EXPERIMENT #8 FREQUENCY MODULATION

Transcription

1 EXPERIMENT #8 FREQUENCY MODULATION INTRODUCTION: Frequency modulaion, or FM, is an imporan mehod of impressing informaion on a carrier. I has many advanages over AM. Firs, since FM only changes he frequency and no he ampliude of he carrier wave, FM receivers can be buil o ignore ampliude (volage) changes. This is imporan because mos exernal noise is in he form of volage variaions ha are "added" o he carrier wave as i makes is way from ransmier o receiver. An AM receiver direcly responds o hese wih he familiar sounds of saic inerference; an FM receiver ignores hese ampliude changes, almos eliminaing he effec of he noise. Second, i is much easier o design sysems o reproduce high-fideliy sound using FM. "High-fideliy" means accurae signal reproducion wih a minimum of disorion. The reproduced informaion signal is a very close replica of he original in an FM sysem. I's difficul o build hi-fi AM receivers, parly due o he inheren nonlinear disorion creaed in a convenional AM diode deecor, and also parly due o he limied ransmission bandwidh (8 khz) alloed for AM broadcas. These advanages do come a a price; ha price is increased bandwidh. A ypical FM broadcas saion uses up o 75 khz of signal deviaion (he peak frequency change), which resuls in a ypical bandwidh of 150 o 200 khz. Because of he high bandwidh requiremens, FM broadcasing is done in he VHF band beween 88 and 108 MHz. FM receivers and deecors are slighly more complex han hose for AM; and he higher frequencies used for FM (VHF) complicae overall ransmier and receiver design. The circuiry in his lab operaes on a carrier frequency of 100 khz, which is in he VLF band. A such a low carrier frequency, he frequency deviaion needs o be limied so ha he available bandwidh is no used up. FM sysems wih limied deviaion are very commonly used where he highes fideliy is no needed. Voice-only communicaion sysems in he amaeur, business, and governmen radio services are ypical "narrowband" FM applicaions. Mos narrowband FM communicaions occurs in he VHF and UHF bands, where he reduced bandwidh requiremen allows more saions o share he available range of radio frequencies. CIRCUIT ANALYSIS: This experimen is in wo pars, an FM modulaor and an FM deecor. The circuiry is designed o operae in he VLF band a a carrier frequency of 100 khz. The VCO porion of an LM565 is used o creae he FM waveform, and an LM565 phase-locked-loop (PLL) operaes as he FM deecor. FM Modulaor Figure 1 illusraes he FM modulaor circui. I's quie similar o he VCO lab, wih wo minor addiions. Lab 8 - FM Modulaion and Deecion Page 8-1

2 R5 Vcc (+15 V) 10 Ohms R4 2.2 K C3 + C4 10 uf R2 5 k R1 1 k R3 22 k R8 1 k R6 4.7K R7 4.7K C uf U1 2 IN 3 IN 7 VCON 9 TCAP 1 VCC- TRES 8 VCC+ 10 REF 6 VOUT 4 VIN 5 LM565 C5 FM Oupu C2 C6 AF Informaion Inpu + 10 uf 0.01 uf Figure 1: FM Modulaor Employing he LM565 The firs addiion o he circui is C2. C2 allows he informaion signal (an audio signal) o be AC-coupled ino he conrol volage inpu of he VCO (pin 7). The signal a pin 7 of he VCO looks like his: AC Informaion Signal LM565 Pin 7 Volage Vaverage Noice ha he audio signal is now riding on op of a DC level, V average. Poeniomeer R2 ses his DC level, which conrols he carrier cener frequency. As he DC volage rises, he VCO frequency decreases in direc proporion o he amoun of rise. The volage "rise" is acually he same hing as he ampliude of he informaion. Thus posiive informaion causes negaive carrier frequency change or deviaion. The opposie effec happens on he negaive peak of he inelligence; when he conrol volage falls, he VCO frequency increases. In oher words, frequency modulaion occurs. Lab 8 - FM Modulaion and Deecion Page 8-2

