EC 6651 Communication Engineering

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3 Communication Engineering
EC6651 COMMUNICATION ENGINEERING LT P C
3 0 0 3
OBJECTIVES:
 To introduce different methods of analog communication and their significance
 To introduce Digital Communication methods for high bit rate transmission
 To introduce the concepts of source and line coding techniques for enhancing rating of
transmission of minimizing the errors in transmission.
 To introduce MAC used in communication systems for enhancing the number of users.
 To introduce various media for digital communication
UNIT I ANALOG COMMUNICATION 9
AM – Frequency spectrum – vector representation – power relations – generation of AM – DSB,
DSB/SC, SSB, VSB AM Transmitter & Receiver; FM and PM – frequency spectrum – power
relations : NBFM & WBFM, Generation of FM and DM, Amstrong method & Reactance
modulations : FM & PM frequency.
UNIT II DIGITAL COMMUNICATION 9
Pulse modulations – concepts of sampling and sampling theormes, PAM, PWM, PPM, PTM,
quantization and coding : DCM, DM, slope overload error. ADM, DPCM, OOK systems – ASK,
FSK, PSK, BSK, QPSK, QAM, MSK, GMSK, applications of Data communication.
UNIT III SOURCE CODES, LINE CODES & ERROR CONTROL (Qualitative only) 9
Primary communication – entropy, properties, BSC, BEC, source coding : Shaum, Fao,
Huffman coding : noiseless coding theorum, BW – SNR trade off codes: NRZ, RZ, AMI, HDBP,
ABQ, MBnBcodes : Efficiency of transmissions, error control codes and applications:
convolutions & block codes.
UNIT IV MULTIPLE ACCESS TECHNIQUES 9
SS&MA techniques : FDMA, TDMA, CDMA, SDMA application in wire and wireless
communication : Advantages (merits) :
UNIT V SATELLITE, OPTICAL FIBER – POWERLINE, SCADA 9
Orbits : types of satellites : frequency used link establishment, MA techniques used in satellite
communication, earth station; aperture actuators used in satellite – Intelsat and Insat: fibers –
types: sources, detectors used, digital filters, optical link: power line carrier communications:
SCADA
TOTAL : 45 PERIODS

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OUTCOMES:
 Ability to understand and analyse, linear and digital electronic circuits.
TEXT BOOKS:
1. Taub & Schiling ―Principles of Communication Systems‖ Tata McGraw Hill 2007.
2. J.Das ―Principles of Digital Communication‖ New Age International, 1986.
REFERENCES:
1. Kennedy and Davis ―Electronic Communication Systems‖ Tata McGraw hill, 4th Edition, 1993.
2. Sklar ―Digital Communication Fundamentals and Applications― Pearson Education, 2001.
3. Bary le, Memuschmidt, Digital Communication, Kluwer Publication, 2004.
4. B.P.Lathi ―Modern Digital and Analog Communication Systems‖ Oxford University Press,
1998.
THIS MATERIAL IS PREPARED WITH THE HELP OF VARIOUS SOURCES AVAILABLE IN INTERNET
AND FROM TEACHING COMMUNITY FOR THE BENEFIT OF STUDENT COMMUNITY.
"THANKS TO ALL THE RESOURCE PERSONS FOR THEIR CONTRIBUTION."

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Unit – I
Analog Communication
Part –A
1. Define amplitude Modulation and draw it’s spectrum.[May-2011, Dec-2012]
Amplitude of the carrier signal varies according to amplitude variations in modulating signal
is called as amplitude modulation.
The AM signal is mathematically represented as
e AM=(Ec+Em sin ωm t)sin ωc t
and the modulation index is given by, m=Em/Ec
The Figure shows the spectrum of AM signal. It consists of carrier (fc) and two sidebands at fc±
fm.
Spectrum of AM wave
2. Define Modulation index and percent modulation for an AM wave. [May-2012
Nov/Dec 14]
Modulation index is a term used to describe the amount of amplitude change present in an
AM waveform .It is also called as coefficient of modulation. Mathematically modulation index is
m = Em/ Ec Where m = Modulation coefficient, Em = Peak change in the amplitude of the output
waveform voltage. Ec = Peak amplitude of the unmodulated carrier voltage.
Percent modulation gives the percentage change in the amplitude of the output wave when the
carrier is acted on by a modulating signal.
3. Distinguish between low level and high level modulation.
In low level modulation, modulation takes place prior to the output element of the final stage of
the transmitter. It requires less power to achieve a high percentage of modulation.
In high level modulators, the modulation takes place in the final element of the final stage
where the carrier signal is at its maximum amplitude and thus, requires a much higher
amplitude modulating signal to achieve a reasonable percent modulation.

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4. Define image frequency.
The local oscillator frequency (fo), input signal frequency (fs), IF(fi) are related as
f0-fs=fi (i.e.) f0= fs+fi
If some other frequency fsi=fs+2fi appears at the input of mixer then it produces fi at the output
of the mixer. This interferes with the desired IF, since it is same as IF. The frequency fsi is called
image frequency. Thus image frequency gets converted to IF range and it is amplified by IF
amplifiers.
5. Define Heterodyning.
Heterodyne means to mix two frequencies together in a nonlinear device or to translate one
frequency to another using nonlinear mixing.
6. What are the disadvantages of conventional (or) double side band full carrier system?
 In conventional AM, carrier power constitutes two thirds or more of the total transmitted
power. This is a major drawback because the carrier contains no information; the sidebands
contain the information.
 Second, conventional AM systems utilize twice as much bandwidth as needed with single
sideband systems.
7. Define Single sideband suppressed carrier AM.
AM Single sideband suppressed carrier is a form of amplitude modulation in which the
carrier is totally suppressed and one of the sidebands removed.
8. Define AM Vestigial sideband.
AM vestigial sideband is a form of amplitude modulation in which the carrier and one
complete sideband are transmitted, but only part of the second sideband is transmitted.
9. What are the advantages of single sideband transmission?
1. Power conservation: only one sideband is transmitted and the carrier is suppressed. So
less power is required to produce essentially the same quality signal.
2. Bandwidth conservation: Single sideband transmission requires half as much bandwidth
as conventional AM double side band transmission.
3. Noise reduction
10.What is the maximum frequency deviation allowed by FCC in standard FM?
FCC allows maximum frequency deviation of 75 KHz and carrier frequency of 100 MHz.

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11. What are the disadvantages of single side band transmission?
1. Complex receivers
2. Tuning Difficulties: receivers require more complex and precise
12. Define direct frequency modulation.
In direct frequency modulation, frequency of a constant amplitude carrier signal is directly
proportional to the amplitude of the modulating signal at a rate equal to the frequency of the
modulating signal.
13. Define indirect frequency Modulation.
In indirect frequency modulation ,phase of a constant amplitude carrier directly proportional
to the amplitude of the modulating signal at a rate equal to the frequency of the modulating
signal.
14. Define frequency deviation.
Frequency deviation is the change in frequency that occurs in the carrier when it is acted on
by a modulating signal frequency. Frequency deviation is typically given as a peak frequency
shift in Hertz (Do).The peak to peak frequency deviation (2Df) is sometimes called carrier
swing.
The peak frequency deviation is simply the product of the deviation sensitivity and the
peak modulating signal voltage and is expressed mathematically as Df=K1 Vm Hz
15. State Carson’s rule of FM bandwidth. [May-2012]
Carson rule states that the bandwidth required to transmit an angle modulated wave as
twice the sum of the peak frequency deviation and the highest modulating signal frequency.
Mathematically Carson‘s rule is BW=2(δ +fm(max)) Hz.
δ = maximum frequency deviation
fm(max)=maximum signal frequency
16. Define Deviation ratio.
Deviation ratio is the worst case modulation index and is equal to the maximum peak
frequency deviation divided by the maximum modulating signal frequency. Mathematically, the
deviation ratio is DR= D f (max) f m(max)

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17. Define transmission efficiency.
It can be defined as the ratio of power in sideband to total power.
% transmission efficiency= power in side band/total power *100
18. Write down the basic principle used in super heterodyne receivers?
[Apr/May-2011]
The super heterodyne receiver converts all incoming RF frequencies to fixed lower frequency
called intermediate frequency. This is then amplitude and detected to get the original signal.
19. What is the need for modulations?
Modulation serves the following purposes:
1. Reduces the height of antenna.
2. Avoids mixing of signals
3. Increase range of communication
4. Allows multiplexing of signals
5. Allows adjustments in the bandwidth
6. Improves quality of reception.
20. What are the advantages of super heterodyne receivers?
1. The selectivity of this receiver is better since its IF amplifiers are narrowband and they
operate only at IF.
2. The design of IF amplifiers is simple since they operate only at IF.
21. Define modulation index for frequency modulation. [Nov-2013]
The modulation index for FM is defined as the ratio of frequency deviation to the
modulating signal frequency.m=
Where, =frequency deviation
Fm=modulating signal frequency
22. State the advantages and disadvantages of FM over AM. [Dec-2012]
FM has following advantages over AM.
i) The amplitude of FM is constant. It is independent of depth of modulation.
ii) Since amplitude of FM constant, the noise interference is minimum in FM.
iii) The depth of modulation has limitation in AM.
Disadvantages:
1. Bandwidth requirement of FM is much higher than AM.
2. FM transmitting and receiving equipment is complex and costly.

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23. Define frequency modulation.
Frequency modulation is defined as the process of changing the frequency of the carrier in
accordance with the modulating s i gn a l . The amplitude of the modulated carrier remains
constant. FM equation is given as, eFM(t)=Ec sin[ωct+msinωmt]
24. What is the required bandwidth of FM signal in term of frequency deviation?
Bandwidth=2(f+fm)hz
Where,f - peak frequency deviation(hz) Fm-modulating signal frequency(hz)
25. For an AM DSBFC modulator with a carrier frequency of 100KHZ and
maximum modulating signal frequency of 5KHZ determine upper and lower side band
frequency and bandwidth.
Upper sideband, fusb=fc+fm
=100+5=105khz
Lower sideband, flsb=fc-fm
=100-5=95 KHz
Bandwidth (B)= 2fm=10KHz
26. The carrier frequency of an FM broadcast transmission is 100 MHz and maximum
frequency deviation is 75 KHz. Find the bandwidth of the signal when the highest audio
modulating carrier is 15 KHz.
Ans:∆f=75 KHz, Fm=15KHz
Bandwidth= 2mf.fm
Mf=∆f/fm= 75*103
/15*103
=5
Bandwidth= 2*5*15*103
=150 KHz
27. Draw the FM and PM waveforms.

