实用电力英语4精编版

1、2考试考试 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，文章或若干段落（英译汉） 评分：所有小组成员分数相同 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）3电力系统通信基础电力系统通信基础4Digital and Analog Communication SystemCommunication system m(t) : electrical signal representing messages m(t): electrical signa

2、l sent to the user. (m prime t), may be not the same as m(t) due to noise or system unlinearity.Signal processingTransmission medium (channel)Information inputm(t)TransmitterS(t)Noisen(t)r(t)ReceiverTo information sink(user)m(t)Carrier circuitsSignal processingCarrier circuits5message: words, pictur

3、es, voices, value of voltage, currentElectric 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%.c

4、ontinuous message: voicesdiscrete message: words, status of circuit breakersAnalog signaldigital signal.sin(2)Aftamplitude6 dealing with the source m(t) for more efficient transmission. For example: analog low pass filter(低通滤波器) in analog communication system to restrict the bandwidth(带宽) of m(t).an

5、alog-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(差错控制编码) to the digital word to achieve error detection and correction caused by noise-channel coding(信道编码)converts the baseband signal(基带信号) into a

6、 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 processingTransmission medium (channel)Information inputm(t)TransmitterS(t)Noisen(t)r(t)ReceiverTo information sink(user)m(t)Carrier circuitsSignal processingCarrier circuit

7、s7twisted-pair telephone lines: 双绞线coaxial cables: 同轴电缆fiber optic cables: 光纤wireless 无线natural electrical disturbances (e.g. lightning)man-made noise (e.g. electromagnetic interference) Signal processingTransmission medium (channel)Information inputm(t)TransmitterS(t)Noisen(t)r(t)ReceiverTo informa

8、tion sink(user)m(t)Carrier circuitsSignal processingCarrier circuits8 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 communicat

9、ion system can also transmit analog source to the sink.9 General digital communication systemDigital sourceTransmission medium (channel)g(t)NoiseEncoder and other signal processingCarrier CircuitsCarrier circuitsDecoder and other signal processingDigital Sinkms(t)r(t)r(t)mTransmitterReceiverEncoder:

10、 including source coding and channel coding10Comparison 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.11Channel+noiserepeater12 Errors can be detected and corrected by the use of

11、 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 f

12、or analog systems. Synchronization (同步) is required.13Communication 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 betwe

13、en the two connected devices in both directions simultaneously14 channelReceiverSimplex modeSender channelReceiverHalf duplex modeReceiverSenderSender channelSender Duplex modeReceiverSenderReceiver15Information MeasureWhat is information exactly?message? information?Information is the valuable part

14、 a message containsRead 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.16How many information does a message have?Definition: The Self-Information of the message j is given bywhere, Pj is the probability o

15、f transmitting the jth message.Ij is called Self-Information of event j)1(log2jjPI17 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”.18Communication systems performance Realiability Equipment

16、 reliability Data transmission reliability Accuracy Real-time19Equipment 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 a

17、nd preventive maintenance(维修) time.Ap = MTBF / ( MTBF + MTTR )where, Ap is predicted availabilityMTBF: 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 operationReference period20Data

18、 transmission reliability:Bit Error Rate(比特差错率): to value the quality of communication. It is given by:Accuracyoverall 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 g

19、iven by: offset: 偏差; full scale: 满量程transferedtotallybitsreceivedbitserrorPe100%offsetfull scale21 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

20、 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 longer22Si

21、gnal 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 codeRZ Bipolar code23Uni

22、polar 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 th

23、e DC-component. Or using modulation and demodulation techniques. This code is used in telecontrol system.24 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 alternati

24、ng polarity.25Manchester code(裂相码):“1”“0”26Signal 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 freq

25、uency analysis.Fourier analysis toolsFourier series / Fourier transform27 “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.2829Features:Envelope has a sha

26、pe of sinx/x.The spectrum of period signal is composed of discrete spectral components.1.Ck decays to zero when f , is zero at multiples of 1/.30Bandwidth:Generally, in engineering definitions, the bandwidth would be f2-f1, for baseband waveform, f1 is usually taken to be zero, and f2 is the first n

27、ull in the envelope of the magnitude spectrum along the positive frequency axis.So, for this periodic rectangular waveform, Bandwidth B=1/.31Channels and noiseWhat is a channel?The propagating medium or electromagnetic path connecting the transmitter and receiver is called the channel.Open wire, sel

28、dom used nowadaystwisted 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)32coaxial cables(同轴电缆): has a single core, with an outer conductor acting as a shield(屏蔽). The sig

29、nal 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.33The atmospheremicro-wave system(微波): a radio system which uses very high frequencie

30、s 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

31、station to its transmitter site, and the sending of video or audio signals from an outside sports event back to a TV broadcasting studio. 34fiber optic cables(光纤): made from fine strands of silica (glass), coated with a plastic sheath.35Communication 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 repr