Analog Communication Test Questions - Set 3

1)   TRF receiver and super heterodyne receiver are used for

a. Detection of modulating signal
b. Removal of unwanted signal
c. Both a and b
d. None of the above
Answer  Explanation  Related Ques

ANSWER: Both a and b

Explanation:
TRF receivers and super heterodyne receivers are used for detection of modulating signal and removal of unwanted signal. The receivers receive the modulated signal and detect the information signal from the received modulated signal.


2)   Disadvantage of using a DSB or SSB signal modulation is

a. Difficult to recover information at the receiver
b. Carrier has to be locally generated at receiver
c. Both a and b are correct
d. None of the above
Answer  Explanation  Related Ques

ANSWER: Both a and b are correct

Explanation:
Disadvantage of using a DSB or SSB signal modulation is that it is difficult to recover information at the receiver. Demodulation depends upon the carrier present in the received signal at the receiver. If the carrier is not present, carrier has to be regenerated at the receiver so a complex circuitry is required.


3)   Calculate the modulation index when the un modulated carrier power is 15KW, and after modulation, carrier power is 17KW.

a. 68%
b. 51.63%
c. 82.58%
d. 34.66%
Answer  Explanation  Related Ques

ANSWER: 51.63%

Explanation:
The total power in an AM is given by
Pt = Pc ( 1 + m2/2)
17 = 15(1 + m2/2)
m2/2 = 0.134
m = 0.5163
= 51.63%


4)   An AM transmitter has an antenna current changing from 5 A un modulated  to 5.8 A. What is the percentage of modulation?

a. 38.8%
b. 83.14%
c. 46.8%
d. 25.2%
Answer  Explanation  Related Ques

ANSWER: 83.14%

Explanation:
Modulation index m is given by
m= √ (2{It/Ic}2-1)
= √ (2 (5.8/5)2 -1)
= √ (2 (5.8/5)2 -1)
= 0.8314
= 83.14%


5)   Calculate the power in a DSB SC signal when the modulation is 60% with a carrier power of 600W.

a. 600 W
b. 540 W
c. 108 W
d. 300 W
Answer  Explanation  Related Ques

ANSWER: 108 W

Explanation:
The total power in an AM is given by
Pt = Pc (1 + m2/2)
Given: m = 0.6
Therefore DSB power = (m2/2)Pc
= 600* (0.6)2/2
= 108 W


6)   Analog communication indicates:

a. Continuous signal with varying amplitude or phase
b. No numerical coding
c. AM or FM signal
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Analog communication means that the information is transmitted in the form of a continuous signal, such as a sound wave. The signal has varying amplitude or phase or frequency.

Coding is not applied to the signal as in case of digital data. Amplitude modulated signal and frequency modulated signals are the examples of analog communication.


7)   Types of analog modulation are:

a. Phase modulation
b. Frequency modulation
c. Amplitude modulation
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Modulation is a process by which some characteristics of signal such as amplitude or phase or frequency are varied in accordance with the instantaneous value of the information signal .

The signal whose characteristics are varied is known as carrier signal, as it carries the information. The signal that carries the information is known as modulating signal as according to it, the characteristics of carrier signal are varied.

Methods of analog modulations are

1) Amplitude modulation
2) Frequency modulation
3) Phase modulation


8)   What is the effect on the transmitted power of AM signal when the modulation index changes from 0.8 to 1?

a. 0.1364
b. 0.3856
c. 1.088
d. 0.5
Answer  Explanation  Related Ques

ANSWER: 0.1364

Explanation:
The total power in an AM is given by
Pt = Pc (1 + m2/2)
Where Pc is the carrier power and m is the modulation index.

Therefore,

Pt1 = Pc (1 + 0.82/2) = 1.32 Pc
Pt2 = Pc ( 1 + 12/2) = 1.5 Pc
Increase in power = (1.5 Pc - 1.32 Pc)/ 1.32 Pc
= 0.1364


9)   Synchronous detection means

a. Extracting week signal from noise
b. Need a reference signal with predetermined frequency and phase
c. Both a and b
d. None of the above
Answer  Explanation  Related Ques

ANSWER: Both a and b

Explanation:
Synchronous detection means demodulation of received signal and extracting information from it. It requires a reference signal at the receivers with predetermined frequency and phase which is generated using extra circuitry. Synchronous detection helps in extracting weak signals from noise.


10)   Analog signal may be converted into digital signal by

a. Sampling
b. Amplitude modulation
c. Filtering
d. Mixing
Answer  Explanation  Related Ques

ANSWER: Sampling

Explanation:
Conversion of analog signal into digital signal converts a continuous time signal in the form of digits. The conversion is done with the help of sampling technique. A sample is a set of values at a point in time space. The average number of samples obtained in one second is known as sampling frequency.


