis Mixer With Integrated Tuning and Coupled Gain

A broadband waveguide SIS (superconductor-insulator-superconductor) mixer with no adjustable mechanical tuning elements for the 75–110 GHz band was constructed and tested. The design is a demonstration of a prototype receiver element in a focal plane array receiver. The large instantaneous bandwidth was accomplished with a broadband waveguide-to-microstrip transition and a scale-modeled microstripline circuitry. The transition to microstripline consisted of a waveguide single ridge 4-step Chebychev transformer. The last step of the ridge connected the waveguide to the microstripline circuit, which contained the SIS element in parallel with a thin film tuning inductor. The operation was double-sideband (DSB). The typical DSB mixer noise temperature, as measured between 79.5 and 110 GHz, was about 35 to 50 K at band center increasing to about 50–60 K at the band edges. The lowest mixer noise we obtained was 20 K at 80 GHz for a single untuned junction. For some junction geometries, gain was also observed between 80 to 110 GHz, with a maximum of +3 dB (DSB) at 100 GHz. No impedance transformers were used on the intermediate frequency (if) side. For several devices, negative dynamic resistance was observed on the first photon induced step below the gap.