iii b.tech. i semester examinations may/june - 2012 analog communications ( electronics and...

8/12/2019 III B.Tech. I Semester Examinations May/June - 2012 ANALOG COMMUNICATIONS ( Electronics and Communication

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Code No: 09A50407/R09

III B.Tech. I Semester Examinations

May/June - 2012ANALOG COMMUNICATIONS

( Electronics and Communication Engineering )

Time: 3 Hours Max. Marks: 75

Answer any FIVE Questions

All Questions carry equal marks

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1. (a) Show giving a mathematical proof, how a square-law device can be used to generate an AM signal. Give

(b) A commerical AM station is broadcasting with an average transmitted power of 6 kW. The modulation index is

set at 0.65 for a sinusoidal message signal. Find the transmission power efficiency and the average power in the

(b) The modulating signal x(t) = 2 cos (2000t) + sin (8000 t) is applied to a DSB modulator operating with a carrier

3. (a) What are different methods of generating single sideband (SSB) signal? Explain Weavers method.

4. (a) Explain the following in connection with frequency modulation,

(i) Frequency deviation

(ii) Practical bandwidth and

(b) An angle modulated signal,

xc(t) = 10 cos [106t + 3 sin(2) (104 t)] is present across a 50 ohm resistive load.

Find,

(i) The total average power

(ii) Peak phase deviation

6. (a) Derive an expression for Signal-to-Noise Ratio (SNR) for double side band modulation scheme using coherent

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complete diagram of the signal inputting and outputting arrangements.

carrier component of the transmitted signal. [15]

2. (a) Draw the circuit diagram of a demodulator for DSB-SC signal.

frequency of 200 kHz. Sketch the power spectral density of the modulator output. [15]

(b) Compare SSB and Vestigial Sideband (VSB) signal.

(b) Distinguish between PAM, PPM, PWM and compare their characteristics.[15]

8. (a) Explain the generation of Pulse Width Modulation (PWM).

(b) What exactly does a noise limiter do in AM receiver? How does it do this? [15]

7. (a) Explain the important of Automatic Gain Control (AGC) and discuss simple AGC.

(b) Derive an expression for an SSB system using coherent demodulation scheme. [15]

demodulation.

(b) Explain the generation of FM using reactance modulator. [15]

5. (a) Explain the operation of ratio detector with help of diagrams.

(iii) Peak frequency deviation. [15]

(iii) Modulation index.

(c) Draw the schematic diagram of demodulator for a VSB signal and explain. [15]

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Code No: 09A50407/R09







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(b) The input to an envelope is an AM signal of the form xc(t) = (1 + k cos

mt) cos

ct, where k is a constant,

0 < k < 1 andc>>

m. If the demodulator is to follow the envelope at all times, show that RC

m

1

21- k

k.

(b) What is the frequency deviation of an FM transmitter, when its modulation index is 7 in a practical bandwidth

(b) Deduce an expression for frequency modulated wave and explain a method for generating such modulation,

7. (a) What do you understand by the following,

(i) Selectivity

(ii) Sensitivity

(iii) Fidelity

(iv) Tracking error as referred to superheterodyne AM radio receiver.

(b) A superheterodyne receiver having an RF amplifier is tuned to 15 MHz. The intermediate frequency is 455 kHz.

The RF amplifier and preselector are equal and are such that the image rejection is 41.58 dB. Calculate Q.

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1. (a) Define amplitude modulation. Explain with an example in time and frequency domain.

[15]

2. (a) Classify radio transmitters and explain.

(b) Give a comparison between AM and DSB-SC systems. [15]

3. (a) What are the advantages of Vestigial Sideband (VSB) transmission?

(b) Explain the generation of USB signal.

(c) Discuss the applications of different AM systems. [15]

4. (a) Distinguish between AM and narrowband FM.

of 160 kHz? [15]

5. (a) Why is an FM system preferred over an AM system? Discuss.

using a FET reactance modulator. [15]

6. (a) Compare AM, DSB-SC, SSB, SSB-SC with respect to bandwidth and SNR performance.

