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DZ160气体泄漏超声检测系统的设计,毕业设计
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毕业设计 (论文 )任务书 课 题： 气体泄漏超声检测系统的设计 院 （系）： 通信与信息工程系 专 业： 电子信息工程 学生姓名： 韦 伟 学 号： 010220530 指导教师单位： 电子工程教研室 姓 名： 晋 良 念 职 称： 讲 师 题目类型： 理论研究 实验研究 工程设计 工程技术研究 软件开发 2006 年 3 月 2 日 nts 一、毕业设计（论文）的内容 目前，工业上和生活中均大量用到用于储存和输送压缩气体的压力容器，如气缸、 气罐、煤气管道等。由于各种原因，容器会产生漏孔，从而发生气体泄漏。工业上气体泄漏不但会造成能源的浪费，而且如果是有害 气体的话，还会对空气造成污染。因此，准确地判断和定位产生泄漏的位置，对于提高企业的生产效率和节约能源具有重大的意义。传统的泄漏检测方法如绝对压力法、压差法、气泡法等，操作复杂并且对技术人员要求较高，而且不具有实时性。目前，工业上广泛利用泄漏产生超声波的原理来进行泄漏检测，利用超声波检测气体泄漏位置，不仅方法简单，而且准确可靠。 本课题的内容是设计一种气体泄漏超声波检测系统，要求能准确地判断和定位产生泄漏的位置。 二、毕业设计（论文）的要求与数据 1、要求掌握气体泄漏产生超声波的原理及其检 测方法； 2、要求掌握单片机的外围扩展原理和软件编程方法； 3、要求从理论上分析声压与泄漏量的关系； 4、要求 能准确地判断和定位产生泄漏的位置，并显示泄漏孔的声强和估算的泄漏值 ； 4、拟定系统的技术方案，设计实验样机；能够通过实物检测； nts 三、毕业设计（论文）应完成的工作 1、论文 (不少于 2 万字，包括 300 500 单词的英文摘要 )。 2、完成软硬件的设计，实现论文的技术要求 。 3、译文（不少于 4 万英文字符）。 四、应收集的资料及主要参考文献 1 王化祥，张淑英 . 传感器原理及 应用 M. 天津：天津大学出版社， 1999.154- 163. 2 于亚非 .用超声波传感器检测气体泄漏 J. 仪器与未来 ,1992(8). 3 李光海，王勇等 .基于声发射技术的管道泄漏检测系统 J. 自动化仪表 ,2002,5. 4 李群芳 .单片微型计算机与接口技术 M. 北京：电子工业出版社， 2001. 5 李进，陈会仓等 .气体泄漏超声波检测装置 J. 工业仪表与自动化装置 ,1996,5. 6 龚其春 ,叶骞 .新型气体泄漏超声检测系统的研究与设计 J.电子技术应用 ,200 5,3. 7 ATMEL Corporation. 8-bit Microcontroller with 8K Bytes Flash AT89C52. 8 Donald A.Neamen. Electronic Circuit Analysis and Design(Second edition). Published by the McGraw-Hill companies,2001. 五、试验、测试、试制加工所需主要仪器设备 1、示波器 电源 信号源 2、单片机烧录器 3、微机 nts 任务下达时间： 2006 年 3 月 2 日 毕业设计开始与完成时间： 2006 年 3 月 2 日至 2006 年 6 月 23 日 组织实施单位：桂林电子科技大学通信与信息工程系 电子工程教研室 教研室主任意见： 签字 2006 年 3 月 2 日 系领导小组意见： 签字 2006 年 3 月 2 日 nts 毕业设计（论文）进度计划表 序号 起止日期 计划完成内容 实际完成内容 检查日期 检查人签名 1 3 月 2 日 3 月 10 日 毕业设计准备，查资 料，翻译外文资料 2 3 月 11 日 4 月 1 日 拟定设计方案 3 4 月 2 日 4 月 29 日 电路的设计与制作 4 4 月 30 日 5 月 29 日 软件编程和系统调 试 5 5 月 30 日 6 月 13 日 撰写论文；教师评阅论文，评阅人评阅。 准备毕业设计答辩 6 6 月 15 日 6 月 20 日 毕业设计验收、答辩 7 指导教师批准日期 年 月 日 签名： nts 桂林电子工业学院 系 专业 学生 毕业设计（论文） 评 语 成 绩： 指导教师 (签名 )： 年 月 日 nts 论文评定人评定意见 成 绩： 论文评定人 (签 名 )： 年 月 日 nts 答辩小组意见 成 绩： 答辩组长 (签 名 )： 答辩小组成员（签名）： 年 月 日 nts ntsuchar adpr(bit chadr)/ad子程序uchar addata = 0,clki;clk = 0;_nop_();csa = 0;di = 1;/csclk = 1;/1_nop_();clk = 0;/di=1;clk=1;_nop_();clk = 0;/di = chadr;clk = 1;_nop_();clk = 0;/*_nop_();clk = 1;_nop_();clk = 0;*/for(clki=0;clki8;clki+)/11clk = 1;_nop_();clk = 0;if(doo)addata+; clk = 0;if(clki7)addata = addata1;ntsA/D 子程序 uchar adpr(bit chadr)/ad 子程序 uchar addata = 0,clki; clk = 0; _nop_(); csa = 0; di = 1;/cs clk = 1;/1 _nop_(); clk = 0; / di=1; clk=1; _nop_(); clk = 0; / di = chadr; clk = 1; _nop_(); clk = 0; /* _nop_(); clk = 1; _nop_(); clk = 0; */ ntsvoid test_busy(void)uchar i=1;do P0=0xff;RS=0;RW=1; E=1;if(P0&0x80)=0)break;E=0;while(-i!=0);/*/void ENABLE(uchar order) P0=order;_nop_();RS=0;_nop_();RW=0;_nop_();E=0;_nop_();test_busy(); _nop_();E=1;_nop_();void writedata(uchar digital) P0=digital;_nop_();RS=1;_nop_();RW=0;_nop_();E=0;_nop_();test_busy();_nop_();E=1;_nop_();/*/void resetlcd(void) /*lcd初始化设置*/DELAY(); /*/清除屏幕*/ENABLE(0X01); /*/8位点阵方式*/ENABLE(0X38); /*/开显示*/ENABLE(0X0c); /*/移动光标*/ENABLE(0X06); /*/显示位置*/ENABLE(0X80);void clear(uchar line) uchar i;nts switch(line) case 1: ENABLE(0x80);for(i=0;i16;i+) writedata( ); /_case 2:ENABLE(0xc0);for(i=0;i16;i+) writedata( ); /_case 3:ENABLE(0x80);for(i=0;i16;i+) writedata( ); /_ENABLE(0xc0);for(i=0;i16;i+) writedata( ); /_/*lcd*/ntsLCD 子程序 void test_busy(void) uchar i=1; do P0=0xff; RS=0; RW=1; E=1; if(P0&0x80)=0) break; E=0; while(-i!