Download ec2252 – communication theory iv semester

DHANALAKSHMI COLLEGE OF ENGINEERING, CHENNAI
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
EC 6402 COMMUNICATION THEORY
UNIT – I : AMPLITUDE MODULATION
PART - A (2 Marks)
1. Define − Amplitude Modulation
(A/M – 09)
Amplitude modulation is defined as a process of varying the amplitude of high frequency carrier signal is varied
in accordance with the amplitude of the modulating signal.
2. Mention the types of analog modulation.
The types of analog modulation are:
a) Amplitude modulation
b) Angle Modulation
Amplitude modulation is further classified into
a. DSB-FC : Double Sideband - Full Carrier
b. DSB-FSC : Double Sideband - Suppressed Carrier
c. SSB-SC : Single Sideband – Suppressed Carrier
d. VSB
: Vestigial Sideband
e. ISB
: Independent Sideband
Angle modulation is further classified into
a. Frequency modulation
b. Phase modulation
3. Define − Depth of Modulation
(A/M – 06)
Depth of modulation is defined as the ratio of the amplitude of the message signal to that of carrier amplitude.
m = Em / Ec
where, m = Depth of modulation
Em = Amplitude of the modulating signal and
Ec = Amplitude of the carrier signal
4. What are the different types of degrees of modulation?
The degrees of modulation are classified as
a. Under modulation : m < 1 when Em < Ec
b. Critical modulation : m = 1 when Em = Ec
c. Over modulation : m > 1 when Em > Ec
5. What is the need for modulation?
The needs of modulation are for
a) Ease of transmission
b) Multiplexing
c) Reducing noise and interference
d) Narrow bandwidth
e) Frequency assignment
6. What are the different types of linear and non-linear AM modulators?
The different types of linear AM modulators are
a) Collector modulator
b) Emitter modulator
c) Switching modulator
The different types of non-linear AM modulators are
a) Square law modulator
b) Product modulator
c) Balanced modulator
7. What are the different types of pulse and digital modulation techniques?
The different types of pulse modulation techniques are
a) Pulse Amplitude Modulation (PAM)
b) Pulse Position Modulation (PPM)
c) Pulse Duration Modulation (PDM)
d) Pulse Code Modulation (PCM)
The different types of digital modulation techniques are
a)Amplitude Shift Keying (ASK)
b)Phase Shift Keying (PSK)
c) Frequency Shift Keying (FSK)
d)Quadrature Amplitude Modulation (QAM)
8. Define − Channel Bandwidth
Channel bandwidth is defined as the range of frequencies a communication channel can transmit the signal with
uniform attenuation and zero phase change.
9. Define – Single Tone Modulation
Single tone modulation is defined as the modulation process performed for a message signal with only one
frequency component.
10. Define – Multi Tone Modulation
Multi tone modulation is defined as the modulation process performed for a message signal with more than one
frequency component.
11. Compare amplitude modulation with DSB-SC and SSB-SC.
Parameter
Bandwidth
AM
DSB-SC
SSB-SC
2 fm
2 fm
fm
Frequency
spectrum
It consists of USB,
LSB and carrier
It consists of USB and
LSB
It consists of USB or
LSB
Power
More power is
required for
transmission
Power required is less
than that of AM
Power required is less
than that of AM and
DSB-SC
12. Distinguish between linear and non-linear modulators.
Linear modulators
Non-linear modulator
Heavy filtering is not required
Heavy filtering is required
These modulators are used in high level
modulation
These modulators are used in low level
modulation.
The carrier voltage is very much greater
than modulating signal voltage.
The modulating signal voltage is very much
greater than the carrier signal voltage.
13. List the methods for generating DSBSC-AM systems.
The two ways of generating DSBSC-AM are
a) Balanced modulator technique
b) Ring modulator technique
14. What are the advantages of ring modulator?
The advantages of ring modulator is given by
a) Ring modulator output is stable
b) It requires no external power source to activate the diodes
c) Virtually no maintenance
d) Long life
15. Compare low level and high level modulation techniques.
Parameters
Power level
Efficiency
Type of amplifier
Design of AF power
amplifier
Devices used
Applications
High level modulation
Modulation takes place at high
power level
Very high
High efficient class C amplifier
Complex due to very high
power level
Vaccum tubes or transistors
High power broadcast
transmitters
(N/D – 07)
Low level modulation
Modulation takes place at low
power level
Low
Linear amplifier (A, AB or B)
used after modulation
Easy as it is operated0 at low
power level
Transistors, JFET or Op-amp
Wireless intercom, remote
control, walkie-talkie and in TV
transmitters
16. What are the advantages of VSB-AM system?
(A/M – 11)
The advantages of VSB-AM system are
a) It occupies higher bandwidth than SSB but less than DSB system.
b) Transmission power is greater than DSB but less than SSB system.
c) No low frequency component present.
