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Solution to Previous Year Exam Questions for Analog Communication Technique - ACT of BPUT by Mitu Baral

  • Analog Communication Technique - ACT
  • 2014
  • PYQ Solution
  • Biju Patnaik University of Technology Rourkela Odisha - BPUT
  • Computer Science Engineering
  • B.Tech
  • 1643 Views
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  • Uploaded 8 months ago
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Subject: Analog Communication Technique Subject Code: PCEC 4302 Semester: Fifth Semester Applicable Branch(S): CSE, IT Question Code: H 180 Year of Examination: 2014 Exam Type: (Special/Regular): Regular NATIONAL INSTITUTE OF SCIENCE &TECHNOLOGY PALUR HILLS, BERHAMPUR, ORISSA – 761008, INDIA

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Analog Communication Technique (PCEC-4302) CSE-IT (Regular Exam-2014) Q.1 Answer the following Questions: (a) What type of filter is used in generation of DSB-SC signal? Justify. Ans:  Generally Notch filter is used for generation of DSB-SC signal.  In DSB-SC transmission to transmit the band frequency while sub-press the carrier frequency, notch filter is used. (b) Find out the frequency spectrum of AM signal. Ans: The equation of single tone AM signal is S AM  f (t ) cos c t  A cos c t Or S AM  1 F (  c )  F (  c )  A (  c )   (  c ) 2 [Frequency Spectrum of AM signal] (c) In a FM wave the frequency deviation is 15 kHz. What is the maximum phase deviation does it represent if modulation signal is 1.5 kHz? Ans: The equation of narrow band FM signal is given by SFM (t)  A c os ct  m f sin m t  , Where m f  f fm Here phase deviation   m f sin m t . Phase deviation will me maximum if sin m t  1 ;  max  m f  f 15KHz   10 f m 1.5KHz National Institute of Science and Technology, Berhampur-761008 Page 2

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Analog Communication Technique (PCEC-4302) CSE-IT (Regular Exam-2014) (d) A NBFM signal has carrier frequency of fc and frequency deviation of Δf. The signal is passed through a frequency doubler circuit. Find out new carrier frequency and deviation. Ans: Here carrier frequency becomes twice (2fc) and frequency deviation remains constant. (e) What happened to the average power of the modulated signal, when the modulation index of FM signal is doubled? Ans: Since the power of the FM signal does not depend upon the modulation index, so there will be no change in the average power if modulation index will be doubled. (f) What do you mean by noise bandwidth? Express the noise bandwidth of simple RC filter in terms of 3-dB frequency. Ans:  Noise bandwidth of a filter may be defined as the bandwidth of an ideal band pass filter which produces the same noise power as the actual system does.  H (f)  1 ,  f  1 j   fc  where f c  1 =3dB frequency. 2 RC (g) Draw the power spectral density of white noise? Ans:  White noise is a random signal with a constant power spectral density. National Institute of Science and Technology, Berhampur-761008 Page 3

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Analog Communication Technique (PCEC-4302) CSE-IT (Regular Exam-2014) (h) How noise figure of AM transmitting system depend on modulation depth? Ans: Let f (t)  m Acos mt is a single tone sinusoidal modulation signal. 2 m 2 A2 f (t) 2  For an AM system,   2 2 2 2 m A A  f (t) A  2 m2  2  m2 (i) What is the threshold effect of FM signal? Ans:  FM threshold is usually defined as a Carrier-to-Noise ratio at which demodulated Signal-to-Noise ratio falls 1dB below the linear relationship. This is the effect produced in an FM receiver when noise limits the desired information signal.  Below the FM threshold point, the noise signal (whose amplitude and phase are randomly varying) may instantaneously have amplitude greater than that of the wanted signal. (j) What is the transmission bandwidth for AM and FM signal? Ans:  Transmission bandwidth of AM signal is BW  2 f m , where “fm” is the frequency of baseband signal.   Transmission bandwidth of FM signal is BW  2 1  m f f m , where “mf” is the modulation index of FM signal. Q.2 (a) What is a signum function? Find out the Fourier transform of the signum function. Ans: Signum Function: The sign function or signum function is an odd mathematical function that extracts the sign of a real number. In mathematical expressions the sign function is often represented as “sgn”. sgn(t)  lim e at u(t)  eat u( t)  a 0 National Institute of Science and Technology, Berhampur-761008 Page 4

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