Conversion gain in millimeter wave quasi-particle heterodyne mixers

The physical mechanisms which give rise to conversion gain in SIS quasi-particle mixers are studied. It is shown that the
S
-shape tunneling structure at the gap voltage of the
I- V
curve is essential in achieving conversion gain. In the development of SIS quasi-particle mixers, a new approach is used to analyze the embedding network of the mixing experiment. The method described in this paper has the advantage over conventional methods that no separate measurements are necessary. In order to obtain a complete picture of the performance of SIS quasi-particle mixers, the photon-assisted tunneling theory used by Tucker to describe quasi-particle mixing is extended here to include pair current contribution. Based on this complete quantum theory, the effects of the Josephson noise on SIS quasi-particle mixing is discussed and an upper frequency limit of SIS quasi-particle mixing is estimated.