Design and Analysis of a Waveguide Structure for 211–275 GHz 2SB SIS Mixer

We present an analysis of a waveguide structure for a 211–275 GHz sideband separating (2SB) mixer based on superconductor–insulator–superconductor (SIS) tunnel junctions. A general analytical model describing the quality of the sideband rejection ratio (SRR) is developed. It shows a crucial influence of reflections from single-ended mixers, reference frequency (RF) load, and the RF hybrid on the SRR level. Due to the intrinsic asymmetry of the 2SB waveguide structure, the reflections strongly affect both the balance of the observed signal and the balance of the local oscillator (LO) pumping signal. The model is verified and confirmed by 3-D electromagnetic simulations showing good qualitative and quantitative agreement. The developed theory gives a practical tool to design 2SB waveguide mixers with a required SRR level. Based on the presented theory, the waveguide structure of the 211–275 GHz 2SB SIS mixer is designed. It is predicted a degradation of the SRR level from 26 dB to about 18 dB due to reflections. The developed model explains some experimental data measured for 2SB SIS mixers developed earlier.