SIS quasiparticle mixers with bow-tie antennas

We have designed and evaluated planar lithographed W-band SIS mixers with bow-tie antennas and several different RF cou;ling structures. Both Pb-In-Au/Pb-Bi and Nb/Pb-In-Au junctions were used, each with R_NC«1. Single junctions and series arrays of five junctions directly attached to bow-tie antennas with no additional coupling structure gave poor performance, as expected. Single junctions with inductive microstrips and five-junction arrays with parallel wire inductors gave good coupling over bandwidths of 5 and 25 percent respectively. Good agreement was found between design calculations based on a simple equivalent circuit and measurements of the frequency dependence of the mixer gain. When good coupling was achieved, typical values of mixer gain G_M (DSB)0 dB, noise T_M(DSB)150 K, and receiver noise 200 K were observed. These measurements are referred to the cryostat window. When corrected for the estimated loss between the cryostat window and the antenna terminals, these values of gain are comparable to those observed for W-band waveguide mixers with IF matching, but the noise is significantly higher. There is evidence that 100 K radiation surrounding the mixer reduces the gain and increases the noise. No systematic difference was observed between the performance of Pb-In-Au/Pb-Bi junctions and Nb/Pb-In-Au junctions when the area of the latter was made three times smaller and the current density three times larger so as to maintain the same capacitance and resistance.Work supported by the U.S. Air Force Office of Scientific Research under Grant #AFOSR 85-0230.