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ECE Objective Questions { Digital Communication System }

85. Which of the following is not true regarding auto-correlation function :
A. it is an average function
B. it is defined only for periodic signals
C. it is a measure of both time variation and statistical dependence
D. it is non-unique description of signals.

86. Probability density function of thermal noise is
A. Binomial
B. Gaussian
C. Poisson
D. None of the above

87. The Gaussian probability density is defined as
A.2fc a e?(x?m)
B.2/2 cr2
C. f(x)=2
D. f(x) = 27c a2

88. The auto-correlation function of white noise is a
A. step function
B. impulse function
C. Gaussian function
D. constant

89. The auto-correlation function for the function x(t) = V sin cot is given by
A. V2 cos cot
B. -2 V', cos cot
C. V2 cos2 cot
D. 2V2 cos cot

90. If x and y are two independent Gaussian random variables, each with average value zero and with variance 152, the joint density function is defined as :
A. gx, y)= f(x) + f(y)
B. gx, y)= f(x) ? gy)
C. gx, y)= f(x)- f(y)
D. gx, y) = f(x)/ f(y)

91. In a TDM system, different signals are distinguished from each other
A. only in time
B. only in frequency
C. both in time and frequency
D. in amplitude

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