Analog Communications MCQs – Double Sideband Suppressed Carrier Modulation ( Analog Communications ) MCQs

Analog Communications MCQs – Double Sideband Suppressed Carrier Modulation ( Analog Communications ) MCQs

Latest Analog Communications MCQs

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Analog Communications MCQs – Double Sideband Suppressed Carrier Modulation ( Analog Communications ) MCQs

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Time Domain Description

1. Distance between direct broadcasting satellite and earth’s surface is ________
a) 4500km
b) 33000km
c) 5200km
d) 36000km
Answer: d
Explanation: Satellite broadcasting takes place between directional parabolic antennae, where the signal is sent via cable to antennae which beams it to the satellite and then the satellite sends back a signal to another location on Earth. A communication satellite amplifies radio telecommunications signals with the help of a transponder. The distance between it and earth’s surface is approximately 36000km.

2. UHF stands for ________
a) Ultra High Electric Field
b) Ultraviolet High Frequency
c) Ultra High Frequency
d) Ultra High Magnetic Field
Answer: c
Explanation: Ultra High Frequency (UHF) are frequencies at ultra high levels, thus consisting of radio frequencies in the range of 300 MHz and 3 GHz. Rest of the options don’t have any existence.

3. What is the ITU designation range for VHF?
a) 30 to 300 Kilohertz
b) 300 to 3000 Kilohertz
c) 30 to 300 Megahertz
d) 3 to 30 Megahertz
Answer: c
Explanation: The VHF (Very High Frequency) are frequencies in the range of 30 MHz to 300 MHz. Frequencies in the range 30 to 300 Kilohertz belong to LF (Low Frequency) range, whereas frequencies in the range 300 to 3000 Kilohertz belong to MF (Medium Frequency) range. Frequencies in the range 3 to 30 Megahertz belong to HF (High Frequency) range.

4. Effective noise at high frequencies is ________
a) Johnson noise
b) Flicker noise
c) transit-time noise
d) Partition noise
Answer: c
Explanation: Transit-time noise is alike to shot noise, and results if the time taken by electrons to reach from emitter to collector is comparable to the period of signals being amplified. Thus, transit-time noise occurs at high frequencies, whereas Flicker noise occurs below a few kilohertz and Johnson noise, also known as thermal noise, occurs at all frequencies.

5. VHF stands for ________
a) very high frequency
b) very high electric field
c) very high magnetic field
d) very high electromagnetic field
Answer: a
Explanation: The VHF (Very High Frequency) are frequencies in the range of 30 MHz to 300 MHz. Rest options don’t have any existence.

6. Which among the following is the Analog Continuous Modulation technique?
a) PAM
b) PCM
c) AM
d) PM
Answer: c
Explanation: Only AM (Amplitude Modulation) is Analog Continuous Modulation technique while rest are Digital Modulation techniques. In AM, the amplitude of a carrier wave is varied with respect to the instantaneous amplitude of the message signal. This is how amplitude modulation takes place.

7. ITU stands for ________
a) Indian Telecommunication Union
b) Indian Telephonic Union
c) International Telephonic Union
d) International Telecommunication Union
Answer: d
Explanation: The International Telecommunication Union (ITU) is an United Nations agency serving to correlate all operations regarding telecommunication and provide services all over the world. Rest options are all invalid.

8. If AM transmitter has low level modulation then it has ________
a) high efficiency
b) poor efficiency
c) high modulating power
d) high resistance towards noise
Answer: b
Explanation: In low-level modulation, the generated amplitude modulated signal utilizes less power, followed by the AM signal being amplified by a chain of linear amplifiers. However, in high power applications, the amplifiers that follow the modulator stage will be linear amplifiers which will render the whole modulation as inefficient.

9. Radiation resistance of an antenna is ________
a) a dc resistance
b) an ac resistance
c) a constant value
d) neither ac nor dc resistance
Answer: b
Explanation: Radiation resistance is that part of an antenna that is caused by the radiation of electromagnetic waves from the antenna. It is considered to be an equivalent resistance dissipating the same amount of power as an actual antenna would have radiated. Radiation resistance of an antenna is usually an ac resistance.

