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## Communications MCQ Set 1

1. The spectrum of white noise and impulse noise is similar in terms of ________
a) magnitude spectrum
b) phase spectrum
c) both magnitude spectrum and phase spectrum
d) amplitude

Answer: a [Reason:] White noise is a random signal that has equal intensities at different frequencies. Thus it has a constant power spectral density. Impulse noise includes instantaneous unwanted signals that adds to our signal. They have only their magnitude in common.

2. UHF stands for ________
a) Ultra High Field
b) Ultraviolet High Frequency
c) Ultra High Frequency
d) Ultraviolet High Field

Answer: c [Reason:] Ultra High Frequency is actually the range of radio frequencies between 300MHz and 3GHz. It is also known as the decimetre range.

3. A long wave AM broadcast transmitter needs _________
a) very small carrier power
b) very large carrier power
c) small carrier power
d) large carrier power

Answer: b [Reason:] AM transmitter used to transmit information with the help of carrier wave. For sending information to a large distance, it is necessary that a very large carrier wave is used.

4. In FM, if we decreases modulating frequency then the modulation index ________
a) will increase, if the modulating voltage amplitude increases
b) will increase, if the modulating voltage amplitude increases
c) will increase, if the modulating voltage amplitude remains constant
d) will decrease, if the modulating voltage amplitude remains constant

Answer: c [Reason:] In frequency modulation, the modulation index can be made inversely proportional to the frequency by keeping amplitude constant.

5. Which of the following modulation system is used for video-modulation?
a) DSB-SC
b) SSB-SC
c) VSB
d) FM

Answer: c [Reason:] A modified form of SSB with carrier called vestigial sideband (SSB) is mainly used for video modulation.

6. Which among the following is Analog Continuous Modulation technique?
a) PAM
b) PCM
c) AM
d) PM

Answer: c [Reason:] AM is a technique used generally for electronic communication, mostly it is used for transmitting information via a radio wave. In today’s world Quadrature Amplitude Modulation is generally used because it makes the efficient use of available bandwidth.

7. In SSB modulation, only a single sideband is transmitted leaving the other sideband and the carrier.
a) True
b) False

Answer: a [Reason:] The two sidebands exist in double side band modulation is found to have exactly same information. So if we remove one of the sidebands then also the same information is transmitted. This also reduces the bandwidth of the modulation.

8. For demodulation of PCM, it is first converted into __________
a) PDM
b) PWM
c) PPM
d) PAM

Answer: d [Reason:] Pulse code Modulation is used to convert an analog signal to digital signal. During its demodulation, it is first converted to PAM and then it gets back to analog signal with the help of a decoder.

9. A noise can be represented as superposition of spectral components.
a) True
b) False

Answer: a [Reason:] Noise is a general term for unwanted signals that gets add to our original signal. It is usually represented as the superposition of spectral components.

10. The amount of power and bandwidth necessary to be transmitted for a given amount of information are reduced in _________
a) Single Sideband Modulation
b) Double Sideband Modulation
c) Vestigial Sideband Modulation
d) Frequency Modulation

Answer: a [Reason:] The two sidebands exist in double side band modulation is found to have exactly same information. So if we remove one of the sidebands then also the same information is transmitted. This also reduces the bandwidth of the modulation.

## Communications MCQ Set 2

1. FSK stands for ________
a) Full Signal Keying
b) Full Shift Keying
c) Frequency Signal Keying
d) Frequency Shift Keying

Answer: d [Reason:] FSK stands for Frequency Shift Keying. In FSK, information is transmitted through discrete frequency changes in carrier signal.

2. When analyzing a transmission line, its inductance and capacitance are considered as _________
a) distributed
b) lumped
c) equal reactances
d) ideal elements

Answer: a [Reason:] For a transmission line, inductance and capacitance are two components that are considered as distributed.

