Dual-Band Lowpass–Bandpass Filter With Three-Element Resonator Configuration

This article explores the synthesis and design of dual-band lowpass–bandpass microwave filters, specifically focusing on a filter configuration that incorporates a three-element resonator. To synthesize a general ladder network, we begin by deriving analytic expressions of its characteristic function, which allow for readily finding the function coefficients from predetermined transmission zeros. We systematically study the general ladder networks and introduce a novel matrix transform method tailored to meet the high precision demands of designing high-order filters. Although this method precisely extracts circuit parameters, it could produce negative-value elements for which no straightforward physical realization currently exists. To address this issue, we shift our focus to the ladder network with a three-element resonator configuration. We formulate an objective function based on the theoretical characteristic function derived during the synthesis phase of the general ladder networks. The circuit synthesis is performed through a numerical optimization method, thereby circumventing the issue of negative-value elements, resulting in circuit parameters that have scattering parameters close to those from the objective ones. This lumped prototype circuit can be readily converted into a distributed circuit with a physically realizable structure, making it an ideal choice for dual-band lowpass–bandpass filters. We confirm the efficacy of our design through experimental validation of a fabricated filter, which operates effectively up to 2700 MHz. This validation verifies the performance of the prototype filter and demonstrates its potential use for the miniaturization of dual-band lowpass–bandpass microwave filters.