Rec101 unit v communication engg

Fundamentals of Communication Engineering

Fundamentals of Communication Engineering: Elements of aCommunication System, Need of Modulation, Electromagnetic spectrumand typical applications. Basics of Signal Representation and Analysis,Introduction of various analog modulation techniques, Fundamentals ofamplitude modulation, Modulation and Demodulation Techniques of AM.

10/16/2017 1REC 101 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

Elements of a Communication System

• Communication refers to information transfer from source todestination. Electronic communication refers to transfer andprocessing 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.




Block diagram of communication system

Source: Source generates information in form of symbols, images,sounds etc, and its physical manifestation is called message. Inputtransducer 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

ModulatorRF

amplifierAmplifier

De-modulator

Channel

Carrier Oscillator

Local Oscillator

Trans-ducer

Trans-ducer

Additive Noise

Sourcedestina

tion

Transmitter Receiver



Transmitter: encodes or modifies message signal into a form suitablefor transmission into channel. Transmitter modulates message signalover a carrier signal, amplifies and after suitable amplificationtransmits into the channel.

Channel: is medium which effectively connects transmitter andreceiver (ex. Coaxial cable, twisted pair, optical fiber, free space etc.).Transmitted signal propagates through channel, attenuated and getdistorted due to channel imperfection, noise and interference.

Receiver: amplifies the received signal and decodes or detectsmessage signal from it through a process called demodulation. Due tonoise, degradation or distortion occurs in reproduced message signal.

Destination: output transduces converts the received message signalinto desired physical form for delivery of information to destination

Need of Modulation

Modulation is a complex process and is done due to somecompelling reasons.

Ease of radiation: for transmission through electromagneticradiation, antenna height required is approximately ¼ ofwavelength.

Consider transmission of a 3KHz signal (wavelength =speed of lightc/ frequency f =100 Km), so antenna height required is 25 Km whichis not feasible.

If message is modulated to frequency 3 MHz ( =c/ f =100 m), itrequires antenna of 25 m for transmission

Thus modulation to high frequency carrier is required to facilitatetransmission


Need of Modulation

Multiplexing: Signals from various sources occupy commonbandwidth and may get mixed up (interference) during transmissionand may not be separable.

Using modulation, each signal can be modulated to a separatefrequency band (using different carrier), can be transmitted on thesame channel (multiplexing) and can be detected by filtering anddemodulation.

Modulation facilitates multiplexing of signals


Electromagnetic spectrum and applications

• Electromagnetic wave can travel through vacuum, space.

• Electromagnetic spectrum is range and spectrum ofelectromagnetic radiation frequencies (wavelength) and photonenergies.

• It extends below low frequencies used for radio communicationto 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.




https://www.flickr.com/photos/advancedphotonsource/5940581568



band Frequency range WavelengthVery 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 KmLF (low frequency) 30 KHz-300 KHz 10 Km-1 KmMF (low frequency) 300 KHz-3 MHz 1 Km-100 mHF (High frequency) 3 MHz-30 MHz 100 m-10 mVHF (Very high frequency) 30 MHz-300 MHz 10 m-1 mUHF (Ultra high frequency) 300 MHz-3 GHz 1 m-10 cmSHF (Super high frequency) 3 GHz-30 GHz 10 cm-1 cmEHF (Extremely high frequency) 30 GHz-300 GHz 1 cm-1 mm

Infra Red Infrared (FIR, MIR, NIR) 300 GHz-400 THz 1 mm-380 nmVisible Visible spectrum 400 THz-789 THz 380 nm-750 nmUltra violet NUV & EUV 789 THz-30 PHz 750 nm-10 nmX-rays Soft and Hard X rays 30 PHz-30 EHz 10 nm-10 pmGamma Ray 30 Ehz-300 EHz 10 pm-1pm

Basics of Signal Representation and Analysis


• Electrical signals are either voltage waveform or currentwaveform which are function of time represented as v(t) or i(t).

• Power dissipated in resister R due to voltage and currentwaveforms are v2(t)/R and i2(t) R respectively.

• For R=1, power dissipated (Normalized power) are v2(t) andi2(t) respectively, i.e. normalized power is square of the signalirrespective of being voltage or current waveform.

• Therefore signal is represented as waveform (voltage & current)which is function of time.


