实用电力英语4

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,实用电力英语,任 惠,华北电力大学电力工程系保定,1,考试,分组,5人一组,自由分组，4月10日上课前提交分组结果,开卷,试题：翻译词汇，文章或假设干段落英译汉,评分：所有小组成员分数一样,考试时间：4月10日发题，4月17日交卷每小组交一份答案，要求提供答案的打印件和电子文档，文档中注明小组成员姓名班级,其他,可以要求单独承担翻译最多10名,2,电力系统通信根底,3,Digital and Analog Communication System,Communication system,m(t): electrical signal representing messages,m(t): electrical signal sent to the user. (m prime t), may be not the same as m(t) due to noise or system unlinearity.,Signal processing,Transmission medium,(channel),Information input,m(t),Transmitter,S(t),Noise,n(t),r(t),Receiver,To information sink(user),m(t),Carrier circuits,Signal processing,Carrier circuits,4,message: words, pictures, voices, value of voltage, current,Electric signal: representing message.,Signals parameters change according to message.,Sine Wave(,正弦波,): amplitude(,幅值,), frequency, phase offset(,相移,).Pulse wave(,脉冲波,): amplitude, period, mark space ratio(,占空比,) = mark time/space timeduty cycle = (high time/period)*100%.,continuous message: voicesdiscrete message: words, status of circuit breakers,Analog signaldigital signal.,amplitude,5,Transmitter: dealing with the source m(t) for more efficient transmission. For example: signal processing:analog low pass filter(低通滤波器) in analog communication system to restrict the bandwidth(带宽) of m(t).analog-to-digital converter(模数转换器), used to transmit an analog information through a digital communication system- source coding(信源编码)error correct coding(过失控制编码) to the digital word to achieve error detection and correction caused by noise-channel coding(信道编码)carrier(载波) circuits:converts the baseband signal(基带信号) into a frequency band signal(频带信号) that is appropriate for the transmission medium(传输媒质).-modulation(调制) and demodulation(解调).,Signal processing,Transmission medium,(channel),Information input,m(t),Transmitter,S(t),Noise,n(t),r(t),Receiver,To information sink(user),m(t),Carrier circuits,Signal processing,Carrier circuits,6,Transmission medium (channel):,twisted-pair telephone lines:,双绞线,coaxial cables:,同轴电缆,fiber optic cables:,光纤,wireless,无线,Noise:,natural electrical disturbances (e.g. lightning)man-made noise (e.g. electromagnetic interference),Signal processing,Transmission medium,(channel),Information input,m(t),Transmitter,S(t),Noise,n(t),r(t),Receiver,To information sink(user),m(t),Carrier circuits,Signal processing,Carrier circuits,7,Definition:,a Digital Communication System,transfers information from a digital source to the sink(user).,an Analog Communication System,transfers information from an analog source to the sink.,Notes: an electronic digital communication system can also transmit analog source to the sink.,8,General digital communication system,Digital source,Transmission medium,(channel),g(t),Noise,Encoder and other signal processing,Carrier Circuits,Carrier circuits,Decoder and other signal processing,Digital Sink,m,s(t),r(t),r(t),m,Transmitter,Receiver,Encoder: including source coding and channel coding,9,Comparison between Digital and Analog Communication system,Advantages of digital Communication:,In long-distance systems, noise does not accumulate from repeater(,中继器,) to repeater.,10,Channel,+,noise,repeater,11,Errors can be detected and corrected by the use of coding (,编码,).,Data from voice, video, and data sources may be merged and transmitted over a common digital transmission system.,Privacy is preserved by using data encryption(,加密,).,Relatively inexpensive digital circuits may be used.,Disadvantages:,Generally, more bandwidth(,带宽,) is required than that for analog systems.,Synchronization (,同步,) is required.,12,Communication Modes,Simplex (,单工,)data can be transmitted in,one direction only,Half duplex (,半双工,),two interconnected devices can exchange data in both direction,alternately,e.g. a walkie-talkie(,步话机,),Duplex (,全双工,)full duplex, data are exchanged between the two connected devices in both directions,simultaneously,13,channel,Receiver,Simplex mode,Sender,channel,Receiver,Half duplex mode,Receiver,Sender,Sender,channel,Sender,Duplex mode,Receiver,Sender,Receiver,14,Information Measure,What is information exactly,message information,Information is the valuable part a message contains,Read the following sentences:,Tomorrow, the sun will rise from the East.,The phone will ring in the next one hour.,It will snow in Hainan this winter.,15,How many information does a message have,Definition:,The Self-Information of the message j is given by,where, Pj is the probability of transmitting the jth message.,Ij is called Self-Information of event j,16,The unit of “I :if the base of the logarithm is 2, the units are “bits;比特,if the base is e, the units are “nats;if the base is 10, the units are “hartley.,17,Communication systems performance,Realiability,Equipment reliability,Data transmission reliability,Accuracy,Real-time,18,Equipment reliability: the probability that a system will perform its intended function under specified conditions during a specified period of time.,Availability(,可用率,):,the reference period of time includes operation time and corrective and preventive maintenance(,维修,) time.,Ap = MTBF / ( MTBF + MTTR ),where, Ap is predicted availability,MTBF: mean-time-between-failure(,平均故障间隔时间,),MTTR: mean-time-to-repair(,修复的平均时间,) (including,administrative, transport, and repair time ),Availability,=,Total time of satisfactory operation,Reference period,19,Data transmission reliability:,Bit Error Rate(比特过失率): to