Rec101 unit v communication engg

Fundamentals of Communication
Engineering
Fundamentals of Communication Engineering: Elements of a
Communication System, Need of Modulation, Electromagnetic spectrum
and typical applications. Basics of Signal Representation and Analysis,
Introduction of various analog modulation techniques, Fundamentals of
amplitude modulation, Modulation and Demodulation Techniques of AM.
10/16/2017 1
REC 101 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

Elements of a Communication System
• Communication refers to information transfer from source to
destination. Electronic communication refers to transfer and
processing of information in form of electrical signals.
• Communication can be described as a series of processes
– Generation of thought at source, converted into message using symbols
– Converting the message into electrical form referred as message signal
– Conversion of message signal into a form suitable for transmission into channel
– Transmission through channel or medium. In medium noise get mixed
– Receiving and detecting the message signal
– Converting the message signal into desired physical form
– Assimilation of message at destination within acceptable level of degradation.
10/16/2017 2

10/16/2017 3
Block diagram of communication system
Source: Source generates information in form of symbols, images,
sounds etc, and its physical manifestation is called message. Input
transducer converts message into electrical form (ex. Microphone).
Information : news or knowledge one wishes to convey
Message : physical manifestation of information
Message signal: electrical analogy of message generated by the source
Modulator
RF
amplifier
Amplifier
De-
modulator
Channel
Carrier
Oscillator
Local
Oscillator
Trans-
ducer
Trans-
ducer
Additive
Noise
Source
destina
tion
Transmitter Receiver

10/16/2017 4
Transmitter: encodes or modifies message signal into a form suitable
for transmission into channel. Transmitter modulates message signal
over a carrier signal, amplifies and after suitable amplification
transmits into the channel.
Channel: is medium which effectively connects transmitter and
receiver (ex. Coaxial cable, twisted pair, optical fiber, free space etc.).
Transmitted signal propagates through channel, attenuated and get
distorted due to channel imperfection, noise and interference.
Receiver: amplifies the received signal and decodes or detects
message signal from it through a process called demodulation. Due to
noise, degradation or distortion occurs in reproduced message signal.
Destination: output transduces converts the received message signal
into desired physical form for delivery of information to destination

Need of Modulation
Modulation is a complex process and is done due to some
compelling reasons.
Ease of radiation: for transmission through electromagnetic
radiation, antenna height required is approximately ¼ of
wavelength.
Consider transmission of a 3KHz signal (wavelength  =speed of light
c/ frequency f =100 Km), so antenna height required is 25 Km which
is not feasible.
If message is modulated to frequency 3 MHz ( =c/ f =100 m), it
requires antenna of 25 m for transmission
Thus modulation to high frequency carrier is required to facilitate
transmission
10/16/2017 5

Need of Modulation
Multiplexing: Signals from various sources occupy common
bandwidth and may get mixed up (interference) during transmission
and may not be separable.
Using modulation, each signal can be modulated to a separate
frequency band (using different carrier), can be transmitted on the
same channel (multiplexing) and can be detected by filtering and
demodulation.
Modulation facilitates multiplexing of signals
10/16/2017 6

Electromagnetic spectrum and applications
• Electromagnetic wave can travel through vacuum, space.
• Electromagnetic spectrum is range and spectrum of
electromagnetic radiation frequencies (wavelength) and photon
energies.
• It extends below low frequencies used for radio communication
to gamma rays at high frequency end
• Electromagnetic spectrum are segmented as
– long waves,
– radio wave & microwave,
– infrared,
– visible light,
– ultraviolet,
– x-rays and
– gamma rays.
10/16/2017 7

10/16/2017 8
https://www.flickr.com/photos/advancedphotonsource/5940581568

10/16/2017 9
band Frequency range Wavelength
Very very low frequency 3 Hz-3 KHz 100 Mm-100 Km
Radio and
Microwave
frequency
VLF (Very low frequency) 3 KHz-30 KHz 100 Km-10 Km
LF (low frequency) 30 KHz-300 KHz 10 Km-1 Km
MF (low frequency) 300 KHz-3 MHz 1 Km-100 m
HF (High frequency) 3 MHz-30 MHz 100 m-10 m
VHF (Very high frequency) 30 MHz-300 MHz 10 m-1 m
UHF (Ultra high frequency) 300 MHz-3 GHz 1 m-10 cm
SHF (Super high frequency) 3 GHz-30 GHz 10 cm-1 cm
EHF (Extremely high frequency) 30 GHz-300 GHz 1 cm-1 mm
Infra Red Infrared (FIR, MIR, NIR) 300 GHz-400 THz 1 mm-380 nm
Visible Visible spectrum 400 THz-789 THz 380 nm-750 nm
Ultra violet NUV & EUV 789 THz-30 PHz 750 nm-10 nm
X-rays Soft and Hard X rays 30 PHz-30 EHz 10 nm-10 pm
Gamma Ray 30 Ehz-300 EHz 10 pm-1pm

