Triple-mode bandpass filters on stub-loaded resonator with novel I/O coupling scheme

In this paper, a novel I/O stage coupling scheme is presented for exploration of a triple-mode bandpass filter with improved out-of-band performance. A triple-mode stub-loaded resonator is first formed by tap-connecting a λ/2 resonator with a pair of open-ended stubs of less than λ/4 in length at its two symmetrical positions. Thus, three resonant modes are excited to make up a three-pole passband, above which a transmission zero is produced as well. Next, these two stubs are flared apart to create an additional pair of transmission zeros in the lower stopband and the U-shaped I/O coupled lines are applied to suppress the inherent spurious response of the stub-loaded resonator. As a result, a triple-mode bandpass filter, which is self-contained with two transmission zeros at both upper and lower stopbands, is constituted. Finally, the two proposed triple-mode bandpass filters are designed and fabricated to provide an experimental validation for the predicted performances.