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Fifth Semester B.E. Degree Examination, May/June 2010Analog Communication

1 a. A carrier wave with amplitude 12 V and frequency 10 MHz is amplitude modulated to 50%level with a modulated frequency of 1 KHz. Write down the equation for the above waveand sketch the modulated signal in frequency domain. (07 Marks)

b. Draw and explain a square-law modulator along with related equations. (07 Marks)c. The antenna current of an AM broadcast transmitter modulated to a depth of 40% by an

audio sine wave is 11 A. It increases to 12 A as a result of sinusoidal modulation by anotheraudio sine wave. What is the modulation index due to second wave? (06 Marks)

2 a. With a neat block diagram, explain the balanced modulator method of generating DSBSCwave. (06 Marks)

b. With block diagram and related equations, explain coherent detection of a DSBSC wave.What is its disadvantage? Explain the synchronous receiving system. (08 Marks)

c. Explain FDM,with a neat block diagram. (06 Marks)

3 a. Show that the output of coherent detector of a SSB modulated, wave is given by :I I A

Va(t) = - Acm(t)cos$ ±-Acm(t) sin $ where $ is the phase error. (07 Marks)4 4b. Consider a two-stage product modulator with a BPF after each product modulator, where the

input signal consists of a voice signal occupying the frequency band 0.3 to 3.4 KHz. Thetwo local oscillator frequencies have the value f1 = 100 KHz and 6 = 10 MHz. Calculate thefollowing:i) Sidebands ofDSBSC modulated waves appearing at the two product modulator outputs.ii) Sidebands of SSB modulated waves appearing at the BPF outputs.iii) The pass-bands of the two BPI< s. (09 Marks)

c. State the properties of Hilbert transform. (04 Marks)

4 a. Show that a VSB modulated wave set) containing a vestige of the lower side band is definedA A Aby : set) = _c met) cos(21tfct) +_c met) sin(21tfct) (10 Marks)2 2

b. With a neat block diagram, explain the receiver section of a colour television system.(10 Marks)

5 a. Show that narrow band frequency modulation and amplitude modulation systems have thesame bandwidth. How do they differ in other aspects? Explain with necessary blockschematics and phasor diagrams. (10 Marks)

b. The equation for a FM wave is set) = 1OCOs[S.7 x 108t + S sin( 12x 103}]. Calculate:

i) Carrier frequency ii) Modulating frequency iii) Modulation indexiv) Frequency deviation v) Power dissipated in a loon resistor load. (06 Marks)

c. A FM signal has sinusoidal modulation with fro = 15 KHz and modulation index ~ = 2.Using Carson's rule, find the transmission bandwidth and deviation ratio. Assumei1f= 75 KHz. (04 Marks)

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6 a. Explain with a neat circuit diagram, the direct method of generating FM waves. Derive anexpression for instantaneous frequency ofFM for sinusoidal modulation. (10 Marks)

b. With a neat block diagram, explain the operation of FM stereo multiplexing anddemultiplexing. (10 Marks)

7 a. Define the following:i) Conditional probability ii) Random variable iii) Mean (06 Marks)

b. State the properties of Gaussian process. (04 Marks)c. For a cascade connection of two port network, show that the overall noise figure is given by:

F = F1+ (F2 -1) + (F3 -1) + .....

G1 G2

Where F 1, F2 are the noise figures andG1, G2 are the gains of the individual two port networks. (10 Marks)

8 a. Derive the SNR equations of SSB system and show that it has a figure of merit equal to 1.(08 Marks)

b. Explain FM threshold effect with related equation and diagram. (06 Marks)c. Discuss pre-emphasis and de-emphasis in FM. (06 Marks)