Basics of Signal Representation and Analysis
10/16/2017 10
• Electrical signals are either voltage waveform or current
waveform which are function of time represented as v(t) or i(t).
• Power dissipated in resister R due to voltage and current
waveforms are v2(t)/R and i2(t) R respectively.
• For R=1, power dissipated (Normalized power) are v2(t) and
i2(t) respectively, i.e. normalized power is square of the signal
irrespective of being voltage or current waveform.
• Therefore signal is represented as waveform (voltage & current)
which is function of time.

Basic signals
Sinusoidal wave:
10/16/2017 11
rad/s
2
frequency,angular
Hz,
1
,,
,
)sin()(
T
T
fperiodTimeTanglePhase
AmplitudeA
tAtx








tT
A
-A
tT
A
-A
2 t
Square wave:
Hz,
1
andrad/s,
2
frequency,angular
,,
2
2
0
)(
T
f
T
periodTimeTAmplitudeA
Tt
T
A
T
tA
tx












One period of Sine wave
One period of Square wave

10/16/2017 12
Unit Step signal






00
01
)(
t
tu
t
1
U(t)
Unit step signal u(t)
Unit Impulse signal
1)iscurveunder thearea(i.e.
1)(and
00)(
-




dtt
tfort

 t
1
(t)
Unit impulse signal (t)
Decaying exponential wave:
,
1
constanttime
)exp()(
a
attx



Decaying exponential
t
1
0.36
exp(-at)
=1/a
t
1
0.36
exp(at)
= -1/a
Rising exponential
Rising exponential wave:
,
1
constanttime
)exp()(
a
attx



Basic signals

10/16/2017 13
Fourier series and Fourier transform
Fourier series (complex form):
A periodic signal with frequency 0 (Time period T0) can be
represented as linear sum of harmonic exponentials
 
  toolanalysisexp)(
1
where
toolsynthesisexp)(
0
0
0
0
dttjntx
T
C
tjnCtx
T
pn
n
np








Cn provides information about spectrum of signal [harmonic
frequency (integer multiple of fundamental frequency) and its
amplitude].

10/16/2017 14
Fourier series and Fourier transform
Fourier transform:
A signal x(t) can be represented in frequency domain X() using
following relationship
toolanalysis)exp()()(
toolsynthesis)exp()(
2
1
)(
pairansformfourier tr)()(









dttjtxX
dtjXtx
Xtx




Fourier transform of signal can exist only iff signal is absolutely
integreable.

10/16/2017 15
Fourier transform pairs

0=2/T0
A
t
T0
A
x(t) X()
-0
t
T0
A

0=2/T0
A
x(t) jX()
-0      000   jtSin
      000  tCos
1)( t 
1
t
T0
1
x(t) X()
   00 2exp  tj












2
T
TSa
T
t
rect

X()
T

T


2
0 
0
t
1
x(t)
T/2-T/2

Introduction of various modulation techniques
10/16/2017 16
• Modulation is defined as a process of changing some characteristics
of high frequency carrier signal in accordance with instantaneous
value of message signal.
• Carrier signal is generally sinusoidal signal. It may be square or
other signal of high frequency.
• Sinusoidal signal is described by its amplitude, frequency and phase.
Changing any one characteristics in accordance with message signal
is a basic modulation technique
• Various modulations techniques are grouped in the figure next slide.

Introduction of various modulation techniques
10/16/2017 17
Baseband communication
Amplitude modulation
(AM)
Frequency Modulation
(FM)
Phae Modulation
(PM)
Angle modulation
Continuous wave modulation
(CW modulation)
Pulse Amplitude modulation
(PAM)
Pulse width modulation
(PWM)
Pulse position modulation
(PPM)
Pulse analog modulation
Pulse Code Modulation
(PCM)
Delta modulation
(DM)
Pulse digital modulation
(waveform coding technique)
Pulse modulation
Amplitude Shift Keying
(ASK)
Phase Shift Keying
(PSK)
Frequency Shift Keying
(FSK)
Digital data transmission
Carrier Communication
Communication system

