A 275-370 GHz SIS mixer for the APEX telescope

Atacama Pathfinder EXperiment (APEX) submillimeter telescope is currently under completion on Chajnator, at an altitude of 5050 m on the Atacama Desert, in the Northern Chile. The telescope facility heterodyne receivers should have 3 bands covering 211-500 GHz. We present design of a 275-370 GHz SIS mixer to be used as a first light APEX Band 2 receiver. A novel waveguide-to-microstrip transition with integrated bias-T is used in this mixer. This structure allows coupling of the RF signal from a full height waveguide to a thin-film superconducting line via E-probe. The wide side of the probe is connected to another port via a specially shaped high impedance line that provides RF/DC isolation. This port is used to extract the IF signal and to inject a DC current that creates a local magnetic field parallel to the plane of the SIS junction to suppress the Josephson effect. The main advantage of this type of Josephson suppression circuit is its compactness as it uses the existing superconducting lines from the SIS integrated tuning circuitry. The entire structure with the probe, SIS junction with its tuning circuitry is placed on a quartz substrate. For more advanced designs, as a sideband separating or balanced mixer that we intend to have for the final version of the APEX telescope heterodyne receiver, the SIS junctions of two balanced or quadrature mixers will be at a very close distance. The standard solution of using superconducting coils to suppress Josephson effect is very difficult to implement and, therefore, this new structure should be of a great advantage