3. What effect will be seen on the resistance of a wire if we increases the frequency?
a) it will also increase
b) it starts to decrease
c) it will remain same
d) it starts changing periodically

Answer: a [Reason:] In accordance to skin effect, if we increases the frequency then it will also increase the resistance of a wire.

4. What is speed of radio waves in free space?
a) 3 x 106 m/s
b) 3 x 109 m/s
c) 300 x 106 m/s
d) 300 x 109 m/s

Answer: c [Reason:] The speed of radio waves in free space is 3 x 108 m/s. It cal also be written as 300 x 106.

5. An antenna has “gain” as compared to ________
a) Ground wave antenna
b) Marconi antenna
c) Vertically polarized radiator

Answer: d [Reason:] Isotropic radiator is considered as an ideal antenna. So all the gains of any antenna is estimated by comparing with it only.

6. Ground waves are most effective when _________
a) it is above about 20MHZ
b) it is below about 3MHz
c) it is at microwave frequencies
d) it is using horizontally polarized waves

Answer: b [Reason:] Ground wave is generally preferred for long distance communication using frequencies which are below 3MHz. It can be used for short distance communication by using frequencies between 3 and 30MHZ.

7. The microwave frequency range is considered to start at ________
a) 1 GHz
b) 10 GHz
c) 100 GHz
d) 5 GHz

Answer: a [Reason:] Microwave frequency range is considered to be start at 1 GHz. It is standard and fixed.

8. In digital microwave systems, additional repeaters increase the ________
a) reliability
b) noise level
c) jitter
d) sensitivity

Answer: c [Reason:] In telecommunication, jitter can be seen as a deviation from true periodicity of a presumably periodic signal. Higher level of jitter are more likely to occur on either slow or heavily congested link. In any circuit, adding additional repeaters finally increase the jitter only.

9. What is the approximate reliability of a microwave system?
a) 90%
b) 99%
c) 99.99%
d) 99.9%

Answer: c [Reason:] Microwave system is mostly known for its reliability. Its reliability is of the order of 99.99%.

10. Which modulation is used by microwave system?
a) FM
b) SSB
c) QAM
d) Either FM or SSB or QAM

Answer: d [Reason:] Microwave are used for point-to-point communication links. It can either use frequency modulation or single sideband modulation or quadrature modulation.

## Communications MCQ Set 3

1. Which among the following is the drawback of Pulse Position Modulation (PPM)?
a) Synchronization is required between receiver and transmitter
b) Synchronization is not required between receiver and transmitter
c) Amplitude is constant
d) Instantaneous power of PPM modulated signals is constant

Answer: a [Reason:] In PPM, position of the pulse of the carrier is varied with respect to the position of a reference pulse which requires synchronization between the transmitter and the receiver. It also requires large bandwidth as compared to Pulse amplitude modulation.

2. Calculate the Nyquist rate for the signal :
x(t) = 12cos50πt + 7 cos75π t – 13 cos100πt
a) 300 Hz
b) 600 Hz
c) 100 Hz
d) 150 Hz

Answer: c [Reason:] On comparing with general Equation, Acosωt and substituting ω = 2×π× f. We have f1 = 25 Hz, f2= 37.5Hz, f3=50Hz. Nyquist rate = 2 × fmax = = 2 × 50 = 100 Hz.

3. What is the need of doing Pre emphasis?
a) For boosting of modulating signal voltage
b) For restoring of original signal power
c) For removing amplitude variations due to noise
d) For removing frequency variations

Answer: a [Reason:] Pre-emphasis is generally done for boosting the amplitudes of modulating voltages at higher audio frequencies.

4. Amount of data transmitted for a given time is called _________
a) Noise
b) Frequency
c) Bandwidth
d) Power

Answer: c [Reason:] Amount of data transmitted for a given time is called bandwidth. It is generally expressed in bits per second.

