Tri-band filter with partly combined stepped-impedance resonators

A novel partly combined two half-wavelength SIR resonators (PCSIR) and its application to tri-band filter are presented in this paper. In the PCSIR, a part of one SIR is loaded in the center of the other SIR. The loaded SIR has the twice geometrical width as the other SIR, but with different length of its high impedance microstrip section. The proposed PCSIR is so constructed that the resonant frequencies can be characterized by controlling the length of the loaded SIR and conveniently designed by use of conventional SIR and excited simultaneously in one structure. Two kinds of PCSIR filter configurations with unfolded and folded formats are proposed and analyzed. The unfolded tri-band PCSIR filter at three resonances is characterized to control the FBWs (fractional bandwidth) by explicit J-inverters and susceptances slope. For the folded PCSIR filter, it has the advantage that the characteristics of coupling degree of the second passband can be conveniently controlled, whereas that of first and third passbands are fixed. It is found that the folded PCSIR filter structure can introduce transmission zeros between passbands and exhibit the tri-band response with high selectivity. To verify the proposed approach, two filters are designed and fabricated. The agreement between measured results and the theoretical prediction validates the proposed structure.