The Design Methods and Experiments for a 220-GHz Quasi-Optical Cryogenic Schottky Subharmonic Mixer of High Performance

In this article, we present systematic design methods and experimental verifications of a 220-GHz quasi-optical cryogenic Schottky subharmonic mixer. To maximize device performance over a wide bandwidth and overcome the difficulties in conjugate matching designs, an impedance optimization-based cooperative design method is proposed for such quasi-optical mixer that includes the on-chip antenna and heterodyne mixing circuit. A coplanar waveguide (CPW) tuning network is utilized and optimized to offset the parasitic reactance effect of Schottky diodes. Besides, a dual-band log-periodic-slot integrated lens antenna is presented for coupling the radio frequency and local oscillator radiations with high efficiency. A reversed field tracing method is proposed to acquire the position and size of the equivalent Gaussian waists at both frequencies, thus offering the guidance for optimizing the substrate thickness and lens dimensions for improved antenna gain and radiation pattern. A 220-GHz quasi-optical mixer prototype was fabricated and experimentally verified at both room and cryogenic temperatures. At 134 K, the single-sideband noise factor and the conversion loss are around 4.2-8.5 and 6-11 dB over a wide intermediate-frequency bandwidth of 0.5-20 GHz, showing the superior device performance at the comparable bands.