Planar multimode resonator bandpass filters with sharp transition and wide stopband

Planar bandpass filters are contrived with a very sharp transition band and wide stopband based on multimode stepped-impedance resonators (SIRs). The SIR is tapped with two open stubs at its center and two more stubs at the junctions of the low- and high-impedance sections. By properly trimming the stub lengths, the leading five resonances of the SIR have a close proximity and constitute the passband. Besides, two transmission zeros can be created at both upper and lower sides of the passband. It is believed that the SIR has the most resonant modes in comparison with those in open literature. Based on the multimode resonator, a single-resonator bandpass filter with five transmission poles is designed to have a fractional bandwidth of 35%. In addition, by cascading two such resonators, a tenth-order filter is designed, fabricated and measured. Good agreement between simulation and measured results is obtained.