PLANAR TRANSMISSION LINE METHOD FOR CHARACTERIZATION OF PRINTED CIRCUIT BOARD DIELECTRICS

Abstract

Abstract|An efiective approach to characterize frequency-dispersivesheet materials over a wide RF and microwave frequency range basedon planar transmission line geometries and a genetic algorithm isproposed. S -parameters of a planar transmission line structure witha sheet material under test as a substrate of this line are measuredusing a vector network analyzer (VNA). The measured S -parametersare then converted to ABCD matrix parameters. With the assumptionof TEM/quasi-TEM wave propagation on the measured line, as wellas reciprocity and symmetry of the network, the complex propagationconstant can be found, and the corresponding phase constant andattenuation constant can be retrieved. Attenuation constant includesboth dielectric loss and conductor loss terms. At the same time,phase term, dielectric loss and conductor loss can be calculatedfor a known transmission line geometry using corresponding closed-form analytical or empirical formulas. These formulas are used toconstruct the objective functions for approximating phase constants,conductor loss and dielectric loss in an optimization procedure basedon a genetic algorithm (GA). The frequency-dependent dielectricproperties of the substrate material under test are represented asone or a few terms following the Debye dispersion law. Theparameters of the Debye dispersion law are extracted using theGA by minimizing the discrepancies between the measured and thecorresponding approximated loss and phase terms. The extracted datais verifled by substituting these data in full-wave numerical modelingof structures containing these materials and comparing the simulatedresults with experimental.1. INTRODUCTIONDevelopment of simple and robust methods for wideband extractionof frequency characteristics of planar sheet materials for variouselectromagnetic applications is an important present-day problem. Inparticular, characterization of dielectric substrates for printed circuitboards (PCBs) is vital to achieve the flrst-pass success in modern high-speed digital system designs. When the on-board data rate is in theGbps (gigabits per second) range or higher, traces and discontinuitiesincluding vias, AC coupling pads, and trace bends on a signal path haveto be modeled to catch the channel response accurately [1{3]. A staticfleld solver is not su–cient to model these discontinuities and traces,and full-wave modeling tools have to be used. To build the full-wavemodel for a given signal path, the detailed structures are known, butthe well-represented dielectric material properties of the correspondingsubstrates are unknown. In general, the dielectric properties (relative