Communications _ Amplitute Modulation

8/2/2019 Communications _ Amplitute Modulation

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MODULATION


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Modulation is the process of putting information

onto a high frequency carrier for transmission

(frequency translation).

1. What is modulation?


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Once this information is received, the low frequency

information must be removed from the high frequency

carrier. This process is known as Demodulation.


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2. What are the reasons formodulation?


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1. Frequency division multiplexing (To support multiple

transmissions via a single channel)

To avoid interference

2. What are the reasons formodulation?


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f

M1(f)

0

f

M2(f)

0

f

M(f)

0 f1 f2

Multiplexed

signal

+


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2. Practicality of Antennas

Transmitting very low frequencies require antennas with

miles in wavelength


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3.What are the Different ofModulation Methods?


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1. Analogue modulation- The modulating signal and

carrier both are analogue signals

Examples: Amplitude Modulation (AM) , Frequency

Modulation (FM) , Phase Modulation (PM)

2. Pulse modulation- The modulating signal is an

analogue signal but Carrier is a train of pulses

Examples : Pulse amplitude modulation (PAM), Pulse

width modulation (PWM), Pulse position modulation

(PPM)

3. What are the Different ofModulation Methods?


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3. Digital to Analogue modulation- The modulating

signal is a digital signal , but the carrier is an

analogue signal.

Examples: Amplitude Shift Keying (ASK), FSK, Phase

Shift Keying (PSK)

4. Digital modulation -

Examples: Pulse Code Modulation, Delta

Modulation,Adaptive Delta Modulation

3.What are the Different ofModulation Methods?


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ANALOG AND DIGITALANALOG AND DIGITAL

Analog-to-analog conversion is the representation ofAnalog-to-analog conversion is the representation ofanalog information by an analog signal. One may askanalog information by an analog signal. One may ask

why we need to modulate an analog signal; it is alreadywhy we need to modulate an analog signal; it is already

analog. Modulation is needed if the medium isanalog. Modulation is needed if the medium is

bandpass in nature or if only a bandpass channel isbandpass in nature or if only a bandpass channel is

available to us.available to us.

Amplitude Modulation

Frequency Modulation

Phase Modulation

Topics discussed in this section:Topics discussed in this section:


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Figure Types of analog-to-analog modulation


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Figure Amplitude modulation


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The total bandwidth required for AM

can be determined

from the bandwidth of the audio

signal: BAM = 2B.

Note


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Figure AM band allocation


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The total bandwidth required for FM canbe determined from the bandwidth

of the audio signal: BFM = 2(1 + )B.

Note


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Figure Frequency modulation


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Figure FM band allocation


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Figure Phase modulation


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The total bandwidth required for PM can

be determined from the bandwidthand maximum amplitude of the

modulating signal:

BPM = 2(1 + )B.

Note


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4. What are the Basic Types ofAnalogue Modulation Methods ?


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Consider the carrier signal below:

sc(t ) = Ac(t) cos( 2fc t + )

1. Changing of the carrier amplitude Ac(t)produces

Amplitude Modulation signal(AM)

2. Changing of the carrier frequency fc produces

Frequency Modulation signal(FM)

3. Changing of the carrier phase producesPhase Modulation signal(PM)

4. What are the Basic Types ofAnalogue Modulation Methods ?


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Analogue Modulation Methods


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5 Wh t th diff t F


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5. What are the different Formsof Amplitude Modulation ?

5 Wh t th diff t F


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1. Conventional Amplitude Modulation (DSB-LC)

(Alternatively known as Full AM or Double Sideband

with Large carrier (DSB-LC) modulation

2. Double Side Band Suppressed Carrier(DSB-SC)

modulation

3. Single Sideband(SSB) modulation

4. Vestigial Sideband(VSB) modulation

5. What are the different Formsof Amplitude Modulation ?


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Conventional Amplitude Modulation

(Full AM)


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6. Derive the Frequency Spectrum for Full-AM

Modulation (DSB-LC)


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1 The carrier signal is

ccccc

ftAts 2where)cos()( ==

2 In the same way, a modulating signal (information

signal) can also be expressed as

tAtsmmm

cos)( =

6. Derive the Frequency Spectrum for Full-AM

Modulation (DSB-LC)


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3 The amplitude-modulated wave can be expressed as

[ ] )cos()()( ttsAts cmc +=

[ ] )cos()cos()( ttAAts cmmc +=

4 By substitution

c

m

A

Am =

5 The modulation index.


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6 Therefore The full AM signal may be

written as

)cos())cos(1()( ttmAts cmc +=

)]cos()[cos(2/1coscos BABABA ++=

tmA

tmA

tAtsmc

c

mc

c

cc)cos(

2)cos(

2)(cos)( +++=


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7. Draw the Frequency Spectrum of the above AM

signal and calculate the Bandwidth


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fC fc+fmfc-fm

2fm

7. Draw the Frequency Spectrum of the above AM

signal and calculate the Bandwidth


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8. Draw Frequency Spectrum for a complex input

signal with AM

8 S f i


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8. Draw Frequency Spectrum for a complex input

signal with AM

fcfc-fm fc+fm


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The frequency spectrum of AM waveform containsthree parts:

1. A component at the carrierfrequency fc

2. An upper side band (USB), whose highest frequency

component is at fc+fm

3. A lower side band (LSB), whose highest frequencycomponent is at fc-fm

The bandwidth of the modulated waveform is twice the

information signal bandwidth.

