Point‐contact Josephson mixers at 130 GHz

摘要

The properties of point‐contact Josephson junctions operated as mixers with an external local oscillator at 130 GHz have been studied, and the results compared with predictions of the resistively shunted junction model. It was found that the junction’s output noise could be within a factor of 1.5 of the model prediction based on the thermal‐driving noise. When the coupling to the rf source was optimized, a factor of 2 discrepancy was typical. The measured conversion efficiency agreed with the model, within the experimental uncertainty in the equivalent microwave circuit. The best coupling was achieved in full‐height waveguide. The best overall performance, measured using the hot/cold source technique, was a single sideband conversion efficiency of 0.30 and a mixer‐noise temperature of 180 K (both ±20%). The best junctions were made of Nb, with carefully prepared points. Nonideal behavior in other junctions is ascribed to heating effects. There is some evidence that junctions whose resistance exceeds a threshold value of ∼60 Ω are no longer well described by the RSJ model. A discussion is given of possible improvements in performance with better junctions or improved rf matching.