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28 .One input to a convention AM modulator is a 500 KHz carrier with an amplitude of 20
Vp. The second input is a 10 KHz modulating signal that is of sufficient amplitude
to cause a change in the output wave of ±7.5 Vp.
Determine: a) Upper and lower side frequency B) Modulation efficiency
Ans: The upper and lower side band frequency is simply the sum and difference of frequencies
FUSB=fc+fm
= 500 khz + 10 Khz
= 510 khz
flSB= fc-fm = 500khz-10KHZ
FLSB=490KHZ
b) Modulation co-efficient, m=Em/Ec m=7.5/20 =>m=0.375
Modulation efficiency, M=m*100
=0.375*100; M=37.5%
29. When a superhetrodyne receiver is tuned 555KHZ, i t s local oscillator provides
the mixer with an input at 1010KHZ. Find the Image frequency.
Ans: Fs=555KHZ Fo=1010KH
Fi= fo – fs =1010-555 =455KHZ
Image frequency, fsi=fs+2fi = [555+2(455)] KHZ =1465KHZ.
30. Why carrier frequencies are generally selected in HF range than low frequency
range?
The antenna size is very large at low frequencies. Such antenna is practically not possible to
fabricate. High carrier frequencies require reasonable antenna size for transmission and
reception.

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High frequency can be transmitted using tropospheric scatter propagation which is used to
travel long distances.
31. The amplitude of an AM wave is , eAM=100[1+0.7cos
Find the amplitude and frequency of various sideband terms.
Ans: The given equation can also be written as,
eAM=[100+70 cos
Here Em1=70 and ω1= rad/sec
Em2=30 and ω2= rad/sec
Ec=100 and ωc= rad/sec
Here m1=Em1/Ec= 70/100=0.7 and m2= Em2/Ec=30/100= 0.3
Figure shows the frequency spectrum:
Frequency spectrum of AM wave
32. Calculate percent modulation in AM if carrier amplitude is 20 V and modulating signal
is of 15 V.
Ans: Here Em=15V Ec=20 V
Modulation Index: m=Em/Ec=15/20= 0.75
Percent modulation: =m*100=75%
33. What do you understand by narrow band FM?
When the modulation index is less than 1 the angle modulated systems are called low index.
The bandwidth requirement of low index systems is approximately twice of the modulating
signal frequency. Therefore low index systems are called as narrow band FM.

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34. Why Armstrong method of FM is superior to reactance modulator?
Reactance modulator is direct FM where as Armstrong method is indirect FM. Armstrong
method generates FM from PM. Hence crystal oscillators can be used in Armstrong method.
Therefore frequency stability is better than reactance modulator.
35. Differentiate between narrow band FM and wideband FM?
 In narrow band FM the frequency deviation is very small. Hence the frequency spectrum
consists of two major sidebands like AM. Other sidebands are negligible and hence they can
be neglected. Therefore the bandwidth is limited only to twice of highest modulating
frequency.
 If the deviation in carrier frequency is large enough so that other sidebands can‘t be
neglected, then it is called as wideband FM. The bandwidth of wideband FM is calculated as
per Carson‘s rule.
36. Define PM.
In phase modulation the phase of the carrier varies according to amplitude variations of the
modulating signal. The Pm signal can be expressed as,
ePM=Ec sin(ωc t+mp sin ωm t)
mp =modulation index for phase modulation It is given by, mp =Φm
Φm = maximum value of phase change.
37. What is meant by indirect method of FM generation?
In the indirect method FM is generated from PM. The phase modulated signal is represented
as, ePM=Ecsin(ωct+mpsinωmt)
The modulated frequency has deviation of ∆f with respect to fc. And ∆f=mfmsin (2𝞹fmt)
Here maximum deviation=∆f=mfm
If fm remains constant then frequency deviation will be directly proportional to m. Thus as long
as modulating frequency does not change phase modulation produces FM output.
38. What is the maximum frequency deviation allowed by FCC in standard FM?
FCC allows maximum frequency deviation of 75 KHz and carrier frequency of 100 MHz.
39. A 107.6 MHz carrier is frequency modulated by a 7 KHz sine wave. The resultant FM
signal has frequency deviation of 50 KHz. Determine the modulation index of FMwave.
Solution: Here δ=50 KHz and fm=7KHz
Modulation index= δ/fm=50Khz/7KHz = 7.142
40. What is AGC?
Automatic gain control (AGC) keeps the output signal level constant irrespective of the
increase or decrease in the signal level at the input of the receiver. The AGC circuit takes part

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of the detected signal and derives a dc control voltage for RF, mixer and IF stages. This control
voltage acts as negative feedback and controls the overall gain of these stages. The gain is
varied such that output signal level is constant.
41. Why FM signal is less susceptible to noise than an AM signal?
In FM the frequency of the carrier is varied as per the amplitude variations of the modulating
signals. The amplitude of FM signal remains constant. The interference of external noise can be
easily removed by amplitude limiter. Hence FM is less susceptible to noise.
42. A carrier signal with power of 40 watts is amplitude modulated by a sinusoidal signal.
Find the power of the modulated signal if the modulation index is 0.7.
Solution: Here Pc=40 W m=0.7
Power of the modulated signal is given by,
Ptotal=Pc (1+ ) =40(1+ )= 49.8 W
43. Draw the pre-emphasis circuit. [Dec-2011]
44. AM transmitter radiates 9 KW with the unmodulated carrier and 10.125 KW when the
carrier is modulated. Calculate the modulation index. [Dec-2011]
Ans: Pc=9KW Ptotal= 10.125 KW
m=
45. Find the transmission power efficiency for a tone modulated signal when modulation
index is 0.5.
46. Define the propagation constant of transmission line. [Apr/May 15]
The propagation constant of an electromagnetic wave is a measure of the change
undergone by the amplitude of the wave as it propagates in a given direction. The quantity

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being measured can be the voltage or current in a circuit or a field vector such as electric field
strength or flux density.
47. Why is VSB preferred for TV video transmission? [Nov/Dec 14]
VSB is mainly used for TV transmission, since low frequencies near fc represent significant
picture details. They are unaffected due to VSB.

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Unit – I
Analog Communication
Part –B
:
1. Explain in detail about the general communication systems?
Definition:
Communication is the process of establishing connection (or link) between two points for
information exchange.
The science of communication involving long distances is called telecommunication (the word
tele standing for long distance)
 The two basic types of communications systems are analog and digital.
 Analog systems, both the information and the carrier are analog signals.
 In digital, the digital pulses are transferred between two or more points in a
communication system.
GENERAL COMMUNICATION SYSTEM
The basic communication components are
 A transmitter, communication channel or medium, and A receiver
Block diagram of a general communication system
Information Source: The information signal can be analog or digital.
 The original source information can be in analog form, such as the human voice, or
music, or in digital form, such as binary coded numbers or alphanumeric codes.
Transmitter:
 A transmitter is a collection of electronic components and circuits designed to convert the
information into a signal suitable for transmission over a given communications medium.
 In addition to that it increases the power level of the signal.
Communication Channel:
 The communication channel is the medium by which the electrical signal is sent from one
place to another.
 Two types of communication systems will exist.
(i) Wire Communication or Line Communication:
The line communication systems use the communication mediums like the simple wires or
cables or optical fibers.
Here message transmission capability is also limited.
(ii) Wireless Communication or Radio Communication:

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―In wireless communication, the information signal is transmitted through free space by
electromagnetic waves called radio waves. Radio waves are radiated from the transmitter in
open space through a device called antenna.
The radio or TV broadcasting, satellite communication are the examples of the wireless
communication.
Wireless communication is mostly used for long distant communication as well as wide
coverage area.
Receiver:
The main functions of the receiver are,
(i) Receive the message signal from channel and Reproduce the message signal in
electrical form from the distorted received signal from channel.
Noise and Distortion:
 Noise is random, undesirable electric energy that enters the communication system via
the communication medium and interferes with the transmitted message.
Drawbacks of Base and Transmission (Without Modulation)
(i) Excessively large antenna heights.
(ii) Signals get mixed up.
(iii) Short range of communication.
(iv) Multiplexing is not possible and
(v)Poor quality of reception
2. Explain the need of modulation in communication systems?
Modulation may be defined as the process by which some parameter of a high frequency
signal termed as carrier, is varied in accordance with the information (modulating or baseband)
signal.
 The carrier is supposed to carry the message signal or information from transmitter to
receiver.
Frequency, Wavelength and Bandwidth
Frequency (f): Frequency is defined as the number of cycles of a waveform per second. It is
expressed in hertz (Hz).
Wavelength (λ): Wavelength (A) is defined as the distance between two points of similar cycles
of a periodic wave.
Wavelength is also defined as the distance traveled by an electromagnetic wave during the time
of one cycle.

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I 1
1
λ = speed of light(c )/frequency ( f) = 3 * 10 8
m/s
f
Bandwidth: Bandwidth is defined as the frequency range over which information signals is
being transmitted.
NEED OF MODULATION (OR) ADVANTAGES OF MODULATION:
The advantages of modulation are:
(i) Easy of radiation,
(ii)Adjustment of bandwidth,
(iii) Reduction in height of antenna,
(iv) Avoids mixing of signals,
(v) Increases the range of communication,
(vi) Multiplexing, and
(vii) Improves quality of reception.

