Liquid Crystal-Based Enclosed Coplanar Waveguide Phase Shifter for 54–66 GHz Applications

Jinfeng Li,Daping Chu

doi:10.3390/cryst9120650

Abstract

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

Highlights

Millimeter-wave phase shifters are of an ever-increasing research and development interest as vital components in phased array electronic beam steering systems, targeted 60 GHz high-data rate wireless communications [1], intersatellite communications, high-precision radar-based autonomous driving, hand-gesture sensing [2], and other forward-thinking applications
The variable polarization allows a continuous tuning of the dielectric constant and, a differential phase shift when interacting with a millimeter-wave signal propagating over the delay line
Differential phase shifts as a function of frequency and bias voltages were measured for devices of the first round and the second iteration designs

Summary

Introduction

Millimeter-wave phase shifters are of an ever-increasing research and development interest as vital components in phased array electronic beam steering systems, targeted 60 GHz high-data rate wireless communications [1], intersatellite communications, high-precision radar-based autonomous driving, hand-gesture sensing [2], and other forward-thinking applications. Existing microwave switchable techniques (e.g., radio frequency micro-electro-mechanical system (RF MEMS) and solid state p-i-n diodes) offer ultrafast but resolution-limited phase modulation due to their binary nature [3]. Continuous phase shifting can be realized by putting continuously tunable dielectrics in a guided structure to vary the wave speed. Nematic liquid crystals (LC) have been demonstrated with decent dielectric tunability and low dissipation factor at millimeter-wavebands [4,5]. The dielectric tunability of LC is attributed to its molecular shape anisotropy and, the dipole moment variation from the perpendicular state (⊥) to the parallel state (||) with an applied low-frequency bias field from below the Fredericks transition voltage to a saturated one. The variable polarization allows a continuous tuning of the dielectric constant and, a differential phase shift when interacting with a millimeter-wave signal propagating over the delay line

Methods

Results

Discussion

Conclusion