10. What do you understand by isotropic antenna?
a) it radiates its power uniformly in all directions
b) it radiates its power non-uniformly in all directions
c) it radiates its power specifically in one direction only
d) it does not radiate any power
Answer: a
Explanation: An isotropic antenna is considered to be an ideal antenna that radiates power uniformly in all directions. Generally, this type of antenna does not exist. However, it is often used to determine certain antenna characteristics like antenna gain.

Frequency Domain Representation

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 spectrum
Answer: a
Explanation: White noise is a random signal having equal intensities of different frequencies. Impulse noise is a type of noise consisting of sudden sounds, which includes unwanted and instantaneous signals. Thus, they have only their magnitude in common.

2. 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) Amplitude Modulation
Answer: a
Explanation: In SSBSC, the carrier is suppressed and only either of the two sidebands is transmitted. This reduces the power consumption and also lessens the bandwidth. While in DSBSC, the carrier is suppressed but both the sidebands are transmitted, whereas in AM, the carrier as well as both the sidebands are transmitted.

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
Explanation: In Communication Systems, a transmitter produces radio waves, which are the modulated message signals, and are transmitted via the antennae. These message signals carry information which are modulated using a carrier wave, having frequency higher than the message signal frequency. Thus, for transmitting information to a large distance, it is necessary that a very large carrier wave is used for modulation.

4. In FM, if we decreases modulating frequency then the modulation index ________
a) will increase, if the modulating voltage amplitude increases
b) will decrease, 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
Explanation: In frequency modulation, the modulation index is the ratio of frequency deviation to modulating frequency. Therefore, modulation index can be made inversely proportional to the modulating frequency by keeping amplitude constant. Thus, if we decrease modulating frequency then the modulation index will increase, with amplitude remaining 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
Explanation: Vestigial Sideband Modulation (VSB) is a type of amplitude modulation in which the carrier and only one sideband is completely transmitted and the other sideband is partly transmitted. Thus, video signals are transmitted using VSB modulation.

6. In SSB modulation, only a single sideband is transmitted leaving the other sideband and the carrier.
a) True
b) False
Answer: a
Explanation: In SSBSC, the carrier is suppressed and only either of the two sidebands is transmitted. This reduces power consumption and also lessens the bandwidth.

7. For demodulation of PCM, it is first converted into __________
a) PDM
b) PWM
c) PPM
d) PAM
Answer: d
Explanation: Pulse Code Modulation deals with converting an analog signal to a digital signal. During it’s demodulation, the wave to be treated is to be parallel. The series pulse wave signal is then converted into a parallel digital signal using a serial to parallel converter, after which it is further converted into the original analog signal with the help of a decoder or detector.

8. A noise can be represented as superposition of spectral components.
a) True
b) False
Answer: a
Explanation: Noise is any undesirable electrical energy that falls under the passband of the signal. It is a general term for the unwanted signals that gets added to our original signal. It is usually represented as the superposition of spectral components.

Generation of DSBSC Waves

1. LCD uses ________
a) sematic crystals
b) twisted nematic crystals
c) nematic crystals
d) cholesteric crystals
Answer: b
Explanation: LCD uses liquid crystal display. It uses twisted nematic crystals which are a type of liquid crystal, consisting of a substance called the nematic. The nematic liquid crystal is placed 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) Mixer
Answer: a
Explanation: Amplitude Limiter circuit is used in FM receiver to remove the noise or any variation in amplitude present in the received signal. Thus, the output of the amplitude limiter has a constant amplitude. So it is only used in frequency modulation and not in amplitude modulation.