5. Consider an AM broadcast station which transmits modulating frequencies up to 10kHz. If it transmits a frequency of 1000 kHz. Find its maximum and minimum upper and lower sidebands frequencies and also the total bandwidth?
a) 900 KHz, 820 KHz, 1000 Hz
b) 720 KHz, 650 KHz, 1020 Hz
c) 1010 KHz, 880 KHz, 15000 Hz
d) 1010 KHz, 990 KHz, 20000 Hz

Answer: d [Reason:] Maximum Frequency, fm = 1000 + 10 = 1010 kHz. Minimum Frequency, fl = 1000 – 10 = 990 kHz and bandwidth = fm – fl = 1010 – 990 =20000 Hz.

6. A superheterodyne receiver receives signal within frequency range of 120 to 180 MHz. Then the required Intermediate frequency is _________
a) 30MHz
b) 60MHz
c) 90MHz
d) 50MHz

Answer: a [Reason:] 7. Carson’s rule is used to calculate ________
a) Bandwidth of FM signal
b) SNR
c) Modulation index
d) Figure of merit

Answer: a [Reason:] Carson’s rule states that the required bandwidth is equal to the twice of sum of the maximum frequency deviation and the maximum modulating frequency, B = 2(fd +fm)Hz.

8. What is the bandwidth of a FM wave when maximum allowed deviation is 50KHz and the modulating signal has a frequency of 15KHz?
a) 32.5 KHz
b) 47.8 KHz
c) 53.1 KHz
d) 63.25 KHz

Answer: a [Reason:] According to Carson s rule, B = 2(fd +fm) = 2 (50 + 15) = 32.5 KHz.

9. For signal, m(t) = 50cos(10 × 90t + 30 sin100t), the power dissipated by the 20Ω resistor is ________
a) 100W
b) 65W
c) 74.7W
d) 62.5W

Answer: d [Reason:] On Comparing with the equation, m(t) = Acos(ωct + mf sinωmt). We have A = 50, dissipated power, 10. Signal and its Hilbert transform have ________
a) same energy density spectrum
b) same power
c) a phase difference of 60°
d) a phase difference of 120°

Answer: a [Reason:] Properties of Hilbert transform states that the signal and its Hilbert transform : 1. have same energy density spectrum 2. are mutually diagonal 3. have same auto correlation function.

## Communications MCQ Set 4

1. Which of the following devices is used to generate AM waves?
a) Square-law modulator
b) Amplifier
c) Transmitter
d) Transducer

Answer: a [Reason:] AM signals is generated by Square-law modulators. The input for generating AM signal should be of type (A + m(t)) instead of m(t). Square-law modulator sums carrier and information signal, then square them using a non linear device.

2. What is the disadvantage of FM over AM ?
a) high modulating power is needed
b) requires high output power
c) large bandwidth required
d) high noise is produced

Answer: c [Reason:] Advantage of FM over AM is that the amplitude of an FM wave remains constant. In FM, power of transmitted wave depends on amplitude of unmodulated carrier wave and hence it is constant.

3. For low level amplitude modulation, amplifier must be ________
a) linear
b) non linear
c) harmonic
d) class A amplifier

Answer: a [Reason:] Low level AM transmitter is same as high level transmitter except that power of carrier are not amplified. The signals are directly applied to the class C power amplifier.

4. Approximate range of a cordless telephone is ________
a) 900m
b) 100m
c) 1000m
d) 475m

Answer: b [Reason:] Cordless telephone is that in which the handset is portable and communicates with the body of the phone by radio waves instead of being attached by a cord. Basically length depends on quality of instrument and is about 90m to 100m.

5. A wave is modulated by two sin waves having modulation indices of 0.3 and 0.5. Find the total modulation index?
a) 0.1
b) 0.7
c) 0.58
d) 0.35

Answer: c [Reason:] Given that m1 = 0.3 and m2 = 0.5. Total modulation index will be equal to . By substituting values we have which is equal to 0.58.

6. Carrier wave carries information.
a) True
b) False

Answer: b [Reason:] It is a high frequency electro-magnetic wave. A carrier wave does not have any information. It is modulated with an input signal for the purpose of conveying information.

