Muhammad Uzair Rasheed2009-CPE-03

UCE&T BZU MULTAN

Amplitude Modulation

CSULB May 22, 2006 3

What is ModulationModulation

In the modulation process, some characteristic of a high-frequency carrier signal (bandpass), is changed according to the instantaneous amplitude of the information (baseband) signal.

Why Modulation is usedSuitable for signal transmission (distance…etc)Multiple signals transmitted on the same

channelCapacitive or inductive devices require high

frequency AC input (carrier) to operate.Stability and noise rejection

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About ModulationApplication Examples

broadcasting of both audio and video signals.

Mobile radio communications, such as cell phone.

• Basic modulation types– Amplitude Modulation: changes the amplitude.– Frequency Modulation: changes the frequency.– Phase Modulation: changes the phase.

Basic Amplitude ModulationAmplitude

Modulation is the simplest and earliest form of transmitters

The information signal varies the instantaneous amplitude of the carrier

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AMPLITUDE MODULATION (AM) In amplitude modulation, the message signal m(t) is

impressed on the amplitude of the carrier signal c(t) = Accos(2fct) This results in a sinusoidal signal whose amplitude is a

function of the message signal m(t) There are several different ways of amplitude

modulating the carrier signal by m(t) Each results in different spectral characteristics for

the transmitted signal We will describe these methods, which are called

(a) Double sideband, suppressed-carrier AM (DSB-SC AM)

(b) Single-sideband AM (SSB AM)Oh-Jin Kwon, EE dept., Sejong Univ., Seoul, Korea: http://dasan.sejong.ac.kr/~ojkwon/

Amplitude ModulationThe condition for envelope detection of the

AM signal for all t

If and A=0 also satisfy the above condition

Let be the peak amplitude of

This condition is equivalent toThe min. carrier amplitude required for

envelope detection is

0)]([ tmA0)( tm

)(tmp)(tm

)()( tmtm p)(tmA p

)(tmp

Modulation indexThe modulation index

Atmp /)(

10

CSULB May 22, 2006 10

Modulation Index of AM Signal

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Modulation Index of AM Signal

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Double-Sideband Suppressed-Carrier AM

A double-sideband, suppressed-carrier (DSB-SC) AM

signal is obtained by multiplying the message signal m(t)

with the carrier signal c(t) = Accos(2fct)

Amplitude-modulated signal

An example of the message signal m(t), the carrier c(t), and the

modulated signal u (t) are shown in Figure 3.1

This figure shows that a relatively slowly varying message signal

m(t) is changed into a rapidly varying modulated signal u(t), and

due to its rapid changes with time, it contains higher frequency

components

At the same time, the modulated signal retains the main

characteristics of the message signal; therefore, it can be used to

retrieve the message signal at the receiver

Oh-Jin Kwon, EE dept., Sejong Univ., Seoul, Korea: http://dasan.sejong.ac.kr/~ojkwon/

)2cos()()()()( tftmAtctmtu cc

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Double-Sideband Suppressed-Carrier AM Figure 3.1 An example of message, carrier, and DSB-SC

modulated signals

Oh-Jin Kwon, EE dept., Sejong Univ., Seoul, Korea: http://dasan.sejong.ac.kr/~ojkwon/

Single-Sideband AMThe two sidebands of an AM signal are mirror images of one

anotherAs a result, one of the sidebands is redundantUsing single-sideband suppressed-carrier transmission results in

reduced bandwidth and therefore twice as many signals may be transmitted in the same spectrum allotment

Typically, a 3dB improvement in signal-to-noise ratio is achieved as a result of SSBSC

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Single-Sideband AMA method, illustrated in

Figure 3.16, generates a

DSB-SC AM signal and then

employs a filter that selects

either the upper sideband

or the lower sideband of the

double-sideband AM signal

.,

.

Figure 3.16 Generation of a single-sideband AM signal by filtering one of the sidebands of a DSB-SC AM signal.

Sideband and carrier powerCarrier term does not carry information, and

hence the carrier power is wasted

The carrier power is the mean sq. value of which is The sideband power is the mean sq. value

of which is

sidebandscarrierttmtAt ccAM cos)(cos)(

cPtA ccos 2/2A

sPttm ccos)( 2/)(2 tm

Power Efficiency

The power efficiency

For the special case of tone modulation

Hence

tAtm m cos)(

%100)(

)(22

2

tmA

tm

PP

P

sc

s

%1002

%1002/

2/2

2

22

2

AA

A

PP

P

sc

s

2/)( 22 Atm

%33,1 max

Quadrature AM Two carriers generated at the same frequency but

90º out of phase with each other allow transmission of two separate signals

This approach is known as Quadrature AM (QUAM or QAM)

Recovery of the two signals is accomplished by synchronous detection by two balanced modulators

Quadrature Operation

Advantages/disadvantagesAdvantages of Amplitude Modulation, AMThere are several advantages of amplitude modulation, and some of these

reasons have meant that it is still in widespread use today: It is simple to implement it can be demodulated using a circuit consisting of very few components AM receivers are very cheap as no specialized components are needed.

Disadvantages of amplitude modulationAmplitude modulation is a very basic form of modulation, and although its

simplicity is one of its major advantages, other more sophisticated systems provide a number of advantages. Accordingly it is worth looking at some of the disadvantages of amplitude modulation.

It is not efficient in terms of its power usage It is not efficient in terms of its use of bandwidth, requiring a

bandwidth equal to twice that of the highest audio frequency It is prone to high levels of noise because most noise is amplitude based

and obviously AM detectors are sensitive to it.