17. Define − Demodulation
Demodulation or detection is defined as the process when the modulating voltage is recovered from the
modulated signal. It is the reverse process of modulation.
18. What are the different types of AM detectors?
The different types of AM detectors are
i.
Nonlinear detectors
a) Synchronous or coherent detector.
b) Envelope or non coherent detector.
ii.
Linear detectors
a) Square law detector.
19. Draw the block diagram of coherent detector.
Modulating Signal
Product Modulator
LPF
Modul
ator
Output
Carrier Signal
20. Define − Multiplexing
Multiplexing is defined as the process of transmitting several message signals simultaneously over a single
channel.
21. Define − Sensitivity
Sensitivity is defined as the measure of its ability to receive weak signals. It depends on the RF and IF stages of
the receiver. Good sensitivity of the system will provide higher output.
22. Define − Selectivity
Selectivity of a receiver is defined as its ability to select the desired signals among the various signals while
rejecting all other frequencies. Selectivity depends on the sharpness of the resonance curve of tuned circuit
involved in the receiver.
23. A transmitter supplies 8 KW to the antenna when modulated. Determine the total power radiated when
modulated to 30%.
Solution:
Given:
Modulation Index m = 0.3;Power Transmitted Pc=8 kW
Pt = Pc (1+m2/2);
Pt = 8*10-3 (1+0.045) =8.36 kW
24. What are the advantages of super heterodyne receiver over TRF receiver?
The advantages of super heterodyne receiver over TRF receiver are as follows
a) Improved selectivity and stability
b) Higher gain per stage because IF amplifiers are operated at a lower frequency
c) Uniform bandwidth because of fixed intermediate frequency
25. Define − Frequency Division Multiplexing
Frequency division multiplexing is defined as the process of transmitting a number of signals over the same
channel and the signals must be kept apart so that they do not interfere with each other and they can be
separated at the receiving end. This is accomplished by separating the signals in frequency or in time domain.
26. List the applications of SSB-SC-AM.
(N/D – 07)
a) Police wireless communication
b) SSB telegraph system
c) Point to point radio telephone communication
d) VHF and UHF communication systems
27. Define – Vestigial Side Band
VSB is defined as the modulation technique in which one of the sideband is partially suppressed and vestige
(portion) of the other sideband is transmitted. The vestige compensates the suppression of the sideband.
28. List the applications of FDM.
(N/D – 10)
The applications of FDM are
a) Efficient spectrum usage
b) Large amount of data transmission
c) Power efficiency
29. Define ─ Frequency Translation
(N/D – 11)
Frequency translation is defined as the modulation process when the carrier frequency f1 is increased to a
specified carrier frequency f2.
PART – B (16 Marks)
1. Explain the modulation and demodulation of AM signals.
(N/D –11)
2. Draw the circuit diagram of Ring modulator and explain its operation
3. Draw the block diagram for the generation and demodulation of a VSB signal and explain the principle of
operation.
4. Explain frequency translation technique and frequency division Multiplexing.
(A/M –10)
5. Draw an envelope detector circuit used for demodulation of AM and explain its operation.
(A/M –10)
6. With neat block diagram explain the generation of SSB in Weaver’s method.
(A/M –10)
7. Compare amplitude modulation and frequency modulation techniques.
(A/M –10)
8. Explain the generation of DSB-SC using balanced modulator with a neat block diagram.
(N/D –10)
9. Explain the generation of DSB-SC SSB-SC using coherent detection method. What happens when there is a
phase mismatch?
(N/D –10)
10. Explain the operation of an envelope detector using neat block diagram. Why does negative clipping takes
place?
(A/M –11)
11. Compare the characteristics of DSBFC, DSBSC, SSBFC, SSBSC and VSB schemes.
(A/M –11)
12. Discuss the frequency components present in a periodic and nonperiodic signal.
(M/J –16 )
13. Derive the equation of an AM wave. Also draw the modulated AM wave for various modulation index. (M/J –16)
14. Derive the equation of an AM wave and explain any one method to generate and demodulate it.
(N/D– 16)
15. Explain the generation of SSB-SC signal using phase shift method.
(N/D –14)
16. Suggest a scheme for recovering the message signal from the signal s(t) = 2 m(t) cos2πfct. Explain the same.
(N/D– 14)
17. Explain the function of switching modulator in the generation of AM signal.
(N/D –14)
18. Determine the average power in the carrier and in the sidebands.
19. Find the modulation index and peak power delivered to the load.
(N/D –14)
20. Explain with suitable diagrams the generation of AM using square law method. Also derive its efficiency.
21. Explain the demodulation of AM using envelope detection.
(A/M –15)
22. With a help of neat diagram explain the operation of envelope detector. Why does negative peak clipping take
place.