=0); /*/ void ENABLE(uchar order) P0=order; _nop_(); RS=0; _nop_(); RW=0; _nop_(); E=0;_nop_(); test_busy(); _nop_(); E=1; _nop_(); void writedata(uchar digital) P0=digital; _nop_(); nts RS=1; _nop_(); RW=0; _nop_(); E=0; _nop_(); test_busy(); _nop_(); E=1; _nop_(); /*/ void resetlcd(void) /*lcd 初始化设置 */ DELAY(); /*/清除屏幕 */ ENABLE(0X01); /*/8 位点阵方式 */ ENABLE(0X38); /*/开显示 */ ENABLE(0X0c); /*/移动光标 */ ENABLE(0X06); /*/显示位置 */ /ENABLE(0X80); void clear(uchar line) uchar i; switch(line) case 1: ENABLE(0x80); for(i=0;i16;i+) writedata( ); /_ case 2: ENABLE(0xc0); for(i=0;i16;i+) nts writedata( ); /_ case 3: ENABLE(0x80); for(i=0;i16;i+) writedata( ); /_ ENABLE(0xc0); for(i=0;i16;i+) writedata( ); /_ /*lcd*/ nts键盘子程序 n=P1; n=n&0x0f; if(n!=15) key_new=n; if(key_new=13) clear(1); t_heat=0; heat=0; ge=ge+1; tt=10*shi+ge; if(ge=9) ge=0; dis_word(set,0xc0,8); display(tt,0xc4); key_new=0; if(key_new=11) clear(1); t_heat=0; heat=0; shi=shi+1; tt=10*shi+ge; if(shi=11) nts shi=0; dis_word(set,0xc0,8); display(tt,0xc4); key_new=0; if(key_new=14) clear(1); tt=10*shi+ge; dis_word(set,0xc0,8); display(tt,0xc4); heat=1; key_new=0; if(key_new=7) clear(1); stop=1; heat=0; t_heat=0; t_cool=0; dis_word(stoped,0xc0,6); key_new=0; ntsn=P1;n=n&0x0f;if(n!=15)key_new=n;if(key_new=13)clear(1);t_heat=0;heat=0;ge=ge+1;tt=10*shi+ge;if(ge=9)ge=0;dis_word(set,0xc0,8);display(tt,0xc4);key_new=0;if(key_new=11)clear(1);t_heat=0;heat=0;shi=shi+1;tt=10*shi+ge;if(shi=11)shi=0;dis_word(set,0xc0,8);display(tt,0xc4);key_new=0;if(key_new=14)clear(1);tt=10*shi+ge;dis_word(set,0xc0,8);display(tt,0xc4);heat=1;key_new=0;if(key_new=7)clear(1);stop=1;heat=0;t_heat=0;t_cool=0;dis_word(stoped,0xc0,6);key_new=0; nts 毕业设计（论文）开题报告 题 目： 气体泄漏超声检测系统的设计 院 （系）： 通信与信息工程 专 业： 电子信息工程 班 级： 0102205 学 号： 010220531 姓 名： 韦 伟 指导教师： 晋良念 填表日期： 2006 年 3 月 16 日 nts毕业设计（论 文）开题报告 1本课题的目的及研究意义 目的：目前，工业上和生活中均大量用到用于储存和输送压缩气体的压力容器。如汽缸、气罐、煤气管道等。由于各种原因，容器会产生漏孔从而发生气体泄漏。据估计，工业上由于泄漏而损失掉的压缩气体平均占到 40%左右。泄漏不但会造成能源的浪费，而且如果是有害气体的话，还会对空气造成污染。因此准确地判断和定位产生泄漏的位置， 对于提高企业的生产效率和节约能源具有重大的意义。 研究意义：传统的泄漏检测方法和绝对压力法、压差法、气泡法等，操作复杂并且对技术人员要求较高，而且不具有实时性。目前，工业上广泛利用泄漏产生超声波的原理来进行泄漏检测。利用超声波检测气体泄漏位置，不仅方法简单，而且准确可靠。基于此，本课题研究并设计了一种气体泄漏超声波检测系统。 2本课题的国内外的研究现状 气体泄漏问题 是目前一个非常活跃的研究课题，它可以广泛存在于 管道、煤气、气罐等的泄漏。 比如作为气体泄漏的一个分支： 管道泄漏不仅造成大的天然气 输差损失，而且严重危及管道安全；随着管道的建设，作为管道监控核心的泄漏检测技术一直受到各国科技工作者的重视。我国管道工业的起步较晚，所以泄漏检测技术也相对比较落后。目前主要依靠巡管工沿管道进行实际观察，无法及时准确地发现泄漏事故。 气体 泄 漏 检测技术 不尽如人 意的原因：由于气体本身的性质特点决定的，因此对于这类问题的研究必须付出更多的努力。 nts毕业设计（论 文）开题报告 3本课题的研究内容 设计一种气体泄漏超声检测系统，在通过对分析小孔气体泄漏产生超声波原理的基础上 ， 研究该检测系统的原理及 设计方案。使该系统能对各种压力容器的孔隙泄漏所产生的微弱超声信号进行精确检测。 并且 利用 单片机 技术对泄漏所产生的超声波信号进行分析处理和声压级计算 ,从而实现对泄漏的检 测及泄漏量的估算 。 4本课题的实行方案、进度及预期效果 nts 实行方案 1 检测原理 1.1 气体泄漏产生超声波 如果一个容器内充满气体 ,当其内部压强大于外部压强时 ,由于内外压差较大 ,一旦容器有漏孔 ,气体就会从漏孔冲出。当漏孔尺寸较小且雷诺数较高时 ,冲出气体就会形成湍流 ,湍流在漏孔附近会产生一定频率的声波 ,如图 1 所示。 图 1 气体泄漏产生超声波 1.2 声压与泄漏量的关系 泄漏超声本质上是湍流和冲击噪声。