17671938 Lecturer1 Analog Modulation

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DTC5038 ANALOG COMMUNICATION SYSTEM Trimester 3 2008-2009

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Subtopic:

3-1 Introduction to Modulation

3-2 Types of Modulation

3-3 Modulation Index

A large number of information sources are analog sources such as speech, images, and videos.

Today, they are transmitted as analog signal transmission, especially in audio and video

broadcast. The transmission of an analog signal is either by modulation of the amplitude, the

phase, or the frequency of a sinusoidal carrier.

Modulation is the process of putting information onto a high frequency carrier for

transmission (frequency translation). Modulation occurs at the transmitting end of the system.

Figure 3-1: Block diagram of modulation process

At the transmitter, modulation process occurs when the transmission takes place at the high

frequency carrier, which has been modified to carry the lower frequency information. At the

receiver, demodulation takes place. Once this information is received, the lower frequency

information must be removed from the high-frequency carrier.

Figure 3-2: Block diagram of modulation and demodulation processes

3-1 INTRODUCTION

LECTURE NOTES 3

CHAPTER 3: AMPLITUDE MODULATION

Modulator

Carrier signal

Modulating signal Modulated signal

Modulator DemodulatorChannelMessagesignal

Messagesignal


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There are several strong reasons why the modulation is important in analog communication

system:

(a)The frequency of the human voice range from about 20 to 30 kHz. If every one transmitted

those frequencies directly as radio waves, interference would cause them to be inefficient.

(so, we need a higher frequency to carry the baseband frequency)

(b)To overcome hardware limitation because transmitting such lower frequencies require

antennas with miles in wavelength

(c)Modulation is to reduce noise which result in the optimization of signal to noise ratio, SNR

(d)To minimize the effects of interference

In analog communication systems, we use the sinusoidal signal as the frequency carrier. And as

the sinusoidal wave can be represented in three parameters; amplitude, frequency and phase,

these parameters may be varied for the purpose of transmitting information giving respectively

the modulation methods:

(a)Amplitude Modulation (AM) - the amplitude of the carrier waveform varies with the

information signal

(b)Frequency Modulation (FM) - the frequency of the carrier waveform varies with the

information signal

(c)Phase Modulation (PM) - the phase of the carrier waveform varies with the information

signal

3-2 TYPES OF MODULATION


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Figure 3-3: Carrier wave

Figure 3-3: Modulating wave

Figure 3-4: Amplitude modulated wave

Figure 3-5: Frequency modulated wave


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Figure 3-6: Amplitude modulation block diagram

3-2-1 Double Sideband Large Carrier (DSB-LC)Also known as full AM. In Amplitude Modulation, the baseband or the information signal is

modulated to the carrier signal to produce the modulated sine wave.

Consider the carrier signal,

)cos()( tAts ccc = where cc f 2=

The modulating signal (information signal),

( )tAts mmm cos)( =

Then, the amplitude-modulated can be expressed as

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

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

The amplitude term of the AM signal )(ts is

( ))cos( tAAA mmc +=

( ))cos( tmAA mcc +=

))cos(1( tmA mc +=

where notation m in expression above is termed the modulation index. Simply a measurement

for the degree of modulation and bears the relationship of the ratio of cm AA to ,

Amplitude Modulator


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c

m

A

Am =

Therefore the full AM signal may be written as

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

or

( ) ( )tmA

tmA

tAts mcc

mc

c

cc +++= cos2

cos2

cos)(

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

The frequency description of the AM signal (i.e. frequency spectrum of AM) DSB-LC:

From the above analysis, we found that the frequency spectrum of AM waveform DSB-LC:

A component of carrier frequency, cf

An upper sideband (USB), whose highest frequency component is at mc ff +

A lower sideband (LSB), whose highest frequency component is at mc ff

The bandwidth of the modulated waveform is twice the information signal bandwidth

tAts mmm cos)( =

tAts ccc cos)( =

Modulator( )

( )tmA

tmA

tAts

mc

c

mc

c

cc

+

++=

cos2

cos2

cos)(

AM( )fS

f mc ff mc ff + cf

2

cmA

cA

mA

( )fSm

mf f

B2 B


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Because of the two sidebands in the frequency spectrum with carrier frequency, thus it is

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

3-2-2 Double Sideband Suppressed Carrier (DSB-SC)As noted earlier, where there are two sidebands in the frequency spectrum, USB and LSB, and it

is called as Double-sided band (DSB).

