module1ec010405 analog communication
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MODULE 1:
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Communication Systems
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A Communications Model
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Basic Communication System
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Basic Communication System
Basic components: Transmitter
Channel or medium
Receiver
Noise degrades or interferes with transmittedinformation.
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Transmitter
The transmitter is a collection of electronic
components and circuits that converts the electrical
signal into a signal suitable for transmission over a
given medium.
Transmitters are made up of oscillators, amplifiers,
tuned circuits and filters, modulators, frequency
mixers, frequency synthesizers, and other circuits.
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Communication Channel
The communication channel is themedium by which the electronic signal is
sent from one place to another.
Types of media include Electrical conductors
Optical media
Free space
System-specific media (e.g., water is the medium for
sonar).
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Physical Transmission Media
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Physical Transmission Media
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Receivers
A receiver is a collection of electronic components
and circuits that accepts the transmitted message
from the channel and converts it back into a form
understandable by humans.
Receivers contain amplifiers, oscillators, mixers,
tuned circuits and filters, and a demodulator or
detector that recovers the original intelligence signal
from the modulated carrier.
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Transceivers
A transceiver is an electronic unit that
incorporates circuits that both send and
receive signals.
Examples are:
Telephones Fax machines
Cell phones
Computer modems
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Signal Attenuation
Signal attenuation, or degradation, exists
in all media of wireless transmission.
It is proportional to the square of the
distance between the transmitter and
receiver.
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Noise
Noise is random, undesirable electronic
energy that enters the communication
system via the communicating medium and
interferes with the transmitted message.
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Types of Electronic Communication
Electronic communications are
classified according to whether they
are
1. One-way (Simplex) or two-way (Half
duplex orFull duplex) transmissions.
1. Analog ordigital signals.
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Simplex
The simplest method of electronic
communication is referred to as simplex.
This type of communication is one-way.
Examples are:
Radio
TV broadcasting
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Half Duplex
The form of two-way communication in
which only one party transmits at a time is
known as half duplex.
Examples are:
Police, military, etc. radio transmissions
Walky Talky
HAM radio
Morse Code
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Full Duplex
Most electronic communication is two-
way and is referred to as duplex.
When people can talk and listen
simultaneously, it is called full duplex.
The telephone is an example of this
type of communication.
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COMMUNICATION
SYSTEM
ANALOG
COMMUNICATION
DIGITAL
COMMUNICATION
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Analog Communication
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Digital Communication
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To be transmitted, data must be transformed to
electromagnetic signals.
Note
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Data
Data can be analog ordigital.
The term analog data refers to
information that is continuous. Digital data refers to information that has
discrete states.
Analog data take on continuous values. Digital data take on discrete values.
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omparison ofanalog and digital signal
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Analog Signal
CycleTime
Signal
Amplitude
Frequency = Cycles/Second
A typical
sine wave
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Analog Signal
3 Basic Parameters of analog signal1. Amplitude
2. Frequency
3. Phase
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Two signals with the same phase and
frequency, but different amplitudes
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Frequency
Frequency is the rate of change of cycle
(Positive and Negative) with respect to time.
Change in a short span of time means high
frequency.
Change over a long span of time means low
frequency.
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If a signal does not change at all, itsfrequency is zero.
If a signal changes instantaneously, its
frequency is infinite.
Note
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Two signals with the same amplitude and
phase, but different frequencies
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3 Sine waves with
frequencies 0, 8 & 16
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Phase
Phase describes the position of thewaveform relative to time 0.
Note
Three sine waves with the same
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Three sine waves with the sameamplitude and frequency, but different
phases
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Units of period and frequency
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Practical Case Composite
Signal
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Frequency Spectrum Defined
Available range of frequencies for
communication
Starts from low frequency communication
such as voice and progresses to high
frequency communication such as satellitecommunication
The spectrum spans the entire bandwidth of
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Frequency Spectrum
Low Frequency High Frequency
Radio
Frequency
Coaxial Cable
MHz
Satellite
Transmission
Microwave
GHz
Voice
KHz
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Bandwidth Definition
Bandwidth, in general, represents a
range of frequencies
300 MHz 700 MHz
Bandwidth is 400 MHz
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Bandwidth and Signal Frequency
The bandwidth of a composite signal
is the difference between the highest
and the lowest frequencies contained
in that signal.
