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DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING

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QUESTION BANK

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SUBJECT : EC3491 – COMMUNICATION SYSTEMS

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SEM / YEAR: IV / II year B.E.

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UNIT I AMPLITUDE MODULATION

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Review of signals and systems, Time and Frequency domain representation of signals, Principles of

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Amplitude Modulation Systems- DSB, SSB and VSB modulations. Angle Modulation, Representation of FM

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and PM signals, Spectral characteristics of angle modulated signals. SSB Generation - Filter and Phase Shift
Methods, VSB Generation - Filter Method, Hilbert Transform, Pre-envelope & complex envelope AM

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techniques, Superheterodyne Receiver. PART A
Q.No Questions
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1.
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BTL 1 Remembering
What are the advantages of VSB-AM?
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2. State heterodyning principle. BTL 1 Remembering
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3. What are the advantages of modulating low frequency signal into BTL 1 Remembering
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high frequency signal?

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4. List the types of AM modulators? BTL 1 Remembering

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5. Define Coherent Detection. BTL 1 Remembering

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6. Why do you need modulation in communication systems? BTL 1 Remembering

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7. Identify the differences between single side band and vestigial BTL 2 Understanding
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side band systems.

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8. BTL 2 Understanding
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Write about diagonal clipping and negative peak clipping?

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9. Suggest a modulation scheme for broadcast video transmission. BTL 6 Creating

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10. Apply the concepts of sensitivity and selectivity in AM receiver. BTL 23 Applying
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11. BTL 2 Understanding

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Draw the AM modulated wave for modulation index=0.5 and its

spectra.
12. Illustrate the applications of Hilbert transform. BTL 3 Applying

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13. Apply the concepts of power relations and find the total power in a BTL 3 Applying
modulated wave if the carrier is 10 watts and amplitude modulated
to a depth of 80%.
14. Compare AM with DSB-SC and SSB-SC. BTL 4 Analyzing

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15. Distinguish between high level and low level modulation? BTL 4 Analyzing

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16. Differentiate between linear and nonlinear modulation BTL 4 Analyzing

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17. A transmitter radiates 9kW without modulation and 10.125kW BTL 5 Evaluating

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after modulation. Determine depth of modulation.

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18. When a signal m(t)=3cos(2 x103t) modulates a carrier BTL 5 Evaluating
c(t)=5cos(πx106t), determine the modulation index and

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transmission bandwidth if the modulation is AM.

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19. BTL 2 Understanding

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Summarize the methods for generating SSB-SC signal.

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20. Can you formulate the theory for modulation index of an AM BTL 6 Creating

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signal and write its classification.

PART – B
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1. What is the need for carrier suppression in AM system? Draw and BTL 2 Remembering
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explain the functioning of such system. (16)
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2. (i)Explain the generation of SSBSC signal using phase shift BTL 2 Understanding
method. (8)
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(ii)Suggest a scheme for recovering the message signal from the

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signal s(t)=2m(t)cos2πfct. Explain the same. (8) BTL 3 Applying

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3. (i) An AM signal is generated by modulating the carrier BTL 65 Evaluating

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fc=800MHz by the signal m(t)=sin3000πt+0.5cos5000πt. The AM

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signal s(t)=100[1+m(t)] cos2πfct is fed to a 50ohm load. (8)

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a) Determine the average power in the carrier and in the

sidebands.
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b) Find the modulation index and peak power delivered to the

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load.
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(ii)Explain the function of switching modulator in the generation

of AM signal. (8) BTL 2 Understanding
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4. (i)Analyze the concepts of AM modulation and derive the BTL 4 Analyzing

equation of an AM wave. Also draw the modulated AM wave for
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various modulation index. (8)

(ii) Summarize the methods of demodulation of DSBSC and BTL 2 Understanding
explain briefly about costas loop. (8)

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5. (i)Draw the VSB spectrum and explain the significance. (8) BTL 2 Understanding
(ii)How do you demodulate AM signal? Explain. (8) BTL 1 Remembering

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6. (i)Compare the characteristics of DSBFC, DSBSC, SSBFC, BTL 4 Analyzing

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SSBSC, SB schemes. (8)

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(ii)What is heterodyning and explain the operation of
Superheterodyne Receiver with a suitable block diagram. (8)

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BTL 1 Remembering

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7. (i)What is Hilbert transform? And how it can be used for various BTL 1 Remembering
applications. (8)

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(ii)Analyze the frequency components present in the output when

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the 1000kHz carrier is simultaneously AM modulated with
BTL 4 Analyzing

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300Hz, 800Hz and 1.5kHz audio sine waves. (8)

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8. (i) Apply the concepts of envelope detection for demodulation of BTL 3 Applying

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AM and explain its operation. (8)

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(ii)How would you generate SSB using Weavers method? Illustrate

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with a neat block diagram. A (8) BTL 1 Remembering
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9. (i)Construct the balanced modulator circuit for the generation of BTL 6 Creating
DSB-SC-AM and explain its operation. (8)
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(ii)Develop the coherent detection method in detail for the
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detection of DSB-SC and SSB-SC. What happens when there is BTL 6 Creating
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phase mismatch? (8)

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10. (i) How do you apply ring modulator for the generation of DSB-SC BTL3 Applying
signal? (8)
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(ii) For an AM DSBFC wave with peak unmodulated carrier voltage

Vc =10V, a load resistance RL=10Ω and a modulation coefficient
iy

ma=1,determine (8)
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a) Powers of the carrier and the upper sidebands

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BTL 5 Evaluating
b) Total sideband power
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c) Total power of the modulated wave

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d) Draw the power spectrum

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UNIT II RANDOM PROCESS & SAMPLING

Review of probability and random process. Gaussian and white noise characteristics, Noise in amplitude
modulation systems, Noise in Frequency modulation systems. Pre-emphasis and De-emphasis, Threshold
effect in angle modulation.