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Electronics and Communication Engineering Department 19 Communication Engineering
Easy of Radiation:
As the signals are translated to higher frequencies, it becomes relatively easier to design
amplifier circuits as well as antenna systems at these increased frequencies.
Adjustment of Bandwidth:
Signal to noise ratio in the receiver, is a function of the signal bandwidth, which can be
improved by proper control of bandwidth at the modulating stage.
Reduction in Height of Antenna:
When free space is used as a communication media, messages are
 transmitted and received with the help of antennas.
 In broadcast systems, the max. audio frequency transmitted from a radio station is 5kHz.
If signals are to be transmitted without modulation, the size of antenna needed for an
effective radiation is the order of half of the wavelength.
λ/2 = c/2f = 3*10 8
/2 * 5 *10 3
=30,000m = 30 km
The antenna of this height is practically impossible to install.
Now consider a modulated signal f = 10MHz.
Minimum antenna height = λ/2 = c/2f = 3*10
8
/2 * 10 *10
3
=30,000m = 15 meter.
Avoids Mixing of Signals:
Each modulating signal (message signal) is modulated with different carrier then they will
occupy different slots in the frequency domain (different channels). Thus modulation avoids
mixing of signals.
Increases the Range of Communication:
 The frequency of the baseband signals is low, and low frequency signals cannot travel
a long distance when they are transmitted
Multiplexing:
 Different message signals can be transmitted over a same channel without
interference using multiplexing techniques which is possible in modulation.
Improves Quality of Reception:
 Due to modulation, the effect of noise is reduced to a great extent. This improves
quality of reception.
There are three types of modulation
(i) Amplitude modulation
(ii) Angle modulation
(iii) Pulse modulation

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Pulse modulation
Analog Digital
(i) Pulse amplitude modulation(PAM) (i) Pulse code modulation(PCM)
(ii) Pulse position modulation(PPM) (ii) Delta modulation(DM)
(iii) Pulse duration modulation (ill) Adaptive delta modulation (ADM)
(iv) Differential pulse code modulation (DPCM)
Figure. Classification of Modulation
Demodulation or Detection:
The process of extracting a modulating signal from the modulated signal is called
demodulation or detection.
3. Explain in detail about the mathematical representation of AM wave and AM
frequency spectrum and bandwidth? (Or) Derive an expression for AM wave.
[May-2012]
Amplitude modulation is the process by which amplitude of the carrier signal is varied
in accordance with the instantaneous value of the modulating signal, but the frequency and
phase remains constant.
Mathematical representation
Let the modulating signal Vm(t) = Vmsinωmt ----------------(1)
Carrier signal Vc(t) = Vcsinωct -----------------(2)
Where
Vm = amplitude of the carrier signal(volts)
Vc = amplitude of the modulating signal (volts)
The amplitude of the carrier signal is changed after modulation.
VAM (t) = VC +Vm(t) ------------------(3)
Sub equation (1) in (3)
=Vc+Vmsinωmt
= Vc (1+Vm / Vc sin ωmt )
VAM (t) = Vc (1+masin ωmt ) --------------------(4)
Hence AM wave is given by
VAM (t) = VAMsinωct -------------(5)
Sub equation (4) in (5)

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Where
ma =modulation index
VAM (t) = Vc(1+ma sinωmt ) sin ωct -----------------(6)
This expression represents the time domain representation of an AM signal.
AM Envelope
 The shape of the modulated signal is AM envelope.
 It contains all the frequency that make up the AM Signal And used communicate the
information through the system.
Nomodulati
"Carrier |
modulation
signal
Expression for AM Frequency Spectrum and Bandwidth
An AM modulator is a non nonlinear device. Therefore, non linear mixing occur and the
output envelope is a complex wave made up of a dc voltage, the carrier frequency, and the
sum (fc + fm) and difference (fc - fm) frequencies
Frequency spectrum of AM:
The AM wave is given by
VAM (t)=VC [1+masinωmt] sinωct
= VCsinωct +masinωmtsinωct
We know that, sinωmtsinωct =
VAM (t)=VCsinωct + maVC/2 [ ]

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VAM (t)=VCsinωct + maVC/2 [ ]--- --(7)
Figure: Frequency domain representation of AM wave Sidebands
Observations:
The expression for the AM wave shows that it consists of three terms:
(i)First term is nothing else but the unmodulated carrier signal
(ii)The second term represents a sinusoidal signal at frequency (fc-fm). It is called as the lower
sideband. Its amplitude is maVC/2.
(iii)The third term is a sinusoidal signal at frequency (fc+fm). It is called as the upper
sideband. Its amplitude is maVC/2
Sidebands:
♦ Whenever a carrier is modulated by an information signal, new signals at different
frequencies are generated as part of the process. These new frequencies are called side
frequencies or sidebands
♦ The sidebands are occurs in the frequency spectrum directly above and below the carrier
frequency.
♦ Assuming a carrier frequency of fc and a modulating frequency of fm, the upper side band
fUSB and lower sideband fLSB are computed as follows:fUSB=(fc+fm)
fLSB= (fc-fm)
Bandwidth of AM:
The bandwidth of the AM signal is given by the subtraction of the highest and the
lowest frequency component in the frequency spectrum.
B=fUSB-fLSB
=(fc+fm) - (fc-fm)
B=2xfm-----------------------(9)
Where, B - Bandwidth in hertz
fm - Highest modulation frequency in hertz.
Thus bandwidth of AM signal is twice of the maximum frequency of modulating signal.

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munication Engineering Department 23 Communicati
Phasor Representation of an AM with Carrier
♦ The amplitude variation in an AM system can be explained with the help of a phasor
diagram.
♦ The phasor for the upper sideband rotates anticlockwise at an angular frequency of ωm.
♦ Similarly, the phasor for the lower sideband rotates clockwise at the same angular
frequency ωm.
♦ The upper side frequency rotates faster than the carrier (ωm>ωC) and the lower side
frequency rotates slower (ωm<ωC).
♦ The resulting amplitude of the modulated wave at any instant is the vector sum of the two
sideband phasors.
♦ VC is carrier wave phasor, taken as reference phasor and the resulting phasor is VAM(t).
♦ The phasors for the carrier and the upper and lower side frequencies combine, sometimes
in phase (adding) and sometimes out of phase (subtracting).
Figure: Phasor representation of AM with carrier
4. Explain about the degree of modulation?
 The modulating signals preserved in the envelope of amplitude modulated signal only if
Vm<Vc, then ma< 1.
Where, Vm = Maximum amplitude of modulating signal.
Vc = Maximum amplitude of carrier signal.
❖ In AM, three types of degree of modulation are available. It depends upon amplitude of the
modulating signal relative to carrier amplitude.
1. Under Modulation: ma< 1 when Vm<Vc
 Here the envelope of amplitude modulated signals does not reach the amplitude axis.
AM wave with ma< 1 i.e., Vm<Vc
Envelope
Electronics and Com on Engineering

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Figure: AM wave for percentage modulation less than 100%

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Hence the message signal is fully preserved in the envelope of the AM wave.
 An envelope detector can recover the message signal without any distortion.
2. Critical modulation: ma = 1 when Vm = Vc
 Here the envelope of the modulated signal just reaches the zero amplitude axis.
 The message signal remains preserved.
AM wave with ma =1,i.e., 100% modulation Vm = Vc
r t ' "
Figure :AM wave with 100% modulation
3. Over Modulation: ma>1 when Vm>Vc
 Here both positive and negative extensions of the modulating signals are cancelled(or)
clipped out.
 The envelope of message signal is not same. Due to this envelope detector provides
distorted message signal.
Distortion due to over
Figure: Over modulation m a > 1
5. Explain in detail about modulation index and percentage modulation?
In AM wave, the modulation index (ma) is defined as the ratio maximum amplitude of
modulating signal to maximum amplitude carrier signal.
ma= --------------(10)

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Value of Vm must be less than value of Vc (Vc>Vm) to avoid distortion in the modulated signal.
Hence the maximum value of ma is 1 (ma= 1).
The modulation index should be a number between 0 and 1.
Percent Modulation:
When modulator index is express in percentage, it is called percentage modulation.
For example, ma = 0.5 corresponds to 50% modulation.
%modulation = x 100 or simply ma x 100-----------------(11)
Calculation of modulation index from AM waveform:
This is also called time domain representation of AM signal.
From the above figure, we can write
2Vm= Vmax- Vmin
Vm= ----------------- (12)
VC= Vmax- Vm-------------- (13)
V =-V -V V =-V +V
Figure: AM wave for calculation of m„
Substitute equation (12) in equation (13)
VC= Vmax-
=Vmax- +
Vm ------------------ (14)
Modulation index ma= ----------------------- (15)
So substituting the values of Vm and Vc from the equations (12) and equation (15), we get
ma=

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ma= -----------------(16)
where
Vmax=VC+Vm
Vmin=VC-Vm
The modulation index is a number lying between 0 and 1, and its very expressed as a
percentage and called the percentage modulation.
% ma= x100---------------------- (17)
The peak change in the amplitude of the output wave Vm is the sum of IP voltages from the
upper and lower side frequencies.
VUSB=VLSB
Vm=2VUSB
Vm=VUSB+VLSB
VUSB= ------------------------- (18)
VLSB = VUSB = ------------------ (19)
Substitute equation (12) in equation (19)
=
VUSB=VLSB= (Vmax- Vmin) --------------- (20)
Where,VLSB- Peak amplitude of the Lower Side Frequency (volts)
VUSB- Peak amplitude of the upper Side Frequency (volts)
For 100% modulation VM = VC, the maximum amplitude of the envelope Vmax
the minimum amplitude of the envelopeVMIN = 0.
vm = vc
=
2VC and
Vmax = 2Vc
Figure: 100% modulated wave
Modulation index for multiple modulating frequencies:

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When two or more modulating signals are modulated by a single carrier. Then the modulation
index is given by,
Where,
2 2
ma = √m1 +m2+ ………….. ----------------------------- (21)
ma = total resultant modulation index
m1, m2…….. = Modulation indices due to individual modulating components
6. Derive the power relationship of amplitude modulated signal. [May-2011,
May-2012.]
This equation relates total power of AM wave to carrier power. Maximum value of modulation
index, m=1 to avoid distortion. At this value of modulation index, PTotal =1.5 Pc. From above
equation we have,
Transmission Efficiency:
Fig: Power spectrum for an AM wave
The amount of useful message power present in AM wave is expressed by a term called
transmission efficiency.
The transmission efficiency of an AM wave is the "ratio of the transmitted power which
contains the information (Le., the total sideband power) to the total transmitted power"
% power in sideband /total power x100 = PLSB + PuSB / Pt x100

VI SEM
Advantages of AM:
(i) Simple modulators and demodulators.
(ii) AM is a relatively inexpensive.
(iii) AM wave can travel a long distance.
Disadvantages of AM:
(i) Poor performance in the presence of noise.
(ii) Inefficient use of transmitter power.
(iii) Wastage in bandwidth
7. Explain method of generation of an amplitude modulated signal and sketch the time
domain waveform of message, carrier and modulated signals? [May-2011]
Generation of AM- DSB
The AM modulator circuits can be of two types: low level or high level modulator. This
depends upon placement of the modulator in transmitter.
Low Level AM Modulator (Emitter Modulator)
It has two inputs:
1) Carrier signal and
2) Modulating signal.
 The modulating signal is applied to the emitter; hence it is also called emitter modulation
 The modulating signal (em) varies the gain of the amplifier according to amplitude
variations. The gain is given as,
 Here, Av is voltage gain with modulation. Aq is voltage gain without modulation.
Since sin (ωmt) varies from 0 to 1,

VI SEM
Advantages of low level modulation:
Waveforms of low level modulator
1. Modulator operates at low voltage level. Hence less power consumption in
the modulator.
2. The circuit is very simple, since it is basically class A amplifier.
Disadvantage of low level modulation:
1) The modulator operates in class A, hence it's power efficiency is very low.
2. Medium Power AM Modulator (Collector Modulator, High Level Modulator)
The modulated output can be obtained by making the voltage on output electrode to vary
according to input modulating signal.
Operation of the circuit

VI SEM
 The RF drive is a carrier signal used for AM. This carrier amplitude is such that it
drives transistor in conduction over part of its cycle. It is applied to the base of
transistor.
 The modulating signal is passed through the power amplifier and applied to the collector
through a low frequency transformer.
 This voltage is shown as vm (t) in figure. This modulating voltage is in series with the
supply voltage Vcc. Hence the collector voltage becomes
 The tuned LC circuit and associated tuned transformer on the collector receives the AM
signal. Because of modulating voltage, the net supply voltage of transistor changes
according to slow variations in vm (t).
 Hence the RF carrier signal amplitude is also varied according to variations in vm (t).
Thus AM signal is produced across the LC circuit at the collector.


