信息工程專業(yè)外文翻譯5

1、<p>　　本科畢業(yè)設(shè)計（論文）</p><p>　　外文參考文獻(xiàn)譯文及原文</p><p>　　學(xué) 院 信息工程學(xué)院 </p><p>　　專 業(yè) 信息工程(電子信息工程方向) </p><p>　　年級班別 2004級（4）班

2、 </p><p>　　學(xué) 號 3104002975 </p><p>　　學(xué)生姓名 陳英權(quán) </p><p>　　指導(dǎo)教師 劉喜英 </p><p&

3、gt;　　2008 年 6 月 5 日</p><p><b>　　目 錄</b></p><p><b>　　外文參考文獻(xiàn)譯文</b></p><p><b>　　1鎖相環(huán)1</b></p><p>　　1.1鎖相特性1</p><p>

4、　　1.2歷史與應(yīng)用2</p><p>　　1.3其它應(yīng)用4</p><p><b>　　2光通信元件5</b></p><p><b>　　2.1光纖5</b></p><p>　　2.2調(diào)制器和檢測器6</p><p><b>　　外文參考文

5、獻(xiàn)原文</b></p><p>　　1Phase Lock Loop9</p><p>　　1.1Nature of Phaselock9</p><p>　　1.2History and Application10</p><p>　　1.3Other Applications13</p><

6、p>　　2Optical Communication Components14</p><p>　　2.1The Optical Fiber14</p><p>　　2.2Modulators and Detectors17</p><p><b>　　鎖相環(huán) </b></p><p><b>

7、;　　鎖相特性</b></p><p>　　鎖相環(huán)包含三個組成部分：</p><p>　　1、相位檢測器（PD）。</p><p><b>　　2、環(huán)路濾波器。</b></p><p>　　3、壓控振蕩器（VCO），其頻率由外部電壓控制。 </p><p>　　相位檢測器將一個周期輸入

8、信號的相位與壓控振蕩器的相位進(jìn)行比較。相位檢測器的輸出是它兩個輸入信號之間相位差的度量。差值電壓由環(huán)路濾波后，再加到壓控振蕩器上。壓控振蕩器的控制電壓使頻率朝著減小輸入信號與本振之間相位差的方向改變。</p><p>　　當(dāng)鎖相環(huán)處于鎖定狀態(tài)時，控制電壓使壓控振蕩器的頻率正好等于輸入信號頻率的平均值。對于輸入信號的每一周期，振蕩器輸出也變化一周，且僅僅變化一周。鎖相環(huán)的一個顯而易見的應(yīng)用是自動頻率控制（AFC）。

9、用這種方法可以獲得完美的頻率控制，而傳統(tǒng)的自動頻率控制技術(shù)不可避免地存在某些頻率誤差。</p><p>　　為了保持鎖定環(huán)路所需的控制電壓，通常要求相位檢測器有一個非零的輸出，所以環(huán)路是在有一些相位誤差條件下工作的。不過實(shí)際上對于一個設(shè)計良好的環(huán)路這種誤差很小。</p><p>　　一個稍微不同的解釋可提供理解環(huán)路工作原理的更好說明。讓我們假定輸入信號的相位或頻率上攜帶了信息，并且此信號不

10、可避免地受到加性噪聲地干擾。鎖相接收機(jī)的作用是重建原信號而盡可能地去除噪聲。</p><p>　　為了重建原始信號，接收機(jī)使用一個輸出頻率與預(yù)計信號頻率非常接近的本機(jī)振蕩器。本機(jī)振蕩和輸入信號的波形由相位檢測器比較，其誤差輸出表示瞬時相位差。為了抑制噪聲，誤差在一定的時間間隔內(nèi)被平均，將此平均值用于建立振蕩器的頻率。</p><p>　　如果原信號狀態(tài)良好（頻率穩(wěn)定），本機(jī)振蕩器只需要極少

11、信息就能實(shí)現(xiàn)跟蹤，此信息可通過長時間的平均得到，從而消除可能很強(qiáng)的噪聲。環(huán)路輸入是含噪聲的信號，而壓控振蕩器輸出卻是一個純凈的輸入信號（的復(fù)本）。所以，有理由認(rèn)為環(huán)路是一種傳輸信號并抑制噪聲的濾波器。</p><p>　　環(huán)路濾波器有兩個重要的特性：其一是帶寬可以非常窄，其二是濾波器能自動跟蹤信號頻率。自動跟蹤和窄帶的特點(diǎn)說明了鎖相接收機(jī)的主要用途。窄帶能夠抑制大量的噪聲，難怪鎖相環(huán)路常用來恢復(fù)深深地淹沒在噪聲中

