This paper presents a method for synthesizing coupled resonator diplexers composed of TX and RX filters (two types of junctions connecting the TX and RX filters are considered). For the first junction type, the common port is directly coupled to the first resonator of the TX and RX filters, respectively. For the second junction type, common node is realized by adding an extra resonator besides those of the TX and RX filters. The method is based on the evaluation of the characteristic polynomials of the diplexer using the proposed linear frequency transformation and well-established method, and then the "N + 3" coupling matrix of overall diplexer can be obtained using hybrid optimization methods. Two diplexers have been designed and fabricated to validate the proposed method.
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