Advanced Direct Synthesis Approach for High Selectivity In-Line Topology Filters Comprising $N$ - 1 Adjacent Frequency-Variant Couplings

In this paper, a direct synthesis approach is presented to realize high selectivity in-line topology filters with adjacent frequency-variant couplings (FVCs). By considering the annihilation of frequency-variant elements during admittance matrix transformation for the first time, this paper provides a deterministic mechanism (no optimization is involved) that can generate FVCs between every two cascaded resonators. In consequence, a high selectivity filtering response where N-1 transmission zeros are implemented and independently controllable can be achieved for an Nth-order in-line network. As the foundation, a novel matrix process is detailed to obtain two adjacent FVCs inside of a 4th-order in-line network. The Nth-order prototype is then realized based on the process in an iterative manner. A synthesis example is illustrated in terms of the proposed approach step by step to show validity. Eventually, a 6th-order in-line band-pass filter, which contains adjacent FVCs in two pairs has been designed and fabricated via coaxial cavity structures. The synthesis results, EM simulation results, and tested results are well matched with each other, which reveals the effectiveness of the presented method during physical implementation.