VI SEM
Advantages of high level modulation:
1) Power efficiency is practically higher than 80 9'0.
2) All the preceding linear amplifiers operate at low power level.
Disadvantages of high level modulation:
1) Requires high amplitude of modulating signal.
2) Amplifier is nonlinear; hence it generates intermodulation components (higher
order harmonics).
8. Explain a method of generating a double sideband SC signal using balance
modulators.[May-2012, Nov-2013]
DSB-SC
If only carrier is suppressed and both the sidebands are transmitted, then it is called
Double Sideband Suppressed Carrier (DSB-SC) or DSB system.
Suppression of the carrier:

VI SEM
The balanced modulator is used to suppress the carrier from the AM signal.
Balanced modulator using diodes:
 It is called lattice type balanced modulator.
 Both the figure a and b are same. The diode bridge is shown in two different ways.
 The modulator consists of input transformer T1, output transformer T2 and four
diodes.
 The modulating signal is applied to the input of transformer T1 and carrier signal is
applied to center tap of two transformer T1 and T2.
 The DSB output is collected at the secondary of transformer T2. Let us consider that
modulating input is zero.
 In positive half cycle of the carrier signal diodes D1 and diode D2 are forward biased
and D3 and D4 are reverse biased.

VI SEM
 That the current divides equally in the upper and lower portions of the primary winding of
T2.
 The current in the upper part of the winding produces a magnetic field that is equal and
opposite to the magnetic field produced by the current in the lower half of the secondary.
 As magnetic fields are equal and opposite, they cancel each other, producing no output at
the secondary of T2. Thus the carrier is suppressed.

VI SEM
 In the negative half cycle, D1 andD2 are reverse biased and diodes D3 and D4 are
forward biased as shown in Fig. Similar to positive half cycle, here also magnetic
fields in primary winding of T2 are equal and opposite cancelling each other.
 Therefore there is no output produced at the secondary of T2. Consider that a
sinusoidal modulating signal is applied to the primary of T1. This signal will also appear
across the T1 secondary.
 In the positive half cycle, the diodes D1 andD2 are forward biased and they will
connect the secondary of T1 to the primary of T2. As a result, the modulating signal at
the secondary of T1 is applied to primary of T2 through diodes Di andD2.
 In the negative half cycle diodes D3 andD4 are forward biased and they will connect
the secondary of T1 to the primary of T2 with reverse connections. This inverts the
polarity of modulating signal when it is applied to primary of T2. Fig. shows DSB signal
at the primary of T2.
 Thus when D3 and D4 conduct, the polarity of the signal is opposite to that of
modulating signal. Figure shows the DSB output at the secondary of T2.Thus carrier is
totally suppressed in the signal.


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Balanced Modulator using FETs
 The balanced modulator can also be built using FETs. Fig. shows the circuit diagram
of balanced modulator using FETs. There are three transformers T1, T2 and T3.
 The carrier signal is applied to the center taps of input transformer T1 and output
transformer T3 through the transformer T2. The modulating signal is applied to the
input transformer T1.
 The carrier signal is applied to primary of transformer T2. This signal is further applied
to two gates of FETs in phase through the secondary of T2.
 The modulating voltage appears 180° out of phase at the gates, since these are the
opposite ends of the center tapped transformer.
 Consider that there is no modulating signal applied. Then FET currents due to carrier
signal are equal in amplitude but opposite in the directions.

VI SEM
 These opposite and equal currents at the primary of the output transformer cancel each
other.
 No output is produced at the secondary of T3. Thus carrier is suppressed. When
modulating signal is applied, the currents id1 andid2 flow in the primary of T3 due to carrier
signal as well as the modulating signal.
 The FET currents due to carrier are equal and opposite and cancel each other. Since the
modulating signal is applied 180° out of phase at the gates, the FET currents due to
modulating signal are equal but not opposite, hence do not cancel each other.
 Therefore output is produced at the secondary of the transformer T3. Thus DSB output is
produced by FET balanced modulator.
To prove that balanced modulator produces DSB output.
Now let us show mathematically that the FET balanced modulator really produces DSB
output. The transfer curve Id versus Vgs of a FET is almost parabolic and may be
approximated .
9. What are the advantages of single sideband modulation technique? Explain any one
method of SSB generation? (or) How does the phase-shift method effectively suppress
the unwanted sideband? Explain with diagram. (nov/dec 14)
If carrier and one of the sideband in AM is suppressed, then only one
sideband remains. It is then called Single Sideband Suppressed Carrier
(SSB-SC) system. Only upper sideband is transmitted. SSB-SC is also called
SSB transmission.
Advantages of SSB:
 This bandwidth is half of that required by DSB-FC system. Thus SSB conserves
bandwidth and allows more number of channels to be transmitted in the same band.
 The power of the suppressed carrier and sideband is saved. Hence transmitter power
IJ
requirement in SSB is reduced.
 Because of narrow bandwidth of SSB, the effect of noise at the receiver circuits is
reduced. This gives better quality of reception in SSB.

VI SEM
 Fading effect is absent because of SSB. Fading effect arises at the receiver
because of two sidebands and carrier interferes with each other at the receiver.
These techniques are (i) filter method, (ii) phase shift method and (iii) The 'third' method.
(i) Filter Method to Produce SSB
 The balanced modulator is provided with both the modulating and the carrier signal and
the output is the sum and differences signal (DSBSC) signal.lt is then given to the filter
which is made to select only one sideband and reject or attenuate the other sideband.
 Usually, the voice signal is transmitter using SSB system. Since the modulation is done at
the low frequency before transmission, it has to be converted to suitable high frequency.
 To achieve this process, a mixer is used which again produces a sum and difference
frequency at the output.
 The filter selects the high frequency component to get single side band high frequency
(approximately 10 MHZ).
Applications
 Since the modulation and demodulation is complex, costlier, this system is not used for
commercial broadcasting.
 It is mainly used in wireless system for ultra-high frequency and very high frequency
communication process.

VI SEM
(ii) Phase Shift Method to Generate SSB
 Balanced modulator M1 generates USB & LSB but each one is shifted by +90°. Balanced
modulator M2, produces USB and LSB but USB is shifted by +90° and LSB by -90°.
 The outputs of the balanced modulator are added by the summing amplifier. Since the
USB of both the modulator are shifted by +90°, they are in phase and add to produce
double amplitude signal.
 But LSB of both the modulators M1 and M2 are 1800
out of phase and hence cancel each
other.
 Thus the output of summing amplifier contains only upper sideband signal. The carrier is
already suppressed by the balanced modulators.
 Hence the output of the summing amplifier is only SSB signal.
 It can be shown mathematically how the sidebands add and cancel each other because of
phase shifts.
 The input to the balanced modulator, M1 are m(t)=sin m t and c'(t) = sin (ct + 90°)
Mathematically output of the balanced modulator M1 is V1(t) which can be expressed as

VI SEM
Vi(t) and V2(t) are added in the summing amplifier .The phase shifted LSB signal
are cancelled whereas the USB signal which are in phase with each other are added as
shown below.
Using above equation we get
Advantages
1. Any desired sideband in a single frequency translation step, regardless of the
carrier frequency can be generated.
2. No need of sharp cut off filter.
Disadvantages
1. The modulator should have equal sensitivity to the base band signal.
2. The Phase shifting network must provide exact 90° phase shift.
10. Illustrate the operation of VSB transmission. Dec-2011

VI SEM
11. Explain in detail about AM transmitters with block diagram?
High Level AM Transmitter
 The crystal oscillator generates carrier frequency. The buffer amplifiers and driver amplifiers
amplify the power level of the carrier to required value.
 The amplified carrier is given to class C modulator amplifier. The modulating signal is audio
signal and given to audio amplifier.
 It is further amplified by audio power amplifier at a level suitable for modulation. The
class C modulator amplifier modulates the carrier input according to modulating audio
signal.
 The output of the class C modulating amplifier is AM and it is given to antenna through some
antenna matching network.
 The antenna matching network is generally tuned LC circuit in collector circuit of modulator
amplifier. In this AM transmitter, the modulator amplifier operates at high power levels and
delivers power directly to the antenna. This is called High level modulated AM transmitter.
Low level AM transmitter:
 In this block diagram, observe that a linear class B power amplifier is used after class
C modulator amplifier.

VI SEM
 The linear class B power amplifier performs the major power amplification and feeds the
amplified AM signal to antenna.
 In this block diagram, the modulator amplifier performs modulation at relatively low
power levels. Hence this is called low level modulated AM transmitter.
 The modulated AM signal is amplified by class B power amplifier to avoid distortion in
the output.
AM broadcast transmitter:
AM broadcast transmitter using partially modulated driver stage
 The crystal oscillator generates carrier signal. The buffer amplifier raises the power
level of this carrier. The driver stage is class C modulator amplifier. It is partially
modulated.
 This signal is further modulated at high power levels by class C modulated power
amplifier.
 This type of modulation has two advantages -
o If one or more tubes in main amplifier fail, the driver stage still provides
modulated output which can be transmitted. This avoids total shut down of
transmitter.
o The total power is distributed in driver and main amplifier.

VI SEM
12. Discuss the principle of AM based radio frequency receiver with block diagram.
[Dec-2013]
 The output of AM detector is audio signal. The detected audio signal is then amplified by
audio power amplifier. The speaker finally produces the sound.
 The RF amplifier is tuned to specific frequency which is to be received. Hence if other
frequency is to received, then another TRF receiver should be used.
 This problem can be solved by using the RF amplifier tuned to wide range of
frequencies. But this reduces selectivity of the receiver.
Advantages:
1) Most simplest type of receiver since it does not involve mixing and IF operation.
2) Very much suitable to receive single frequency.
3) TRF receivers have good sensitivity.
Disadvantages:
1) Bandwidth changes with center frequency, when TRF receiver is used to receive
wide range of frequencies.
2) Multiple stages of RF amplifiers are used.
Due to above disadvantages, TRF receivers are not practically used in AM reception.
Super heterodyne receivers are commonly used since they offer many advantages.

