1.Analog Data Analog Signal

8/2/2019 1.Analog Data Analog Signal

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BASIC CONCEPTS &

ANALOG DATA, ANALOGSIGNAL


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q Analog Data, Analog Signals (AM, FM, PM)

Telephone

Analog Data(soundwaves)

Analog Signal

Modem

Digital Data (Binary VoltagePulses)

Analog Signal (Modulated on carrier frequency)

CODEC

Analog Data

(Voice orVideo) Digital Data

Encoder

Digital Data Digital Signal

q Digital Data, Analog Signals (ASK, FSK, PSK)

q Analog Data, Digital Signals (PAM, PWM, PCM)

q Digital Data, Digital Signals (NRZ, Bipolar, Manchester)

THE FOUR SCENARIOS OF COMMUNICATIONS


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BASIC CONCEPTS

Base band and Pass band signals.

Base band SignalsThe original message/signal generated from aninformation source is known as Base band signal. Thissignal may be a combination of two or more messages!

The base band signal may be analog as well asdigital. The base band transmission is preferred at lowfrequencies and short distances.


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Pass band signal :

The modulated signal is called as Pass band (orBand pass) signal a Pass band transmission. Thegenerated frequencies (lower! higher side

bands) are called Band pass frequencies. If a IkHz signal amplitude modulates a 200 KHzcarrier, the pass band frequencies generatedare 199 KHz (200-1) to 201 (200+1) kz. Thepass band transmission is preferred at high

frequencies and for long distances.

The T.V. and satellite transmission are examplesof pass band transmission. This transmission

may be analog as well as digital.

BASICCONCEPTS


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Time domain and frequency domain.

(a) In time domain representation of a signal, Thesignal is a time varying quantity. Time domain

specifies the amplitude of the signal at aparticular instant of time.

(b) The Time domain signal can be converted intoFrequency domain by Fourier series.

BASIC CONCEPTS


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Errors in communication systems

Errors In Communication System : When a distortionoccurs in the received message, an error is said to haveoccurred. The distortion may be a frequency, phase orany other distortion. . .These are following reasons which produce errors:1. Band Width : When the band width allowed to atransmission is not sufficient it causes an error. The bandwidth allowed to the AM transmission is only 10 kHz buta human ear requires a band width of 15 kHz for full

satisfaction. Hence AM. Transmission lacks fidelity. Thebandwidth allowed to F.M. transmission is 200 kHz and itcan reproduce a transmitted signal to our fullsatisfaction.

2. Noise : The noise is another reason that produce error.The unwanted sound is called noise. There are man

BASIC CONCEPTS

S C CO C S


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Advantage of using a high frequency carrier for

transmissionFor transmission h.f. carrier is used so that the signalsgains strength and go to large distance.

BASIC CONCEPTS

AM and FM broadcast bands AM broadcasting-kilohertz.FM broadcasting-megahertz.


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B.W. requirement for various signals.By limiting the band width for a signal, more channels can

be accommodated. The band width requirement for asignal mainly depends upon the modulating signals. Theaudio signal occupies a band width upto 15 KHz but whena carrier is modulated by the audio signal the modulatedsignal will certainly need more band width. Here we are

giving B.W. of few signals.

1. The audio signal need B.W. of about 20 Hz to 15 kHz fortransmission.2. The video signal needs a BW of about 4 MHz, while a fax

signal needs a BW of I kHz only. In T.V. the picture isscanned in 1/30 seconds while a FAX needs 10 minutes to

BASIC CONCEPTS


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BASIC CONCEPTSRadio broadcasting, Transmission and Reception.

