Control of Transmission Zero by Mixed-Coupling in a Two-Stage Coupled-Resonator Filter

A transmission zero is generated near the passband for the filter consisting of two coupled resonators. The transmission zero can be placed in upper or lower rejection bands by designing the electric and magnetic components of the inter-resonator coupling. An equivalent circuit is developed for explaining the creation of zero and the control of filter bandwidth. Y- matrix parameters of the proposed equivalent circuit are formulated. The electric and magnetic components of the coupling associated with parallel-coupled half-wavelength microstrip resonators are extracted for the purpose of filter synthesis. For validating the theory, two experimental filters based on microstrip resonators with mixed-coupling structures are designed and measured.