Integrated Dual-Band BPF and Single-Band BSF for Tri-Band Filter Design

Abstract

This paper presents a tri-band bandpass filter in which a dual-band bandpass filter (BPF) and a single-band bandstop filter (BSF) are integrated into a single circuit of short length. The dual-band BPF contains several two-section stepped-impedance stubs which are mounted onto a main transmission line. The BSFs are constructed using capacitor-loaded shunt stubs which are connected by transmission lines. After circuit embedding, one wide passband of the dual-band BPF is converted into two passbands by the stopband of the BSF, resulting in a total of three passbands. A prototype tri-band filter is designed according to prescribed specifications: the required passbands are 1–1.4, 1.8–2.0 and 2.42–2.66 GHz with return loss above 10 dB, while the expected isolations between bands are above 40 dB. Optimization method is adopted for extracting the circuit parameters. The simulated and measured results are compared, and good features such as low insertion loss and high isolation are demonstrated.