Transmission of Analog Signal - II

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1 CS311: DATA COMMUNICATION Transmission of Analog Signal - II Dr. Manas Khatua Assistant Professor Dept. of CSE IIT Jodhpur manaskhatua@iitj.ac.in

2 Transmission of Analog Signal-II On completion, the student will able to: Explain the basic concept of analog modulation. Distinguish between FM and PM. Explain the basic concepts of digital data to digital signal conversion Explain different aspects of ASK, FSK, PSK and QAM conversion techniques. Explain bandwidth and power requirement Dr. Manas Khatua 2

3 Modulation Technique Dr. Manas Khatua 3

4 Frequency Modulation The modulating signal e m (t) is used to vary the carrier frequency. The change in frequency is proportional to the modulating voltage ke m (t), K is the constant known as frequency deviation constant, expressed in Hz/V. The instantaneous frequency of the modulated signal is f i (t)= f c + ke m (t), where f c is the carrier frequency Dr. Manas Khatua 4

5 Sinusoidal FM For sinusoidal modulation e m (t)= E m cos2 f m t and f i (t)= f c + ke m (t) = f c + ke m cos2 f m t= f c + fcos2 f m t Therefore s(t)=e c cosθ(t) = E c cos(2 f c t + 2 fʃ 0t cos2 f m tdt) = E c cos(2 f c t + ( f/f m )sin2 f m t) The modulation index, denoted by β, is given by β=( f/f m ) Or s(t)= E c cos(2 f c t + βsin2 f m t) Dr. Manas Khatua 5

6 Bandwidth The modulated signal will contain frequency components f c + f m, f c + 2f m, and so on Carson s Rule B T =2(β+1)B m Where β= f/b=n f A m /2 B B T =2 f+2b FM requires greater bandwidth than AM Dr. Manas Khatua 6

7 Bandwidth Peak Deviation= f=(1/ 2 )n f A m Hz Where A m is the maximum value of m(t) Dr. Manas Khatua 7

8 Power As the amplitude remains constant, total average power is equal to the unmodulated carrier power. Power =A c2 /2 Although A m increases the bandwidth, it does not affect power. Transmission power for FM is less at the expense of high bandwidth Dr. Manas Khatua 8

9 Phase Modulation Representation of modulated signal s(t)=a c cos[w c t+ᴓ(t)] The angle w c t+ᴓ(t) goes under modulation around the angle θ= w c t The signal is therefore an angular-velocity modulated signal. When the phase is directly proportional to the modulating signal, i.e, ᴓ(t)= n p m(t), we call it phase modulation, where n p is phase modulation index Dr. Manas Khatua 9

10 Relation Between FM and PM The instantaneous frequency of the phase modulated signal s(t)=e c cos[w c t+k m(t)], Where k is constant Dr. Manas Khatua 10

11 Relation Between FM and PM Let m(t) be derived as an integral of the modulated signal e m (t), so that m(t)=k ʃe(t), then with k=k k, we get s(t)=e c cos(w c t+kʃe(t)), The instantaneous angular frequency of s(t) is 2 f i (t)=d/dt(2 f c t+kʃe(t)) or f i (t)=f c +(1/2 )ke(t) The waveform is therefore modulated in frequency In summary, these two together are referred to as angle modulation and modulated signals have similar characteristics Dr. Manas Khatua 11

12 Analog Data to analog signal modulation technique at a glance Dr. Manas Khatua 12

13 Why digital data to analog signal conversion Quite often we have to send digital data through analog transmission media Dr. Manas Khatua 13

14 Digital Data-Analog Signal Types of digital-to-analog modulation Dr. Manas Khatua 14

15 Amplitude Shift keying(ask) The unmodulated signal can be represented by e c (t)=e c cos2 f c t The modulated signal can be written as s(t)=ke m cos2 f c t s(t)=a 1 cos2 f c t for 1 s(t)=a 2 cos2 f c t for 0 Special case: on off keying(ook), A 2 is 0 ASK is susceptible to sudden gain change. OOK is used to transmit digital data over optical fibers Dr. Manas Khatua 15

16 Dr. Manas Khatua 16

17 Frequency Spectrum of ASK Signal If B m is the overall bandwidth of the binary signal, the bandwidth of the modulated signal is B T = N b, Where N b is the baud rate Dr. Manas Khatua 17

18 Frequency Shifting Keying Frequency of the carrier is used to represent 0 or 1. s(t)=acos2 f c1 t for binary 1 s(t)=acos2 f c2 t for binary 0 It is much less susceptible to noise and gain change Dr. Manas Khatua 18

19 FSK Dr. Manas Khatua 19

20 Frequency Sprectrum of The FSK Signal FSK may be consider as combination of two ASK spectra centered around f c1 and f c2. Bandwidth= f c2 - f c1 + N b Dr. Manas Khatua 20

21 Phase Shifting Keying(PSK) The phase of carrier is used to represent 0 or 1 s(t)=acos(2 f c t + ) for binary 1 s(t)=acos2 f c t for binary 0 2-PSK Dr. Manas Khatua 21

22 For more efficient use of bandwidth Quadrature Phase Shifting keying QPSK s(t)=acos(2 f c t ) for 00 =Acos(2 f c t +90) for 01 =Acos(2 f c t +180) for 10 =Acos(2 f c t +270) for Dr. Manas Khatua 22

23 8-PSK The idea can be extended to have 8-PSK. The phase is shift by Dr. Manas Khatua 23

24 Ability of equipment to distinguish small difference in phase limits the potential bit rate By combining ASK and PSK it is possible to obtain higher data rate. This combine technique is known as QAM(Quadrature Amlitude Modulation) Dr. Manas Khatua 24

25 Bit Rate and Baud Rate Dr. Manas Khatua 25

26 Modem Modem: Modulator + DEModulator Converts digital signal into analog signal using ASK, FSK, PSK and QAM. Important parameters Transmission rate Bandwidth(Baud rate) Dr. Manas Khatua 26

27 Dr. Manas Khatua 27

Mobile Communication An overview Lesson 03 Introduction to Modulation Methods

Mobile Communication An overview Lesson 03 Introduction to Modulation Methods Oxford University Press 2007. All rights reserved. 1 Modulation The process of varying one signal, called carrier, according