Impedance matching and spurious-response suppression in stepped-impedance low pass filters

Abstract

In this article, the method of least squares is used to design a stepped-impedance low pass filter with arbitrary specifications on the pass, transition, and stop bands, having different input and output impedances.The equivalent circuits of the line sections and step discontinuities are used (with appropriate dispersion relations) to determine the filter transmission matrix and the scattering parameters, which are then used to construct an error function. Its minimization is performed by the combination of particle swarm optimization and conjugate gradient methods. The conversion of lumped filter designs, by Butterworth, Chebyshev, and elliptic methods to the equivalent microstrip filters (namely conversion of L and C components to strip lines) may be difficult to realize. The proposed method of least squares procedure leads to the design characteristics having sharper transition bands and a wider pass band. The rectangular split ring resonators, as patterned ground plane, will remove the spurious responses in the stop band and actually increase the stop bandwidth. The results obtained by full-wave simulation softwares, fabrication, and measurements verify the proposed method for low pass stepped-impedance filter design. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26184