I. Introduction
Electronically tunable/reconfigurable filters are one of the most essential microwave components for multiband communication systems due to their attractive features, i.e., miniaturizing the overall system size, relaxing the system complexity, and reducing the cost in fabrication. Microwave tunable filters have been studied for several decades, and, in general, they can be classified into three categories: 1) yittrium–iron–garnet (YIG) filters [1]; 2) varactor diode filters [2]–[4]; and 3) radio-frequency microelectromechanical system (RF-MEMS) filters [5], [6]. Although the YIG filters have large tuning range and small in-band insertion loss, their high power consumption, slow tuning rate, large size, and large weight block them from many applications in modern highly integrated communication systems. RF-MEMS filters utilize RF-MEMS capacitors with the advantages of high -factor at RF and low distortion levels. Varactor-diode filters are widely studied due to their small size, nanosecond tuning speed, and low cost.