Design of Microstrip Cross-Coupled Bandpass Filter With Multiple Independent Designable Transmission Zeros Using Branch-Line Resonators

This letter proposes a novel branch-line resonator to design a fourth-order cross-coupled bandpass filter (BPF). Theoretically, the proposed branch-line resonator plays two crucial roles. First, the arbitrary number transmission zeros can be designed in the stopband of a fourth-order cross-coupled BPF. Second, the required external quality factor can also be designed without an impedance transformer. This study examines a novel fourth-order cross-coupled BPF with a single pair of transmission zeros produced by a cross-coupled mechanism near the passband and five independent designable transmission zeros created by the proposed resonator in the stopband.