This paper presents a direct matrix synthesis for in-line diplexers constructed by general Chebyshev channel filters. The finite transmission zeros of the channel filters are generated and independently controlled by a set of frequency-variant couplings (FVC) sections. The network only involves resonators cascaded one by one without any auxiliary elements (such as cross-coupled or extracted-pole structures), and this paper provides the best synthesis solution in configuration simplicity for narrowband contiguous diplexers. For the channel filters, considering both the couplings and capacitances matrices of a traditional low-pass prototype, a generalized transformation on the admittance matrix is introduced as the basis of the synthesis, which allows more than one cross-coupling to be annihilated in a single step, while generating an FVC section simultaneously. Two examples of diplexer are synthesized to show the validation of the method presented in this paper.
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