Design of Quasi-Elliptic Bandpass Filters Based on Short-Circuited Coupled-Line Stubs

This paper presents a new method to design quasi-elliptic bandpass filters (BPFs). A short-circuited coupled-line stub (SCCLS) is proposed to serve as a resonator of a BPF and also generate a pair of symmetrical transmission zeros (TZs) at the stopband frequencies. An equivalent circuit model of the SCCLS is derived to facilitate the design procedure. To demonstrate an example, a 3rd-order quasi-elliptic BPF is designed and fabricated. The implemented quasi-elliptic BPF is based on a coupled-resonator filer design, except a connected coupling mechanism is introduced to incorporate the SCCLS into a BPF. The measured results agree well with the simulations thus successfully verify the design concept.