电子信息与通信 专业英语 Deyelopment Platform ardware 赵藏清主编 哈尔滨工业大学出版社
电子信息与通信 专业英语 赵淑清主编 哈尔滨工业大学出版社 哈尔滨
内容提要 本书以培养学生专业英语阅读能力为主要目标。内容包括:基础电子 学、信号的信息处理及通倍的理论、计算机原理及应用信号处理专题以及一 些电子仪器设备及部件的说明书 本书可作为大学电子信息工程和通信工程专业三四年级本科生的专业 英语教村,也可供广大工程技术人员使用。 电子信息与通僧专业英语 tanzi Ximi yu Tongxin Zhanye Yingyu 赵淑清主编 哈尔滨工业大学出版社出版发行 哈尔滨工业大学印別厂印刚 开本850×11681B2印张975字数286千字 200年3月第1版2000年3月第1次印刷 印数1-500 ISBN75603-14503H141定价12.00元
Content Chapter 1 Basic Electronic 1. 1 Receiver Circuits 1.2 Digital Design 1.3 Digital Signal and Discrete- Time Systems Chapter 2 Signal Processing and Electronie System.50 2.1 Digital Filter Design……………………… Image Enhan 2.3 Speech Coummunications…………………" 24 PCM and digital Tramsmission…… 3. i Introduction to unix e++.. 3.2 Data Base 131 33 Boran C………………… Chapter4spec| TopIcs in signal Processing……175 4.2 The Least Mean squares(M) algorithm…………190 4.3 Pattem Classification by Distance Functions……208 4.4 The Delta Rule and Leaming by Back-Propagation 18 Chapter5 struction Manual………… 5.1 Mobile phone…… 5.2 Specificationg for So 246 ………246 5.4 SKYe 5.5 ADSP-2106x SHARC DSP Micro-Computer Family 5.6TMS320C5X…
Chapter 1 Basic Electronics 1.1 Receiver Circuits The purpose of a receiver is to select a desired group of frequencies from one transmitter, get rid of all unwanted signals and noise, and then demodulate the signal to obtain the modulating infomation. The better the receiver does its job, the closer the demodulated sigmal will resemble the original signal from the transmitter. Regardless of the type of demodula 40n required, the main functions performed by a receiver are filtering and amplifying. The superheterodyne receiver is the logical choice for the job 1.1.1 Superheterodyne Receiver Since it is easier to desigm narnow-bend, steep-akirt filters and ob tain high gains at lower frequencies, the ahet" recever is an e伍 cient design. All incoming signals are mixed with the output of a local os- illator and the difference frequency is selected and amplified by the termediate frequency amplifiers. The big benefit is that these amplifiers remain at a fixed frequency and only the RF amplifier and local oscillator need be tunable. Fig. 1. 1 is a block diagram of a typical superhet receiv- er. One further benefit is the fact that the gain is concentrated at two or sametimes three different fequencies. This reduces the gain required at RF AGC x hg. 1.I Block dagra of a superheterodyne recelver
any ne frequency and leads to more stabte amplifier. When over 120 dB o rf gain is imvolved, every little bit helps. The function of each item in Fig. 1. 1 can be explained as follows 1. RF amplifier It should have just enough gain, usually about 10 dB, to establish he overall noise figure of the receiver. The tuned circuits at the input and output need only be selective enough to reject image signals and other spurious signals that could intermodulate and appear at the imtermediate frequency. Some AGC may be needed to prevent overloading on strong signals. The RF amplifier may also be called on to suppress amy tenden for the local oscillator to radiate out to the antenna and interfere with other 2. Mixer and local ascillato The mixer has two inputs, one from the RF amplifier and one from the local oscillator. The nonlinearities of the mixer will create numerous demodulation products, and one of the sum or difference fre quency, will occur at the F frequency. Usually, there will be a second frequency, the image, that can also mix with the oscillator fre produce an output at the IF. Depending on the type of mixer used, conv- from-10 dB to +30 dB are common. The local oscillat must be tunable, yet have a low drift rate and relatively low sides noise, since this could increase the noise level of the receiver This section establishes the overall bandwidth and selectivity of the receiver. The bulk of the receivers gain will be concen- trated here and some type o automatic gain control will be included to ad- just for variations in received signal strength. The IF is usually at a lower frequency than the RF, but, in some special cases, the IF may be higher to reduce spurious intermodulation and image problems 4. Demodulators For each type of modulation used(i. e, AM, FM, SSB, PM),a number of different circuits exist. Some will have gin others a loss. 2
