ET275 Unit 2- Lesson Plan- Slides

Amplitude Modulation 2.1

Introductory Communications Systems

Chapter 2:

Amplitude Modulation


Describe amplitude modulation with reference to its physical appearance.

Identify AM waveforms for various modulating signals.

Derive the equation of an AM wave.

Evaluate the bandwidth of an AM wave from its frequency spectrum.

Identify the importance of the modulation index of an AM wave, and evaluate its practically feasible value.

Evaluate the power distribution in an AM wave.

Objectives


Derive an expression for the transmitting current of an AM wave.

Identify the forms of AM signals.

Describe DSB-SC, SSB, ISB, and VSB signals with respect to:

Power saving in the signal

Equation of the signal

Physical appearance of the signal

Frequency spectrum of the signal

Objectives (cont.)


Compare the forms of AM signals with respect to their powers and bandwidths.

Identify the applications of AM signals.

Objectives (cont.)


Amplitude modulation (AM) is the modulation process that varies the instantaneous amplitude of the carrier signal in accordance with the instantaneous amplitude of the modulating signal.

Waveforms related to AM are:

Modulating signal

Carrier signal

Introduction


AM waveforms for various modulating signals are:

AM Waveforms for Various Modulating Signals

e1

t

(d)

e c

t

(c)

E c

em 1

t

(a)

em 2

(b)

t

Carrier S ignal

envelope(em 1)

E c

e2

t

(e)

Carrier signal

envelope(em 2)

E c


The amplitude-versus-frequency plot of the signal is called the frequency-spectrum of the signal.

The frequency-spectrum of an AM signal is:

Frequency Spectrum and Bandwidth of an AM Wave

Frequency

LSB

m a

E c

Carrier

USB

2

m a

E c

0

Em

(f - f )c m fc (f + f )c m

BW = 2 fm

fm

Ec

Am plitude

M odulating signal

fm fm

2


The modulation index of an AM wave is the ratio of amplitudes of the modulating signal and the carrier signal.

The modulation index is designated as ma, where

a signifies amplitude modulation.

The three possible values of ma are:

ma = 1 called 100-percent modulation

ma > 1 called over modulation

ma < 1 called under modulation

Modulation Index

c

ma E

Em


The two permissible conditions of ma are:

(ma = 1)

(ma < 1)

According to the definition of the modulation index, the percentage modulation is given as:

Modulation Index (cont.)

%100% c

ma E

Em


The capability of the transmitted signal to counter the channel noise and attenuation depends on the transmitted power.

The rule is: the more the transmitted power, the larger the servicing area; and the lesser the transmitted power, the smaller the servicing area.

The total side band power is the sum of the individual powers of LSB and USB. If Psb is equal

to the total side band power, then:

Power Distribution in an AM Wave

usblsbsb PPP


Transmission efficiency is the percentage of useful signal power in the total transmitting power.

Transmission efficiency indicates the percentage of total transmitting power that is converted to useful power.

If transmission efficiency of the AM transmitter, then transmission efficiency is:

Power Distribution in an AM Wave (cont.)

AM

%100ermittingPowTotalTrans

werSidebandPoAM


The various forms of AM signals are:

Double Side Band-Suppressed Carrier (DSB-SC)

Single Side Band (SSB)

Independent Side Band (ISB)

Vestigial Side Band (VSB)

Forms of an AM Signal


Power saving in a DSB-SC signal:

DSB-SC Signals

21

2a

T

m

PPC


Physical appearance of a DSB-SC signal:

DSB-SC Signals (cont.)

t

Phase- reversed at zero - crossings

e D S BS C

0

Carrier signal envelope


Physical appearance of a DSB-SC signal:

DSB-SC Signals (cont.)

Am plitude

Frequency(fc - fm ) fc

LSB

(fc + fm )

Suppressedcarrier

USB

(BW )D SBSC = 2 fm

2

m a

E c

2

m a

E c

O


Power saving in SSB signals: % Power Saving in SSB = % Power in carrier + % Power in one sideband

SSB Signals


Physical appearance of an SSB signal:

SSB Signals (cont.)

t

(a)

eSSB-L

0

2

m a

E c

- S ingle - toneLSBfrequency = (f - f )c m

t

(b)

eSSB-U

0

2

m a

E c

- S ingle - toneUSBfrequency = (f + f )c m

t

(c)

eSSB-L

0

- M ulti - toneLSB


Frequency-spectrum of an SSB signal:

SSB Signals (cont.)

