# Digital Communications Questions and Answers – Classification of Signals

This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Classification of Signals”.

1. Find the periodicity of the following signal. x(t)=cos((2π/7)t)sin((3π/5)t)
a) 30
b) 7
c) 35
d) 5/3

Explanation: T1=2π/2π/7 = 7, T2=2π/3π/5=5/3, T1/T2=5/21, T=21T1 or 5T2, T=35.

2. Find the fundamental period of 1+sin^2 (3π/5)n.
a) 10/3
b) 5
c) 3π/5
d) None of the mentioned

Explanation: sin2(3π/5)n = (1-cos(6π/5)n)/2 = 1/2-1/2cos(6π/5)n ; period = 6π/(5*2π) = k/N = 3/5 ; N=5.

3. Which signal is called as energy signal?
a) Finite energy and zero power
b) Finite energy and non-zero power
c) Infinite energy and zero power
d) Infinite energy and non-zero power

Explanation: A signal is said to be an energy signal if it has finite energy and zero average power.

4. Which signal is said to be power signal?
a) Infinite power and zero energy
b) Infinite power and non-zero energy
c) Finite power and infinite energy
d) Finite power and zero energy

Explanation: If a signal has finite power but infinite energy, it is considered to be a power signal.

5. Determine the periodicity and also find whether it is a power or energy signal?
e(-at)u(t)
a) Periodic and energy signal
b) Non periodic and energy signal
c) Periodic and power signal
d) Periodic and energy signal

Explanation: This is non-periodic because it does not repeat itself after a certain amount of time. And when we calculate the energy, we obtain 1/2a, which is finite. As a result, it is an energy indicator.

6. Continuous Impulse signal is a power or energy signal?
a) Power signal
b) Energy signal
c) Both power and energy
d) Neither power nor energy signal

Explanation: We’ll find out it’s neither power nor energy signal if we use the power and energy calculation formula.

7. Discrete impulse signal is a power or energy signal?
a) Power signal
b) Energy signal
c) Both power and energy signal
d) Neither power or energy signal

Explanation: When we calculate the energy of this signal using the energy formula, we get 1 as the result, which is a finite value. As a result, it is an energy indicator.

8. A unit impulse function has?
a) Large amplitude
b) Zero pulse width
c) Unity weight
d) All of the mentioned

Explanation: The amplitude of an impulse function is infinite or huge, the pulse width is 0, and the weight is concentrated at zero.

9. What is the period of a signal x(t)?
a) T
b) 2T
c) T/2
d) None of the mentioned

Explanation: A signal is said to be periodic if one complete cycle lasts T seconds and it repeats itself after that time.

10. Which of the given signals are periodic?
a) x(t) = 4 cos(5πt)
b) x(t) = u(t) – 1/2
c) x(t) = 4u(t) + 2sin(3t)
d) x[n] = 2sin(3n)

Explanation: T=2π / 5π = 2/5, periodic with period 2/5.

11. Check whether the signal is periodic or not?
x(t) = cos(4t) + 2sin(8t)
a) Periodic with period π/2
b) Periodic with period 2
c) Periodic with period 2/π
d) Not periodic

Explanation: T1=2π /4 T2=2π /8, T1/T2 = 2 T=T1 or 2T2, T=π /2.

The following are the different types of signals: Signals in both continuous and discrete time. Signals that are both deterministic and non-deterministic. Signals that are both periodic and aperiodic. Analog and digital signals are the two most common types of signals found in practise. The graphic depicts a digital signal created by approximating an analogue signal using its values at specific time intervals. Analog signals are continuous, but digital signals are quantized. If a signal has finite power but infinite energy, it is considered to be a power signal.