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Communications MCQ Set 1

1. Frequency difference between WDM (Wavelength Division Multiplexing) and FDM (Frequency Division Multiplexing) is ________
a) low
b) high
c) slightly greater than 1
d) infinity

View Answer

Answer: d [Reason:] WDM (Wave Division Multiplexing) uses some basic principles of FDM (Frequency Division Multiplexing). FDM is used for analog signals transmitted through twisted pair cable while WDM is used to transmit signals through optical fiber cables. Frequency difference between WDM and FDM is infinity.

2. FDM stands for ________
a) Frequency Determine Multiplexing
b) Finite Difference Method
c) Frequency Division Multiplexing
d) Frequency Data Manager

View Answer

Answer: c [Reason:] FDM stands for Frequency Division Multiplexing. The total bandwidth is divided into a series of non-overlapping frequency bands and each band is used to carry a separate signal.

3. TDM stands for ________
a) Time Division Multiplexing
b) Time Difference Multiplexing
c) Time Division Method
d) Time Data Method

View Answer

Answer: a [Reason:] TDM stands for Time Division Multiplexing. It is used to transmit and receive independent signals over a common signal path with the help of synchronized switches.

4. TDM and FDM are used to multiplex multiple signals into a single carrier.
a) False
b) True

View Answer

Answer: b [Reason:] Both TDM and FDM are used to multiplex different signals into a single carrier. Multiplexing is the method of combining two or more than two signals into one, in such a way that each individual signal can be retrieved at the receiver.

5. What is Synchronous TDM?
a) gives same amount of time to each device
b) gives same amount of frequency to each device
c) gives variable time to each device
d) gives variable frequency to each device

View Answer

Answer: a [Reason:] Synchronous TDM gives exactly the same amount of time to each connected device. It allocates time to every device even if a device has nothing to transmit.

6. FDM is an analog multiplexing technique used to combines ________
a) analog signals
b) digital signals
c) both analog and digital signals
d) alternatively passes analog and digital signals

View Answer

Answer: a [Reason:] FDM is used to multiplex multiple analog signals. FD is applied when the bandwidth of the link is greater than the combined bandwidth of signals to be transmitted.

7. Frequency Division Multiplexing (FDM) is based on orthogonality.
a) True
b) False

View Answer

Answer: a [Reason:] Orthogonal frequency division multiplexing is used to encode multiple carrier frequencies. It is mainly used for latest wireless telecommunications.

8. Which multiplexing technique transmits digital signals?
a) FDM
b) TDM
c) WDM
d) Both FDM and TDM

View Answer

Answer: b [Reason:] Time Division Multiplexing is used to transmit digital signals. FDM and WDM techniques are used to transfer analog signals.

9. To get constant time delay, we should use ________
a) FDM technique
b) WDM technique
c) Synchronous TDM
d) Non synchronous TDM

View Answer

Answer: c [Reason:] Synchronous Time Division Multiplexing is used to provide constant time delay. It can also provide constant bandwidth.

10. Time Division Multiplexing (TDM) is based on orthogonality.
a) True
b) False

View Answer

Answer: a [Reason:] Both Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM) are based on orthogonal multiplexing.

11. As distance increases ________
a) Packet size also increases
b) Packet size decreases
c) Packet size becomes twice of its previous value
d) Packet size becomes half

View Answer

Answer: a [Reason:] Distance is directly proportional to packet size. So as distance increases packet size also increase.

12. If 32 equally probable events exists and we have to select one of them, number of required bits is?
a) 2
b) 4
c) 8
d) 5

View Answer

Answer: d [Reason:] 25 = 32, so number of bits required is 5.

Communications MCQ Set 2

1. What is the auto-correlation function of a signal (t) at t0=0?
a) It is equal to average voltage of the signal
b) It is equal to average power of the signal
c) It is equal to zero
d) It is equal to infinite

View Answer

Answer: b [Reason:] Auto-correlation function is also known as serial-correlation. It is the correlation of a function with a delayed copy of itself. So it is non-unique description of signals. For a signal (t) at t0=0, it is equal to the average power of the signal.

2. The auto-correlation function is _________
a) exponential function
b) linear function
c) odd function
d) even function

View Answer

Answer: d [Reason:] Auto-correlation is an even function. Sometimes it is also known as serial-correlation. It is the correlation of a function with a delayed copy of itself.

3. In TV transmission, sound signal is ________ modulated.
a) Phase
b) Pulse
c) Frequency
d) Amplitude

View Answer

Answer: c [Reason:] Amplitude Modulation is invariably used for picture transmission while frequency modulation is generally used for transmission of sound due to its inherent advantages over amplitude modulation. It is not suitable for transmitting videos due to its large bandwidth.

4. A signal is called ergodic if all type of ensemble averages are constant.
a) True
b) False

View Answer

Answer: b [Reason:] A random signal is ergodic if all type of ensemble averages can be interchange.

