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## Communications MCQ Set 1

1. What is the bandwidth required in SSB signal?
a) fm
b) 2fm
c) > 2fm
d) < 2fm

Answer: a [Reason:] In an AM modulated system, total bandwidth required is from fc + fm to fc – fm i.e. bandwidth is equal to 2fm. In SSB-SC transmission, one of the sideband gets suppressed so the bandwidth becomes fm only.

2. One of the advantage of using a high frequency carrier wave is that it dissipates very small power.
a) True
b) False

Answer: a [Reason:] The main advantage of using high frequency signals is that the signal gets transmitted over very long distances and thus dissipates very less power. The antenna height required for transmission also gets reduced at high frequencies. This is the reason for sending the audio signals at high frequency carrier signals for communication purpose.

3. What is the function of RF mixer?
a) Addition of two signals
b) Multiplication of two signals
c) Subtraction of two signals
d) To reduce the amount of noise

Answer: b [Reason:] RF mixer translates the frequencies of the two incoming signals from band to another by multiplying them.

4. The antenna current is 10A. Find the percentage of modulation when the antenna current increases to 10.4A?
a) 50%
b) 30%
c) 28.5%
d) 23%

Answer: c [Reason:] which gives m = 0.285 or 28.5%.

5. All the output pulses are at full transmitter power for a strong signal in all of the following except ________
a) PAM
b) PFM
c) PCM
d) PWM

Answer: a [Reason:] Pulse amplitude modulation varies amplitude of pulses for transmission of data. Its output pulses are not at full transmitter power for a strong signal and thus it is a exception.

6. In pulse transmitter, the ratio of time on to time off is called ________
a) base
b) bandwidth
c) duty cycle
d) efficiency

Answer: c [Reason:] Duty cycle is the fraction of one period in which the system is active. It can also be defined as the ratio of time on to time off of a system.

7. What is the carrier power of an un-modulated carrier signal when it is transmitted through an antenna having an impedance of 50ῼ and it produces a current of 5.4A?
a) 1458W
b) 1468W
c) 1478W
d) 1488W

Answer: a [Reason:] PC=I2 R = (5.4)2 X (50) = 1458 W.

8. Signal to quantization noise ratio in PCM system depends on _________
a) sampling rate
b) signal bandwidth
c) number of quantization levels
d) nature of carrier wave

Answer: c [Reason:] Pulse code modulation (PCM) is a digital form of communication. For demodulation of PCM, it is necessary to convert it into PAM. Quantization noise occurs in PCM only. Signal to quantization noise ratio depends on number of quantization levels.

9. Find the total power, if the carrier of an AM transmitter is 800W and it is modulated to 50%?
a) 100W
b) 800W
c) 500W
d) 900W

Answer: d [Reason:] PT=PC (1+m22), according to the problem PC = 800W and m = 0.5. On substituting values in the equation we get PT=800(1+ 0.522) = 900W.

10. Aliasing refers to ________
a) Sampling of signals less than at Nyquist rate
b) Sampling of signals at Nyquist rate
c) Sampling of signals greater than at Nyquist rate
d) Unsampled the already sampled signal

Answer: a [Reason:] Aliasing refers to the sampling of signals less than at Nyquist rate. Nyquist rate states that the rate of sampling of signals should be greater than or equal to twice the bandwidth of modulating signal. It gets reduced if sampling is done at a higher rate than nyquist rate of sampling.

## Communications MCQ Set 2

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 [Reason:] Selectivity is the property of a radio to respond only to the signals it is tuned to and reject other signals. If a radio receiver has poor selectivity then it also has poor blocking.

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 [Reason:] A push pull amplifier is generally used whenever a large amount of power is required. For getting a large power it is essential to operate push pull amplifier as a class B amplifier.

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 [Reason:] In a Vidicon camera tube, the side of target plate facing the light has a coating of 〖Sno〗_2 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 noise
d) extraterrestrial noise

Answer: a [Reason:] White noise or thermal noise or Johnson noise is an interior type of noise in communication system. It is a random signal having equal intensities at different frequencies which gives it a constant power and autocorrelation function.

5. Thermal noise, shot noise and transit time noise is part of internal noise in communication.
a) True
b) False

Answer: a [Reason:] Internal noise in communication is the noise that gets generated within the receiver or communication system. It may be put into the following categories: thermal noise, shot noise, transit time noise.

