part 03-1 eee309 amplitude modulation

8/10/2019 Part 03-1 EEE309 Amplitude Modulation

1/15

EEE 309 Communication Theory

r. . ar a ossa nAssistant Professor

Department of EEE, BUET

Email: [email protected]

Office: ECE 331, ECE Building


2/15


3/15

Part 03-1Double-sideband with Carrier

-

3


4/15

DSB-WC / AM (1)

tfAtAtc cccc 2coscos TheCarrierSignal: , fc = Carrier frequency (Hz)

tmThebasebandmessage(modulating)signal:

ttmtAttmAt cccccAM coscoscos TheAMsignal:

tctmkttmkAt

A

tmA acacc

c

c

1cos1cos1

Thus, we transmit an unmodulated carrier in addition to the modulated carrier

4

Ka = 1/Ac:Amplitude sensitivity of the modulator


5/15

AM (2): Frequency Domain Thebasebandmessage(modulating)signal:

USBLSB

AmplitudespectrumofAMsignal:2B 0 2B

ccccc

AM ffMffMffffA

f 2

1

2

cccccAM wwMwwMwwwwAw 1Or,

CarrierCarrier

USBLSBLSBUSB

5

BandwidthofAMsignal:BAM=2B,Hz

c2B c c+2B(c+2B) c (c2B)


6/15


7/15

AM (4): Modulation Index,

minmax

minmax

Modulation Index:

max = Maximum of AM(t)

min = Minimum of AM(t)

Case I:m(t) with zero offset(i.e.,m(t)|max = -m(t)|min =mp): c

p

A

m

Case II:m(t) with non-zero offset (rare

case) (i.e.,m(t)|max

-m(t)|min

):

minmax

minmax

||2

||

tmtmA

tmtm

c

< 1: Undermodulation

= 1: 100% modulation

= nve ope e ec on crea es s or on

An example of over modulation

7Acmp0

=> 0 1


8/15

AM (5): Example

ExampleforSingleToneModulation

tAtm mm cosLet,

ttAt cmcAM coscos1

Then,

50%modulation 100%modulation

8

Amplitudespectrum(tryyourself verysimple!!)


9/15

AM (6): Sideband and Carrier Power

CarrierPower:2

2

cC

AP

ttmtt cccAM coscos Transmittedsignal:

Sidebandpower:

Powerefficiency:

ttA

tAt mcmcc

ccAM coscos2cos

SpecialCase:SingleToneModulation

USBpower:8

22

cUSB

AP

8

22cLSB

AP LSBpower: Totalsidebandpower:

4

22

cS

AP

Underthebestcondition(=1):

9

Thus,fortonemodulation,underthebestconditions,onlyonethirdofthepowerisusedfor

carryingmessage,whichisevenlower(lessthan25%orworse)underpracticalconditions


10/15

AM (7): ModulationbyPracticalSpeech

Signals

GeneralCase:ModulationbyPracticalSpeechSignals

tAtAtAtAtm nn cos...coscoscos)( 332211 Messagesignal:

tttttAttAtAtAtAtAt

CnnC

CnnCCAM

coscos...coscoscos1coscos...coscoscoscos)(

332211

332211

AMsignal:

44

... 22232

2

2

1c

Tc

nSP Total sideband power:

Total transmitted power:c

TT PP 1

2

22

3

2

2

2

1

2 ... nT where

22

3

2

2

2

1 ... nT Overall modulation index:

10

To prevent overmodulation: T 1 , 2 , , n


11/15

Generation of AM SignalSwitching Modulator:

Input:Ac cosct+ m(t)

Condition:Ac >> m(t): Thus the switching action of the diode is controlled by {Ac cosct}

This switching action results in a multiplication of [Ac cosct+ m(t)] by w(t)

Thus,

'

11

cc


12/15

Demodulation of AM Signals (1)1. Rectifier Demodulator:

12


13/15

Demodulation of AM Signals (2)2. Envelope Detector:

13

,

mustbechosenproperly

Differencebetweentherectifierdetectorandtheenvelopedetector?(Thinkfirstandconsultwiththetextbooks)


14/15

Demodulation of AM Signals (3)3. Synchronous/ Coherent / Homodyne Detector:

td tp

M

DC blocking

tccos

ttmttt coscos 2

A

ttmA

c

cc

11

2cos12

1

er an oc ng:

tmCtd

cc

222

Phaseandfrequencyofthelocalcarrierhavetobesameasthoseof thecarrier:

S nchronizationre uiredbetweentransmitterandreceiver

14

Morecomplexandexpensivethanofanenvelopdetector

Rectifierdetectoriseffectivelyacoherentdetector


15/15

AM: Summary

Wasteful of transmitted power: power efficiency

Wasteful of channel bandwidth: twice of the

Simpler modulator and demodulator

Less expensive modulator and demodulator

Easy to be affected by noise

15

part 03-1 eee309 amplitude modulation

Documents