Synthesis of diplexers based on the evaluation of suitable characteristic polynomials

This work describes a method for the synthesis of diplexers based on characteristic polynomials evaluation; the method is particularly suited when the separation between the two passbands is very small. In particular the proposed method allows the evaluation of the characteristic polynomials associated to the two filters composing the diplexer (the input node is assumed as a shunt connection of the two filters input, with the possible addition of a frequency-invariant susceptance of arbitrary value). The method offers a noticeable design flexibility as it allows the synthesis of the two composing filters independently of their connection to the duplexer; moreover the choice of the filters topology is absolutely arbitrary, being allowed general cross-coupled architectures (only the order of the two filters and the imposed transmission zeros are required). The method proposed has been verified through the design and fabrication of a diplexer for GSM base stations applications