VI SEM
Super heterodyne Receiver
 The problems of TRF receiver are overcome in this receiver. The super heterodyne
receiver converts all incoming RF frequencies to a fixed lower frequency, c alled
Intermediate Frequency (IF).
 This IF is then amplified and detected to get the original signal.
Block diagram of super heterodyne receiver.
 The antenna receives all the frequency signals and gives it to RF amplifier. The RF
stage consists of preselector and RF amplifiers.
 Preselector:
o It is broad-tuned band pass filter. The center frequency of preselector is
adjustable and it is tuned to desired carrier frequency. Preselector provides
enough band limiting such that image frequency does not enter the receiver.
o Preselector reduces noise bandwidth of the receiver and reduces overall receiver
bandwidth.
 RF Amplifier :
o The RF amplifier amplifies the signals in the required range of frequencies. It
provides initial gain and selectivity.
o The output of the RF amplifier is given to the mixer stage. The local oscillator
output is also applied to the mixer.
 Let us assume that local oscillator frequency is f0 and signal frequency is fs. The signal
frequency fs, and local oscillator frequency f0 are mixed in the mixer in such a way that
frequency difference (f0 —fs) is produced at the output of mixer.

VI SEM
 This difference (f0 —fs) is called Intermediate Frequency (IF). The signal at this IF
contains the same modulation as the incoming signal. The IF is amplified by one or
more IF amplifier stages and given to the detector.
 Most of the gain and selectivity is provided by these IF amplifiers. Normally IF is fixed for
the AM receivers.
 To select a particular station, the local oscillator frequency A, is changed in such a way
that the frequency 'fs,' of that station and 'f0.
has the difference equal to IF. Thus
whatever is the station being tuned, the IF is fixed.
 Thus the IF amplifiers and detector operate at the single frequency i.e. IF. Hence the
bandwidth of the IF amplifiers is relatively narrow.
 A part of output is taken from the detector and it is applied to RF amplifier, mixer and
IF amplifiers for gain control. This is called Automatic Gain Control or AGC. This AGC
maintains the constant output voltage level over a wide range of RF input signal
levels.
 The detector obtains the modulating signal from the modulated IF. The output of
detector is amplified and given to speaker.
Advantages of Super heterodyne Receiver
1. The selectivity of this receiver is better since its If amplifiers are narrow band and
the operate only at IF.
2. The design of IF amplifiers is relatively simple since they operate only at IF.
Performance Parameters of Receivers
The performance of a Radio receiver is measured on the basis of its selectivity, sensitivity,
fidelity and image frequency rejection.
 Selectivity:The selectivity is the ability of the receiver to select a signal of a desired
frequency while rejecting all others.
 Sensitivity:The ability of the receiver to pick up weak signals and amplify them, is called
sensitivity.
 Fidelity: The ability of the receiver to reproduce all the range of modulating frequencies
equally is called fidelity of the receiver.
V

VI SEM
13. Explain in detail about FM and PM frequency spectrum? (Or) CO1-H2 -Nov/Dec 14
Derive the equation for the spectrum of FM signal.[ Nov/Dec 13]
FM and PM Waveforms
Fig. shows the waveforms of FM and PM.
In this figure following observations can be noted:
(i) For FM signal, the maximum frequency deviation takes place when modulating signal is
at positive and negative peaks.
(ii) For PM signal the maximum frequency deviation takes place near zero crossings of the
modulating signal.
(iii) Both FM and PM waveforms are identical except the phase shift.

VI SEM
14. Explain the generation of an frequency modulated signal using reactance
modulation scheme with neat diagram? [May-2011]
 The frequency of the carrier is varied according to amplitude changes in the modulating
signal. The carrier frequency is generated by LC oscillators.
 The carrier frequency can be changed by varying either the inductance or capacitance of
the tank circuit.
There are two types of FM modulators:
I) Direct FM: In this type of angle modulation, the frequency of the carrier is varied directly by
the modulating signal. This means, an instantaneous frequency deviation is directly
proportional to amplitude of the modulating signal.
II) Indirect FM: In this type of angle modulation FM is obtained by phase modulation of the
carrier. Instantaneous phase of the carrier is directly proportional to the amplitude of the
modulating signal.
Direct FM
Direct FM can be obtained by using FET and varactor diode.
1. FET Reactance Modulator
 It behaves as reactance across terminals A-B. The terminals A-B of the circuit may be
connected across the timed circuit of the oscillator to get FM output.
 The varying voltage (modulating voltage) V, across terminals A-B changes
reactance of the FET. This change in reactance can be inductive or capacitive.
From the circuit, impedance of the FET

VI SEM
 Here Ceq =gm CR. Thus the impedance of FET is capacitive reactance. By
varying the modulating voltage across FEI. the operating point gm, can be
varied.
 Hence this varies C. This change in the capacitance will change t h e
frequency of the oscillator. If we connect inductance instead of capacitor, we
get inductive reactance in the circuit.
2. Frequency modulation using varactor diode:
 All the diodes exhibit small junction capacitance in the reverse biased condition.
The varactor diodes are specially designed to optimize this characteristic.
 The junction capacitance of the varactor diode changes as the reverse bias across it
is varied.
 The variations in capacitance of this diode are wide and linear. The varactor
diodes provide the junction capacitance in the range of 1 to 200 pF.
 L1 and C1 form the tank circuit of the carrier oscillator.
 The capacitance of the varactor diode depends upon the fixed bias set by R1 and
R2 and the AF modulating signal.
 Either R1 or R2 is made variable so that the center carrier frequency can be
adjusted over a narrow range.
 The Radio Frequency Choke (RFC) has high reactance at the carrier frequency to
prevent the carrier signal from getting into the modulating signal circuits.
 At positive going modulating signal adds to the reverse bias applied to the
varactor diode D, which decreases its capacitance and increases the carrier
frequency.

VI SEM
 A negative going modulating signal subtracts from the bias, increasing the
capacitance, which decreases the carrier frequency.
Indirect FM generation
15. Explain in detail about direct method of FM transmitter?
FM Transmitters
Depending upon the modulation there are two types of FM transmitters:
Direct and Indirect.
i) Direct FM Transmitter :
 Such transmitters produce the FM signal whose frequency deviation is directly
proportional to the modulating signal. Therefore carrier oscillator frequency is
directly deviated.
 For this purpose crystal oscillators cannot be used since their frequency
cannot be varied significantly. Therefore other oscillators are used.
ii) Indirect FM Transmitter :
 Such transmitters produce the FM signal whose phase deviation is directly
proportional to amplitude of modulating signal.
 The frequency of the oscillator is not directly varied by modulating signal voltage.
Hence crystal oscillation can be used in indirect FM transmitters.
 Need for automatic frequency correction: The frequency of the oscillator is
directly varied in direct FM.

VI SEM
 Hence such oscillations do not produce stable frequency. This problem can be
overcome with the help of AFC.
Automatic Frequency Correction (AFC)
The automatic frequency correction is incorporated in FM transmitter to keep
carrier frequency stable. Fig. 23.1 shows the block diagram of AFC circuit.
 The discriminator reacts only to small changes in the carrier frequency but not to
the frequency deviations in the carrier (since it is too fast).
 Suppose frequency of the carrier increases. This higher frequency is fed to the
mixer for which the other input frequency is from the stable crystal oscillator.
 A somewhat higher frequency will be fed to the discriminator. Since the
discriminator is timed to the correct frequency difference which should exist
between the LC oscillator and crystal oscillator, and its input frequency is
now somewhat higher, the discriminator will develop a positive d.c. voltage.
 This voltage is applied to the reactance modulator whose transconductance is
increased by the positive voltage developed by the discriminator. This
increases the equivalent capacitance of the reactance modulator thereby
decreasing the oscillator frequency.
 The frequency increase in the carrier frequency is thus lowered and brought to
the correct value.
 The correcting d.c. voltage developed by the discriminator may be fed to a
varactor diode connected across the tank circuit of the oscillator and be used for
AFC purposes.
 In directly modulated FM transmitters, many times the frequency modulation is
carried out at a lower frequency and with a smaller frequency deviation.
Then passing this frequency modulated wave through frequency multiplier circuit,
the desired carrier frequency and desired frequency deviation is achieved.
Direct FM Transmitters

VI SEM
Here we will discuss two transmitters that generate FM by modulating the carrier
frequency directly.
Crosby Direct FM Transmitter (Broadcast Band FM Transmitter)
 Fig. shows the block diagram of cross by direct FM transmitter. The modulating signal is
given to frequency modulator and oscillator.
 The frequency modulator can be reactance modulator or voltage controlled oscillator. The
frequency of the unmodulated carrier is fc = 5.1 MHz.
 This frequency is multiplied by 18 to generate the transmitted frequency of 91.8 MHz. It is
the center frequency of the FM signal.
 The AFC loop is similar to that shown in Fig. It is used to maintain the center frequency of
unmodulated carrier stable.
 Note that the multiplier output given to the mixer is 6fc = 30.6 MHz. The crystal reference
oscillator generates 28.6 MHz. The mixer generates the 2 MHz difference of these two
frequencies and gives it to the discriminator through amplifier.
 The discriminator is tuned to 2 MHz. If there is difference in the mixer output frequency,
then discriminator generates DC correction voltage.
 If the multiplier frequency is exactly 6fc = 30.6 MHz (i.e. fc = 5.1 MHz), then no frequency
correction is required and hence DC correction voltage must be zero. However the
frequency of 6fc contains FM.
 This means there is frequency deviation of 6f, depending upon modulating signal. Hence
DC correction voltage also contains corresponding variation.
 Therefore DC correction voltage is passed through lowpass filter to remove the effect of
modulation. Such filtered voltage is then used for frequency correction.

