A photomixer local oscillator (LO) operating at the 630-GHz difference frequency of two laser diodes was successfully demonstrated with a heterodyne detector based on a niobium superconducting tunnel junction. The low-temperature-grown GaAs photomixer generated 0.20 uW in the input spatial mode of the receiver. Using the photmixer LO, the double-sideband noise temperature of the receiver was 331 K - in good agreement with the 323-K noise temperature obtained when a multiplied Gunn oscillator generating 0.25 uW was substituted for the photomixer.