TABLE OF CONTENTS In this first chapter, we introduce the concept of a signal as a real or complex function of time. We pay special attention to sinusoidal signals and to real and complex exponential signals, as they have the fundamental property of keeping their "identity" under the action of a linear time-invariant (LTI) system. We also introduce the concept of a system as a relationship between an input signal and an output signal.
The word system refers to many different things in engineering. It can be used to designate such tangible objects as software systems, electronic systems, computer systems, or mechanical systems. It can also mean, in a more abstract way, theoretical objects such as a system of linear equations or a mathematical input-output model. In this book, we greatly reduce the scope of the definition of the word system to the latter; that is, a system is defined here as a mathematical relationship between an input signal and an output signal. Note that this definition of system is different from what we are used to. Namely, the system is usually understood to be the engineering device in the field, and a mathematical representation of this system is usually called a system model.
Signals are functions of time that represent the evolution of variables such as a furnace temperature, the speed of a car, a motor shaft position, or a voltage. There are two types of signals: continuous-time
