Rethinking Liquid Crystal Tunable Phase Shifter Design with Inverted Microstrip Lines at 1–67 GHz by Dissipative Loss Analysis

Abstract

Growing 5G/6G phased-array beam-steering applications, for which liquid crystal (LC) is one of the enabling technology candidates, have sparked interest in the modulation of the phase (and amplitude) of microwave and millimeter-wave signals. In this communication, fresh insights into the systematic design analysis of a 1–67 GHz passive inverted microstrip line (IMSL) phase shifter filled with highly anisotropic LC as tunable dielectric media are obtained. Based on waveguide disturbance tests to characterize the dielectric properties of the non-tunable PCB and tunable LC used in the IMSL phase shift device filled with a GT3-24002 LC layer (125 µm thick) partially enclosing a 220 µm wide, 17 µm thick, 1.35 cm long copper core line, a 0–π differential phase shift in the 1–67 GHz range with less than 2 dB insertion loss is reported. Dissipative loss analysis shows that the dielectric absorption of the LC is 21.28% of the input signal power at 60 GHz. Further investigation is performed to quantify the impacts of dielectric substrate thicknesses (PCB and LC) on the wave-occupied volume ratio (and hence the phase tuning range), as well as on dissipative losses (including conductor loss and dielectric loss). Specifically, conductor loss is observed to follow a linear relationship with the reciprocal of the LC thickness.

Liquid Crystal-Filled 60 GHz Coaxially Structured Phase Shifter Design and Simulation with Enhanced Figure of Merit by Novel Permittivity-Dependent Impedance Matching

“…Similar equations have been reported extensively in a majority of papers [26,29] concerning LC-based mmW phase shifting devices, whereas very seldom does it fundamentally understand the quintessence underpinning the tuning problem, i.e., the distinction between the tuning range (tunability) of LC materials and the tuning range of the whole delay line (phase shifter) device. As such, the concept of wave-occupied volume ratio (colloquially referred to as mmW power concentration ratio) between tunable dielectrics and nontunable dielectrics was first theorized in our past works on multidielectric-encompassing delay line systems (e.g., inverted microstrip [29], floating-electrode-free coplanar [37], and enclosed coplanar [24]), which demystifies the distinction between a device's tuning range and a materials' tunability.…”

“…The nanoscopically molecular orientations of LCs can be macroscopically represented by directors [22,27] that are controllable by these external stimuli (electric, magnetic, temperature, or deformation fields). Fundamentally, their molecular shape anisotropy [24] and the resulting produced variable dipole moments can be exploited to develop a variable dielectric constant (dependent on the interacted field direction) for reconfigurable phase and wavefront steering applications (e.g., tunable filters [28], variable phase shifters [29], tunable antennas [30], and antenna arrays [31]). Arguably, the 5G/6G, IoT, vehicle, and satellite markets are becoming growth engines for the LC mmW industry.…”

Modeling 0.3 THz Coaxial Single-Mode Phase Shifter Designs in Liquid Crystals with Constitutive Loss Quantifications

“…At the millimeter-wave domain, our recent research efforts have introduced the first LC coaxial phase shifter at 60 GHz [25], representing a notable advancement. The work [25] also highlights the trade-offs in performance metrics compared to previously proposed planar transmission line structures such as the inverted microstrip [32] and enclosed coplanar waveguide [20] configurations. Despite the compromises in performance metrics, the fully enclosed and symmetric electromagnetic structure and the polyimide (PI)-free manufacturing process associated with the proposed LC-filled coaxial phase shifters [25] offer significant advantages over the traditional planar transmission lines as accommodating structures for LCs (requiring time-consuming and thermally stringent PI processing and a rubbing process [20] for mechanically aligning LC molecules).…”

“…In electromagnetic parlance, the coaxial structure radially filled with LCs features a single-dielectric encompassed device topology that eliminates the multi-dielectric competition and interface effects (coupling and radiation) as encountered by the planar solutions, hence the coaxial approach is less susceptible to undesirable higher-order modes. Compared with waveguides (single conductor) that require power-consuming magnets for biasing LC materials (e.g., >100 V [23]), the coaxial approach retains the advantage exhibited by the two-conductor system, i.e., enabling low-power electronic driving (e.g., 10 V) such as our previously demonstrated planar transmission line solutions [20,32] for accommodating LCs. The ease of noise-free unimodal operation, and the potential for PI-free rapid prototyping and mass production for arrayed devices on a panel, jointly position the coaxial approach as a compelling solution for the deployment of LC-based phase-reconfigurable components (mainly functioning by continuously varying the differential phase shifts between radiation elements) in 5G and future generation (6G) communication systems.…”

Impact of laser polarization and angle of incidence on phase shifting efficacy of an all-optically addressed liquid crystal delay line at X band

“…The demonstration in this work furthers the potential of the all-optical approach in holding the promise for various gamechanging applications (both terrestrial and space environments to be discussed in a future paper). The insertion loss dependency on polarizations and incident angles will be investigated and reported in our later papers, based on which we can conclude the overall performance comparison with other stimuli, e.g., applying a magnetic field [8], temperature field [9], as well as the mainstream electrical field biasing [10] [11]. Upscaling techniques from microwave to millimeterwave [12] and terahertz [13] will also be undertaken in future endeavors as inspired by the upcoming 6G [14] and intelligent reflective surfaces [15] that advance at a rapid pace.…”