A 400–500 GHz Balanced SIS Mixer with a Waveguide Quadrature Hybrid Coupler

Abstract

We have developed a 400-500 GHz low-noise balanced SIS (Superconductor Insulator Superconductor) mixer, which is based on a waveguide RF quadrature hybrid coupler. The RF quadrature hybrid was designed and fabricated as a broadband hybrid with good performance at 4 K. The fabricated RF quadrature hybrid was measured at room temperature with a submillimeter vector network analyzer to check amplitude and phase imbalance between two output ports. Then the balanced mixer was assembled with the RF hybrid, two DSB mixers, and a 180 IF hybrid. Several important parameters such as noise temperature, LO power reduction, and IF spectra were measured. The LO power reduction is defined as how much LO power the balanced mixer saves compared with a typical single-ended mixer. The measured noise temperature of the balanced mixer was similar to 55 K at the band center which corresponds to similar to 3 times the quantum noise limit (hf/k) in DSB, and similar to 120 K at the band edges. The noise performance over LO frequency was almost the same as that of the worse DSB mixer used in the balanced mixer. In addition the LO power required for the balanced mixer is similar to 11 dB less than that of the single-ended mixers.