Multiple Choice Questions and Answers on Analog Communication (Part-2)

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1)   Square law modulators

a. Have non linear current-voltage characteristics
b. Are used for Amplitude Modulation
c. Are used for frequency modulation
d. Both a and b are correct

ANSWER: (d) Both a and b are correct

2)   AM demodulation techniques are

a. Square law demodulator
b. Envelope detector
c. PLL detector
d. Both a and b are correct

ANSWER: (d) Both a and b are correct

3)   The process of recovering information signal from received carrier is known as

a. Detection
b. Modulation
c. Demultiplexing
d. Sampling

ANSWER: (a) Detection

4)   Ring modulator is

a. Is used for DSB SC generation
b. Consists of four diodes connected in the form of ring
c. Is a product modulator
d. All of the above

ANSWER: (d) All of the above

5)   What is the maximum transmission efficiency of an AM signal?

a. 64.44%
b. 33.33%
c. 56.66%
d. 75.55%

ANSWER: (b) 33.33%

6)   In synchronous detection of AM signal

a. Carrier is locally generated
b. Passed through a low pass filter
c. The original signal is recovered
d. All of the above

ANSWER: (d) All of the above

7)   Requirements of synchronous detection of AM signal are:

a. Local generation of carrier
b. The frequency of the locally generated carrier must be identical to that of transmitted carrier
c. The phase of the locally generated carrier must be synchronized to that of transmitted carrier
d. All of the above

ANSWER: (d) All of the above

8)   Disadvantages of using synchronous detection of AM signal are:

a. Needs additional system for generation of carrier
b. Needs additional system for synchronization of carrier
c. Receiver is complex and costly
d. All of the above

ANSWER: (d) All of the above

9)   Quadrature Amplitude Modulation (QAM) is

a. Have same bandwidth used for two DSB-SC signals
b. Is also known as Bandwidth Conservation scheme
c. Is used in color television
d. All of the above

ANSWER: (d) All of the above

10)   Pilot carrier is

a. Used with DSB-SC signal
b. A small carrier transmitted with modulated signal
c. Used for synchronization with local carrier at the receiver
d. All of the above

ANSWER:(d) All of the above

11)   Generation of SSB SC signal is done by

a. Phase discrimination method
b. Frequency discrimination method
c. Product modulator
d. Both a and b

ANSWER: (d) Both a and b

12)   Limitations of Frequency discrimination method are:

a. Cannot be used for video signals
b. Designing of band pass filter is difficult
c. Both a and b
d. None of the above

ANSWER:(c) Both a and b

13)   Phase shift method is

a. Includes two balanced modulators
b. Two phase shifting networks
c. Avoids the use of filters
d. All of the above

ANSWER: (d) All of the above

14)   Vestigial side band signals are detected by

a. Filters
b. Synchronous detection
c. Balanced modulator
d. None of the above

ANSWER: (b) Synchronous detection

15)   Automatic gain control is

a. Provides controlled signal amplitude at the output
b. Adjusts the input to output gain to a suitable value
c. Is used in AM radio receiver
d. All of the above

ANSWER: (d) All of the above

16)   In an Amplitude Modulation

a. Amplitude of the carrier varies
b. Frequency of the carrier remains constant
c. Phase of the carrier remains constant
d. All of the above

ANSWER: )(d) All of the above

17)   If modulation index is greater than 1

a. The baseband signal is not preserved in the envelope of the AM signal
b. The recovered signal is distorted
c. It is called over modulation
d. All of the above

ANSWER: (d) All of the above

18)   Examples of low level modulation are

a. Square law diode modulation
b. Switching modulation
c. Frequency discrimination method
d. Both a and b

ANSWER: (d) Both a and b

19)   Frequency components of an AM wave (m = modulation index) are

a. Carrier frequency (ωc ) with amplitude A
b. Upper side band (ωc + ωm) having amplitude mA/2
c. Lower side band (ωc – ωm) having amplitude mA/2
d. All of the above

ANSWER: (d) All of the above

20)   Squelch circuit is

a. Suppresses output audio
b. Works when there is insufficient desired input signal
c. Is used to suppress the unwanted channel noise when there is no reception by the receiver
d. All of the above

ANSWER: (d) All of the above

21)   In Automatic gain control of the AM receiver

a. Gain of the receiver is adjusted
b. The gain adjustment depends upon the strength of the received signal
c. The output provided is a DC voltage
d. All of the above

ANSWER: All of the above

22)   The factors that determine the sensitivity of super heterodyne receiver are

a. Gain of the IF amplifier
b. Noise figure of the receiver
c. Gain of RF amplifier
d. All of the above

ANSWER: (d) All of the above

23)   Selectivity of a receiver:

a. Changes with incoming signal frequency
b. Is poorer at high frequencies
c. Is the rejection of the adjacent channel at the receiver
d. All of the above