(A/M –11)
23. Draw an envelope detector circuit used for demodulation of AM and explain its operation.
(A/M– 10)
24. How do you demodulate AM signal ? Explain.
(M/J –16)
25. (a) Explain with block diagram super hetrodyne receiver.
(b) Explain the Hilbert transform with an example.
(A/M –15)
26. Explain about super heterodyne receiver with block diagram.
(M/J –16)
27. With neat block diagram explain the function of super heterodyne receiver in detail.
(N/D –14)
28. Illustrate the superiority of super heterodyne receiver over single tuned receiver.
(N/D –14)
29. Explain the working of a AM transmitter and that of receiver with block schematic.
(M/J –14)
30. Explain the need for carrier suppression in AM system. Draw and explain the functioning of one such system.
(M/J –14)
31. Compare the characteristics of DSBFC, DSBSC, SSBFC, SSBSC and VSB schemes.
32. Derive the expression for DSB-SC AM and calculate its power and efficiency. Explain a method to generate and
detect it.
(N/D –16) (M/J –16)
33. Name the methods used for suppression of unwanted sidebands in AM transmission.
34. Discuss the working of any one of them.
(N/D –14)
35. How SSB can be generated using Weaver’s method? Illustrate with a neat block diagram.
36. With suitable block diagrams and equations show how will you generate:
(a) DSBSC and
(b) VSB signals
37. Determine the modulated wave after transmission through the circuit. Assume suitable
data.
38. Draw the VSB spectrum and explain the significance.
(M/J –16)
39. Explain the concept of FDM with a suitable block diagram
(A/M –15)
UNIT – II : ANGLE MODULATION SYSTEMS
PART – A : (2 Marks)
1. State the disadvantages of FM.
The disadvantages of FM are
i) Bandwidth requirement of FM is high.
ii) FM transmitting and receiving equipment is more complex and costly.
iii) Distance of reception is limited only to line of sight.
(A/M – 08)
2. What are applications of phase locked loop?
The applications of phase locked loop are
(N/D – 06)
i.
ii.
Direct FM transmitter
FM demodualtors
3. Differentiate Phase Modulation from FM.
(M/J – 07)
Frequency modulation
Phase modulation
The maximum frequency deviation depends
upon amplitude of modulating voltage and
modulating frequency
The maximum frequency deviation depends
only on amplitude of modulating voltage
Better noise immunity is better than AM and
PM
Better noise immunity is better than AM but
worse than FM
Frequency of the carrier is modulated by
modulating signal
Phase of the carrier is modulated by
modulating signal
4. Draw block diagram of PLL.
The block diagram of PLL is
FM wave
(M/J – 07)
error e(t)
output v(t)
Loop filter
Feedback signal r(t)
VCO
5. Define − Phase Modulation
(N/D – 07)
Phase modulation is defined as the process of varying the phase of the carrier signal in accordance to the
amplitude of the modulating signal.
6. What is Narrowband FM?
(N/D – 07)
When the modulation index is less than 1, the angle modulated systems are called low index.For low index FM,
minimum bandwidth required is approximately twice of the highest modulating signal frequency. Hence low index is
also called narrowband FM.
7. A 80 MHz carrier is frequency modulated by a sinusoidal signal of 1-v amplitude and the frequency sensitivity is
100Hz/V. Find the approximate bandwidth of the FM waveform if the modulating signal has a frequency of 10KHz.
(A/M– 08)
Given:
Carrier frequency, fc =80MHz; Amplitude=1 V; Sensitivity=100Hz/V
Maximum frequency deviation, =1X1OOHz=100Hz
Maximum modulation frequency=10 KHz =100x
Hz
Formula:
BW=2( + m (max)) = 2(100+ (10x
)) =2(100+10000) =––0 =–.2KHZ.
Bandwidth=–.2 KHz
8. State the principle of FM stereo receivers.
(A/M– 08)
The principle of FM stereo receiver is used for stereo transmission. It is basically called as frequency division
multiplexing. It is used for FM radio broadcasting.
9. What is diversity reception?
(N/D– 08)
Diversity reception is used when the signal fades into noise level. There are two types 1) Space diversity 2)
Frequency diversity.
10. State the disadvantages of FM.
The disadvantages of FM are
(i) Wider channel bandwidth is required by FM.
(ii) Increased complexity at receiver side.
