COMMUNICATION COMMUNICATION SYSTEMSYSTEM

COMMUNICATION : COMMUNICATION :

It is a process of transmitting information It is a process of transmitting information

or signal from one point known as source or signal from one point known as source

to another point known as destination. to another point known as destination.

Information can be continuous such as Information can be continuous such as

music, picture etc. or discrete signals like music, picture etc. or discrete signals like

data from computer etc. data from computer etc.

BLOCK DIAGRAM OF A COMMUNICATION BLOCK DIAGRAM OF A COMMUNICATION SYSTEM:SYSTEM:

II I I NN N N FF F F OO O O RR R R MM M M AA A A TT T T I I I I OO O O N N N N

TX

Noise

TX

CHANNEL

RX OUTPUT

transducer

INPUT

transducer

As shown in the figure above, the first block As shown in the figure above, the first block at the source is an input transducer which at the source is an input transducer which is used to convert physical quantity (non is used to convert physical quantity (non electrical) to electrical quantity. For electrical) to electrical quantity. For example voice is converted to electrical example voice is converted to electrical quantity using microphone. Similarly at the quantity using microphone. Similarly at the destination output transducer is used to destination output transducer is used to convert electrical back to physical quantity. convert electrical back to physical quantity. For example a loudspeaker is used to For example a loudspeaker is used to convert voice signal in the form of electrical convert voice signal in the form of electrical back to physical quantity.back to physical quantity.

There are basically three essential blocks There are basically three essential blocks in a communication system.in a communication system.

Transmitter (TX)Transmitter (TX)

Transmission ChannelTransmission Channel

Receiver (RX)Receiver (RX)

Transmitter : Transmitter :

The output signal of the transducer is a The output signal of the transducer is a complex signal. It is restricted to desired complex signal. It is restricted to desired range of frequencies. On this signal range of frequencies. On this signal modulation is performed. Modulation is a modulation is performed. Modulation is a process of altering the characteristics of process of altering the characteristics of carrier signal in accordance with the carrier signal in accordance with the information. There are basically three information. There are basically three types of modulation techniquetypes of modulation technique

Amplitude modulation.Amplitude modulation. Frequency modulation.Frequency modulation. Phase modulation.Phase modulation.

Transmission channel:Transmission channel:

It is a medium over which the electronic signal is It is a medium over which the electronic signal is transmitted from one point to another. This medium can be transmitted from one point to another. This medium can be either wired or wireless.either wired or wireless.

An example for wired communication is telephony where a An example for wired communication is telephony where a pair of physical wires is running parallel between pair of physical wires is running parallel between transmitter and receiver. Now a days optical fibers are used transmitter and receiver. Now a days optical fibers are used in between transmitter and receiver in which light carries in between transmitter and receiver in which light carries the information. Similarly an example for wireless the information. Similarly an example for wireless communication is radio communication where two communication is radio communication where two antennas are employed, one at the transmitter and other at antennas are employed, one at the transmitter and other at the receiver. The transmitter antenna transmits the the receiver. The transmitter antenna transmits the modulated information into free space and the receiver modulated information into free space and the receiver antenna picks up the modulated information which is later antenna picks up the modulated information which is later demodulated to get the information back. demodulated to get the information back.

The modulated signal is then transmitted over a The modulated signal is then transmitted over a transmission channel.transmission channel.

Noise:Noise: It is a random, undesirable electrical It is a random, undesirable electrical

energy that interferes with the energy that interferes with the transmitted signal. It can be either transmitted signal. It can be either natural noise such as noise caused by natural noise such as noise caused by lightning during rainy season or man lightning during rainy season or man made noise produced by ignition made noise produced by ignition system of cars etc. Noise is a serious system of cars etc. Noise is a serious problem which cannot be eliminated problem which cannot be eliminated but one can reduce the effect caused but one can reduce the effect caused by it on the signal.by it on the signal.

Receiver:Receiver:

It is a collection of electronic circuits It is a collection of electronic circuits designed to convert the modulated designed to convert the modulated signal back to modulating signal. This signal back to modulating signal. This process is known as demodulation. process is known as demodulation. Finally an output transducer is Finally an output transducer is employed to convert back the employed to convert back the information in electrical form to information in electrical form to physical form.physical form.

