A 350-GHz SIS antipodal finline mixer

In this paper, we describe the design and operation of a 350-GHz finline su perconductor-insulator-superconductor mixer. The mixer is fed by a horn-ref lector antenna, and the superconducting circuit is fabricated using planar- circuit technology and fully integrated tuning. An important feature of the mixer is that it employs an antipodal finline section, deposited on one si de of a quartz substrate, which transforms the high impedance of the wavegu ide (approximate to 300 Omega) to the low impedance of the microstrip (appr oximate to 20 Omega). The Nb/Al-oxide/Nb tunnel junction is fabricated at t he same time as the finline circuit. In this paper, we describe the design procedure in some detail. We pay particular attention to the synthesis of t he finline taper and the electromagnetic design of the horn-reflector anten na. We have tested a finline mixer over the frequency range of 330-370 GHz and measured a receiver noise temperature of 90 K, which remained unchanged over the whole frequency range. Our investigation has demonstrated that it is possible to make superconducting finline mixers for frequencies as high as 350 GHz.