Tunable Bandstop Filter Using Distributed Coupling Microstrip Resonators With Capacitive Terminal

This letter presents a varactor-based tunable bandstop filter (BSF) using distributed coupling microstrip resonators with the capacitive terminal. Each distributed coupling microstrip resonator consists of a coupling microstrip line, a varactor, and a capacitive terminal which is a floating pad connected to the cathode of the varactor. The capacitive terminal serves not only as a bias pad for the control voltage but also as an intrinsic capacitive path to ground. Compared to the conventional inductive terminal with the metallized via, the capacitive terminal can be beneficial to realize much higher frequency responses by taking advantage of the capacitance to ground from the floating pad. High stopband suppression level is achieved by cascading multiple distributed coupling microstrip resonators. For demonstration, a tunable BSF prototype with the frequency tuning range of 11.3-16.5 GHz (37) is designed and fabricated. Measured and simulated results agree well and validate the design principle.