Fundamentals of amplitude modulation
Amplitude modulation is process of varying amplitude of carrier
(high frequency sinusoidal signal) in accordance with instantaneous
value of message signal. Carrier’s frequency/phase is unchanged.
10/16/2017 18
 
C
m
mm
C
C
C
C
CCC
aaC
C
CC
A
A
mt)(Atm
A
m
tctm
A
ttm
A
AttmAs(t)
ktmkAA
tmAtAts
tAtcm(t)

















,sinmessagesinosoidalfor,)(
1
indexmodulation
)()(
1
1)sin()(
1
1)sin()(Thus
1)astaken(normallymodulatorofysensitivitamplitudewhere);(
)(AM,ofdefinitionperasthen),sin()(signalmodulatedand
),sin()(signalCarrier,besignalmessageLet
max




Modulation index is always kept less than 1 (100%). Overmodulation
(m>1) leads to distortion in demodulation due to phase reversal at
zero crossing

AM: Single tone message
10/16/2017 19
    

(USB)sidebandUpper(LSB)sidebandLower
carrier
)cos(
2
)cos(
2
)sin(
index,modulationwhere
)sin()sin()sin(signalAM
)sin()(Let
mCmCCC
C
m
CmCCC
mm
mm
tAs(t)
A
A
m
ttmAtAs(t)
tAtm







• Modulated signal has three components; carrier, LSB and USB
• Bandwidth (range of frequencies covering modulated signal) =2m
• Upper side Bands have message information while carrier
component carriers no message information

Fundamentals of amplitude modulation
10/16/2017 20
m(t)
T
Am
-Am
t
c(t)
AC
-AC
t
S(t)
Amax
Amin
minmax
minmax
AA
AA
m



C

Spectrum of c(t)
-C
m
Spectrum of m(t)
Am

-m
C
C+mC-m

Spectrum of s(t)
-C
-C-m -C+m
AC
AC
mAC/2

AM: Power relation
10/16/2017 21
























2
1
)(Icurrentofcomponentcarrierwith)(IcurrentantennaofIn terms
effectivei.e,
222
1
indexmodulationwithmodulationAMtonemultifor
2
1
222882
powerTotal
)cos(
2
)cos(
2
)sin(AMmodulatedtoneSingle
2
22
ct
22
2
2
1
22
2
2
1
21
2
222
PpowersidebandUpper
22
PpowersidebandLower
22
PpowerCarrier
2
(USB)sidebandUpper(LSB)sidebandLower
carrier
USBLSBC
m
II
mmmm
mmm
PP
,---,m,mm
m
PP
AmAAmAmA
P
mm
tAs(t)
Ct
n
n
Ct
n
Ct
CCCCC
t
mCmCCC

    



AM: Modulation
AM generation block diagram
10/16/2017 22
Carrier
Oscillator
m(t)
AC sin(Ct)
AC m(t)sin(Ct)
[AC + AC m(t)]sin(Ct)
AM wave
AM
Switching modulator
m(t)
c(t)
Band pass filter
(BPF)
Passband 2m
Cantered at C
 ttmAtS CC 

sin)(
4
2
1
)( 






   ttmAAtS CCC sin)()( 

AM Demodulation: Envelope or diode detector
10/16/2017 23
t
S(t)
t
S(t)
Amax
Amin
t
S(t)
AM RL
C Detected
signal
• Envelope detector is very easy circuit
• High frequency ripples can be removed by
passing through LPF stages
• Low cost receiver; one of the key
requirement of broadcast.
• m<1 to make detection possible using
envelope detector.
• Overmodulation (m>1) will result in distortion t
S(t)

AM Demodulation: Synchronous detection
10/16/2017 24
Synchronous demodulation
Local
Oscillator
AM
sin(Ct)
Low
pass
filter Detected
message
m(t)/2
 
   
)(
2
1
capacitorblockingusingremovedDCwithLPFofoutputThus
)2cos(
2
)(
2
)(
22
)2cos(1
)(
)(sin)(
multiplierofoutput
2
tm
t
tmAtmAt
tmA
ttmA
C
CCC
C
CC







 





• Detection requires synchronization of local oscillator with carrier

Limitations of AM Wave
10/16/2017 25
Two important limitation of AM
• In AM, more than 2/3 power is involved in carrier component,
which does not bear any message information.
• Bandwidth of AM wave is twice of message bandwidth.
Advantages of AM
• Simplicity of system.
• Easy and low cost receiver

DSB-SC modulation
(Double Side Band Suppressed Carrier)
10/16/2017 26
DSB-SC, S(t)=m(t) c(t)
•It solves the power wastage in AM.
•DSB-SC has two sidebands and carrier component is suppressed.
•In DSB-SC, phase reversal occurs at zero crossing of message signal
•Due to phase reversal at zero crossing, DSB-SC demodulation can
not be performed using envelope detector.
•Its demodulation circuit is more complex.

