Design of liquid crystal based coplanar waveguide tunable phase shifter with no floating electrodes for 60–90 GHz applications

Abstract

A continuously tunable millimeter wave (mmwave) phase shifter for 60-90 GHz applications was proposed using a coplanar waveguide (CPW) structure without the use of a floating electrode (FE). In contrast to conventional CPW-FE structures, the proposed FE-free CPW device can be modulated by the nematic liquid crystal (LC) materials confined in two symmetric feeding channels. The nearly true-TEM nature of this CPW design enables wideband and low-loss operations, particularly in high frequencies up to 90 GHz. In order to optimize between high tunability and low loss, the aspect ratio of the CPW structure was optimized to maximize the defined Figure-of-Merit (FoM). By taking into account different loss mechanisms in the designed structure and the effect of LC orientations, the driving-voltage dependent impedance matching was examined to minimize the return and insertion losses. As an example, the design of a phase shifter aimed to operate at 79 GHz with low bias voltages (0-10 V) is presented, showing a wide phase shift range of 0-408° and a low insertion loss from-6.15dB to-4.56dB. The corresponding FoM is 66.3°/dB, which is expected to outperform over other LC-based phase shifters as reported within the targeted frequency range of 60-90 GHz.

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

“…This work contributed a passive millimeter-wave broadband (54-66 GHz) analogue phase shifter implemented in a liquid crystal-based enclosed coplanar waveguide (ECPW) structure which screened out interference and stray modes, allowing resonance-free quasi-TEM wave propagation from 54 GHz to 66 GHz. The structure not only retained the true-TEM advantage of the FE-free CPW we initially designed [3], but also achieved a high tuning range based on modulating a planar LC layer on top of the CPW's core line in addition to the two CPW slots. More fundamentally, the continuous sidewalls of the enclosed cavity served as a unified grounding path to substantially address the stray modes and energy leakage problems posed by conventional CPW with floating electrodes.…”

“…The phase shift-to-loss optimization was performed in recognition of this nonlinearity. According to our previous work on loss derivation [3], the peak phasor forms of the LC volumetric loss per unit length at the 0 V biasing state, and the total metal core line loss (thickness T strip considered) per unit length are given by Equations (4) and (5), respectively.…”

“…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.…”

“…more stringent requirement regarding phase shifting range and resolution, insertion loss, operating bandwidth, linearity, signal-to-noise figure, power consumption, response time, robustness, size, large-scale integration, manufacturing tolerance, and cost, etc. We have explored the use of LC as low-loss tunable media for passive microwave analog components [11,12] and extended our phase shifter design into 60-90 GHz [3]. Different approaches of electrically-tunable device configurations combining with LC were studied for the millimeter-wavelength range, among which coplanar waveguide (CPW) was desirable as compared with other transmission lines [5,13,14] or waveguide structures [15][16][17][18] in terms of overall performance, ease of control, and circuit size.…”

“…Phase modulation of the FE-free CPW is entirely based on the two vertical slots instead of a planar cavity. We did a pilot study [3] on the tuning range limit of the FE-free CPW structure parameterized with geometrical aspect ratios and pointed out that such thin-Cu patterned structure suffers from low wave-occupied volume ratio of tunable LC to nontunable substrates, while a countermeasure we proposed using thick-Cu patterning and surface anchoring inside a deep channel challenged the status of precision manufacturing and LC alignment technology. Instead, the CPW-FE structure with a floating electrode on top [20][21][22][23] modulated the LC predominately at a planar cavity with the wave largely occupying tunable dielectrics, and, hence, reasonably higher tuning ranges and phase shifts were reported at microwave frequencies.…”