MODULATION:MODULATION:

Modulation is a process of altering Modulation is a process of altering the characteristics of carrier signal in the characteristics of carrier signal in accordance with the information or accordance with the information or modulating signal. modulating signal.

Carrier signal:Carrier signal: It is a high frequency It is a high frequency signal that carries the information signal that carries the information from transmitter to receiver.from transmitter to receiver.

Need for modulation:Need for modulation:

The height of the antenna required to transmit and receive radio The height of the antenna required to transmit and receive radio waves is a function of wavelength of the frequency used. i.e.waves is a function of wavelength of the frequency used. i.e.λ = λ = c/fc/f. At low frequency, λ is high and hence the height of the . At low frequency, λ is high and hence the height of the antenna should be more to transmit the signal (sinceantenna should be more to transmit the signal (since ‘ ‘λ’ is λ’ is proportional to ‘h’). Therefore high frequencies are used to proportional to ‘h’). Therefore high frequencies are used to transmit the information which requires antenna of lesser height.transmit the information which requires antenna of lesser height.

At low frequency radiation is poor and signal gets highly At low frequency radiation is poor and signal gets highly attenuated. Therefore signal cannot be transmitted over longer attenuated. Therefore signal cannot be transmitted over longer distance. Modulation effectively increases the frequency of the distance. Modulation effectively increases the frequency of the signal to be radiated and thus increases the distance over which signal to be radiated and thus increases the distance over which signal can be transmitted faithfully.signal can be transmitted faithfully.

The modulation permits multiplexing to be used. Multiplexing is The modulation permits multiplexing to be used. Multiplexing is method of transmitting two or more informations method of transmitting two or more informations simultaneously over a single channel. In this method each simultaneously over a single channel. In this method each message signal is modulated using different carrier signal message signal is modulated using different carrier signal frequencies and then transmitted over a single channel. At the frequencies and then transmitted over a single channel. At the receiver the message signals are extracted individually by tuning receiver the message signals are extracted individually by tuning to their respective carrier frequencies.to their respective carrier frequencies.

AMPLITUDE MODULATION:AMPLITUDE MODULATION:

It is defined as a process of varying It is defined as a process of varying the the

amplitude of the carrier wave amplitude of the carrier wave

proportional to the instantaneous proportional to the instantaneous

amplitude of modulating signal. amplitude of modulating signal.

CARRIER AND MODULATING SIGNALSCARRIER AND MODULATING SIGNALS

AMPLITUDE MODULATED SIGNALAMPLITUDE MODULATED SIGNAL

Time domain analysisTime domain analysis

Let the equation of carrier signal be Let the equation of carrier signal be c(t) = Ac(t) = Acccos(2пfcos(2пfcct)t) where A where Acc is the peak amplitude of carrier signal and fc is the frequency of the is the peak amplitude of carrier signal and fc is the frequency of the carrier signal. carrier signal.

Let the equation of modulating signal be Let the equation of modulating signal be m(t) = Am(t) = Ammcos(2пfcos(2пfmmt)t) where Awhere Amm is the peak amplitude of modulating signal and fm is the is the peak amplitude of modulating signal and fm is the frequency of the modulating signal.frequency of the modulating signal.

Then by the definition of AM:Then by the definition of AM:

VVAMAM (t) (t) = [A = [Acc+A+Ammcos(2пcos(2пffmmt)]cos(2пt)]cos(2пffcct)t)

= A= Acccos(2пcos(2пffcct) + (At) + (Amm) cos(2п) cos(2пffmmt) cos(2пt) cos(2пffcct)t)

= A= Acccos(2пcos(2пffcct) + (mAt) + (mACC/2) cos[2п(/2) cos[2п(ffcc + + ffmm)t] + cos[2п()t] + cos[2п(ffcc - - ffmm)t] )t]

Where ‘m’ is the modulation index of AM signal which is defined as Where ‘m’ is the modulation index of AM signal which is defined as ratio of amplitude of modulating signal to that of carrier signal. ratio of amplitude of modulating signal to that of carrier signal.

