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Dr. Ahmed El-Mahdy COMM 601: Modulation I
COMM 601: Modulation I
Lecture 4
Double Side Band Suppressed Carrier
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Double Side Band (DSB)
Compare between this equation and the equation of
conventional AM
Dr. Ahmed El-Mahdy COMM 601: Modulation I
3
0 0.05 0.1 0.15 0.2 0.25
-20
0
20
Time[Sec.]
Sc(t
)
0 0.05 0.1 0.15 0.2 0.25
-20
0
20
Time[Sec.]
m(t
)
0 0.05 0.1 0.15 0.2 0.25
-20
0
20
Time[Sec.]
SD
SB
(t)
CARRIER SIGNAL
MODULATING BASEBAND SIGNAL
DOUBLE SIDEBAND MODULATED SIGNAL
Dr. Ahmed El-Mahdy COMM 601: Modulation I
4
0 0.02 0.04 0.06 0.08 0.1 0.12
-25
-20
-15
-10
-5
0
5
10
15
20
25
Time[Sec.]
SD
SB
(t)
DOUBLE SIDEBAND MODULATED SIGNAL
Phase Reversal
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Phase Reversal in DSB
Dr. Ahmed El-Mahdy COMM 601: Modulation I
DSB Spectrum
Dr. Ahmed El-Mahdy COMM 601: Modulation I
tfatm mm 2cos
tfftffa
ts mcmcm 2cos2cos2
tftfats mcm 2cos2cos
mcmcm
mcmcm
ffffffa
ffffffa
fS
2
1
2
1
2
2
1
2
1
2
mmm ffff
afM
2
MODULATING SIGNAL
MODULATED (DSB) SIGNAL
DSB Spectrum for Sinusoidal Modulating Signal
Dr. Ahmed El-Mahdy COMM 601: Modulation I
8
0 0.05 0.1
-20
-10
0
10
20
Time[Sec.]
m(t
)
0 0.05 0.1
-20
-10
0
10
20
Time[Sec.]
m(t
)
0 50 100 150 200 2500
2
4
6
Frequency[Hz]
M(f
)
0 50 100 150 200 2500
2
4
6
Frequency[Hz]
SD
SB
(f)
MODULATING SIGNAL MODULATED (DSB) SIGNAL
Hzf
Hzf
va
c
m
m
200
10
10
UPPER SIDEBAND
(USB)
LOWER SIDEBAND
(LSB)
Dr. Ahmed El-Mahdy COMM 601: Modulation I
tftmAts cc 2cos)(
tfjcetCts
2Re
tfj
ccetmAts
2)(Re
)(tmAtC c
)(tmAta c
INSTANTANEOUS
AMPLITUDE
INSTANTANEOUS
PHASE
)(tmAt c
INSTANTANEOUS
FREQUENCY
tdt
dtf
2
1
Instantaneous Amplitude, Phase, and Frequency of DSB
Dr. Ahmed El-Mahdy COMM 601: Modulation I
tftfats cmm 2cos2cos
tfj cetCts2
Re
tfj
mmcetfats
22cosRe
tfatC mm 2cos
tfata mm 2cos
INSTANTANEOUS
AMPLITUDE
INSTANTANEOUS
PHASE
02cos
02cos0
2cos
tf
tf
tfat
m
m
mm
INSTANTANEOUS
FREQUENCY
tdt
dtf
2
1
1cAAsume
Instantaneous Amplitude, Phase, and Frequency of DSB(single Tone)
Dr. Ahmed El-Mahdy COMM 601: Modulation I
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
-20
0
20
Time[Sec.]
XD
SB
(t)
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
-20
0
20
Time[Sec.]
XD
SB
(t)
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
-20
0
20
Time[Sec.]
a(t
)
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16-200
0
200
Time[Sec.]
phai(t)
MODULATING SIGNAL
MODULATED (DSB) SIGNAL
INSTANTANEOUS AMPLITUDE
INSTANTANEOUS PHASE
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Transmitted Power of DSB
)()(signalDSB of envelopeComplex tmAtC c
)(2
)(2
1
22
2
tmA
tCP
c
avg
22
2
2
])([max2
])([max2
1
)(max2
1
tmA
tmA
tCP
c
c
PEP
Using Complex Envelope
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Calculation of Transmitted Power of DSB using power spectrum
Sp(f)
fc -fc
f 4
2
cA
Power Spectrum
)(2
)()(4
)()(4
)(
:then2 than lessmuch contentsfrequency with signal baseband a is m(t) If
22
222
2
c
tmA
tmtmA
dfffMffMA
dffSP
f
c
c
ccc
pavg
)(2 tm
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Generation of DSB
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Generation of DSB
(a)Block diagram of the product modulator
(b)Baseband signal (modulating signal) (c) DSB
modulated signal
(1) Product Modulator
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Generation of DSB (Cont.) (2) Balanced Modulator
s
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Coherent Demodulation of DSB
S(t)
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Coherent Demodulation of DSB (Cont.)
S(f)
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Effect of Phase Error on Coherent Demodulation of DSB
S(t)
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Remarks on DSB Coherent demodulation
Phase Locked Loop
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Squaring Loop
S(t)
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Voltage Controlled Oscillator (VCO)
If the control signal is negative, then the VCO phase increases.
If the control signal is positive, then the VCO phase decreases.
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Voltage Controlled Oscillator (VCO)
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Costas Phase Locked Loop
_
_
_
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Explanation of Costas Phase Locked Loop
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Explanation of Costas Phase Locked Loop
_
_
_
_
_
)]4sin())sin(()[(2
1
)]4sin())()[sin((2
1
)]4sin())[sin((2
1
0
0
0
cvcvcc
cvcvcc
cvccvc
tftmAA
tftmAA
tftmAA
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Explanation of Costas Phase Locked Loop It has cut off
freq. near zero.
It acts as
integrator to
produce DC
control voltage
to VCO
_
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Phase Locked Loop (PLL)
S(t) LPF Filter
H(f)
)]4sin()[sin(2
1)( 000 icic tfAAtx
)][sin(2
1)][sin(
2
1)( 0000 ecic AAAAte
Phase Shift 90
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Notes: Phase Locked Loop (PLL) Note that the PLL is locked -90 degree out of phase with
respect to the incoming signal. Then the output of the PLL
needs to be shifted by 90 degree so that it will be in phase
with the carrier of the input signal.
Dr. Ahmed El-Mahdy COMM 601: Modulation I
PLL Can Track Phase or Frequency
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Conventional AM versus DSB
Dr. Ahmed El-Mahdy COMM 601: Modulation I
Remember: Bandwidth of AM & DSB