泄漏驻点压力 P 与泄漏孔口直径 D 决定了湍流声的声压级 L。著名学者马大猷教授推出如下公式 1: 式中 ,L 为垂直方向距离喷口 1m 处的声压级 (单位 :dB);D 为喷口直径 (单位 :mm);D0 1mm;P0为环境大气绝对压力 ;P 为泄漏孔驻压。 图 2 声压级与雷诺数的 关系 2. 系统硬件实现 小孔气体泄漏所发出的超声波强度是极其微弱的 ,而且在工业场合 ,环境噪声是相当大的。所以要检测出在恶劣环境下的气体泄漏所发出的超声 ,必须对系统信号放大部分进行精心的设计。在本系统中只检测 40kHz 点的泄漏超声波的强度 ,原因是通过实验得出 ,在 40kHz 点的泄漏超声波能量都是比较大的 ,而且泄漏声和本底噪声能量差值也最大 (如图 3 所示 )。这样选择可以增加系统灵敏度。 nts 系统原理如图 4 所示。系统分为模拟和数字两部分 ,模拟部分包括信号放大电路和音频处理电路等。信号放大电路由前置放大电路、带通 滤波电路和二次放大电路组成。音频处理电路由本振电路、混频器、功率驱动电路组成。数字部分主要由 单片机 和 数码管 、 RAM、键盘等外围设备组成。传感器信号经过放大滤波以后 ,一路交由 单片机 处理 ,另一路通过降频转化为可听声。 图 3 本底噪声与泄漏声声压图 图 4 系统原理图 2.1 信号放大电路 图 5 所示为模拟电路的信号放大部分。 图 5 信号放大电路 nts2.2 音频处理电路设计 设计音频处理电路的目的是能够比较方便地判断哪里有泄漏的产生。人耳的听觉范围大约在 1kHz 到 20kHz 之间。因此检测到的超声信号必须通过降频才能为人耳所听到。降频的原理是利用差分信号的乘法特性 : 然后在 Uo 后接上低通滤波器 ,则可得差频信号。如选用本振电路的频率为37kHz,那么得到的差频信号为 3kHz,可为人耳听到。音频处理电路的原理图如图 6 所示。 图 6 音频处理电路原理图 2.3 单片机 2.4 数码管显示部分设计 2.5 键盘电路设计 3. 系统软件部分设计 因为系统要完成测量泄漏超声的声压级、估算泄漏量以及完成显示功能 ,所以软件主要由信号采集子程序、滤波子程序、 FFT 变换程序、泄漏估算子程序、 LCD 显示子程序、键盘服务子程序等组成。限于篇幅 ,在 此只列出程序设计的总体思路 ,如图9 所示。本文所介绍的超声波泄漏检测系统具有精度高、体积小、便于携带和具有很好的人机交互界面等特点。该系统还利用 单片机 等技术实现了对泄漏量的估算。 nts本课题进度计划表 3 月 2 日 3 月 10 日 毕业设计准备，查资料，翻译外文资料 3 月 11 日 4 月 1 日 拟定设计方案 4 月 2 日 4 月 29 日 电路的设计与制作 4 月 30 日 5 月 29 日 软件编程和系统调试 5 月 30 日 6 月 13 日 撰写论文；教师评阅论文，评阅人评阅。准备毕业设计答辩 6 月 15 日 6 月 20 日 毕业设计验收、答辩 预期效果 作出 一种气体泄漏超声波检测 系统。 nts毕业设计（论 文）开题报告 5、已查阅参考文献 ： 1 王化祥，张淑英 . 传感器原理及应用 M. 天津：天津大学出版社， 1999. 2 于亚非 .用超声波传感器检测气体泄漏 J. 仪器与未来 ,1992(8). 3 李光海，王勇等 .基于声发射技术的管道泄漏检测系统 J. 自动化仪表 ,2002,5. 4 李进，陈会仓等 .气体泄漏超声波检测装置 J. 工业仪表与自动化装置 ,1996,5. 5 龚其春 ,叶骞 .新型气体泄漏超声检测系统的研究与设计 J.电子技术应用 ,200 5,3. 6 Donald A.Neamen. Electronic Circuit Analysis and Design(Second edition). Published by the McGraw-Hill companies,2001. 指导教师意见 该同学能正确理解课题的要求和任务，提出的方案基本可行，进度安排符合教学计划，达到开题要求。课题中超声波的检测及声压与泄露量关系的理论分析是难点 。 指导教师： 2006 年 3 月 12 日 答辩小组意见 该生课题通过答辩小组考核，同意开题。 答辩组长： 小组成员： 2006 年 3 月 12 日 院（系）审查意见 院（系）领导（公章）： 2006 年 3 月 15 日 nts 桂林电子科技大学毕业设计（论文）译文用纸 第 1 页 共 20 页 RF and Microwave Fiber-Optic Design Guide Introduction Agere Systems Inc., through its predecessors, began developing and producing lasers and detectors for linear fiber-optic links nearly two decades ago. Over time, these optoelectronic components have been continually refined for integration into a variety of systems that require high fidelity, high frequency, or long-distance transportation of analog and digital signals. As a result of this widespread use and development, by the late 1980s, these link products were routinely being treated as standard RF and microwave components in many different applications. There are several notable advantages of fiber optics that have led to its increasing use. The most immediate benefit of fiber optics is its low loss. With less than 0.4 dB/km of optical attenuation, fiber-optic links send signals tens of kilometers and still maintain nearly the original quality of the input. The low fiber loss is also independent of frequency for most practical systems. With laser and detector speeds up to 18 GHz, links can send high-frequency signals in their original form without the need to downconvert or digitize them for the transmission portion of a system. As a