This set of Signals & Systems Multiple Choice Questions & Answers (MCQs) focuses on “Properties of Systems – II”.

1. What is the following type of system called? y[n] = x[n] + y[n-1]
a) Subtractor system

b) Adder system

c) Product System

d) Divisor System

View Answer

Explanation: If we write for n-1, n-2, .. we will obtain y[n] = x[n] + x[n-1] + x[n-2] …,

thus obtaining an adder system.

2. Which of the following systems is linear?

a) y(t) = sin(x(t))

b) y(t) = log(x(t))

c) y(t) = cos(x(t))

d) y(t) = dx(t)/dt

View Answer

Explanation: Only d satisfies both the scaling and the additivity properties.

3. Which of the following systems is stable?

a) y(t) = log(x(t))

b) y(t) = exp(x(t))

c) y(t) = sin(x(t))

d) y(t) = tx(t) + 1

View Answer

Explanation: Stability implies that a bounded input should give a bounded output. In a,b,d there are regions of x, for which y reaches infinity/negative infinity. Thus the sin function always stays between -1 and 1, and is hence stable.

a) y(t) = x(2t) + x(t)

b) y(t) = x(t) + x(1-t)

c) y(t) = -x(t) + x(1-t)

d) y(t) = x(t) + x(t-1)

View Answer

Explanation: In each of a, b and c there is a negative sign of t involved, which means a backward shift of t-0 in time, would mean a forward shift in each of them. However, only in d, the backward shift will remain as backward, and undiminished.

5. State whether the differentiator system is causal or not.

a) Causal system

b) Non causal system

View Answer

Explanation: The derivative of a function can be written in forward difference and in backward difference form, hence the derivative would depend on a slightly forward value of the function, thus making it non causal.

6. State whether the differentiator system is a stable system or not.

a) Stable system

b) Unstable system

View Answer

Explanation: The derivative of a function can be unbounded at some bounded inputs, like tan(x) at x=pi/2, hence the differentiator system is unstable in general, when the input is not specified.

7. Which of the following systems is memoryless?

a) y(t) = x(2t) + x(t)

b) y(t) = x(t) + 2x(t)

c) y(t) = -x(t) + x(1-t)

d) y(t) = x(t) + 2x(t+2)

View Answer

Explanation: A system possessing no memory has its output depending upon the input at the same time instant, which is prevalent only in option b.

8. For what value of k, will the following system be time invariant?y(t) = x(t) + x(kt) – x(2t) + x(t-1)

a) 1

b) 2

c) 3

d) 2.5

View Answer

Explanation: A system possessing no memory has its output depending upon the input at the same time instant, which is prevalent only in option b.

a) No

b) Yes

View Answer

Explanation: The function y = sin(nx) is periodic only for rational ‘n’.

10. State whether the following system is periodic or not. y(t) = log(sin(x(t)).

a) Yes

b) No

View Answer

Explanation: Sin x is a periodic function, but log x is not a periodic function. Thus y is log t, where t= sin x, thus y is not periodic.

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