(N/D– 08)
11. Obtain the bandwidth of the FM signal.
(M/J– 09)
C (t) =10xcos (2x
Given:
C (t) =10cos (2*
x t+8cos (1000x t)).
x t+8cos (1000* t)); Ac =10; fc =
Hz; fm =500Hz,
=8.
Formula used:
F= .fm=8*500=4000;
Transmission bandwidth, BT=2
BT=9 KHz
)=24000(1+1/8) = 9000 Hz
12. What is meant by FM stereo multiplexing?
(M/J– 09)
Stereo multiplexing is a form of FDM designed to transmit two separate signals via the same carrier. It is widely
used in FM radio broadcasting to send two different elements of a program so as, the give a spatial dimension to
its perception by a listener at the receiving end.
13. A carrier wave of frequency 100MHz is frequency modulated by a sinusoidal wave of amplitude 2 volts and
frequency 100 KHz. The frequency sensitivity of the modulator is 2.5 KHz/ volt. Determine the Bandwidth of FM signal.
(N/D– 09)
Ans:
Bandwidth to the FM signal is
Carrier frequency, f c=100MHz
Amplitude, Ec =2v;
Frequency of modulating signal=100 KHz; kf=2.5 KHz/volt
ΔF=A×kJ=5 KHz
Bandwidth=2(� f+fm) =210 KHz.
14. List the non-linear effects in FM system.
The non-linear effects in FM system are
i)Amplitude Non-linearity
ii)Phase Non-linearity
iii)AM-PM conversion.
(N/D– 09)
15. Draw the block diagram of a method for generating a narrowband FM signal.
(A/M– 10)
The block diagram of a method for generating a narrowband FM signal
Message Signal
Integrator
Modulato
r
Phase Modulator
FM Signal
Carrier Signal
16.A carrier wave of frequency 100MHZ is frequency modulated by a signal – sin(–0 x t).What is bandwidth of FM signal
if the frequency sensitivity of the modulation is 25KHz/v?
(A/M– 10)
Given:
Frequency sensitivity, kf=25 KHz/V; Amplitude modulation signal=–V
Maximum frequency deviation=25 KHz/Vx–V = 500 KHz.
Frequency of the Modulation signal =100MHz
BW=2(f+fm (max)) =402KHZ.
PART – B (16 Marks)
1. Derive an expression for a single tone FM signal with necessary diagrams and draw its frequency spectrum.
(A/M –15) ( M/J –16) (N/J –16)
2. Derive the mathematical representation of FM wave
(M/J –13)
3. Derive the time domain expressions for FM and PM signals.
(A/M –10)
4. Derive the expression for the single tone frequency modulated signal and hence prove that it is constant envelope
modulation requiring infinite bandwidth.
(N/J –14)
5. Using suitable mathematical analysis show that FM modulation produces infinite sideband.
Also deduce an expression for FM output and its frequency spectrum.
(A/M –10)
6. Draw the typical spectrum of the FM
7. Explain the Armstrong method of FM generation.
(N/J –14)
(M/J –13)(M/J–16)(M/J –14)
8. Explain with diagrams generation of FM by direct method.
(N/D –16) (A/M –15)
9. Explain the indirect method of generation of FM signals.
(N/D –14) (M/J –16)
10. Describe how FM wave is generated by indirect method and give a suitable demodulating scheme for the same.
(N/D –13)
11. Explain how FM is achieved using varactor diodes.
(M/J –14)
12. How can you generate FM from PM and PM from FM.
(A/M –10)
13. Draw the circuit diagram of Foster-Seeley discriminator and explain its working with relevant
phase diagrams.
14. Explain about the basic principles of FM detection and explain about Ratio detector
(N/J –16)
15. Explain the working operation of balanced slope detector.
(N/J–16)
16. With the necessary diagrams explain the working of slope detector for demodulating FM signals.
17. Explain the functions of any FM detector circuit.
18. Explain the operation of PLL as FM demodulator.
19. What are the applications of PLL ?
20. Discuss the effects of nonlinearities in FM systems.
21. Make atleast five comparisons of AM and FM systems.
(M/J –14)
UNIT – III : RANDOM PROCESS
PART – A (2 Marks)
1. When is random process called deterministic?
(A/M– 10)(N/D – 11)
A random process or stocharter process is an assemble or sample space . Such sample consists of sample points
whose outcome is random function of him. Hence random processes are called deterministic.
2. A receiver connected to an antenna of resistance of 50 ohms has an equivalent noise resistance of 30 ohms . Find
the receiver noise figure.