result, signal conversion equipment can be placed in convenient locations or even eliminated altogether, which often leads to significant cost and maintenance savings. Savings are also realized due to the mechanical flexibility and lightweight fiber-optic cable, approximately 1/25 the weight of waveguide and 1/10 that of coax. Many transmission lines can be fed through small conduits, allowing for high signal rates without investing in expensive architectural supports. The placement of fiber cable is further simplified by the natural immunity of optical fiber to electromagnetic interference (EMI). Not only can large numbers of fibers be tightly bundled with power cables, they also provide a uniquely secure and electrically isolated transmission path. The general advantages of fiber-optics first led to their widespread use in long-haul digital telecommunications. In the most basic form of fiber-optic communications, light from a semiconductor laser or LED is switched on and off to send digitally coded information through a fiber to a photodiode receiver. By comparison, in linear fiber-optic systems developed by Lucent, the light sent through the fiber has an intensity directly related to the input electrical current. While this places extra requirements on the quality of the lasers and photodiodes, it has been essential in many applications to transmit arbitrary RF and microwave signals. As a result, tens of thousands of Agere Systems transmitters are currently in use. nts 桂林电子科技大学毕业设计（论文）原文用纸 第 2 页 共 20 页 The information offered here examines the basic link components, provides an overview of design calculations related to gain, bandwidth, noise, and dynamic range and distortion. A section on fiber-optic components discusses a number of key parameters, among them wavelength and loss, dispersion, reflections, and polarization and attenuation. Additional information evaluates optical isolators, distributed-feedback lasers and Fabry- Perot lasers, predistortion, and short- vs. long-wavelength transmission. One of linear optical fiber relation main usages or receives between the electronic installation and the remote localization antenna in the transmission transmits RF and the microwave signal。 Because of the optical fiber chain flexibility， possibly can for the simulation or digital signal design some antennas， including the military and the commercial communications satellite， the global localization satellite， the remote sensing with traces the lighthouse， or wireless cell network 。 Another kind of type chain is the optical fiber delay line， installs in a package it including a transmitter， a receiver， with long textile fiber。 