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

A continuously tunable millimeter wave (mm-wave) 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 make it possible to outperform over other LC-based phase shifters as reported within the targeted frequency range of 60-90 GHz.