12、的信號。</p><p><b>　　歷史與應(yīng)用</b></p><p>　　關(guān)于鎖相的早期論述（思想）是Bellescize于1932 年提出的，并在處理無線電信號同步接收中得到應(yīng)用。20世紀(jì)20年代開始使用超外差接收機(jī)，但人們一直努力尋求更簡單的接收技術(shù)。一種方法就是同步接收機(jī)或零差接收機(jī)。這種接收機(jī)本質(zhì)上只是由一個本機(jī)振蕩器，一個混頻器和一個音頻放大器組成。為了

13、正常工作，必須調(diào)節(jié)振蕩器使其輸出頻率與輸入的信號載波頻率完全一致，于是載波被變換成0Hz的“中頻”?；祛l器輸出含有解調(diào)出來的，由信號邊帶攜帶的信息。干擾與本地振蕩器不同步，因此由干擾信號引起的混頻器輸出是一個拍音，可用音頻濾波器加以抑制。</p><p>　　對于同步接收，本振的正確調(diào)諧至關(guān)重要，任何一點(diǎn)頻率誤差都將嚴(yán)重?fù)p壞信號。此外，本振的相位必須與接收的載波相位一致，其間的誤差限于周期的很小一部分。就是說，本

14、振與輸入信號之間必須實(shí)現(xiàn)相位鎖定。</p><p>　　由于各種原因簡單的同步接收機(jī)從未廣泛應(yīng)用過?，F(xiàn)在鎖相接收機(jī)幾乎無例外地運(yùn)用超外差原理，并趨于高度復(fù)雜化。鎖相接收機(jī)最重要的應(yīng)用之一是接收來自遙遠(yuǎn)的宇宙飛行器的極微弱信號。鎖相技術(shù)的首次廣泛使用是在電視接收機(jī)中的行和幀的同步掃描。與視頻信號一起傳送的脈沖發(fā)出電視圖像每一行的開始信號和隔行掃描的半幀開始信號。作為一種非常粗糙的重建電視顯象管掃描光柵的方法，這些脈

15、沖可以剝離出來單獨(dú)用于觸發(fā)一對掃描發(fā)生器。</p><p>　　一個較為復(fù)雜的途徑是利用一對自由振蕩的張弛振蕩器驅(qū)動掃描發(fā)生器。用這種方法，即使失去同步（消失），掃描還是存在的。</p><p>　　將振蕩器的自由振蕩頻率設(shè)置得略低于水平和垂直（掃描）脈沖頻率，剝離出來的脈沖用于提前觸發(fā)振蕩器從而使振蕩器與行頻和半幀頻同步（由于美國電視在交替的垂直掃描時進(jìn)行隔行交織，所以是半幀頻）。<

16、;/p><p>　　在噪聲不存在的情況下這種方案可提供良好的同步，這就完全可以了。不幸的是噪聲總是存在的，并且任何觸發(fā)電路對噪聲都是特別敏感的。在極端情況下觸發(fā)掃描將完全失效，盡管在這樣的信噪比條件下電視圖像雖然較差卻還能辯認(rèn)。</p><p>　　在不是極端惡劣的條件下，噪聲將造成起始時間抖動和偶爾的誤觸發(fā)。行抖動將降低行清晰度,并使得垂直線條呈現(xiàn)鋸齒狀。嚴(yán)重的水平誤觸發(fā)通常會造成畫面出現(xiàn)狹

17、窄的水平黑帶。</p><p>　　幀掃描抖動會引起圖像的垂直滾動。另外，相繼半幀之間的隔行掃描行還會相對移動，使圖像進(jìn)一步惡化。 </p><p>　　將兩個振蕩器與剝離出來的同步脈沖鎖相可大大減小噪聲起伏。鎖相技術(shù)靠檢查各振蕩器和許多同步脈沖之間的相位關(guān)系來調(diào)節(jié)振蕩頻率，使得平均相位偏差很小，而不是僅用一個脈沖進(jìn)行觸發(fā)。由于鎖相同步器檢測許多脈沖，因此它不會被偶發(fā)的破壞同步器觸發(fā)的大幅