 

SSB Modulation MCQs

 

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
Explanation: A duplexer is being an electronic unit, allows bi-directional communication over the same path. The transmitter and receiver can communicate simultaneously. In radar, the duplexer isolates the receiver from the transmitter while allowing 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
Explanation: A point-to-point communication refers to bidirectional communication between only one transmitter and one receiver. In SSB-SC modulation technique, the carrier is suppressed and only one of the two side-bands are transmitted. Thus, it reduces power consumption and lessens bandwidth. Thus, it is preferred 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
Explanation: In an AM transmitter, the required transmission power is obtained from class C amplifier, as it is a power amplifier, for low-level or high-level modulation. 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
Explanation: In low-level modulation, the generation of amplitude modulated signal takes place at low power levels. The generated AM signal is then amplified using a chain of linear amplifiers, which are required to avoid waveform distortion. Thus, linear amplified modulated stage is used in low level amplitude modulated system.

7. When noise is passed through a narrow band filter, the output of filter should be?
a) triangular
b) square
c) parabolic
d) sinusoidal
Answer: d
Explanation: 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
Explanation: 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 to pass a band of frequencies or to attenuate a band of frequencies.

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
Answer: a
Explanation: Upper sideband frequency will be (30 + 5) = 35 KHz and Lower sideband frequency will be (30 – 5) = 25 KHz.

10. Phase array radar can track many targets together.
a) True
b) False
Answer: a
Explanation: A phased array radar is an array of radiating elements, with each having a phase-shifter. The phase of the signal being emitted from the radiating element is changed to produce beams, thereby producing constructive or destructive interference for steering the beams in the required direction. Thus, it can track many targets together.

Coherent Detection of DSBSC Waves

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
Explanation: Pulse Position Modulation (PPM) is a type of modulation process in which position of the pulse of the carrier wave is varied with respect to the instantaneous position values of the message signal. In PPM, there is minimum noise interference but the main disadvantage of the PPM modulation technique is that synchronization between transmitter and receiver must be needed.

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
Explanation: On comparing with general Equation, Acosωt and substituting ω = 2×π× f. We have f1 = 25 Hz, f2 = 37.5Hz, f3 = 100π/2π = 50Hz.
Taking the maximum modulating frequency, fmax = 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 boosting of modulating signal frequency
c) For removing amplitude variations due to noise
d) For removing frequency variations
Answer: a
Explanation: Pre-emphasis is vastly applied in communication systems to improve signal strength before transmission. It refers to boosting the amplitudes of the weak modulating voltages for high audio frequencies in the range of 2 to 15KHz.

4. Amount of data transmitted for a given time is called _________
a) Noise
b) Frequency
c) Bandwidth
d) Power
Answer: c
Explanation: Bandwidth is the maximum amount of data transmitted over a particular period of time. It is also defined as the difference of high frequency and low frequency, of a given band.

5. Consider an AM broadcast station that 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
Explanation: Maximum Frequency, fm = (1000 + 10) KHz = 1010 KHz.
Minimum Frequency, fl = (1000 – 10) KHz = 990 KHz and,
Bandwidth = 2 * Modulating frequency = Maximum Frequency – Minimum Frequency
= (1010 – 990) KHz = 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
Explanation:
analog-communications-questions-answers-freshers-q6

7. Carson’s rule is used to calculate ________
a) Bandwidth of FM signal
b) SNR
c) Modulation index of FM signal
d) Figure of merit
Answer: a
Explanation: Carson’s rule states that only (β+1) (where β = modulation index) upper and lower sidebands along with the carrier, have significant magnitude and contain 99% of total power. Thus, as per Carson’s rule, required bandwidth is equal to the twice of sum of the maximum frequency deviation (fd) and the maximum modulating frequency(fm), 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) 130 KHz
b) 260 KHz
c) 65 KHz
d) 50 KHz
Answer: a
Explanation: According to Carson s rule, B = 2(fd +fm) = 2 (50 + 15) = 130 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
Explanation: In angle modulation, the amplitude of carrier remains constant and thus power depends on amplitude only. On Comparing with the equation, m(t) = Acos(ωct + mf sinωmt). We have A = 50, dissipated power, analog-communications-questions-answers-freshers-q9

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
Explanation: Properties of Hilbert transform states that the signal and its Hilbert transform :
i) have same energy density spectrum
ii) are mutually orthogonal
iii) have same auto correlation function
iv) have same magnitude
v) have a phase difference of “-90” degree.