Radio (wireless) Broadcasting, transmission and Reception : Theprocess of sending radio T.V. signals by an antenna to multiplereceivers which can simultaneously pick up the signal is calledbroadcastingIn simple words to radiate radio wave from a station into space isbroadcasting or, to send signal in all directions (broad) is calledbroadcasting.After the waves are thrown into the space, the transmission start andall the receivers in the range can simultaneously pick up the signal.This is called reception. There is a little difference betweenbroadcasting and transmission. However, the process of reception is

quite different.Important components of this network are shown in next slide:


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a o roa cas ng,Transmission and

Reception


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BASIC CONCEPTS

Radio does not mean radio receiver. The radiomeans wireless.(ii) Radio means radiations for wireless transmission.The principle of radio broadcasting, transmission andreception described above are same for radio, as well

T.V. signals and also for all such wireless devices.(iii) Radio is the abbreviated form of radio telegraphor radio telephone.(iv) Broadcasting means to send out in all directions.

(v) (a) AM. radio broadcast band: Its range is 540-1600 kHz. The stations are a assigned every 10 kHz inthe above band.(b) F.M. radio broadcast band: Its range is 88-108

MHz. The stations are assigned every 200 Hz in theabove band.


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BASIC CONCEPTS

Transmission and reception of T.V. signals.

The T.V. signal has sound as well as picture;Both are processed through separate circuits.

When a person speaks in a T.V. station before aT.V. (video) camra, his sound as well as picture,both are converted into electric pulses. Note thatthe camera has an in built microphone. These

electrical Pulses are modulated and transmittedthrough the antenna. The signal transmitted iscalled the composite video signal, whichcontains sound as well as picture; in the form ofelectrical ener . -


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DIGITAL IS PREFERRED METHODOF TRANSMISSION

Reseasonsq Digital Technology: Continuing drop in

cost and size.

.

q Data Integrity: Repeaters rather than amplifiers, so effects of noise

are not cumulative. Possible to transmit over longer distance

keeping integrity of data.q Capacity Utilization: Economical to build transmission links of very

high bandwidth. A high degree of multiplexing is needed.

q Security and privacy: Encryption Techniques

q Integration: By treating analog and digital data digitally, voice, video and

data can be integrated


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It is a sinusoidal wave, at higher frequency (than that ofmodulating signal) used for modulation purpose.Mathematically it is expressed as

where,e = instantaneous value of the voltage

Ec = Maximum amplitude of carrier wavefc = frequency of carrier wave = Phase of carrier wave

)2sin()sin( +=+= tfEtEe cccc

WHAT IS CARRIER?


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Varying one of the characteristics(i.e. frequency, amplitude, or phase) ofthe carrier wave, with the modulatingsignal is called modulation.

So, there are three types of modulations1. Amplitude Modulation (AM): where the amplitude of the carrier waveis varied according to

the modulating signal.2.Frequency Modulation (FM): where the frequency of the carrier wave is

varied according tothe modulating signal.

3. Phase Modulation (AM): where the phase of the carrier wave is variedaccording to the

modulating signal.

MODULATIO

N


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There are 3 main reasons to modulate a signal on to ahigh frequency carrier1. A large number of radio transmitters are trying to

transmit at the same time. It is necessary for thereceiver to pick up only the wanted signal and toreject the rest. One way to do to this is to assign a

carrier with a known frequency to each transmitter,modulate this carrier with the signal, and thendesign the receiver to pick up only that knowncarrier frequency and reject the rest, using

appropriate filtering methods. Then the originalsignal is removed from the received carrier. Thesame concept is used in carrying a large number oftelephone conversations over a single pair of wiresor optical fibre.

WHY DO WE NEED MODULATION ?


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WHY DO WE NEED MODULATION ?3.Audio is in the range approx. 20 - 20000 Hz...o The size of antenna should be at least /4,

This much size of antenna is practically impossible.

If more than one station starts transmitting at the same AF (AudioFrequency) there

would be a lot of interference.

kmm

x

f

c

75.337504

1500020000

1038

== ==

=


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AMPLITUDE MODULATION (AM)One of the most fundamental forms of modulation used intelecommunications is am-plitude modulation (AM). The transmissionof music and speech in the commercial AM broadcast band is anexample of the widespread use of AM.

