High-Q-Factor Dual-Band Bandpass Filter and Filtering Switch Using Stub-Loaded Coaxial Resonators

In this paper, a dual-band bandpass filter (BPF) and a dual-band filtering switch are designed based on high-Q-factor dual-mode stub-loaded coaxial resonators. A novel coaxial resonator is constructed in a 3-D cavity structure with two open studs and one centrally-loaded short stub. Resonant frequencies of the coaxial resonator are studied to obtain the desired passband frequencies. The required external quality factors and coupling coefficients of a dual-band BPF can be achieved by properly arranging the feeding probes and controlling the coupling windows between the two resonators. Accordingly, a high-Q-factor dual-band BPF with good filtering responses is realized. Moreover, a transmission zero can be generated to obtain high isolation between two passbands. Based on this dual-band combine resonator filter, two pieces of printed circuit boards mounted with PIN diodes are embedded into the cavity filter structure to switch on and off the two passbands, resulting in a dual-band filtering switch. The proposed dual-band BPF and filtering switch have the advantages of high Q factors, which are attractive in the multi-band wireless systems.