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## Communications MCQ Set 1

1. Which of the following oscillator is not found in FM?
a) Hartley Oscillator
b) Crystal Oscillator
c) Armstrong Oscillator
d) Dynatron Oscillator

Answer: b [Reason:] In frequency modulation, frequency of carrier varies with the modulated wave. Crystal oscillator is used to create an electric signal with a precise frequency. But it has no use in frequency modulation.

2. What is the approximated range of audio frequencies?
a) 20Hz to 20KHz
b) 20Hz to 200KHz
c) 10Hz to 10KHz
d) 20Hz to 20MHz

Answer: a [Reason:] The standard fixed range of audio frequencies is from 20HZ to 20 KHz.

3. What is duty cycle?
a) ratio of time on to time off
b) ratio of time off to time on
c) 12 × (ratio of time on to time off)
d) 12 × (ratio of time off to time on)

Answer: a [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.

4. Which of the following is used for demodulation of PAM signal?
a) a band reject filter
b) a band pass filter
c) a low pass filter
d) a high pass filter

Answer: c [Reason:] Pulse Amplitude Modulation is the transmission of data by varying the amplitude of different pulses. A low pass filter is always used when we have to demodulate it.

5. Which of the following is not a part of analogue communication?
a) PPM
b) AM
c) PCM
d) PWM

Answer: c [Reason:] Pulse code modulation (PCM) is a digital form of communication. It has no relation with analogue communication.

6. For demodulation of PCM, it is first converted into ________
a) PAM
b) PWM
c) PPM
d) FM

Answer: a [Reason:] Pulse code modulation (PCM) is a digital form of communication. For demodulation of PCM, it is necessary to convert it into PAM.

7. There is no requirement of IF amplifier stages for a video monitor.
a) True
b) False

Answer: a [Reason:] Video amplifier is used to amplify video signals before passing them to a Cathode ray tube. And there is no need of IF amplifier stages for a video monitor.

8. Inherently most noise resistant modulating system is _________
a) PCM
b) DSB
c) SSB
d) VSB

Answer: a [Reason:] Noise is an unwanted signal that causes disturbance. Generally most of the PCM system are noise resistant.

9. Which of the following modulation is used for radio navigation?
a) Pulse Modulation
b) Amplitude Modulation
c) Phase Modulation
d) Frequency Modulation

Answer: a [Reason:] In pulse modulation, message information is encoded in the amplitude of series of signal pulse. It is mostly used for radio navigation and for many others.

10. Frequency modulated broadcast band generally lies in ________
a) VHF
b) HF
c) SHF
d) LF

Answer: a [Reason:] VHF (very high frequency) is the ITU designation for the range of radio frequency from 30 to 300MHz. Frequency modulated broadcast band generally lies in it only.

## Communications MCQ Set 2

1. Quantization noise occurs in ________
a) Frequency Division Multiplexing
b) Time Division Multiplexing
c) Delta Modulation
d) Pulse Code Modulation

Answer: d [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.

2. Which is the greatest disadvantage of Pulse Code Modulation?
a) high error rate
b) its incompatibility with TDM
c) its inability to handle analog signals
d) large bandwidth is required for it

Answer: d [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. Its greatest disadvantage is its requirement for large bandwidth.

3. Inductance and capacitance of a line is 0.8 μHm and 32 pFm. Find Z0?
a) 158
b) 166
c) 143
d) 127

Answer: a [Reason:] 4. Pulse communication system that is inherently highly immune to noise is ________
a) PCM
b) PPM
c) PAM
d) PWM

Answer: a [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. It is inherently highly immune towards noise signals.

5. What the main advantage of PCM?
a) lower bandwidth
b) higher bandwidth
c) lower noise
d) good reception

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. Its greatest advantage is that it is immune towards noise.

6. In AM pilot carrier, transmission has ________
a) carrier and part of one side band
b) two side bands and a trace of carrier
c) two side bands
d) carrier one side band and part of other side band

Answer: b [Reason:] In amplitude modulated pilot carrier, the transmission wave has two side bands and a trace of carrier.

7. Quantization noise depends upon both sampling rate and number of quantization levels.
a) True
b) False

Answer: b [Reason:] Quantization noise in pulse code modulation (PCM) depends upon only on number of quantization levels.

8. Which of the following frequency is not transmitted in AM transmission?
a) Upper side frequency
b) Carrier frequency
c) Lower side frequency
d) Audio frequency

Answer: d [Reason:] Audio frequency is the frequency that is not transmitted in AM transmission.

9. Companding is used in PCM transmitters to allow amplitude limiting in the receivers.
a) True
b) False

Answer: b [Reason:] It decreases the number of bits required to record the strongest signal. Companding also improves signal to noise ratio.