VI SEM
16. With a neat diagram explain the operation of Armstrong frequency modulation
system (or) Indirect FM Transmitter. [Dec-2011, May-2012, Nov-2013]
 Direct methods of FM generation are not suitable practically for broadcasting
purposes. We have seen earlier that FM is one form of phase modulation. Hence it
is possible to obtain FM from PM. This method is called indirect method to generate FM.
Block diagram of Armstrong method
 The crystal oscillator generates the carrier frequency fc. This is highly stable
frequency source. The modulating signal is amplified and given to balanced
modulator.
 The balance modulator generates DSB AM signal at carrier frequency This
DSB signal is phase shifted by 90° in the phase shifter.
 This phase shifted AM signal is added (vector addition) with the carrier signal in the
combining network. The combining network produces the FM signal at its output.
 Observe that AM signal is having frequency 1- with amplitude variations. Hence
resultant vector addition is phase modulated which is basically FM signal.
 The buffer isolates the crystal source from combining network so that its
stability is not disturbed.
 In the above equation observe that if modulating frequency fm remains
constant, then frequency deviation will be directly proportional to in.
 Thus as long as the modulating frequency does not change, phase modulation
produces FM output. This technique is employed in indirect method.
 The phase modulated signal is obtained by vector addition of carrier and
modulating signal as shown in Fig. The modulating signal vector AB adds to the

VI SEM
carrier signal vector OA with 90' phase shift. The resultant phase modulated
vector is OB with phase shift of ∆0.
 But this type of scheme works only if both the vectors OA and AB have same
frequency. This means carrier and modulating signal should have same
frequency. Under this condition phase modulation produces FM output as we
have stated earlier.
 To obtain the modulating signal of same frequency as that of carrier, amplitude
modulation is used.
 The AM signal is first produced which amplitude modulates the carrier of
frequency fc. This AM signal vector is shifted by 90' and added to carrier (f,)
signal vector. Then as shown in Fig., the resultant vector is produced which is
phase modulated.
 Since both the vectors, AM and carrier, have same frequency fc, the output is
FM. Here the frequency fc, can be thought of as frequency of the modulating
signal (Note here that modulating signal is AM signal). Hence phase
modulation produces FM output.
17. Write the comparison between AM and FM.
S.No
.
Amplitude Modulation Frequency Modulation
1. Amplitude of the carrier is varied according
to amplitude of modulating signal.
Frequency of the carrier is varied according to
amplitude of the modulating signal.
2.
AM has poor fidelity due to narrow
bandwidth.
Since the bandwidth is large, fidelity is better.

VI SEM
3. Most of the power is in carrier hence loss
efficient.
All the transmitted power is useful.
4. Noise interference is more. Noise interference is minimum.
5. Adjacent channel interference is present. Adjacent channel interference is avoided duo
to wide bandwidth.
6. AM broadcast operates in NW and HF
range
FM broadcast operates in VHF and UHF
range.
7. In AM only carrier and two sidebands are
present
Infinite numbers of sidebands are present.
8. The transmission equipment is simple.
The transmission equipment is complex.
9. Transmitted power varies according to
modulation index.
Transmitted power remains constant
irrespective of modulation index.
10. Depth of modulation has limitation. It
cannot be increased above 1.
Depth of modulation has no limitation. It can
be increased by increasing frequency
18. Write the comparison between FM and PM.
Sr. No. Frequency Modulation Phase Modulation
1. The maximum frequency deviation depends
upon amplitude of modulating voltage and
modulating frequency.
The maximum phase deviation depends only
upon the amplitude of modulating voltage.
2. Frequency of the carrier is modulated by
modulating signal.
Phase of the carrier is modulated by modulating
signal.
3.
Modulation index is increased as modulation
frequency is reduced and vice versa.
Modulation index remains same if modulating
frequency is changed.
19. Describe the relationship between FM and PM. [May-2011]
Relationship / Difference between FM and PM
 The basic difference between FM and PM lies in which property of the carrier is
directly varied by modulating signal.
 Note that when frequency of the carrier varies, phase of the carrier also varies and
vice versa.

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 But if frequency is varied directly, then it is called FM. And if phase is varied directly,
then it is called PM. The instantaneous phase deviation is denoted by 0(It is the
instantaneous change in phase of the carrier with respect to reference phase.
 The instantaneous phase of the carrier is precise phase of the carrier at a given
instant. It is mathematically expressed as,
Instantaneous phase = ωct+θ(t) rad ... (1.8.1)
Here θ (t) is instantaneous phase deviation and
ωc is carrier frequency.
20. What are the relative merits of high levels modulation and low level modulation in
AM transmission? The anode dissipation of a class C power amplifier is 944 walts
when modulation depth is 60%, the efficiency of a power amplifiers is 60 % while that
the modulator is 25%
Find
1. Carrier power and modulator tube dissipation when modulation depth is 100%
2. AF output and rating of the modulation value to affect 100% modulation.
3. Overall efficiency at 60% modulation depth. (Dec-2012)
Solution:
i) Carrier power and tube dissipation:
Since the efficiency of power amplifier is 60% the remaining power vis tube dissipation.
Hence tube dissipation is 40%
Tube dissipation= (944*60)/40= 1416 W
Input of power amplifier=Output of modulator=1416W/0.6
Modulated output= 2360W
Pt=Modulated input=Modulated output/Modulator efficiency= 2360/0.25= 9440
Carrier power, Pc= Pt/(1+(m2
/2))= 9440/(1+(12
/2)), since m=100%
=6293.3 W
Tube loss in modulator= Modulator input power- Modulator output power
=9440-2360= 7080 W
ii) AF output :
AF output= Modulator input power- Carrier power= 9440-6293.3= 3146.7 W
iii) Overall efficiency= η= ηPA* ηmod=0.6 *0.25 =0.15 or 15%

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24. Explain the operation of foster seeley discriminator with diagram.(or) FM
demodulator. [Apr/May 15]

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Unit – II
Digital Communication
Part –A
1. State sampling theorem [Nov/Dec-2012, Nov/Dec -2013]
A bandwidth signal of infinite energy which has no frequency components higher than W
Hz is completely described by specifying the values of signal at instants of time separated by
Ts seconds.
2. Define selectivity and sensitivity of a receiver.[ Apr/may-2009]
Selectivity:
It is a receiver parameter that is used to measure the ability of the receiver to accept a
given band of frequencies and all other.
Sensitivity:
Sensitivity of receiver is the maximum RF signal level that can be detected at the input to
the receiver and sill produce a usable demodulate signal.
3. Why is delta phase shift keying the most common form of PSK? [Nov/Dec-
2009]
 In DPSK, the binary input information is contained in the difference between two
successive signaling elements rather than the absolute phase.
 The difference in the phase of 2 signaling elements determine the logic condition of the
data, it not required to coherent demodulation, DPSK is commonly used.
4. What are the advantages of super heterodyne receiver? [Apr/May-2009]
1) The selectivity of this receiver is better since its IF amplifiers are narrowband and
they operate only at IF 2) The design of IF amplifiers is simple since they operate only at
IF.

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5. Define differential transmission line .[Apr/May-2010]
With two –wire balanced lines ,both conductors carry current however, one conductor
carrier the signal, other conductor is return path, this type transmission is called differential
transmission. or balanced signal transmission. The signal propagating down the wire
measured as the potential difference between the two wire.
6. Define maximum usable frequency. [Apr/May-2010]
The maximum usable frequency is the highest frequency is the highest frequency that
can be used for sky wave propagation between specific points on earth surface
MUF = Critical frequency * SPC I
7. What is the difference between standard FSK and MSK? [Apr/May-2010]
S.NO FSK MSK
1. In the FSK modulating signal shifts
the output frequency between
predetermined levels
The MSK yields orthogonally of I and Q channels
is that which results in a phase shift of 𝞹/2
radians/sec. FSK with this derivation is called MSK
2. Used for higher frequency Used in GSM
8. What are the elements of digital communication systems? [Nov/Dec-2010]
Digital communication system consists of following elements:
1. Source encoder: The symbols generated by the source are converted to digital form.
2. Channel encoder: The channel encoder adds parity bits to the source symbol for error
detection and correction.
3. Modulator: The encoded digital data modulates the carrier. This modulation helps to
transmit the signal over long distances.
4. Demodulator: The demodulator converts the modulated signal to encoded digital signal at
the receiver.
5. Channel decoder: The channel decoder detects and corrects errors in the received digital
data.

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6. Source decoder: The source symbol is obtained back from its digital form.
9. How does the phase of carrier vary for the message m(n) ={1,0,1,1.0,1}?.
[Nov/Dec-2010] m(n)
={1,0,1,1.0,1} phase
= { 0, ,0,0, ,0}
10. What are the advantages of digital transmission?
The advantage of digital transmission over analog transmission is noise immunity.
Digital pulses are less susceptible than analog signals to variations caused by noise.
• Digital signals are better suited to processing and multiplexing than analog signals.
• Digital transmission systems are more noise resistant than the analog transmission
systems.
• Digital systems are better suited to evaluate error performance.
11. Define pulse code modulation.
In pulse code modulation, analog signal is sampled and converted to fixed length, serial
binary number for transmission. The binary number varies according to the amplitude of the
analog signal.
12. Define Heterodyning. [Nov/Dec-2011]
Heterodyne means to mix two frequencies together in a nonlinear device or to translate
one frequency to another using nonlinear mixing.

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13. What is the purpose of the sample and hold circuit?
The sample and hold circuit periodically samples the analog input signal and converts
those samples to a multilevel PAM signal.
14. What is the Nyquist sampling rate?[ Nov/Dec-2009]
Nyquist sampling rate states that, the minimum sampling rate is equal to twice the
highest audio input frequency.
15. Define coding efficiency.
Coding efficiency is the ratio of the minimum number of bits required to achieve a certain
dynamic range to the actual number of PCM bits used. Mathematically, coding efficiency is
Coding efficiency= Minimum number of bits (including sign bit) X 100 Actual number of bits
(including sign bit)
16. Define slope overload. How it is reduced.
The slope of the analog signal is greater than the delta modulator can maintain, and is
called slope overload. Slope overload is reduced by increasing the clock frequency and by
increasing the magnitude of the minimum step size.
17. Define granular noise. How it is reduced. [Nov/Dec-2010]
When the original input signal has relatively constant amplitude, the reconstructed signal
has variations that were not present in the original signal. This is called granular noise.
Granular noise can be reduced by decreasing the step size.
18. Define QAM.
Quadrature amplitude modulation is a form of digital modulation where the digital
information is contained in both the amplitude and phase of the transmitted carrier
19. Define slope overload distortion. [May-2012]
In delta modulation, the rate of rise/fall of input signal is very high at some time instants.
This rapid change of input signal cannot be achieved by staircase signal generated by the
predictor. The step size ‗δ ‗is too small for the predictor to follow the rapid changes in input
signal. Hence there is large difference between the actual signal and predicted signal. The
difference introduces the distortion. It is called as slope overload distortion.

