Analog Communications 4

JNTUWORLD

Code: 9A04501

B.Tech III Year I Semester (R09) Regular & Supplementary Examinations December/January 2013/14 ANALOG COMMUNICATIONS

(Electronics and Communication Engineering) Time: 3 hours Max Marks: 70

Answer any FIVE questions All questions carry equal marks

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1 (a) Discuss about different modulation methods. (b) Explain the benefits of coding. 2 (a) How low pass signals are converted into band pass signals? (b) A DSBSC transmitter radiates 1 KW when the modulation percentage is 45%.How much carrier

power is required if we want to transmit the same message by an AM transmitter? 3 (a) Give the comparison of different AM systems. (b) Describe the demodulation of AM SSB signal using filter method. 4 (a) What is the need for amplitude limiter circuit in FM? (b) In an FM system, when an audio frequency is 500 Hz, and the AF voltage is 2.4 V, the

frequency deviation is 4.8 KHz. If the AF voltage increased to 6.4 V, what is the new deviation? Find the modulation index in each case.

5 Draw the block diagram of indirect FM transmitter and explain the necessity of each block. 6 (a) Explain about super hetero dyne tracking. (b) In a broadcast super heterodyne receiver having no RF amplifier, the loaded Q of the antenna

coupling circuit is 100. If the Intermediate frequency is 455 kHz, further in order to have same image frequency rejection at IF frequencies of 1 MHz and 25 MHz, calculate (i) The loaded Q which an RF amplifier for this receiver would have to have and (ii) The new inter mediate frequency that would be needed.

7 (a) Explain about the sources of thermal noise. (b) Obtain the expression for output SNR of AMDSB-SC system. 8 (a) With block diagram explain the generation of PWM signal. (b) What sampling rate would be appropriate for a telephone channel limited to 3.3 kHz bandwidth?

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Code: 9A04501




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1 (a) What are the applications of modulation? Explain in detail. (b) What are the applications of coding methods? Explain in detail. 2 (a) With the help neat block diagram discuss about Balanced Modulator (b) The antenna current of an AM transmitter is 8 A when only the carrier is sent, but it increases

to 8.93 A when the carrier is modulated by a single sine wave. Find the percentage modulation. Determine the antenna current when the percentage of modulation changes to 0.8

3 (a) Discuss the benefits of suppressed carrier AM systems (b) Describe the generation AMSSB using phase shift method 4 (a) Sketch the waveforms of AM, FM and PM. Assuming carrier sinusoidal and modulating signal

is sine wave pulse. (b) Explain the relation of Bessel Coefficients in terms of modulation index 5 (a) What is the need for pre-emphasis, discuss in detail. (b) A differentiator circuit behaves like an FM slope detector, Justify the statement 6 (a) Explain about double spotting in SRF receiver (b) In a broadcast Super Heterodyne Receiver having no RF amplifier, the loaded Q of the

antenna coupling circuit is 150. If the Intermediate frequency is 455 kHz, calculate (i) The image frequency and its rejection ratio at 1 MHz (ii) The image frequency and its rejection ratio at 25 MHz.

7 (a) Derive an expression for equivalent noise bandwidth of an RC circuit (b) Derive an expression for input signal and noise power of AM system. 8 (i) Sketch a channel interleaving scheme for TDM, the following PAM channels: four 4 kHz

telephone channels and one 20 kHz music channel. (ii) Find the pulse repetition rate of the multiplexed signal (iii) Estimate the minimum system bandwidth required

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JNTUWORLD

Code: 9A04501




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1 (a) Define and describe following terminologies of communication systems: (i) Broadband signal. (ii) Demodulation. (iii) Modulation. (iv) Baseband signal.

(b) Discuss the functionality of radio receiver. 2 (a) Draw and explain the low pass and band pass transfer functions. (b) Sketch and explain the AM signal waveform when it is sinusoidally modulated for following

cases: (i) Under modulation. (ii) Critical modulation. (iii) Over modulation 3 (a) Sketch the spectrum of suppressed carrier AM systems when one sideband is also

suppressed. Assume sinusoidal carrier and sinusoidal modulating signal. (b) Calculate the percentage power saving when the carrier and one of the side bands are

suppressed in an AM wave modulated to a depth of (i) 55% (ii) 75%. 4 (a) Explain about hard limiter used in FM circuits. (b) Certain FM system is having maximum frequency deviation of 75 kHz and modulating

frequencies are cover from 30 Hz to 15 kHz. Find modulation index and bandwidth. 5 (a) Explain the generation FM using direct method. (b) Compare slope detector and balanced slope detector. 6 (a) Discuss about tracking curves of radio receiver. (b) In a broadcast super heterodyne receiver having no RF amplifier, the loaded Q of the antenna

coupling circuit is 100. If the Intermediate frequency is 455 kHz, further in order to have same image frequency rejection at IF frequencies of 1 MHz and 20 MHz, calculate (i) The loaded Q which an RF amplifier for this receiver would have to have and (ii) The new inter mediate frequency that would be needed.

7 Write short notes on: (a) Threshold effect in FM. (b) Noise equivalent bandwidth. (c) Band pass noise representation. 8 (a) Discuss the constraints in pulse transmissions. (b) Sketch a channel interleaving scheme for TDM, the following PAM channels: five 4 kHz

telephone channels and one 20 kHz music channel.

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JNTUWORLD

Code: 9A04501




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1 (a) With the help of block diagram explain the elements of communication system. (b) Explain how modulation can be used for efficient transmission. 2 (a) Draw and explain the band pass signal and spectrum. (b) Find percentage power saving when only carrier is suppressed in AM signal. Assume modulation

index is 1. 3 (a) Compare the different AM systems. (b) Discuss the frequency translation techniques using square wave carrier. 4 (a) Discuss about Carson’s rule. (b) In an FM system, when an audio frequency is 500 Hz, and the AF voltage is 2.4 V, the frequency

deviation is 4.8 KHz. If the AF voltage increased to 10 V while audio frequency dropped to 200 Hz, then what is the new deviation? Find the modulation index in each case.

5 It is required to provide a maximum frequency deviation of 75 kHz for the 88 MHz carrier

frequency of a certain FM transmitter. A FET is used as a capacitive reactance modulator, and the linear portion of its gm –vgs curve lies from 220 µS (at which Vgs = - 2 V) to 730 µS (at which Vgs = - 0.5 V). Assuming that Rgs is one-tenth of XCgd. Calculate (i) The rms value of the required modulating voltage (ii) The value of the fixed capacitance and inductance of the oscillator tuned circuit across which the reactance modulator is connected.

6 (a) Discuss the draw backs of tuned radio frequency receiver. (b) Discuss about linearized PLL models in time domain. 7 (a) Derive the expressions for input and output signal powers in FM system with noise. (b) Obtain the expression for rms thermal noise current generated by two resistors when they are

connected in parallel. 8 (a) Discuss the issues of practical sampling and aliasing. (b) Consider PAM transmission of a voice signal with maximum frequency content of 3 kHz.

Calculate bandwidth of a system if sampling frequency is 8 kHz and pulse width is 0.1 times of sampling period.

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Analog Communications 4