10. What is the use of Companding?
a) in PCM transmitters to allow amplitude limiting in the receivers
b) in PCM receiver to overcome impulse noise
c) to overcome quantizing noise in PCM
d) to protect small signals in PCM from quantizing distortion

Answer: d [Reason:] It decreases the number of bits required to record the strongest signal. Companding also improves signal to noise ratio. It is mainly used to protect small signals in PCM from quantizing distortion.

## Communications MCQ Set 3

1. What will be the effect if we loading down a tuned-circuit amplifier?
a) it will raise the Q factor of tuned circuit
b) it will lower the Q factor of tuned circuit
c) it will multiply the Q factor by a constant
d) it will have no effect on Q factor

Answer: b [Reason:] Loading down a tuned circuit will automatically lower the Q factor of any circuit. Q factor characterizes resonator’s bandwidth relative to its center frequency.

2. What is the baseband bandwidth for a voice grade signal?
a) atleast 10KHz
b) atleast 5KHz
c) approximately 3KHz
d) approximately 5MHz

Answer: c [Reason:] The fixed standard baseband bandwidth for a voice grade signal is approximately close to 3KHz.

3. Miller effect can reduce the bandwidth of an amplifier.
a) True
b) False

Answer: a [Reason:] Miller effect is responsible for the increase in equivalent input capacitance of an inverting voltage amplifier. The increase equivalent input capacitance is given by, CM = C(1 + AV). Miller effect can be avoided by using a common-base amplifier.

4. What will be the reason of miller effect in BJT?
a) inductance of collector load
b) collector-to-emitter capacitance
c) base-to-emitter capacitance
d) base-to-collector capacitance

Answer: d [Reason:] Miller effect is responsible for the increase in equivalent input capacitance of an inverting voltage amplifier. The increase equivalent input capacitance is given by, CM = C(1 + AV). In BJT, miller effect is due to base-to-collector capacitance.

5. Trimmers and padders are ________
a) two types of adjusting tools
b) small adjustable resistors
c) small adjustable inductors
d) small adjustable capacitors

Answer: d [Reason:] Trimmers and padders are the two types of small adjustable capacitors. Its capacitance can be adjusted to the required value.

6. A class D amplifiers is very efficient than other amplifiers.
a) True
b) False

Answer: a [Reason:] Class D amplifier is also known as switching amplifier. It is operate as electronic switches, and not electric gain device which is commonly used in most amplifiers.

7. What is the full form of IF?
a) Intermediate Frequency
b) Internal Frequency
c) Indeterminate Frequency
d) Image Frequency

Answer: a [Reason:] IF stands for intermediate frequency. It is a frequency to which a carrier wave is shifted as an intermediate step in transmission and reception.

8. What is the full form of AGC?
a) Automatic Gain Control
b) Audio Gain Control
c) Automatic Gain Conversion
d) Active Gain Control

Answer: a [Reason:] AGC stands for Automatic Gain Control. It is a closed loop feedback regulating circuit. Its purpose is to provide a controlled signal amplitude at its output.

9. Sensitivity measures ________
a) weakest signal that is received at the receiver
b) strongest signal that is received at the receiver
c) weakest frequency signal that is received at the receiver
d) strongest frequency signal that is received at the receiver

Answer: a [Reason:] Sensitivity and Selectivity are two key components of any circuit. Sensitivity is used to measure the weakest signal that is received at the receiver.

10. Selectivity measures ________
a) the range of frequencies that receiver can select
b) with two signals close in frequency, the ability to select one and reject other
c) how well adjacent frequencies are separated by the demodulator
d) how well adjacent frequencies are separated in the mixer

Answer: a [Reason:] Sensitivity and Selectivity are two key components of any circuit. Selectivity is used to measure the range of frequencies that receiver can select.

## Communications MCQ Set 4

1. Who originated the theory of radio waves?
a) Marconi
b) Bell
c) Hertz
d) Maxwell

Answer: d [Reason:] Maxwell is the first person who originated the theory of radio waves. After some years, it was Hertz who applied Maxwell theories to the production and reception of waves.

2. If we compared to the USB, the power in the LSB is ________
a) different
b) twice of USB
c) same
d) half of USB

Answer: c [Reason:] The two bands i.e. upper sideband and lower sideband generally contains same information. They also has same power.

3. Which effect is characteristic of FM reception in a noisy environment?
a) threshold effect
b) capture effect
c) bessel effect
d) carson’s effect

Answer: a [Reason:] Threshold effect has low output signal to noise ratio. It contains large noise as compared to input signal to envelope detector. In threshold effect, detection of message signal is difficult.

4. Mathematically, the number of sidebands in frequency modulated system is _________
a) infinite
b) one
c) two
d) zero

Answer: a [Reason:] If we consider the number of sidebands in frequency modulated system, we found that the number of sidebands is infinite.

