Communications Principles Analog Communications · Analog Communications Ch3: Amplitude Modulation:...

Philadelphia UniversityPhiladelphia Universityp yp yDepartment of Communication & Electronics Department of Communication & Electronics

EngineeringEngineering

Analog CommunicationsAnalog CommunicationsCh3: Amplitude ModulationCh3: Amplitude Modulation

InstructorInstructor

Ibrahim N. AbuIbrahim N. Abu--IsbeihIsbeih

Email: [email protected]

Website: www.abusbeih.com/ecourse

Communications Principles

Prince Sattam Bin Abdulaziz University Electrical Engineering Department

Dr. Moustafa M. Nasralla

Email: [email protected] Website: https://faculty.psau.edu.sa/m.nasralla

Modulation ProcessModulation Process

Modulation: The process of varying some parameter of a carrier wave in accordance with the message signal. i f iinformation

Why modulation?

1. Reducing the size of the antennas:

- The optimal antenna size is related to wavelength:

- Voice signal: 3 kHz

2. Allowing transmission of more than one signal in the same channel (multiplexing)

3 Allowing better trade off between bandwidth and signal to3. Allowing better trade-off between bandwidth and signal-to-noise ratio (SNR)

Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [2]

ModulationModulation

InformationWhat is it?

Modulating SignalMessage Signal

m(t)(B b d)

Modulated Signals(t)

(Bandpass)(Baseband)

( p )

Modulatormessage: m(t) s(t): radio signal

carrier: c(t)

[3]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih

ModulationModulationModulationModulation

Continuous-Wave (CW) Modulation Pulse Modulation

1. Amplitude Modulation (AM) Analog Pulse M d l ti

Digital Pulse M d l ti

2. Angle Modulation:

- Frequency Modulation (FM)

Ph M d l ti (PM)

Modulation:

1. Pulse Amplitude Modulation (PAM)

Modulation:

- Pulse Code Modulation (PCM)- Phase Modulation (PM)

2. Pulse Width Modulation (PDM)

3. Pulse Position

(PCM)


Modulation (PPM)

11. Amplitude Modulation (AM). Amplitude Modulation (AM)

An amplitude-modulated (AM) wave is described as a function of time as follows:

)2cos()(1)( tftmkAts cac

signalg)(modulatinmessagetheism(t)in volt

measuredmodulator theofy sensitivit amplitude theis where1-

ka

vecarrier wa theis )2cos()(c and

signalg)(modulatinmessagetheis m(t) ,in volt

tfAt cc


Fig. Illustrating the amplitude modulation process:

(a) Baseband signal m(t). (b) AM wave for | kam(t) | < 1 for all t. (c) AM wave for | kam(t) | > 1 for some t.

The percentage modulation:

100%)(max

tmka max


©2000, John Wiley & Sons, Inc. Haykin / Communication Systems, 4th Ed

We observe that the envelope of s(t) has essentially the same shape as the baseband signal m(t) provided that two requirements are

i fi dsatisfied:1. The amplitude of kam(t) is always less than unity, that is,

ttmk allfor1)(

2. The carrier frequency fc is much greater than the highest frequency component W of the message signal m(t) that is

ttmka allfor 1)(

frequency component W of the message signal m(t), that is

Wfc

• When |ka m(t)| > 1 for any t, the carrier becomes overmodulated.


AM SpectrumAM Spectrum

)]()([)]()([)( cac ffMffMAk

ffffA

fS

)2cos()()2cos()2cos()(1)( tftmkAtfAtftmkAts caccccac

)]()([2

)]()( [2

)( ccca

ccc ffMffMfffffS

2BT=2W

The transmission bandwidth for an AM wave is twice the b d id h h i 2


message bandwidth W, that is, BT=2W.

Example: Example: Single Single Tone AMTone AM

m(t)

A)2cos()( tfAtm mm

t

Am

c(t)

)()( fmm

t

Ac

)2cos()( tfAtc cc

s(t)Ac[1+kaAm]

)2cos()2cos(1)( tftfAkAts cmmac

t

Ac )2cos()2cos(1 tftfA cmc

(index)factor modulation theis ma Ak


Modulation index (factor)Modulation index (factor)

minmax

minmax

AA

AA

s(t) 1max cAA

1min cAAcA

maxA

t

min c

minA

t


Spectrum (Single Tone AM)Spectrum (Single Tone AM)

mT fB 2 :Bandwidthon Transmissi

Message Signal

Carrier Wave

AM WaveAM Wave


AM Power & EfficiencyAM Power & Efficiency

Single tone AM wave:

tftfAts cmc )2cos()2cos(1)(

tffA

tffA

tfA

tftfAtfA

ff

cc

cmccc

cmc

)(2cos)(2cos)2cos(

)2cos()2cos()2cos(

)()()(

tfftfftfA mcmccc )(2cos2

)(2cos2

)2cos(

2

2

1powerCarrier cA

22222

8

1

8

1power sideupper and sideLower cc AA

power total thepower to sideband total theof ratio The

2

2

22222

2222

211181

81

cc AA1power band side total

then,modulation 100%1 if 2


22222 281

81

21 ccc AAA 3

2power total 2

Switching ModulatorSwitching Modulator

The generation of AM wave may be accomplished using various devices; here we describe one such device called switching

d lmodulator.

