PSpice Application Notes

Introduction A transmission line can be broken down into a network of distributed L, C, and resistance (R) elements. It's important to achieve an intuitive and physical understanding of how and why a transmission line behaves as it does, to demystify the reasons for its characteristics, and to gain an in-depth understanding in terms of its fundamental elements. In other words, L and C (for a lossless transmission line) is very important. In order to gain a deeper understanding of the way that a transmission line functions and what makes it unique compared to a normal conductor, I performed some SPICE simulations, where I plugged in simple LC networks to model a transmission line. I experimented with different kinds of stimuli and observed the propagation of signals at each point on the transmission line. In this paper, I describe the experiments I've done and the deeper insights I've gained as a result of accurately modeling full transmission line behavior using a simple LC network. All of the expressions, such as the reflection coefficient, can be derived fully using fundamental properties of capacitors and inductors, as well as energy conservation principles. Such work provides a physical idea as to how the mechanism of reflections in the transmission line occur. Scope of Work In addition to modeling the transmission line as a simple passive LC network, I also used PSpice ® technology from Cadence to perform simulations (See Figure 1 for a depiction of a typical transmission line circuit). I attempted to analyze both the forward and reflected traversals of the signal input. To understand the derivation of expression for the reflection coefficient for reflection at the load, I tapped into the principle of energy conservation. For this work, I examined the following cases: • Open transmission line • Shorted transmission line Accurately Modeling Transmission Line Behavior with an LC Network-based Approach By Anoop Veliyath, Design Engineer, Cadence Design Systems In high-speed signal transmission, understanding transmission line behavior is imperative to achieving proper impedance matching and proper termination to minimize loss stemming from reflections, as well as to maximize signal integrity. This paper shows that transmission line behavior can be accurately modeled using simple SPICE simulations with an inductance (L) and capacitance (C) network, and also provides a good physical understanding of the mechanism of reflections and derivation of key formulas. Contents Introduction ..........................................1 Scope of Work ......................................1 Characteristic Impedance ......................2 Model Used ...........................................2 A Step Voltage Input to a Transmission Line: Start of Travel of the Step .............3 Derivation of the Reflection Coefficient Expression Using the Principle of Energy Conservation .........................................5 When the Reflected Wave from the Load Side Reaches at the Source: Source Reflections ...........................................6 When the Step Reaches the End of the Transmission Line (Transmission Line Terminated in R=Z0) ..............................8 When the Step Reaches the End of the Transmission Line (Transmission Line Terminated in R>Z0) ..............................8 When the Step Reaches the End of the Transmission Line (Transmission Line Terminated in R

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