Application of the high-order S21 fitting strategy in coupling-matrix-extraction methods
Fitting the measured or simulated sampled data by a rational function model is an important step in a filter coupling-matrix-extraction method.A high-order transmission coefficient(S21)fitting strategy is proposed to address the problem of the deviation between the fitted and sampled data near the transmission zeros with small amplitudes.This strategy achieves a high fitting accuracy by fitting the sampled transmission coefficient with a rational function with a numerator polynomial of Nth-order(N being the order of filter)to accurately locate the transmission zeros.Then,the numerator polynomial of the transmission coefficient is reconstructed by selecting the Nz(Nz being the number of transmission zeros of the real filter)desired transmission zeros among the N fitted transmission zeros to guarantee that the number of transmission zeros of the extracted coupling matrix is the same as that of the real filter.For verification,a ninth-order coaxial filter with three transmission zeros is used as an example to validate the conventional Cauchy method,the Cauchy method that applies the higher-order transmission coefficient fitting strategy,and the model-based vector fitting method(MVF).The results show that this strategy can improve the fitting accuracy near the transmission zeros.Since the robustness of the Cauchy method is not high enough,a coupling-matrix-extraction method by identifying the zeros of S-parameters using vector fitting is proposed in this paper by consisting the operation steps of the Cauchy method and MVF,this method can fit the zero of S-parameters more accurately than MVF and has a less influence by noise than the Cauchy method.
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