Defining AM

Amplitude modulation is defined as a carrier frequency whoseamplitude is varied in proportion to the instantaneous amplitude of amodulating voltage. The carrier frequency much higher than that of

the modulating voltage. It is usually a Radio Frequency (RF) signal inthe Mid-Frequency (MF) range of 300 kHz to 3 MHz or, higher. Incontrast, the frequency of the modulating voltage is typically an AudioFrequency (AF) signal in the range of 20 Hz to 20 kHz.


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Modulating signal (sine wave) and modulatedcarrier

Modulating signal (Audio) and modulatedcarrier


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1. Amplitude Modulation DerivationA carrier is described by

v = Vc Sin ( c t + )To amplitude modulate the carrier its amplitude is changed in accordancewith the level of the audio signal, which is described by

v = Vm Sin ( m t )The amplitude of the carrier varies sinusoidally about a mean of Vc. When thecarrier is modulated its amplitude is varied with the instantaneous value of the

modulating signal. The amplitude of the variation of the carrier amplitude isVm and the angular frequency of the rate at which the amplitude varies is

m. The amplitude of the carrier is then:Carrier amplitude = Vc + Vm Sin ( m t )and the instantaneous value (value at any instant in time)is

v = {Vc + Vm Sin ( m t )} * Sin ( c t )

= Vc Sin ( c t ) + Vm Sin ( m t ) * Sin ( c t )Using Sin A * Sin B = Cos (A - B) - Cos (A + B) this becomes

v = Vc Sin ( c t ) + Vm Cos ( ( c - m) t ) - Vm Cos (( c + m)t)


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This is a signal made up of 3 signal components

carrier at c (rad/s) Frequency isfc = c/2 Hz

upper side frequency c + m (rad/s) Frequency is ( c + m)/2 = fm + fc Hz

lower side frequency c - m (rad/s)Frequency is ( c - m)/2 = fm - fc Hz

The bandwidth(the difference between the highest and the lowestfrequency) isBW = ( c + m ) - ( c - m)=2 * m Rad/s ( =

m/ Hz)

Carrier


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Amplitude(V)

AngularFrequency

c c+mc -m

Lowerside

frequency

Carrier Upperside

frequency

Bandwidth=2 * m

The spectrum of these signals is shown. This isdescribed as the signal in the frequency domain, asopposed to the signal in the time domain. In this case

the audio signal is made up of a single frequency.In this example the angular frequencies (expressedin Radians/sec, or kRad/sec, or Mrad/sec) are show. Inmost cases however the frequency is shown (expressedin Hz, or kHz, or MHz).


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Modulation Index (or Modulation Factor or Depth of

Modulation)

The Modulation Index or degree of modulation is the ratio of

the peak amplitude of the modulation voltage, to that ofcarrier voltage. It is also called depth of modulation. It is anumber between 0 and 1.

Previous Eqn can be re-written asv = Vc Sin ( c t ) + (Vm Cos ( ( c - m) t ) - Vm Cos (( c +

m)t) ) * Vc /Vcv = Vc { Sin ( c t ) + m [ Cos ( ( c - m) t ) + Cos (( c +

m)t) ] }

cV

mV

mIndexModulation ==

Remember if the modulating signal is too high, (i.e. the modulation index is

greater than 1), it will cause the carrier to cutoff for a period of each cycle. Asa result a part of the envelope will be chopped-off; this is called overmodulation, which must be avoided.


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50%Modulation

-1.5

-1

-0.5

0

0.5

1

1.5

1 9 17

25

33

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100%modulation

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

1 9 17

25

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150%Modulation

-3

-2

-1

0

1

2

3

1 9 17

25

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Carrier

-1

-0.5

0

0.5

1

1 9 17

25

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ModulatingSignal

-1

-0.5

0

0.5

1

1 9 17

25

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ANLAYSIS OF AM WAVE


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The carrier is represented by

And the sinusoidal modulating signal is represented by

The resultant AM wave iswhere

tVvccc

sin=

tVvmmm

sin=

tmv

tmv

tVvmc

c

mc

c

ccAM)cos(

2)cos(

2sin ++=

SB)Sideband(UUpper)t

cos(

2

mv

(LSB)SidebandLower)tcos( 2

mv

waveCarrierofvalueousInstantanetsinV

waveAMofvoltageousInstantanev

mc

c

mcc

cc

AM

=+

=

=

=


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POWER DISTRIBUTION IN THE AMWAVEFORM

PT =Pc + PLSB + PUSB

Since power is proportional to the square of voltage, the power versus-Frequency spectrum for amplitude modulated wave is shown,each sideband has a power content equal to m2 Pc/4, where Pc ispower content of the signal at the carrier frequency.

