chapter 2 amplitude modulation (am) 2-2 am single side band communications
TRANSCRIPT
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