Chapter 5 Amplitude Modulation

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1 Chaper 5 pliude Modulaion 68

2 nalog Couniaion Syse Inoraion Soure Signal Modulaor Propagaion Channel Signal Deodulaor Inoraion Desinaion nalog signals ay be ransied direly via arrier odulaion over he propagaion hannel and o be arrier-deodulaed a he reeiver. ransier Modulaor Reeiver Deodulaor 69

3 Modulaion: he proess by whih soe haraerisis o a arrier signal (i.e. odulaed signal) is varied in aordane wih essage signal (i.e. odulaing signal) (): essage signal bandliied signal whose requeny onen is in he neighbourhood o =0 (DC) baseband signal (): he arrier signal, independen o () ()= os(π +Ө ) : Carrier apliude : Carrier requeny w =π (radian requeny) Ө : Carrier phase () odulaes () in eiher apliude, requeny or phase. In ee, odulaion onvers () o a bandpass or, in he neighborhood o he ener requeny. 70

4 Why is Modulaion Required? o ahieve easy radiaion: I he ouniaion hannel onsiss o ree spae, anennas are required o radiae and reeive he signal. Diension o he anennas is liied by he orresponding wavelengh. Exaple: Voie signal bandwidh =3kHz /4=5000!! 3 0 I we odulae a arrier = 00MHz wih he voie signal /4=

5 Why is Modulaion Required? (Con d) o aoodae or siulaneous ransission o several signals s C s C s 3 C3 Exaple: Radio/V broadasing 7

6 Why is Modulaion Required? (Con d) o expand he bandwidh o he ransied signal or beer ransission qualiy (o redue noise and inererene) C B log SNR Channel apaiy Bandwidh Signal-o-noise raio Channel apaiy: Maxiu ahievable inoraion rae ha an be ransied over he hannel SNR C B B he required SNR (or ixed noise level, orresponding signal power) dereases 73

7 pliude Modulaion (M) (Ch. 5 in exbook) Objeives: o sudy dieren apliude odulaion shee o sudy generaion and deeion o M signals o sudy appliaion o M We will sudy Double Sideband Large Carrier (DSB-LC) Modulaion: Coerial broadas saions use his ype and i is oonly known as jus apliude odulaion (M). Double Sideband Suppressed Carrier (DSB-SC) Modulaion Single Sideband (SSB) Modulaion Vesigial Sideband (VSB) Modulaion 74

8 Double Side Band Large Carrier (DSB-LC) (5. in exbook) F F F Modulaor os os os os 75 j j e e os F F F os os F j j j j e e e e F F F

9 () F()= F [()] F(0) baseband -B B () os F [() os ] F(0)/ passband - -B +B os π F [ os ] π - os + ()os envelope F [ os + ()os ] - 76

10 baseband F()= F [()] passband ()= F [ os + ()os ] -B F(0) B π F(0)/ Lower sideband π Upper sideband - -B +B Bandwidh=B [Hz] Bandwidh=B Observaions: Modulaion shis he onen o F() o he neighbourhood o. F() or [-B, 0] is shied o () or [ -B, ] and alled as lower sideband. F() or [0, B] is shied o () or [, +B] and alled as upper sideband. Le B denoe he highes requeny oponen o (). ssue >> B () is deined as a narrowband signal (i.e. is speral onen is loaed in he iediae viiniy o soe high ener requeny) 77

11 F()= F [()] ()= F [ os + ()os ] -B F(0) B π F(0)/ Lower π Upper sideband sideband - -B +B Bandwidh=B [Hz] Bandwidh=B Observaions (on d) he bandwidh o essage signal is B. he ransission bandwidh =B (i.e. DSB-LC is waseul o bandwidh) he arrier er does no arry any inoraion and hene he arrier power is wased. 78

12 Envelope has he sae shape o () Envelope disorion Observaions (on d) Zero rossings I +()>0 or all, he envelope o () o has essenially he sae shape as he (). I +()<0 or any, he arrier wave beoes over-odulaed, resuling in arrier phase reversal whenever +() rosses zero. () has envelope disorion 79

13 80

14 overodulaed envelope dis. Ees o varying odulaion indexes 8

15 Carrier and Sideband Power in DSB-LC os os os os os os os ssue and () varies slowly wih respe o os 0 8 os os d / / os li os d / / os li os li li / / / / d = 0

16 Carrier Power Sideband Power (arries inoraion) Modulaion (Power) Eiieny µ= useul power oal power 83

17 8 os os os os os Exaple (Con d) Upper sideband power Lower sideband power 84 8 os power oal useul power oal power µ= 4 4 For µ 33%. Under he bes ondiion, i.e. =, 67% o he oal power is used in he arrier and represens wased power.

18 Generaion o DSB-LC Signals Ciruiry aspes o his opi will no be in any exainaion, aheaial aspes ay be esed. Given (), how will be he odulaed signal () be generaed? Modulaor os os Chopper Modulaor os () D () V () V () Bandpass V 0 ()=() 85

19 D V V g V () V () V () Gain g () V () V os ssue D is an ideal diode, and () <, hen V V, i 0 0, i 0 i.e. he load volage V () varies periodially beween he values V () and 0 a a rae equal o he arrier requeny. When he diode is on, g () = is o, g () = 0 hopper odulaion 86

20 Expand g () using Fourier Series, we have os n n g n n g () /4 / - -/4 -/ he diode an be represened by a reangle pulse generaor. = / = / 87 os os n n V n n g V V ers oher 3 os 3 3 os os 3 os os os

Principles of Communications Lecture 3: Analog Modulation Techniques (1) Chih-Wei Liu 劉志尉 National Chiao Tung University

Principles of Communications Lecture 3: Analog Modulation Techniques (1) Chih-Wei Liu 劉志尉 National Chiao Tung University Priniples of ouniaions Leure 3: nalog Modulaion Tehniques 1 hih-wei Liu 劉志尉 Naional hiao Tung Universiy wliu@wins.ee.nu.edu.w Oulines Linear Modulaion ngle Modulaion Inerferene Feedbak Deodulaors nalog