Compact Multi-Band Differential Bandpass Filters Using Microstrip Multi-mode Resonators

Three types of miniaturized multi-band differential bandpass filters (BPFs) using microstrip multi-mode resonators are presented in this paper. The first one is a dual-band differential BPF using four-mode stepped-impedance square ring loaded resonators (SI-SRLRs) with a controllable frequency separation between its common-mode (CM) and differential-mode (DM). Next, by adding two additional open-circuited stubs to the square ring of the SI-SRLR, a new six-mode SI-SRLR is then obtained. By using two coupled six-mode resonators, a tri-band differential BPF is designed. Finally, to reduce the insertion loss due to the metal loss, high-temperature superconducting (HTS) technology is applied to develop a fourth-order HTS dual-band differential BPF using modified SRLRs. All these BPFs shown compact size and favorable performance.