Liquid crystal-based enclosed coplanar waveguide phase shifter for 54–66 GHz applications

Abstract

A 0-10V bias voltage driven liquid crystal (LC) based 0-180° continuously-variable phase shifter is designed, fabricated and measured with insertion loss less than -4dB across the spectrum from 54GHz to 66GHz. The phase shifter is structured in an enclosed coplanar waveguide (ECPW) with LC as tunable dielectrics encapsulated by a unified ground plate in the design, which significantly reduces the instability due to floating effects and losses due to stray modes. By competing spatial volume distribution of the millimetre-wave signal occupying lossy tunable dielectrics versus low-loss but non-tunable dielectrics, the ECPW’s geometry and materials are optimised to minimise the total of dielectric volumetric loss and metallic surface loss for a fixed phase-tuning range. The optimised LC-based ECPW is impedance matched with 1.85mm connectors by time domain reflectometry (TDR) method. Device fabrication features the use of rolled annealed copper foil of lowest surface roughness with nickel-free gold-plating of optimal thickness. Measured from 54GHz to 66GHz, the phase shifter prototype presents a tangible improvement in phase shift effectiveness and signal-to-noise ratio, while exhibiting lower insertion and return losses, more ease of control, high linearity as well as lower-cost fabrication as compared with up-to-date documentations targeting 60GHz applications.