3 Modulaor Sensiiviy The amoun of frequency deviaion produced in he VCO is direcly proporional o he ampliude of he informaion signal. The amoun of deviaion can be prediced by using he following equaion: δ = V m K 0 Where: K 0 is he modulaor sensiiviy, in Hz / Vol. (For his circui, K 0 = 72,727 Hz/Vol.) V m = he peak informaion signal ampliude. δ = he deviaion or peak frequency swing, in Hz. For example, if V m = 500 mv peak, hen he amoun of deviaion produced will be: δ = V m K = (0.5Vpk)(72.727KHz / V) kHz 0 = This would probably be oo much deviaion, since he carrier frequency is only 100 khz! Therefore, he deviaion in he experimen will be limied o 10 khz. The FM oupu of he VCO could be direcly ransmied over he air if wo circui addiions were made. Firs, since he wave shape a he oupu pin is square i would need o be shaped ino a clean sine wave using a low-pass filer; and second, some sor of power amplifier would be necessary since he signal oupu is quie small. Noe ha if you have access o a VLF receiver in your laboraory ha can une down o 100 khz, you can monior he oupu of his circui. FM Deecor: Figure 2 shows he FM deecor. I's based on he LM565 phase-locked-loop. The loop is se o free-run a 100 khz by C104 and he series combinaion of R107 and R106. Poeniomeer R106 allows adjusmen of he free-running frequency, which should be he same as he carrier or cener frequency of he FM ransmier. R103 R102 R V 4.7K R104 R K C102 C101 R106 VCO Free Run Adjusmen R107 1K 10 Ohms FM INPUT C IN 1 V+ R T IN 2 VCO OUT U101 CTRL LM565 VLTG PD VCO IN V- C T 7 C pf R108 47K R109 R110 + C uf AF OUT 1 9 C pf C pf C pf C C uf Figure 2: FM Demodulaor Using he LM565 Lab 8 - FM Modulaion and Deecion Page 8-3

4 The FM signal is coupled ino he number 1 reference inpu of he phase deecor (pin 2) hrough C103. Since he PLL is operaing from a single supply, resisors R102 and R103 are used as a volage divider o spli he power supply in half. Approximaely 7.5 vols appears a he juncion of R102 and R103 when he circui is working properly. C102 is an RF bypass for he bias poin, and R104 and R105 serve o isolae he wo phase deecor inpus (since signal should only be inroduced ino one inpu a a ime.) The VCO conrol volage of he loop on pin 7 of he 565 conains wo componens. One is a DC level corresponding o he "average" frequency going ino he PLL from he FM modulaor, and he oher is an AC level ha is acually he deeced informaion signal. This AC signal arises because of he PLL's self-correcing acion; as he ransmier deviaes up or down in frequency, he PLL aemps o force he VCO o follow his frequency exacly by varying is conrol volage. Thus he conrol volage is a copy of he original informaion signal. Componens C106, C107, and R108 form he loop filer which ses up he loop operaing parameers (capure range, damping raio, naural frequency) appropriaely for demodulaion of an FM signal. The signal a pin 7 of he 565 conains some carrier frequency in addiion o he AC and DC levels jus discussed; componens R109, C108, R110, and C109 form a low-pass filer o remove any races of 100 KHz carrier signal. The final oupu is AC coupled by C110, leaving only demodulaed informaion a he oupu. Lab 8 - FM Modulaion and Deecion Page 8-4

5 LABORATORY PROCEDURE: Name Sign-off In his experimen you'll have wo circuis o build. I is suggesed ha hey be buil on separae breadboards, if possible. 1. Build he circui of figure 1. Don' connec anyhing o he AF INPUT ye. 2. Connec a frequency couner and scope o he FM OUTPUT of he circui, and adjus R2 unil a 100 KHz carrier wave frequency is obained. 3. Now connec a signal generaor o he AF INPUT. Adjus i for 1 khz frequency, and correc peak volage for 10 khz deviaion. (Remember ha K 0 = 72,727 Hz/Vol.) Show your calculaion below: AF INPUT -- Peak volage for 10 KHz deviaion: V pk 4. Connec scope channel 1 o he AF INPUT, and scope channel 2 o he FM OUTPUT. Trigger off scope channel 1. Adjus he conrols appropriaely, and draw wha you see below: AF INPUT FM OUTPUT 5. How does he FM OUTPUT look wih respec o he AF INPUT in quesion 4? Can you see he FM modulaion aking place? Why or why no? 6. Build he FM demodulaor circui of figure 2. Keep componen lead lenghs as shor as possible. Lab 8 - FM Modulaion and Deecion Page 8-5