ANSWER: (d) All of the above

24)   Advantages of using an RF amplifier are:

a. Better selectivity
b. Better sensitivity
c. Improved signal to noise ratio
d. All of the above

ANSWER: (d) All of the above

25)   Intermediate frequency (IF) should be carefully chosen as

a. High IF results in poor selectivity
b. High IF results in problems in tracking of signals
c. Image frequency rejection becomes poor at low IF
d. All of the above

ANSWER: (d) All of the above

26)   Example of continuous wave analog modulation is

a. PCM
b. DM
c. AM
d. PAM

ANSWER:  (c) AM

27)   The standard value for Intermediate frequency (IF) in AM receivers is

a. 455 KHz
b. 580 KHz
c. 10.7 MHz
d. 50 MHz

ANSWER: (a) 455 KHz

28)   The functions of radio receiver are

a. Receive the Incoming modulated carrier by antenna
b. Select the wanted signal and reject the unwanted signals and noise
c. Detection and amplification of the information signal from the carrier
d. All of the above

ANSWER: (d) All of the above

29)   Function of frequency mixer in super heterodyne receiver is

a. Amplification
b. Filtering
c. Multiplication of incoming signal and the locally generated carrier
d. None of the above

ANSWER:  (c) Multiplication of incoming signal and the locally generated carrier

30)   The advantages of using an RF amplifier are

a. Better sensitivity
b. Improved signal to noise ratio
c. Better selectivity
d. All of the above

ANSWER: (d) All of the above

31)   The costas receiver is used for

a. FM signal
b. DSB-SC signal
c. PCM signal
d. DM signal

ANSWER:  (b) DSB-SC signal

32)   Cross talk is –

a. The disturbance caused in the nearby channel or circuit due to transmitted signal
b. Adjacent frequency rejection
c. Generation of closely lying side bands
d. None of the above

ANSWER:  (a) The disturbance caused in the nearby channel or circuit due to transmitted signal

33)   In terms of signal frequency (fs) and intermediate frequency (fi), the image frequency is given by

a. fs + fi
b. fs + 2fi
c. 2fs + fi
d. 2( fs + fi)

ANSWER:  (b) fs + 2fi

34)   In Frequency Modulation –

a. Amplitude of the carrier remains same
b. Frequency of the carrier varies in accordance with the modulating signal
c. The number of side bands are infinite
d. All of the above

ANSWER:  (d) All of the above

35)   Frequency deviation in FM is

a. Change in carrier frequency to the frequency above and below the centre frequency
b. Formation of side bands
c. The variation of the instantaneous carrier frequency in proportion to the modulating signal
d. All of the above

ANSWER:  (d) All of the above

36)   Carrier swing is defined as

a. The total variation in frequency from the lowest to the highest point
b. Frequency deviation above or below the carrier frequency
c. Width of the side band
d. None of the above

ANSWER:  (a) The total variation in frequency from the lowest to the highest point

37)   The amount of frequency deviation in FM signal depends on

a. Amplitude of the modulating signal
b. Carrier frequency
c. Modulating frequency
d. Transmitter amplifier

ANSWER: ( a) Amplitude of the modulating signal

38)   Drawbacks of using direct method for generation of FM signal are

a. Does not give high stability to FM signal frequency
b. Distorted FM signal is generated due to harmonics of modulating signal
c. Cannot be used for high power FM generation
d. Both a and b

ANSWER: (d) Both a and b

39)   Advantage of using direct method for generation of FM signal is

a. It gives high stability to FM signal frequency
b. Distortion free FM signal is generated
c. High power FM generation is possible
d. None of the above

ANSWER:  (c) High power FM generation is possible

40)   What are the disadvantages of using balanced slope detector for demodulation of FM signal?

a. The detector operates only for small deviation in frequency
b. Low pass filter of the detector produces distortion in the detection
c. Both a and b
d. None of the above

ANSWER:  (c) Both a and b

41)   Drawbacks of Tuned Radio Receiver are

a. Oscillate at higher frequencies
b. Selectivity is poor
c. Bandwidth of the TRF receiver varies with incoming frequency
d. All of the above

ANSWER: (d) All of the above

42)   Sensitivity is defined as

a. Ability of receiver to amplify weak signals
b. Ability to reject unwanted signals
c. Ability to convert incoming signal into Image Frequency
d. Ability to reject noise

ANSWER: (a)  Ability of receiver to amplify weak signals

43)   In radio receivers, varactor diodes are used for

a. Tuning
b. Demodulation
c. Mixing
d. None of the above

ANSWER:  (a) Tuning

44)   The standard value for Intermediate frequency (IF) in double conversion FM receivers is

a. 455 KHz
b. 580 KHz
c. 10.7 MHz
d. 50 MHz

ANSWER:  (c) 10.7 MHz

45)   Amplitude limiter in FM receivers are used to

a. Remove amplitude variations due to noise
b. Filteration
c. Demodulation
d. Amplification

