电气工程及其自动化专业英语 Chapter 2 Electronics

Chapter 2ElectronicsSection 1 Introductionn Textn New Words and Expressionsn Exercisesn Endn Vocabulary Characteristic IIISection 1 IntroductionTo say that we live in an age of electronics is an understatement. From the omnipresent integrated circuit to the equally omnipresent digital computer, we encounter electronic devices and systems on a daily basis. In every aspect of our increasingly technological society whether it is science, engineering, medicine, music, maintenance, or even espionagethe role of electronics is large, and it is growing.In general, all of the tasks with which we shall be concerned can be classified as “signal-processing“Section 1 Introductiontasks. Let us explore the meaning of this term. SignalA signal is any physical variable whose magnitude or variation with time contains information. This information might involve speech and music, as in radio broadcasting, a physical quantity such as the temperature of the air in a room, or numerical data, such as the record of stock market transactions. The physical variables that can carry information in an electrical system are voltage and current. When we speak of “signals“, therefore, Section 1 Introductionwe refer implicitly to voltages or currents. However, most of the concepts we discuss can be applied directly to systems with different information-carrying variables. Thus, the behavior of a mechanical system (in which force and velocity are the variables) or a hydraulic system (in which pressure and flow rate are the variables) can often be modeled or represented by an equivalent electrical system. An understanding of the behavior of electrical systems, therefore, provides a basis for understanding a much broader range of phenomena. Section 1 IntroductionAnalog and Digital signals A signal can carry information in two different forms. In an analog signal the continuous variation of the voltage or current with time carries the information. An example, in Fig.2-l, is the voltage produced by a thermocouple pair when the two junctions are at different temperatures. As the temperature difference between the two junctions varies, the magnitude of the voltage across the thermocouple pair also varies. The voltage thus provides an analog representation of the temperature difference. Section 1 IntroductionThe other kind of signal is a digital signal. A digital signal is one that can take on values within two discrete ranges. Such signals are used to represent ON-OFF or YES-NO information. An ordinary household thermostat delivers a digital signal toFig.2-1 (a) An example of an analog signal; (b) Voltage-time characteristic Section 1 Introductioncontrol the furnace. When the room temperature drops below a preset value, the thermostat switch closes turning on the furnace. Once the room temperature rises high enough, the switch opens turning off the furnace. The current through the switch provides a digital representation of the temperature variation: ON equals “too cold“ while OFF equals “not too cold“. Section 1 IntroductionSignal-Processing SystemsA signal-processing system is an interconnection of components and devices that can accept an input signal or a group of input signals, operate on the signals in some fashion either to extract or improve the quality of the information, and present the information as an output in the proper form at the proper time. Fig.2-2 illustrates the components in such a system. The central circles represent the two types of signal processing (digital and analog), while theSection 1 Introductionblock between the two signal- processing blocks represents the conversion of an analog signal to equivalent digital form (A/D=Analog-to-Digital) and the reverse conversion of a digital signal to the corresponding analog form (D/A=Digital-to- Analog). The remaining blocks involve inputs and outputs getting signals into and out of the processing system. Section 1 IntroductionFig.2-2 Components of a signal system Section 1 IntroductionMany electrical signals derived from physical systems are obtained from devices called transducers. We have already encountered an example of an analog transducer, the thermocouple pair. It converts temperature difference (the physical variable) to a voltage (the electrical variable). Generally, a transducer is a device that converts a physical or mechanical variable to an equivalent voltage or current signal. Unlike the thermocouple example, however, most transducers require some form of electrical excitation to operate. Section 1 IntroductionThe output from a system can be in many forms, depending on the use to be made of the information contained in the input signals. One can seek to display the information, either in analog form (using a meter, for example, in which the needle position indicates the size of the variable of interest) or in digital form (using a set of digital display