Hilbert Transform

1. If a receiver has poor selectivity then it also have poor ________
a) sensitivity
b) blocking of unwanted signals
c) double spotting
d) reception
Answer: b
Explanation: Selectivity is the ability of a radio receiver to reject unwanted signals and allow only the desired signal to pass. Thus, it’s the blocking of unwanted signals. Sensitivity is the ability of a receiver to amplify weak signals to the desired level, whereas reception means to receive a signal.

2. For a large amount of power, a push pull amplifier is designed for operation as ________
a) class C
b) class B
c) class AB
d) class A
Answer: b
Explanation: A push pull amplifier is an amplifier whose output is a cumulative sum of the outputs of the currents produced by the two different stages of the load. It is generally used when a large amount of power is to be consumed. Of all the power amplifiers, only Class B amplifier is capable to produce such large amount of power.

3. In a Vidicon camera tube, the side of target plate facing the light has a coating of antimony trisulphide and the side facing electron gun has a coating of tin oxide.
a) True
b) False
Answer: b
Explanation: A vidicon camera tube is a type of camera having a photoconductive surface within. In a Vidicon camera tube, the side of target plate facing the light has a coating of Sno2 and the side facing electron gun has a coating of antimony trisulphide.

4. Autocorrelation function of which noise is a constant?
a) white noise
b) shot noise
c) transit time noise
d) extraterrestrial noise
Answer: a
Explanation: White noise is a random signal having equal intensities at different frequencies. The autocorrelation of white noise signal has a straight vertical line at origin and is zero for rest points. Thus the Autocorrelation function of white noise is constant.

5. Thermal noise shot noise and transit time noise is part of internal noise in communication.
a) True
b) False
Answer: a
Explanation: Internal noise in communication is the noise that gets generated within the receiver or communication system, like thermal noise, shot noise, transit time noise, partition noise and such like. External noise is that noise which is generated from an external source, that is not a part of the communication system.

6. In amplitude modulation, magnitude of side bands is 2ma times the carrier amplitude.
a) True
b) False
Answer: b
Explanation: In amplitude modulation, magnitude of side bands is m/2 times the carrier amplitude (‘a’), where ‘m’ is the modulation index.

7. In frequency modulation, there is a large increase in noise and hence decrease in the signal to noise ratio.
a) True
b) False
Answer: b
Explanation: In communication system, Signal-To-Noise ratio (SNR) is a measure of the amount of signal present compared to the surrounding noise. It is the ratio of signal power to noise power. Thus in frequency modulation, there is a large decrease in noise and hence increase in the signal to noise ratio.

8. The output of a battery eliminator is closed to ________
a) 70V DC
b) 70V AC
c) 6V AC
d) 6V DC
Answer: d
Explanation: A battery eliminator is an electronic device which is powered by an external source (except battery) such that it converts the source to a particular DC voltage which can be used as power supply for other devices. Its output is approximately 6V.

9. In India, the subcarrier frequency for transmission of color difference signals in television is approximately ________
a) 10.47 MHZ
b) 5.4 MHZ
c) 7.67 MHZ
d) 1.3 MHZ
Answer: d
Explanation: A carrier wave is a signal with a high frequency which is modulated for transmitting the information signal. A subcarrier is a sideband of the carrier wave, which is also modulated to send additional information. In India, the subcarrier frequency for transmission of color difference signals in television is approximately 1.3MHz.

10. The advantage of using mechanical filter in filter system of sideband suppression is good attenuation characteristics.
a) True
b) False
Answer: a
Explanation: The advantage of using mechanical filter in filter system of sideband suppression is smaller size, good band pass, good attenuation characteristics.

Costas Loop

1. Costa’s receiver is used for ________
a) coherent detection of AM-SC signal
b) asynchronous detection of AM-SC signal
c) amplification of signal
d) frequency mixing of signal
Answer: a
Explanation: A Costa’s loop is based on phase-locked loop (PLL). The circuit recovers carrier frequency from the suppressed-carrier modulated signals like DSBSC and SSBSC and phase modulated signals like FSK and BFSK. It is generally used for coherent detection of AM-SC signal in receivers.

