MEASUREMENTS OF NOISE IN JOSEPHSON-EFFECT MIXERS

We present new heterodyne receiver results obtained at 100 GHz using r esistively-shunted Nb and NbN tunnel junctions, In addition, we have c arried out accurate measurements of the available noise power of these devices at the L-band (1.5 GHz) IF frequency. Both the heterodyne and the output noise measurements show that the noise of these devices ca n be a factor of five or more higher than that predicted by the simple current-biased RSJ model, The noise approaches the appropriate therma l or thermal and shot noise limits for bias voltages where the nonline arity is not strong (i.e., V > I(C)R(N)), but as expected from the RSJ model, can be significantly higher at the low voltages where the mixe rs are typically biased. The bias voltage dependence of the noise show s structure which is associated with resonances in the RF embedding ci rcuit, Surprisingly, we find that changes in the high-frequency (100 G Hz) impedance presented to the junction can dramatically affect the ma gnitude and voltage dependence of the low-frequency (1.5 GHz) noise, T his emphasizes the necessity of very closely matching the junction to free space over a wide frequency range.