5. A ransom signal is defined as ergodic, if ________
a) all type of ensemble averages are interchangeable
b) all type of ensemble averages are not interchangeable
c) all type of ensemble averages are constant
d) all type of ensemble averages are double than their previous value

View Answer

Answer: a [Reason:] A stochastic process is said to be ergodic if its properties can be reduced from a single. A non ergodic process changes erratically at an inconsistent rate. Ergodic process exists if many random signals have identical time and ensemble averages.

6. Which statement is correct about ergodic process?
a) ensemble and time averages are identical
b) ensemble and time averages are opposite to each other
c) ensemble and time averages are different
d) ensemble average are exactly twice of time averages

View Answer

Answer: c [Reason:] A stochastic process is said to be ergodic if its properties can be reduced from a single. A non ergodic process changes erratically at an inconsistent rate. Ergodic process exists if many random signals have identical time and ensemble averages.

7. What is the maximum transmission efficiency?
a) 67.88%
b) 33.33%
c) 73%
d) 54.03%

View Answer

Answer: b [Reason:] The maximum transmission efficiency is 33.33%. Since 2/3 of power is in carrier which conveys no useful information. So we left with only 1/3 of power.

8. Communication consists of electrical means of sending information only.
a) True
b) False

View Answer

Answer: b [Reason:] Communication consists of electrical means of sending, receiving and processing information.

9. In communication system, redundancy ________
a) Helps to detect error
b) Helps to correct error
c) Reduces efficiency of communication
d) helps to detect and correct error and also reduces efficiency

View Answer

Answer: d [Reason:] Redundancy helps in many ways in communication. It not only helps to detect error but also helps in correcting it. It sometimes reduces efficiency also. It is basically the duplication or can be say that it is the backup of some important components with the intention of increasing reliability of the system.

10. Which statement is true about emitter modulator amplifier for amplitude modulation?
a) operates in class A mode
b) operates in class C mode
c) has a high efficiency
d) has a high output power

View Answer

Answer: a [Reason:] Emitter modulator amplifier for AM operates in class A mode and also has a very low efficiency. The output of this modulator is very small and therefore it is not suitable for modulation at high level.

Communications MCQ Set 3

1. FM and PM are two ways of angle modulation.
a) True
b) False

View Answer

Answer: a [Reason:] In angle modulation, phase or frequency of the carrier is varied with respect to the transmitting signal.

2. FM transmitting and receiving equipment is __________ as compared to AM equipment.
a) costly
b) cheaper
c) almost equally costly
d) available with ease

View Answer

Answer: a [Reason:] Frequency transmitting and receiving equipment is generally costly when compared to amplitude modulated equipment.

3. Which one of the following is analog?
a) PCM
b) PWM
c) Delta modulation
d) Differential PCM

View Answer

Answer: b [Reason:] Pulse code modulation (PCM), Differential pulse code modulation (DPCM), and Delta modulation comes under digital modulation. Only the PCM comes under analog communication.

4. Schmitt trigger is used to generate PDM.
a) True
b) False

View Answer

Answer: b [Reason:] Pulse density modulation (PDM) is used to represent an analog signal with a binary signal. For generating PDM, monostable multi-vibrator is used.

5. Which of the following is used to generate PDM?
a) Monostable multi-vibrator
b) Free running multi-vibrator
c) Either monostable of free running multi-vibrator
d) JK flip-flop

View Answer

Answer: a [Reason:] Pulse density modulation (PDM) is used to represent an analog signal with a binary signal. For generating PDM, monostable multi-vibrator is used.

6. In case of FM, if the modulating frequency is lowered and the modulating voltage remains constant then the amplitude of distant sidebands decreases.
a) True
b) False

View Answer

Answer: b [Reason:] If the modulating frequency in FM lowered and the modulating voltage remains constant then the amplitude of distant sidebands increases.

7. Skip distance depends on time of day and angle of incidence.
a) True
b) False

View Answer

Answer: a [Reason:] A skip distance is the distance between the two points where radio waves from a transmitter refracted downwards by different layers of the ionosphere. It basically depends on time of day as well as on angle of incidence.

8. Leak type bias is used in plate modulated amplifier to prevent tuned circuit damping.
a) True
b) False

View Answer

Answer: b [Reason:] Leak bias is mainly used to prevent excessive grid current in plate modulated amplifiers.

9. For telegraphy pulse code modulation (PCM) is widely used.
a) True
b) False

View Answer

Answer: b [Reason:] Frequency Shift Keying (FSK) is used to transmit digital information through discrete frequency changes of a carrier signal. It is mainly used for telegraphy.

10. DVD uses laser beam for recording and video head for playback.
a) True
b) False

View Answer

Answer: b [Reason:] Digital Versatile Disc (DVD) uses laser beam for both recording and playback.

Communications MCQ Set 4

1. TDMA stands for ________
a) Time Division Multiple Access
b) Time Domain Multiple Access
c) Tone Division Multiple Access
d) Tone Domain Multiple Access

View Answer

Answer: a [Reason:] TDMA stands for Time Division Multiple Access. It can be seen as a channel access method for shared-medium networks.

2. Which term is used when signals move from one line to another?
a) time switching
b) space switching
c) line switching
d) cross-point switching

View Answer

Answer: b [Reason:] Space switching is the used term for signals moving from one line to another.

