Broadband microstrip bandpass filters using triple-mode resonator

A broadband microstrip bandpass filter using a stub-loaded resonator is proposed. The basic block of the proposed filter is a triple-mode resonator that consists of a half-wave resonator and a shunt open-circuited stub. A parameter study of how to control the frequencies of the three modes is presented. Two bandpass filters using the proposed resonator have been designed, fabricated and tested to validate the design concept. Design guidelines and a comparison with previous work are also included. For the first bandpass filter, from 1.75 to 3.25 GHz, the measured return loss is greater than 16 dB and the measured insertion loss is less than 0.9 dB. The maximum group delay variation is 0.6 ns. Furthermore, there are two transmission zeros at 1.3 and 3.8 GHz for a better cut-off response. For the second one, from 1.6 to 3.7 GHz, the measured return loss is greater than 14 dB and the measured insertion loss is less than 1 dB. The maximum group delay variation is 0.6 ns, and there are two transmission zeros at 1.36 and 4.03 GHz.