Conversion gain and noise in a Josephson mixer

Fundamental mixing at 36 GHz in non-hysteretic Nb point contacts has been observed with overall conversion efficiencies in the range 0.33 ≤ η ≤ 4 and mixer noise temperatures in the neighborhood of TM ~ 200 K. The observed conversion efficiencies are in good agreement with the Resistively Shunted Junction model. The observed noise can be accounted for by assuming that the Johnson noise in the shunt resistor is the only source of noise driving the junction. Somewhat larger noise is seen from resonantly coupled mixers than is seen when the mixer is driven from an rf current source. Fundamental mixing in hysteretic junctions is limited to a narrow if bandwidth and gives much larger conversion gain but also larger noise. Parametric amplification at the if frequency is seen in hysteretic junctions. Fourth order harmonic mixing with ωLO/2 π at 9 GHz has been achieved with performance almost comparable to fundamental mixing. Cavity mode mixing appears less promising