18、度脈沖噪聲所干擾。目前電視接收機(jī)中使用的飛輪同步器實(shí)際上就是鎖相環(huán)路。使用飛輪一詞是因為此電路能夠跟蹤增加的噪聲或微弱信號的周期。通過鎖相可以獲得同步性能的重大改進(jìn)。</p><p>　　在彩色電視接收機(jī)中彩色副載波是由鎖相環(huán)路同步的。</p><p>　　宇宙飛行的需要強(qiáng)烈地刺激了鎖相技術(shù)的應(yīng)用。鎖相的空間應(yīng)用是隨著早期美國人造衛(wèi)星的發(fā)射而開始的。這些飛行體攜帶低功率（10毫瓦）的連續(xù)波

19、發(fā)射機(jī)，相應(yīng)的接收信號很微弱。由于多普勒頻移和發(fā)射振蕩器的頻率漂移，接收信號的精確頻率難以確定。在最初使用的108MHz頻率上，多普勒頻移可在3kHz 范圍內(nèi)。</p><p>　　因此使用普通的固定調(diào)諧接收機(jī)時，帶寬至少應(yīng)為6kHz，然而信號本身卻只占非常窄的頻譜，大約在6Hz帶寬內(nèi)。</p><p>　　接收機(jī)中的噪聲功率與帶寬成正比，所以如果使用傳統(tǒng)的技術(shù)，就不得不接受1000倍（3

20、0dB）噪聲的代價。隨著技術(shù)的進(jìn)步這些數(shù)字變得更加驚人。發(fā)射頻率上升到了S波段，使多普勒頻移范圍達(dá)到75kHz，而接收機(jī)帶寬則已減小到3Hz。這樣一來常規(guī)技術(shù)的代價就將是47dB左右。這是無法接受的，也就是要使用窄帶的鎖相跟蹤接收機(jī)的原因所在。</p><p>　　窄帶濾波器能抑制噪聲，但是如果濾波器被固定，則信號將幾乎總是落在通帶之外。一個可用的窄帶濾波器必須有跟蹤信號的能力。鎖相環(huán)路既提供了窄帶，又提供了所需

21、的跟蹤能力。而且，非常窄的帶寬也能方便地獲得（對于空間應(yīng)用典型的是３到1000Hz）。如果需要的話，還能容易地改變帶寬。</p><p>　　對于多普勒信號，用于確定飛船速度的信息是多普勒頻移。鎖相接收機(jī)很適合用于多普勒恢復(fù)，因為當(dāng)鎖相環(huán)路鎖定時不存在頻率誤差。</p><p><b>　　其它應(yīng)用</b></p><p>　　以下的應(yīng)用闡述了

22、目前鎖相技術(shù)的一些應(yīng)用，這些應(yīng)用將在本書其他章節(jié)進(jìn)一步討論。</p><p>　　1、跟蹤運(yùn)動飛船的一種方法涉及到將相干信號發(fā)射到飛船上，將信號頻率偏移并轉(zhuǎn)發(fā)回地面。飛船上的相干應(yīng)答器必須如此工作以使輸入和輸出頻率嚴(yán)格地成m/n的比例關(guān)系，此處m和n 都是整數(shù)。鎖相技術(shù)經(jīng)常被用來建立相干性。</p><p>　　2、鎖相環(huán)可用作頻率解調(diào)器，鎖相環(huán)在其中比傳統(tǒng)的鑒頻器具有更優(yōu)越的性能。<

23、;/p><p>　　3、帶有噪聲的振蕩器可被包圍在環(huán)路內(nèi)，并使之鎖定在一個純凈的信號上。如果環(huán)路帶有的帶寬，振蕩器檢測出自已的噪聲，其輸出被大大凈化。</p><p>　　4、用鎖相環(huán)路可構(gòu)成頻率倍乘器和分頻器。</p><p>　　5、數(shù)字信號的發(fā)射通常應(yīng)用鎖相技術(shù)實(shí)現(xiàn)。</p><p>　　6、頻率合成器可方便地用鎖相環(huán)路構(gòu)成。</p&

24、gt;<p><b>　　光通信元件 </b></p><p><b>　　光纖</b></p><p>　　正如先前所討論的，大氣不能被用來作為地面光通信的傳輸信道。最有前途的信道是光纖波導(dǎo)。光纖基本上由一個中心透明的稱為纖芯的區(qū)域和一個環(huán)繞纖芯的稱為包層的折射率較低的區(qū)域所組成。纖芯的折射率既可以是均勻的，也可以是從中心向外具有

