PSpice Application Notes

9 At high frequencies R dc can often be neglected. Inductance Per Unit Length, L It has been shown for 2-wire lines (twisted pair, parallel wire) that, as the frequency is increased, the skin effect and proximity effect cause a slight reduction in the effective per-unit length self-inductance of the line. This frequency effect can often be neglected in models, and can lead to non-causality. Capacitance Per Unit Length, C This depends primarily on the dielectric constant of the insulating medium and the geometry of the conductor. Capacitance per unit length is constant over a wide range of frequencies for most dielectrics, such as Polyethylene. Conductance Per Unit Length, G If the loss tangent is available, G may be modeled by use of Where C is capacitance per unit length, is the angular frequency, and tan is a dielectric material coefficient known as the "loss tangent". Obtaining R and G from the attenuation vs. frequency curve It is often necessary to obtain R and G simultaneously from the attenuation vs. frequency curve. The following methods have been used successfully: 1. Using two points at the lower and upper bound of the signal frequencies of interest, simultaneously solve two equations of the form: 2. Attribute 90% of the loss to R, and 10% of the loss to G [3]. The rational for this is that most of the loss at high frequencies comes from the resistance of the center conductor. Specific examples of modeling the attenuation curve are given later. Both ideal and lossy transmission lines may be modeled as either distributed or lumped. The internal transmission line device in PSpice is distributed, but often a lumped macromodel can be used to advantage. For this reason both are provided in OrCAD's libraries. The following are guidelines for model selection. Model Selection Networks of transmission lines will typically span multiple categories. Each transmission line should be modeled as simply as possible, for faster simulation speed. tan C G 2 2 0 0 GZ Z R 1 0 , m Kf R R m dc

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