Transcript chapter 2 - Portal UniMAP

CHAPTER 2
AMPLITUDE MODULATION
(AM)
2.3 AM Single Side Band
Communications
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:
fc-fm
fc
fc+fm
Bandwidth=2 x fmmax
Total Power=Pcarrier +Pusb +Plsb
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:
fc-fm
Bandwidth=fm(max)
fc
Total Power=Pcarrier +Pusb
fc+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:
fc-fm
Bandwidth=fm(max)
Total Power=+Pusb
fc
fc+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)
2
(V / 2 ) V
(10)
P


 5W
R
2 R 2 10
mP
P P 
 1.25W
4
2
c
2
c
c
2
c
usb
ii)
lsb
m
m
P P 
P
P
4
4
1
1
 5  (5)  (5)  7.5W
4
4
2
t
c
2
c
2
c
2
iii) Bandwidth=2xfmmax=2(10kHz)=20kHz
EXAMPLE 1..CONT’D

Solution:For DSB-SC
ii)
m
m
P
P
4
4
1
1
 (5)  (5)  2.5W
4
4
2
P 
t
2
c
2
c
2
iii)Bandwidth=2xfmmax=2(10kz)=20kHz
iv)
90kHz
110kHz
 7.5W  2.5W
Power
saved
 5W
% Power
saved
 66.67%

5W
x100%
7.5W
EXAMPLE 1..CONT’D

Solution:For SSB-FC
ii)
m
P P
P
4
1
 5  (5)  6.25W
4
2
t
c
c
2
iii)Bandwidth=fmmax=10kHz
iv)
fc-fm 100kHz
110kHz
 7.5W  6.25W
Power
saved
 1.25W
% Power
saved
 16.67%

1.25W
x100%
7.5W
EXAMPLE 1..CONT’D

Solution:For SSB-sC
ii)
m
P 
P
4
1
 (5)  1.25W
4
2
t
c
2
saved
 6.25W
% Power
saved
 83.33%
iii)Bandwidth=fmmax=10kHz
iv)
fc-fm
fc
 7.5W  1.25W
Power
110kHz

6.25W
x100%
7.5W
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
Antenna
DSB
signal
Carrier
oscillator
Balanced
modulator
SSB
signal
Sideband
filter
Linear
amplifier
Microphone
Audio
amplifier
Filter
response
curve
Upper
Lower
sidebands sidebands
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
Am cos wmt
Balanced
Modulator 1
Information signal
A1(t)
Ac cos (wct + 90)
Output Signal, aot
Phase shifter
+
Carrier signal
Phase shifter
Am cos (wmt + 90)
Balanced
Modulator 2
A2(t)
MATHEMATICAL ANALYSIS OF PHASING
METHOD
a (t )  a (t )  a (t )      (1)
0
1
2
a (t )  A cos( w t  90 ) * A cos w t
0
1
c
c
m
m
1
 A A cos( w t  90  w t )  A A cos( w t  90  w t )   (2)
2
0
c
m
c
0
m
c
m
c
m
a (t )  A cos( w t ) * A cos( w t  90 )
0
2
c
c
m
m
1
 A A cos( w t  90  w t )  A A cos( w t  90  w t )   (3)
2
a (t )  (2)  (3)
 A A cos( w t  90  w t )
0
c
m
c
0
m
c
m
c
m
0
0
c
m
c
m
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
VSB
Carrier
LSB
MSB
fc
ADVANTAGES/BENEFITS OF SSB
Power consumption
 Bandwidth conservation
 Selective fading
 Noise reduction

DISADVANTAGES OF SSB
Complex receivers
 Tuning difficulties

AT THE END OF THIS CHAPTER,
YOU SHOULD BE ABLE,
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

END OF CHAPTER 2.3
AM SINGLE SIDEBAND
COMMUNICATIONS