Quasi-optical 0.5 THz SIS receiver with twin junction tuning circuit

A quasi optical SIS mixer for the frequency band 400-550 GHz has been designed and successfully testedin a laboratory receiver setup. The intrinsic SIS junction capacitance was tuned out by a novel pair (twin) SISjunction compensation circuit. The 4µm2 area Nb-A10x-Nb junctions, with (oRnC product as high as 7.4 at500 GHz, were patterned by optical lithography and integrated with a equiangular spiral antenna to give broad- band SIS mixer operation. The receiver demonstrated flat response within the tuning range of the available LOsource (440 - 520 GHz) with DSB noise temperatures below 300 K over the entire band and with the best re- ceiver noise temperature below 250 K DSB at 453 GHz (measured at 3.5 K physical temperature). No mechani- cal tuning was employed in the SIS mixer. These noise temperatures include all losses and was measured atan IF center frequency of 1.5 GHz.