Tunable balanced liquid crystal phase shifter based on spoof surface plasmon polaritons with common-mode suppression

In this paper, a novel design approach for microwave tunable balanced phase shifters with the common-mode (CM) suppression is proposed based on the liquid crystal (LC) technology. The proposed tunable balanced liquid crystal phase shifter (LCPS) is designed by the LC-based microstrip differential line with periodical T-shaped stubs, which supports the spoof surface plasmon polaritons (SSPP) mode. Thanks to the slow-wave characteristic induced by the SSPP mode, both tunable phase shifts and figure of merit (FoM) of the balanced LCPS are effectively enhanced. In addition, the defected ground structure based on slotted dumbbell-shaped resonators is also employed in this design and the CM suppression is integrated into tunable balanced phase shifters for the first time. Finally, the corresponding prototype is fabricated and tested to verify the above theoretical analysis. The measured results demonstrate that this proposed tunable balanced LCPS can simultaneously achieve tunable phase shifts with FoM of more than 12.4°/dB and the CM noise suppression of more than 17 dB attenuation from 2.5 to 5.5 GHz.