1. What is the time period of the function x[n] = exp(jwn)?
a) pi/2w
b) pi/w
c) 2pi/w
d) 4pi/w
Explanation: Using Euler’s rule, exp(2pi*n) = 1 for all integer n. Thus, the answer can be derived.
2. What is the nature of the following function: y[n] = y[n-1] + x[n]?
a) Integrator
b) Differentiator
c) Subtractor
d) Accumulator
Explanation: If the above recursive definition is repeated for all n, starting from 1,2.. then y[n] will be the sum of all x[n] ranging from 1 to n, making it an accumulator system.
3. Is the function y[n] = x[n-1] – x[n-4] memoryless?
a) True
b) False
Explanation: Since the function needs to store what it was at a time 4 units and 1 unit before the present time, it needs memory.
4. Is the function y[n] = x[n-1] – x[n-56] causal?
a) The system is non causal
b) The system is causal
c) Both causal and non causal
d) None of the mentioned
Explanation: As the value of the function depends solely on the value of the input at a time presently and/or in the past, it is a causal system.
5. Is the function y[n] = y[n-1] + x[n] stable in nature?
a) It is stable
b) It is unstable
c) Both stable and unstable
d) None of the mentioned
Explanation: It is BIBO stable in nature, i.e. bounded input-bounded output stable.
6. We define y[n] = nx[n] – (n-1)x[n]. Now, z[n] = z[n-1] + y[n]. Is z[n] a causal system?
a) No
b) Yes
Explanation: As the value of the function depends solely on the value of the input at a time presently and/or in the past, it is a causal system.
7. Discrete-time signals are _________________
a) Continuous in amplitude and continuous in time
b) Continuous in amplitude and discrete in time
c) Discrete in amplitude and discrete in time
d) Discrete in amplitude and continuous in time
Explanation: A discrete-time signal is continuous in amplitude and discrete in time. It can either be present in nature or is sampled from an analog signal. A digital signal is discrete in amplitude and time.
8. Determine the discrete-time signal: x(n)=1 for n≥0 and x(n)=0 for n<0
a) Unit ramp sequence
b) Unit impulse sequence
c) Exponential sequence
d) Unit step sequence
Explanation: Unit step is defined by: x(n)=1 for n≥0 and x(n)=0 for n<0.
9. Determine the value of the summation: ∑∞n= -∞ δ(n+3)(n2+n).
a) 3
b) 6
c) 9
d) 12
Explanation: ∑∞n= -∞ δ(n+3)(n2+n)
⇒ δ(n+3)=1 when n= -3 otherwise 0.
Therefore, the limit reduces to n = -3
⇒ ∑∞n = -∞ δ(n+3)(n2+n) = (n2+n)|n = -3 = (-3)2-3 = 9 – 3 = 6
10. Determine the product of two signals: x1 (n) = {2,1,1.5,3}; x2 (n) = { 1,1.5,0,2}.
a) {2,1.5,0,6}
b) {2,1.5,6,0}
c) {2,0,1.5,6}
d) {2,1.5,0,3}
Explanation: Product of discrete-time signals is computed element by element.
⇒ x(n) = x1 (n) * x2 (n) = {2×1, 1×1.5, 1.5×0, 3×2} = {2,1.5,0,6}.