Miniaturized Tri-Band Bandpass Filter Design Using Quarter-Wavelength Shunted-Line Stepped-Impedance Resonators (slsirs) with Multi-Transmission Zeros

In this paper, a miniaturized tri-band bandpass filter (BPF) is proposed with a pair of quarter-wavelength shunted-line stepped-impedance resonators (SLSIRs). The resonant characteristics, high design freedom and tuning relationship of multi-dimension parameters of SLSIR are studied and applied to the tri-band design. To obtain the great enhancement of skirt slope, multi-transmission zeros (multi-TZs) are introduced below and above the passbands and the selectivity is improved significantly. The miniaturization of subwavelength and electric/magnetic coupling are introduced and the propagation of electromagnetic wave is manipulated. Moreover, different kinds of generation mechanisms of the produced five TZs are investigated, illustrated and classified. Based on the multi-path transmission of signal, self-coupling effect and self-resonance of feed lines, the multi-TZs are excited and produced. The electric/magnetic coupling and current density distributions are plotted and analyzed. For validation, the designed second-order tri-band BPF centered at 2.35/4.89/7.42 GHz is fabricated and measured. Good agreement between the measured multi-band performances and the simulated frequency responses are observed and can be used to verify the feasibility of design concept.