Differential- and common-mode characterization of coupled interconnect transmission lines using four-port S-parameter measurements

In this paper, the differential- and common-mode characteristics of coupled interconnect transmission lines are presented using four-port scattering parameter (S-parameter) measurements for a section of coupled line, a series resistor and a shunt resistor, where the impedances of the resistors need not to know. Firstly, the differential- and common-mode propagation constants are solved by the determining equations consisted of three measured differential/common mode transmission matrixes, in which the transition and parasitic effects between the coaxial connectors and the coupled lines can be removed. With the low conductance loss condition for the substrate, the differential/common mode characteristic impedances can be further determined by the propagation constants and the unit-length capacitances which are measured by the dc resistance and low frequency reflection data of the series/shunt test key. The measured results for the coupled microstrip lines on FR-4 substrates are shown from 40 MHz to 8.5 GHz with comparisons of the other measurement method.