)()2cos()( tmtfAtv )()2cos()(1 tmtfAtv cc

0)(0

0)( , )()( 1

2 tc

tctvtv


0)( , 0 tc

Switching Modulator (Cont.)Switching Modulator (Cont.)

)( )()2cos()(02 tgtmtfAtv Tcc

cfT

10

hiF iitb)(R ti t

122cos12

12

2

1)(

haveweseries,Fourier itsby )( ngRepresenti

1

1

0

0

ntfn

tg

tg

c

n

T

T

122 1 nn

tv

component The .1

:components twoof sum theof consists )( that find on wesubstitutiAfter 2

cc

cT tftm

A

Atg

4

2cos)(4

12

)( 0


ca A

k

4 ofy sensitivit amplitude with waveAM desired theis which

Switching Modulator (Cont.)Switching Modulator (Cont.)

2 width of intervalsfrequency which and on, so and ,4 ,2 ,0at

functionsdeltaofconsistswhich ofspectrumthe,components d Unwante.2

cc Wff

filter pass-band a of meansby removed are termsunwanted The

on.soand,5,3 ,0at centered cc ff

BPF[v2(t)]

m(t)

2


Envelope DetectorEnvelope Detector

The charging time constant (rf + RS )C must be short for rapid charge that ischarge, that is,

The discharging time constant R C must be long enough toc

sf fCRr

1)(

The discharging time constant RLC must be long enough to ensure slowly discharging, that is,

CR11


WCR

f l

c

Envelope Detector (Cont.)Envelope Detector (Cont.)


Envelope Detector Envelope Detector (Example)(Example)

The following numerical values can be used in a practical envelope detector:

Rs 75 resistance Source

FC

kR

r

l

f

01.0eCapacitanc

10 resistance Load

25 resistance Forwared

c

sf fCRr

1)(

kHzf

kHzW

FC

c 20 frequency Carrier

1 bandwidth Message

01.0 eCapacitanc

WCR

f l

c

11


Virtues, Limitations Virtues, Limitations and Modifications of and Modifications of Amplitude Amplitude ModulationModulation

Virtue of amplitude modulation– Amplitude modulation system is very cheap to build and maintain

Practical Limitation Practical Limitation– Amplitude modulation is wasteful of transmitted power

The transmission of the carrier wave therefore represents a waste of power

– Amplitude modulation is wasteful of channel bandwidthAmplitude modulation is wasteful of channel bandwidth

Insofar as the transmission of information is concerned, only one sideband is necessary, and the communication channel therefore needs to provide only the same bandwidth as the message signal.

i i i b d id h l i h b d id h It requires a transmission bandwidth equal to twice the message bandwidth

Modifications of amplitude modulation– Double sideband-suppressed carrier (DSB-SC) modulation

The transmitted wave consists of only the upper and lower sidebands But the channel bandwidth requirement is the same as before

– Single sideband (SSB) modulation

Th d l t d i t l f th id b d th l

19

The modulated wave consists only of the upper sideband or the lower sideband

Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih

22. Double Sideband. Double Sideband--Suppressed CarrierSuppressed Carrier(DSB(DSB--SC) ModulationSC) Modulation

Double sideband-suppresses carrier (DSB-SC) modulation consists of the product of the message signal m(t) and the carrier

( )wave c(t):

)2cos()()()()( tftmAtctmts )2cos()( )()()( tftmAtctmts cc

Its only advantage is saving transmitted power, g p ,which is important enough when the available transmitted power is at a

i


premium

DSBDSB--SC SC SpectrumSpectrum

)] ( ) ([2

)( ccc ffMffM

AfS

The transmission bandwidth for DSB-Sc wave is twice the

BT=2W

The transmission bandwidth for DSB Sc wave is twice the message bandwidth W, that is, BT=2W.

Its only advantage is saving transmitted power.


Ring ModulatorRing Modulator One of the most useful product modulators, well suited for

generating a DSB-SC wave, is the ring modulator


Coherent DetectionCoherent Detection Coherent detection or synchronous demodulation is used to

recover the message signal m(t) from the DSB-SC wave

)()2()( ' fA

)()cos(1

)()4cos(1

)()2cos()2cos(

)()2cos()(

''

'

tmAAtmtfAA

tmtftfAA

tstfAtv

cccc

cc

)()cos(2

)()4cos(2

tmAAtmtfAA ccccc

)()cos(1

)()( '

LPF0 tmAAtvtv cc )cos(

2

1)cos(

2

1)cos()cos( 212121

)()(2

)()(LPF0 cc


Coherent Detection (Quadrature null effect)Coherent Detection (Quadrature null effect)

)()cos(2

1)()( '

LPF0 tmAAtvtv cc

- The quadrature null effect of the coherent detector

The zero demodulated signal, when occurs for Φ=±π/2g , f / The phase error Φ in the local oscillator causes the detector

output to be attenuated by a factor equal to cosΦ


Costas ReceiverCostas ReceiverCostas Receiver

– A practical synchronous receiver system used for demodulating DSB SC wavesDSB-SC waves.