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20. How can we reduce the slope overload distortion.
Slope overload error in DM system can be eliminated by
(i)Filtering the signal to limit the maximum rate of change.
(ii) Increasing step size.
(iii) Increasing sample rate. (fs)
21. When granular noise occurs?
When the step size is too large compared to small variations in the input signal, then
granular noise occurs.
22. Define adaptive delta modulation?
 Adaptive delta modulation is a delta modulation system where the step size of the
AC is automatically varied depending on the amplitude characteristics of the
analog input signal.
 The performance of delta modulator can be improved significantly by making the step
size of the modulator as a time-varying form i.e., the step of the modulator is increased,
when the input is varying rapidly and the step size of the modulator is decreased, when
the input is varying slowly.
 Then the system which uses the above technique for reducing the quantization error is
called adaptive delta modulation (ADM) system.
23. Define bit time and baud rate? [ Apr/May-2009,Apr/may-2015]
Bit time: it is the reciprocal of the bit rate.Baud rate (Nb): The rate of change of a signal on
the transmission medium after encoding and modulation have occurred. Baud=1/ts
24. What are the advantages of QPSK?
(i) Low error probability
(ii) Very good noise immunity
(iii) Effective utilization of available bandwidth is possible because baud rate is half the bit
rate.

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25. Draw the ASK and FSK signal for the binary signal s(t)=1011001.
FSK signal
26. Draw the phasor diagram of QPSK .
27. Compare BPSK and QPSK .
Binary PSK
1. Two different phases are used to represent two binary values.
2. Each signal element represents only one bit.
QPSK
1. Four different phases are used to represent two binary values.
2. Each signal element represents two bits

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28. For an 8 PSK system operating with and information bit rate of 48 kbps.
Determine 1) baud rate 2) minimum bandwidth 3) bandwidth efficiency.
Given: fb=48kbps
Solution:
(a) Baud = fb = 48000 = 16000
N 3
(b) Bandwidth, B = fb = 48000 = 16000
N 3
(C)Bandwidth efficiency = Transmission bit rate (bps) = 48000bps = 3bps
Minimum bandwidth (hz) 16000hz
29. What is binary phase shift keying?
The carrier phase is changed between 00
and 1800
by the bipolar signal. With BPSK two
phases are possible for the carrier. One represents logic 1 and the other phase represents
logic 0.
30. List out the advantages and disadvantages of FSK.
Advantages
(i)Easy to implement.
(ii)It has better noise immunity than ASK .So probability of error free reception of data is high.
Disadvantages
(i)High bandwidth requirement.
(ii)It is used only for low bit rate (1200bps) applications.
31. Compare QAM and QPSK. [CO2-L2]
Parameter QPSK QAM
Type of modulation Quadrature phase modulation Quadrature amplitude and phase
modulation
Noise immunity Better than QASK Poorer than QPSK
Probability of error Less than QASK more than QPSK

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complexity Less complex than QASK More complex than QPSK
32. For the signal m(t)=3cos500t +4sin1000t, determine the nyquist sampling rate
Ans: Ts = 1/2f m sec
= 1 /2 x500 =1/1000 =0.001 Sec
33. Define quantization error [Dec-2011]
Quantization error is defined as the difference between amplitudes of quantized sample
and original sample amplitude. i.e.
Quantization error (ε)=xq(nTs)-x(nTs)
xq(nTs)= quantized value of the sample.
x(nTs)=original sample amplitude.
34. Define intersymbol interference (ISI).
The transmitted signal will undergo dispersion and gets broadened during its
transmission through the channel. So they happen to collide or overlap with the adjacent
symbols in the transmission. This overlapping is called Inter Symbol Interference.
35. What is nyquist rate, nyquist interval and aliasing [Apr/May-2011, Apr/May-2012
Nyquist rate
When the sampling rate becomes exactly equal to 2W samples per second, for a signal
bandwidth of W Hertz, then it is called Nyquist Rate.
Aliasing

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When the signals are sampled at the rate less than nyquist (i.e. fs < 2W), then aliasing
takes place. Frequencies higher than ‗W‘ takes of lower frequencies in sampled spectrum.
This is called aliasing. Aliasing can be reduced by sampling at a rate higher than nyquist rate.
Nyquist Interval
It is the time interval between any two adjacent samples when sampling rate is Nyquist
rate.
Ts = 1 sec
2fm
36. What is aperture effect?
During the flat top sampling, to convert varying amplitudes of pulses to flat top pluses we use
a sinc function. Because of this, there would be decrease in amplitude. This is named as
aperture effect.
37. Define pulse code modulation.
In pulse code modulation, analog signal is sampled and converted to fixed length, serial
binary number for transmission. The binary number varies according to the amplitude of the
analog signal.
38. Define frequency deviation?
It is half the difference between mark and space frequencies. F=|fm-fs|/2
39. Compare PAM, PWM and PPM.
S.No PAM PWM / PDM PPM
1. Amplitude of the Width of the pulse is proportional The relative position of the

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pulse is proportional
to the amplitude of the
modulating signal
to the amplitude of the
modulating signal
pulse is proportional to the
amplitude of the modulating
signal.
2. The bandwidth of the
transmission channel
depends
on width of the pulse.
The bandwidth of the transmission
channel depends on rise time of
the pulse.
The bandwidth of the
transmission channel
depends on rising time of
the pulse.
3. The instantaneous
Power of the transmitter
varies.
The instantaneous
power of the
transmitter varies.
The instantaneous power of
the transmitter
remains constant.
4. Noise is interference is
high.
Noise is interference is minimum. Noise is interference is
minimum.
5. Similar to amplitude
modulation.
Similar to
Frequency modulation.
Similar to phase modulation.
40. Define PSK and BPSK? [Dec-2012]
PSK: The phase of the carrier signal is varied according to the binary input signal.
BPSK: The carrier phase is changed between 0and 180 by the bipolar signal. With BPSK
two phases are possible for the carrier. One represents logic 1 and the other phase
represents a logic 0.
As the input digital signal changes state (from 1 to 0 or from 0 to 1), the phase of the output
carrier shift between two angles that are separated by 180.

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41. Write a short note on Baseband and pass band data transmission?
Base band: The digital data is transmitted over the channel directly. There is no carrier or
any modulation. This is suitable for short distance transmission.
Pass band: The digital data modulates high frequency sinusoidal carrier. Hence it is also
called as digital continuous wave modulation. It is suitable for transmission over long
distances.
42. Write the advantages of MSK system?
Constant envelope.
Spectral efficiency.
Good BER performance.
Self-synchronizing capability.
MSK is a spectrally efficient modulation scheme and it is particularly attractive for use in
mobile radio communication systems.
43. Define pulse modulation.
In pulse modulation some parameter of a carrier pulse train is varied in accordance with the
message signal.
44. What is sampling process?
The sampling process is a process of converting a continuous time signal into an equivalent
discrete time signal.
In sampling process an analog signal is converted into a corresponding sequence of samples
that are usually spaced uniformly in time.

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45. Define sampling theorem.
A continuous time signal can be completely represented in its sample and received back. If
the sampling frequency is twice of the highest frequency content of the signal.i.e. fs≥2fm
Here fs= sampling frequency
Fm=maximum frequency of the continuous signal
46. Write the types of sampling method.
There are three sampling methods that can be employed
i. Ideal or instantaneous sampling
ii. Natural sampling
iii. Flat top sampling
47. Define Nyquist rate of sampling.
When the sampling rate becomes exactly equal to 2 fm samples /sec, for a signal bandwidth
fm. Hence then it is called Nyquist rate.
It is the minimum sampling rate required to represent the continuous signal faithfully in its
sampled form. fs=2fm samples/sec.
Nyquist rate= 2W Hz.
48. Define companding with respect to PCM.
The signal is amplified at low voltage levels and attenuated at high voltages level. This is
called as compression. Uniform quantization is used after compression. This is equivalent to
more step size at low voltage levels and small step size at high voltage levels. That is signal
is attenuated at low voltage levels and amplified at high voltage levels to get original signal.
Thus the compression of signal at transmitter and expansion at receiver is called
companding.

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49. Why binary ASK is called as on-off keying?
In binary ASK the carrier is transmitted for binary ‗1‘ and no carrier for binary‘0‘. Thus the
carrier is simply turned on for binary‘1‘ and it is turned off for binary‘0‘. Therefore it is called
as on-off keying.
50. Sketch the PSK waveform for the bit sequence: 1 0 0 1 0 0 1 1 [CO2-H2]
51. List any two advantages and limitations of DPCM.
Advantages: 1. DPCM requires less bandwidth compared to PCM.
2. Its signal to noise ratio is better than DM and ADM.
Limitations: 1. Implementation of DPCM is complex compared to PCM.
2. Slope overload distortion and quantization noise is present in DPCM.
1. DPCM requires high sampling frequency.
52. State the principle of working of differential pulse code modulated systems.
The differential pulse code modulation works on the principle of prediction. The value of the
present sample is predicted from the past samples. The prediction may not be exact but it is
very close to the actual sample value. The difference between unquantized input sample and
its predicted value is obtained. This error signal is encoded and transmitted.

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53. What is meant by coherent reception?
In coherent reception, the local carrier generated at the receiver is phase locked with the
carrier at the transmitter. The detection is done by correlating received noisy signal and
locally generated carrier. The coherent reception is also called as synchronous detection.
54. What is quadrature phase shift keying?
 In QPSK two successive bits are taken together. Such two bits form four distinct symbols.
When the symbol is changed to next symbol the phase of the carrier is changed by 45‘ (
/4 radians). Since QPSK transmits two bits at a time, its bandwidth requirement is
reduced. The phase changes in QPSK are not as abrupt as BPSK.
55. Define bandwidth efficiency.
It is the ratio of transmission bit rate to minimum required bandwidth. i.e.
BW efficiency= bits/cycle.
56. Give the difference between coherent and non-coherent receiver.
Coherent receiver: Non-coherent receiver:
1. Locally generated carrier at the receiver
is phase locked with the carrier at the
transmitter.
No need of carrier to be phase locked with
the carrier at the transmitter.
2. System is complex but error probability
is less.
System is simple but error probability is
more.
57. State shannon’s capacity limit.
Shannon‘s capacity limit is given by,
C=B log2(1+ )

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This is highest capacity that can be achieved for the channel of bandwidth ‗B‘ and signal to
noise power ratio of
58. Compare the performance of FSK and PSK based on the power and bandwidth
efficiency.
S.No Modulation
method
Transmission rate Minimum
bandwidth
Bandwidth
efficiency
1 BPSK 2fb 2fb 1
2 BFSK 2fb 4fb 0.5
3 QPSK 2fb fb 2
4 DPSK 2fb fb 2
5 QASK 2fb 2fb/N N
59. What is guard band?
In frequency division multiplexing the signals from adjacent channels tend to interfere with
each other. It is called as adjacent channel interference. It can be reduced by isolating the
adjacent cells with enough frequency gap. This gap is called as guard band.
60. How is PPM obtained from PWM? [May-2011]
The width of the carrier pulses various in proportion with the amplitude of modulating signal.
The amplitude and width of the pulses are kept constant but the position of each pulse is
varied in accordance with the amplitude of the sampled values of the modulating signal
Figure shows the scheme to obtain PPM from PWM. The PWM signal given as a triggering
signal to monostable multivibrator. The multivibrator triggers on falling edge of PWM.

