Tri-band filter using combined E-type resonators

A compact microstrip tri-band bandpass filter using novel combined E-type resonators (CETRs) is presented. The CETR consists of a quarter-wavelength (λ/4) stepped-impedance resonator (SIR) and a quarter-wavelength (λ/4) unit impedance resonator through a via-hole. These two components are relatively independent. The first and the third passband centre frequencies can be flexibly controlled by tuning the impedance ratio R_Z of the SIR, whereas the second passband frequency is fixed. The centre frequency of the second passband can be independently controlled by tuning the lengths of L₂. The filter is designed to operate at 1.95, 5.2, and 3.5 GHz. The simulated and measured results show good agreement.