(A/M– 10)
Req=30 ohms
Ro =50 ohms
F= 1 + (Req/ Ro) = 1+(30/50) = 1.6 ohms
3. What is white noise? Give its characteristics.
(N/D– 10)
The noise which has Gaussian distribution and flat spectral density over a wide range of frequencies is
called white noise.
The characteristics of white noise are:
 The PSD of white noise is independent of the operating frequency is S(f)=No/2.
 The auto correlation function of white noise is delta function is Rw( t)=No/2 d(t).
4. Define – Random Variable. Specify the sample and the random variable for a coin tossing experiment.
(N/D– 10) (M/J– 12)
A function which has takes on any value from the sample space and its range is some set of real numbers is called a
random variable of the experiment. If a coin is tossed, the possible outcomes are head (H) and tail(T). Therefore the
sample space S contains two sample points {H,T}.
If we define the function X such that
X = 1 when S=H, -1 when S=T Then
X ={ 1,-1}.
5. Define – Probability Distribution
The probability distribution of a discrete random variable is a list of probabilities associated with each of its possible
values. It is also sometimes called the probability function or the probability mass function.
6. State central limit theorem.
The central limit theorem (CLT) states that, given certain conditions, the arithmetic mean of a sufficiently large
number of iterates of independent random variables, each with a well-defined expected value and well-defined
variance, will be approximately normally distributed.
7. Define – Stationary Process
A stationary process is a stochastic process whose joint probability distribution does not change when shifted in
time
8. List the requirements of Wide Sense Staionary Process.
WSS random processes only require that 1st moment and covariance do not vary with respect to time.Any strictly
stationary process which has a mean and a covariance is also WSS.
9. Define – Correlation
Correlation refers to any of a broad class of statistical relationships involving dependence.
10. Define – Covariance
Covariance is a measure of how much two variables change together, and the covariance function, or kernel,
describes the spatial covariance of a random variable process or field.
11. Represent a Mean function of a process.
12. Represent a Auto correlation function.
13. When a process can be defined as a Gaussian process?
A random process X(t) is a Gaussian process if for all n and all (t1 ,t2 ,…,tn ), the random variables have a jointly
Gaussian density function.
14. What are the application of Gaussian process?
(i) A Gaussian process can be used as a prior probability distribution over functions in Bayesian inference.
(ii)Wiener process (aka Brownian motion) is the integral of a white noise Gaussian process.
It is not stationary, but it has stationary increments.
15.At what condition a random process becomes a ergodic process?
A random process is a ergodic process if
PART- B (16 Marks)
1.Let X and Y be real random variables with finite second moments. Prove the Cauchy-Schwarz
inequality. (E(XY))2 E(X2)E(Y2).
2. Differentiate the strict-sense stationary with that of wide sense stationary process.
(A/M –15)
(A/M –15)
3. Let X(t) and Y(t) be both zero-mean and WSS random processes. Consider the random
process z(t) = X(t) + Y(t). Determine the auto correlation and power spectrum of z(t) if X(t)
and Y(t) are jointly WSS.
(A/M –15)
4. Let X(t) = A cos (ωt + φ) and Y(t) = A sin (ωt + φ) where A and ω are constants and φ is a
uniform random variable (0,2π). Find the cross correlation of x(t) and y(t).
(A/M –15)
5. Two random processes X(t) = A cos (ωt + φ) and Y(t) = A sin (ωt + φ) where A and ω are
constants and φ is a uniformly distributed random variable in (0,2π). Find the cross
correlation function.
6. Explain in detail about the transmission of a random process through a linear time invariant filter. (M/J –16) (N/D –16)
7. When is a random process said to be a strict sense stationary (SSS), wide sense stationary (WSS) and
ergodic process.
(N/D –16)
8. Given a random process, X(t) = A cos (ωt + µ) where A and w are constants and µ is a uniform
random variable. Show that X(t) is ergodic in both mean and auto correlation.
9. Define the following terms mean, correlation, covariance and ergodicity.
(N/D –16)
10. What is a Gaussian random process an mention its properties.
(N/D –16)
UNIT- IV: NOISE CHARACTERIZATION
PART – A (2 Marks)
1. What is known as extended threshold demodulator?
(N/D– 06)
In communication system using FM, noise threshold in FM receiver must be reduced, so as to operate receiver
with minimum signal power this may be achieved by using FM demodulator with negative feedback or by using PLL
demodulator. Such devices are referred as extended threshold demodulator.
2. What is the purpose of pre-emphasis & de-emphasis in FM?
(M/J – 07)
The combined use of pre-emphasis and de emphasis filter will improve the signal to noise ratio of FM receiver.
For the efficient use of allowed frequency band to depends on the pre-emphasis in the transmitter & de emphasis in the
receiver.
