Measurement of the characteristic impedance of balanced twisted pairs using scattering parameters

The concept of scattering parameters has previously been used to describe the behavior of circuit networks in an effort to predict their behavior under varied external conditions. This paper describes the use of scattering parameters to lump an impedance test setup, including connectors, cables, baluns, and even internal test equipment circuitry into one two-port network. Scattering parameters are then derived to describe the behavior of that network at one port as a function of varying loads that may be connected at the other port. After the network is characterized, the cable impedance is calculated from corrected short and open circuited cable impedance measurements. The basic theory of scattering parameters and their application to this procedure are described, and equations are presented which are used to implement this method. A brief description of a computer program that was written to accomplish this procedure is included. Sample plots of the impedance of balanced cables which have been produced by the program are also presented.