A 385-500 GHz sideband-separating (2SB) SIS mixer based on a waveguide split-block coupler

We have developed a 385 - 500 GHz sideband-separating (20) mixer, which is based on a waveguide split-block coupler at the edge of the H-plane of the 508 mu m x 254 mu m (WR 2.0) waveguide, for the Atacama Large Millimeter/submillimeter Array (ALMA). An RF/LO coupler, which contains an RF quadrature hybrid, two LO couplers, and an in-phase power divider, was designed with the issue of mechanical tolerance taken into account. The RF/LO coupler was measured optically with a microscope and electrically with a submillimeter vector network analyzer. The image rejection ratio (IRR) and the single-sideband (SSB) noise temperature of the receiver using the RF/LO coupler have also been measured. The IRR was found to be larger than 8 dB and typically similar to 12 dB in the 385 - 500 GHz band. The SSB noise temperature of this receiver is 80 K at the band center, which corresponds to 4 times the quantum noise limit (hf/k) in SSB, and 250 K at the band edges.