Balanced bandpass filter with high common-mode suppression based on dual-mode substrate integrated waveguide

In this paper, a balanced bandpass filter (BPF) based on dual-mode substrate integrated waveguide (SIW) is proposed. The degenerate dual-mode characteristic in the SIW cavity is utilized to construct a high-order filtering response, contributing to miniaturization and the improvement of frequency selectivity. Meanwhile, benefiting from the balanced feeding structure and the symmetry of the SIW, the balanced BPF achieves excellent differential-mode (DM) transmission while suppressing the excitation of common-mode (CM) signals. For the purpose of demonstrating the theoretical design, the proposed balanced BPF with a center frequency of 8.23 GHz was fabricated and measured. The simulated and measured results are in good agreement. The design realizes a filtering response with two transmission zeros, and its CM suppression is higher than 41 dB over an ultrawide frequency range from 6 GHz to 12 GHz.