2. Helical antenna is mainly used for ________
a) bandwidth calculation
b) production of video signals
c) radio transmission
d) satellite tracking
Answer: d
Explanation: A helical antenna is made of one or more conducting wires wound in the form of a helix. It is used for the communication (transmitting and receiving) Very High Frequency (VHF) signals through the different layers of atmosphere. Thus, it is used in satellite tracking.

3. Sampling frequency of a compact disk is about ________
a) 30 KHz
b) 50 KHz
c) 10 KHz
d) 70 KHz
Answer: c
Explanation: Sampling frequency is the number of samples per second which helps in extracting a discrete-time signal from a continuous-time signal. For a CD, it is about 10 KHz.

4. In frequency modulation, there is a large increase in noise and hence increase in the signal to noise ratio.
a) True
b) False
Answer: b
Explanation: In communication, Signal-To-Noise ratio (SNR) is defined as measure of the amount of signal present compared to the surrounding noise. It is the ratio of signal power to noise power. Thus, in frequency modulation, if there is a large decrease in noise, then there will be increase in the signal to noise ratio.

5. Image frequency is given by ________
a) fs + fi
b) fs ± fi
c) fs + 2fi
d) fs ± 2fi
Answer: c

Explanation: In the superheterodyne receiver, during the frequency conversion process, the local oscillator and the mixer often allow an undesired frequency signal, in addition to the incoming frequency signal. This results in the production of image frequency which is equal to the sum of signal frequency and double intermediate frequency.

6. Atmospheric noise, extraterrestrial noise and man-made noise is part of internal noise in communication.
a) True
b) False
Answer: b
Explanation: Atmospheric noise, extraterrestrial noise and man-made noise is part of external noise in communication. External noise are the ones which are produced by external sources, that are not part of the communication system. Internal noise are the ones which are produced by internal components of the communication system.

7. Thermal noise is a synonym of ________
a) Atmospheric noise
b) shot noise
c) Johnson noise
d) transit time noise
Answer: c
Explanation: Thermal noise is a type of internal noise in communication. It is produced due to thermal excitation of the free electrons in the circuit. It is also known as Johnson noise.

8. Distance at which a sky wave is received back on earth ________ with an angle of incidence.
a) increases
b) decreases
c) can be increase or decrease
d) can neither be increase or decrease
Answer: b
Explanation: The distance at which a sky wave is received back on earth is inversely proportional to the angle of incidence. Thus, the distance at which a sky wave is received back on earth decreases with an angle of incidence.

9. In a Vidicon camera tube, both sides of the target plate have the coating of Sno2.
a) True
b) False
Answer: b
Explanation: A vidicon camera tube is a type of camera tube having a photoconductive surface within. In Vidicon camera tube, the side of target plate facing the light has a coating of Sno2 and the side facing electron gun has a coating of antimony trisulphide.

10. Ground wave can be used for communication upto ________
a) 5 KHz
b) 12 KHz
c) 16 KHz
d) 25 KHz
Answer: c
Explanation: Ground wave is that wave that reaches a receiver from a transmitter directly without reflecting from ionosphere. It can be used for communication upto 16 KHz.

Problems of DSBSC

1. A Marconi antenna has length equal to ________
a) ʎ4
b) ʎ2
c) 1
d) ʎ8
Answer: a
Explanation: Marconi antenna is like a vertical wire having length one fourth of the working wavelength. Thus, it is a quarter-wave monopole antenna. Since, it was invented by Guglielmo Marconi, so it’s also called Marconi Antenna. It requires a ground plane with very good conductivity. Hence it is also called as a grounded antenna.

2. In a Vidicon camera tube, both sides of the target plate have the coating of antimony tri-sulphide.
a) True
b) False
Answer: b
Explanation: A vidicon camera tube is a type of camera having a photoconductive surface within. In a Vidicon camera tube, the side of target plate facing the light has a coating of Sno2 and the side facing electron gun has a coating of antimony trisulphide.

