A 385 - 500 GHz SIS Mixer for APEX Telescope

摘要

The Atacama Pathfinder Experiment (APEX) is a 12 m antenna operating at the Atacama Desert on the Chilean Andes at about 5000 m altitude. APEX would be equipped with a suit of single-pixel heterodyne receivers covering 211 - 1500 GHz frequency range, We present here a design of a sideband-separating superconductor-insulator-superconductor (SIS) mixer for the APEX, receiver Band 3, operating in 385 - 500 GHz band. The receiver uses quadrature scheme with the RF signal passing via a 90-degree waveguide 3 dB hybrid and the LO is divided by a waveguide E-plane Y-junction. The outputs of the waveguide hybrid are coupled to the mixer SIS junctions through an E-probe with integrated bias-T. For the LO coupler, conventional branch waveguide couplers are difficult to manufacture at this high frequency with required accuracy as the branch waveguides become extremely narrow. In order to solve this problem, we propose an on-chip LO injection, where the LO coupler is integrated onto the mixer chip and fabricated together with the SIS junction and the tuning circuitry. The on-chip LO coupler is made of superconducting lines, which gives almost a lossless solution and provides fabrication accuracy better than 0.5 urn by using optical lithography only. Furthermore, the mixer design includes a novel component, an ellipse termination for the idle LO port, made of thin-film resistive material with sheet resistance equal to the transmission line characteristic impedance, which gives very broadband performance using extremely compact area.