Low-loss tunable dielectrics for millimeter-wave phase shifter: from material modelling to device prototyping

Jinfeng Li

doi:10.1088/1757-899x/892/1/012057

Abstract

Passively controllable millimeter-wave phase shifters based on continuously tunable dielectrics offer promising alternatives to conventional switch-based topologies (e.g. active semiconductor devices and MEMS) with digital resolution. However, only a handful of tunable dielectrics have thus far been identified with high tunability and low loss, among them, liquid crystal (LC) in the nematic phase is of research and development interest. This work presents the modelling of LC materials at 66 GHz based on a novel shielded coplanar device structure in place of traditional waveguides. Phase shifters of 0-π tunability are fabricated and measured, with an insertion loss of -4 dB and return loss lower than -15 dB demonstrated.

Highlights

Liquid crystal (LC) in the nematic phase has been reported extensively in electronically tuned microwave devices [1] that are key components of a phased antenna array system
Only a handful of tunable dielectrics have far been identified with high tunability and low loss, among them, liquid crystal (LC) in the nematic phase is of research and development interest
Working as a reconfigurable tuning dielectric material based on molecular shape anisotropy [2], LC offers highly attractive properties [3] over competing technologies, e.g. appreciable tunability [4], low polarisation loss [5], ease of control [6], transparency, and possible integration with printed and flexible circuit technologies

Summary

Introduction

Liquid crystal (LC) in the nematic phase has been reported extensively in electronically tuned microwave devices (e.g. resonators, filters, antennas, frequency-selective surfaces, and phase shifters) [1] that are key components of a phased antenna array system. This work proposes a novel planar device configuration (Figure 3) realised in shielded coplanar waveguide (SCPW) for 66 GHz signal and investigates how to optimally combine the structure with LC as tunable dielectrics for a phase-shifting device. With all the ground planes physically and electrically bonded, the top metal enclosure provides mechanical strength and serves as a heat sink for high-power applications. In a summary, this via-free and bond wire-free SCPW topology including an enclosed ground plate provides a low-loss and low-cost device-on-substrate solution for millimeter-wave devices.

Surface-anchoring alignment direction for maximum tunability

LC director calculations based on finite element simulations

Conclusion