# 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:** sin^{2}(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.