Chapter 2 Continuous-Wave Modulation. 2.1 Introduction

Transcription

1 Chaper Coninuous-Wave Modulaion.1 Inroduion 1

2 . mpliude Modulaion os : arrier ampliude : arrier requeny 1+k a m S X.1 os The oupu o he modulaor s 1 kam os. Where m is he baseband signal, k a is he ampliude sensiiviy. 1. k a m 1, or all.3. W where W is he highes reqeny o m.4

3 3 Reall 1.Negaive requeny omponen o m beomes visible.. -W M lower sideband M +W upper sideband 3.Transmission bandwidh B T =W. os os m k s a he Fourier Transorm o is where.5 1 os 1 os m M M M k s M M m a

4 Virues and Limiaions o mpliude Modulaion Transmier Reeiver Major limiaions 1.M is waseul o power..m is waseul o bandwidh. 4

5 .3 Linear Modulaion Shemes Linear modulaion is deined by s s s I Q s I os s In - phaseomponen Quadraure omponen Q sin.7 Three ypes o linear modulaion: 1.Double sideband-suppressed arrier DSB-SC modulaion.single sideband SSB modulaion 3.Vesigial sideband VSB modulaion 5

6 Noes: 1.s I is solely dependen on m.s Q is a ilered version o m. The speral modiiaion o s is solely due o s Q. 6

7 7 Double Sideband-Suppressed Carrier DSB-SC Modulaion The Fourier ransorm o S is.8 os m s.9 1 M M s

8 8 Coheren Deeion Synhronous Deeion The produ modulaor oupu is Le V be he Fourier ransorm o v.10 os ' 1 os4 ' 1 os os ' os ' m m m s v.11 os ' 1 0 m v ilered ou Low pass ilered

9 Cosas Reeiver I-hannel and Q-hannel are oupled ogeher o orm a negaive eedbak sysem o mainain synhronizaion os sin m m sin m sin 4 The phase onrol signal eases wih modulaion. muliplier + very narrow band LF 9

10 Quadraure-Carrier Muliplexing or QM Two DSB-SC signals oupy he same hannel bandwidh, where pilo signal one may be needed. s m1 os m sin 10

11 Single-Sideband Modulaion SSB The lower sideband and upper sideband o M signal onain same inormaion. The requeny-disriminaion mehod onsiss o a produ modulaor DSB-SC and a band-pass iler. The iler mus mee he ollowing requiremens: a.the desired sideband lies inside he passband. b.the unwaned sideband lies inside he sopband..the ransiion band is wie he lowes requeny o he message. To reover he signal a he reeiver, a pilo arrier or a sable osillaor 11 is needed Donald Duk ee.

12 1 Vesigial Sideband Modulaion VSB When he message onains near DC omponen The ransiion mus saisy or 1 : linear b.the phase responseis 1 a. W B W W H H H H ν T

13 Consider he negaive requeny response: H v v W v v W Here, he shi response H- is H v v W v 0 v W 13

14 and H+ is H W v v v 0 v W 14

15 So, we ge H- + H+ is H W v 0 v v v H v v v 0 v W 15

16 Consider W<<W we ge: Whih is equal o W v 0 W v W W So, H- + H+ =1 or -W<<W 16

17 1 1 s m os m' sin ± orresponds o upper or lower sideband.15 m H Q m H Q H H or W.16 j W 17

18 Television Signals NTSC 18

19 .4 Frequeny Translaion os Up onversion = 1 + l, l = - 1 Down onversion = 1 - l, l = 1-19

20 .5 Frequeny-Division Muliplexing FDM 0

21 1.6 ngle Modulaion Basi Deiniions: Beer disriminaion agains noise and inererene expense o bandwidh. The insananeous requeny is.19 os s i onsan is where. is unmodulaed arrier, For an.1 1 lim lim 0 Δ Δ 0 Δ i i i i i i d d

22 1. Phase modulaion PM k m k i p : phasesensiiviy o he modulaor.3 s os k pm. Frequeny Modulaion FM k m i π πk m d i p.4 s os π π k m d.6 0 k :requeny sensiiviy o he modulaor ompare.3 and.6 0 k m' πk m d p.5 0 generaing FM signal generaing PM signal

23 .7 Frequeny Modulaion FM is a nonlinear modulaion proess, we an no apply Fourier ransorm o have speral analysis direly. 1.Consider a single-one modulaion whih produes a narrowband FM k is small.nex onsider a single-one and wideband FM k is large le m i os os.7 os m.8 Δ k : requeny deviaion m m k m m m deerminisi 3