SIGNIFICANCE OF MODULATION INDEXSIGNIFICANCE OF MODULATION INDEX

The significance ofThe significance of modulation index is, it modulation index is, it decides the depth of modulation. If it is decides the depth of modulation. If it is less than one, then AM signal is known as less than one, then AM signal is known as under modulated signal. If it is more than under modulated signal. If it is more than one, then AM signal is known as over one, then AM signal is known as over modulated signal. If it is equal to one , modulated signal. If it is equal to one , then AM signal is known as perfect then AM signal is known as perfect modulated signal. To obtain the original modulated signal. To obtain the original information, modulation index should information, modulation index should always be less than or equal to one. always be less than or equal to one.

Spectrum of AM signal: Spectrum of AM signal:

AAcc

mAmAcc/2 mA/2 mAcc/2 /2

fc -fmfc -fm fc fc fc + fm f (Hz) fc + fm f (Hz)

As shown in the figure above, the spectrum As shown in the figure above, the spectrum consisted of three frequency components, one consisted of three frequency components, one at fc and other two at fat fc and other two at fcc+ f+ fmm , f , fcc- f- fmm. The . The frequencies ffrequencies fcc+ f+ fmm and f and fcc- f- fmm are known as are known as sideband frequencies i.e. fsideband frequencies i.e. fcc+ f+ fmm is known as is known as upper sideband frequency and fupper sideband frequency and fcc- f- fmmis known is known as lower sideband frequency . The separation as lower sideband frequency . The separation between these two frequencies is defined as between these two frequencies is defined as bandwidthbandwidth of AM signal. Therefore the of AM signal. Therefore the bandwidthbandwidth of AM signal is of AM signal is 2f2fmm..

Total power required to Total power required to transmit AM signaltransmit AM signal

The total power required to transmit AM signal (The total power required to transmit AM signal ( P PTT ) is sum of ) is sum of power power

required to transmit carrier signal ( required to transmit carrier signal ( PPCC ) and power required to ) and power required to transmit side band signals ( transmit side band signals ( PPTSBTSB ). ). Therefore Therefore PPTT = P = PCC + P + PTSBTSB

= P= PCC + P + PLSBLSB + P + PUSBUSB

= (A= (ACC22/2R) + (m/2R) + (m22AACC

22/8R) + (m/8R) + (m22AACC22/8R) /8R)

= P= PCC 1 +( m 1 +( m22/2) /2)

The above equation gives the total power The above equation gives the total power required to transmit AM signal in terms of required to transmit AM signal in terms of carrier power and modulation index.carrier power and modulation index.

For 100% modulation : m = 1, For 100% modulation : m = 1,

Therefore PTherefore PTT = P = PCC 1 +( ½) 1 +( ½)

PPCC = 0.6666 P = 0.6666 PTT Or P Or PCC = 66.66%P = 66.66%PTT

i.e. 66.66% of total power is wasted in i.e. 66.66% of total power is wasted in transmitting carrier signal. transmitting carrier signal.

Current calculation:Current calculation:

PPTT = P = PCC 1 + (m 1 + (m22/2) /2)

IITT22R = IR = ICC

22R 1 + (mR 1 + (m22/2) /2)

IITT = I= ICC 1 + (m 1 + (m22/2) /2) 0.50.5

where ‘Iwhere ‘ITT’ is the current with modulation , ‘I’ is the current with modulation , ‘ICC’ is the ’ is the current without modulation and R is the resistance current without modulation and R is the resistance of the antenna. of the antenna.

Modulation by several Modulation by several sine wavessine waves

In modulation by several sine waves, In modulation by several sine waves, modulating signal consists of several sine modulating signal consists of several sine waves i.e.waves i.e.

m(t)m(t) = A = Am1m1cos(2пfcos(2пfm1m1t) + At) + Am2m2cos(2пfcos(2пfm2m2 t) t)

+ . . .+ . . . For modulation by several sine waves overall For modulation by several sine waves overall

modulation index will bemodulation index will be

mmtt = (A = (Am1m12 2 + A+ Am2m2

22 + …..)/A + …..)/Acc220.50.5

== (m(m112 2 + m+ m22

22 +…….) +…….)0.50.5

Therefore Total power will be: Therefore Total power will be:

Similarly current with modulation Similarly current with modulation will will

be:be:

PT = PC 1 + (mt2/2)

IT = IC [ 1 + (mt2/2) ]0.5

Problems:Problems:

1. 1. An audio signal 10sin (2п1000t) amplitude An audio signal 10sin (2п1000t) amplitude modulates a carrier of 40sin (2п2000t)modulates a carrier of 40sin (2п2000t). . FindFind