Frequency Spectrum of an AM signal


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Because of the two side bands in the frequency spectrum its

often called Double Sideband with Large Carrier.(DSB-LC)

The information in the base band (information) signal is

duplicated in the LSB and USB and the carrier conveys no

information.


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ExampleExample

We have an audio signal with a bandwidth of 5 KHz.

What is the bandwidth needed if we modulate the signal

using AM?


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ExampleExample

We have an audio signal with a bandwidth of 5 KHz.

What is the bandwidth needed if we modulate the signal

using AM?

SolutionSolution

An AM signal requires twice the bandwidth of the original

signal:

BW = 2 x 5 KHz = 10 KHz


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AM Radio Band


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Modulation Index (m)

m is merely defined as a parameter, which determines the

amount of modulation.

What is the degree of modulation required to establish a

desirable AM communication link?

Answer is to maintain m


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Modulation Index (m)9. What is the significance of modulation index ?


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If the amplitude of the modulating signal is higher than the

carrier amplitude, which in turn implies the modulation

index . This will cause severe distortion to the

modulated signal.

%)100(0.1m


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The ratio of useful power, power efficiency :

2

2

2

2

22/12/

mm

mm

powertotalpowersidebands

+=

+=

In terms of power efficiency, for m=1 modulation, only33% power efficiency is achieved which tells us that only

one-third of the transmitted power carries the useful

information.

10. Calculate the power efficiency of AM signals

Double Side Band Suppressed Carrier


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The carrier component in full AM or DSB-LC does not convey any

information. Hence it may be removed or suppressed during the

modulation process to attain higher power efficiency.

The trade off of achieving a higher power efficiency using DSB-SC is

at the expense of requiring a complex and expensive receiver due to

the absence of carrier in order to maintain transmitter/receiversynchronization.

Double Side Band Suppressed Carrier(DSB-SC) Modulation

11 Derive the Frequency Spectrum for Double Sideband


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1 Consider the carrier

ccccc ftAts 2where)cos()( ==

2 modulated by a single sinusoidal signal

mmmm ftAts 2wherecos)( m ==3 The modulated signal is simply the product of these two

( )

LSB

mc

cm

USB

mc

cm

mcmc

mmcc

tAA

tAA

BABABA

ttAA

tAtAts

)cos(

2

)cos(

2

)cos()cos(2

1coscossince

)cos()cos(

)cos()cos()(

++=

++=

=

=

11. Derive the Frequency Spectrum for Double Sideband

Suppressed Carrier Modulation (DSB-SC)


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tAts mmm cos)( =

tAts ccc cos)( =

)cos()cos()( tAtAts mmcc =X

fcfc-fm fc+fm

Frequency Spectrum of a DSB-SC AM Signal


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All the transmitted power is contained in the two sidebands

(no carrier present).

The bandwidth is twice the modulating signal bandwidth.

USB displays the positive components ofsm(t) and LSB

displays the negative components ofsm(t).


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Generation and Detection of DSB-SC

The simplest method of generating a DSB-SC signal is

merely to filterout the carrier portion of a full AM (or DSB-

LC) waveform.

Given carrier reference, modulation and demodulation

(detection) can be implemented using product devices or

balanced modulators.


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BALANCED MODULATOR

AM Modulator 1

AM Modulator 2

Carrier

Sm(t)

Sm(t)

-Sm(t)

Accos(ct)

Accos(ct)

S2(t)

S1(t)

S(t)

DSB-SC


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The two modulators are identical except for the sign reversal

of the input to one of them. Thus,

)cos())cos(1()(1 ttmAts cmc +=

)cos())cos(1()(2 ttmAts cmc =

)cos()cos(2

)()()( 21

ttmA

tststs

cmc =

=

COHERENT (SYNCHRONOUS) DETECTOR OR


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DSB-SC Signal s(t)

Local Oscillator

LPFX

Cosct

v(t) vo(t)

COHERENT (SYNCHRONOUS) DETECTOR OR

DSB-SC (PRODUCT DETECTOR)

Since the carrier is suppressed the envelope no longer

represents the modulating signal and hence envelope

detector which is of the non-coherent type cannot be used.