24 4 larger ha n one radian. is, FM Wideband is smaller han one radian., Narrowband FM.33 sin os.3 sin.31 index Modulaion.30 sin.5, Reall 0 s π π d m m i m m m i i.19 =>

25 5 Narrowband FM.35 sin sin os sin sin sin 1 sin os is small, Beause.34 sin sin sin sin os os sin os s s m m m m m m m

26 The oupu o Fig.1 is s' os k s diers rom ideal ondiion in wo respes: 1.The envelope onains a residual M. FM has onsan envelope. i onains odd order harmoni disorions 3 5 x x sin x x 3! 5! For narrowband FM, β m d sin 7 x 7! 0.3 radians. 6

27 7.37 os os 1 os os os os. os 1 os wave, modulaing For M wih sinusoidal.36 os os 1 os.35 sin sin os.35 Reall M k m k s m s m m m a a m m m m Narrow band FM M

28 Wideband FM large β s exp jx os x j sin x s Re Re os ~ s exp exp sin j where Re denoes he real par and ~ s is he omplex envelope deined by ~ s exp[ j sin ] j m m j sin m ~ s n n exp j n m Complex Fourier Transorm.40 8

29 1 m n m 1 m m s exp j n d 1 m m 1 m m m exp j sin j n d.41 Le x.4 m exp sin n j x nx dx.43 Deine he nh order Bessel union o he irs kind as d y dy x x n y 0 3, x dx dx 1 J exp sin n j x nx dx J n n s J exp j n n n m

30 30.49 is The Fourier ransorm o.48 os.47 exp Re m m n m n m n n n J S s n J n j J s Figure.3 Plos o Bessel unions o he irs kind or varying order.

31 Properies o J n 1. J 1 J, or all n.50 n.i is small - J 0 n 1 3. J 1 n Observaion o FM 1.n FM signal onains n n J1 J 0 n.51,,, 3, m m m omponens..for small, he FM signal is eeively omposed o a arrier and a single pair o side reqenies a 3.The ampliude o arrier depends on 1 P J n m narrowband FM.54 31

32 Example. 3

33 Transmission Bandwidh o FM signals Wih a speiied amoun o disorion, he FM signal is eeively limied o a inie number o signiian side requenies..carson s rule 1 BT m 1, =, m.55 m 33

34 B. B n, J 0.01, B n T max m n T max max Universal urve or evaluaing he 1 peren bandwidh o an FM wave 34

35 Example.3 In norh meria, he maximum value o requeny deviaion is ixed a 75kHz or ommerial FM broadasing by radio. I we ake he modulaion requeny W=15kHz, whih is ypially he maximum audio requeny o ineres in FM ransmission, we ind ha orresponding value o he deviaion raio is 75 D 5 15 Using Carson s rule o Equaion.55, replaing by D, and replaing m by W, he approximae value o he ransmission bandwidh o he FM signal is obained as B T =75+15=180kHz On he oher hand, use o he urve o Figure.6 gives he ransmission bandwidh o he FM signal o be B T =3. =3.x75=40kHz In praie, a bandwidh o 00kHz is alloaed o eah FM ransmission. On his basis, Carson s rule underesimaes he ransmission bandwidh by 10 peren, whereas he universal urve o Figure.6 overesimaes i by 0 peren. 35

36 Generaion o FM signals Frequeny Muliplier v a s a s a s n 1 n.56 s os k m d 0 The requeny muliplier oupu s ' 'os n nk m d.58 0 i ' n nk m.59 36

37 Varaor diode VCO FM modulaor 3-1

38 Crosby Dire FM Transmier 3-

39 Demodulaion o FM signals The requeny disriminaion onsiss o a slope irui ollowed by an envelope deeor Consider Fig.9a, he requeny response o a slope irui is elsewhere 0,,, 1 T T T T T T B B B a j B B B a j H.60 33

40 H H, H H, 0 34

41 ppendix.3 Hilber Transorm Fourier Transorm-requeny-seleive Hilber Transorm-phase-seleive Le gg 1 g gˆ d ±90 0 shi Denoe he Hilber ransorm o g as 1 g gˆ d The inverse Hilber ransorm

42 1 j sgn 1 0 sgn The Fourier ransorm o Gˆ jsgn G g is g gˆ H 36

43 Properies o he Hilber Transorm ime domain operaion I g is real 1. gˆ and g have he same magniude sperum.hilber ransorm o gˆ is g 3. - g ĝ d 0 g gˆ g ake H.F o and ompare wih.3 37

