Electrically Tunable Phase Shifter with Fast Response and Improved Phase Shifting Capability per Wavelength Based on Liquid Crystal

In this paper, an electrically tunable phase shifter (PS) with fast response and improved phase shifting capability is proposed based on liquid crystal (LC). In the LCPS, an opposed planar waveguide (OPW) is proposed as the main transmission line structure, which maintains the quasi-TEM operating mode of the coplanar waveguide (CPW). In addition, the staggered periodic stubs are stretched out from both sides of OPW ground. Compared to CPW-based LCPS, the OPW-based one avoids the metalized via and thus is easier to fabricate and miniaturize. The overlapping area in the vertical projection direction between the centerline and the loading stubs can be equated to variable capacitances. When a bias voltage is applied to the centerline and ground, the LC relative dielectric constant changes, leading to a variation in the variable capacitance. This, in turn, alters the phase delay of the phase shifter, ultimately resulting in a phase shift. Besides, to apply different DC bias voltages, a DC bias network is designed. A prototype is fabricated using LC display technology and measured using an Agilent vector network analyzer. The measured results indicate that the proposed LCPS realizes a 300° phase shifting angle, improved $289^{\circ} / \lambda 0$ phase shifting per wavelength, and figure-of-merit of $59^{\circ} / \text{dB}$ at 14.5 GHz. The reflection coefficient remains below -10 dB and the transmission coefficient above -6 dB from 9.3 GHz to 15 GHz.