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Philadelphia UniversityPhiladelphia Universityp yp yDepartment of Communication & Electronics Department of Communication & Electronics
EngineeringEngineering
Analog CommunicationsAnalog CommunicationsCh3: Amplitude ModulationCh3: Amplitude Modulation
InstructorInstructor
Ibrahim N. AbuIbrahim N. Abu--IsbeihIsbeih
Email: [email protected]
Website: www.abusbeih.com/ecourse
Communications Principles
Prince Sattam Bin Abdulaziz University Electrical Engineering Department
Dr. Moustafa M. Nasralla
Email: [email protected] Website: https://faculty.psau.edu.sa/m.nasralla
Modulation ProcessModulation Process
Modulation: The process of varying some parameter of a carrier wave in accordance with the message signal. i f iinformation
Why modulation?
1. Reducing the size of the antennas:
- The optimal antenna size is related to wavelength:
- Voice signal: 3 kHz
2. Allowing transmission of more than one signal in the same channel (multiplexing)
3 Allowing better trade off between bandwidth and signal to3. Allowing better trade-off between bandwidth and signal-to-noise ratio (SNR)
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [2]
ModulationModulation
InformationWhat is it?
Modulating SignalMessage Signal
m(t)(B b d)
Modulated Signals(t)
(Bandpass)(Baseband)
( p )
Modulatormessage: m(t) s(t): radio signal
carrier: c(t)
[3]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
ModulationModulationModulationModulation
Continuous-Wave (CW) Modulation Pulse Modulation
1. Amplitude Modulation (AM) Analog Pulse M d l ti
Digital Pulse M d l ti
2. Angle Modulation:
- Frequency Modulation (FM)
Ph M d l ti (PM)
Modulation:
1. Pulse Amplitude Modulation (PAM)
Modulation:
- Pulse Code Modulation (PCM)- Phase Modulation (PM)
2. Pulse Width Modulation (PDM)
3. Pulse Position
(PCM)
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [4]
Modulation (PPM)
11. Amplitude Modulation (AM). Amplitude Modulation (AM)
An amplitude-modulated (AM) wave is described as a function of time as follows:
)2cos()(1)( tftmkAts cac
signalg)(modulatinmessagetheism(t)in volt
measuredmodulator theofy sensitivit amplitude theis where1-
ka
vecarrier wa theis )2cos()(c and
signalg)(modulatinmessagetheis m(t) ,in volt
tfAt cc
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [5]
Fig. Illustrating the amplitude modulation process:
(a) Baseband signal m(t). (b) AM wave for | kam(t) | < 1 for all t. (c) AM wave for | kam(t) | > 1 for some t.
The percentage modulation:
100%)(max
tmka max
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [6]
©2000, John Wiley & Sons, Inc. Haykin / Communication Systems, 4th Ed
We observe that the envelope of s(t) has essentially the same shape as the baseband signal m(t) provided that two requirements are
i fi dsatisfied:1. The amplitude of kam(t) is always less than unity, that is,
ttmk allfor1)(
2. The carrier frequency fc is much greater than the highest frequency component W of the message signal m(t) that is
ttmka allfor 1)(
frequency component W of the message signal m(t), that is
Wfc
• When |ka m(t)| > 1 for any t, the carrier becomes overmodulated.
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [7]
AM SpectrumAM Spectrum
)]()([)]()([)( cac ffMffMAk
ffffA
fS
)2cos()()2cos()2cos()(1)( tftmkAtfAtftmkAts caccccac
)]()([2
)]()( [2
)( ccca
ccc ffMffMfffffS
2BT=2W
The transmission bandwidth for an AM wave is twice the b d id h h i 2
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [8]
message bandwidth W, that is, BT=2W.
Example: Example: Single Single Tone AMTone AM
m(t)
A)2cos()( tfAtm mm
t
Am
c(t)
)()( fmm
t
Ac
)2cos()( tfAtc cc
s(t)Ac[1+kaAm]
)2cos()2cos(1)( tftfAkAts cmmac
t
Ac )2cos()2cos(1 tftfA cmc
(index)factor modulation theis ma Ak
[9]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
Modulation index (factor)Modulation index (factor)
minmax
minmax
AA
AA
s(t) 1max cAA
1min cAAcA
maxA
t
min c
minA
t
[10]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
Spectrum (Single Tone AM)Spectrum (Single Tone AM)
mT fB 2 :Bandwidthon Transmissi
Message Signal
Carrier Wave
AM WaveAM Wave
[11]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
AM Power & EfficiencyAM Power & Efficiency
Single tone AM wave:
tftfAts cmc )2cos()2cos(1)(
tffA
tffA
tfA
tftfAtfA
ff
cc
cmccc
cmc
)(2cos)(2cos)2cos(
)2cos()2cos()2cos(
)()()(
tfftfftfA mcmccc )(2cos2
)(2cos2
)2cos(
2
2
1powerCarrier cA
22222
8
1
8
1power sideupper and sideLower cc AA
power total thepower to sideband total theof ratio The
2
2
22222
2222
211181
81
cc AA1power band side total
then,modulation 100%1 if 2
[12]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
22222 281
81
21 ccc AAA 3
2power total 2
Switching ModulatorSwitching Modulator
The generation of AM wave may be accomplished using various devices; here we describe one such device called switching
d lmodulator.