Thus the total power isPT=m2Pc/4 + m2Pc/4 + PC

Vc

mVc/2 mVc/2

Fc fa fc fc +fa

Pc

m2Pc/4 m2Pc/4

Fc fa fc fc +fa

Voltage and Power Frequency


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Percent Modulation in an AM transmission.

Combining terms, PT = m2Pc/2 + PcPc/PT = 1/(1+m2/2)

for m = 1Pc/ PT = 2/3

Therefore for 100% modulation 2/3rd of thetotal power goesin carrier


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TYPES OF AMPLITUDEMODULATION

The power of an AM signal is given as

Since the information lies only with the modulating signal, not with the carrierso some times we Suppress Carrierto save the transmission power. On theother hand, the frequency spectrum of astandard AMwaveform, shows that

the frequency response is symmetric about the carrier frequency, i.e. theupper and lower sidebands are of same amplitude and at the same distance(on frequency scale) to the carrier. So, some times we suppress a sideband tosave further power. Depending on these facts the following are some types ofAM.

o Single Sideband Suppressed Carrier (SSBSC)

USBLSBcarrierAM PPPP ++=


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v Double Sideband Suppressed Carrier (DSBSC)


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Radio receivers for receiving Full Wave signals are cheap to produce but thetransmitter must be capable of transmitting a lot of power. It is used for

broadcast radio stations in the Medium Wave band because there will beonly one transmitter for a country the size of Ireland but millions of receiversso that the aim is to keep the receivers as cheap as possible.Radio receivers for SSB or Carrier Suppressed signals are expensive toproduce, but the transmitter need not be capable of outputting a high powerlevel. It is used for ship to shore communications (e.g. between a fishing

boat and the harbour master) or for other one-to-one communications. In thiscase there are as many transmitters as receivers, one per boat, so that thereare no major cost savings if the receiver is made a little cheaper, but therewill be major gains if the transmitter can be made cheaper. In addition thepower on the boat (or plane, or other vehicle) may be limited and will beneeded for lighting and other functions, so that it is desirable that as little of

it as possible is needed for communications.


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An unusual form of AM is that used in televisionbroadcasting . A TV signal consists of the picture , videosignal and the radio signal which have different carrier

frequencies. The audio carrier is frequency modulated,but the video information amplitude modulates thepicture carrier. The picture carrier is transmitted, butone sideband is partially suppressed.Video information typically contains frequencies as high

as 4.2 MHz. A fully amplitude modulated television signalwould then occupy 2(4.2) = 8.4 MHz. This is anexcessive amount of bandwidth that is wasteful ofspectrum space because not all of it is required to

reliably transmit a TV signal. To reduce the bandwidth tothe 6 MHz maximum allowed by the FCC or TV signals, aportion of the lower sideband of the TV signal issuppressed leaving only a small vestige of the lower

sideband. Such an arrangement is known as avestigial sidebandsi nal.
http://electrapk.com/what-is-vestigial-sideband-vsb/http://electrapk.com/broadcasting/http://electrapk.com/frequency-modulation/http://electrapk.com/am-amplitude-modulation/http://electrapk.com/what-is-vestigial-sideband-vsb/http://electrapk.com/what-is-vestigial-sideband-vsb/http://electrapk.com/am-amplitude-modulation/http://electrapk.com/frequency-modulation/http://electrapk.com/broadcasting/


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v

Vestigial Sideband (used commonly for TV video signaltransmission)


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1.Analog Data Analog Signal