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

---