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[ ]

2cos)

)cos()(since

)2cos()cos()cos(

2

2cos1)cos(2

)(cos)cos(2

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

2

d by LPF)erm(removeUnwanted t

cmm

mmm

cmmmm

cmm

cmc

c

m

ccmcc

t)((ts(t)s

tAts

ttAtA

ttA

ttAA

A

tttmAttstv

+=

=+=

+=

=

==


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It is necessary to have synchronization in both frequency

and phase between the transmitter (modulator) & receiver

(demodulator), when DSB-SC modulation ,which is of the

coherent type, is used.Both phase and frequency must be known to demodulate

DSB-SC waveforms.

LACK OF PHASE SYNCHRONISATION


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LACK OF PHASE SYNCHRONISATION

Let the received DSB-SC signal be( ) ccmSCDSB Attsts += cos)()(

if is unknown,

( )

( )[ ]

++=

+==

tts

A

tttsA

ttstv

cm

c

ccmc

cSCDSB

2coscos)(2

coscos)(

cos)()(

Output of LPF

cos)(2)( ts

A

tv mc

o=

But we want just


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But we want just

)(

2

)( tsA

tv mc

o =

Due to lack of phase synchronization, we will see that the

wanted signal at the output of LPF will be attenuated by an

amount of cos .In other words, phase error causes an attenuation of the

output signal proportional to the cosine of the phase error.

The worst scenario is when = /2, which will give rise tozero or no output at the output of the LPF.


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LACK OF FREQUENCY SYNCHRONISATION

Suppose that the local oscillator is not stable at fc but at

fc+ f,then ( )( )

( )[ ]

++=

+=

+=

tttsA

tttsA

ttstv

cmc

ccmc

cSCDSB

2coscos)(2

coscos)(

cos)()(

Output of LPF

ttsAtv mc

o = cos)(2

)(

Thus, the recovered baseband information signal will vary

sinusoidal according to cos t

This problem can be overcome by adding an extra


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This problem can be overcome by adding an extra

synchronization circuitry which is required to detect and t and by providing the carrier signal to the receiver.A synchronizer is introduced to curb the synchronization

problem exhibited in a coherent system.

Let the baseband signal be

tAtsmmm

cos)(=

Received DSB-SC signal

ttsAts cmc cos)()( =


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( )2 PLL BPF 2

SYNCHRONISER

Mathematical analysis of the synchronizer is shown below:

[ ][ ]

[ ]

( ) ( )

+++++=

+++=

++=

=

ttttAA

ttttAA

ttAA

ttAAts

mcmccmmc

cmcmmc

cmmc

cmmc

2cos2

12cos

2

12cos2cos1

4

2cos2cos2cos2cos14

2cos12cos14

coscos)(

22

22

22

22222

Output of BPF

t

AA

c

mc

2cos4

22


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Output of frequency dividertk ccos

where kis a constant of proportionality.

DISADVANTAGE OF USING COHERENT SYSTEMS

The frequency and phase of the local oscillator signal must

be very precise which is very difficult to achieve.

It requires additional circuitry such as synchronizer circuit

and hence the cost is higher.

Single-Sideband Modulation

Single Side Band Modulation (SSB)


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g

How to generate SSB signal?

Generate DSB-SC signal

Band-pass filter to pass only one of the sidebandand suppress the other.

For the generation of an SSB modulated signal

to be possible, the message spectrum must have

an energy gap centered at the origin.

Single Side Band Modulation (SSB)


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Example of signal with -300 Hz ~ 300 Hz energy gap

Voice : A band of 300 to 3100 Hz gives good articulation Also required for SSB modulation is a highly selective filter

Vestigial Sideband Modulation

Vestigial Side Band Modulation (VSB)


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Instead of transmitting only one sideband as SSB, VSB

modulation transmits a partially suppressed sideband and avestige of the other sideband.

Vestigial Side Band Modulation (VSB)

C i f A li d M d l i h d


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Comparison of Amplitude Modulation methods

C i f A lit d M d l ti th d


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Full AM (or DSB-LC)

- Sidebands are transmitted in full with the carrier.

- Simple to demodulate / detect

- Poor power efficiency

- Wide bandwidth ( twice the bandwidth of the information

signal)- Used in commercial AM radio broadcasting, one transmitter

and many receivers.


C i f A lit d M d l ti th d


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DSB-SC

- Less transmitted power than full AM and all the transmitted

power is useful.- Requires a coherent carrier at the receiver; This results in

increased complexity in the detector(i.e. synchroniser)

- Suited for point to point communication involving one

transmitter and onereceiver which would justify the use of

increased receiver complexity.




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SSB

- Good bandwidth utilization (message signal bandwidth =

modulated signal bandwidth)- Good power efficiency

- Demodulation is harder as compares to full AM; Exact filter

design and coherent demodulation are required

- Preferred in long distance transmission of voice signals


Comparison of Amplit de Mod lation methods


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VSB

- Offers a compromise between SSB and DSB-SC

- VSB is standard for transmission of TV and similar signals- Bandwidth saving can be significant if modulating signals

are of large bandwidth as in TV and wide band data signals.

For example with TV the bandwidth of the modulatingsignal can extend up to 5.5MHz; with full AM the

bandwidth required is 11MHz


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