3. A trigonometric function is a synonym of ________
a) triangular function
b) orthogonal function
c) singular function
d) rectangular function
Answer: a
Explanation: Trigonometric function is the angle expressed as the ratio of two sides of a right-angled triangle that contains the angle. It can also be seen as a synonym of triangular function.

4. White noise is a synonym of ________
a) partition noise
b) shot noise
c) johnson noise
d) transit time noise
Answer: c
Explanation: White noise is an internal noise of communication system having different frequencies present with equal intensities. It can also be called Thermal noise or Johnson noise.

5. Local programmes in a cable TV can be telecast at any frequency.
a) True
b) False
Answer: a
Explanation: There is not a fixed frequency range or bandwidth set for broadcasting of local programs in a cable TV. It can be telecasted at any frequency, depending on the channel of bandwidth available.

6. Minimum viewing angle of resolution of human eye is ________
a) \(\frac{ʎ}{d}\)
b) \(\frac{1.22ʎ}{d}\)
c) \(\frac{3ʎ}{d}\)
d) \(\frac{2ʎ}{d}\)
Answer: b
Explanation: Minimum resolution of human eye for clear observation is generally \(\frac{1.22ʎ}{d}\). The diameter of human eye is about 25mm.

7. Power density spectrum of a resistor R can be expressed as ________
a) 2 kTR
b) 1 kTR
c) 0.75 kTR
d) 3 kTR
Answer: a
Explanation: Power Spectral Density is the quantity of power present in a signal in the frequency domain. The Power Density Spectrum of a resistor R can be expressed as 2 kTR where k is Boltzmann’s constant, T is temperature and R is given resistance.

8. VSB is generally used in ________
a) television production
b) radio transmission
c) telephonic conversations
d) long distance conversations
Answer: a
Explanation: Vestigial Sideband Modulation (VSB) is a type of amplitude modulation in which the carrier and only one sideband are completely transmitted and the other sideband is partly transmitted. Thus, television production is done using VSB modulation.

9. A helical antenna is used for satellite tracking due to its __________
a) broad bandwidth
b) circular polarization
c) linear polarization
d) good front to back ratio
Answer: d
Explanation: Helical antenna consists conducting wire. These conducting wire is wound around it in the form of helix. It is mainly used for satellite tracking due to its good front to back ratio.

10. What is the typical bandwidth of a 2-wire telephone line?
a) 20 Hz to 6 KHz
b) 300 Hz to 3.4 KHz
c) 20 Hz to 20 KHz
d) 200 Hz to 6.5 KHz
Answer: b
Explanation: A helical antenna is an antenna made of one or more conducting wires wound in the form of a helix. It produces circular polarization, because of which it is highly used in radio telemetry. It is mainly used for satellite tracking due to its good front to back ratio.

Quadrature Carrier Multiplexing

1. The output stage in a television transmitter is likely to be ________
a) screen modulated
b) plate modulated
c) rectangular modulated
d) grid modulated
Answer: d
Explanation: We generally use vestigial sideband modulation (VSB) for television production as it reduces the required bandwidth. However, the output stage in a television transmitter is generally grid modulated using a Class C amplifier, In grid modulation, the carrier wave to the message signal is provided by the grid circuit of the electron tube.

2. The advantage of using mechanical filter in filter system of sideband suppression is good band pass.
a) True
b) False
Answer: a
Explanation: Generally, the filters have sharp cut-off frequency as a result of which either of the sidebands are not completely suppressed. Thus, the advantage of using mechanical filter in filter system of sideband suppression is smaller size, good band pass and good attenuation characteristics.

3. Television transmission is possible through ________
a) DSB-SC
b) SSB-SC
c) VSB
d) AM
Answer: c
Explanation: We generally use vestigial sideband (VSB) for television production. In VSB, one of the sidebands is completely transmitted whereas the other sideband is partly transmitted, along with the carrier. VSB is used as it reduces the required bandwidth to half.

