Tri-band filter with multiple transmission zeros and controllable bandwidths

This paper presents a compact microstrip tri-band bandpass filter (BPF) using two multimode stepped-impedance resonators (SIRs) with a 0° feed structure. The fundamental odd-mode and even-mode resonant frequencies and the third resonant frequency are utilized to determine the center frequencies of the tri-band filter. The mode-splitting technique is used by combining two SIRs with electrical coupling. Therefore, two modes generate one passband and the bandwidths can be controlled by the electrical coupling strength. The 0° feed network is applied to create one pair of transmission zeros at each side of all the triple passbands, resulting in high selectivity. Finally, a tri-band BPF with the central frequencies of 1.8, 2.4, and 5.8 GHz, and respective fractional bandwidths of 8.9, 12.5, and 5.3% are designed and fabricated. The simulated and measured results show a good agreement.