Communication System MCQ || Communication System Questions and Answers

Ques.1. 1. An odd and imaginary signal always has an odd and real Fourier transform.

2. The convolution of an odd Fourier transform with an even Fourier transform is always even.

Which of the above statements is/are true:

1. 1 & 2
2. 1 only
3. 2 only
4. None of the above

Explanation:-

Let F(ω) is the Fourier transform of f(t).

 f(t) F(ω) Real → Conjugate symmetric Conjugate symmetric → Real Imaginary → Conjugate antisymmetric Conjugate Anti symmetric → Imaginary Real + Even → Real + Even Imaginary + Even → Imaginary + Even Real + odd → Imaginary + odd Imaginary + odd → Real + odd Discrete → Periodic Periodic → Discrete Continuous → Aperiodic Aperiodic → Continuous Continuous + periodic → Discrete + Aperiodic Continuous + Aperiodic → Continuous + Aperiodic Discrete + Periodic → Discrete + Periodic Discrete + Aperiodic → Continuous + Periodic

Hence an odd and imaginary signal always has an odd and real Fourier transform

Hence statement (1) is Right

Convolution:

Convolution is a mathematical operation used to express the relation between input and output of an LTI system. It relates the input, output, and impulse response of an LTI system as

y(t) = x(t) * h(t)

Where y (t) = output of LTI

x (t) = input of LTI

h (t) = impulse response of LTI

1. Convolution of two even signals or two odd signals always results in an even signal.

2. Convolution of odd signal and even signal always results in the odd signal.

Hence statement (2) is also false.

So option (1) is the correct answer.

2. In a communication system noise is most likely to affect the signal

1. At the transmitter
2. In the channel
4. Both 2 and 3

Explanation:-

Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system when a transmitted signal propagates along the channel it may get distorted due to channel imperfection also noise adds to the transmitted signal. The addition of noise in the channel is most likely to affect signal recovery due to low signal strength caused by path loss.

When the modulating frequency is doubled, the modulation index is halved and the modulating voltage remains constant, the modulation system is frequency modulated.

For AM, the Modulation index is given by:

µ = Am/AC

The modulation index is independent of frequency

For PM, the Modulation index is given by:

β = KpAm

For FM the modulation index is

β = KfAm/fm

3. In a modulation system, if the modulating frequency is doubled, the modulation index also becomes double, the system is

1. FM
2. AM
3. PM
4. None of these

Explanation:-

When the modulating frequency is doubled, the modulation index is halved and the modulating voltage remains constant, the modulation system is frequency modulated.

For AM, the Modulation index is given by:

µ = Am/AC

The modulation index is independent of frequency

For PM, the Modulation index is given by:

β = KpAm

For FM the modulation index is

β = KfAm/fm

4. When the modulating frequency is doubled, the modulation index is halved and the modulating voltage remains constant, the modulation system is

1. FM
2. AM
3. PM
4. None of these

Explanation:-

When the modulating frequency is doubled, the modulation index is halved and the modulating voltage remains constant, the modulation system is frequency modulated.

For AM, the Modulation index is given by:

µ = Am/AC

The modulation index is independent of frequency

For PM, the Modulation index is given by:

β = KpAm

For FM the modulation index is

β = KfAm/fm

5. If two tones f1 and f2 are amplified by a non-linear amplifier, which frequency components would be present in output?

1. f1, f2
2. f1, f2, f1 + f2, f1 – f2
3. f1 + f2, f1 – f2
4. nf1 ± mf2, where n and m are integers

Answer.4. nf1 ± mf2, where n and m are integers

Explanation:-

When two frequency signals (say f1 and f2) are fed as input to any nonlinear device such as an amplifier, it will generate amplified output at f1 and f2. In addition, it will generate IMD (Intermodulation distortion) products at other unwanted frequencies (+/-nf1 ± mf2).

6. Consider the following types of modulation:

1. Amplitude modulation

2. Frequency modulation

3. Phase modulation

4. Pulse modulation

Which of the above modulation are used for telecasting TV programs?

1. 2 and 3
2. 3 and 4
3. 1 and 2
4. 1 and 4

Answer.4. nf1 ± mf2, where n and m are integers

Explanation:-

In TV Transmission FM is used for Audio transmission and AM for Video transmission.

Amplitude Modulation (AM) is preferred for picture transmission in TV because of the following reasons:

• The distortion which arises due to interference between multiple signals is more in FM than AM because the frequency of the FM signal continuously changes.
• Steady production of the picture is affected because of this.
• If AM were used, the ghost image, if produced is steady.
• Also, the circuit complexity and bandwidth requirements are much less in AM than in FM.

7. Calculate the modulation percentage if the modulating signal is 8 V and carrier is of 12 V?

1. 50
2. 67
3. 150
4. 33

Explanation:-

modulating signal = Vm = 8 V

carrier signal = Vc = 12 V

Formula:

Modulation percentage =  (Vm × 100)/Vc

Calculation:

Modulation percentage = (8 × 100)/12

∴ Modulation percentage = 66.66 ≈ 66.67

8. Which of the following devices is used for modulation & demodulation?

1. Multiplexer
2. Serial port
3. Modem
4. Gateway

Explanation:-

• Modem stands for Modulator-demodulator.
• It converts the analog signals from the transmission wires into digital signals which can be read by the computer devices (modulation).
• Then conversion of digital signal into analog and extracting out the useful information (demodulation) takes place.

9. Which one of the following is not a positive communication vehicle?

1. Being more personal
2. Immediate feedback to both parties
3. Tunnel vision
4. Quick achievement of a solution

Explanation:-

Being more personal is not a positive communication vehicle.

• When the communication becomes more personal then it will try to achieve personal interests rather than organizational interests.
• A healthy relationship is good for communication, but an over-healthy relationship is bad for communication.

Positive communication brings effective and quick solutions to problems.

• Feedback is important in the process of communication.
• It completes the process of communication.
• It determines whether the message was clearly understood by the receiver or not.
• It is important for effective communication and helps in closing the communication loop.

Tunnel Vision is again an important vehicle of positive communication.

• One must remain focussed on a single point or point of view.
• In a communication, the sender must focus his attention on achieving the goal i.e. the information sent by him is clearly understood by the receiver or not.

10. Radio channel used for transmission of information from the base station to the mobile is called:

1. Reverse Channel
2. Page
3. Handoff
4. Forward Channel