Basic signalsSinusoidal wave:


rad/s 2

frequency,angular

Hz, 1

, ,

,

)sin()(

T

TfperiodTimeTanglePhase

AmplitudeA

tAtx

tT

A

-A

tT

A

-A

2 t

Square wave:

Hz, 1

and rad/s, 2

frequency,angular

, ,

2

20

)(

Tf

T

periodTimeTAmplitudeA

TtT

A

TtA

tx

One period of Sine wave

One period of Square wave


Unit Step signal

00

01)(

ttu

t

1

U(t)

Unit step signal u(t)

Unit Impulse signal

1) is curve under the area (i.e.

1)( and

0 0)(

-

dtt

tfort

t

1

(t)

Unit impulse signal (t)

Decaying exponential wave:

,1

constant time

)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

constant time

)exp()(

a

attx


Basic signals



Fourier series and Fourier transformFourier series (complex form):

A periodic signal with frequency 0 (Time period T0) can berepresented as linear sum of harmonic exponentials

toolanalysisexp)(1

where

toolsynthesisexp)(

0

0

0

0

dttjntxT

C

tjnCtx

Tpn

n

np

Cn provides information about spectrum of signal [harmonicfrequency (integer multiple of fundamental frequency) and itsamplitude].



Fourier series and Fourier transformFourier transform:

A signal x(t) can be represented in frequency domain X() usingfollowing relationship

toolanalysis)exp()()(

toolsynthesis)exp()(2

1)(

pair ansformfourier tr)()(

dttjtxX

dtjXtx

Xtx

Fourier transform of signal can exist only iff signal is absolutelyintegreable.



Fourier transform pairs

0=2/T0

A

tT0

Ax(t) X()

-0

tT0

A

0=2/T0

Ax(t) jX()

-0 000 jtSin

000 tCos

1)( t

1

tT0

1

x(t) X()

00 2exp tj

2

TTSa

T

trect

X()

T

T

20

0t

1

x(t)

T/2-T/2

Introduction of various modulation techniques


• Modulation is defined as a process of changing some characteristicsof high frequency carrier signal in accordance with instantaneousvalue of message signal.

• Carrier signal is generally sinusoidal signal. It may be square orother signal of high frequency.

• Sinusoidal signal is described by its amplitude, frequency and phase.Changing any one characteristics in accordance with message signalis a basic modulation technique

• Various modulations techniques are grouped in the figure next slide.

Introduction of various modulation techniques


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 instantaneousvalue of message signal. Carrier’s frequency/phase is unchanged.


C

mmm

C

C

C

C

CCC

aaC

C

CC

A

Amt)(Atm

Am

tctmA

ttmA

AttmAs(t)

ktmkAA

tmAtAts

tAtcm(t)

,sin message sinosoidalfor ,)(1

index modulation

)()(1

1)sin()(1

1)sin()( Thus

1) astaken (normally modulator ofy sensitivit amplitude where);(

)( AM, of definitionper asthen ),sin()( signal modulated and

),sin()( signalCarrier , be signal messageLet

max

Modulation index is always kept less than 1 (100%). Overmodulation(m>1) leads to distortion in demodulation due to phase reversal atzero crossing

AM: Single tone message


(USB) sidebandUpper (LSB) sidebandLower carrier

)cos(2

)cos(2

)sin(

index, modulation where

)sin()sin()sin( signal AM

)sin()(Let

mCmCCC

C

m

CmCCC

mm

mmtAs(t)

A

Am

ttmAtAs(t)

tAtm

• Modulated signal has three components; carrier, LSB and USB

• Bandwidth (range of frequencies covering modulated signal) =2m

• Upper side Bands have message information while carriercomponent carriers no message information

Fundamentals of amplitude modulation


m(t)

TAm

-Am

t

c(t)

AC

-AC

t

S(t)Amax

Amin

minmax

minmax

AA

AAm

C

Spectrum of c(t)

-C

m

Spectrum of m(t)

Am

-m

C

C+mC-m

Spectrum of s(t)

-C

-C-m -C+m

AC

AC

mAC/2

AM: Power relation


21

)(Icurrent ofcomponent carrier with )(Icurrent antenna of In terms

effective i.e ,222

1

index modulation with modulation AM tonemultifor

21

222882power Total

)cos(2

)cos(2

)sin( AM modulated toneSingle

222

ct

22

2

2

1

22

2

2

1

21

2

222

Ppower sidebandUpper

22

Ppower sidebandLower

22

Ppower Carrier

2

(USB) sidebandUpper (LSB) sidebandLower carrier

USBLSBC

mII

mmmmmmm

PP

,---,m,mm

mPP

AmAAmAmAP

mmtAs(t)

