Dual-Bandpass Filters With Individually Controllable Passbands

A novel dual-band filter based on split-ring resonators (SRRs) and double-slit complementary SRRs is presented. The size of the filter is small and the two passbands can be individually designed. The basic cell of the filter is presented and analyzed, and then the multistage dual-passband filter is achieved by cascading basic cells. The design graphs for external quality factors of the resonators at input and output stages and the coupling coefficient between the adjacent resonators are constructed. The design graphs are utilized to determine the proper geometric parameters of each filter stage for a given filter specification. As an example, a prototype three-stage Chebyshev filter with a fractional bandwidth (FBW) of 2% at 0.9 GHz and a FBW of 3% at 1.3 GHz is demonstrated. The prototyped filter has an equal ripple of 0.4 dB at both passbands. The measurements of the prototyped filter agree well with the simulation results. The center frequencies and the FBWs of the two passbands of the prototyped filter can be individually designed with more flexibility compared to dual-band filters that utilize resonances of higher order modes. The overall size reduction of the proposed filter can be as high as by a factor of 3 compared to that of edge-coupled microstrip filters.