An improved matrix synthesis approach for inline filter is presented in this paper. Frequency-variant couplings (FVC) can generate and control multiple finite transmission zeros (TZs). As the resultant network only involves resonators cascaded one by one without any auxiliary elements (such as cross-coupled or extracted-pole structures), this paper provides the best optimizatised synthesis solution in configuration simplicity for narrowband filters based on genetic algorithm (GA) and solvopt optimization method. Compared with the conventional synthesis method for inline topology filters, the method presented in this paper has following advantages: First, it is unnecessary to consider both the couplings and capacitances of a traditional low-pass prototype. Second, there is no need to use similar transformation, and the adjacent FVCs can be implemented. Third, the approach presented can implement more TZs than the previous works. The maximum number of TZs can be as many as the filter order. Two examples with different topologies and specifications are synthesized to show the validation of the method presented in this paper.
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