COMMUNICATION THEORY JNTU UNIVERSITY PREVIOUS YEAR QUESTION PAPER COLLECTION 3rd year second semester ECE students

COMMUNICATION THEORY JNTU UNIVERSITY PREVIOUS YEAR QUESTION PAPER COLLECTION 3rd year second semester ECE students

COMMUNICATION THEORY JNTU UNIVERSITY PREVIOUS YEAR QUESTION PAPER COLLECTION 3rd year second semester ECE students

II B.Tech II Semester Supplimentary Examinations, Apr/May 2008

COMMUNICATION THEORY

(Electronics & Communication Engineering)

Time: 3 hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

⋆ ⋆ ⋆ ⋆ ⋆

1. (a) Derive an expression for AM wave and draw its frequency spectrum.
(b) Draw the one cycle of AM wave and calculate the modulation index of it in
terms of Vmax and Vmin voltages.
(c) A 500-W carrier is modulated to a depth of 80%. Calculate the total power
in the modulated wave. [6+6+4]
2. (a) Explain with the help of block diagram the phase-shift method of sideband
suppression.
(b) Explain the operation of square law modulator. [8+8]
3. (a) What are the non-coherent methods of AM demodulation? Explain those
clearly using suitable sketches and expressions.
(b) With neat sketches, explain about the vestigial sideband (VSB) modulation
system and mention its application(s). [9+7]
4. (a) A 500-Hz modulating voltage fed into a PM generator produces a frequency
deviation of 2.25 KHz. What is the modulation index? If the amplitude of
the modulating voltage is kept constant, but its frequency is raised to 6 KHz,
what is the new deviation?
(b) Explain the effect of multiple frequency modulations in FM System with an
example. [8+8]
5. (a) Draw the schematic diagram for basic reactance modulator using FET and
explain how this can be used as FM generator?
(b) Explain with block diagram the Armstrong method of FM generation. [8+8]
6. Draw the practical circuit of a balanced ratio detector, and show how it is derived
from the basic circuit. Explain the improvement effected by each of the changes.
[16]
7. Explain the SNR’s at Input and output of demodulators of DSB-SC and SSB-SC
and compare them. [16]
8. (a) What is the purpose of pre-emphasis and de-emphasis filtering? Explain the
filtering process with suitable sketches.
(b) Draw the phasor diagram of signal and noise in an angle modulated system
showing both the signal and noise components. [8+8]
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II B.Tech II Semester Supplimentary Examinations, Apr/May 2008

COMMUNICATION THEORY

(Electronics & Communication Engineering)

Time: 3 hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

⋆ ⋆ ⋆ ⋆ ⋆

1. (a) An AM transmitter radiates 8KW with carrier unmodulated and 9KW when
the carrier is sinusoidally modulated. Calculate the modulation index. If
another sine wave, corresponding to 50% modulation, is transmitted simulta-
neously, determine the total radiated power.
(b) What is Frequency Translation and discuss various methods of frequency
translation techniques. [8+8]
2. (a) Explain the modulation of VSB wave using Filter method with neat block
diagram and waveforms.
(b) What does modulation actually do to a message and carrier? [10+6]
3. (a) Write notes on amplitude limiting in ratio detector.
(b) Describe the detection of AM with carrier using Rectifier detector using neat
waveforms. What are its drawbacks compared to Envelope detector. [7+9]
4. An angle modulated signal has the form u(t) = 100 cos [2π fc t + 4 sin 2000πt ]
Where fc = 10 MHz.
(a) Determine the average transmitted power.
(b) Determine the peak-phase deviation.
(c) Determine the peak-frequency deviation.


(d) Is this an FM or a PM signal? Explain. [4+4+4+4]
5. A carrier 100 cos 2 π 1×0t volts is frequency modulated by a signal 5 cos 2 π ×
102t volts. The modulation index β is 0.3. The modulated wave is passed through
a band pass filter whose transfer function is:
H(f) = 0.5+0.25(f-9) when 9<f<11
= 0 otherwise
and frequency is in KHz.
The filtered signal; is passed throughout an ideal envelope detector which is followed
by an ideal low pass filter with cut off at 1.5 KHz. Find the signal power at the
output of the low pass filter. Use the following values of Bessel functions wherever
required. J0(0.3)=0.99, J1(0.3)=0.1, J2(0.3)=0.01, J3(0.3)=0.001, J4(0.3)=0.0001.
[16]
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Code No: RR220401 Set No. 2
6. Using neat circuit diagram and relevant waveforms discuss how an FM signal can
be detected using phase-discrimination method. What are its advantages and dis-
advantages? [16]
7. Explain the SNR’s at Input and output of demodulators of DSB-SC and SSB-SC
and compare them. [16]
8. Obtain the expression for SNR in the PM reception. [16]
⋆ ⋆ ⋆ ⋆ ⋆

