EXPERIMENT #9 FIBER OPTIC COMMUNICATIONS LINK

Transcription

1 EXPERIMENT #9 FIBER OPTIC COMMUNICATIONS LINK INTRODUCTION: Much of daa communicaions is concerned wih sending digial informaion hrough sysems ha normally only pass analog signals. A elephone line is such a sysem. For such sysems, modems are used o conver he digial signals ino an analog form suiable for ransmission. A common medium used for ransferring boh digial and analog signals is he opical fiber. Fiber opic sysems use a beam of ligh (which is really a high-frequency elecromagneic wave) as a carrier of informaion. Jus like in radio, his "carrier" can be ampliude, frequency, or phase modulaed. Fiber opics have many advanages over wires. Firs, since here is no elecrical signal, he securiy of communicaions is quie good. I's pracically impossible o "ap" a fiber opic line wihou generaing a sizeable signal disurbance, which would be easily deeced a he receiver. Furhermore, because he communicaion isn' elecrical, i is immune o induced noise such as AC power line hum or auomobile igniion noise. Finally, since here is no elecrical connecion in a fiber opic cable, here's no need o share a common ground beween ransmier and receiver. This effecively DC isolaes he ransmier and receiver, which furher reduces noise. To achieve effecive FM or PM in fiber opics requires precise conrol of he lighwave carrier frequency. Since LEDs are no sable frequency sources, lasers are used as he ransmi source in FM or PM fiber opic applicaions. A frequency-modulaed ligh wave is demodulaed using opical heerodyning; he received ligh signal is "mixed" wih a local oscillaor (anoher laser signal) in a non-linear opical maerial. The resul is a UHF radio signal which is hen downconvered and deeced using convenional superheerodyne echniques. Tremendous bandwidh (informaion capaciy) can be obained using hese echniques, bu hey are quie unusual! Ampliude modulaion of lighwave signals is much easier o achieve because convenional LEDs and phooransisors can be used as emiers and deecors. LEDs are inexpensive compared o LASERs, and have a longer life expecancy. However, he opical power oupu of LEDs isn' as grea as ha of LASERs, and he ligh emied by LEDs is specrally "diry" (many frequencies are presen). In AM, he informaion signal can be eiher a sine wave (analog modulaion) or a square wave (digial modulaion). Boh echniques will be explored in his experimen. The digial form of he circui will be used again for laer experimens. To send analog informaion, he operaing curren of a ransmiing LED is made o vary in sep wih he ampliude of he informaion signal. This causes he ligh oupu o vary up and down wih he informaion signal. This is exacly he same as he working of a convenional AM radio ransmier. To deec he AM signal, a phooransisor is used. The phooransisor acs as a ligh-dependen resisor. When he ligh level changes, he phooransisor's resisance changes, hus changing he receiver's curren in sep wih he original informaion. A resisor in he receiver circui convers his curren back ino a volage ha is a copy of he original informaion signal. Digial informaion is also easy o send. Mos sysems simply urn he ligh on o indicae a logic "1" or mark, and urn he ligh off o indicae a logic "0" or space. Thus, a digial ransmier is nohing more han a swich ha urns an LED on or off! A he receiver, he presence or absence of ligh is allowed o urn a phooransisor on and off; he resuling signal is condiioned (amplified) ino a proper digial (usually TTL-level) signal. The fiber opic emier in his experimen uses infra-red ligh insead of visible ligh. This is done in order o reduce fiber opic signal loss, because he maerials used for fiber opic cable ransmi hese lower frequencies beer han visible ligh frequencies. Also, silicon phoodeecors end o have a "peak" in sensiiviy somewhere in he IR region. Lab 9 - Fiber Opic Communicaions Link Page 9-1