Modulation indexModulation index Sideband frequenciesSideband frequencies.. Bandwidth.Bandwidth. Total power delivered if RTotal power delivered if RLL = 1KΩ. = 1KΩ. Amplitude of each side band Amplitude of each side band

componentscomponents

Solution:Solution: i) i) Modulation index:Modulation index: mm = =0.25.0.25. ii)ii) Sideband frequencies : Sideband frequencies : Upper side band = fC + fm = 300Hz.Upper side band = fC + fm = 300Hz. Lower side band = fC - fm = 1000Hz.Lower side band = fC - fm = 1000Hz. iii)iii) Bandwidth = 2fm = 2KHz. Bandwidth = 2fm = 2KHz. iv) iv) Total power delivered:Total power delivered:

PPTT = 1 + (0.25 = 1 + (0.2522)/2)/2 == 1.03125 1.03125

v) v) Amplitude of each sideband = mAAmplitude of each sideband = mAcc/2=5V /2=5V

2. The antenna current of an AM transmitter is 2. The antenna current of an AM transmitter is 8A when only carrier is transmitted, but 8A when only carrier is transmitted, but increases to 8.93A when carrier is modulated increases to 8.93A when carrier is modulated by a single sine wave. Find the percentage by a single sine wave. Find the percentage modulation. Determine the antenna current modulation. Determine the antenna current when the depth of modulation changes to when the depth of modulation changes to 0.8A.0.8A.

Solution:Solution: i)i) Given : Given : IITT = 8.93A. = 8.93A.

IICC = 8A. = 8A.

IITT = I = ICC 1+ m 1+ m22/2/2 m = 0.701 =70.1%.m = 0.701 =70.1%.

ii)ii) IITT = ?, when m = 0.8 = ?, when m = 0.8

IITT = 8 1+(.7) = 8 1+(.7)22/2 = 9.19A ./2 = 9.19A .

EXERCISE:EXERCISE:1. A certain transmitter radiates 9KW with 1. A certain transmitter radiates 9KW with

carrier unmodulated and 10.125KW carrier unmodulated and 10.125KW when carrier is sinusoidally modulated. when carrier is sinusoidally modulated. Calculate modulation index. If another Calculate modulation index. If another sine wave corresponding to 40% sine wave corresponding to 40% modulation is transmitted modulation is transmitted simultaneously, determine the total simultaneously, determine the total power radiated.power radiated.

FREQUENCY FREQUENCY MODULATIONMODULATION

It is defined as a process of altering It is defined as a process of altering thethe

frequency of the carrier signal w.r.t.frequency of the carrier signal w.r.t.

instantaneous amplitude of instantaneous amplitude of modulatingmodulating

signal.signal.

CARRIER AND MODULATING SIGNALSCARRIER AND MODULATING SIGNALS

FREQUENCY MODULATED SIGNALFREQUENCY MODULATED SIGNAL

Time domain analysis: Time domain analysis: From the definition: From the definition:

ffFMFM= f= fCC + K + Kffm(t) m(t)

Where KWhere Kff is known as frequency sensitivity. is known as frequency sensitivity.

ffFMFM= f= fCC + K + Kff A Ammcos(2Πfcos(2Πfmmt)t)

= f= fCC + Δf cos(2Πf + Δf cos(2Πfmmt)t) where Δf is known as frequency deviation. Its where Δf is known as frequency deviation. Its

signifies , by how much amount carrier signifies , by how much amount carrier frequency gets deviated.frequency gets deviated.

Multiplying by 2Π on both sides :Multiplying by 2Π on both sides :

2Πf2ΠfFMFM = 2Πf = 2ΠfCC + 2ΠΔf cos(2Πf + 2ΠΔf cos(2Πfmmt)t)

ωωFMFM = = ωω CC + Δ + Δ ωω cos(2Πf cos(2Πfmmt)t)

ωω FM FM =d=dθθ(t)/dt , integrating both the sides : (t)/dt , integrating both the sides :

ωω FMFMt = t = ωω CCt +(t +(Δ Δ ωω / / ωω mm)sin(2Πf)sin(2Πfmmt)t)

Therefore equation of FM Signal: Therefore equation of FM Signal:

VVFMFM(t) = A(t) = ACCcos[cos[θθ(t)](t)]

= = AACCcos[cos[ωω CCt +t +ββ sin(2Πf sin(2Πfmmt)]t)]

where β = where β = ΔΔf/ff/fm m is defined as modulation index of is defined as modulation index of FM. Unlike AM modulation index is not restricted to FM. Unlike AM modulation index is not restricted to one. It can be more than unity. one. It can be more than unity.