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Communication Capacity
Bandwidth is indicative of the
communication capacity
Communication speed is proportional
to bandwidth
Units used to represent bandwidth
are Hz, bps etc.
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The Electromagnetic Spectrum
Figure 1-13: The electromagnetic spectrum.
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Electromagnetic Frequency Spectrum
Frequency : f [Hertz]Wavelength: [m]
c : velocity of light: 3 108 m/sec
f
1 kHz 3 105 m
100 kHz 3 103 m10 MHz 3 101 m = 30 m
1 GHz 3 10-1 m = 30 cm
c
f
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Electromagnetic Frequency Spectrum
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Introduction
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Topics to be covered
Need for Modulation
What is Modulation?
Types of Modulation
Amplitude Modulation (AM)
Angle Modulation
Frequency Modulation (FM)
Phase Modulation (PM)
B b d P b d
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Baseband vs Passband
Transmission
Baseband Signal:- Information bearing Signalor Message Signal.
The term Baseband refers to the band of
frequencies representing the original signalobtained from the source (orBase). Voice (0-4kHz)
TV (0-6 MHz)
A signal may be sent in its baseband formatwhen a dedicated wired channel is available.
Otherwise, it must be converted to passband.
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Need for Modulation
Size of the antenna For efficient radiation, the size of the antenna should be
/10 or more (preferably around /4 ), where is thewavelength of the signal to be radiated.
Easy to Multiplex Several message signals can be transmitted on a given
channel, by assigning to each message signal anappropriate slot in the channel.
Channel Selectivity Each station can be assigned a suitable carrier so that
the corresponding program material can be received bytuning to the station desired.
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Need for Modulation
Improved Signal to Noise Ratio Will be dealt in future lectures
Less Fading of transmitted signal As the energy of a signal is proportional to its frequency,
fading by the atmospheric particle is less
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What is Modulation?
The message signal is called MODULATINGSIGNAL or BASEBAND SIGNAL.
The word modulation means the systematicalteration of onewaveform, called the carrier,according to the characteristic of another
waveform, the modulating signal or themessage.
We use c(t )andm(t ), to denote thecarrierand the messa e waveforms res ectivel .
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What is Modulation?
The resultant signal after modulation is called
MODULATED SIGNAL.
For study purpose, the commonly used carrier
and message signal is SINUSOIDAL WAVE.
Transmitter Side - Modulation
Receiver Side - Demodulation
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Definition for Modulation
Modulation is defined as the process
by which some characteristic of acarrier wave is varied in accordance
with the message signal.
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Modulation and Demodulation
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Types of Modulation
Modulation - Characteristics of Carrier Wave is
varied in accordance with the characteristics
of message signal.
Consider a Carrier wave:
c(t) = Ac Cos ( )
InstantaneousValue
Maximum
AmplitudeAngle
( 2fc t + )
Frequency
Phase
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Types of Modulation
MODULATION
Angle
Modulation
Amplitude
Modulation (AM)
Phase
Modulation (PM)
Frequency
Modulation (FM)
AM DSB FC
AM DSB SC
SSBVSB
NBFM
WBFM
NBPM
WBPM
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AMPLITUDEMODULATION
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INTRODUCTION
Amplitude Modulation is the simplest and
earliest form of transmitters
AM applications include broadcasting in
medium- and high-frequency applications,CB radio, and aircraft communications
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The information signal varies the
instantaneous amplitude of the carrier
Basic Amplitude Modulation
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AM Characteristics
AM is a nonlinear process
Sum and difference frequencies are
created that carry the information
Amplitude Modulation
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p
g( t)= Ac[1+m( t)]
The Complex Envelope of an AM signal is given by
Ac indicates the power level of AM and m(t) is the Modulating Signal
Ac[1+m(t)] In-phase component x(t)
Ifm(t) has a peak positive values of +1 and a peak negative value of -1
AM signal 100% modulated
Representation of an AM signal is given by
() [1 ()]cosc cst A mt
Envelope detection can be used if % modulation is less than 100%.
Amplitude Modulation
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An Example of a message signal m(t)
Waveform for Amplitude modulation of the message signal m(t)
Amplitude Modulation
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An Example of message energy spectral density.
Energy spectrum of the AM modulated message signal.