– Consists of two coherent detectors supplied with the same input signal Two local oscillator signals that are in phase quadrature with

respect to each other I-channel : in-phase coherent detector

h l d h h d Q-channel : quadrature-phase coherent detector

– Phase control in the Costas receiver ceases with modulation, Which means that phase-lock would have to be re-established with

the reappearance of modulation


Costas ReceiverCostas Receiver


Quadrature Amplitude Modulation (QAM)Quadrature Amplitude Modulation (QAM)(Quadrature(Quadrature--Carrier Multiplexing)Carrier Multiplexing)

QAM scheme enables two DSB-SC modulated waves to occupy the same channel bandwidth. It is therefore a bandwidth-conservation schemescheme.

)2sin()()2cos()()( 21 tftmAtftmAts cccc


Filtering of SidebandsFiltering of Sidebands

We can recover the original baseband signals without distortion if the transfer function of the filter satisfies the condition:

WfWffHffH cc ,constant )()(

)(1

)( '0 tmAAtv


)(2

)(0 tmAAtv cc

33. Single Sideband (SSB) Modulation. Single Sideband (SSB) Modulation

The single-sideband (SSB) signal is generated by filtering the DSB-SC signal.

Generation of SSB Waves:1- Frequency Discrimination Method (Filtering Method)

For the design of the band pass filter to be practically feasible there must be aFor the design of the band-pass filter to be practically feasible, there must be a certain separation between the two sidebands that is wide enough to accommodate the transition band of the band-pass filter.

2- Phase Discrimination MethodWide-band phase-shifter is designed to produce the Hilbert transform in

t th i i i lresponse to the incoming message signal.To interfere with the in-phase path so as to eliminate power in one of the two sidebands, depending on whether upper SSB or lower SSB is the requirement.


a. Frequency Discrimination (Filtering) Method


Transfer Function for BPF to Generate SSB (LSB or USB):


DSB Spectrum

SSB/USB Spectrum

SSB/LSB Spectrum


b. Phase Discrimination MethodThe SSB wave may be written in the canonical form:

11

for USB )2sin()(ˆ2

1)2cos()(

2

1)( tftmAtftmAts cccc

LSBfor )2sin()(ˆ2

1)2cos()(

2

1)( tftmAtftmAts cccc

)( of ansformHilbert tr theis )(ˆ tmtm

)sgn()( fjfHQ

)()()(ˆ fHfMfM Q


Demodulation of SSB Signals : (Coherent Detector)Demodulation of SSB Signals : (Coherent Detector)

Coherent detection or synchronous demodulation is used to recover the message signal m(t) from the SSB-SC wave


44. Vestigial. Vestigial--Sideband (VSB) ModulationSideband (VSB) ModulationThe Vestigial sideband (VSB) Modulation is a technique used for commercial video broadcasting which lies some where in the middle of SSB and DSB and it is a compromise between SSB andmiddle of SSB and DSB and it is a compromise between SSB and DSB.


Television (TV) Signals:

Television Signals:- The video signal exhibits a large

bandwidth and significant low-bandwidth and significant low-frequency content, which suggest the use of vestigial sideband modulation.

- The circuitry used for demodulation in the receiver should be simple and therefore inexpensive. This suggest the use of envelope detection, whichthe use of envelope detection, which requires the addition of a carrier to the VSB modulated wave.

Fig ( ) Id li d it d t fFig (a): Idealized magnitude spectrum of a transmitted TV signal.

Fig (b): Magnitude response of VSB shaping filter in the receiver


shaping filter in the receiver.

Frequency Translation (Mixing):Frequency translation, mixing, or hetrodyning is used to translate the spectrum of a signal from one frequency to another.

downwardtion transla

upwardation transl

1

12

l

l

ff

fff


Frequency Division Multiplexing (FDM)Frequency Division Multiplexing (FDM)

Multiplexing is used to transmit a number of these signals over the same channel, the signals must be kept apart so that they do not interfere with each other and thus they can be separated at theinterfere with each other, and thus they can be separated at the receiving end.

Multiplexing techniques:

1. Frequency-Division Multiplexing (FDM):

(A technique of separating signals in frequency)

2 Time-Division Multiplexing (TDM):2. Time-Division Multiplexing (TDM):

(A technique of separating signals in time)


Block diagram of FDM systemBlock diagram of FDM system


Example: FDM of SSB for Telephone Systems


Example: FDM of SSB for Telephone Systems (Cont.)


End of Chapter ThreeEnd of Chapter Three


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