Frequency spectrum:Frequency spectrum: The frequency spectrum of FM signal consisted of The frequency spectrum of FM signal consisted of

infinite number of sideband components ( using infinite number of sideband components ( using Fourier Transform ). Hence theoretically the Fourier Transform ). Hence theoretically the bandwidth of FM signal is infinity. But practically, the bandwidth of FM signal is infinity. But practically, the bandwidthbandwidth of FM signal is restricted using of FM signal is restricted using Carson’s rule.Carson’s rule. According to According to Carson’s ruleCarson’s rule the the bandwidth of FM signal is given by bandwidth of FM signal is given by 2(Δf + fm).2(Δf + fm).

Problems:Problems:

Given a FM equationGiven a FM equation VVFMFM(t) =(t) = 10 cos [2Π1010 cos [2Π1088t + 5 sin(2Π15000t)] , t + 5 sin(2Π15000t)] ,

CalculateCalculate Carrier frequency.Carrier frequency. Modulating frequency.Modulating frequency. Frequency deviation.Frequency deviation. Bandwidth using Carson’s rule.Bandwidth using Carson’s rule.

Solution: Solution: Carrier frequency: fCarrier frequency: fCC = 108Hz. = 108Hz. Modulating frequency : fModulating frequency : fmm = 15KHz. = 15KHz. Frequency deviation : Δf = β fFrequency deviation : Δf = β fmm = 5 * 15 = 75KHz. = 5 * 15 = 75KHz. Bandwidth = 2(Δf + fBandwidth = 2(Δf + fmm ) = 2( 75 + 15 ) = 180KHz. ) = 2( 75 + 15 ) = 180KHz.

In an FM system when the audio frequency is In an FM system when the audio frequency is 50Hz , modulating voltage is 2.5V , the deviation 50Hz , modulating voltage is 2.5V , the deviation produced is 5KHz. If the modulating voltage is now produced is 5KHz. If the modulating voltage is now increased to 7.5V , calculate the new value of increased to 7.5V , calculate the new value of frequency deviation. If the AF voltage is raised to 10V frequency deviation. If the AF voltage is raised to 10V while the modulating frequency is dropped to 250Hz , while the modulating frequency is dropped to 250Hz , what is the frequency deviation produced. Also what is the frequency deviation produced. Also calculate modulation index in each case. calculate modulation index in each case.

Solution:Solution:

Given : fGiven : fmm = 50Hz , A = 50Hz , Amm = 2.5V , Δf = 5KHz. = 2.5V , Δf = 5KHz.

Modulation index: β = Δf/fModulation index: β = Δf/fmm= 5*10= 5*1033/50 = 100/50 = 100

ii) If Aii) If Amm = 7.5V , Δf = ? = 7.5V , Δf = ?

KKff = Δf/A = Δf/Amm=2KHz/V.=2KHz/V.

Δf = KΔf = Kff A Amm = 2*7.5 = 15KHz. = 2*7.5 = 15KHz.

Modulation index: β = Δf/fModulation index: β = Δf/fmm= 300.= 300.

iii) Δf = Kiii) Δf = Kff A Amm = 2*10KHz = 20KHz. = 2*10KHz = 20KHz.

Modulation index: β = Δf/fModulation index: β = Δf/fmm= 800. = 800.

EXERCISE:EXERCISE:

A carrier of amplitude 5V and frequency A carrier of amplitude 5V and frequency 90MHz is frequency modulated by 90MHz is frequency modulated by asinusoidal voltage of amplitude 5V and asinusoidal voltage of amplitude 5V and frequency 15KHz. The frequency sensitivity frequency 15KHz. The frequency sensitivity is 1Hz/V. Calculate the frequency deviation is 1Hz/V. Calculate the frequency deviation and modulation index. and modulation index.

Compare and contrast AM and FM.Compare and contrast AM and FM.

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