Research On The Design Method Of Tunable Liquid Crystal Phase Shifters With Filtering Functions

As a novel type of tunable material,liquid crystal(LC)material has been introduced into the microwave and millimeter wave fields from the optical field in recent years.Due to its excellent dielectric properties,continuous tuning and high linearity,tunable microwave devices have been further developed.Tunable phase shifters are one of the important microwave devices,which are widely used in phased array systems,beamforming networks,circularly polarized antennas,new smart antennas,and phase modulation communication systems.Tunable liquid crystal phase shifters(LCPSs)formed by combining the LC material and the tunable phase shifter technology have the advantages of compact structure,simple design,and low cost.However,the tunable phase shifters based on the LC technology also have many challenges.In this dissertation,theoretical solutions and design methods are proposed to solve the problem of low phase shift tuning rate and single function in LCPS fields.These works lay a foundation for the engineering application of liquid crystal phase shifters with filtering functions.Firstly,in view of the low figure of merit(Fo M)of tunable LCPSs,the LCPS design method based on the spoof surface plasmon polariton(SSPP)technology is proposed by analyzing the working mechanism of the LCPS and the propagation characteristics of the SSPP mode.The slow-wave transmission theory and equivalent circuit model are utilized to verified the method theoretically.On the basis,the slot-line LCPS and inverted microstrip line(IMSL)LCPS based on the SSPP mode are designed,respectively.Compared with the traditional LCPS based on slot-line and IMSL structure,the Fo M can be are effectively improved.Secondly,the design method of tunable LCPSs with the band-stop filtering function is proposed to solve the problem that reported LCPSs only regulate the phase and realize a single function.The self-embedded band-stop filtering functions and pass-band phase shifts can be simultaneously achieved by introducing the magnetic coupling between resonant structures and LC-based transmission lines.Band-stop filtering LCPSs based on split-ring resonators(SRRs)and hair-pin resonators are designed to verify the proposed design method and theoretical analysis,respectively.Thirdly,in order to achieve the design of band-pass filtering LCPSs,the equivalent circuit model of the LC-based transmission line combined of SRRs and periodic short circuit branches is established and the transmission characteristics are analyzed by the composite right/left handed transmission line theory.According to the above analysis,the narrowband band-pass filtering LCPS is proposed.On the basis,the balance characteristic of composite right/left handed transmission lines are utilized to enhance the bandwidth of the band-pass filtering LCPS and design the wideband band-pass filtering LCPS,which further improves the practical application value.Lastly,the design method of tunable balanced LCPS with the common-mode(CM)filtering function is proposed,which can effectively suppression the CM noise interference for balanced RF circuit systems.The equivalent circuit model and the odd/even-mode analysis method are carried out to comprehensively declare the CM suppression mechanism of the tunable balanced LCPS.More importantly,the slow-wave characteristic of differential mode is firstly employed in the design of tunable balanced LCPSs.To further realize the compact structure and miniaturized design,the defected ground structure(DGS)is utilized to realized the CM suppression.Simulation and measurement results verify the effectiveness of the proposed method.To sum up,the research content of this dissertation aims at the problems encountered in the practical application of tunable LCPSs.Based on the theoretical research,combined with full wave simulation and experimental verification,comprehensive scientific research is carried out,which provides theoretical basis and research value for the research and potential application of tunable LCPSs.