5. How an AM radio station increase its signal-to-noise ratio?
a) by increasing its modulation index
b) by decreasing its modulation index
c) by maintaining a constant modulation index
d) by increasing the frequency

Answer: a [Reason:] Signal to noise ratio is the ratio of signal power to the noise power. It is often described in decibels. It is directly proportional with modulation index and thus it increases with modulation index.

6. Which statement is true about an SCA signal?
a) it can use amplitude modulation
b) it can use frequency modulation
c) is monaural
d) it can use amplitude as well as frequency modulation and is also monaural

Answer: d [Reason:] STC stands for Subsidiary Communication Authority. SCA is a separated data channel which is transmitted along with the main signal over a broadcast station. It can either be amplitude modulated or frequency modulated.

7. What is the use of pre-emphasis?
a) to increase the signal to noise ratio for higher audio frequencies
b) to allow stereo audio to be carried by FM stations
c) to increase the signal to noise ratio for all audio frequencies
d) to increase the signal to noise ratio for lower audio frequencies

Answer: d [Reason:] Pre emphasis is used to equalize the modulating signal drive power in terms of deviation ratio. It is also helpful in increasing S/N ratio but for lower audio frequencies only.

8. In FM system, capture effect is observed when an FM receiver switching suddenly between two stations having nearby frequencies.
a) True
b) False

Answer: a [Reason:] Capture effect is related with FM reception. In capture effect, only the stronger of two signals at, or near, the same frequency will be demodulated.

9. What is the full form of NBFM?
a) Narrow band frequency modulation
b) National broadcast frequency modulation
c) Near band frequency modulation
d) Non broadcast frequency modulation

Answer: a [Reason:] NBFM stands for Narrow band frequency modulation. For NBFM, bandwidth should be less than 1.

10. Why the bandwidth of an FM signal is considered to be limited?
a) because there can only be a finite number of sidebands
b) because it is equal to the frequency deviation
c) because it is band-limited at the receiver
d) because the power in the outer sidebands is negligible

Answer: d [Reason:] In frequency modulated system, frequency of carrier is varied in proportion with the wave which is transmitted. Its bandwidth kept limited because the power of outer sidebands in frequency modulated system is negligible.

## Communications MCQ Set 5

1. What is the reason of “envelope” in an amplitude modulated signal?
a) amplitude signal
b) carrier signal
c) nematic signal
d) baseband signal

Answer: d [Reason:] Envelope is basically a smooth curve that outlines the extremes of any baseband signal. So basically it is message or baseband signal that determines the envelope.

2. AM stands for ________
a) Amplitude Modulation
b) Audio Modulation
c) Antenna Modulation
d) Amplified Modulation

Answer: a [Reason:] AM stands for amplitude modulation. Amplitude modulation is the change in amplitude of carrier wave in proportion to the waveform being transmitted.

3. What is the equation for full-carrier AM?
a) V(t) = (Ec+Em) ×(sin⁡ ωc t)
b) V(t) = (Ec+Em) ×(sin⁡ ωm t) + (sin⁡ ωc t)
c) V(t) = (Ec ×Em) ×(sin ⁡ωm t) × (sin⁡ ωc t)
d) V(t) = (Ec+Em sin⁡ ωm t) × (sin ⁡ωc t)

Answer: d [Reason:] Amplitude modulation is the change in amplitude of carrier wave in proportion to the waveform being transmitted. A carrier can be seen as a waveform that is modulated with input signal for the purpose of transmitting information. The equation for full-carrier AM is V(t) = (Ec +Em sin⁡ωm t) × (sin ⁡ωc t).

4. What is the cause of Overmodulation?
a) distortion
b) splatter
c) both distortion and splatter
d) half reception of signals

Answer: c [Reason:] Overmodulation is observed when the value of modulation index exceeds the value which is necessary to produce 100% modulation. So overmodulation is observed for modulation index greater than 1. It results in both distortion and splatter of waveform.

5. If AM radio station increases its modulation index then the audio gets louder at the receiver.
a) True
b) False

Answer: a [Reason:] Modulation index describes the variation of carrier signal modulation around its unmodulated level. If we increase the modulation index then audio signal gets louder.

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. The modulation index can be derived from ________
a) frequency-domain signal
b) time-domain signal
c) both frequency and time domain signal
d) a highly modulated carrier wave

Answer: c [Reason:] Modulation index describes the variation of carrier signal modulation around its unmodulated level. It can be derived for frequency-domain signals as well as for time-domain signals.

8. A single sideband modulation system is more efficient than a plain amplitude modulated system?
a) True
b) False

Answer: a [Reason:] Single side band uses only one of its sideband among two sidebands. The two sidebands carries exactly the same information and thus removing a sideband does not have any effect on our signal. Due to the removal of one sideband total bandwidth required also decreases which makes it more efficient than others.

9. At peak modulation an SSB transmitter radiate 1000W, what will it radiate with no modulation?
a) 1000 watts
b) 500 watts
c) 250 watts
d) 0 watts