Balanced Bandpass Filter Using Stub-Loaded Ring Resonator and Loaded Coupled Feed-Line

A balanced bandpass filter is proposed based on stub-loaded ring resonators (SLRRs) and loaded coupled feed-lines (CFLs) with high selectivity and common-mode (CM) rejection. By controlling the coupling coefficient of the CFLs, three transmission poles within the differential passband are achieved. The CFLs are loaded with open-and short-circuited stubs at the end of the traditional parallel-coupled feed-lines, which resulting in multiple transmission zeros (TZs) in differential mode (DM) and CM to improve the passband selectivity and CM suppression, simultaneously. Finally, the proposed filter is designed and fabricated, the simulated and measured results are in good agreement. The filter shows low insertion loss, multiple transmission zeros, high selectivity and enhanced CM rejection.