3. What is threshold effect with respect to noise?
(M/J – 07)
Loss of message in a envelope detector that operates at a low carrier to noise ratio below which noise
performance of a detector deteriorates much more rapidly than proportionately to the carrier to the noise ratio.
4. What is pre-emphasis and De-emphasis?
(N/D – 07)(N/D – 10)
The premodulation filtering in the transistor, to raise the power spectral density of the base band signal in its
upper-frequency range is called pre emphasis (or pre distortion). The FM signal is then transmitted. Noise adds to
this signal before it reaches the receiver. The filtering at the receiver to undo the signal pre-emphasis and to
suppress noise is called de-emphasis.
5. What do you understand by capture Effect in FM?
(A/M– 08)
The FM system minimizes the effect of noise interference. This can be effective when
interference is week compared to FM signal. But if the interference is stronger than FM signal, then FM receiver
locks to interference. This suppresses then the Fm receiver locking fluctuates between them. This phenomenon
is called Capture effect.
6. Compare the noise performance of AM and FM systems.
(A/M 08) (M/J 09)
The figure of merit of AM system is 1/3 when the modulation is 100 percent and that of
FM is (3/2) mf2 .The
use of FM offers improved noise performance over AM when (3/2)
mf2 > 1/3.mf –modulation index in FM.
7. What is capture effect?
(N/D– 08)
The FM system minimizes the effect of noise interference .This can be effective when
interference is weak
compared to FM signal. But if the interference is stronger than FM
signal, then FM receiver locks to
interferences. This suppresses FM signal. When
noise interference as well as FM signal are of equal strength,
then the FM receiver locking fluctuates between them .this phenomenon is called capture effect.
8. Write the expression of the SNR for a synchronous detector.
(N/D– 08)
The expression of the SNR for a synchronous detector is
(SNR)0 = power at receiver output = A2c K2a P
Noise power at receiver output
2No B
9. What is threshold effect in FM?
(M/J– 09) (N/D– 09)
As the carrier to noise ratio is reduced, clicks are heard in the receiver output. As the
carrier to noise ratio
reduces further, cracking or sputtering sound appears at the receiver
output. Near the breaking point the
theoretically calculated output signal to noise ratio
becomes large, bur its actual value is very small. This
phenomenon is called threshold effect in FM.
10. What is noise figure of AM receiver?
(N/D– 09)
The noise figure of AM receiver is defined as the ratio between signals to noise ratio at
to noise ratio at the input
(SNR)O / (SNR) I=
P/1+
the output to signal
P
11. What are the characteristics of superheterodyne receiver?
(A/M– 10)
The important characteristics of superhetrodyne receiver are.
a) Sensitive
b) noise selectivity
c) fidelity
12. What are the methods to improve threshold reduction?
(A/M– 10)
The methods to improve threshold reduction are
a) Pre-emphasis and de-emphasis method.
b) Frequency modulation with feedback
c) Voltage controlled oscillator.
13. Define Threshold effect in AM receiver.
(N/D–10)
When the carrier to noise ratio reduces below certain value, the message information is lost. The performance
of envelope detector determines rapidly and it has no proportional to carrier to noise ratio. This is called threshold effect
in AM.
14. How is threshold reduction achieved in FM system?
Threshold reduction is achieved in FM system by using an FM demodulator with
using a phase locked loop demodulator
(A/M– 11)
negative feedback or by
15. Compare the noise performance of AM receiver with that of DSB-SC receiver.
(A/M– 11)
The figure of merit of DSB-SC or SSB-SC receiver using coherent detection is always
unity, the figure of
merit of AM receiver using envelope detection is always less
than unity. Therefore noise performance of AM
receiver is always inferior to that of
DSBSC due to the wastage of power for transmitting the carrier.
16. What is meant by figure of merit of a receiver?
(N/D– 06)
The figure of merit of receiver is given by
Figure of merit= (SNR)o / (SNR)c
Higher the value of figure of merit, better will be the noise performance of receiver. It may be equal to one, less than
one or greater than one, depending on the type of
modulated used.
17. What is white noise?
(M/J – 07) (N/D – 07)
White noise power spectral density of which is independent of operating frequency adjective white is used in
the sense that while light contains equal amount of all frequencies within the visible band of EM radiation.
Sw(f)=No/2
18. Define noise - figure of communication system
(M/J– 07)
The noise figure of 2 port device as ratio of total available output noise power per unit band width to
portion thereof due solely to source.
19. Define – Shot noise
(N/D– 07)
The fluctuations of electrons or holes in a device constitute shot noise. The shot noise depends upon
operating conditions of the device.
