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Solution to Previous Year Exam Questions for Analog Communication Technique - ACT of by sumit saurabh

  • Analog Communication Technique - ACT
  • 2018
  • PYQ Solution
  • Silicon Institute of Technology - SIT-SBP
  • Electronics and Communication Engineering
  • B.Tech
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Probable Q & A (Short type) Sub-AC Sem-5th Sec-ECE Prepared by- Mr. M. R. Jena 1. What is envelope distortion? The baseband signal is preserved in the envelope of AM signal only if () m(t)max ≤ A i.e. modulation index = m = ≤1 Where m(t) is original message signal and A is carrier amplitude. The envelope preserves the baseband signal only if the percentage modulation is less than or equal to 100 percentage. When m>1, the envelope does not preserve the baseband signal. Here the baseband signal is recovered from the envelope is distorted. This type of distortion is called as envelope distortion. 2. What is noise temperature? It is defined as the temperature at which a noisy resistor has to be maintained such that by connecting this resistor to the input of the noiseless version of the system. It produces the same available noise power at the output of the system as is produced by all the sources of noise in the actual system. 3. What is noise bandwidth? How it is related with 3-dB bandwidth? It is an important parameter used for specifying the noise power at the output of a bandpass linear system. 3-dB bandwidth of a system is defined as range of frequencies for which power does not fall below half of maximum power. In actual systems 3-dB bandwidth (BN) is less than equivalent noise bandwidth (B3dB). So BN > B3-dB But for ideal systems both are same. 4. Define Shot noise. Ans: The noise caused by random fluctuations in the motion of charge carriers in a conductor. In semiconductor devices, it is caused due to random diffusion of minority carriers or random generation & recombination of electron-hole pairs. The current in(t) fluctuates around a mean value(I0) and this current is called as shot noise. So the total current is expressed as i(t) = I0 + in(t) 1

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5. What is White noise? Ans: Many types of noise sources are Gaussian and have flat spectral density over a wide frequency range. Such spectrum has all frequency components in equal portion, and is therefore called white noise. The power spectral density of white noise is independent of the operating frequency. 6. Calculate noise figure and equivalent noise temperature for a receiver connected to an antenna whose resistance is 100Ω and equivalent noise resistance is 50Ω. Ans: F = (1 + (Req / Ra)) = ( 1 + ( 50 / 100) ) = 1.5. 7. Define Noise equivalent Bandwidth. Ans: The noise equivalent bandwidth of the filter is defined as the bandwidth of an ideal filter at which the noise power passed by real filter and ideal filter is same. 8. Define Thermal noise. Give the expression for the thermal noise voltage across a register. Ans: Thermal noise is the name given to the electrical noise arising from the random motion of electrons in a conductor. 9. What is meant by figure of merit of a receiver? Ans: The ratio of output signal to noise ratio to channel signal to noise ratio is called figure of merit. Figure of Merit = (SNR)o/(SNR)c = (So/No)/(Si/Ni) 10. Define Noise Figure. Ans: It is defined as the ratio of input signal to noise ratio to the output signal to noise ratio. F = Input (s/n) / output (s/n). 11. Define Noise Temperature. Ans: The available noise power is directly proportional to temperature and it is independent of value of resistance. this power specified in terms of temperatures is called as noise temperature. 2

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12. What is partition Noise? Ans: In an electron tube having one or more positive grids, this noise is caused by irratic partition of the cathode current among the positive electrodes. In a transistor, the partition noise is created from the random fluctuation in the division of current between the collector and base. 13. Define a random variable? Ans: Random variable is defined as a rule or mapping from the original sample space to a numerical sample space subjected to certain constraints. Random variable is also defined as a function where domain is the set of outcomes ωεs and whose range is R, is the real line. 14. What is a random process? Ans: A Random process X (s,t) is a function that maps each element of a samples space into a time function called sample function. Random process is a collection of time functions. 15. What is a stationary random process? Ans: When the statistical properties of a process do not change with time, it is called Stationary process. 16. A receiver connected to an antenna of resistance of 50Ω has an equivalent noise resistance of 30Ω. Find the receiver noise figure. Ans: F = (1 + (Req / Ra)) = ( 1 + ( 30 / 50) ) = 1.6. 17. Compare narrow band FM and Wideband FM. Ans: 3

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18. What is pre-emphasis? Why is it used? Ans: The premodulation filtering in the transistor, to raise the power spectral density of the base band signal in its upper-frequency range is called pre emphasis (or pre distortion) Pre emphasis is particularly effective in FM systems which are used for transmission of audio signals. 19. Define phase modulation. Ans: It is a type of modulation, used in communication systems, in which the phase of a carrier wave is varied by an amount proportional to the instantaneous amplitude of the modulating signal. 20. What is Narrowband FM? Ans: Narrowband FM has only a single pair of significant sidebands. 21. What are the applications of phase locked loop? Ans: Phase-locked loops are widely used in radio telecommunications, computer and other electronic applications. communication, 22. State the frequency in an FM system is 500 Hz and modulating voltage is 3 V,modulation index is 60. Calculate maximum deviation and bandwidth. Ans: Modulation index m = δ/fm 60 = δ/ 500 δ = 60 x 500 = 30 kHz Bandwidth BW = 2 δ = 2 x 30 = 60 kHz. 23. Mention advantages of angle modulation over amplitude modulation. Ans: 1. The amplitude of FM is constant. It is independent of depth of modulation. 2.Hence transmitter power remains constant in FM whereas it varies in AM. 3.Since amplitude of FM is constant, the noise interference is minimum in FM. 3. FM uses UHF and VHF ranges, the noise interference is minimum compared to AM which uses MF and HF ranges. 24. A 80MHz carrier is frequency modulated by sinusoidal signal of 1V amplitude and the frequency sensitivity is 100Hz/V. Find the approximate bandwidth of the FM waveform if the modulating signal has a frequency of 10KHz. 4

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