Differential Tri-Band Bandpass Filter Based on Tri-section Stepped Impedance Resonator

A differential tri-band bandpass filter based on trisection stepped impedance resonators (TSSIRs) is proposed in this letter. Three desirable operating passbands can be allocated by adjusting the impedance ratios of the TSSIRs. The proposed filter shows simple and effective design process. In addition, insertion loss of proposed differential tri-band bandpass filter will be extremely low and the passband symmetry is very good, which is owing to the inherent characteristics of the conventional optimum bandpass filter. To verify the design concept, a differential tri-band filter with three differential-mode passbands centered at 1.35/4.5/7.6 GHz is fabricated and measured. The minimum insertion losses of the passbands are 0.15/0.5/1.2 dB. Good match between the simulated and measured results is realized in this design, which indicates the design approach is practicable.