Dimensional Synthesis of Coupled-Resonator Pseudoelliptic Microwave Bandpass Filters With Constant and Dispersive Couplings

In this paper, we propose a novel technique for the dimensional synthesis of coupled-resonator pseudoelliptic microwave filters with constant and dispersive couplings. The proposed technique is based on numerical simulations of small structures, involving up to two adjacent resonators, and it accounts for a loading effect from other resonators by replacing them with terminations coupled through appropriately scaled inverters. The dimensions of the resonators and coupling elements are determined by an optimization process that matches the zeros and poles of the simulated response of a low-order circuit consisting of one or two coupled resonators connected to a source and to matched loads with the eigenvalues of a submatrix obtained from the coupling matrix of the lumped-element model of the filter. To assure high accuracy and rapid convergence, a zero-pole optimization algorithm was used. The application of this method is illustrated with four examples involving a simple fourth-order open-loop microstrip filter with two transmission zeros, through a triplet with one transmission zero, a quadruplet with two asymmetric transmission zeros located on both sides of the passband, to a fourth-order waveguide filter with three transmission zeros. The validity of the method is confirmed by an experiment involving a filter with dispersive coupling realized in substrate-integrated waveguide technology.