It may provide the long delay time, high band width, with low weight. These high-frequency RF and the microwave product has obtained benefits from the use linear optical fiber equipment cable television raging tide middle earth. In here, the textile fiber expands the TV signal the transmitting range, improves their quality and the system reliability, but when merely only has the electric cable, even with the system which used compares has saved the expense Typical Linear Link Components In each of these applications, as well as many others, the Agere Systems transmitters and receivers comprising the links are similar and can be treated as standard microwave components. Focusing on these common elements, this design guide describes the general technical considerations and equations necessary for engineers to choose the most appropriate Agere Systems components for their systems. These equations also have been incorporated into various programs, which an Agere Systems applications engineer can use to provide an analysis for a specific link application. Figure 1 shows the three primary components in a fiber-optic link: an optical transmitter, a fiber-optic cable, and an optical receiver. In the transmitter, the input signal modulates the light output from a semiconductor laser diode, which is then focussed into a fiber-optic cable. This fiber carries the modulated optical signal to the receiver, which then reconverts the optical signal back to the original electrical RF signal. nts 桂林电子科技大学毕业设计（论文）原文用纸 第 3 页 共 20 页 Basic Link Applications and Components For RF systems, distributed feedback (DFB) lasers are used for low-noise, high-dynamic range applications, and Fabry-Perot lasers for less demanding applications. The wavelength of these lasers is either 1310 nm or 1550 nm. The intensity of the laser light is described by the simplified light-current (L-I) curve in Figure 2. When the laser diode is biased with a current larger than the threshold current, ITH, the optical output power increases linearly with increasing input current. Analog links take advantage of this behavior by setting the dc operating point of the laser in the middle of this linear region. Typically, this bias current for Agere Systems transmitters is set somewhere between 40 mA to 90 mA. The threshold current ranges from 10 mA to 30 mA.The efficiency with which the laser converts current to usable light is given by the slope of the L-I curve and is called the modulation gain. For typical Agere Systems lasers, this dc modulation gain ranges from 0.02 W/A to 0.3 W/A, depending on the model chosen. The wide variation is largely due to differing methods of coupling the light into the optical fiber. The modulation gain also varies somewhat with frequency, so it must be specified whether a particular value is a dc or higher-frequency gain. In addition to the laser diode, transmitters also contain a variety of other components, depending on the specific application or level of integration desired. nts 桂林电子科技大学毕业设计（论文）原文用纸 第 4 页 共 20 页 The most basic laser module package contains the laser chip, optical fiber, and impedance-matched electrical connections in a hermetically sealed container such as the one shown in Figure 3. Modules also may contain a photodiode for monitoring the laser power, a thermistor and a thermoelectric (TE) cooler for monitoring and controlling the laser temperature, and an optical isolator for reducing the amount of light reflected back to the laser from the fiber. The basic laser module, although