B
2B
Carrier component together
with the message
AM
Percentage Modulation
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Amax
- Maximum value ofAc
[1+m ( t) ]
Amin
- Minimum value ofAc [ 1+m ( t) ]
Ac - Level of AM envelope in the absence of modulation [ i .e . , m ( t)= 0 ]
Definition: The percentage of positive modulation on an AM signal is
max%PositiveModulation 100max()cc
AAt
A
min100min()1cc
AAmt
A
The percentage of negative modulation on an AM signal is
maxmin
max()min()%Modulation 100
2 2c
mt tAA
A
The percentage of overall modulation is
Ifm(t) has a peak positive values of +1 and a peak negative value of -1
AM signal 100% modulated
AM Signal Waveform
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Amax = 1.5Ac
Amin = 0.5 Ac
% Positive modulation= 50%
% Negative modulation =50%
Overall Modulation = 50%
AM
Percentage Modulation
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g
Under modulated (100%)
AM
Normalized Average Power
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s2(t)=
1
2g (t)
2=
1
2Ac
2[1+m (t)]
2
1
2A
c2[1+2m (t)+m2(t)]
1
2
Ac2+A
c2m(t)+
1
2
Ac2 m2(t)
s2(t) =
1
2Ac
2 +
1
2Ac
2m
2(t)
The normalized average power of the AM signal is
If the modulation contains no dc level, then m(t)= 0
The normalized power of the AM signal is
Discrete Carrier Power Sideband power
AM Modulation Efficiency
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ff y
Translated Message Signal
Definition : The Modulation Efficiency is the percentage of the total power
of the modulated signal that conveys information.
Only Sideband ComponentsConvey information
Modulation Efficiency:
2
210
1
mt
E
mt
Highest efficiency for a 100% AM signal : 50% - square wave modulation
Normalized Peak Envelope Power (PEP) of the AM signal:
PPEP
=Ac
2
2
{1+max [m (t)]}2
Voltage Spectrum of the AM signal:
S(f)=Ac2 [ (f fc)+M(f fc)+ (f+fc)+M(f+fc)]
Unmodulated CarrierSpectral Component
Example 1 Power of an AM signal
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Example 1. Power of an AM signal
Suppose that a 5000-W AM transmitter is connected to a 50 ohm load;
1
2
Ac2
50 = 5,000Ac= 707 VThen the constant Acis given byWithout
Modulation
If the transmitter is then 100% modulated by a 1000-Hz test tone ,
the total (carrier + sideband) average power will be
1.5
[12(A
c
2
50 )]= (1 .5 ) (5000)= 7,500W [m
2(t)= 12 for 100% modulation]
The peak voltage (100% modulation) is (2)(707) = 1414 V across the 50 ohm load.
The peak envelope power (PEP) is 4[12(Ac
2
50 )]= (4) (5000)= 20,000W
The modulation efficiency would be 33% since < m2(t) >=1/2
Single Sideband (SSB) Modulation
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g ( )
An upper single sideband (USSB) signal has a zero-valued spectrum for ffc
SSB-AMpopular method ~ BW is same as that of the modulating signal.
Note: Normally SSB refers to SSB-AM type of signal
USSB LSSB
Single Sideband Signal
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Theorem :A SSB signal has Complex Envelopeand bandpass form as:
g(t)= Ac [m(t) j m (t)]m(t) cos
ct {m
s (t)= Ac[( t) sinc t ]
Upper sign (-) USSB
Lower sign (+) LSSB
m ( t) Hilbert transform ofm(t) m(t)m(t) h(t) Where h (t)= 1t
H(f)= [h(t)] j , f>0
j , f
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2AcM (f), f>0
0, ffc0, f
fc
M (f+fc), f
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g g
2Ac M (f), f>00, f
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The normalized average power of the SSB signal
s2(t)=
1
2
g ( t)2=
1
2
Ac2m
2(t)+[m (t)]2
m(t)2= m2(t)Hilbert transform does not change
power.
