Dual-Mode Frequency Tunable Planar Filter Design with Capacitive Coupling Technique

Abstract

This paper proposed and demonstrated a novel dual-mode frequency tunable band-pass filter (BPF) with capacitive coupling. The proposed three-pole microstrip-type combline BPF can tune both a center frequency and an operating frequency bandwidth with varactors. The varactors connected at the edge of the resonators and between the resonators tune the center frequency and the operating bandwidth, respectively. The varactors between the resonators make strong capacitive coupling, resulting in less magnetic coupling. The dual-mode tunable BPF also realizes high out-of-band signal suppression due to the capacitive coupling and the optimum varactor location connected at the resonator. The proposed configuration effectively realizes high level stopband rejection and wide range tunings. The fabricated tunable BPF achieved 26% tuning range, 8-16% variable bandwidth, and a maximum loss of 4.6dB. The stopband rejection range more than 40dB is 134%. The proposed three-pole dual-mode tunable BPF is the top-level performances in planar type tunable filters.