value the quality of communication. It is given by:,Accuracy,overall accuracy(总准确度): is the offset between the data source and data sink through transform and processing ( transformer, analog-to-digital converter, digital-to-analog converter, ), is given by:,offset: 偏差; full scale: 满量程,20,Real-time: to be valued by,overall transmission time(,总传输时间,),. For example: for tele-metering or tele-indication, it refers to the period of time from a event happened or a value changed at substation or power plant to this information displayed on the screen of operators computer.,for important analog inputs, 3s,for less important analog inputs, 10s or longer,for status, less than 3s.,for control or adjusting command, less than 3s.,for others, such as dispatching plans, logs, minutes or longer,21,Signal and spectra - Line Codes,Bipolar code(双极性码):,using,positive pulse,and,negative pulse,to represent logical 1 and 0.,Non-Return-to-Zero (NRZ, 不归零码): signal doesnt drop to zero between each pulse.,Return-to-Zero (RZ, 归零码): signal drops tozero between each pulse.,NRZ Bipolar code,RZ Bipolar code,22,Unipolar code(单极性码): using positive pulse and grounded pulse to represent digit 1 and 0.,Advantage: easy to create.,Disadvantage: Unipolar code will create a significant DC-value, which is not possible to transport a DC-component through long wires. Solution: RZ unipolar code could be used to reduce the DC-component. Or using modulation and demodulation techniques.,This code is used in telecontrol system.,23,Alternate Mark Inversion(交替极性码)：using positive, negative and grounded pulses to represent logical values. Logical 0 is represented with a grounded pulse, and a logical 1 by pulses of alternating polarity.,24,Manchester code(,裂相码,):,“1”,“0”,25,Signal and Spectrum frequency analysis,white light can be break up by a prism into the colors of the rainbow.,Spectrum is first used to describe the continuous bands of colors produced by a prism.,The analysis of light into colors is actually a form of frequency analysis.,Fourier analysis tools,Fourier series / Fourier transform,26,“spectrum is used when referring to the frequency content of a signal.,Frequency or spectrum analysis: the process of obtaining the spectrum of a given signal using the basic mathematical tools.,27,28,Features:,Envelope has a shape of,sinx/x,.,The spectrum of period signal is composed of discrete spectral components.,C,k,decays to zero when f, is zero at multiples of 1/,.,29,Bandwidth:,Generally, in engineering definitions, the bandwidth would be f,2,-f,1, for baseband waveform, f,1,is usually taken to be zero, and f,2,is the first null in the envelope of the magnitude spectrum along the positive frequency axis.,So, for this periodic rectangular waveform, Bandwidth B=1/,.,30,Channels and noise,What is a channel,The propagating medium or electromagnetic path connecting the transmitter and receiver is called the channel.,Open wire, seldom used nowadays,twisted Pair Cable(双绞线): Two insulated wires are twisted around each other, and combined with others into a cable . Two forms: UTP (unshielded Twisted Pair), STP (shielded twisted pair),31,coaxial cables(同轴电缆): has a single core, with an outer conductor acting as a shield(屏蔽). The signal is transmitted on the inner core. The inner core and the outer shield are separated by an insulator (normally plastic). It has a larger bandwidth than twisted cable, used in television, radio, and Ethernet LANs.,32,The atmosphere,micro-wave system(微波): a radio system which uses very high frequencies to send and receive data. Because of the high frequencies involved, stations are located about 30 kilometers apart and in line of sight (目视距离).,microwave systems are used in a number of areas, such as linking local area networks together between campus buildings, sending radio signals from a radio station to its transmitter site, and the sending of video or audio signals from an outside sports event back to a TV broadcasting studio.,33,fiber optic cables(光纤): made from fine strands of silica (glass), coated with a plastic sheath.,34,Communication systems Engineering,Organization of the book,Before we embark on the analysis and design of communication systems, we provide a brief review of basic,frequency-domain,characteristics of signals and,linear systems,in Chapter 2. Emphasis is placed on the,Fourier series,and the,Fourier transform,representation of signals and the use of,transforms,in linear systems analysis. The process of,sampling,a,bandlimited,analog signal is also considered.,35,In Chapter 3, we treat the,modulation,and,demodulation,of analog signals. This chapter provides detailed description of,amplitude modulation,(AM),frequency modulation,(FM), and,phase modulation,(PM). As examples of analog signal transmission and reception, we consider radio and television broadcasting, and mobile radio cellular communication systems.,36,In Chapter 4, we present a review of the basic definitions and concepts in,probability,and,random,processes,. These topics are particularly important in our study of electrical communications, because information sources produce,random signals,at their output and,communication channels,generally corrupt the transmitted signals in a random manner, through the,addition of noise,and other channel,distortion,. Special emphasis is placed on,Gaussian random processes, which provide mathematically tractable models for,additive noise disturbances,.,37,