Wideband Tri-band Filter Design using Stepped-impedance Stub Resonators

This letter presents a wideband tri-passband filter in which tri-section stepped-impedance shunt-stubs are used as tri-mode resonators, connected by transmission lines. Developed from the topology of quarter-wavelength stub filters, these filters have much wider passbands compared to many other designs published. The stepped-impedance stubs also contribute transmission zeros between the passbands, thus increasing the isolation. A prototype filter with passbands located at 0.6-1, 1.5-1.8 and 2.2-2.4 GHz is designed by an optimization method. Good agreement between simulation and measurements in respect of low insertion loss and high isolation is demonstrated.