Broadband IF matching for quasioptical mixers

Abstract

Everyone recognizes the need to drive symmetric quasioptical antennas in a symmetric way to maintain clean antenna patterns; in this note we report on the advantages of bringing out the IF in a symmetric (balanced) way as well. The main difference in IF circuits between waveguide and open structure mixers is that the quasioptical mixers are usually also open at IF wavelengths, so IF currents can flow on the outside of the mixer mounting structures. We measured these surface currents and their associated resonances on a scale model of our mixer block for a 690 GHz SIS mixer. Bringing the IF off the mixer with a balanced circuit solves the surface current problems, yielding a broad bandwidth with predictable impedances. We successfully tested an octave bandwidth IF matching circuit for open structure mixers that incorporates a commercial 180° hybrid at cryogenic temperatures. We also found that surface currents are not significant for corner cube mixers because they generate their own balancing currents.