6. In amplitude modulation, magnitude of side bands is 2ma times the carrier amplitude.
a) True
b) False

Answer: b [Reason:] In amplitude modulation, magnitude of side bands is 4ma times the carrier amplitude.

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 [Reason:] 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 [Reason:] Battery eliminator is used to deliver electrical power without the need for multiple batteries. Its output power 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 [Reason:] Subcarrier is actually a sideband of a frequency carrier wave. It is modulated to send additional information. In India the subcarrier frequency for transmission of color difference signals in television is approximately 1.3 MHz.

10. The advantage of using mechanical filter in filter system of sideband suppression is good attenuation characteristics.
a) True
b) False

Answer: a [Reason:] The advantage of using mechanical filter in filter system of sideband suppression is smaller size, good band pass, good attenuation characteristics.

## Communications MCQ Set 3

1. What is the full-form of UTP?
a) Unshielded Twisted-Pair copper wire
b) Untwisted-Pair copper wire
c) Uniquely Twisted-Pair copper wire
d) Unshielded Thermal-Pair copper wire

Answer: a [Reason:] UTP stands for Unshielded Twisted-Pair copper wire. Unshielded in UTP refers to the lack of metallic shielding around copper wires. The twisted pair helps to minimize electronic interference.

2. NIC stands for ________
a) Network Interface Card
b) Network Interface Cable
c) Network Interface Code
d) Network Internal Card

Answer: a [Reason:] NIC stands for Network Interface Card. It is a computer circuit board that is installed in a computer so that it can be connected to a network.

3. Why CSMA/CD network has a maximum length for cables?
a) to increase the data rate
b) to decrease the data rate
c) to prevent packets from reaching all other nodes
d) to make sure that all other nodes hear a collision in progress

Answer: d [Reason:] CSMA/CD is a media access control used in early Ethernet technology for local area networking. Cables used in CSMA/CD has maximum length when compared to others to make sure that all other nodes hear a collision in progress.

4. The Clapp oscillator is _________
a) an advanced version of FETs
b) a type of crystal controlled oscillators
c) modified form of Colpitts oscillator
d) modified form of Hartley oscillator

Answer: c [Reason:] Clapp oscillator is just an advanced form of Colpitts oscillator.

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. What is the total voltage, when two noise voltage V1 and V2 are combined?

Answer: a [Reason:] When two noise voltage V1 and V2 are combined, the resulting total voltage VT comes out to be .

7. Shot noise is generated in ________
a) resistors
b) copper wires
c) transistors and diodes
d) capacitors

Answer: c [Reason:] Shot noise noise originate from discrete nature of electric charge. It is mostly generated in transistors and diodes.

8. Radians per second is equal to _________
a) 2πf
b) phase angle
c) phase deviation
d) bandwidth

Answer: a [Reason:] Radians per second is equal to the value 2πf.

9. Repeaters are not required for fiber optic cable lengths upto ________
a) 10km
b) 100km
c) 5km
d) 12km

Answer: b [Reason:] If the length of fiber optic cable is upto 100 km, the there is no need of repeaters in that circuit.

10. FTTC stands for ________
a) Fiber To The Curb
b) Fiber Telephone Transmission Cable
c) Fiber Transmission Timing Cable
d) Fiber Transmission Timing Constraint

Answer: a [Reason:] FTTC stands for Fiber To The Curb. FTTC refers to the installation and use of optical fiber cable directly to the curbs near homes as a replacement for plain old telephone service (POTS). It is basically the removal of all telephone lines and replacing them with optical fiber lines.

## Communications MCQ Set 4

1. Discone antenna is mainly used in UHF range.
a) True
b) False

Answer: a [Reason:] Discone antenna is omnidirectional so it is mainly used in UHF range. It is especially used in airports. Discone antenna is a combination of disc and cone.

2. Squelch circuit is normally inserted in receiver ________
a) after detector
b) before detector
c) before mixer
d) after power amplifier

Answer: a [Reason:] Squelch is added to suppress noise. It is added after detector when there is no carrier.

3. Each frequency gives rise to four side bands.
a) True
b) False

Answer: b [Reason:] Each frequency in communication gives rise to two side bands.

