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8/11/2019 AM (Amplitude Modulation)
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AMPLITUDE MODULATION
8/11/2019 AM (Amplitude Modulation)
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8/11/2019 AM (Amplitude Modulation)
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Two major Disadvantages/Drawbacks
of DSBFC
◦ Large power consumption, where carrier power constitutes >2/3transmitted power.{remember:carrier does not contain anyinformation}
◦ Large bandwidth utilized.
Thus, DSBFC is both power and bandwidth inefficient
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Single Side Band Full Carrier (SSB-
FC)Frequency spectrum:
Bandwidth=fm(max)
Total Power=Pcarrier +Pusb f cf c+f m
f c-f m
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Single Side band SuppressCarrier (SSB-SC)
Frequency spectrum:
Bandwidth=fm(max)
Total Power=+Pusb f cf c+f m
f c-f m
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Cont’d…
◦ A form of amplitude modulation in which the carrier is totallysuppressed and one of the sidebands removed.
◦ Therefore, SSBSC requires half as much bandwidth as
conventional DSB AM and considerably less transmitted power
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Comparison of time domain representation ofthree common AM transmission systems:
Tomasi
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Example 1
For an AM DSCFC wave with a peak unmodulatedcarrier voltage Vc = 10 Vp,frequency of 100kHz, aload 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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Example 1..cont’d
◦ For the same given values, determine questions (ii)-(iv) for a AMDSB-SC, AM SSB-FC and AM SSB-SC systems. Determine also thepercentage of power saved in each of the system design.
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Example 1..cont’d Solution for DSBFC;i)
ii)
iii) Bandwidth=2xfmmax=2(10kHz)=20kHz
W
P m
P m
P P ccct
5.7)5(4
1)5(
4
15
4422
22
W
P m
P P
W R
V
R
V P
c
lsbu sb
cc
c
25.14
5102
)10(
2
)2/(
2
222
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Example 1..cont’d
◦
Solution:For DSB-SCii)
iii)Bandwidth=2xfmmax=2(10kz)=20kHz
iv)
%67.66
%1005.7
5%
5
5.25.7
xW
W Power
W
W W Power
sa ved
sa ved
W
P m
P m
P cct
5.2)5(
4
1)5(
4
1
4422
22
110kHz
90kHz
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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
W Power
W
W W Power
sa ved
sa ved
W
P m
P ct
25.1)5(4
1
42
2
f c
110kHz f c-f m
M th d f G ti SSB
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Methods of Generating SSB
i) Filtering method
◦ A filter removes the undesiredsideband producing SSB.
◦ Balanced modulators is used to
suppress the unwanted carrier andfilters to suppress the unwantedsidebands
◦ Quartz crystal filters are the most
widely used sideband filters since theyare very selective and inexpensive.
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Block diagram of filtering
method
Filterresponse
curve
Sidebandfilter
Balanced
modulator
Carrier
oscillator
Microphone Audio
amplifier
Linear
amplifier
Antenna
Upper
sidebands
DSB
signal
SSB
signal
Lower
sidebands
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Block diagram of phasing
methodBalanced
Modulator 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)
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Mathematical analysis of
phasing method
)2()90cos()90cos(21
cos*)90cos()(
)1()()()(
00
0
1
210
t wt w A At wt w A A
t w At w At a
t at at a
mcmcmcmc
mmcc
)3()90cos()90cos(21
)90cos(*)cos()(
00
0
2
t wt w A At wt w A A
t w At w At a
mcmcmcmc
mmcc
)90cos(
)3()2()(0
0
t wt w A A
t a
mcmc
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VESTIGIAL SIDEBAND (VSB)
Also called asymmetric sideband system.
Compromise between DSB & SSB.
Easy to generate.
Bandwidth is only ~ 25% greater than SSBsignals.
Derived by filtering DSB, one pass band ispassed almost completely while just a traceor vestige of the other sideband is included.
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Cont’d
AM wave is applied to a vestigial sideband filter, producing amodulation scheme – VSB + C
Mainly used for television video transmission.
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Cont’d…vsb
VSB Frequency Spectrum
fc fc
LSB MSB
Carrier
VSB
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Advantages/Benefits of SSB
◦ Power consumption
◦ Bandwidth conservation
◦ Selective fading
◦ Noise reduction
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Disadvantages of SSB
◦ Complex receivers
◦ Tuning difficulties
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