Coupling dispersion of parallel-coupled microstrip lines for dual-band filters with controllable fractional pass bandwidths

Dual-band bandpass filters with controllable fractional bandwidths (FBWs) are constructed by cascading the multiple λ/2 stepped-impedance resonators (SIRs) through the distributed parallel-coupled microstrip lines (PCMLs). By suitably choosing the aspect ratio of two strip widths or impedances in the SIR, the first two resonant frequencies are allocated to 2.4 and 5.2 GHz for dual-band filter application. These PCMLs with different overlapped lengths are investigated to show their distributed coupling performance in terms of explicit J-inverter susceptances. For the first time, it implies that the coupling degrees at two resonances can be adjusted with freedom to control the FBWs of these dual pass bandwidths. Then, the three one-stage dual-band SIR circuits are characterized to initially exhibit the dual-band performances in terms of S-parameters. In final, the three two-stage filters are optimally designed and fabricated to demonstrate in reality the dual-band performance with controllable FBW parameters.