Performance of Arrays of SIS Junctions in Heterodyne Mixers

We have made a systematic experimental study of the performance of millimeter-wave quasiparticle heterodyne mixers which use arrays of SIS tunnel junctions. Sets of arrays with N =1, 5, 10, 25, and 50 junctions in series were fabricated by photolithography. All of the arrays in a given set were made on a single silicon wafer so that their response time parameter omega_SR_NC would be the same. Junction areas were scaled so that the total impedance was the same for each array in a set. Sets of arrays from four wafers with values of omega_SR_NC ranging from 2.6 to 13 were evaluated in mixers at 33 and 36 GHz. These measurements showed that the signal power required to saturate the mixers varies as N² and that the conversion efficiency is nearly independent of N for all values of omega_SR_NC. The mixer noise temperature is independent of N for large values of omega_SR_NC. Therefore, the dynamic range of an SIS quasiparticle mixer can increase in proportion to N². For small values of as omega_SR_NC, however, the mixer noise increases systematically with N. This correlation suggests that the junction capacitance affects the coupling between junctions that can contribute to the noise.