CHAPTER 2

AMPLITUDE MODULATION(AM)

2-2 AM Single Side Band Communications

Applications of SSB

1) For wireless communications, especially to amateur radio band, citizen band and navigational communication equipment.

2) In telephone system between an exchange station to other station (trunk-call) to maximize (double) the useful allocated medium or band.

Review: conventional AM(DSB-FC)

Frequency spectrum:

Bandwidth=2Xfmmax

Total Power=Pcarrier +Pusb +Plsb

fc

fc+fmfc-fm

Two major Disadvantages/Drawbacks of

DSBFC

Large power consumption, where carrier power constitutes >2/3 transmitted power.{remember:carrier does not contain any information}

Large bandwidth utilized.

Thus, DSBFC is both power and bandwidth inefficient

Double side band suppressed carrier(DSB-SC)

Frequency spectrum:

Bandwidth:2 x fmmax

Total Power= Pusb + Plsb

fc

fc+fmfc-fm

Single Side Band Full Carrier (SSB-FC)

Frequency spectrum:

Bandwidth=fmmax

Total Power=Pcarrier +Pusb

fc

fc+fmfc-fm

Single Side band Suppress Carrier (SSB-SC)

Frequency spectrum:

Bandwidth=fmmax

Total Power=+Pusb

fc

fc+fmfc-fm

Comparison of time domain representation of three common AM transmission systems:

TomasiElectronic Communications Systems, 5e

Copyright ©2004 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458All rights reserved.

AM DSBFC Waveforms

SSB Waveforms

Example 1

For an AM DSBFC wave with a peak unmodulatedcarrier voltage Vc = 10 Vp,frequency of 100kHz,

a loadresistor of RL = 10 , frequency of modulating

signal of10kHz and m = 1, determine the following

i) Powers of the carrier and the upper and lower sidebands.

ii) Total power of the modulated wave.

iii) Bandwidth of the transmitted wave.

iv) Draw the power and frequency spectrum.

Example 1..cont’d

For the same given values, determine questions (ii)-(iv) for a AM DSB-SC, AM SSB-FC and AM SSB-SC systems. Determine also the percentage of power saved in each of the system design.

Example 1..cont’d Solution for DSBFC;

i)

ii)

iii) Bandwidth=2xfmmax=2(10kHz)=20kHz

W5.7)5(4

1)5(

4

15

P4

mP

4

mPP

22

c

2

c

2

ct

W25.14

PmPP

W5102

)10(

R2

V

R

)2/V(P

c2

lsbusb

22c

2c

c

Example 1..cont’d Solution:For DSB-SC

ii)

iii)Bandwidth=2xfmmax=2(10kHz)=20kHz

iv)

W5.2)5(4

1)5(

4

1

P4

mP

4

mP

22

c

2

c

2

t

fc

110kHz90kHz

%67.66

%100xW5.7

W5Power%

W5

W5.2W5.7Power

saved

saved

Example 1..cont’d Solution:For SSB-FC

ii)

iii)Bandwidth=fmmax=10kHz

iv)

W25.6)5(4

15

P4

mPP

2

c

2

ct

%67.16

%100xW5.7

W25.1Power%

W25.1

W25.6W5.7Power

saved

saved

100kHz 110kHzfc-fm

Example 1..cont’d Solution:For SSB-sC

ii)

iii)Bandwidth=fmmax=10kHz

iv)

W25.1)5(4

1

P4

mP

2

c

2

t

%33.83

%100xW5.7

W25.6Power%

W25.6

W25.1W5.7Power

saved

saved

fc

110kHzfc-fm

Methods of Generating SSB

There are 2 common methods:

i) Filtering method A filter removes the undesired sideband producing

SSB. Quartz crystal filters are the most widely used

sideband filters since they are very selective and inexpensive.

ii) Phasing method

A balanced modulator eliminates the carrier and provides DSB.

Filtering method

Filterresponsecurve

Sidebandfilter

Balancedmodulator

Carrieroscillator

Microphone Audioamplifier

Linearamplifier

Antenna

Uppersidebands

DSBsignal

SSBsignal

Lowersidebands

Phasing methods-using two balance modulator Another way to produce SSB uses a

phase shift method to eliminate one sideband.

Two balanced modulators driven by carriers and modulating signals 90º out of phase produce DSB.

Adding the two DSB signals together results in one sideband being cancelled out.

Phasing method..cont’d

BalancedModulator 1

Balanced Modulator 2

Phase shifter

Phase shifter

+

Information signal

Carrier signal

Output Signal, aot

Am cos wmt

Am cos (wmt + 90)

Ac cos (wct + 90)

A2(t)

A1(t)

Phasing method..cont’d

)2()tw90twcos(AA)tw90twcos(AA2

1

twcosA*)90twcos(A)t(a

)1()t(a)t(a)t(a

m0

cmcm0

cmc

mm0

cc1

210

)3()tw90twcos(AA)tw90twcos(AA2

1

)90twcos(A*)twcos(A)t(a

m0

cmcm0

cmc

0mmcc2

)tw90twcos(AA

)3()2()t(a

m0

cmc

0

VESTIGIAL SIDEBAND (VSB) Also called asymmetric sideband system. Compromise between DSB & SSB. Easy to generate. Bandwidth is only ~ 25% greater than

SSB signals. Derived by filtering DSB, one pass band is

passed almost completely while just a trace or vestige of the other sideband is included.

Cont’d…vsb

AM wave is applied to a vestigial sideband filter, producing a modulation scheme VSB + fc

Mainly used for television video transmission.

Cont’d…vsb

VSB Frequency Spectrum

fcfc

LSB MSB

CarrierVSB

Advantages/Benefits of SSB

Power consumption Bandwidth conservation Immune to or less affected by

Selective Fading Noise reduction

Disadvantages of SSB

Complex receivers Tuning difficulties