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A compact 780–950 GHz sideband separating (2SB) superconductor-insulator-superconductor (SIS) mixer measuring 22 mm × 27 mm × 11 mm is designed in this study. In this mixer block, all components such as a radio frequency (RF) 90° hybrid coupler, a local oscillator (LO) power splitter, two LO couplers, two identical SIS chips, and an intermediate frequency (IF) 90° hybrid coupler are integrated. To minimize the waveguide length for the RF signal path, we separate the placement of the waveguide components into two layers in parallel. One layer contains the RF hybrid and LO couplers, and another layer contains the LO power splitter located above the RF hybrid coupler. They are connected by waveguides fabricated via wire electric discharge machining. We performed three-dimensional electromagnetic simulations and confirmed the results. Furthermore, a 4-12-GHz IF 90° hybrid coupler to combine the IF signals from each SIS chip is designed with an alumina substrate having a relatively high dielectric constant to be integrated in the mixer block. The preliminary test result of single sideband noise temperatures of the fabricated 2SB SIS mixer partly complied with the current Atacama Large Millimeter/submillimeter Array (ALMA) specifications without any loss correction in front of the receiver. Because the RF and LO interfaces of the mixer block are the same as that of the current ALMA band 10 mixer block, band 10 cartridges are expected to be upgraded to 2SB configurations without significant changes in optics.
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