Compact triple-mode microstrip bandpass filter

In this paper, a novel I/O stage coupling scheme is presented for exploration of a microstrip triple-mode bandpass filter with improved out-of-band rejection performance. Firstly, a modified microstrip triple-mode stub-loaded resonator is formed by tap-connecting a λ/2 microstrip line resonator with a pair of open-circuited stubs of less than λ/4 in length at its two symmetrical positions. As such, three resonant modes are properly excited to make up a three-pole passband, above which a transmission zero attributed to the virtual ground of open-circuited stub is produced as well. Next, these two stubs are flared apart not only to avoid harmful parasitic coupling, but also to create an additional pair of transmission zeros in the lower stopband. As a result, a triple-mode bandpass filter, which is self-contained with two transmission zeros at both upper and lower stopbands, is theoretically constituted. Finally, the proposed microstrip triple-mode bandpass filter is designed and fabricated to provide an experimental validation for the predicted frequency responses.