A Direct Matrix Synthesis Approach for High Selectivity In-Line Topology Dual-Band Filters

In this paper, a direct matrix approach is proposed for the first time to synthesize a coupling matrix for a frequency-variant coupled dual-pass band filter based on two low-pass prototype functions, which achieves a highly selective in-line topology with adjacent frequency-variant coupling in addition to realizing different passbands with different orders, return losses and bandwidths. An N-order dual passband filter can generate up to N-1 transmission zeros. Ultimately, a fifth-order in-line dual-pass band filter is synthesized by this method, which illustrates the effectiveness of the method.