Synthesis of wideband bandpass filters using a hybrid ring shunts with an open-circuited stub

Wideband bandpass filters using a hybrid ring with a shunt open-circuited stub is proposed and developed. The impedances of the hybrid ring and open-circuited stub determine the location of the transmission zero pair in the bandpass domain. This controllable transmission zero pair can be designed to flatten the group delay or improve the selectivity of the filter. Two experimental filters using the same circuit topology are realized to verify the improvement of group delay flatness and selectivity respectively. These two design examples are implemented by balanced microstrip line on a 20-mil RO4003 substrate. The measurement results show excellent agreements with the electromagnetic simulation.