Communication Engineering - MCQs from AMIE exams (Winter 2018)

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Choose the correct answer for the following (10 x 2)

1. For a linear electromagnetic circuit, which statement is true?
(a) Field energy is equal to the co-energy
(b) Field energy is greater than co-energy
(c) Field energy is less than co-energy
(d) Co-energy is zero

2. If E̅ is the electric field intensity, V(V x E̅) is equal to
(a) E̅
(b) |E̅|
(c) Null vector
(d) Zero

3. For the scalar field u = \frac{1}{2}{x^2} + \frac{1}{3}{y^2} the magnitude of the gradient at the point (1, 3) is
(a) \sqrt {13/9}
(b) \sqrt {9/2}
(c) \sqrt 5
(d) 9/2

4. A regression model is used to express K as a function of another variable X. This implies
(a) There is a causal relationship between Y and X
(d) A value of X may be used to estimate a value of F
(c) The values of X exactly determine values of Y
(d) There is no causal relationship between F and X

5. The Fourier transform of f(t) = e-a|t| where a > 0 is
(a) \frac{s}{{{a^2} + {s^2}}}
(b) s.({a^2} + {s^2})
(c) \frac{1}{{\sqrt {2\pi } }}.\frac{s}{{{a^2} + {s^2}}}
(d) None of the above

6. An example of a noise that is not naturally occurring is
(a) due to atmospheric disturbances
(d) due to extraterrestrial radiation
(c) due to random electron motion
(d) due to ignition and commutator sparking

7. Carson’s formula for estimating FM transmission bandwidth states that (symbols have usual meaning)
(a) BT = DW
(b) BT = 2(D + 1)W
(c) BT = 2(W + 1)D
(d) BT = (W + D)D

8. The ideal sampling function is defined as
(a) {s_\delta }(t) = \sum\limits_0^\infty {\delta (t - kTs)}
(b) {s_\delta }(t) = \sum\limits_{ - \infty }^0 {\delta (t - kTs)}
(c) {s_\delta }(t) = \sum\limits_{ - \infty }^\infty {\delta (t - kTs)}
(d) {s_\delta }(t) = \sum\limits_{ - \infty }^\infty {\delta (t - 2kTs)}

9. Comma code ensures decipherability by
(a) ensuring that no codeword appears as a prefix of another codeword
(d) ensuring that all codewords are of equal length
(c) ensuring that some codewords appear as the prefix of some other codewords
(d) marking the start of each codeword with the same bit.

10. Linear predictive coding (LPC) is a novel approach to a digital representation of analog signals. This method uses a
(a) Transversal filter
(b) Quadrature filter
(c) Switched capacitor filter
(d) Equiripple filter

Answers

1. (a) Co-energy is the dual of energy, a non-physical quantity useful for theoretical analysis of systems storing and transforming energy. It is expressed in the same units as energy and is especially useful for the calculation of magnetic forces and torque in rotating machines. The co-energy is zero for systems incapable of storing energy. For linear electromagnetic circuit field energy is equal to co energy.

2. (d) 

3. (c) \nabla u = \left( {\frac{\partial }{{\partial x}}i + \frac{\partial }{{\partial y}}j} \right)u = \frac{{2x}}{2}i + \frac{{2y}}{3}j = xi + \frac{2}{3}yj
{\left. {\nabla u} \right|_{(1,3)}} = 1xi + \frac{2}{3}x3j = i + 2j
{\left| {\nabla u} \right|_{(1,3)}} = \sqrt {{1^2} + {2^2}} = \sqrt 5

4. (a)

5. (d) F(\omega ) = \int_{ - \infty }^0 {{e^{at}}{e^{ - j\omega t}}dt + \int_0^\infty {{e^{at}}{e^{ - j\omega t}}dt} }
= \frac{1}{{a - j\omega }} + \frac{1}{{a + j\omega }} = \frac{{2a}}{{{a^2} + {\omega ^2}}}

6. (c) 
  • Natural noise gets generated due to either natural phenomena or atmospheric actions like solar flares, radiation in space, electronic storms etc.
  • Internal Noise is the fundamental noise that gets generated by the electronic equipment involved in the system itself.
7.  (b) Carson's rule is given by B = 2 (D + 1) W where B = Bandwidth, D =  modulation depth ‘or’ the modulation index and W = maximum modulating frequency.

8. (c)

9. (a) A comma code is a type of prefix-free code in which a comma, a particular symbol or sequence of symbols, occurs at the end of a code word and never occurs otherwise. For example, Fibonacci coding is a comma code in which the comma is 11. 11 and 1011 are valid Fibonacci code words, but 101, 0111, and 11011 are not.

10. (c) 
  • The technique that has recently emerged as the  leading approach to speech synthesis is linear prediction coding  (LPC). 
  • LPC offers  several advantages to the chip designer. First, it offers high  quality of speech for a relatively low bit rate. Second is the  flexibility of LPC in improving speech quality at higher bit  rates with no change in hardware, as well as compatibility y  with a wide range of speakers. Another advantage is the ease  of analysis which can be implemented in real time on silicon  to creat~ a complete analysis/synthesis system.
  • This chip uses a high-resolution  switched-capacitor analog multiplier in conjunction with sample-and-hold stages to implement the ten-pole LPC filter.
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