Development of a Dual Polarization SIS Mixer With a Planar Orthomode Transducer at 350 GHz

A multipixel array equipped with dual-polarization superconductor-insulator-superconductor junction mixers at approximately 350 GHz is considered to improve the efficiency and survey speed of a telescope. This study develops a compact mixer with a planar orthomode transducer as a key component. Signals coming from a circular waveguide are separated into two polarizations via orthogonal waveguide probes, and then transmitted to the junctions through the tuning circuit. To suppress resonance, metallic blocks confine the RF signals within the circular waveguide, and a 2-μm-thick silicon nitride membrane, on which all devices were implemented, was suspended across the circular waveguide. The orthomode transducer, simulated with a three-dimensional electromagnetic simulator, has a cross-polarization of less than -20 dB between 306 and 367 GHz. The IF circuit consists of a low pass filter, coplanar waveguides, and a transition to coaxial. The return loss of the IF circuit is less than -20 dB up to 16 GHz from both simulations and measurements. The superconductor-insulator-superconductor junction mixers have been fabricated with quality factor (R_sg/R_n) of 8 and the double-side-band receiver noise temperature T_rx is approximately 120 K between 320 and 360 GHz.