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CHAPTER 2
AMPLITUDE MODULATION
(AM)
2-2 AM Single Side Band
Communications
Review: Conventional AM
(DSB-FC)
Frequency spectrum:
fc-fm
fc
fc+fm
Bandwidth=2Xfmmax
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
Double Side Band Suppressed
Carrier (DSB-SC)

Frequency spectrum:
fc-fm


fc
fc+fm
Bandwidth:2 x fmmax
Total Power= Pusb + Plsb
Single Side Band Full Carrier
(SSB-FC)
Frequency spectrum:
fc-fm
fc
fc+fm
Bandwidth=fmmax
Total Power=Pcarrier +Pusb
Single Side Band Suppressed
Carrier (SSB-SC)
Frequency spectrum:
fc-fm
Bandwidth=fmmax
Total Power=+Pusb
fc
fc+fm
Comparison of time domain representation of three
common AM transmission systems:
Tomasi
Electronic Communications Systems, 5e
Copyright ©2004 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Example 1
For an AM DSBFC 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

i)
Solution for DSBFC;
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

ii)
Solution for DSB-SC
m
m
P 
P
P
4
4
1
1
 (5)  (5)  2.5W
4
4
2
t
2
c
2
c
2
iii) Bandwidth=2xfmmax=2(10kHz)=20kHz
iv)
90kHz
110kHz
Power  7.5W  2.5W
saved
 5W
5W
% Power 
x100%
7.5W
 66.67%
saved
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
saved
 6.25W
2
iii)
% Power
saved
Bandwidth=fmmax=10kHz
 83.33%
iv)
fc-fm
fc
 7.5W  1.25W
Power
c
110kHz

6.25W
x100%
7.5W
Methods of Generating SSB
2 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
DSB
signal
Carrier
oscillator
Balanced
modulator
Antenna
SSB
signal
Sideband
filter
Linear
amplifier
Microphone
Audio
amplifier
Filter
response
curve
Upper
Lower
sidebands sidebands
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
Am cos (ωmt)
Balanced
Modulator 1
A1(t)
Ac cos (ω ct + 90)
Information signal
Phase shifter
+
Carrier signal
Phase shifter
Balanced
Modulator 2
A2(t)
Am cos (ω mt + 90)
Output
Signal, aot
Phasing method..cont’d
a0 (t )  a1 (t )  a2 (t )      (1)
a1 (t )  Ac cos(c t  90 0 ) * Am cos  m t


1
 Ac Am cos(c t  90 0   m t )  Ac Am cos(c t  90 0   m t )    (2)
2
a2 (t )  Ac cos(c t ) * Am cos(mt  90 0 )


1
 Ac Am cos(c t  90 0  mt )  Ac Am cos(c t  90 0  mt )    (3)
2
 a0 (t )  (2)  (3)


 Ac Am cos(c t  90 0   m t )
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 + C
Mainly used for television video
transmission.
Cont’d…vsb

VSB Frequency Spectrum
VSB
Carrier
LSB
MSB
fc
fc
Advantages/Benefits of SSB




Power consumption
Bandwidth conservation
Selective fading
Noise reduction
Disadvantages of SSB


Complex receivers
Tuning difficulties