ELEC 243 Lab

Introduction

The terms DC (short for Direct Current) and AC (Alternating Current) originated in the early days of electric power to differentiate the system developed by Thomas Edison from that invented by Nicola Tesla and supported by George Westinghouse. In Edison's system the current flowed continuously in one direction, while the direction of Tesla's current alternated at a frequency of 60 Hz. Today the terms are used somewhat more loosely, with DC referring to signals which are constant or only slowly time varying, while AC can mean any rapidly changing signal.

Strictly speaking an AC signal should be bipolar, i.e. having a sign which alternates with time. While it's certainly possible to have a time-varying signal whose sign never changes, it's not possible to have one which is strictly monotonic. I.e. what goes up must come down, even if it never hits the ground. In this case we sometimes refer to the DC and AC components of a signal, more precisely defined as the average value and the variations about that average.

An important class of time-varying signals are periodic signals for which $f(t)=f(t+nT) \; \forall n,$ where $T$ is the period of the signal. We can completely characterize a periodic signal by describing its waveshape (e.g. a sinusoid), its amplitude, and its frequency.

The signals we measured last week were DC signals and could be described by a single number. This week we will look at a variety of AC signals which, in addition to being more interesting, can contain a greater amount of information.