Superconducting 4-8 GHz Hybrid Assembly for 2SB Cryogenic THz Receivers

Paper in proceeding, 2013

In the 2SB receivers, the rejection ratio is determined, among other factors, by the performance of the quadrature hybrids. We present here the design and characterization of an intermediate frequency (IF) assembly comprising a compact 90° hybrid chip (coupled line coupler – Lange coupler– coupled line coupler), two bias-T circuits in order to bias the Superconductor-Insulator-Superconductor (SIS) mixers. Specifically, the miniaturized 3-section hybrid chip made using thin-film technology utilizes superconducting Niobium (Nb) transmission lines and air bridges to connect the fingers of the Lange coupler (middle section). The assembly is complemented with two bias-T circuits with integrated MIM capacitors. The assembly was designed to have the amplitude and phase imbalance better than 0.6 dB and ±2.5° respectively. Experimental verification of the assembly at 4 K shows good agreement between the measurements and simulations with amplitude imbalance of 0.5 dB and maximum phase imbalance of ±2°.