44 For a band-pass sysem, we onsider x X X is limied wihin ± W Hz W x x os x sin I Q The omplex evelope o x is x x j x I x H Q band pass sysem, B y h h os h sin I Q

45 ' ' ~, rom ~ We an obain.55 0, ~ wih o limied is and H ~ * real is Sine.54 * ~ ~ Fourier ransorm o.53 pply.53 exp * ~ exp ~ *, From.5 we have are low - passunions h ~ and,.5 exp ~ Re The omplex represenaion o.51 ~ he omplex impluse response Deine Q I Q I H H H H H H B B H H h H H H j h j h h z z v ju v z h h j h h h j h h h 39

46 x band-pass h sysem y.57 y Re y exp j h x d Deine he pre-envelope o h h j h, H H sgn H H H H 0 0 h 0 as 0.58 h : Hilber T. o h y Re h Re x d.59 40

47 Reall h h jhˆ - h Re h x Re x To prove.60 Re h x d Re [ h jhˆ ][ x jxˆ ] d h x d hˆ xˆ d h x- d 1 1 u h u xˆ ddu, 1 1 ˆ u h x d x d h u du h x d h u x u du h x d Re[ h ]Re[ x ] d, d d 41

48 ~ ~ exp Re 1 exp ~ exp ~ Re 1 Re 1.59 Re Re.58 beomes d x h j d j x j h d x h d x h y 4

49 Comparing.57 and.61 we have ~ ~ y h ~ x d.6 ~ or ~ y h * ~ x.63 We an represen bandpass signals and sysems by heequivalen lowpassunions x~, y~ ~ and h wihou he aor exp j 43

50 .68 y.67 y.66 ~ ~ le ~ ~ Q I x h x h x h x h jy y y x h x h j x h x h jx x jh h y Q I I Q Q Q I I Q I Q I I Q Q Q I I Q I Q I 44

51 45

52 Proedure or evaluaing he response o a band-pass sysem 1. Replae x by ~ x x Re ~ x exp j ~. h Re h exp j ~ 3. Obain ~ y h * ~ x 4. y Re ~ y exp j 46

53 To simpliy he analysis 1. shi H 1 o he righ by o align o he band-pass requeny. se H 1 H 1, or 0.61 Reall H BT BT BT j4 a 0 elsewhere 1 BT BT BT j πa BT BT BT H 1 j πa.60 0 elsewhere From.60 and.61, we ge.6 47

54 Reall FM signal s s os k m d 0 The omplex envelope is s exp j k m d.63 0 Le s 1 denoe he omplex envelope o he slope k. response oupu. Reall.63 y h x, we have 1 S1 H 1 S upper arm o Fig.30 in ex BT BT BT j a S.64 0 elsewhere d s s1 a j BT s d.65 From.63 and.65, we have k s1 j B a 1 m exp j k m d T B 0 T.66 48

55 s1 Re ~ s 1 exp j k 1 os BT a m k m d.67 0 BT sin k m d 0 s1 is a hybrid-modulaed signal ampliude, requeny However, provided ha we hoose using an envelope deeor, we have k m 1, or all B k s1 BTa 1 m.68 BT The bias erm B a an be removed by a seond requeny T disriminaor wih H, where H H. 1 T 49

56 .71 4 ~ ~.70 1 ~.69 ~ ~ a m k s s s m B k a B s H H T T Balaned Frequeny Disriminaor Le he ranser union o he seond branh o Fig.30 be omplemenary slope irui 50

57 FM Sereo Muliplexing Two aors whih inluene FM sereo sandards 1.Operaion wihin he alloaed FM hannels..compaible wih monophoni radio reeiver..7 os os4 K m m m m m r l r l 51

58 Sereo FM Figure FM sereo generaion blok diagram. 51-1

59 Sereo FM In Figure 9-40, audio signals rom boh le and righ mirrophones are ombined in an linear marixing nework o produe an L+R signal and an L-R signal. Boh L+R and L-R are signals in he audio band and mus be separaed beore modulaing he arrier or ransmission. This is aomplished by ranslaing he L-R audio signal up in he sperum. s seen in Figure 9-40, he requeny ranslaion is ahieved by ampliude-modulaing a 38-kHz subsidiary arrier in a balaned modulaor o produe DSB-SC. 51-