25、遞減梯度的。前一種光 纖也稱為勻芯光纖由于在纖芯包層的界面處的全內(nèi)反射現(xiàn)象而形成光導(dǎo)。后一種光纖稱為漸變率光纖是由光束朝纖芯中央連續(xù)折射而產(chǎn)生光導(dǎo)。</p><p>　　在光波導(dǎo)中，存在著不改變場結(jié)構(gòu)并以固定的相位和群速傳播的特殊的場分布。這些場結(jié)構(gòu)稱為光波導(dǎo)的模。這些模以不同的傳播常數(shù)和不同的群速度為特征。在多模光波導(dǎo)中，存在著大量的這種傳播模式，而在單模光波導(dǎo)中，只存在一種傳播模式。每種模式的大部分能量都在纖

26、芯內(nèi)部，但由于纖芯外部存在的迅衰場（泄漏場），一部分能量也在包層中傳播。通過將包層做得足夠厚，可使傳播模式的場在包層－空氣界面處很弱，使得光纖便于處置和支撐而不會嚴(yán)重地擾亂傳播模式。</p><p>　　正如已經(jīng)討論過的那樣，在光纖通信系統(tǒng)中，信息以離散脈沖的形式編碼，通過光纖傳輸。系統(tǒng)的信息容量將由單位時間內(nèi)可發(fā)送的脈沖數(shù)來確定。為了在輸出端恢復(fù)信息，各個脈沖必須能在時間上被正確分辨。在光纖中由于不同模式之間的

27、群速度不等，以及各模式的傳輸常數(shù)依賴于波長等因素，光脈沖會在光纖傳輸過程中變寬。 </p><p>　　因此，即使兩個脈沖在輸入端可以很好地分辨，因脈沖展寬他們在輸出端可能無法分辨。在這種情況下，就不能在輸出端恢復(fù)信息。因此，對于某一給定的展寬，脈沖之間必須以一 個最小的時間間隔分開，這個時間間隔就確定了系統(tǒng)的最大信息容量。</p><p>　　當(dāng)發(fā)射脈沖射入光纖時，就會激勵光纖的各種模式

28、。由于每一種模式一般都以不同的特征群速度傳播，所以入射光脈沖隨著傳播而展寬，稱為模間展寬。當(dāng)光纖能只傳播一種模式時，即在單模 光纖中，這種展寬不存在。但是，由于傳播常數(shù)依賴于波長，仍然存在著某種展寬，稱為模內(nèi)展寬。模間擴(kuò)散和模內(nèi)擴(kuò)散都是由于波導(dǎo)效應(yīng)和材料效應(yīng)引起的。材料效應(yīng)是由于光源為有限帶寬，以及對不同波長的光有不同的折射率而產(chǎn)生的一種效應(yīng)。這里要提一下，由于激光器與發(fā)光二極管（LED）相比具有更小的頻譜寬度，因此使用激光器的系統(tǒng)與使

29、用LED的系統(tǒng)相比其材料色散更小。例如，使用LED，由于材料色散脈沖展寬可能是每公里４毫微秒左右，而使用激光器，材料色散每公里小于0.2毫微秒。</p><p>　　可以利用幾何光學(xué)概念形象地說明光脈沖在光纖中傳播時的展寬現(xiàn)象。光脈沖注入勻芯光纖時會產(chǎn)生與軸線成不同夾角的光線。由于與軸線夾角較大的光線必須經(jīng)過較長的光程，因此它們要用更長的時間到達(dá)輸出端。所以，光脈沖在光纖中傳播時就會展寬。與此相對應(yīng)的是，在漸變率

30、光纖中，盡管與軸線夾角較大的光線必須通過較長的光程，但它們是在折射率較低的媒介中傳播。于是較長的光程被較高速的傳播部分地補(bǔ)償。所以與勻芯光纖比較，漸變光纖中的脈沖展寬必定更小。事實(shí)上情況確實(shí)如此，對于寬帶應(yīng)用漸變光纖比勻芯光纖更適用。</p><p>　　這里要提到，纖芯半徑很小，同時纖芯與包層間折射率差也較小時，可以制成只存在一種傳播模式的光纖。這類光纖稱為單模光纖。因為只有一種模式存在，所以這些光纖中的色散很