)()2cos()( tmtfAtv )()2cos()(1 tmtfAtv cc
0)(0
0)( , )()( 1
2 tc
tctvtv
[13]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
0)( , 0 tc
Switching Modulator (Cont.)Switching Modulator (Cont.)
)( )()2cos()(02 tgtmtfAtv Tcc
cfT
10
hiF iitb)(R ti t
122cos12
12
2
1)(
haveweseries,Fourier itsby )( ngRepresenti
1
1
0
0
ntfn
tg
tg
c
n
T
T
122 1 nn
tv
component The .1
:components twoof sum theof consists )( that find on wesubstitutiAfter 2
cc
cT tftm
A
Atg
4
2cos)(4
12
)( 0
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [14]
ca A
k
4 ofy sensitivit amplitude with waveAM desired theis which
Switching Modulator (Cont.)Switching Modulator (Cont.)
2 width of intervalsfrequency which and on, so and ,4 ,2 ,0at
functionsdeltaofconsistswhich ofspectrumthe,components d Unwante.2
cc Wff
filter pass-band a of meansby removed are termsunwanted The
on.soand,5,3 ,0at centered cc ff
BPF[v2(t)]
m(t)
2
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [15]
Envelope DetectorEnvelope Detector
The charging time constant (rf + RS )C must be short for rapid charge that ischarge, that is,
The discharging time constant R C must be long enough toc
sf fCRr
1)(
The discharging time constant RLC must be long enough to ensure slowly discharging, that is,
CR11
[16]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
WCR
f l
c
Envelope Detector (Cont.)Envelope Detector (Cont.)
[17]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
Envelope Detector Envelope Detector (Example)(Example)
The following numerical values can be used in a practical envelope detector:
Rs 75 resistance Source
FC
kR
r
l
f
01.0eCapacitanc
10 resistance Load
25 resistance Forwared
c
sf fCRr
1)(
kHzf
kHzW
FC
c 20 frequency Carrier
1 bandwidth Message
01.0 eCapacitanc
WCR
f l
c
11
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [18]
Virtues, Limitations Virtues, Limitations and Modifications of and Modifications of Amplitude Amplitude ModulationModulation
Virtue of amplitude modulation– Amplitude modulation system is very cheap to build and maintain
Practical Limitation Practical Limitation– Amplitude modulation is wasteful of transmitted power
The transmission of the carrier wave therefore represents a waste of power
– Amplitude modulation is wasteful of channel bandwidthAmplitude modulation is wasteful of channel bandwidth
Insofar as the transmission of information is concerned, only one sideband is necessary, and the communication channel therefore needs to provide only the same bandwidth as the message signal.
i i i b d id h l i h b d id h It requires a transmission bandwidth equal to twice the message bandwidth
Modifications of amplitude modulation– Double sideband-suppressed carrier (DSB-SC) modulation
The transmitted wave consists of only the upper and lower sidebands But the channel bandwidth requirement is the same as before
– Single sideband (SSB) modulation
Th d l t d i t l f th id b d th l
19
The modulated wave consists only of the upper sideband or the lower sideband
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
22. Double Sideband. Double Sideband--Suppressed CarrierSuppressed Carrier(DSB(DSB--SC) ModulationSC) Modulation
Double sideband-suppresses carrier (DSB-SC) modulation consists of the product of the message signal m(t) and the carrier
( )wave c(t):
)2cos()()()()( tftmAtctmts )2cos()( )()()( tftmAtctmts cc
Its only advantage is saving transmitted power, g p ,which is important enough when the available transmitted power is at a
i
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [20]
premium
DSBDSB--SC SC SpectrumSpectrum
)] ( ) ([2
)( ccc ffMffM
AfS
The transmission bandwidth for DSB-Sc wave is twice the
BT=2W
The transmission bandwidth for DSB Sc wave is twice the message bandwidth W, that is, BT=2W.
Its only advantage is saving transmitted power.
[21]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
Ring ModulatorRing Modulator One of the most useful product modulators, well suited for
generating a DSB-SC wave, is the ring modulator
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [22]
Coherent DetectionCoherent Detection Coherent detection or synchronous demodulation is used to
recover the message signal m(t) from the DSB-SC wave
)()2()( ' fA
)()cos(1
)()4cos(1
)()2cos()2cos(
)()2cos()(
''
'
tmAAtmtfAA
tmtftfAA
tstfAtv
cccc
cc
)()cos(2
)()4cos(2
tmAAtmtfAA ccccc
)()cos(1
)()( '
LPF0 tmAAtvtv cc )cos(
2
1)cos(
2
1)cos()cos( 212121
)()(2
)()(LPF0 cc
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [23]
Coherent Detection (Quadrature null effect)Coherent Detection (Quadrature null effect)
)()cos(2
1)()( '
LPF0 tmAAtvtv cc
- The quadrature null effect of the coherent detector
The zero demodulated signal, when occurs for Φ=±π/2g , f / The phase error Φ in the local oscillator causes the detector
output to be attenuated by a factor equal to cosΦ
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [24]
Costas ReceiverCostas ReceiverCostas Receiver
– A practical synchronous receiver system used for demodulating DSB SC wavesDSB-SC waves.
