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We present the design, modeling, construction, and characterization of a sideband separating heterodyne receiver that covers the frequency range from 600 to 720 GHz. The receiver has been constructed using waveguide technology in the split-block technique. The core of the mixer consists of a quadrature hybrid, two directional couplers to inject the local oscillator signal, two superconductor-insulator-superconductor (SIS) junctions, three signal-termination loads, and two planar IF/bandpass-filter/dc-bias circuits. The instrument that we have constructed presents excellent performance as demonstrated by two important figures of merit: receiver noise temperature and sideband ratio. Across the entire band, the uncorrected single-sideband noise temperature is below 500 K and reaches 190 K at the best operating point. The sideband ratio is greater than 10 dB over most of the frequency operating range. Superconducting junctions containing AlO∞ - and AlN-tunnel barriers were tested.