High selectivity and common-mode suppression combline balanced BPF

In this study, new balanced microstrip bandpass filter (BPF) based on stepped-impedance combline structure is proposed with high selectivity and high common-mode (CM) suppression performance In the beginning, a single-ended compact stepped-impedance resonator (SIR) combline BPF with enhanced performance including low insertion loss, sharp skirt and wide stopband response is designed by consisting of an array of short quarter-wavelength bi-section SIRs with a new scheme of tapped-transformer coupling at input and output ports. Under CM excitation, no special consideration or design procedure is required to suppress the CM, and the CM suppression is the nature of the parallel-coupled filter. Finally, the second- and third-order SIR combline balanced BPFs for WLAN application, without loss of generality, have been designed and implemented. The simulated and measured results show a good agreement and validate the proposed theory.