31、小，而且色散只是由模內(nèi)展寬造成的。這種光纖確實(shí)可望用于將來的超帶寬系統(tǒng)中。使用光波的系統(tǒng)除了具有極大的信息容量外，與同軸電纜等傳統(tǒng)金屬系統(tǒng)相比，通過光纖通信 或傳輸還有另一些優(yōu)點(diǎn)。</p><p>　　1、由于實(shí)際可獲得的光纖傳輸損耗極低，人們可以獲得更大的中繼距離，從而節(jié)約大量資金。</p><p>　　2、光纖的平均直徑大約是100 微米，基本上由石英或玻璃制成。這使光纖的重量和所占空

32、間都大大縮減，這一點(diǎn)對于敷設(shè)在已經(jīng)布滿電纜的管道中十分重要。重量和體積的這種節(jié)約 對于船用和航空使用光纖傳送數(shù)據(jù)也十分重要。</p><p>　　3、光纖不受電磁干擾影響，而且沒有串音。這對于國防上的保密通信十分重要。</p><p>　　4、由于不存在任何由短路等原因造成的危險，光纖可使用在易爆和高壓環(huán)境中。</p><p>　　除了在電通信方面的主要應(yīng)用，預(yù)期光

33、纖也將在計算機(jī)網(wǎng)絡(luò)，宇宙飛船，工業(yè)自動化和過程控制等領(lǐng)域中發(fā)揮重要作用。事實(shí)上，目前光纖已經(jīng)被用來在Lawrence Livemore 實(shí)驗室和Los Alamos 科學(xué)實(shí)驗室的大型聚變激光器中傳送數(shù)據(jù)和控制信息，也使用在Nevada 試驗場監(jiān)控地下核爆炸。使用光纖的額外優(yōu)點(diǎn)包括價格低廉和不受噪聲的影響。</p><p><b>　　調(diào)制器和檢測器</b></p><p&

34、gt;　　上面我們所討論的只是光波通信系統(tǒng)各組成部分之一。除了光纖，人們還需要能把信息編碼成 光波的調(diào)制器和能在接收端檢測光脈沖并把光脈沖解譯還原成信息的檢測器。我們將簡單討論調(diào)制器和檢測器的原理。</p><p>　　光源既可以通過改變其某個輸入?yún)?shù)如輸入電流來直接調(diào)制，也可以讓輸出的光通過稱為調(diào)制器的器件實(shí)現(xiàn)外部調(diào)制。用在光纖通訊系統(tǒng)中最有前途的光源即半導(dǎo)體激光源很容易通過改變輸入電流來調(diào)制。事實(shí)上，在數(shù)字系

35、統(tǒng)中光源必須被鍵控調(diào)制。實(shí)際上半導(dǎo)體光源能以上升時間小于1毫微秒的高速來鍵控調(diào)制。鍵控調(diào)制是通過把激光二極管偏置在稍低于門限值上來實(shí)現(xiàn)的，門限值一般為100毫安左右。這里激光二極管起LED的作用，以較低的輸出功率發(fā)射非相干光。由高速驅(qū)動器加入一個20mA左右的附加電流，將二極管激光器從非相干光發(fā)射狀態(tài)轉(zhuǎn)換成具有較大輸出光功率的相干光發(fā)射狀態(tài)。通過將“關(guān)閉”狀態(tài)保持略低于閾值可使加在激光二極管上的電脈沖與所產(chǎn)生的光輸出之間的延遲最小。這一

36、延遲必須不大于比特之間的間隔從而使光脈沖能精確重建輸入信號。</p><p>　　值得注意的一個重要因素是輸出光功率的溫度靈敏度。在上述方案中，這一點(diǎn)可通過用一個光 反饋回路改變直流偏置來保證，這樣可以兼顧環(huán)境溫度的緩慢變化和激光器本身逐漸老化兩種因素。通常由收集激光器背面射出的光來實(shí)現(xiàn)輸出功率監(jiān)控，激光器前面發(fā)出的光則全部耦合到光纖中去。</p><p>　　對于非半導(dǎo)體激光源，使用外部

