Coupled-Line Filters With Stub-Embedded Resonators Using Accurate Admittance-Transformer Feeds for Flexible Terminations

Accurate admittance-transformer (Y-TF) feeds along with short-circuited stub-embedded resonators (SCSERs) are proposed in this study to design the improved parallel coupled-line (PCL) filters with structure-inherent transmission zeros (TZs). The proposed Y-TF feeds for flexible terminations are thoroughly analyzed and found more accurate than conventional approximate tapped-line feedings. Specifically, two basic parallel-coupled bandpass filters with conventional λ/2 stepped-impedance resonators are first successfully implemented to validate the proposed Y-TF feedings. The ideal frequency responses of the two filters by a circuit simulator are compared with those transformed from the coupling matrix. Furthermore, an SCSER-based filter with multiple TZs is designed to verify the feasibility of the described design procedure. The adopted resonating components are composed of two short-circuited stepped-impedance stubs and multiple open-circuited ones all extended from a common joining point. The interstage couplings of the filter are achieved by the short-ended PCLs, while the input/output couplings are realized by the proposed Y-TF feedings. Through the fabrication of the proposed filters, the measured results along with the simulated ones validate the theoretical derivations.