60 Sereo FM Transmier Sereo FM ransmier using requeny-division muliplexing. 51-3

61 Sereo FM Transmier Sereo FM ransmier: a blok diagram; b resuling sperum. SC: Subsidiary Communiaion uhorizaion 51-4

62 Sereo FM The sereo reeiver will need a requeny-oheren 38-kHz reerene signal o demodulae he DSB-SC. To simpliy he reeiver, a requeny- and phase-oheren signal is derived rom he subarrier osillaor by requeny division o produe a pilo. The 19-kHz pilo is niely beween he L+R and DSB-SC L- R signals in he baseband requeny sperum. 51-5

63 Sereo FM s indiaed by is relaive ampliude in he baseband omposie signal, he pilo is made small enough so ha is FM deviaion o he arrier is only abou 10% o he oal 75-kHz maximum deviaion. er he FM sereo signal is reeived and demodulaed o baseband, he 19-kHz pilo is used o phase-lok an osillaor, whih provides he 38-kHz subarrier or demodulaion o he L-R signal. simple example using equal requeny bu unequal ampliude audio oned in he L and R mirophones is used o illusrae he ormaion o he omposie sereo wihou pilo in Figure

64 Sereo FM Figure Developmen o omposie sereo signal. The 38 khz alernaely muliplies L-R signal by +1 and 1 o produe he DSB-SC in he balaned M modulaor par d. The adder oupu shown in e wihou pio will be ilered o redue higher harmonis beore FM modulaion. 51-7

65 Sereo FM Sperum o sereo FM signal. SC: Subsidiary ommuniaion auhorizaion ommerial-ree program 51-8

66 Reerene : G. M. Miller Modern Eleroni Communiaion 5h Ediion, Prenie Hall 51-9

67 .8 Nonlinear Ees in FM Sysems 1.Srong nonlineariy, e.g., square-law modulaors, hard limier, requeny mulipliers..weak nonlineariy, e.g., impereions Nonlinear inpu-oupu relaion 3 v0 a1v i avi a3vi.73 v i Nonlinear Channel devie v 0 5

68 os os 1 os os os os os For FMsignal a a a a a a a a v d m k v i 53

69 Carson' s rule, BT m W W W W 4 W In order o seperae he desired FM signal rom he seond harmoni, we have W W 3 W.76 The oupu o he band-pass iler is 3 3 v 0 ' a1 a3 os no ee o m 4 n FM sysem is exremely sensiive o phase nonlineariies. Common ype o soure : M-o -PM onversion. 54

70 .9 Super Heerodyne Reeiver Carrier-requeny uning, ilering, ampliiaion, and demodulaion M radio reeiver IF = LO - RF.78 FM sysem may use a limier o remove ampliude variaions. 55

71 Commerial FM Broadas lloaions and Sidebands 56

72 .10 Noise in CW modulaion Sysem 1.Channel model: addiive whie Gaussian noise WGN.Reeiver model: a band-pass iler ollowed by an ideal demodulaor The PSD o w is denoed by N 0. 57

73 The ilered noise in narrowband noise represenaion : n n The ilered signal x The hannel signal - o- noise raio SNR The oupu signal SNR Figure C O I o os s n average average average power o he demodulaed signal average power o noise a he oupu meri n or demodulaion is power o power o - o- noise raio SNR SNR O C Q sin s n

74 .11 Noise in Linear Reeiver Using Coheren Deeion The DSB-SC sysem s C os m m S W M P SM d W.83 C P SNR C,DSB WN 0 C P WN 0 baseband.84 C:sysem dependen saling aor 59

75 by heoheren deeor. ompleely rejeed is. are addiive a he reeiver oupu. and 1. :.86 indiaes Low - passiler high requeny omponens sin4 1 os os.85 sin os os n n m n C m y n n C m n C m x v n n m C n s x Q I I Q I I Q I 60

76 The average Le The average SNR SNR SNR 1. Coheren SSB has. B T O C No rade- o Serious W O,DSBSC DSB-SC oupu signal 1 noise C 1 problem! n W N I power P C P WN 0 he same igure C m power C 1 o WN meri o DSB -SC beween perormane and bandwidh. 0 1 P WN

77 .1 Noise in M Reeivers Using Envelope Deeion os os.89 os 1 m k m k s a a.91 sin os : he iler o heoupu.90 1 SNR 0 M C, n n m k n s x WN P k Q I a a 6

78 .9 envelope o 1 n n m k x y Q I a.95 1 SNR SNR.94 SNR ssume C O 0 M O, 0 P k P k WN P k k WN n m k y n n m k a a M a a I a Q I a 63 arrier power > noise power

