Chapter 17 - Transmission Lines

Abstract

The signal integrity of high-speed signals via transmission lines is dependent on the frequency-related signal and dispersion losses of the transmission lines. Signal power loss is attributed to the increase in conductor resistance (skin effect) and the increase in dielectric conductance (dielectric loss) with an increase in frequency. Dispersion is the distortion of the signal wave shape resulting from delays introduced by the distributed frequency-dependent inductance and capacitance of the transmission line. Ideal and lossy transmission lines are modeled in PSpice using Tline distributed models and TLUMP lumped line segment models. The parameters required for an ideal transmission line are the characteristic impedance, and either the transmission line delay or the normalized line length, which is the number of wavelengths along the line at a given frequency. Transmission lines can be considered to consist of a number of identical sections known as RLCG lumped line segments. The R represents the line resistance, L the line inductance, C the dielectric capacitance, and G the dielectric conductance. For long transmission lines, one solution would be to use a number of lumped RLCG segments connected together. Simpler RC transmission line models are also available in the TLine library, as are over 40 coaxial cable models and twisted wire pair models. An alternative approach for lossy transmission lines is to use a distributed model, which relies on an impulse response convolution method to determine the transmission line response.