available as a subcomponent, is usually integrated into a complete transmitter housing such as the flange-mount and plug-in packages shown in Figure 4. These transmitters also may include dc electronics to control the laser temperature and bias current, amplifiers and other circuitry to precondition the RF signal, and various indicators for monitoring the overall transmitter performance. Because analog fiber-optic transmitters are used in a variety of applications, the exact implementation of these product features varies as well. nts 桂林电子科技大学毕业设计（论文）原文用纸 第 5 页 共 20 页 Basic Link Applications and Components (continued) At the other end of the fiber-optic link, the light is detected by the receiver PIN photodiode, which converts the light back into an electrical current. The behavior of the photodiode is given by the responsivity curve shown in Figure 5. Once again, note that the response is very linear. The slope of this curve is the responsivity, which typically is greater than 0.75 mA/mW for a photodiode chip without any impedance matching. Similar to Lucents laser diodes, photodiodes are packaged in a hermetic module containing an impedance matching network and electrical lines to provide dc bias and RF output. However, unlike the laser, the photodiode is relatively insensitive to temperature so a thermoelectric cooler (TEC) is not required. Special precautions also are made to minimize optical reflections from returning back through the fiber, which otherwise could degrade a links performance. nts 桂林电子科技大学毕业设计（论文）原文用纸 第 6 页 共 20 页 These photodiode modules are often integrated into more complete receiver packages similar to the flange-mount and plug-in varieties of the transmitters. In these receivers, circuitry reverse biases the diodes to increase the response speed. Receivers also contain monitor and alarm outputs. Some receivers may include a post amplifier, broadband current transformers, and/or impedance matching networks to improve the link gain. Due to such circuitry, the efficiency of a receiver generally will differ from the responsivity of the photodiode chip alone. A fiber-optic cable is the third primary component in a linear optical link. Single-mode fiber, as opposed to multimode fiber, is always used with Agere Systems links because of its low dispersion and low loss. At a wavelength of 1310 nm, the fiber attenuates the optical signal by less than 0.4 dB/km; at 1550 nm, less than 0.25 dB/km。 Typically, the fiber is cabled in rugged yet flexible 3 mm diameter tubing and connected to the transmitter and receiver with reusable optical connectors. The modular nature of the cable simplifies the design of the physical architecture of the system and enables a wide range of configuration possibilities. Although Agere Systems does not supply optical fiber, several important considerations should be followed in selecting these components. The section entitled Selection of Optical Fiber Components, page 19, describes these issues in detail. When selecting the proper components for a fiber-optic link, there are several critical quantities