In this paper we present results of heterodyne mixing measurements of a phonon-cooled hot electron bolometric mixer in the 600-1100 GHz frequency range. The devices are made out of a 3 nm thick NbN film on a high-resistivity Si substrate. A two-step electron beam lithography process is used to form a 0.3#mu#m long and 5 #mu#m wide bolometer strip across the center gap of an Au spiral antenna. The mixer chip is placed on an extended hemispherical substrate lens with antireflection coating. The best results of the DSB noise temeprature are 500 K at 650 GHz and 980 K at 900 GHz. The measured total single sideband conversion gain is about -9.6 dB. The absorbed local oscillator power is estimated to be 100 nW.
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