11)   The minimum antenna height required for transmission in reference to wavelength λ is

a. λ
b. λ/4
c. λ/2
d. 4 λ
Answer  Explanation  Related Ques

ANSWER: λ/4

Explanation:
For effective transmission of signal by the transmitter, the antenna height should be at least quarter length of the signal wavelength i.e., λ/4

L= λ/4
= c/4f

Required antenna height decreases with increase in frequency of the signal so modulation is done. With modulation, the low frequency signals are shifted to high frequency signals.


12)   Advantages of analog communication over digital communication are:

a. Data rate is low
b. Less transmission bandwidth is required
c. Synchronization is not needed
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
Due to analog to digital conversion, the data rate becomes high in digital communication. So the transmission bandwidth is also increased. Digital transmission also needs synchronization in certain applications which is not needed in case of analog communication systems.


13)   Radio waves travel through

a. Electromagnetic waves
b. Water
c. Wires
d. Fiber optic cable
Answer  Explanation  Related Ques

ANSWER: Electromagnetic waves

Explanation:
Radio communication or wireless communication takes place through electromagnetic waves.

The message is transmitted through open space via electromagnetic waves or radio waves. The waves are transmitted in the open space through antenna after processing of signal.


14)   AM wave may be represented as E(t) cos ωct where E(t) is

a. Envelope of the AM wave
b. Carrier signal
c. Amplitude of modulating signal
d. None of the above
Answer  Explanation  Related Ques

ANSWER: Envelope of the AM wave

Explanation:
An AM wave may be represented by
s(t) = [A+ x(t)] cos ωct
= E(t) cos ωct

Where x(t) is the modulating signal

A is the amplitude of carrier wave
E(t) is the Envelope of the AM wave

The envelope consists of the modulating signal that may be recovered from AM signal using demodulation techniques.


15)   USB (Upper Side Band) is the band of frequency

a. Above the carrier frequency
b. Includes the carrier frequency
c. That lies in AM spectrum
d. Both a and c are correct
Answer  Explanation  Related Ques

ANSWER: Both a and c are correct

Explanation:
Upper Side Band is the band of frequency

USB (Upper Side Band) is the band of frequency above the carrier frequency that lies in AM spectrum. It is a part of the shifted spectrum obtained after amplitude modulation. It is denoted by ωc+ ωm where ωc is the carrier frequency and ωm is the modulating frequency.


16)   LSB (Lower Side Band) is the band of frequency

a. Below the carrier frequency
b. Includes the carrier frequency
c. That lies in AM spectrum
d. Both a and c are correct
Answer  Explanation  Related Ques

ANSWER: Both a and c are correct

Explanation:
Lower Side Band

LSB (Lower Side Band) is the band of frequency below the carrier frequency that lies in AM spectrum. It is a part of the shifted spectrum obtained after amplitude modulation. It is denoted by ωc- ωm where ωc is the carrier frequency and ωm is the modulating frequency.


17)   Bandwidth (B) of an AM signal is given by

a. B = 2 ωm
b. B = (ωc + ωm) - (ωc - ωm)
c. ωm
d. None of the above
e. Both a and b are correct
Answer  Explanation  Related Ques

ANSWER: Both a and b are correct

Explanation:
Bandwidth of the AM wave is the difference in the two extreme frequencies of the AM signal. It is given by

B = (ωc + ωm)- (ωc - ωm)
= ωm
i.e., the bandwidth of the AM wave is twice the highest frequency present in the modulating signal.


18)   An oscillator for an AM transmitter has a 100μH coil and a 10nF capacitor. If a modulating frequency of 10 KHz modulates the oscillator, find the frequency range of the side bands.

a. 149 KHz to 169 KHz
b. 184 KHz to 296 KHz
c. 238 KHz to 296 KHz
d. 155 KHz to 166 KHz
Answer  Explanation  Related Ques

ANSWER: 149 KHz to 169 KHz

Explanation:
Carrier frequency fc = 1/2Π√LC
= 1/ 2Π√100 * 10 - 6 * 10 * 10-9
= 1/2Π * 10-6
= 1.59 * 105 Hz
= 159 KHz
The modulating frequency fm is 10KHz
Therefore the range of AM spectrum is given by (fc fm ) to (fc + fm )
= (159 - 10) to (159 + 10)
= 149 KHz to 169 KHz


19)   In Low level Amplitude Modulation

a. Modulation is done at lower power of carrier and modulating signal
b. Output power is low
c. Power amplifiers are required to boost the signal
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
In a Low level Amplitude Modulation system, modulation is done at lower power of carrier and modulating signal. Therefore the output power of modulation is low. So power amplifiers are required to boost the signal up to the desired power level.


20)   In High level Amplitude Modulation

a. Modulation is done at high power of carrier and modulating signal
b. Collector modulation method is High level Amplitude Modulation
c. Power amplifiers are used to boost the carrier and modulating signals before modulation
d. All of the above
Answer  Explanation  Related Ques

ANSWER: All of the above

Explanation:
In High level Amplitude Modulation system, Modulation is done at high power of carrier and modulating signals so Power amplifiers are used to boost the carrier and modulating signals before modulation. Collector modulation method is the example of High level Amplitude Modulation.