But the carrier component in full AM or DSB-LC does not convey any information, it may be

removed or suppressed during the modulation process to attain a higher power efficiency, hence

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

Consider the carrier,

)cos()( twAts ccc = where cc f 2=

The modulating signal (information signal),

( )cos)( tAts mmm = where mm f 2=

Then, the amplitude-modulated can be expressed as

( ) ( )tAtAts mmccm coscos)( =

( ) ( )tAA

tAA

mc

cm

mc

cm ++= cos

2cos

2

( )tAts mmm cos)( =

( )tAts ccc cos)( =

Modulator( ) ( )t

AAt

AAmc

cm

mc

cm ++= cos

2cos

2


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The frequency description of the AM signal (i.e. frequency spectrum of AM) DSB-SC:

Note: Notice that there is no carrier frequency (band).

From the above analysis, we found that the frequency spectrum of AM waveform DSB-SC:

No component of carrier frequency, cf

An upper sideband (USB), whose highest frequency component is at mc ff +

A lower sideband (LSB), whose highest frequency component is at mc ff

The bandwidth is twice the modulating signal bandwidth

Because of the two sidebands in the frequency spectrum without carrier frequency, thus it is

often called Double Sideband with Suppressed Carrier (DSB-SC)

3-2-3 Single Sideband (SSB)Third type of amplitude modulation namely the SSB will be introduced here. Note that

conventional amplitude modulation (Full AM) and DSB-SC modulation require a transmission

bandwidth equal to twice the information signal bandwidth. One half the transmission

bandwidths is occupied by the upper sideband of the modulated signal. Whereas the other half

is occupied by the lower sideband. The basic information is transmitted twice, once in each

sideband. Since the sidebands are the sum and difference of the carrier and modulating signals,

the information must be contained in both of them. There is absolutely no reason to transmit

both sidebands in order to convey the information. One sideband may be suppressed. Theremaining sideband is called a single-sideband suppressed carrier (SSSC or SSB) signal. In

practical systems the carrier is also suppressed in SSB and should be described as SSB-SC.

( )fSm

mA

mf f

DSB-SC

f mc ff mc ff +

( )fS

2

cmAA

B B2


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Therefore, from DSB-SC waveform equation:

)cos2

:LSB

)cos2

:USB

tt(AA

(t)s

tt(AA

(t)s

mc

cm

LSB

mc

cm

USB

=

+=

Either the USB or LSB is used to carry information.

The frequency description of the AM signal (i.e. frequency spectrum of AM) DSB-SC:

Note: Notice that it is either USB or LSB being transmitted.

From the above analysis, we found that the frequency spectrum of AM waveform SSB-SC:

No component of carrier frequency, cf

( )tAts mmm cos)( =

( )tAts ccc cos)( =

)cos2

:LSB

)cos2

:USB

tt(AA

(t)s

tt(AA

(t)s

mc

cm

LSB

mc

cm

USB

=

+=

Modulator

SSB-SC

( )fSm

mA

mf f

B

f mc ff +

( )fS

2

cmAA

f

mc ff

( )fS

2

cmAA


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It is either upper sideband (USB), whose highest frequency component is at mc ff + or

lower sideband (LSB), whose highest frequency component is at mc ff being transmitted

The bandwidth is equal to the modulating signal bandwidth

Because of the only one sideband in the frequency spectrum without carrier frequency, thus

it is often called Single Sideband with Suppressed Carrier (SSB-SC)

The degree of modulation is an important parameter and is known as the modulation index. It is

the ratio of the peak amplitude of the modulation signal, mA to the peak amplitude of the carrier

signal, cA .

c

m

A

Am =

The modulation index, m is also referred as percent modulation, modulation factor and depth of

modulation. It is a number lying between 0 and 1 and is typically expressed as a percentage. The

modulation index can be determined by measuring the actual values of the modulation voltage

and the carrier voltage and computing the ratio.

In practice, the modulation index of an AM signal can be computed from Amax and Amin. as

below:

0 5 10 15 20 25 30 35 40 45

-1.5

-1

-0.5

0

0.5

1

1.5

=maxA half the peak-to-peak value of the AM signal

=minA half the peak-to-peak value of the AM signal

3-3 MODULATION INDEX

( )ptpA max

( )ptpA min


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=mA half the difference of maxA and minA .

=cA half the sum of maxA and minA .

The values for maxA and minA can be obtained directly from the oscilloscope.

The evaluation of the modulation index m can be achieved by invoking the following expression:

( )

( )minmax

minmax

21

21

AA

AAm

+

=

c

m

A

A=

Modulation index can determine the behavior of modulation index:(a)under modulation

(b)ideal modulation

(c) over modulation

3-3-1 Under ModulationWhen 1


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3-3-3 Over ModulationWhen 1>m , we call this as over modulation. If the amplitude of the modulating signal is

higher than the carrier amplitude, this will cause severe distortion to the modulated signal.

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17671938 Lecturer1 Analog Modulation