4. Field strength (E) is directly proportional to the square of transmission power.
a) True
b) False

Answer: b [Reason:] Actually the Field strength (E) is directly proportional to the square root of transmission power i.e. E ∝ √Pt

5. If the transmitted power is 100KW then the field at a distance ‘R’ is 60mV/m. Suppose if the transmitted power is reduced to 50KW then the field at same distance ‘R’ will be equal to ________
a) 50mV/m
b) 42mV/m
c) 45mV/m
d) 55mV/m

Answer: b [Reason:] Since which gives x = 42mV/m

6. If the value of resistor becomes 16 times than its previous value then its noise voltage will become ________
a) 16 times
b) 8 times
c) 4 times
d) 2 times

Answer: c [Reason:] Since the noise voltage is directly proportional to square root of resistance so if the value of resistor increased to 16 times then its noise voltage will become 4 times.

7. What is the modulation index for a single tone modulation, given that positive peak of AM wave is 20V and minimum value is 2V?
a) 0.81
b) 0.91
c) 0.73
d) 1

Answer: a [Reason:] Average value = 11V. So, modulation index = (20-11)/11 = 0.81

8. TV remote control used ultra violet light.
a) True
b) False

Answer: b [Reason:] Each time we press any button of TV remote it sends a beam of infrared rays. For different commands it sends out different beam which is detected by a microchip lying inside TV.

9. How capacitance is related with thickness?
a) it is inversely proportional to thickness
b) it is directly proportional to thickness
c) it is inversely proportional to twice of thickness
d) it is directly proportional to twice of thickness

Answer: a [Reason:] Capacitance, C = A×∈t so it is inversely proportional to thickness.

10. Noise gets mixed with signal at ________
b) transmitter
c) transducer
d) channel

Answer: d [Reason:] It acts as a path for taking signals to the receiver. There is a high probability of involving noise at the channel.

## Communications MCQ Set 5

1. Which of the following oscillator is not used in frequency modulation?
a) Colpitts Oscillator
b) Crystal Oscillator
c) Hartley Oscillator
d) Both Colpitts and Crystal Oscillator

Answer: b [Reason:] Crystal Oscillator uses the mechanical resonance of a vibrating crystal which is made up of piezoelectric material to create an signal with a precise frequency. It has no role in frequency modulated systems.

2. In frequency modulation, noise gets decreased if we decrease deviation.
a) True
b) False

Answer: b [Reason:] In frequency modulation, noise also starts decreasing if we increase deviation.

3. Which device is most commonly used for detection in radio receiver?
a) Triode
b) Amplifiers
c) Diode
d) Transistor

Answer: c [Reason:] In radio receivers, diode is most preferably used for detection purposes.

4. An input resistance of 20 Kῼ causes a noise voltage of 10μV. Suppose two input resistance each of 20 Kῼ are connected in parallel, then find the total noise voltage?
a) 8μV
b) 10μV
c) 20μV
d) 13μV

Answer: a [Reason:] Total resistance, 1R = 120 + 120 = 110 so R = 10kW = 1000W. Noise voltage will be equal to (10)√1020 = 8μV.

5. ________ modulator is an indirect way of generating FM.
a) Varactor FET
b) Reactance FET
c) Armstrong
d) Reactance bipolar transistor

Answer: c [Reason:] FM can be generated by two ways. One is direct and other is indirect way. In indirect way, there is an Armstrong method which is used to generate FM system.

6. Three point tracking is achieved by __________
a) Double conversion
b) Variable selectivity
c) Double spotting

Answer: d [Reason:] To achieve three point tracking a capacitor is connected in series with local oscillator coil. This capacitor is called padder capacitor.

7. The household radio receiver uses __________ detector.
a) Synchronous
b) Asynchronous
d) Envelope

Answer: d [Reason:] Household receivers generally uses envelope detector.

8. High IF in superheterodyne receiver, ____________
a) reduces adjacent channel rejection
b) improves selectivity
c) improves sensitivity
d) increases tracking problems

Answer: d [Reason:] In superheterodyne receiver, most of the received signals gets amplified from intermediate frequency (IF). High intermediate frequency helps in tracking problems.

9. Find the step size of a signal with peak to peak amplitude of 8V and is quantized into 40 levels?
a) 0.8
b) 1
c) 0.2
d) 0.6