Frequency

Suppressed com ponentseSSB - L

0 (f - f )c m fc (f + f )c m

BW = fm

Ec

2

m a

E c

2

m a

E c

Frequency

LSB2

Suppressed com ponents

U SB 's

eSSB - L

0

Em 1

Em 2

(f - f )c m 2 fc (f + f )c m 1

BW = fm 2

fm 1 fm 2

Ec

2

m a 2

E c

2

m a 1

E c

LSB

Frequency

Suppressed com ponentseSSB - U

0 (f - f )c m fc (f + f )c m

BW = fm

Ec

2

m a

E c

2

m a

E c

U SB

(a) (b)

2

m a 1

E c

BW = fm 2

(f + f )c m 2

2

m a 2

E c

(f - f )c m 1

LSB 's

M ulti-tone SSB

(c)

LSB1 U SB1

U SB2


The power in an ISB signal is equal to the power in a DSB-SC signal because both the side bands are transmitted with the carrier signal suppressed. The only difference is that the two side bands correspond to two different modulating signals, whereas in a DSB-SC signal both the side bands are generated by a single modulating signal.

ISB Signals


Frequency-spectrum of an ISB signal:

ISB Signals (cont.)

Frequency

LSB1

Suppressedcarrier

U SB2

eISB

0

Em 1

Em 2

(f - f )c m 1 fc (f + f )c m 2

BW =( + ) fm 1 fm 2

fm 1 fm 2

Ec

2

m a 1

E c

2

m a 2

E c


VSB modulation is exclusively used in transmitting audio and video signals in TV transmission.

Frequency-spectrum of a VSB signal is:

VSB Signals

eV S B

LSB U SBE C

C arrier

m aEc2

1.25

0.5 0.75 4.0 0.5

VSB Transm ission

(BW )V S B = 5.75M H z

(fc+4) (fc+4.5) Frequency(M H z)(fc-1 .25) (fc-0 .75) fc(fc - 4 .5)Fully Suppressed Part of LSB Vestige of

LSB

Fully Transm itted Part of U SB Part of U SB

(Transm itted w ith gradual attenuation)


Comparison and Application of Various AM Schemes

AM scheme

Bandwidth Power at 100-percent modulation as percentage of total transmitting

power

Percentage of power savings as

compared to a DSB-FC signal

Application

Carrier power Sideband power

DSB-FC 2fm 66.66% 33.33% NIL AM Radio Broadcast

DSB-SC 2fm NIL 33.33% 66.66% Non-commercial Systems

SSB fm NIL 16.66% 83.33% Carrier Telephony Systems

ISB fm1 + fm2 NIL 33.33% 66.66% Radio Telephony and Radio Telegraphy

VSB Between DSB-FC and SSB

Signals

66.66% Between DSB-FC and SSB Signals

More than SSB but less than

DSB-FC Signal

Video Transmission in

TV Communication

System


Amplitude modulation is defined as the modulation process that varies the instantaneous amplitude of the carrier signal in accordance with the instantaneous amplitude of the modulating signal.

The carrier signal is represented as .

The amplitude-versus-frequency plot of the signal is called the frequency-spectrum of the signal.

The modulation index of an AM wave is defined as the ratio of amplitudes of the modulating signal and the carrier signal and is designated as ma.

Summary

)sin( cccc tEe


The first possible value of ma (=1) is obtained

when the amplitudes of the modulating and carrier signals are equal. The resultant modulation under these conditions is called 100-percent modulation.

When ma > 1. This is known as over modulation.

The modulation index is greater than 1 when the modulating amplitude, Em, is greater than the

carrier amplitude, Ec. The percentage modulation

in this case is more than 100 percent .

Summary (cont.)


Amplitude modulation is called under modulation when the value of ma is less than 1, and the

percentage modulation is less than 100 percent. This type of modulation occurs when the modulating amplitude is less than the carrier amplitude.

The service area is also related to the power of the transmitted signal, especially in case of radio communication systems, in which, the signal is transmitted through electromagnetic waves. The rule is, the more the transmitted power, the larger the service area; and the lesser the transmitted power, the smaller the service area.

Summary (cont.)


The normal AM wave or DSB-FC signal contains three constituents:

The carrier signal

LSB

USB

The information is carried only by the side bands while the carrier signal, present in an AM wave, does not carry any information. This is defined as the percentage of the useful signal power in the total transmitting power.

Summary (cont.)


In a DSB-FC signal, the carrier signal does not contain any useful information. Thus, only the two side bands can be transmitted by suppressing the carrier signal, which leads to a savings of 66.66 percent of the total transmitting power at 100-percent modulation.

In SSB, only one sideband is transmitted by suppressing the other sideband and the carrier signal.

The same carrier signal can be modulated by many modulating signals. This is known as multi-tone modulation.

Summary (cont.)

Documents

ET275 Unit 2- Lesson Plan- Slides