20. What is noise factor?
(N/D– 08)
Noise factor = Available S/N power ratio at the input/ Available S/N power ratio at the output.
21. What is thermal noise?
(N/D– 08)
The electrons in a conductor possess varying amounts of energy by virtue of temperature of conductor. The
small fluctuation in Energy are sufficient to produce small noise voltages in the conductor .These random fluctations
produced by the thermal agitation of the electrons are called thermal noise.
22.A white noise has a power spectral density of -170dB /Hz..calculate the power at the output of a band pass filter.of
central frequency 1–MHZ and bandwidth – KHZ.If white noise is fed as the input.
(M/J– 09)
The o/p power = bandwidth*power spectral density =–*(-170) =-3400db
23.List the characteristics of Shot Noise.
(M/J– 09)
Shot noise has a uniformly density.
Mean square noise current depends directly on the direct components of current.
Shot noise is dependent upon the operating conditions of the device.
24. List the characteristics of white noise.
(N/D– 09)
The PSD of white noise is independent of the operating frequency is S(f)=No/2.
The auto correlation function of white noise is delta function is Rw( t)=No/2 d(t).
25. What is thermal noise? Give the expression for the thermal noise.
(N/D– 09)
Thermal noise is the electrical noise arising from the random motion of electrons in a conductor.
The mean square value of the thermal noise voltage E(VTH2) appearing across the terminals of a resistor (R),
Measured in a bandwidth of delta f hertz is
E(VTH2)=4KTR delta f volts.
PART- B(16 Marks)
1. Define noise. Explain the various types of internal noise.
(A/M –15)
2. Explain with derivation the effect of noise in cascaded amplifier circuit.
(A/M –15)
3. Derive the SNR performance of DSB system and the AM system. Also prove that the output
SNR in AM is atleast 3 dB worse than that of DSB system.
(A/M –15)
4. Explain preemphasis and Deemphasis in FM.
5. Define noise and write notes on shot noise, thermal noise and white noise.
(N/D –16)
6. Derive the figure of merit for AM system. Assume coherent detection.
(N/D –16)
7. Explain the noise in DSB-SC receiver using synchronous or coherent detection and calculate
the figure of merit fpr a DSB-SC system.
(16)
8. Define narrow band noise and explain the representation of Narrow Band Noise in terms of
In-Phase and Quadrature Components.
(8)
9. Explain the noise in FM receiver and calculate the figure of merit for FM system.
(N/D –16)
10. What is meant by Narrow Band Noise? Explain the characteristics of Narrow Band Noise.
(N/D –14)
11. Discuss on thermal noise.
(N/D – 14)
12. Define and explain the following:
(a) Gaussian noise and Gaussian distribution. What type of PDF does Gaussian
distribution follow?
(b) Thermal noise
(c) Shot noise
13. Explain the significance of preemphasis and Deemphasis in FM .
14. Explain the noise in AM receiver using noisy model block diagram ?
(8)
15. Explain the capture effect and FM threshold effect.(6)
(N/D –06)
16. Draw the block diagram of FM demodulator and explain the effect of noise in detail and
compare the performance of AM and FM in the presence of noise.(10)
(N/D –06)
17. Explain Hilbert transform with a suitable example. (6)
16. Explain the method of generation an detection of SSB waver.(10)
17. Compare the noise performance of AM and FM systems.(16)
18. Derive the output SNR for envelope detection.(8)
19. Explain the FM receiver with block diagram.(8)
20. Compare the noise performance of noise in AM and FM systems.(16)
(N/D –07)
(N/D –07)
(N/D –07)
21. What is the function of pre-emphasis and De-emphasis in FM? Draw the circuit diagram of pre-emphasis and Deemphasis and explain its operation.(16)
(A/M –08)
22. Derive the expression for the power spectral density at the output of a FM discriminator when the received signal
is corrupted by additive white Gaussian noise.(16)
23. Define and explain FM Threshold effect. With suitable diagram, explain threshold reduction by FMFB
demodulator.(16)
(N/D –10)
UNIT- V: INFORMATION THEORY
PART – A (2 Marks)
1. Define – Entropy Function
(A/M– 10)
Entropy Function is defined as the measure of an average information content per second. The
expression for the entropy function is
H(X) =∑I P (xi) log2P (xi) bits/sample
2. Define – Information rate
(A/M– 10)
Information rate is defined as the measure of total number of bits that is being transmitted per second
from the source to destination.
3. Define – Bandwidth Efficiency
(A/M– 10)
Bandwidth Efficiency is defined as the ratio of channel capacity to the available bandwidth.