that must be defined and calculated prior to its implementation, just as would be done with any RF or microwave communication link. Topics of discussion in this section include link gain, bandwidth, noise, dynamic range, and distortion, and the use of that information as an example for a typical link. The detailed equations in this section also have been incorporated into various design programs, which a Agere Systems applications engineer can use to provide the predicted performance of a link in a specific application. Link Design Calculations (continued) Gain The RF loss (or gain) of an optical link is a function of four variables, including transmitter efficiency, fiber loss, receiver efficiency, and the ratio of the output to input impedances. In its most basic form, the power gain of the link can be written in terms of the input and output currents as equation 1： where ROUT is the load resistance at the receiver output and RIN is the input resistance of the nts 桂林电子科技大学毕业设计（论文）原文用纸 第 7 页 共 20 页 laser transmitter. The (IOUT/IIN) term can be expanded in terms of the link characteristics as: equation 2 where Tx, RF is the efficiency of the total transmitter, including any amplifiers and matching networks, in converting input RF currents into optical power modulations. Rx, RF is the efficiency of the total receiver in converting optical power modulations into RF output current. (This RF value is not the same as the dc photodiode responsivity, as described in the section on Bandwidth, page 8.) The units for Tx, RF and Rx, RF are W/A and A/W, respectively. LO is the optical loss of the fiber portion of the link measured as: equation 3 Substituting equation 2 into equation 1 then gives the total gain of a link: equation 4, The factors of Tx, RF and Rx, RF are sometimes converted to a form more similar to traditional RF gains by taking 20log, so that equation 4 can be simplified to: equation 5, where TG is the transmitter gain in dB2W/A and RG is the receiver gain in dB2A/W. TG and RG are related to the units total RF efficiency expressed in W/A or A/W as follows: equation 6, equation 7, For example, combining a 75 transmitter with a TG of 1 dB2W/A, a 75 receiver with an RF of +20 dB2A/ W and a 12 dB optical loss, would give an RF gain for the link of: Figure 6 shows the effects of optical loss and transmitter RF efficiency for a receiver with an efficiency of 0.375 mA/mW (RG of 8.51dB2A/W), as calculated with equation 4. The Appendix, page 28, contains additional sets of curves for other typical transmitter and receiver efficiencies. nts 桂林电子科技大学毕业设计（论文）原文用纸 第 8 页 共 20 页 An interesting and often overlooked aspect of equation 4 is the 2 LO term. As this indicates, for each additional dB of optical loss, there is an additional 2 dB of RF loss. This oddity occurs as a result of converting optical power to RF energy. Here, the RF current is directly proportional to the optical power, but the RF power equals the square of the RF current. When taking the log, this squared term turns into a factor of 2 in front of the optical loss. For example, a transmitter and