– Consists of two coherent detectors supplied with the same input signal Two local oscillator signals that are in phase quadrature with
respect to each other I-channel : in-phase coherent detector
h l d h h d Q-channel : quadrature-phase coherent detector
– Phase control in the Costas receiver ceases with modulation, Which means that phase-lock would have to be re-established with
the reappearance of modulation
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [25]
Costas ReceiverCostas Receiver
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [26]
Quadrature Amplitude Modulation (QAM)Quadrature Amplitude Modulation (QAM)(Quadrature(Quadrature--Carrier Multiplexing)Carrier Multiplexing)
QAM scheme enables two DSB-SC modulated waves to occupy the same channel bandwidth. It is therefore a bandwidth-conservation schemescheme.
)2sin()()2cos()()( 21 tftmAtftmAts cccc
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [27]
Filtering of SidebandsFiltering of Sidebands
We can recover the original baseband signals without distortion if the transfer function of the filter satisfies the condition:
WfWffHffH cc ,constant )()(
)(1
)( '0 tmAAtv
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [28]
)(2
)(0 tmAAtv cc
33. Single Sideband (SSB) Modulation. Single Sideband (SSB) Modulation
The single-sideband (SSB) signal is generated by filtering the DSB-SC signal.
Generation of SSB Waves:1- Frequency Discrimination Method (Filtering Method)
For the design of the band pass filter to be practically feasible there must be aFor the design of the band-pass filter to be practically feasible, there must be a certain separation between the two sidebands that is wide enough to accommodate the transition band of the band-pass filter.
2- Phase Discrimination MethodWide-band phase-shifter is designed to produce the Hilbert transform in
t th i i i lresponse to the incoming message signal.To interfere with the in-phase path so as to eliminate power in one of the two sidebands, depending on whether upper SSB or lower SSB is the requirement.
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [29]
a. Frequency Discrimination (Filtering) Method
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [30]
Transfer Function for BPF to Generate SSB (LSB or USB):
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [31]
DSB Spectrum
SSB/USB Spectrum
SSB/LSB Spectrum
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [32]
b. Phase Discrimination MethodThe SSB wave may be written in the canonical form:
11
for USB )2sin()(ˆ2
1)2cos()(
2
1)( tftmAtftmAts cccc
LSBfor )2sin()(ˆ2
1)2cos()(
2
1)( tftmAtftmAts cccc
)( of ansformHilbert tr theis )(ˆ tmtm
)sgn()( fjfHQ
)()()(ˆ fHfMfM Q
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [33]
Demodulation of SSB Signals : (Coherent Detector)Demodulation of SSB Signals : (Coherent Detector)
Coherent detection or synchronous demodulation is used to recover the message signal m(t) from the SSB-SC wave
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [34]
44. Vestigial. Vestigial--Sideband (VSB) ModulationSideband (VSB) ModulationThe Vestigial sideband (VSB) Modulation is a technique used for commercial video broadcasting which lies some where in the middle of SSB and DSB and it is a compromise between SSB andmiddle of SSB and DSB and it is a compromise between SSB and DSB.
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [35]
Television (TV) Signals:
Television Signals:- The video signal exhibits a large
bandwidth and significant low-bandwidth and significant low-frequency content, which suggest the use of vestigial sideband modulation.
- The circuitry used for demodulation in the receiver should be simple and therefore inexpensive. This suggest the use of envelope detection, whichthe use of envelope detection, which requires the addition of a carrier to the VSB modulated wave.
Fig ( ) Id li d it d t fFig (a): Idealized magnitude spectrum of a transmitted TV signal.
Fig (b): Magnitude response of VSB shaping filter in the receiver
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [36]
shaping filter in the receiver.
Frequency Translation (Mixing):Frequency translation, mixing, or hetrodyning is used to translate the spectrum of a signal from one frequency to another.
downwardtion transla
upwardation transl
1
12
l
l
ff
fff
[37]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
Frequency Division Multiplexing (FDM)Frequency Division Multiplexing (FDM)
Multiplexing is used to transmit a number of these signals over the same channel, the signals must be kept apart so that they do not interfere with each other and thus they can be separated at theinterfere with each other, and thus they can be separated at the receiving end.
Multiplexing techniques:
1. Frequency-Division Multiplexing (FDM):
(A technique of separating signals in frequency)
2 Time-Division Multiplexing (TDM):2. Time-Division Multiplexing (TDM):
(A technique of separating signals in time)
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [38]
Block diagram of FDM systemBlock diagram of FDM system
[39]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
Example: FDM of SSB for Telephone Systems
[40]Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih
Example: FDM of SSB for Telephone Systems (Cont.)
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [41]
End of Chapter ThreeEnd of Chapter Three
Course: Analog Communications - Ch.3 - Instructor: Ibrahim Abu-Isbeih [42]