7. What can we do to eliminate distortion in the picture?
a) use a longer transmission line
b) change the antenna orientation
c) twist the transmission line
d) connect a booster

Answer: b [Reason:] According to modulation, length of antenna should be one fourth of wavelength. If distortion occurs whether in television or at any other place, simplest way to eliminate this is to change the orientation of antenna.

8. For 100% amplitude modulation, the power in upper sideband when carrier power is to be 100W?
a) 100W
b) 75W
c) 25W
d) 50W

Answer: c [Reason:] Modulation index, m = 100% = 1. Power in sidebands is (Pcx m24) . By substituting the values, we have (100 x 14) which is equal to 25W.

9. Modem is considered as high speed if data rate is ________
a) 10000
b) 20000
c) 5000
d) 30000

Answer: c [Reason:] For data rate in the range 2000 to 10500 bps, modem is designated as high speed. Modem is combination of modulator and demodulator.

10. Scanning always takes place from ________
a) right to left
b) left to right
c) left to right for odd fields and right to left for even fields
d) left to right for even fields and right to left for odd fields

Answer: b [Reason:] Scanning is mostly used in TV to obtain continuous frames and to provide motion of picture. Scanning is always take place from left to right.

## Communications MCQ Set 5

1. LCD uses ________
a) sematic crystals
b) twisted nematic crystals
c) nematic crystals
d) cholesteric crystals

Answer: b [Reason:] Twisted nematic display consist a substance called nematic which is placed in between two plates of polarized glass.

2. Which of the following stage is present in FM receiver but not in AM receiver?
a) Amplitude limiter
b) Demodulator
c) AM amplifier
d) AF amplitude

Answer: a [Reason:] Amplitude limiter clips or removes the amplitude of output signals to a desired margin. So it is only used in amplitude modulation and not in frequency modulation.

3. Function of duplexer in a RADAR is to permit the use of same antenna for transmission and reception.
a) True
b) False

Answer: a [Reason:] Duplexer in a RADAR allows bi-directional communication over the same path. Its function is to isolate the receiver from transmitter while permitting them to share a common antenna.

4. Single Sideband Modulation (SSB) is generally reserved for point-to-point communication.
a) True
b) False

Answer: a [Reason:] The two sidebands exist in double side band modulation is found to have exactly same information. So if we remove one of the sidebands then also the same information is transmitted. This also reduces the bandwidth of the modulation. So it is basically used for point-to-point communication.

5. For an AM transmitter, class C amplifier can be used after the modulation stage.
a) True
b) False

Answer: b [Reason:] In an AM transmitter, the required transmission power is obtained from class C amplifier. So it is not used after the modulation stage.

6. For which of the modulated system, the linear amplified modulated stage is used?
a) low level amplitude modulated system
b) high level amplitude modulated system
c) high level frequency modulated system
d) low level frequency modulated system

Answer: a [Reason:] Linear amplified modulation is used to amplify the amplitude. Thus it is used for low level modulated system.

7. When noise is passed through a narrow band filter, the output of filter should be?
a) triangular
b) circular
c) rectangular
d) sinusoidal

Answer: d [Reason:] Narrow band filter is used to isolate a narrow band of frequencies from a wider bandwidth signal. It is a combination of band pass and band reject filter. When noise gets passed through it, the output of it should be sinusoidal.

8. A narrow band noise can exist in _________
a) AM only
b) PCM only
c) FM only
d) AM and FM both

Answer: d [Reason:] Narrow band filter is used to isolate a narrow band of frequencies from a wider bandwidth signal. It is a combination of band pass and band reject filter. So it can be used in both AM and FM.

9. The upper and lower sideband frequencies for 5KHz amplitude modulation with a 30KHz carrier frequency will be?
a) 35KHz and 25KHz
b) 34KHz and 24KHz
c) 25KHz and 35KHz
d) 0.35KHz and 0.25KHz