6 7. Apply power o he demodulaor circui, bu don' connec is FM INPUT o anyhing ye. Firs, adjus R106 so ha he free-running VCO oupu on pin 4 of he LM565 PLL is 100 KHz. Use a frequency couner for his measuremen. 8. Now connec he ransmier and receiver circuis ogeher. Don' forge o hook ogeher he grounds! Connec he scope probes as follows: Channel 1 o he AF INPUT of he FM modulaor, and Channel 2 o he AF OUT of he FM deecor. 9. Make sure he signal generaor is se he same as i was in sep 3; hen record he scope readings below: AF INPUT AF OUTPUT 10. How accurae is he reproducion of he informaion a he deecor, when compared o he original informaion (scope channel 1?) 11. Increase he deviaion a he modulaor (by adjusing he ampliude of he signal generaor) unil he deeced oupu jus begins o become disored. Remember, you can predic he deviaion if you know he peak informaion signal volage. Show your calculaion below. How much deviaion could his sysem suppor? Lab 8 - FM Modulaion and Deecion Page 8-6

7 Troubleshooing Hins FM Modulaor 1. Check he power supplies a he U1 IC pins (Vcc = 15V a pin 8, GND = 0 V a pin 1). 2. Check he DC volage a he following locaions: Circui Locaion DC volage reading Commens and olerance U1 Pin 2,3 7.5 V +/- 0.5 V Supplied by divider R6 & R7. Circui will no oscillae if his poin is no properly biased. U1 Pin V +/- 1 V Measured wih R2 in cener of roaion. 3. If he above checks are OK and he circui oscillaes a he wrong frequency (canno adjus o 100 KHz), check he values of C1 and R4 (RC ime-consan for VCO). 4. If you canno ge he circui o oscillae and you've checked all of he above, ry replacing he LM565. You may have a defecive IC. FM Deecor 1. Check he power supplies a he U101 IC pins (Vcc = 15V a pin 8, GND = 0 V a pin 1). 2. Check he DC volage a he following locaions: Circui Locaion DC volage reading Commens and olerance U101 Pin 2,3 7.5 V +/- 0.5 V Supplied by divider R102 & R103. U101 Pin 7 8 V +/- 2 V Measured wih R106 a cener of roaion, and no signal applied a he FM INPUT. 3. Check he free-run frequency of he VCO on pin 4; i should be 100 KHz +/- 5 KHz. If i is no so, check he values of R106, R107, and C104, which are he RC ime-consan of he VCO. 4. Using he benchop signal generaor, apply a seady sine wave of 100 mv RMS over he frequency range of 90 KHz o 110 KHz o he FM INPUT. The frequency a he VCO OUT pin of U101 (pin 4) should exacly follow he applied frequency (PLL acion). If his does no happen, hen he PLL is no aaining lock and you should check: * Is pin 4 conneced o pin 5 ( VCO OUT o PD IN)? * Is he FM INPUT signal geing o pin 2? (Check wih scope a U101 pin 2.) * Is here anyhing miswired on pin 7 (possibly shoring he VCO conrol volage o ground?) Lab 8 - FM Modulaion and Deecion Page 8-7

8 QUESTIONS 1. Wha characerisic of he carrier is varied during Frequency Modulaion? 2. Wha conrols he amoun of deviaion in an FM ransmier? 3. Wha does R2 adjus in he modulaor circui? Why is his imporan? 4. Wha causes he volage a pin 7 of he FM demodulaor (LM565) o follow he original informaion signal? Lab 8 - FM Modulaion and Deecion Page 8-8

9 5. Wha else have you learned in his experimen? Lab 8 - FM Modulaion and Deecion Page 8-9