2 CIRCUIT ANALYSIS: Figure 1 shows he analog fiber opic sysem. I's amazingly simple! R1 biases D1 (he SFH-450V LED) o mid-poin bias a around 10 ma. C1 allows he AC inpu signal o ener from he generaor, bu blocks he DC which would inerfere wih he Q-poin of D1. R2 ses he AC gain and inpu impedance of he circui. Since he forward-biased diode has a small AC resisance (recall ha r'd = 25 mv / ID = 25 mv/10 ma = 2.5 Ohms), a resisor is needed in order o limi he AC inpu curren. As a bonus, R2 inroduces negaive feedback which linearizes he circui, reducing disorion. R1 330 ANALOG INPUT C uf R2 330 Fiber opic cable Q1 SFH350V C2 D1 SFH450V + ANALOG OUTPUT R3 5K 10 uf R4 10K R5 100 Ohms Figure 1: Analog Fiber Opic Sysem The analog receiver is also sraighforward. Q1 (he SFH-350V phooransisor) receives he ligh ha was launched by D1 ino he opical fiber. This ses up a curren in Q1 proporional o he opical inpu power (ligh level). This curren creaes a volage drop across R3 and R5 which will follow he original inpu signal. There will be a DC level presen because he ligh beam is "on" wih zero informaion volage; C2 removes he DC level, while leing he AC informaion signal pass. R4 is presen o provide a DC reurn for he negaive side of C2 so ha a DC poenial won' appear a he ANALOG OUTPUT. Variable resisor R3 adjuss he DC Q-poin of he receiver circui. This adjusmen is needed because of he possible variaion in inpu opical power from uni o uni, as well as variaion in he power gain of Q1. Lab 9 - Fiber Opic Communicaions Link Page 9-2

3 Figure 2 is he digial version of he sysem. The digial signal eners pin 1 of U1a where i is invered. Therefore, a mark or logic "1" causes U1a pin 2 o go low and urn on D1 hrough R1. A space or logic "0" conversely causes D1 o urn off. Thus, a digial signal causes he ligh beam o urn on and off in sep wih he 1 and 0 paern being sen. R1 470 Ohms R2 150 Ohms R4 4.7K C1 0.1 uf + C uf D1 SFH450V Fiber opic cable Q1 SFH350V U2A LS14 DIGITAL OUTPUT U1A 2N3904 DIGITAL INPUT LS04 R3 4.7K Q2 Figure 2: Digial Fiber Opic Sysem The digial receiver consiss of Q1 (he phoo deecor diode), Q2 (a curren amplifier), and U1b. Recall ha a logic "1" is represened by he ligh beam being urned on. This means ha when a logic "1" is sen, Q1 will be "on" and will saurae. When Q1 is urned on by he ligh wave, Q2 is also urned on (Q1's emier curren flows hrough he base of Q2). When Q2 urns on, i pulls he inpu of inverer U2a "low." U2a herefore oupus a logic "1" when he ligh beam is urned on. When he ligh is urned off, Q1 urns off (i acs like an open circui). Wih Q1 urned off, R3 pulls he base of Q2 owards ground, which urns off Q2. Q2 also appears as an open circui a his poin, and resisor R4 is now able o pull he inpu of U2a up o 5 Vols ("high.") The oupu of U2a is now low. Therefore, when he ligh source is off (a logic 0 is being sen), he receiver oupu is a logic zero. For your reference, here are he pin-ous of he FO emier and deecor pair. Noe ha he wo "spare" pins on he boom of each device are no elecrically conneced. They improve mechanical sabiliy when he device is mouned on a circui board. Cahode Emier Collecor Base Anode SFH-450V Phooemier SFH-350V Phooransisor Figure 3: Pin-ou of Fiber Opic Emier and Deecor Pair Lab 9 - Fiber Opic Communicaions Link Page 9-3