37、調(diào)制器來實(shí)現(xiàn)調(diào)制。外部調(diào)制器利用不同材料所具有的不同特性。這樣,某些晶體具有隨外加電場變化的雙折射，于是讓光通過這樣的晶體可以改變光線的偏振狀態(tài)。如果把這個晶體放在正交的偏振鏡之間，人們就可以進(jìn)行光強(qiáng)調(diào)制。</p><p>　　類似地，聲－光調(diào)制器是建立在聲束與光波相互作用基礎(chǔ)上的。傳播的聲波產(chǎn)生一個折射率光柵，反過來使光波發(fā)生衍射。</p><p>　　在接收終端或中繼站，人們需要光檢測

38、器來接收輸入光信號并把它變換成電信號。光波通信中應(yīng)用的三種重要檢測器是光電倍增管，PIN光二極管和雪崩光二極管。盡管光電倍增管具有較大增益，后二種類型可望獲得更廣泛的應(yīng)用，因為他們體積小，不需要高的偏置電壓，而且更便宜。最簡單的固態(tài)光檢測器由一個具有開闊中心區(qū)域的反偏P-N結(jié)構(gòu)成，為了接收入射光，該區(qū)域涂有抗反射的涂層。所吸收的光子把電子從價帶激勵到導(dǎo)帶。由此產(chǎn)生的電子和空穴被外加電場分離，產(chǎn)生通過P-N結(jié)的光電流。</p>

39、<p>　　為了檢測極低的光功率，人們使用雪崩光檢測器。光子產(chǎn)生的電子-空穴對在這種器件中被加速，釋放出更多的電子-空穴對，以此獲得增益。</p><p>　　光纖通信系統(tǒng)中所需要的光檢測器在工作波長上必須具有高響應(yīng)度，為了適應(yīng)系統(tǒng)的信息率，還要有足夠的帶寬。對于0.80微米波長范圍的最有前景的光檢測器似乎是硅晶體光二極管。它們具有極快的響應(yīng)時間（<0.1 毫微秒）。量子效率即所產(chǎn)生的一次光電

40、子與入射在檢測器上的光子之比也很大。</p><p>　　Phase Lock Loop</p><p>　　Nature of Phaselock </p><p>　　A phaselock loop contains three components: </p><p>　　1. A phase detector (PD). </

41、p><p>　　2. A loop filter. </p><p>　　3. A voltage-controlled oscillator (VCO) whose frequency is controlled by an external voltage. </p><p>　　The phase detector compares the phase of a

42、periodic input signal against the phase of the VCO. Output of PD is a measure of the phase difference between its two inputs. The difference voltage is then filtered by the loop filter and applied to the VCO. Control vol

43、tage on the VCO changes the frequency in a direction that reduces the phase difference between the input signal and the local oscillator. </p><p>　　When the loop is locked, the control voltage is such that t

44、he frequency of the VCO is exactly equal to the average frequency of the input signal. For each cycle of input there is one, and only one, cycle of oscillator output. One obvious application of phaselock is in automatic

45、frequency control (AFC). Perfect frequency control can be achieved by this method, whereas conventional AFC techniques necessarily entail some frequency error . </p><p>　　To maintain the control voltage need

46、ed for lock it is generally necessary to have a nonzero output from the phase detector. Consequently, the loop operates with some phase error present. As a practical matter, however, this error tends to be small in a wel

47、l-designed loop . </p><p>　　A slightly different explanation may provide a better understanding of loop operation. Let us suppose that the incoming signal carries information in its phase or frequency; this

48、signal is inevitably corrupted by additive noise. The task of a phaselock receiver is to reproduce the original signal while removing as much of the noise as possible. </p><p>　　To reproduce the signal the r

49、eceiver makes use of a local oscillator whose frequency is very close to that expected in the signal. Local oscillator and incoming signal waveforms are compared with one another by a phase detector whose error output in

50、dicates instantaneous phase difference. To suppress noise the error is averaged over some length of time, and the average is used to establish frequency of the oscillator. </p><p>　　If the original signal is

51、 well behaved (stable in frequency), the local oscillator will need very little information to be able to track, and that information can be obtained by averaging for a long period of time, thereby eliminating noise that

52、 could be very large. The input to the loop is a noisy signal, whereas the output of the VCO is a cleaned-up version of the input. It is reasonable, therefore, to consider the loop as a kind of filter that passes signals

53、 and rejects noise. </p><p>　　Two important characteristics of the filter are that the bandwidth can be very small and that the filter automatically tracks the signal frequency. These features, automatic tra

