Measurement of Dielectric Constant and Cross-Sectional Variations of a Wire

A new measurement method is proposed for identifying small differences between the relative permittivity of data wires of a twisted pair used in high-data-rate USB cables with <1% uncertainty. This method can also detect small variations in the cross section of the insulated wires. Both the variations are the root causes of the differential mode to common-mode conversion, skew, and possibly electromagnetic interference. The method is based on forming a coaxial structure around the insulated wire under test using metals with low melting points, such as Field's metal or mercury. Using time-domain reflectometry and knowing the exact length of the structure, the dielectric constant can be derived from the propagation delay measurement. The local cross-sectional variations are detected by observing local impedance variations. Two examples are provided to verify the method and illustrate how to use the method to determine the dominant sources of skew.