Module1ec010405 Analog Communication

7/29/2019 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%.



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An Example of a message signal m(t)

Waveform for Amplitude modulation of the message signal m(t)



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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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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



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BT

J0(1.0)

J1(1.0)

J2(1.0)

(S( f)

1

2A

c )

f

1.0



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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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Module1ec010405 Analog Communication