Walt Kester

16.1 Origins of Real-World Signals and Their Units of Measurement

Let's first look at a few key concepts and definitions required to lay the groundwork for things to come.

Webster's New Collegiate Dictionary defines a signal as "a detectable (or measurable) physical quantity or impulse (as voltage, current, or magnetic field strength) by which messages or information can be transmitted." Key to this definition are the words: detectable, physical quantity, and information.

By their very nature, signals are analog, whether DC, AC, digital levels, or pulses. It is customary, however, to differentiate between analog and digital signals in the following manner: Analog (or real-world) variables in nature include all measurable physical quantities. In this book, analog signals are generally limited to electrical variables, their rates of change, and their associated energy or power levels. Sensors are used to convert other physical quantities such as temperature or pressure to electrical signals. The entire subject of signal conditioning deals with preparing real-world signals for processing, and includes such topics as sensors (temperature and pressure, for example), isolation amplifiers, and instrumentation amplifiers (Ref. 16.1).

Some signals result in response to other signals. A good example is the returned signal...