Planar multiband bandpass filters are implemented based on the versatile multimode stepped-impedance resonators (SIRs). The resonant spectrum of a SIR can be calculated as functions of the length ratios for various impedance ratios of the high- and low-impedance sections. Thus, by properly selecting the geometric parameters and designing the input/output coupling structure, the SIRs are feasible to realize multiband multimode filters. Using a single SIR, a dual-mode dual-band, a dual-mode triple-band or a hybrid dual-/triple-mode dual-band bandpass filter can be realized. Emphasis is also placed on designing specified ratios of center frequencies and fractional bandwidths of the passbands. To extend the design flexibility, extra shunt open stubs are used to adjust the ratio of the passband frequencies. In addition, sharpness of the transition bands is improved by designing the input/output stages. Simulation results are validated by the measured responses of experimental circuits.
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