High Selectivity In-Line Topology Dual-Band Filters Based on Direct Synthesis Approach
A new class of dual-band filters that possess an in-line topology is presented in this paper. As each resonator is directly connected one after another, the determined network allows the two sub-filters to be separately implemented in two in-line blocks, which can provide favorable flexibility during the design and tuning process, especially for the asymmetric response cases. Moreover, high selectivity is achieved by utilizing frequency-variant couplings, whose design procedure is based on a direct approach. Note that, the transmission zeros can be generated and controlled individually. The validity of this work is demonstrated by two synthesis examples, where both symmetric and asymmetric response requirements are considered.