Author Topic: LTspice Lesson 3: Transmission lines part 1  (Read 3870 times)

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Offline Kashif

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LTspice Lesson 3: Transmission lines part 1
« on: August 30, 2019, 07:52:12 pm »
Here is the third installment of LTspice Lesson focus on simulating transmission line, if interested in this topic, please check it out!

In this lesson we will focus on single element Lossless Transmission line (T-line) as shown in Figure 1. Lossless T line simulation will be introduced here. One of the goal of these lessons are to give out practical examples from real world. Transmission line model real world signals very well. This lesson focus on the Lossless T-line where no power gets dissipated during transmission. In a future lesson, we will focus on lossy line and loss mechanics.

What is transmission line? A transmission line is any conductor along with a return path where a signal travel from source to load. Because of finite speed of light, a signal takes a finite amount of time called Time Delay (TD) to go from source to load. Signal sees a characteristic impedance (Zo) based on geometry and dielectric around the conductor and return path. For an in-depth review of transmission line, please refer some excellent references as given at the end.

In any simulation we need to build confidence on the models given. One way to do this is find solution of easy problems by hand for cases a formula or numerical values can be derived and then compare them with results from simulation. Once 100% agreement is established, one can go beyond simple situation and start modeling real world effects. In the case of transmission line, let’s look at 4 simple but common cases, where a 50Ω T-line is driven by a CMOS driver with 5Ω source impedance (typical for newer drivers) with an open load, short load, 100Ω load and a properly terminated 50Ω load.

For each case, we will look at the load voltage waveform using bounce diagram. If you haven’t learned or played with bounce diagrams, do not fret, we will walk through it step by step.

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