SSB signal power is:
s
2
(t)= Ac2
m
2
(t)
1
2max g ( t)
2=
1
2Ac
2m
2(t)+[m (t)]2
The normalized peak envelope (PEP) power is:
Power gain factor Power of the modulating signal
Generation of SSB
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Generation of SSB
R(t)=g (t)= Ac
m2(t)+[m(t)]2
(t)= g (t)= tan 1[m(t)m(t) ]
SSB signals have bothAM and PM.
g (t)= Ac [m(t) j m (t)]The complex envelope of SSB:
For the AM component,
For the PM component,
Advantages of SSB
Superior detected signal-to-noise ratio compared to that of AM
SSB has one-half the bandwidth of AM or DSB-SC signals
AM and FM Modulation
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(a) Carrier wave.
(b) Sinusoidal modulating signal.
(c) Amplitude-modulated signal.
(d) Frequency modulated signal.
Angle Modulation
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We have seen that anAM signal can be represented as
s( t)= Ac [1+m( t)]cosc t
Now we will see that information can also be carried in the angle of the
signal as
Note that in this type of modulation the amplitude of signal carries information.
s (t)= Accos[c t+(t)]
Here the amplitudeAc remains constant and the angle is modulated.
This Modulation Technique is called theAngle Modulation
Angle modulation: Vary either the Phase or the Frequency of the carrier signal
Phase Modulation and Frequency Modulation are special cases of Angle
Modulation
Angle ModulationR i f PM d FM i l
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Representation of PM and FM signals:
The Complex Envelope for an Angle Modulation is given by g (t)= Ac ej(t)
R(t)=g (t)= Ac Is a constant Real envelope,
(t) - linear function of the modulating signal m(t)
TheAngle-modulated Signalin time domain is given by s (t)= Accos[c t+(t)]
g(t) - Nonlinearfunction of the modulation.
Special Case 1:
For PM the phase is directly proportional to the modulating signal. i.e.;
WhereDpis the Phase sensitivity of the phase modulator, having units of radians/volt.
Special Case 2:
For FM, the phase is proportional to the integral ofm(t) so that
where the frequency deviation constantDfhas units of radians/volt-sec.
Angle Modulation
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s( t)= Ac cos[c t+Dpm( t) ]Resulting PM wave:
Phase Modulationoccurs when the instantaneous phase varied in proportion to that of
the message signal.
(t)= Dpm(t) Dp is the phase sensitivity of the modulator
Frequency Modulation occurs when the instantaneous frequency is varied linearly
with the message signal.
i( t)= c+Dfm( t)
(t)= Df
t
m()d
s ( t)= Accos
[c t+Df
t
m ( )d]
Resulting FM wave:
Dfis the frequencydeviation constant
Instantaneous Frequency (fi) of a signal is defined by
i (t)=
d(t)
dt (t)=
t
i () dwhere (t)=
c
t+( t)
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Phase and Frequency Modulations
Phase Modulation Frequency Modulation
Comparing above two equations , we see that if we have a PM signal modulated
by mp(t), there is also FM on the signal, corresponding to a different modulation
wave shape that is given by:
Similarly if we have a FM signal modulated by mf(t),the corresponding phase
modulation on this signal is:
Wherefandpdenote frequency
and phase respectively.
Generation of FM from PM and vice versa
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mp(t)=
Df
Dp
t
mf()d
mf (t)=DpDf[dmp (t)dt ]
Integrator Phase Modulator
(Carrier Frequencyfc)
Differentiator Frequency Modulator
(Carrier Frequencyfc)
mp(t)
mf(t) mp(t)
mf(t)
s (t)
s (t)
FM Signal
PM signal
Generation of FM using a Phase Modulator:
Generation of PM using a Frequency Modulator:
Gainf
p
D
D
Gainf
p
D
D
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FM with sinusoidal modulating signal
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fi
(t)= fc
+1
2
[d(t)
dt
]But,
Vp BW
Average Power does not change
with modulation
Average Power=
Ac2
2
Angle Modulation
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Angle Modulation
Advantages:
Constant amplitude means Efficient Non-linear Power Amplifiers can be used.
Superior signal-to-noise ratio can be achieved (compared to AM) if bandwidth is
sufficiently high.
Disadvantages:
Usually require more bandwidth than AM
More complicated hardware
Modulation Index
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The Peak Phase Deviationis given by: = max [(t)]
is related to the peak modulating voltage by: = DpVp Vp= max [m(t)]Where
The Phase Modulation Indexis given by: p= Where is the peak
phase deviation
The Frequency Modulation Indexis given by:
f=F
B
FPeak Frequency Deviation
B Bandwidth of the modulating signal
Spectra of Angle modulated signals
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Spectra of Angle modulated signals
Spectra for AM, DSB-SC, and SSB can be obtained with simple formulasrelating S(f) to M(f).
