Design of Dual-Band Interdigital Bandpass Filters Using Both Series and Shunt Resonators

In this letter, we present a simple design method for dual-band bandpass filters (BPFs) with two independently controllable bandwidths. The proposed design is based on hybrid resonators which have both series and shunt resonances. For compact size, an interdigital BPF type is utilized. The simple design procedures are established based on an equivalent lumped circuit of the proposed dual-band BPF. Moreover, the proposed BPF has good skirt characteristics through transmission zeros created by the series resonance in the hybrid resonator and the cross-coupling. The third-order dual-band BPF is designed at center frequencies of 1.85 and 2.85 GHz with fractional bandwidths of 11% and 7.3%.