ANSWER:  (a) Remove amplitude variations due to noise

46)   Pre emphasis is done

a. For boosting of modulating signal voltage
b. For modulating signals at higher frequencies
c. In FM before modulation
d. All of the above

ANSWER:  (d) All of the above

47)   De emphasis is

a. is restoring of original signal power
b. is done at the detector output of the receiver
c. is the inverse process of Pre emphasis
d. All of the above

ANSWER:  (d) All of the above

48)   Pre emphasis is done before

a. Before modulation
b. Before transmission
c. Before detection at receiver
d. After detection at receiver

ANSWER:  (a) Before modulation

49)   What is the effect on the deviation d of an FM signal when it is passed through a mixer?

a. Doubles
b. Reduces
c. Becomes half
d. Remains unchanged

ANSWER:  (d) Remains unchanged

50)   Armstrong method is used for the generation of

a. Direct FM
b. Indirect FM
c. SSB-SC
d. DSB-SC

ANSWER:  (b) Indirect FM

51)   The modulation index of FM is given by

a. μ = frequency deviation/ modulating frequency
b. μ = modulating frequency /frequency deviation
c. μ = modulating frequency/ carrier frequency
d. μ = carrier frequency / modulating frequency

ANSWER:(a) μ = frequency deviation/ modulating frequency

52)   Disadvantages of FM over AM are

a. Prone to selective fading
b. Capture effect
c. Poorer signal to noise ratio at high audio frequencies
d. All of the above

ANSWER:  (d) All of the above

53)   What is the required bandwidth according to the Carson’s rule, when a 100 MHz carrier is modulated with a sinusoidal signal at 1KHz, the maximum frequency deviation being 50 KHz.

a. 1 KHz
b. 50 KHz
c. 102 KHz
d. 150 KHz

ANSWER: (c) 102 KHz

Explanation:
According to Carson’s rule, bandwidth of FM is given by 2(Δf+ fm) where Δf is the deviation in frequency and fm is the frequency of sinusoidal signal. The required bandwidth is therefore calculated as
2 * (50KHz + 1KHz)
= 2 * 51 KHz
= 102 KHz

54)   The audio signal having frequency 500Hz and voltage 2.6V, shows a deviation of 5.2KHz in a Frequency Modulation system. If the audio signal voltage changes to 8.6V, calculate the new deviation obtained.

a. 17.2 KHz
b. 19.6 KHz
c. 25.6 KHz
d. 14.6 KHz

ANSWER:  (a) 17.2 KHz

Explanation:
Deviation in FM is given by Δf = kf * Am
Therefore, kf = Δf/ Am
= 5.2/2.6
= 2
When voltage changes to 8.6V = Am
New frequency deviation Δf = kf * Am
= 2* 8.6
= 17.2 KHz

55)   According to Carson’s rule, Bandwidth B and modulating frequency fm are related as

a. B = 2(Δf + fm) Hz
b. B = fm Hz
c. B < 2fm Hz
d. B > 2fm Hz

ANSWER:  (a) B = 2(Δf + fm) Hz

56)   What is the change in the bandwidth of the signal in FM when the modulating frequency increases from 12 KHz to 24KHz?

a. 40 Hz
b. 58 Hz
c. 24 Hz
d. Bandwidth remains unaffected

ANSWER:  (c) 24 Hz

Explanation:
According to Carson’s rule, the bandwidth required is twice the sum of the maximum frequency deviation and the maximum modulating signal frequency. Or,
B=2(Δf +fm) Hz
B= 2(Δf +12) Hz = 2 Δf + 24 Hz (1)
Assuming Δf to be constant,
B = 2 Δf + 48 Hz (2)
(2)-(1),
= 24Hz
Therefore the bandwidth changes by 24Hz.

57)   What is the maximum frequency deviation allowed in commercial FM broadcasting?

a. 100 KHz
b. 75 KHz
c. 15 KHz
d. 120 KHz

ANSWER:  (b) 75 KHz

58)   What is the maximum modulating frequency allowed in commercial FM broadcastings?

a. 40 KHz
b. 75 KHz
c. 15 KHz
d. 120 KHz

ANSWER: (c) 15 KHz

59)   The ratio of actual frequency deviation to the maximum allowable frequency deviation is called

a. Multi tone modulation
b. Percentage modulation
c. Phase deviation
d. Modulation index

ANSWER:  (b) Percentage modulation

60)   The range of modulating frequency for Narrow Band FM is

a. 30 Hz to 15 KHz
b. 30 Hz to 30 KHz
c. 30 Hz to 3 KHz
d. 3 KHz to 30 KHz

ANSWER:  (c) 30 Hz to 3 KHz