Compact Balanced FSIR Bandpass Filter Modified for Enhancing Common-Mode Suppression

A novel balanced bandpass filter (BPF) based on folded stepped impedance resonators (FSIR´s) with modified ground plane is presented in this work. By symmetrically introducing a series-LC resonant structure below the FSIR´s, common-mode (CM) propagation can be rejected without affecting differential-mode (DM) performance. The filter presents two main advantages with respect to the conventional solid-ground-plane FSIR filter: i) an important improvement of the CM rejection level within the differential passband and ii) an enhanced filter selectivity due to the inclusion of an extra transmission zero in the differential passband. Both the conventional and the novel filters have been modeled as lumped-element circuits that fully account for DM and CM operation. Simulation and measurement results confirm the benefits of the proposed balanced BPF.