4. The rms value of a thermal noise voltage is proportional to (where k is Boltzmann’s constant) _______
a) √k
b) k
c) k2
d) k1
Answer: a

Explanation: Thermal noise is the noise generated in electronic devices at frequencies below a few kilohertz, due to thermal excitation of electrons. The root mean square value of thermal noise is proportional to root k times, where k represents Boltzmann’s constant.

5. For a human eye the minimum viewing angle of resolution should be ʎD where D is iris opening and ʎ is wavelength of light.
a) True
b) False
Answer: b
Explanation: For a human eye the minimum viewing angle of resolution should be 1.22ʎ/D. The diameter of human eye is about 25mm.

6. In amplitude modulation, magnitude of side bands is ________ times the carrier amplitude.
a) ma2
b) 4ma
c) ma
d) 2ma
Answer: b
Explanation: In amplitude modulation, for a good transmission and reception, the magnitude of side bands must be 4ma times the amplitude of carrier wave, (where, m = Modulation Index and a = Amplitude of Message signal).

7. Noises which get generated within the receiver is called ________
a) Internal noise
b) External noise
c) Man-made noise
d) Extraterrestrial noise
Answer: a
Explanation: Internal noise in communication is the noise that gets generated within the receiver or communication system.

8. Autocorrelation function of white noise is ________
a) an impulse function
b) a step function
c) a constant
d) a variable
Answer: c
Explanation: White noise is an internal noise of communication system having different frequencies present with equal intensities. It has a peak only at zero and are zero at all points. Thus, the autocorrelation function of white noise is a constant.

9. In a Vidicon camera tube, the side of target plate facing the light has a coating of Sno2 (tin oxide) and the side facing electron gun has a coating of antimony trisulphide.
a) True
b) False
Answer: a
Explanation: A vidicon camera tube is a type of camera having a photoconductive surface within. In a Vidicon camera tube, the side of target plate facing the light has a coating of Sno2 and the side facing electron gun has a coating of antimony trisulphide.

10. Marconi antenna ________
a) is a grounded antenna
b) is an ungrounded antenna
c) can either be grounded or ungrounded antenna
d) can neither be grounded nor ungrounded antenna
Answer: a
Explanation: Marconi antenna is like a vertical wire having length one fourth of the working wavelength. Thus, it is a quarter wave monopole antenna. Since it was invented by Guglielmo Marconi, so it’s also called Marconi Antenna. It requires a ground plane with very good conductivity. Hence it is also called as a grounded antenna.

Carrier Acquistion Techniques in DSB-SC

1. Flyback switched mode is used in television because it can supply many voltage outputs.
a) True
b) False
Answer: b
Explanation: In frequency modulation, power is determined from carrier amplitude and as carrier amplitude does not change, thus power remains constant. So if we increase the modulation index, then also it does not affect power and it will remain constant.

2. What will be the effect on power if the modulation index of a frequency modulated signal is increased?
a) increases
b) decreases
c) remains constant
d) first increases and then decreases
Answer: c
Explanation: In frequency modulation, power remains constant. So if we increase the modulation index then also it does not affect power, it will remain constant.

3. The equation 20sin⁡(108 t + 3 sin⁡ 109 t) represents which modulation?
a) AM
b) DSB-SC
c) PM
d) FM
Answer: d
Explanation: On comparing with the general equation of each modulation, we found that it represents Frequency modulation.
FM (t) = Ac sin (2πfct + βsin(2πfmt)), where symbols have their usual meaning.

4. Monopole antenna is a type of travelling wave antenna.
a) True
b) False
Answer: b
Explanation: Monopole antenna is a type of wire antenna like Marconi antenna, also known as quarter-wave antenna. Travelling wave antenna makes use of a travelling wave as a radiating guide and they consist of helical antenna, spiral antenna.

5. In which modulation, frequency of carrier signal is varied with the amplitude of modulating signal?
a) FM
b) AM
c) PM
d) VSB
Answer: a
Explanation: In Frequency Modulation, the frequency of carrier wave is varied with the instantaneous amplitude of modulating signal. The general FM modulated expression is :
FM (t) = Ac sin (2πfct + βsin(2πfmt)), where symbols have their usual meaning.