But for angle modulation signaling, becauseg(t) is a nonlinear function ofm(t).
Thus, a general formula relating G(f) toM(f) cannot be obtained.
To evaluate the spectrum for angle-modulated signal, G(f) must be evaluated on acase-by-case basis for particular modulating waveshape of interest.
S(f)=1
2[G(f fc)+G
( f fc)]
G(f)= [g(t)]= [Acej(t)]Where
Spectrum of Angle modulated signal
Spectrum of PM or FM Signal with Sinusoidal Modulating Signal
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Assume that themodulation on the PMsignal is
mp(t)= Am sinmt (t)= sinm tThen
p=D
pA
m= Where is the phaseModulation Index.
Same (t) could also be obtained ifFM were used
mf(t)= Amcosmtwhere
= f= DfAm/m
F=
1
2 DfAm
The Complex Envelope is:
and
Thepeak frequency deviation would be
g(t)= Acej(t)
= Acejsin
mt
which is periodic with period Tm=1
fm
Using discrete Fourier series that is valid over all time, g(t) can be written as
Spectrum of PM or FM Signal with Sinusoidal Modulating Signal
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Using discreteFourier series that is valid over all time, g(t) can be written as
g (t)= n=
n=
cnejn
mt
cn=Ac
Tm T
m
/ 2
Tm/ 2(ejsinm t)e jnm tdtWhere
cn= A
c[12
ej (sin n)]= AcJn()Which reduces to
Jn()Bessel function of thefirst kindof thenth order
Taking thefourier transform of the complex envelopeg(t), we get
J n()= ( 1)nJ
n() Is a special property of Bessel Functions
G (f)= n=
n=
cn (f nfm) or
n
c n
n
Gf AJ f n
Bessel Functions of the First Kind
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J0()=0 at =2.4, 5.52 & so on
Bessel Functions of the First Kind
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Frequency spectrum of FM
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S( t)= Ac n=
Jn ()cos[(c+nm) t]
The FM modulated signal in time domain
From this equation it can be seen that the frequency spectrum of an FM
waveform with a sinusoidal modulating signal is a discrete frequency
spectrum made up of components spaced at frequencies ofcn
m.
By analogy with AM modulation, these frequency components are called
sidebands.
We can see that the expression for s(t) is an infinite series. Therefore the
frequency spectrum of an FM signal has an infinite number of sidebands.
The amplitudes of the carrier and sidebands of an FM signal are given by
the corresponding Bessel functions, which are themselves functions of the
modulation index
Observations:
Spectra of an FM Signal with Sinusoidal Modulation
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Spectra of an FM Signal with Sinusoidal Modulation
BT
(S( f)
1
2A
c )
f
1.0
The following spectra show the effect of modulation index, , on the
bandwidth of an FM signal, and the relative amplitudes of the carrier and
sidebands
Spectra of an FM Signal with Sinusoidal Modulation
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BT
J0(1.0)
J1(1.0)
J2(1.0)
(S( f)
1
2A
c )
f
1.0
Spectra of an FM Signal with Sinusoidal Modulation
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BT
(S( f)
1
2A
c )
f
1.0
Carsons rule
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Although the sidebands of an FM signal extend to infinity, it has been found
experimentally that signal distortion is negligible for a bandlimited FM signal
if 98% of the signal power is transmitted.
Based on the Bessel Functions, 98% of the power will be transmitted when
the number of sidebands transmitted is 1+ on each side.
(1+fm
Carsons rule
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Therefore the Bandwidth required is given by
phase modulation index/ frequency modulation index
Bbandwidth of the modulating signal
BT= 2(+1)fm
For sinusoidal modulation B= fm
Carsons rule :Bandwidth of an FM signal is given by
Note: When =0 i.e. baseband signals BT= 2f
m
2 1TB
Narrowband Angle Modulation
N b d A l M d l i i i l f l d l i h ( ) i
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Narrowband Angle Modulation is a special case of angle modulation where (t) is
restricted to a small value.
(t)
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