S.E. (Electronics/Electronics and Telecommunication Engg.) (Second Semester) EXAMINATION, 2014 COMMUNICATION THEORY (2008 PATTERN)

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1 Total No. of Questions 12] [Total No. of Printed Pages 7 Seat No. [4657]-49 S.E. (Electronics/Electronics and Telecommunication Engg.) (Second Semester) EXAMINATION, 2014 COMMUNICATION THEORY (2008 PATTERN) Time : Three Hours Maximum Marks : 100 N.B. : (i) Answers to the two Sections should be written in separate answer-books. (ii) (iii) (iv) Neat diagrams must be drawn wherever necessary. Figures to the right indicate full marks. Use of logarithmic tables and electronic pocket calculator is allowed. (v) Assume suitable data, if necessary. Section I 1. (a) Define Amplitude modulation. What are its types? Draw time domain and frequency domain representation for AM (DSBFC, DSBSC, SSB, VSB). [8] For each of the following baseband signals : (i) m(t) = cos 2 * 1000 t P.T.O.

2 (ii) m(t) = 2cos(2 * 1000 t) + cos(2 * 2000 t) (iii) m(t) = cos(2 * 1000 t) * cos(2 * 3000 t). (1) Sketch the spectrum of m(t). (2) Sketch the spectrum of the DSBSC signal m(t) * cos(2 * t). (3) Identify the USB, LSB and baseband frequencies in each case. [10] 2. (a) Draw and explain switching modulator with time domain and frequency domain representation. [6] The antenna current of an AM broadcast transmitter, modulated to a depth of 40% by an audio sine wave is 11 Amps. It increases to 12 Amps as a result of simultaneous modulation by another audio sine wave. What is the modulation index due to this second wave? [8] (c) Enlist and explain AM broadcast standards. [4] [4657]-49 2

3 3. (a) Draw and explain the Direct method of FM generation using Varactor diode. [6] Sketch FM and PM waves for the digital modulating signal m(t) shown in Fig. 1. The constants Kf and Kp are 2 * * 10 5 and /2 respectively and fc = 100 MHz. [10] Fig (a) Compare FM and PM. [4] Design (only the block diagram) an Armstrong indirect FM modulator to generate an FM carrier with a carrier frequency [4657]-49 3 P.T.O.

4 of 98.1 MHz and f = 75 khz. A narrow band FM generator is available at a carrier frequency of 100 khz and f = 10 Hz. The stock has an oscillator with an adjustable frequency in the range of MHz. There are also plenty of frequency doublers, triplers and quintuplers. [12] 5. (a) Explain various performance characteristics of radio receiver. [8] How is FM signal demodulated? Explain FM demodulation PLL method in detail. [8] 6. (a) In radio receiver frequency span to be received is from 525 khz to 1650 khz. If Cmin of tuning circuit is limited to 50 pf by trimmer of 25 pf, calculate value of padder capacitor. If the maximum possible capacitor is 450 pf., the IF used is 465 khz. [6] Differentiate coherent and non-coherent detection. [4] (c) Explain practical diode detector used for AM demodulation. [6] [4657]-49 4

5 SECTION II 7. (a) Two resistors of 20 K and 50 K are at room temperature for bandwidth of 100 khz, calculate thermal noise voltage generated by : [8] (i) (ii) (iii) Each resistor Resistors in series Resistors in parallel. Derive Friss formula for noise factor of cascaded amplifier. [8] 8. (a) Define noise. What are its types? Explain internal noises present in radio receiver. [8] An amplifier circuit having noise figure of 9 db and power gain of 25 db is followed by a mixer having a noise figure of 16 db, calculate overall noise figure and equivalent noise temperature at the input of the combination. [8] [4657]-49 5 P.T.O.

6 9. (a) With the help of a block diagram explain basic model of communication system in the presence of noise. [6] Time average of square of a modulating signal of 60 khz bandwidth is calculated as 0.1 W. The signal is used in DSB- FC modulation with carrier power 10 W. If additive white Gaussian noise power spectral density is 10 6 W/Hz, find output SNR if carrier power is reduced by 100 times. [10] 10. (a) Explain the performance of FM receiver in the presence of noise. [10] Define and explain preemphasis and deemphasis. [6] 11. (a) State and explain the sampling theorem for band limited low pass and band pass signal. [6] Enlist the disadvantages of Delta modulation. How are these overcome in ADM? [6] (c) What is PAM? How is it generated? Explain it with the help of suitable waveform. [6] [4657]-49 6

7 12. (a) Compare PAM, PWM and PPM systems. [8] Explain the following terms : [10] (i) (ii) PCM Flat top and natural sampling. [4657]-49 7 P.T.O.

B.Tech II Year II Semester (R13) Supplementary Examinations May/June 2017 ANALOG COMMUNICATION SYSTEMS (Electronics and Communication Engineering)

Code: 13A04404 R13 B.Tech II Year II Semester (R13) Supplementary Examinations May/June 2017 ANALOG COMMUNICATION SYSTEMS (Electronics and Communication Engineering) Time: 3 hours Max. Marks: 70 PART A