1 chapter 2 amplitude modulation (am) 2-2 am single side band communications
TRANSCRIPT
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1
CHAPTER 2
AMPLITUDE MODULATION(AM)
2-2 AM Single Side Band Communications
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EKT 231 Communication System 2
Review: conventional AM(DSB-FC)
Frequency spectrum:
Bandwidth=2Xfmmax
Total Power=Pcarrier +Pusb +Plsb
fc
fc+fmfc-fm
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EKT 231 Communication System 3
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
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EKT 231 Communication System 4
Double side band suppressed carrier(DSB-SC)
Frequency spectrum:
Bandwidth:2 x fmmax
Total Power= Pusb + Plsb
fc
fc+fmfc-fm
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EKT 231 Communication System 5
Single Side Band Full Carrier (SSB-FC)
Frequency spectrum:
Bandwidth=fmmax
Total Power=Pcarrier +Pusb
fc
fc+fmfc-fm
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EKT 231 Communication System 6
Single Side band Suppress Carrier (SSB-SC)
Frequency spectrum:
Bandwidth=fmmax
Total Power=+Pusb
fc
fc+fmfc-fm
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EKT 231 Communication System 7
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.
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EKT 231 Communication System 8
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.
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EKT 231 Communication System 9
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.
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EKT 231 Communication System 10
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
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EKT 231 Communication System 11
Example 1..cont’d Solution:For DSB-SC
ii)
iii)Bandwidth=2xfmmax=2(10kHz)=20kHz
iv)
W
Pm
Pm
Pcct
5.2)5(4
1)5(
4
14422
22
fc
110kHz90kHz
%67.66
%1005.7
5%
5
5.25.7
xW
WPower
W
WWPower
saved
saved
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EKT 231 Communication System 12
Example 1..cont’d Solution:For SSB-FC
ii)
iii)Bandwidth=fmmax=10kHz
iv)
W
Pm
PPcct
25.6)5(4
15
42
2
%67.16
%1005.7
25.1%
25.1
25.65.7
xW
WPower
W
WWPower
saved
saved
100kHz 110kHzfc-fm
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EKT 231 Communication System 13
Example 1..cont’d Solution:For SSB-SC
ii)
iii)Bandwidth=fmmax=10kHz
iv)
W
Pm
Pct
25.1)5(4
14
2
2
%33.83
%1005.7
25.6%
25.6
25.15.7
xW
WPower
W
WWPower
saved
saved
fc
110kHzfc-fm
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EKT 231 Communication System 14
Comparison
DSB-FC DSB-SC SSB-FC SSB-SCPower (W)
7.5 2.5 6.25 1.25
BW(kHz) 20 20 10 10
Power Saving(%)
66.67 16.67 83.33
Modulating signal = 10kHzUnmodulated carrier frequency=100kHzUnmodulated carrier voltage = 10Vp
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EKT 231 Communication System 15
Methods of Generating SSB
4 methods,i) Balanced Ring Modulator
ii) FET Push-Pull Balanced Modulator
iii) Balanced Bridge Modulator
iv) Linear IC Balanced Modulators
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EKT 231 Communication System 16
(a) schematic diagram; (b) D1 and D2 biased on; (c) D3 and D4 biased on
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Copyright ©2004 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458All rights reserved.
(i) Balanced ring modulator
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EKT 231 Communication System 17
TomasiElectronic Communications Systems, 5e
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Balanced Modulator Waveforms
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EKT 231 Communication System 18
TomasiElectronic Communications Systems, 5e
Copyright ©2004 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458All rights reserved.
(ii) FET Push-Pull Balanced Modulator
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EKT 231 Communication System 19
TomasiElectronic Communications Systems, 5e
Copyright ©2004 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458All rights reserved.
FET Push-Pull Balanced Modulator Phasor Diagrams
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EKT 231 Communication System 20
TomasiElectronic Communications Systems, 5e
Copyright ©2004 by Pearson Education, Inc.
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(iii) Balanced Bridge Modulator
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EKT 231 Communication System 21
TomasiElectronic Communications Systems, 5e
Copyright ©2004 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458All rights reserved.
(iv) Linear IC Balanced Modulators
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EKT 231 Communication System 22
SSB Transmitters 2 types,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.
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EKT 231 Communication System 23
Filtering method
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EKT 231 Communication System 24
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.
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EKT 231 Communication System 25
Phasing method..cont’dBalanced
Modulator 1
Balanced Modulator 2
Phase shifter
Phase shifter
+
Information signal
Carrier signalOutput Signal, ao(t)
Am cos wmt
Am cos (wmt + 90)
Ac cos (wct + 90)
a2(t)
a1(t)
Blok digram 1
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EKT 231 Communication System 26
Phasing method..cont’d
)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
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EKT 231 Communication System 27
Phasing method..cont’d
Blok digram 2
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EKT 231 Communication System 28
AM 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.
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EKT 231 Communication System 29
Cont’d…vsb
AM wave is applied to a vestigial sideband filter, producing a modulation scheme – VSB + C
Mainly used for television video transmission.
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EKT 231 Communication System 30
Cont’d…vsb
VSB Frequency Spectrum
fcfc
LSB MSB
CarrierVSB
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EKT 231 Communication System 31
Advantages/Benefits of SSB
Power consumption Bandwidth conservation Selective fading Noise reduction
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EKT 231 Communication System 32
Disadvantages of SSB
Complex receivers Tuning difficulties