Digital Circuit Boards: Mach 1 GHz

2.1 Introduction

The language of transmission lines was developed in the early days of radio when rf (radio frequency) signals had to be transported from a transmitter to an antenna. The signals were basically a modulated sine wave called a carrier. The problem was to match impedances so that the transported energy reached the antenna and was effectively radiated. In digital electronics, the signals are step functions and the hope is that very little of the energy is radiated. Signals are often transported over and between conducting planes, and this practice was not a prime consideration in early radio.

Transmission line theory uses many terms common to circuit theory. We will be working mainly with step functions and not sine waves. A transmission line can be considered a distributed parameter circuit. Words such as bandwidth and frequency spectrum will apply. We will often use the term impedance but not in a sinusoidal sense.

N.B.

Circuit traces on a printed circuit board are transmission lines; and circuit traces over or between conducting planes are transmission lines.

When a voltage is placed on a transmission line, it is an invitation for nature to use this conductor geometry to move available energy away from the source. At the moment of connection, both voltage and current begin their travel down the line. We will often call this voltage and current flow a signal or a wave.

When a signal (wave) propagates down a transmission line, the source of signal has no way ...