II B.Tech II Semester Supplimentary Examinations, Apr/May 2008

COMMUNICATION THEORY

(Electronics & Communication Engineering)

Time: 3 hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

⋆ ⋆ ⋆ ⋆ ⋆

1. (a) A carrier wave of frequency 12MHz and the peak value of 12V is amplitude
modulated by a 6KHz sine wave with amplitude of 6V. Determine
i. Modulation index
ii. Amplitude in each side bands
iii. Frequency spectrum
iv. Band width required.
(b) Derive an expression for DSB-SC wave, if the message is m(t) and carrier is
Ac cos ωct. [9+7]
2. (a) How modulation can be achieved using non-linear characteristics of an active
device and hence show the circuit diagram of Modulator.
(b) Draw the schematic diagram of chopper type modulator. Hence, explain the
generation of DSB-SC using the Ring Modulator. [8+8]
3. (a) Write notes on amplitude limiting in ratio detector.
(b) Describe the detection of AM with carrier using Rectifier detector using neat
waveforms. What are its drawbacks compared to Envelope detector. [7+9]
4. (a) Define Modulation Index in FM. Discuss the spectra of NBFM and WBFM
for various Modulation Indices.
(b) Derive equation for FM carrier from fundamentals. Also differentiate FM and
PM. [7+9]
5. An RC capacitive reactance modulator is used to vary the frequency of a 10-MHz
oscillator by ±100KHz. An FET whose trans-conductance varies linearly with the
gate voltage from 0 to 0.625mS, is used in conjunction with a resistance whose
value is one-tenth of the capacitive reactance used. Calculate the inductance and
capacitance of the oscillator tank circuit. [16]
6. Draw the practical circuit of a balanced ratio detector, and show how it is derived
from the basic circuit. Explain the improvement effected by each of the changes.
[16]
7. (a) Explain the Signal-to-noise ratio (SNR) calculations for conventional AM.
(b) Derive an expression for effect of noise on SSB AM system. [8+8]
8. Explain the effect of random noise on the output of FM demodulator filtered with
an amplitude limiter. Develop the concept of noise triangle. [16]
1 of 2
Code No: RR220401 Set No. 3
⋆ ⋆ ⋆ ⋆ ⋆

II B.Tech II Semester Supplimentary Examinations, Apr/May 2008

COMMUNICATION THEORY

(Electronics & Communication Engineering)

Time: 3 hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

⋆ ⋆ ⋆ ⋆ ⋆

1. (a) Explain the two methods for calculation of total modulation index, when the
carrier is modulated by more than one modulating signal.
(b) What is VSB, where it is used? Explain its advantages and disadvantages over
SSB. [8+8]
2. (a) Draw the block diagram of the square law modulator and explain its operation
with the help of waveforms.
(b) Write short notes on VSB generation. [8+8]
3. (a) Explain the principle and operation of “slope Detector”.
(b) Give the block diagram for coherent detection of DSB-SC modulated wave
and explain the operation of it. Explain the process of frequency translation.
[7+9]
4. (a) A 25-MHz carrier is modulated by a 400-Hz audio sine wave. If the carrier
voltage is 4V and the maximum deviation is 10 KHz, write the equation of
this modulated wave for
i. FM and
ii. PM.
If the modulating frequency is now changed to 2 KHz, all else remaining
constant, write a new equation for
iii. FM and
iv. PM.
(b) Explain, how the bandwidth in FM is calculated. [12+4]
5. Explain the generation of FM signals
(a) Indirect method
(b) Direct methods. [8+8]
6. Explain the phase discrimination method of detection of FM signal. Using phasor
diagrams obtain the discriminator response. What are its advantages and disad-
vantages?
16]
7. A noise signal with a power density spectrum Sn(ω) is shown in figure 7 is trans-
mitted through an ideal band pass filter shown in figure 7 . Express the output
signal. [16]
1 of 2
Code No: RR220401 Set No. 4
Figure 7
Figure 7
8. What are the observations made from the signal to noise ratio (SNR) expressions
of FM system? Explain them in detail. What do you know about the threshold
extension in FM? [16]
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2 of 2