The properties of a heterodyne receiver at 450 GHz using a Josephson point-contact mixer and an external local oscillator

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Abstract
Adjustable niobium Josephson point contacts, mounted in fundamental-mode waveguide cavities, have been used as mixers in an experimental heterodyne receiver with a 450 GHz laser as the local oscillator. It was possible to estimate separately the various factors, such as intrinsic mixer conversion efficiency, mixer noise, and input coupling efficiency that contribute to the overall receiver sensitivity. Mixer performance agreed reasonably well with qualitative predictions of resistively-shunted-junction theoretical models, and there was broad quantitative agreement for some aspects of performance. Spurious, and potentially very misleading non-heterodyne responses to thermal signals were observed under some regimes of applied DC bias and local oscillator power. The lowest overall receiver noise temperature measured was 2100K (double side band), although reduction to 1000K should be possible with quite simple improvements. The intrinsic mixer noise temperatures were usually in the range 500-1000K (single side band), and local oscillator power required by the receiver was usually less than one microwatt. This performance makes these mixers competitive as the basis of very sensitive near-millimetre-wave receivers.