OBJECTIVES To define and describe AM single sideband To compare single sideband transmission to

conventional double sideband AM The explain the methods of generating SSB To define and describe AM vestigial sideband To describe the advantages and

disadvantages of SSB

LECTURE OVERVIEW Single sideband and double sideband Power calculation in SSB Method of generating SSB Vestigial sidebands Advantages and disadvantages of SSB

REVIEW: CONVENTIONAL AM(DSB-FC)

Frequency spectrum:

Bandwidth=2 x fmmax

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

SINGLE SIDE BAND FULL CARRIER (SSB-FC)

Frequency spectrum:

Bandwidth=fm(max)

Total Power=Pcarrier +Pusb

fc

fc+fmfc-fm

CONT’D…

A form of amplitude modulation in which the carrier is transmitted at full power but only one of the sidebands (either the upper or lower) is transmitted

Requires less bandwidth than DSBFC but also produces a demodulated signal with a lower amplitude

SINGLE SIDE BAND SUPPRESS CARRIER (SSB-SC)

Frequency spectrum:

Bandwidth=fm(max)

Total Power=+Pusb

fc

fc+fmfc-fm

CONT’D…

A form of amplitude modulation in which the carrier is totally suppressed and one of the sidebands removed.

Therefore, SSBSC requires half as much bandwidth as conventional DSB AM and considerably less transmitted power

COMPARISON OF TIME DOMAIN REPRESENTATION OF THREE COMMON AM TRANSMISSION SYSTEMS:

Tomasi

EXAMPLE 1

For an AM DSCFC wave with a peak unmodulated carrier voltage Vc = 10 Vp,frequency of 100kHz, a load resistor of RL = 10 , frequency of modulating signal of 10kHz 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

W

Pm

Pm

PPccct

5.7)5(4

1)5(

4

15

4422

22

WPm

PP

WR

V

R

VP

c

lsbusb

cc

c

25.14

5102

)10(

2

)2/(

2

222

EXAMPLE 1..CONT’D

Solution:For DSB-SC

ii)

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

iv)

%67.66

%1005.7

5%

5

5.25.7

xW

WPower

W

WWPower

saved

saved

W

Pm

Pm

Pcct

5.2)5(4

1)5(

4

14422

22

110kHz90kHz

EXAMPLE 1..CONT’D Solution:For SSB-FC

ii)

iii)Bandwidth=fmmax=10kHz

iv)

%67.16

%1005.7

25.1%

25.1

25.65.7

xW

WPower

W

WWPower

saved

saved

W

Pm

PPcct

25.6)5(4

15

42

2

100kHz 110kHzfc-fm

EXAMPLE 1..CONT’D Solution:For SSB-sC

ii)

iii)Bandwidth=fmmax=10kHz

iv)

%33.83

%1005.7

25.6%

25.6

25.15.7

xW

WPower

W

WWPower

saved

saved

W

Pm

Pct

25.1)5(4

14

2

2

fc

110kHzfc-fm

METHODS OF GENERATING SSB

i) Filtering method A filter removes the undesired

sideband producing SSB. Balanced modulators is used to

suppress the unwanted carrier and filters to suppress the unwanted sidebands

Quartz crystal filters are the most widely used sideband filters since they are very selective and inexpensive.

BLOCK DIAGRAM OF FILTERING METHOD

Filterresponsecurve

Sidebandfilter

Balancedmodulator

Carrieroscillator

Microphone Audioamplifier

Linearamplifier

Antenna

Uppersidebands

DSBsignal

SSBsignal

Lowersidebands

CONT’D…

ii) Phasing method 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.

BLOCK DIAGRAM OF PHASING METHOD

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)

MATHEMATICAL ANALYSIS OF PHASING METHOD

)2()90cos()90cos(2

1

cos*)90cos()(

)1()()()(

00

0

1

210

twtwAAtwtwAA

twAtwAta

tatata

mcmcmcmc

mmcc

)3()90cos()90cos(2

1

)90cos(*)cos()(

00

0

2

twtwAAtwtwAA

twAtwAta

mcmcmcmc

mmcc

)90cos(

)3()2()(0

0

twtwAA

ta

mcmc

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

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

Mainly used for television video transmission.

CONT’D…VSB

VSB Frequency Spectrum

fcfc

LSB MSB

Carrier

VSB

ADVANTAGES/BENEFITS OF SSB Power consumption Bandwidth conservation Selective fading Noise reduction

DISADVANTAGES OF SSB Complex receivers Tuning difficulties