54、cking and narrow bandwidth, account for the major uses of phase lock receivers. Narrow bandwidth is capable of rejecting large amounts of noise; it is not at all unusual for a PLL to recover a signal deeply embedded in n

55、oise. </p><p>　　History and Application </p><p>　　An early description of phaselock was published by de Bellescize in 1932 and treated the synchronous reception of radio signals. Superheterodyne

56、 receivers had come into use during the 1920s, but there was a continual search for a simpler technique; one approach investigated was the synchronous, or homodyne, receiver. In essence, this receiver consists of nothing

57、 but a local oscillator, a mixer, and an audio amplifier. To operate, the oscillator must be adjusted to exactly the same frequency as t</p><p>　　Correct tuning of the local oscillator is essential to synchr

58、onous reception; any frequency error whatsoever will hopelessly garble the information. Furthermore, phase of the local oscillator must agree, within a fairly small fraction of a cycle, with the received carrier phase. I

59、n other words, the local oscillator must be phaselocked to the incoming signal. </p><p>　　For various reasons the simple synchronous receiver has never been used extensively. Present-day phaselock receivers

60、almost invariably use the superheterodyne principle and tend to be highly complex. One of their most important applications is in the reception of the very weak signals from distant spacecraft. </p><p>　　The

61、 first widespread use of phaselock was in the synchronization of horizontal and vertical scan in television receivers. The start of each line and the start of each interlaced half-frame of a television picture are signal

62、ed by a pulse transmitted with the video information. As a very crude approach to reconstructing a scan raster on the TV tube, these pulses can be stripped off and individually utilized to trigger a pair of single sweep

63、generators. </p><p>　　A slightly more sophisticated approach uses a pair of free-running relaxation oscillators to drive the sweep generators. In this way sweep is present even if synchronization is absent.&

64、lt;/p><p>　　Free-running frequencies of the oscillators are set slightly below the horizontal and vertical pulse rates, and the stripped pulses are used to trigger the oscillators prematurely and thus to synchr

65、onize them to the line and half-frame rates (half-frame because United States television interlaces the lines on alternate vertical scans). </p><p>　　In the absence of noise this scheme can provide good sync

66、hronization and is entirely adequate. Unfortunately, noise is rarely absent, and any triggering circuit is particularly susceptible to it. As an extreme, triggered scan will completely fail at a signal-to-noise ratio tha

67、t still provides a recognizable, though inferior, picture. </p><p>　　Under less extreme conditions noise causes starting-time jitter and occasional misfiring far out of phase. Horizontal jitter reduces hori

68、zontal resolution and causes vertical lines to have a ragged appearance. Severe horizontal misfiring usually causes a narrow horizontal black streak to appear. </p><p>　　Vertical jitter causes an apparent ve

69、rtical movement of the picture. Also, the interlaced lines of successive half-frames would so move with respect to one another that further picture degradation would result. </p><p>　　Noise fluctuation can b

70、e vastly reduced by phaselocking the two oscillators to the stripped sync pulses. Instead of triggering on each pulse a phase-lock technique examines the relative phase between each oscillator and many of its sync pulses

71、 and adjusts oscillator frequency so that the average phase discrepancy is small. Because it looks at many pulses, a phaselock synchronizer is not confused by occasional large noise pulses that disrupt a triggered synchr

72、onizer. The flywheel synchronizers in</p><p>　　In a color television receiver, the color burst is synchronized by a phase-lock loop. </p><p>　　Spaceflight requirements inspired intensive applica

73、tion of phaselock methods. Space use of phaselock began with the launching of the first American artificial satellites. These vehicles carried low-power (10 mw) CW transmitters; received signals were correspondingly weak

74、. Because of Doppler shift and drift of the transmitting oscillator, there was considerable uncertainty about the exact frequency of the received signal. At the 108MHz frequency originally used, the Doppler shift could r

75、ange over</p><p>　　With an ordinary, fixed-tuned receiver, bandwidth would therefore have to be at least 6kHz, if not more. However, the signal itself occupies a very narrow spectrum and can be contained in

76、something like a 6Hz bandwidth. </p><p>　　Noise power in the receiver is directly proportional to bandwidth. Therefore, if conventional techniques were used, a noise penalty of 1000 times (30 dB) would have

77、to be accepted. The numbers have become even more spectacular as technology has progressed; transmission frequencies have moved up to S-band, making the Doppler range some 75kHz, whereas receiver bandwidths as small as 3