Fundamentals of DSB-SC modulation
(Double Side Band Suppressed Carrier)
10/16/2017 27
t
S(t)
m(t)= Amsin(mt)
T
Am
-Am
t
c(t)=sin(Ct)
1
-1
C

Spectrum of c(t)
-C
m
Spectrum of m(t)
Am

-m
C
C+mC-m

Spectrum of s(t)
-C
-C-m -C+m

Am/2

DSB-SC modulation
10/16/2017
28
Carrier
Oscillator
sin(Ct)
Amplitude
Modulator
m(t)
Amplitude
Modulator
1800
phase
shift
+
-
[1+m(t)]sin(Ct)
[1-m(t)]sin(Ct)
2m(t)sin(Ct)
Balanced Modulator
DSB-SC
• Balanced modulator use two
amplitude modulators with input
signal m(t) and –m(t).
• Output of amplitude modulators
are added so that carrier
components are cancelled, thus
generating DSB-SC
• DSB-SC can be demodulated
using synchronous detector.
• two sidebands of DSB-SC carry
similar information thereby
having double bandwidth than
message bandwidth.

SSB (Single side band) modulation
10/16/2017
29
•SSB modulation has only one sideband (LSB or USB)
•SSB can be generated by
–filtering method (filtering one sideband of DSB-SC)
–Phase shift method
•Filtering method can be used only for messages whose bandwidth
starts from some high frequency (Ex. Speech signal whose band is
300-3400 Hz). For such messages practical filters can be designed.
•For message which start form dc (Ex. video message whose band is
dc to 5 MHz), phase shift method is used to generate SSB
•SSB is demodulated using synchronous demodulator used for AM
version]shiftedphase[-90m(t)ofansformhilbert tr)(ˆiswhere
)sin()()cos()(ˆ)(SSB,
0
tm
ttmttmts CC  

SSB-SC modulation
10/16/2017
30
sin(Ct)
Balanced
Modulator
Balanced
Modulator
-900 phase
shift
+
LSB +
USB -
Amsin(mt)sin(Ct)
Phase shift method of SSB generation
SSB-SC
-900 phase
shift
-cos(Ct)
Amcos(mt)cos(Ct)
Carrier
Oscillator
)]sin()sin()cos()[cos(
)sin()()cos()(ˆ
ttttA
ttmttm
CmCmm
CC




)cos()(ˆ tAtm mm 
)sin()( tAtm mm 

VSB (Vestigial Side band) modulation
10/16/2017
31
•Filtering of one sideband is
difficult for some messages.
•In VSB modulation, One sideband
and a portion of other sideband is
transmitted. The portion of
sideband (vestige band) is used to
design a practical filter.
•Bandwidth required is slightly
higher than SSB
•Used for Video modulation in
commercial TV transmission
H()

1
C C+mC-m
C+V
C-V
0.5
Frequency response of filer for VSB
filtering of USB with vestige band V,

FM (Frequency modulation)
10/16/2017
32
Frequency modulation is process of
varying amplitude of carrier (high
frequency sinusoidal signal) in
accordance with instantaneous value
of message signal.
• FM has large bandwidth 2(+W)
Where  is frequency deviation
and W is message bandwidth
• Transmitted power is equal to
carrier power
• FM has good noise performance.
m(t)
T
Am
-Am
t
c(t)
AC
-AC
t
c(t)
AC
-AC

Modulation Technique’s comparison
10/16/2017 33
ModulationParameter
Bandwidth
[Message
Bandwidth-
W]
power
Noise
performance
Application
AM Amplitude 2W PAM=PC+PLSB+PUSB poor Radio broadcast
DSB-SC Amplitude 2W PDSBSC=PLSB+PUSB average
In Analog TV for colour
information
SSB Amplitude W PSSB=PLSB=PUSB average
Point to point
communication, military
VSB Amplitude Slightly >W PSSBPLSB average TV video component
FM Frequency
High
2(+W)
 is
frequency
deviation
PFM=PC
Best, reduces
the noise
FM Radio, TV Audio

Rec101 unit v communication engg

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Rec101 unit v communication engg