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The monostable multivibrator generates its output pulse of fixed duration after being triggered
by falling edge of PWM.
61. Compare MSK and QPSK systems. [Dec-2011,Nov/Dec 14]
S.No QPSK MSK
1 There is abrupt phase shift There is continuous phase shift
2 There are amplitude variations There are no amplitude variations
3 Main lobe contains 90% of total energy. Main lobe contains 99% of total energy.
62. Consider an audio signal em(t)=2cos(1000𝞹t).Find the signal to noise ratio when the
signal is quantized using 8 bits.
Ans: The audio signal is sinusoidal. It is quantized using v=8nbits. The signal to noise ratio is
given by,
( = 1.8 + (6*8) = 49.8 dB
63. What is the principle of delta modulation? [Nov-2013]
Delta modulation transmits only one bit per sample. That is the present sample value
is compared with the previous sample value and the indication, whether the amplitude is
increased or decreased is sent. Input signal x(t) is approximated to step signal by the delta
modulator. This step size is fixed.
The difference between the input signal x(t) and staircase approximated signal
confined to two levels, i.e. +δ and—δ. If the difference is positive, then approximated signal is
increased by one step i.e. δ. If the difference is negative, then approximated signal is reduced
by ' δ‘

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64. What is shape factor? [Apr/May 15]
The shape factor of a filter is defined as the ratio between its 60 dB bandwidth and 6 dB
bandwidth. It gives a measure of how steeply the filter‘s attenuation increases beyond the
passband. An ideal bandpass filter has a shape factor of unity.
65. What are the most four common method of pulse modulation? [Apr/May 15]
1. Pulse code modulation
2. Delta modulation
3. Adaptive delta modulation
4. Differential pulse code modulation

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Unit – II
Digital Communication
Part –B
1. Explain in detail about sampling theorem? (or) State and prove sampling
theorem. [May-2012]
Sampling Process: The first operation in digital communication is the sampling.
Representation of CT Signals by its Samples: A CT signal cannot be processed in the digital
processor or computer. the CT signal is sampled at t = 0, Ts, 2Ts, 3Ts ... and so on.
Here sampling theorem gives the criteria for spacing Ts between two successive samples.
The samples xδ (t) must represent all the information contained in x (t).
The sampled signal xδ (t) is called discrete time (DT) signal. It is analyzed with the help of
DTFT and z-transform.
Sampling Theorem for Low pass (LP) Signals
A low pass or LP signal contains frequencies from 1 Hz to some higher value.
Statement of sampling theorem:
A continuous time signal can be completely represented in its samples and recovered back if
the sampling frequency is twice of the highest frequency content of the signal. i.e., fs ≥ 2W

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Here fs is the sampling frequency and W is the higher frequency content
Proof of sampling theorem
There are two parts: (I) Representation of x(t) in terms of its samples (II) Reconstruction of
x(t) from its samples.
Part I: Representation of x(t) in its samples x(nTs)
Step 1: Define xδ (t)
Step 2: Fourier transform of xδ (t) i.e. xδ (f)
Step 3: Relation between X(f) and xδ (f)
Step 4: Relation between x(t) and x(nTs)
Part II: Reconstruction of x(t) from its samples:
Step1: Take inverse fourier transform of X(f) which is in terms of xδ (f)
Step2: Show that x(t) is obtained back with the help of interpolation function.
Comments:
1. The samples x(nTs) are weighted by sinc functions.
2. The sinc function is the interpolating function. The figure shows how x(t) is
interpolated.
Step 3: Reconstruction of x(t) by low pass filter:
When the interpolated function signal is passed through the low pass filter of bandwidth -
W≤f≤W, Then the reconstructed waveform is shown below:
Definition of Aliasing:
When the high frequency interferes with low frequency and appears as low frequency then
the phenomenon is called as aliasing.
Effects of aliasing: i) Since high and low frequencies interfere with each other distortion is
generated.
ii)the data is lost and it cannot be recovered.
Different ways to avoid aliasing:
Aliasing can be avoided by two methods :
i) Sampling rate fs≥2W
ii) Strictly bandlimit the signal to ‗W‘
i) Sampling rate fs≥2W
When the sampling rate is made higher than 2W then the spectrum will not overlap and

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there will be sufficient gap between the individual spectrums. This is shown I n figure:
ii) Bandlimiting the signal
Fig. Band limiting the signal.
The band limiting LPF is called prealins filter
Reconstruction Filter (Interpolation Filter)
Definition
In section we have shown that the reconstructed signal is the succession of sine pulses
weighted by x(nTs). These pulses are interpolated with the help of a lowpass filter. It is also
called reconstruction filter or interpolation filter.
2. Explain in detail about the uniform quantization concept? [Dec-2012]
(Linear Quantization)
Quantization:
It is a process of ―converting an infinite number of possibilities to a finite number of
conditions‖.
Folded binary code:
 The PCM code shown in table is a 3bit sign magnitude code with eight possible
combinations. The left most bit is sign bit (1= + and 0 = -), and two right most bit
represent magnitude.
 The type of code is called folded binary code because the codes on the bottom half of
the table is mirror image of codes on the top half, except for the sign bit.
Quantization interval or quantum:
The magnitude difference between adjacent steps is called the quantum.

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Table: 3 bit PCM code
Overload distortion or peak limiting
If the magnitude of the sample exceeds the highest quantization interval, overload distortion
occurs.
Quantizing
Assigning PCM codes to absolute magnitude is called quantizing.
Resolution:
The magnitude of quantum is called resolution. The resolution is equal to the voltage
of at least significant bit ( ) of PCM code.

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Figure:(a) Analog input signal (b) sample pulse (c) PAM signal (d)PCM code
Each sample voltage is round off to the closest available level and then converted to
its corresponding PCM code.
The PAM signal in the transmitter is essentially same PAM signal produced in the
receiver. Therefore, any round off error in the transmitted signals are reproduced when code
is converted back to analog in the receiver. This error is called quantization error ( ) or
quantization noise).
For sample 3 in the figure a) the voltage at is +2.6 v. the folded PCM code is
There is no PCM code for 2.6; therefore 2.6 is rounded off to 3which is 111. The
rounding process can result in quantization error of 0.4V.

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Figure:Linear input versus output transfer curve
The quality of PAM signal can be improved by using a PCM code with more bits or
sampling the analog signal at faster rate. The above shows the same analog signal as in
figure 7 except the signal being sampled at a much higher rate. As the figure 7 shows, the
signal resembles the analog signal rather closely.
Linear quantizer:
The above figure shows the input output transfer function for a linear analog to
digital converter. As the figure shows for a linear function for a linear analog input signal (i.e. ,
a ramp), the quantized signal is a staircase function. So the maximum quantization error is
same for any magnitude input signal.

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2. Compare different speech coding techniques.
 All encoding method provides telephone quality speech. So in quality wise there is no
distinction between the speech produced by any of this method. The choice of a particular
system then lies in its attraction as a bandwidth conserving scheme.
 ADPCM and ADM are particularly efficient waveform encoding techniques.
 With continuously variable slope delta modulation (CVSD), it is possibly to reduce the bit
rate to 9.6kbps all bit with some noticeable distortion. In fact waveform encoders are not
used below 9.6kbps because of significant distortion.
 For the rate below this, synthesized encoding is used. In table, the summarization of
performance parameter of the encoding methods is tabulated.
Encoding method Quantizer Coder(bits) Transmission
Rate(kbps)
PCM
PCM
DPCM
ADPCM
DM
ADM
LPC
Linear
Logarithmic
Logarithmic
Adaptive
Binary
Adaptive Binary
12
7-8
4-6
3-4
1
1
96
56-64
32-48
24-32
32-64
16-32
2.4-4.8
Performance comparison of encoding methods

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3. Explain in detail about PCM generation (Temporal waveform encoding) and
reception with neat diagrams. (or) Explain a pulse code modulation system with its
block diagram. [Dec-2013]
PULSE CODE MODULATION
PCM generation:
 The pulse code modulator technique samples the input signal x(t) at frequency fs> 2w.
This sampled variable amplitude pulse is then digitized by the analog to digital converter.
The parallel bits obtained at are converted to a serial bit stream. Shows the PCM
generator.
 In the PCM generator of above figure, the signal x(t) is first passed through the low-pass
filter of cutoff frequency W Hz. This low-pass filter blocks all the frequency components
above 'W Hz. Thus x(f) is band limited to 'W Hz.
 The sample and hold circuit then samples this signal at the rate of L. Sampling frequency
I; is selected 3ufficiently above Nyquist rate to avoid aliasing i.e.,fs≥ 2W
 In figure, output of sample and hold is called x(n Ts). This x(n Ts) is discrete in time and
continuous in amplitude. A q-level quantizer compares input x(n Ts) with its fixed digital
levels. It assigns any one of ' the digital level to x(n Ts) with its fixed digital levels.
 It then assigns any one of the digital level to x(nTs) which results in minimum distortion or
error. This error is called quantization error. Thus output of quantizer is a digital level
called xq (n Ts).
 quantization error is given by, ε=xq(nTs)-x(nTs)
Transmission Bandwidth in PCM
 Let the quantizer use 'v' number of
binary digits to represent each level. Then the number of levels that can be represented

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by digits will be, q= 2v
 Here 'q' represents total number of digital levels of q-level quantizer.
 For example if v=3 bits, then total number of levels will be, q = 23
= 8 levels
 Each sample is converted to binary bits. i.e. Number of bits per sample = v .
 We know that, Number of samples per second = fs
PCM Receiver:
a) Shows the block diagram of PCM receiver
b) Shows the reconstructed signal.
The regenerator at the start of PCM receiver reshape the pulse and removes the noise. This
signal is then converted to parallel digital words for each sample.
The digital word is converted to its analog value Xq(t) along with sample and hold. This
signal at the output of S/H is passed through low pass reconstructed filter to get yD(t). As
shown in reconstructed signal.

EC 6651 Communication Engineering

EC 6651 Communication Engineering

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EC 6651 Communication Engineering