78、 Hz have been achieved. The penalty for conventional techniques would thus be about 47 dB. Such penalties are intol</p><p>　　Noise can be rejected by a narrowband filter, but if the filter is fixed the signa

79、l almost never will be within the pass-band. For a narrow filter to be usable it must be capable of tracking the signal. A phase-locked loop is capable of providing both the narrow bandwidth and the tracking that are nee

80、ded. Moreover, extremely narrow bandwidths can be conveniently obtained (3 to 1000 Hz are typical for space applications); if necessary, bandwidth is easily changed. </p><p>　　For a Doppler signal the inform

81、ation needed to determine vehicle velocity is the Doppler shift. A phase-lock receiver is well-adapted to Doppler recovery, for it has no frequency error when locked. </p><p>　　Other Applications</p>

82、<p>　　The following applications, further discussed elsewhere in the book, represent some of the current uses of phase-lock. </p><p>　　1. One method of tracking moving vehicles involves transmitting a c

83、oherent signal to the vehicle, offsetting the signal frequency, and retransmitting back to the ground. The coherent transponder in the vehicle must operate so that the input and output frequencies are exactly related in

84、the ratio m/n, where m and n are integers. Phase-lock techniques are often used to establish coherence. </p><p>　　2. A phase-locked loop can be used as a frequency demodulator, in which it has superior perfo

85、rmance to a conventional discriminator. </p><p>　　3. Noisy oscillators can be enclosed in a loop and locked to a clean signal. If the loop has a wide bandwidth, the oscillator tracks out its own noise and it

86、s output is greatly cleaned up. </p><p>　　4. Frequency multipliers and dividers can be built by using PLLs. </p><p>　　5. Synchronization of digital transmission is typically obtained by phase-l

87、ock methods. </p><p>　　6. Frequency synthesizers are conveniently built by phase-lock loops. </p><p>　　Optical Communication Components </p><p>　　The Optical Fiber </p><

88、p>　　As discussed earlier, the atmosphere cannot be used as a transmission channel for terrestrial communications using light beams. The most promising channel is the optical fiber waveguide. An optical fiber essential

89、ly consists of a central transparent region called the core which is surrounded by a region of lower refractive index called the cladding . The core could either be homogeneous or could have a gradient in refractive inde

90、x with the refractive index decreasing away from the center of the </p><p>　　In an optical waveguide, there exist specific field distributions which propagate without changing their form and with a definite

91、phase and group velocity. These field configurations are referred to as the modes of the optical waveguide. These modes are characterized by different propagation constants and different group velocities. In a multimode

92、waveguide, there exist a large number of these propagating modes while in a single-mode waveguide there exists only one mode. Each mode has most of th</p><p>　　As already discussed, in a fiber optic communic

93、ation system the information is coded in the form of discrete pulses which are transmitted through the fiber. The information capacity of the system will be determined by the number of pulses that can be sent per unit ti

94、me. For the information to be retrieved at the output end, the various pulses must be well resolved in time. In an optical fiber due to various factors like the differences in group-velocity between the different modes a

95、nd the depend</p><p>　　When a pulse of radiation is injected into a fiber, it excites various modes of the fiber. Since each mode propagates with, in general, a different characteristic group velocity, the i

96、ncident pulse of light broadens as it propagates through the fiber. This is referred to as intermodal broadening. When the fiber can carry only one propagating mode, i.e., in a single-mode fiber, this broadening is absen

97、t, but due to the dependence of the propagation constant on wavelength, there is still some broa</p><p>　　The broadening of a pulse of light as it propagates through an optical fiber can also be visualized b

98、y using the concept of geometrical optics. When a pulse of light is injected into a homogeneous core optical fiber, it excites rays traveling at different angles with the axis. As can be seen from Figure 16.2 since rays

99、making larger angles with the axis have to traverse a longer optical path length, they take a longer time to reach the output end. Consequently the pulse of light broadens as it p</p><p>　　It may be mentione

100、d here that in optical fibers having very small core radii and small index difference between the core and cladding, it can be so arranged that only one mode of propagation exists in the fiber. Such fibers are therefore

101、referred to as single-mode fibers. Because of the presence of just one mode, the dispersion in these fibers is very small and is only due to intramodal broadening. Such fibers are indeed expected to be used in future sup