4 LABORATORY PROCEDURE: Name Sign-off PART I: Analog Fiber Opic Link 1. Build he analog circui of Figure 1. Make sure o properly dress he fiber opic cable ends as shown in Figure 4. Failure o do his may make your circuis work marginally, or no a all! Cladding Opical fiber ype Eska SH-4001 Plasic Core - be careful no o scrach or nick Polish end of fiber smooh and fla using #500 sandpaper 0.100" Figure 4: Dressing he Fiber Opic Cable Ends 2. Apply power o he circui, bu no signals ye. Check he volage a he emier of Q1: - Adjus R3 unil he volage is 2.5 V +/- 0.5 V. If his adjusmen goes OK, your receiver is working properly and you can proceed o sep 3. - If here is NO volage (or less han 0.5 V DC), here is somehing wrong! Eiher here is no opical oupu coming from D1 (measure is curren o es), or Q1 is no geing urned "on" (no geing opical inpu, in backwards, ec.) Find he problem before coninuing! - If he volage seems o be "suck" a or near 5 Vols, check o make sure ha R3 is wired correcly, and ha Q1 isn' shored or in backwards. 3. Connec scope channel 1 o he circui ANALOG INPUT, and channel 2 o he circui ANALOG OUTPUT. Connec a signal generaor o he ANALOG INPUT se for a 1 KHz, 1 V p-p SINE wave oupu. Lab 9 - Fiber Opic Communicaions Link Page 9-4

5 4. Graph wo cycles of he inpu and oupu waveforms. Show all imporan volages and imes. (If he oupu is no a clean sine wave, ry adjusing he inpu ampliude up or down slighly). ANALOG INPUT ANALOG OUTPUT 5. Compue and repor he volage gain of he sysem. Show your calculaion. Recall ha: V A V = V ou in Volage gain: V/V 6. Increase he ampliude (volage) of he signal generaor unil he ANALOG OUTPUT becomes disored. Repor how much signal could be received wihou disorion. There will be grea variance from uni o uni! Maximum ANALOG OUTPUT signal wihou disorion: Vpp 7. Wih he inpu ampliude adjused for a clean sine wave oupu, increase he frequency of he signal generaor unil he oupu signal decreases by 3 db from he 1 KHz oupu volage value. Noe: Recall ha a 3 db power decrease means 70.7% of he original volage. Repor his frequency; i is he bandwidh of he analog sysem. -3dB Bandwidh of Analog Sysem: KHz PART II: Digial Fiber Opic Link 8. Build he digial fiber opic daa link of Figure 2. Lab 9 - Fiber Opic Communicaions Link Page 9-5

6 9. Connec scope channel 1 o he circui DIGITAL INPUT, and channel 2 o he circui DIGITAL OUTPUT. Connec a signal generaor o he DIGITAL INPUT and se i for a 300 BPS, TTL SQUARE wave oupu. TIP: Recall ha since here are wo bis in each square wave cycle, he frequency and equivalen daa rae of a digial wave are relaed as follows: BPS = 2 F Where F is he inpu frequency in Hz. 10. Apply power o he circui, and graph wo cycles of he inpu and oupu waveforms. Boh should appear as valid TTL signals. Make sure o use DC coupling on he oscilloscope inpus. Show all imporan volages and imes on he graphs. DIGITAL INPUT DIGITAL OUTPUT 11. See how fas i can go! Increase he frequency of he daa signal from he signal generaor unil he recovered square wave becomes disored. You can consider he wave disored if i: - Has a duy cycle of less han 25% (or more han 75%) - Has any round pars - Has any oher obvious defecs (missing porions, ec). Repor his as a maximum equivalen daa rae. How does i compare wih commonly available elephone modems (e.g., 56 kbps unis) or your home Inerne connecion? Remember o use he relaionship given in sep 9 o relae signal generaor frequency o equivalen daa rae. Maximum equivalen daa rae of digial link: kbps Lab 9 - Fiber Opic Communicaions Link Page 9-6

7 QUESTIONS 1. Wha are wo advanages of fiber opics over convenional ransmission lines? 1) 2) 2. Wha ype of modulaion does he circui of Figure 1 employ? 3. How does he circui of Figure 2 represen 1s and 0s on he opical fiber? 4. Wha else have you learned in his experimen? Lab 9 - Fiber Opic Communicaions Link Page 9-7