6. In Phase Modulation the phase of carrier signal is varied.
a) True
b) False
Answer: a
Explanation: In phase modulation, the phase shift of carrier wave is varied with respect to the instantaneous amplitude of modulating signal. The general PM modulated expression is:
PM (t) = Ac sin (2πfct + βsin(2πfmt)), where symbols have their usual meaning.

7. Power spectral density of thermal noise remains uniform upto frequency ___________
a) 1015 HZ
b) 105 HZ
c) 102 HZ
d) 1013 HZ
Answer: d
Explanation: Power Spectral Density is the quantity of power present in a signal in the frequency domain. Power spectral density of thermal or white noise gets affected when the frequency becomes greater than 1013 Hz and thus, it remains uniform upto 1013 Hz.

8. Carrier frequency remains constant in cell communication.
a) True
b) False
Answer: b
Explanation: In cell communication, the carrier frequency does not remain constant. Instead, the carrier frequency get changes continuously with respect to the instantaneous amplitude of the message signal.

9. Waves of which frequency can penetrate the atmosphere?
a) higher than 10 MHz
b) higher than 20 MHz
c) higher than 30 MHz
d) higher than 100 MHz
Answer: c
Explanation: Most Radio Waves penetrate the atmosphere. Thus, any wave having frequency greater than 30 MHz can penetrate the atmosphere. The waves having frequency less than it cannot penetrate the atmosphere and get absorbed into it.

10. Armstrong modulator can only generate FM signal.
a) True
b) False
Answer: b
Explanation: Armstrong Modulator can also generate PM along with FM. So the statement is incorrect. Thus, collectively Armstrong modulator is an Angle Modulator, where the angle of the carrier wave is varied with respect to the instantaneous amplitude of modulating signal.

Demodulation of DSB-SC

1. Flyback switched mode is used in television because it does not require a filter choke.
a) True
b) False
Answer: a
Explanation: Flyback switched mode is used in television because it does not require filter choke. It is also cheap as compared to others.

2. Waves of frequency 60MHz can penetrate the atmosphere.
a) True
b) False
Answer: a
Explanation: Most Radio Waves penetrate the atmosphere. Thus, frequencies above 30MHz can penetrate the atmosphere. So 60 MHz can penetrate the atmosphere.

3. Ground wave travels along surface of ground.
a) True
b) False
Answer: a
Explanation: Ground wave propagation occurs in the area between the surface of the Earth and the Ionosphere. The wave travels parallel or along the surface of the ground. It is usually used for transmission of low frequency signals.

4. TM stands for _________
a) Trans positional Modulation
b) Time Multiplexing
c) Transit Modulation
d) Tuned Modulation
Answer: a
Explanation: TM stands for Transpositional Modulation. In TM, the message signal is modulated by inserting inflections in the original waveform. The waveform is modified resulting in a modulated signal where each quarter cycle is transposed in modulation process.

5. In which of the modulation, the waveform is modified resulting in a signal where each quarter cycle is transposed.
a) AM
b) FM
c) PM
d) TM
Answer: d
Explanation: In TM, the message signal is modulated by inserting inflections in the original waveform. The waveform is modified resulting in a modulated signal where each quarter cycle is transposed in modulation process.

6. Which of the following is a reflector antenna?
a) Marconi reflector antenna
b) Corner reflector antenna
c) Helical antenna
d) Bow tie antenna
Answer: b
Explanation: It is corner reflector antenna that comes under reflector antenna. Helical antenna and bow tie antenna comes under travelling wave antenna and log antenna respectively. Marconi reflector antenna comes under grounded antenna.

7. Carbon microphone has a bidirectional feature.
a) True
b) False
Answer: b
Explanation: Carbon microphone does not have a bidirectional feature. It is ribbon microphone that has bidirectional feature.

Analog Communications MCQs – Double Sideband Suppressed Carrier Modulation ( Analog Communications ) MCQs

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