Extraction of dielectric constant from measured S-parameters

摘要

Dielectric materials are mostly used as a substrate in fabrication of radio frequency circuits. It is important to know dielectric properties before fabrication because changes in these values may result in frequency shift, frequency dispersion, etc. This paper introduces two methods for extraction of the dielectric constant from measured S-parameters.;The first method is based on a genetic algorithm, which optimizes the value of the dielectric constant by minimizing discrepancies between the measured and simulated S--parameters. Measured S-parameters data was collected from a network analyzer. For simulated S-parameters data, a circuit template was designed in sonnet EM simulation software and was simulated each and every time by changing the value of the dielectric constant. In order to change the dielectric constant, a Matlab script file is generated for circuit template and the value is changed in script file.;In the second method, which is called as transmission line method, four pairs of transmission lines are fabricated on a substrate whose dielectric constant is to be determined. An effective dielectric constant was calculated from measured S-parameters data for each pair of transmission lines by using a special equation designed from transmission line theory. After the calculation of an effective dielectric constant, Appcad RF design calculator was used to find the appropriate dielectric constant from the effective dielectric constant.