Tunable phase shifters using composite inductive-capacitive loaded slow-wave transmission lines

This article presents the analysis and design of a continuously tunable phase shifter based on composite inductive/capacitive loaded slow-wave transmission lines. Slow-wave transmission lines are suitable candidates for the design of compact phase shifters due to their inherent miniaturized structures and highly dispersive characteristics. The design is started by considering the conventional capacitive loaded slow-wave transmission line, where the capacitors are replaced with varactor diodes for tuning the phase response. Then, more practically implementable phase shifter design is proposed by considering composite inductive/capacitive loaded slow-wave transmission lines. A systematic circuit model-based design method has been developed for the design of the tunable phase shifters using an accurate dispersion analysis. Finally, the efficiency of the design procedure is validated by fabricating and measuring a tunable phase shifter prototype using a four unit cell slow-wave transmission line. The measurements show a 121° phase tunability at 2 GHz design frequency with a maximum insertion loss (IL) of 2.1 dB, whereas a 140° phase tunability at 2 GHz can be achieved at a cost of a higher insertion loss around 2.7 dB.