79 Supplemens Deine he pre-demodulaion SNR as The average power o he modulaed signal SNR pre-de = The average noise power a he inpu o he demodulaor modulaed signal s Band pass iler n SNRpre-de SNRo The Bandwidh o he bandpass iler is he demodulaor is N o B T S N BT demodulaor m hen he average noise power a he inpu o For an M sysem B T BT I =W SNR M pre de N 0 M SNR pre de 1 4N o B k W T a 1 N p k o a B T p 1 N o k B a T p

80 Supplemens For a DSB-SC sysem, For an FM sysem 為與課本一致加 C SNR I using Carson s rule, we have SNR DSB SC pre de FM pre de B T N o C P N B B T =Δ+m>> m =w o T N For he purpose o omparing dieren CW modulaion sysems, we deine The average power o he modulaed signal SNR= The average power o hannel noise in he message band o B T C 4N o W p Message signal wih LP iler he same power as modulaed wave wih bandwidh w noise n The equivalen baseband ransmission model. oupu

81 Supplemens More preisely, we may express he DSB-SC as m S R osπ +θ θ is uniormly disribued over 0, π S'= m osπ +θ he reeiver we may wrie m P S=C m osπ +θ s 0 P w w The average noise power in w<<w E S Rs 0 S x d E C m C E C R 0 C os os E m m S m d P n w w N 0 P d N o W

82 Supplemens SNR= = The average power o S The average power o hannel noise in he message band The average power o he modulaed signal The average power o hannel noise in he message band Ps = = Pn N For onveniene we wrie he modulaed signal as C θ 不出現 Sine is ergodi and we ake as a sample union o W P S C m os os os P s C R 0 [ime average o [ os ]] C m P SNR C N 0 P W C N 0 P W

83 64

84 Threshold Ee noise power > arrier power 65

85 .13 Noise in FM Reeivers The disriminaor onsiss o a slope nework and an envelope deeor. sin os Le n n n Q I.130 os r n.131 envelope is The 1 n n r Q I.13 an The phaseis 1 n n I Q , , exp. uniorm disribued over is and Rayleigh disribued, is where R Ψ r r r r Ψ r 66

86 The inoming FM signal s is deined by.133 os 0 d m k s.134 where 0 d m k.135 os.136 os os hebandpass iler oupu r n s x.137 os sin an where 1 r r r 67 r

87 Noe ha he envelope o x is o no ineres o us limier.141 sin 1 where r d d n d.138 sin Beause r r.139 sin 0 r d m k addiive noise message Fig.40 is The disriminaor oupu n m k d d v d 68

88 ssume hen We may simpliy n d n d is independen o message signal. 1 d d r sin.14 From deiniion o r and, we have n n Q d n d is uniormly disribued over 0, as r sin dn Q d The quadraure omponen appears.144, 69

89 From.140 The average oupu signal power = k P Reall n Q S Q d d F. T j 1 d d n d N N S d SN S.145 d NQ noise is enhaned a high requeny 70

90 ssume ha n Q has ideal low-pass haraerisi wih bandwidh B T N SN d I BT W hereeiver oupu S N, B 0 T N0, W

91 N 0 W verage power o n0 d W 3 N0W noise quieing ee 3 k P when inreasing arrier power SNR O,FM N0W The average power o s is, he average noise power in message bandwidh is WN SNR C,FM.150 SNR SNR O C WN FM k.9 SNR m o, FM 3 k W P

92 Example.5 Single-Tone Modulaion s os sin m m We may wrie, k m d sin 0 d boh side m os m d k The average power o m aross1 load is P k 3 3 From.149, SNR O,FM, 3 4N W 4N W W SNR SNR SNR O 1 ompare o M, rom Example SNR C 3 3 M 1 When, FM has beer perormane. 3 0 m 0 m O C FM 3 W Deine 0.5 as he ransiion beween narrowband FM and wideband FM. 73

93 FM Threshold Ee When CNR is low When here is no signal, i.e., arrier is unmodulaed. The omposie signal a he requeny disriminaor inpu an 1 x n os n sin I Q n Q n I '.153 Oasionally, P1 may sweep around he origin, r > inreases or dereases The disriminaor oupu is equal o P 1 n Q x r 0 P n I 74

