# Digital Communications Questions and Answers – Signals

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

**1. A band-pass signal has a Fourier transform equal to**

a) One

**b) Zero**

c) Infinity

d) Cannot be determined

**Explanation:**In both the frequency and temporal domains, the Fourier transform of a band-pass signal is zero.

**2. A band-pass signal has**

a) DC component

**b) No DC component**

c) No side lobes

d) Cannot be determined

**Explanation:** There are no DC components in a band-pass signal, and its Fourier transform is 0. It will not be exactly zero outside of the band. As a result, there are side lobes present.

**3. Which are orthonormal signal representation?**

a) Sine and cosine at same frequency

b) Wavelets

c) Complex sinusoids at a different frequency

**d) All of the mentioned**

**Explanation:** Sine and cosine at the same frequency, Fourier series, sinc functions centred at sampling periods, wavelets, and other orthonormal signal representations are some of the most frequent.

**4. Constellation diagram is plotted in**

a) Constellation space

**b) Signal space**

c) Orthogonal space

d) Boundary space

**Explanation:** The constellation diagram is plotted in a space called as signal space.

**5. Cumulative distributive function is**

a) Non negative

b) Non decreasing

**c) Non negative & decreasing**

d) None of the mentioned

**Explanation:** Cumulative distribution function is non negative and non decreasing function.

**6. Which are non negative functions?**

a) PDF

b) PMF

**c) PDF & PMF**

d) None of the mentioned

**Explanation:** PDF, PMF and CDF are non negative functions.

**7. A base-band signal can be up-converted using**

a) Sine wave

**b) Cosine wave**

c) Filtering

d) None of the mentioned

**Explanation**: A base-band signal can be up-converted to band-pass filter by applying cosine wave.

**8. A band-pass signal can be down-converted using**

a) Sine wave

**b) Cosine wave**

c) Time delayed wave

d) None of the mentioned

**Explanation:** For down-conversion of a band-pass signal also cosine signal is used and multiplied with it.

**9. In down-conversion multiplication with cosine wave is followed by**

**a) Low pass filter**

b) High pass filter

c) Bandpass filter

d) Bandstop filter

**Explanation:** Multiplication using a sine wave is followed by low pass filtering in the downconversion of a bandpass signal.

**10. ADSL has _____ information channels.**

a) One

**b) Three**

c) Four

d) None of the mentioned

**Explanation:** The information channels on an asymmetric digital subscriber line are a high-speed downstream channel, an ISDN channel, and a medium-speed duplex channel.

**11. Fourier transform of a signal gives the**

a) Frequency content

b) Bandwidth

**c) Frequency content & Bandwidth**

d) None of the mentioned

**Explanation**: The frequency content of a signal is determined via the Fourier transform, which also determines the signal’s bandwidth.

**12. Random things in a communication system are**

a) Timing offset

b) Device frequency

c) Attenuation

**d) All of the mentioned**

**Explanation:** Noise in the channel, attenuation, fading, channel filtering, device frequency, phase, and timing offset are all examples of random events in the communication system.

**13. Which can be used for periodic and non periodic?**

a) Fourier series

**b) Fourier transforms**

c) Fourier series & transforms

d) None of the mentioned

**Explanation**: Fourier series are only applicable to periodic signals, but Fourier transforms and laplace transforms are applicable to both periodic and non-periodic signals.

**14. In the digital communication system, signals in different frequency bands are**

**a) Orthogonal**

b) Non orthogonal

c) Orthogonal & Non orthogonal

d) None of the mentioned

**Explanation:** Signals from different frequency bands are orthogonal in a digital communication system, therefore interference is avoided.

**15. Properties of impulse function are**

a) Symmetry

b) Time scaling

c) Shifting

**d) All of the mentioned**

**Explanation**: Some of the properties of impulse function are symmetry, time scaling and shifting.

**16. Properties of Fourier transform are**

a) Duality property

b) Time shifting property

c) Modulation property

**d) All of the mentioned**

**Explanation**: The duality property, time scaling property, time shifting property, modulation property, and many more properties of the Fourier transform are among them.

In both the frequency and temporal domains, the Fourier transform of a band-pass signal is zero. Signals in both continuous and discrete time. Signals that are both deterministic and non-deterministic. Signals that are even and odd. 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.