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Low pass sampling - Aliasing- Signal Reconstruction-Quantization - Uniform & non-uniform quantization -
quantization noise - Nyquist criterion- Logarithmic Companding -PAM, PPM, PWM, PCM - TDM, FDM

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PART A

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Q.No Questions BT Level Domain

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1. Define random variable. Specify the sample space and the BTL 1 Remembering

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random variable for a coin tossing experiment.

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2. List the properties of the cumulative distributive function. BTL 1 Remembering

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3. Demonstrate when random process is called deterministic? BTL 3 Applying

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4. Classify random process? Give one example for each. BTL 4 Analyzing
5.
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Express the auto correlation function and power spectral density of BTL 2 Understanding
white noise.
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6. State central limit theorem. BTL 1 Remembering
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7. Write the Rayleigh and Rician probability density functions. BTL 2 Understanding
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8. Describe mean, autocorrelation and covariance of a random BTL 1 Remembering

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process.
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9. Give the conditions to be satisfied for wide sense stationary. BTL 2 Understanding
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10. What are the properties of an autocorrelation function? BTL 1 Remembering

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11. Quote Ergodic processes and Gaussian processes. BTL 1 Remembering

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12. BTL 4 Analyzing

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Infer the cross correlation of random processes of X(t) and Y(t)?

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13. Show the input output relation for a power spectral density and BTL 3 Applying
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cross spectral density.

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14. Investigate when random processes X(t) is called white process ? BTL 6 Creating
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15. Summarise an expression for noise equivalent bandwidth. BTL 5 Evaluating

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16. Generalize the power spectral density of X(t). BTL 6 Creating

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17. BTL 4 Analyzing

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Pointout the properties of Gaussian process.

18. The pdf of a random variable is given as fX(x) =k for a ≤ x ≤b BTL 3 Applying
and 0 otherwise where k is a constant. Calculate the value of k.

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19. Evaluate when a random process is called as stationary, BTL 5 Evaluating

deterministic and ergodic.
20. Distinguish between random variable and random process. BTL 2 Understanding

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PART – B

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1. Explain the following, BTL 4 Analyzing

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(i) Random variable (5)

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(ii) Gaussian process (5)

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(iii) Central limit theorem (6)

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2. (i) Describe and prove the properties of Gaussian Process. (8) BTL 2 Understanding

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(ii) Differentiate the strict-sense stationary with that of wide sense BTL 2 Understanding

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stationary process. (8)

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3. (i) Analyze the following terms mean, correlation, covariance, BTL 4 Analyzing
and ergodicity.
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(ii) Quote the properties of the auto correlation function. (8)
BTL 1 Remembering

4. (i) An AWGN of power spectral density 1µW is fed through a BTL 3 Applying
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filter with frequency response
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H(f) = 1/2 ; |f| < 40 kHz

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0 ; elsewhere
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Calculate the noise power at the output of the filter. (8)

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(ii)Describe about stationary processes and its classifications. (8) BTL 1 Remembering
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5. Generalize the equation for finding the probability density BTL 6 Creating
function of a one to one differential function of a given random
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variable. (16)
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6. (i) Examine about Transmission of random process through a BTL 1 Remembering

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Linear Time Invariant (LTI) filter. (8)

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(ii) Find the autocorrelation of a sequence x(t) =A

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cos(2πfc(t+θ)) where A and fc are constant and θ is a random BTL 1 Remembering

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variable that is uniformly distributed over the interval [-π π]. (8)
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7. (i) Define autocorrelation. Discuss the properties of BTL 1 Remembering

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autocorrelation function. (8)

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(ii) Let X(t) and Y(t) be both zero-mean and WSS random
processes. Consider the random process z(t) = X(t) + Y(t). BTL 1 Remembering
Determine the auto correlation and power spectrum of z(t) if X(t)
and Y(t) are jointly WSS. (8)

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8. Discuss and prove the properties of power spectral density. (16) BTL 2 Understanding
9. (i) Let X have the uniform distribution given by BTL 3 Applying
⎧1 2π 0 ≤ x ≤ 2π
f X (x) = ⎨
⎩0 Otherwise

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Calculate mean, mean square value and variance. (8)

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(ii) Let X(t) = A cos (ωt+Φ) and Y(t) = A sin (ωt+Φ), where A and
BTL 3 Applying

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ω are constants and Φ is a uniform random variables [0, 2π].
Discover the cross correlation of x(t) and y(t). (8)

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10. Given a random process X(t)=A Cos(ωt+θ), where A and ω are BTL 5 Evaluating

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constants and θ is a uniform random variable. Judge that X(t) is

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Ergodic. (16)

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