Novel Tri-Band Bandpass Filter with High Selectivity

This paper presents a tri-band bandpass filter (BPF) consisting of a wide BPF and two narrow bandstop filters (BSFs). BPF is a ninth-order interdigital structure, and BSFs are designed fifth-order series resonators with quarter wavelength transmission line connected. The selectivity of BPF and the bandwidth of BSF are analyzed using simulation software. A tri-band BPF, made of metal cavities, is designed and measured. Three passbands are located in 1.332 similar to 1.401 GHz, 1.443 similar to 1.458 GHz, 1.506 similar to 1.660 GHz, and insertion losses in passbands are less than 2.1 dB. Ratios of Delta f(40dB)/Delta f(3dB) are 1.9, 2.67 and 1.33, respectively, and isolation between passbands is more than 75 dB. The good agreement between the simulated and measured results validated the proposed structure.