Bandwidth Efficiency = Channel capacity (C) / Available bandwidth (B)
4. A source generates 3 messages with probability 0.5, 0.25 and 0.25. Calculate source entropy.
(N/D– 10)
Given data
P1 = 0.5;
P2 = 0.25;
P3 = 0.25
Entropy H = p1log2(1/p1) + p2log2(1/p2) + p3log2(1/p3)
H = 0.5 log2(1/0.5) + 0.25 log2(1/0.25) + 0.1 log2(1/0.25) = 1.1 bits/message
5. Calculate entropy of the source with symbol probabilities 0.6, 0.3 and 0.1.
(N/D–11)
Given data
P1 = 0.6; P2 = 0.3; P3 = 0.1
Entropy H = p1log2(1/p1) + p2log2(1/p2) + p3log2(1/p3)
H = 0.6 log2(1/0.6) + 0.3 log2(1/0.3) + 0.1 log2(1/0.1) = 1.295 bits/message
6. What is prefix coding?
(N/D–03)
Prefix coding is a variable length coding algorithm in which the binary digits are assigned to the
messages depending on their probabilities of occurrence.
7.
Define − Lossless Channel
The channel described by a channel matrix with only one nonzero element in each column is called a lossless
channel. In the lossless channel no sources information is lost in transmission
8. Define − Deterministic Channel
A channel described by a channel matrix with only one nonzero element in each
deterministic channel and this element must be unity.
9. List the types of source coding techniques.
The various source coding techniques are
a) Prefix coding
b) Shannon-fano coding
c) Huffman coding
row is called a
(N/D– 04)
10. State the expression for the channel capacity of Binary Symmetric Channel and Binary Encrypted Channel.
For BSC the channel capacity is given by
C=1+P log2 P +(1-P) P log2 (1-P).
For BEC the channel capacity is given by
C = (1-P)
11. Define − Noiseless Channel
A channel is called noiseless if it is both lossless and deterministic. The channel matrix has
only
one element in each row and in each column and this element is unity. The input and output alphabets are of the
same size.
12. What is memory less source?
The source in which the alphabets transmitted does not depend upon previous alphabets is called
memory less source.
13. What is channel diagram and channel matrix?
The transition probability diagram of the channel is called the channel diagram and its
representation is called the channel matrix.
matrix
14. What is source coding and entropy coding?
A conversion of the output of a DMS into a sequence of binary symbols is called source coding. The design of a
variable length code such that, its average cod word length approaches the entropy of the DMS is often referred
to as entropy coding.
15. List the advantages of channel coding.
The advantages of channel coding are
a) Error probability is reduced
b) Channel capacity is fully utilized
c) Forward error correction
d) Automatic repeat request
PART – B (16 Marks)
1. What is entropy? Explain the important properties of entropy.(8)
( N/D –06)
2. Define mutual information. Find the relation between the mutual information and the
joint entropy of the channel input and channel output.(12)
( N/D –06)
3. What are the implications of information capacity theorem?(4)
(N/D –06)
4. Write short notes on
a. Differential entropy.(3)
b. Derive the channel capacity theorem.(7)
c. Discuss the implication of the information capacity theorem.(6)
5.
(i)Define entropy and discuss its properties.(5)
(ii) Define mutual information and channel capacity.(5)
(iii) Explain the different conditional entropies.(6)
6. (i) State source coding theorem.(2)
(N/D –08)
(ii) List the properties of prefix codes with an example of prefix codes.(4)
(iii) Write a note on rate distortion theory.(10)
7. Explain 1)Binary Symmetric Channel 2)Binary Erasure Channel (16)
8. Explain the source coding theorem. (6)
(A/M –10)
9. State Shannon’s various theorems and explain. (16)
(N/D –16)
10. Derive the mutual information I(x;y) for a binary symmetric channel, when the probability of source is equally
likely and the probability of channel p = 0.5.
11. State and prove Mutual information and write the properties of mutual information.
12. Derive Shannon-Hartley theorem for the channel capacity of a continuous channel having an average power
limitation and perturbed by an additive band – limited white Gaussian noise.
(8)
(M/J –16)
13. Write short notes on:
14. (a) Mutual Information and (b) Rate distortion theory
15. Explain the Huffman source coding algorithm.
16. Explain Huffman coding system with an example. (16)
(M/J –14)
17. Design a Huffman code for the source given in PART-A. Determine the average code
18. Length and coding efficiency. (10)
(N/D –14)
19. Briefly discuss about the Lossy source coding schemes. (6)
(N/D –14)
20. Design a Shannon Fano code for the source given in PART-A. Determine the average code length and coding
efficiency.
21. Explain the need for source coding and channel coding.
22. Explain how channel capacity could be improved?
Discuss the source coding theorem.