Ct

nn

Ct

n

Ct

CCCCCt

mCmCCC

AM: Modulation

AM generation block diagram


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 2m

Cantered at C

ttmAtS CC

sin)(4

2

1)(

ttmAAtS CCC sin)()(

AM Demodulation: Envelope or diode detector


t

S(t)

t

S(t)Amax

Amin

t

S(t)

AM RLC Detected

signal

• Envelope detector is very easy circuit

• High frequency ripples can be removed bypassing through LPF stages

• Low cost receiver; one of the keyrequirement of broadcast.

• m<1 to make detection possible usingenvelope detector.

• Overmodulation (m>1) will result in distortion t

S(t)

AM Demodulation: Synchronous detection


Synchronous demodulation

Local Oscillator

AM

sin(Ct)

Low pass filter Detected

message

m(t)/2

)(2

1

capacitor blocking using removed DC with LPF ofoutput Thus

)2cos(2

)(

2

)(

22

)2cos(1)(

)(sin)(

multiplier ofoutput

2

tm

ttmAtmAt

tmA

ttmA

CCCC

C

CC

• Detection requires synchronization of local oscillator with carrier

Limitations of AM Wave


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)


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 cannot be performed using envelope detector.

• Its demodulation circuit is more complex.

Fundamentals of DSB-SC modulation

(Double Side Band Suppressed Carrier)


t

S(t)

m(t)= Amsin(mt)

TAm

-Am

t

c(t)=sin(Ct)

1

-1

C

Spectrum of c(t)

-C

m

Spectrum of m(t)

Am

-m

C

C+mC-m

Spectrum of s(t)

-C

-C-m -C+m

Am/2

DSB-SC modulation

10/16/2017REC 101 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

28

Carrier Oscillatorsin(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 twoamplitude modulators with inputsignal m(t) and –m(t).

•Output of amplitude modulatorsare added so that carriercomponents are cancelled, thusgenerating DSB-SC

•DSB-SC can be demodulatedusing synchronous detector.

• two sidebands of DSB-SC carrysimilar information therebyhaving double bandwidth thanmessage bandwidth.

SSB (Single side band) modulation


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 bandwidthstarts from some high frequency (Ex. Speech signal whose band is300-3400 Hz). For such messages practical filters can be designed.

•For message which start form dc (Ex. video message whose band isdc to 5 MHz), phase shift method is used to generate SSB

•SSB is demodulated using synchronous demodulator used for AM

version]shifted phase [-90 m(t) of ansformhilbert tr )(ˆ is where

)sin()()cos()(ˆ)( SSB,

0tm

ttmttmts CC

SSB-SC modulation


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


31

•Filtering of one sideband isdifficult for some messages.

• In VSB modulation, One sidebandand a portion of other sideband istransmitted. The portion ofsideband (vestige band) is used todesign a practical filter.

•Bandwidth required is slightlyhigher than SSB

•Used for Video modulation incommercial TV transmission

H()

1

C C+mC-m

C+V

C-V

0.5

Frequency response of filer for VSBfiltering of USB with vestige band V,

FM (Frequency modulation)


32

Frequency modulation is process ofvarying amplitude of carrier (highfrequency sinusoidal signal) inaccordance with instantaneous valueof message signal.

• FM has large bandwidth 2(+W)Where is frequency deviationand W is message bandwidth

• Transmitted power is equal tocarrier power

• FM has good noise performance.

m(t)

TAm

-Am

t

c(t)

AC

-AC

t

c(t)

AC

-AC

Modulation Technique’s comparison


Modulation Parameter

Bandwidth[Message Bandwidth-W]

powerNoise performance

Application

AM Amplitude 2W PAM=PC+PLSB+PUSB poor Radio broadcast

DSB-SC Amplitude 2W PDSBSC=PLSB+PUSB averageIn Analog TV for colour

information

SSB Amplitude W PSSB=PLSB=PUSB averagePoint to point

communication, militaryVSB Amplitude Slightly >W PSSBPLSB average TV video component

FM Frequency

High 2(+W) is

frequency deviation

PFM=PC

Best, reduces the noise

FM Radio, TV Audio