Transmission lines - Part I: Introduction

A transmission line is a channel able to carry some information at a very high frequency, so high that parasitics and stray components in the practical realization of the circuit cannot be neglected and must be understood and controlled to have a reliable communication system. Transmission lines can be realized in different ways: on a PCB, using a waveguide, or through a bifilar line; all of these communication media have specific physical and electrical properties and they must be modelled properly before doing any calculation. An important aspect to keep in mind when talking about transmission lines: forget about ground (or earth or reference)! When frequencies are very high you don't have a ground anymore (signal, trace, plane or wire, according to the type of support used to realize the transmission line) but a 'return path': the current in the return path will follow, in fact, the forward path and will not distribute, instead, following the lowest impedance path. This must be taken into consideration especially when designing high-frequency layouts.

Every communication system is made of a transmitter, one channel and one or more receivers. Transmission line theory focuses mainly on the physics behind the channel and the output stage (input stage) of the transmitter (receiver).

There are various models of a communication channel: distributed C, lumped RC, distributed RC, lumped RLC, and transmission lines. In the table below you can see when it is convenient to use them.

When studying transmission line theory, typically one starts to study the 'lumped RLC' model, which, in the frequency domain, allows one to find a solution for a wave equation in a well-defined media. This solution well describes voltage and current waves along the line, as well as impedance change and reflection phenomena at ends and in correspondence with discontinuities.