94 Figure.44 Illusraing impulselike omponens in d /d produed by hanges o in ; a and b are graphs o and, respeively. 75

95 posiive-going lik ours, when d r, d, 0 d negaive-going lik ours when d r, d, d 0 The arrier-o -noise raio is deined by BN T The oupu signal-o-noise raio is alulaed as 1. The average oupu signal power is alulaed assuming BT a sinusoidal modulaion whih produes. noise ree. The average oupu noise power is alulaed when no signal is presen The arrier is unmodulaed. 76

96 Figure.45 Dependene o oupu signal-o-noise raio on inpu arrier-o-noise raio or FM reeiver. In urve I, he average oupu noise power is alulaed assuming an unmodulaed arrier. In urve II, he average oupu noise power is alulaed assuming a sinusoidally modulaed arrier. Boh urves I and II are alulaed rom heory. When 0 or 0BT N0 B N T hreshold ees may be avoided 0.155, 77

97 The proedure o alulae minimum 1. Given and W, deermine using Figure.6 or Carson's rule. Given N, we have 0BT N Capure Ee: The reeiver loks ono he sronger signal and suppresses he weaker one. B T 78

98 FM Threshold Reduion raking iler FM demodulaor wih negaive eedbak FMFB Phase loked loop Figure.47 FM demodulaor wih negaive eedbak. Figure.46 FM hreshold exension. 79

99 Pre-emphasis and De-emphasis on FM Figure.48 a Power speral densiy o noise a FM reeiver o b Power speral densiy o a ypial message signal. Figure.49 Use o pre-emphasis and de-emphasis in an FM sysem. 80

100 .16 3 is The improvemen aor.158 power wihde - emphsis noise oupu verage.157,.146, is The PSD a hedisriminaor oupu.156, 1 w -w 3 de 0 de 0 de 0 pe de d H W I I d H N B H N S H B N S W W H H de W W T N T N d d 81

101 .161 an de - emphsis iler responseis 1 simple pre- emphsis iler responseis de 0 pe W W W d W I j H j H W W Example.6 Figure.50 a Pre-emphasis iler. b De-emphasis iler. 8

102 Preemphasis or FM The main dierene beween FM and PM is in he relaionship beween requeny and phase. = 1/. d/d. PM deeor has a la noise power and volage oupu versus requeny power speral densiy. This is illusraed in Figure 9-38a. However, an FM deeor has a paraboli noise power sperum, as shown in Figure 9-38b. The oupu noise volage inreases linearly wih requeny. I no ompensaion is used or FM, he higher audio signals would suer a greaer S/N degradaion han he lower requenies. For his reason ompensaion, alled emphasis, is used or broadas FM. 83

103 Preemphasis or FM Figure Deeor noise oupu spera or a. PM and b. FM. 84

104 Preemphasis or FM preemphasis nework a he modulaor inpu provides a onsan inrease o modulaion index m or high-requeny audio signals. Suh a nework and is requeny response are illusraed in Figure Fig apremphasis nework, and b Frequeny response. 85

105 Preemphasis or FM Wih he RC nework hosen o give = R 1 C = 75s in Norh meria 150s in Europe, a onsan inpu audio signal will resul in a nearly onsan rise in he VCO inpu volage or requenies above.1 khz. The larger-han-normal arrier deviaions and m will preemphasize high-audio requenies. he reeiver demodulaor oupu, a low-pass RC nework wih = RC = 75s will no only derease noise a higher audio requenies bu also deemphasize he high-requeny inormaion signals and reurn hem o normal ampliudes relaive o he low requenies. The overall resul will be nearly onsan S/N aross he 15- khz audio baseband and a noise perormane improvemen o abou 1dB over no preemphasis. Phase modulaion sysems do no require emphasis. 86

106 Pre-emphasis and De-emphasis on FM Preemphasis and deemphasis: a shemai diagrams; b aenuaion urves 87

107 Pre-emphasis and De-emphasis on FM Example o S/N wihou preemphasis and deemphasis. 88

108 Pre-emphasis and De-emphasis on FM Example o S/N wih preemphasis and deemphasis. 89

109 Dolby dynami preemphasis 90

110 Figure.55 Comparison o he noise perormane o various CW modulaion sysems. Curve I: Full M, = 1. Curve II: DSB-SC, SSB. Curve III: FM, =. Curve IV: FM, = 5. Curves III and IV inlude 13-dB pre-emphasis, deemphasis 91 improvemen.

111 In making he omparison, i is inormaive o keep in mind he ransmission bandwidh requiremen o he modulaion sysems in quesion. Thereore, we deine normalized ransmission bandwidh as B n BT W Table.4 Values o B n or various CW modulaion shemes FM M, DSB-SC SSB 5 B n

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