receiver pair that have a 35 dB RF gain when they are directly connected with 0 dB of optical loss, would have a 39 dB RF gain when connected with a 2 dB loss fiber. Resistively Matched Components To calculate the total insertion loss for a specific link, consider the broadband resistively matched link shown in Figure 7. In this case, the laser transmitter includes the laser diode, with a typical impedance of 5 , and a resistor to raise the total input impedance, RIN, up to the impedance of the external signal source. The photodiode module includes the photodiode, with a typical impedance of several k, and a resistor RPD to match to the output impedance RL. Such matching resistors ubstantially improve the VSWR of the link over that of an unmatched link. Due to this extra photodiode resistor, the current output from the receiver, IOUT, will be less than the total current produced by the photodiode chip, IPD. The RF efficiency of the receiver, Rx, RF, is therefore correspondingly smaller than the responsivity of the photodiode chip alone, RPD: GLINK = 35 dB equation 8, nts 桂林电子科技大学毕业设计（论文）原文用纸 第 9 页 共 20 页 For a 50 matched system, RPD and ROUT each would be approximately 50 , therefore, the receiver RF efficiency would be half that of the photodiode chip on its own. This decreases the overall link gain by 6 dB. For a resistively-matched photodiode receiver: equation 9, The transmitter RF efficiency, on the other hand, does not experience such a drop of 6 dB due to the fact that the matching resistor is placed in series rather than in parallel. Therefore, within the bandwidth of a transmitter, its RF efficiency is approximately equal to the dc modulation gain of the laser diode. As an example, consider a transmitter with a dc modulation gain of 0.1 W/A, a resistively matched receiver with a dc responsivity of 0.75 and a fiber with an optical loss of 3 dB. To the first order, RF efficiency of the transmitter will be 0.1 W/A and the RF efficiency of the receiver will be 0.375 A/W. If both the transmitter and receiver are matched to 50 , then the impedance matching term of equation 4 drops out, leaving an RF link gain of approximately: equation 10, Link Design Calculation (continued) To overcome such a loss, many links incorporate additional amplifiers, which are described more fully in the sections on Receiver Noise, page 10; Placement of Amplifiers, page 15; and in the section entitled Example, page 15. As an alternative for narrowband systems, the link gain can be improved by impedance matching so that the laser diode and nts 桂林电子科技大学毕业设计（论文）原文用纸 第 10 页 共 20 页 photodiode see an effective ROUT/RIN 1. The matching electronics used in such links are carefully designed to produce this extra gain without creating reflections or poor VSWR. The range of frequencies over which a fiber-optic link can transmit is limited by the bandwidth of the transmitter and receiver and by the dispersion of the optical fiber. The bandwidth limit of a link generally is defined as the frequency at which the microwave modulati
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