Some will require a reference input(i. e others wont. The demodulation may also red to produce output to aGC or AFC circuits. The recovered audio leved (or video, etc )will detemine the amount of gain required in the following audio or video am- 1.1.2 Specilications Before beginning the design of a receiver, it is necessary to consider the specifications required of the final result. In most cases this ands up as a compromise between what the designer would like and what is possi ble. The determining factor will usually be financial limitations, The fol lowing should then be considered before proceeding 1. Tuning range What range of frequencies must be tuned and will it be tuned contin- uously or in discrete chapels? A short-wave receiver, for example,must continuously tune from 3 to 30 MHz and will usually require some band switching. The locad oscillator will be a continuously tunable type. De modulators will be needed for AM, SSB, and CW. and IF bandwidths should correspond. For CB, a narrow range of frequencies from 26.965 to 27.405 are needed and will be trned as 40 discrete charnels, The local oscilator will therefore likely be a phase-locked loop synthesizer. Demod ulation could be either am or SSB Often, too much emphasis is put on sensitivity without attention to other details. For example, a 100-kHz navigation receiver will pick up so much atmospheric noise that a 100-gv desired signal from the antenna could be obscured at times. On the other hand, ao.1-uV signal at 150 MHz wil often be readily distinguishable from background noise When the modulation type and channel spacing are mown, it is pos- sible to determine the IF bandwidth and its skirt characteristics For FM- stereo broadcasting, a bandwidth of 350 khz is required. For AM aircraft communications, a bandwidth of 30 kHz is coumon--not to provide wide
bandwidth for high audio-frequency response but to accomodate frequen cy tolerances in the transmitters and receivers. The filter-skirt characteris tics will be set to reject adjacent channel signals as require 4.界 punzous signa An oherwise good design can be useless if unwanted signals can sneak into the reoeiver at the IF frequency(s) arough croag-modulatioa problems [J Typical specifications for several good receivers are as fallows (1)M stereo ner: frequency range 88-108 MHz Sensitivity: 1.8 uv acrose 300-0 input for 20 dB electivity: 100 dB for channels 400 kHz either side of center frequency Bandwidth 350kH敏-6- dB point Image rejection: 90 dB Spurious rejection: 90 dB I 90 dB 65 dB capture ratio: 1.5 dB/4) (2)Shortwave receiver: frequency nange 3.0-30 MHz Sensitivity: 0.5 gV for 10 dB S+N/N ratio(5) 2. 3 kHz at -6dB, 5.5 kHz at -60 dB (SSB mode) 60 dB IF rejection: 75 dB (3)CB receiver: frequency range 26.965-27.405 MHz Sensitivity 0.5 uV for 10 dB S+N/N ratio Bandwidth 6 kH at-6 dB 20 kHz at -60 dB age rejection: 60 dB Once the specifications are carefully determined, it is time to start
the design. But what is the best starting point? Generally, the most sensi tive points will be the two nonlinear circuits, the mixer and the detector The IF amplifier takes up the slack between the two, and the RF amplifier picks up the deficiencies of the mixer 1.1.3 Mxer The mixer section of the receiver should ideally produce an IF only at the difference (or sum, for up-conversion )of the two inp quencies. One of these inputs will be the local oscillator signal other will be the desired RF signal. Again, ideally, no other combination of input sigals should produce an IF output. If such frequencies do ex- t, filters must be provided to remove them before they reach the mixer The closest thing to an ideal mixer is any circuit In addition to the their second harmonics appearing at the output, the sum and the differ ence will also appear. The diierence is usually the one signal desired and so is selected by IF fltering. The amplitude of the difference signals will be proportional to the product of the ariginal RF signal level and the local oscillator level. any other two signals at the input oould also produce am output &t the IF if they are separated by an amount equal to the difference frequency. However, the output level they produce will be proportional to their signal levels be had if all RF imput levels to the mixer are kept as low as possihle and the local oscillator sigmal kept as high as poaaible. The one desired sig al will therefore be stronger than all the undesired ones. This is de- ibed in Fig. 1. 2. Thermixer circuit has four signals at its input, all of the same level. The local oscillator and is much higher than the other four. The IF filters only pass signals between0. 4 and 0. 6 MHz, RF frequencies C and d can mix with the illator and produce outputs at 0. 45 and 0. 55 MHz, respectively, well within the IF pasaband. One will be the desired signal and the other is the image, which should be removed by filtering before reaching the mixer
2.02.5 3.5540455 (a)Input spectr B-A 045050055 (MHz) g.1.2S《到s 盘m血whh能邛ps en to be separated by 0.5 MH they will also produce a mixing product (which contains the combined modulation of each)within the IF passband. However, the amplitude of this signal will be much lower than the desired IF signal. Therefare, best obtained by: (2)Using high local oscillator levels (3)Maintaining low RF signal levels (4)Providing proper filtering ahead of the mixer. 1.14 If the ideal square-law mixer can be built, what is the minimum fil ter that is required ahead of it? We have already seen that the image has to be removed and also any group of frequencies that could themselves mix The limiting case is shown in Fig ..3.The