Compact quasi-elliptic filter using mixed EM coupling λ/4 stepped-impedance resonators

A compact third-order bandpass filter with quasi-elliptic function response is synthesized with λ/4 stepped-impedance resonators (SIRs). The filter has enhanced transition response since two transmission zeros are generated on both sides of the passband. Existence of these two transmission zeros is investigated by the mixed electric and magnetic (EM) coupling. Strong and controllable electric and magnetic couplings are realizing by the open-end and short-end coupling sections, respectively. This property leads to creation transmission zero very close to passband. Simulation and measurement results show good performances of the proposed filter such as low insertion loss, high rejection level and compact size.