Design of multiple-passband filters using coupling matrix optimisation

This study presents the design of microwave filters based on the coupling matrix approach; determination of the matrix is based on a hybrid optimisation algorithm which may be applied to cross-coupled filters having diverse topologies. Various filter responses from dual-band to quad-band are given as examples of the approach. The optimisation is performed on the coupling matrix and a genetic algorithm (GA) is employed to generate initial values for the control variables for a subsequent local optimisation (sequential quadratic programming search). The novel cost function in this study measures the difference of the frequency locations of reflection and transmission zeros between the response produced by the coupling matrix and the ideal response. The ideal response in the form of characteristic polynomials is determined from the filter specifications and generated by a recently developed iterative technique which is capable of realising multi-band filters with different return loss levels. Convergence of the coupling matrix optimisation is fast, and no initial values for the control parameters are required by the GA. An eighth-order X-band dual-band waveguide filter with all-capacitive-coupling irises has been fabricated and measured to verify the design technique. Excellent agreement between simulation and experimental result has been achieved.