(b) Derive an expression for an SSB system using coherent demodulation. [15]

[15]

8. (a) Explain the generation of PAM signal.

(b) With a neat diagram explain the generation of Pulse Width Modulation (PWM). [15]

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Code No: 09A50407/R09







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(b) A carrier is amplitude modulated by a combination of two sinusoidal modulating signal of different frequencies

with individual modulation indices 0.3 and 0.4. Find the power in the sidebands as percentage of the total

(b) Discuss with a block diagram of a communication system using vestigial sideband transmission. [15]

4. (a) Derive an expression for an FM signal with carrier frequency fcand a modulating signal 10 cos (100t) + 20 cos

(200

t). Obtain an expression for the spectrum.

(b) Consider an angle modulated signal xc(t) = 10 cos (

ct + 3 cos

mt), f

m= 1 kHz. Assume the modulation to be

FM. Determine the modulation index and find the transmission bandwidth when,

(i) m

is increased by a factor 4 and

m

(b) With a neat block diagram discuss the demodulation of pulse position modulation signal. [15]

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1. (a) Explain need for modulation with examples.

power of the modulated signal. [15]

2. (a) With the help of a block diagram explain AM transmitter.

(b) Draw the circuit of a demodulated for DSB-SC signal and explain. [15]

3. (a) Discuss the generation of SSB using phase discrimination method.

(ii) When is decreased by a factor of 4. [15]

5. (a) With the help of neat block diagram explain FM transmitter.

(b) Explain the generation of FM using armstrong method. [15]

6. (a) Explain the importance of pre-emphasis and de-emphasis.

(b) Derive an expression for an SSB system using coherent demodulation scheme. [15]

8. (a) Explain the generation of Pulse Amplitude Modulation (PAM) signal.

(b) List and discuss the factors influencing the choice of the IF for a radio frequency. [15]

7. (a) Compare tuned radio frequency receiver and superheterodyne receiver.

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Code No: 09A50407/R09







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3. (a) A signal x(t) = 2 cos(2000 t) + cos(4000 t) is multiplied by a carrier 10 cos(106t). Write down the expression

(b) Explain what is meant by Vestigial Sideband (VSB) transmission. What are its special characteristics and how

(b) An audio signal x(t) has a bandwidth of 15 kHz. The maximum value of |x(t)| is 1 volt. This signal is used to

frequency modulate a carrier at 50 MHz. Estimate the bandwidth of the FM signal if the modulator has afrequency deviation.

(i) 2000 Hz/volt

(ii) 4000 Hz/volt

5. (a) Write down the expression for a FM signal with sinusoidal modulation, carefully defining each symbol used.

Obtain an expression for its spectrum.

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(b) Explain the generation of single polarity PAM and double polarity PAM. [15]

8. (a) Explain the generation of Pulse Width Modulation (PWM) signal.

(b) List and discuss the factors influencing the choice of the IF for a radio frequency. [15]

7. (a) What is a simple automatic gain control? What are its functions.

(b) Derive an expression for SNR for an DSB using coherent demodulation. [15]

6. (a) Explain the role of threshold effect in angle modulation system.

(b) Explain the generation of FM signal using a FET reactance modulator.[15]

(iii) 25000 Hz/volt.[15]

4. (a) Why is an FM system preferred over an AM system? Discuss.

for the upper sideband terms of the product signal.

can these be achieved in practice. Discuss the specific examples where VSB is used. [15]

(b) Draw block diagram of AM transmitter and explain each block. [15]

2. (a) Explain the generation of DSB-SC using ring modulator.

(b) With a neat diagram explain square law detector. [15]

1. (a) What is frequency translation? Explain its importance with an example.

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iii b.tech. i semester examinations may/june - 2012 analog communications ( electronics and...

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