3. PSK stands for Pulse Shift Keying.
a) True
b) False

View Answer

Answer: b [Reason:] PSK stands for Phase Shift Keying. It is a modulation scheme that conveys information by changing the phase of carrier.

4. Which term is used for moving PCM samples from one time slot to another?
a) time switching
b) space switching
c) line switching
d) cross-point switching

View Answer

Answer: a [Reason:] Time switching is the used term for moving PCM samples moving from one time slot to another.

5. Power can be coupled into or out of a waveguide with a magnetic field probe.
a) True
b) False

View Answer

Answer: b [Reason:] Power can be coupled into or out of a waveguide not only with a magnetic field probe. It can also be coupled with an electric field probe. It can also be coupled through a hole in the waveguide.

6. What is the full form of LOS?
a) Line Of Signal
b) Line Of Sight
c) Loss Of Signal
d) Loss Of Sight

View Answer

Answer: b [Reason:] Line Of Sight is a line between two points. It is a straight path between a transmitting antenna and a receiving antenna.

7. How we can define satisfactory performance of an analog microwave system?
a) carrier to noise ratio that exceeds a given value
b) carrier to noise ratio that is below a given value
c) an ERP value that exceeds a given value
d) an ERP value that is below a given value

View Answer

Answer: a [Reason:] We can measure performance of an analog microwave system by calculating the carrier to noise ratio that exceeds a given value.

8. RGB stands for ________
a) Red Green Brown
b) Red Green Black
c) Red Gold Blue
d) Red Green Blue

View Answer

Answer: d [Reason:] RGB stands for Red Green Blue. It is an additive color model in which red, green and blue light are added together in various ways to reproduce a broad variety of colors.

9. How many lines are there in an NTSC signal?
a) 1024
b) 1856
c) 625
d) 525

View Answer

Answer: d [Reason:] In NTSC, it is standardized fixed that it has total 525 lines.

10. Luminance refers to ________
a) contrast
b) color
c) brightness
d) chroma

View Answer

Answer: c [Reason:] Luminance refers to brightness. It is a photometric measure of luminous intensity per unit area of light travelling in a given direction.

Communications MCQ Set 5

1. What is Amplitude Modulation?
a) Change in amplitude of carrier according to modulating signal
b) Change in frequency of carrier according to modulating signal
c) Change in amplitude of carrier according to modulating signal frequency
d) Change in amplitude of modulating signal according to carrier signal

View Answer

Answer: a [Reason:] Amplitude modulation is most commonly used for transmitting information via a carrier wave. In amplitude modulation, amplitude of carrier wave is varied with respect to waveform being transmitted.

2. Frequency components of an AM wave are?
a) Carrier frequency (ωc) with amplitude A
b) Lower side band (ωc + ωm) having amplitude mA4
c) Upper side band (ωc – ωm) having amplitude mA4
d) Carrier frequency (ωc) with amplitude A2

View Answer

Answer: a [Reason:] Actually the frequency components of AM waves are – Carrier frequency (ωc) with amplitude A, Lower side band (ωc – ωm) having amplitude mA2 and Upper side band (ωc + ωm) having amplitude mA2 (where m represents modulation index).

3. In amplitude modulation frequency and phase of carrier ________
a) varies
b) same
c) initially varies but become same after sometime
d) remains constant

View Answer

Answer: d [Reason:] In AM, amplitude of carrier varies according to instantaneous value of baseband signal. The frequency and phase of carrier remains constant.

4. Envelope of AM wave has the same shape as the message of baseband signal.
a) True
b) False

View Answer

Answer: a [Reason:] Modulation Envelope is observed when carrier, upper sideband and lower sideband are combined in a single impedance and observed on time versus amplitude curve. For AM, it has same shape as message of baseband signal.

5. When aliasing takes place?
a) Sampling signals less than Nyquist Rate
b) Sampling signals more than Nyquist Rate
c) Sampling signals equal to Nyquist Rate
d) Sampling signals at a rate which is twice of Nyquist Rate

View Answer

Answer: a [Reason:] Aliasing causes different signals to become indistinguishable when sampled. It happens when sampling rate is less than Nyquist rate.

6. Synchronous detection of AM signal is considered as a disadvantage. Why?
a) Available at low prices
b) Needs additional system for generation of carrier
c) Needs less number of system as estimated for generation of carrier
d) Receiver is not complex

View Answer

Answer: a [Reason:] Disadvantage of synchronous detection of AM signal are that it needs an additional system for generation of carrier. It also needs additional system for synchronization of carrier. Moreover receiver is complex and costly.

7. Which devices we used for AM Demodulation?
a) Envelope detector and Square law demodulator
b) PLL detector and Square law demodulator
c) Envelope detector and PLL detector
d) Only square law demodulator

View Answer

Answer: a [Reason:] AM signals are demodulated using Square law demodulator or envelope detector. The demodulator extract information from received AM signal.

8. Square Law modulators ________
a) used for amplitude modulation
b) have non linear current-voltage characteristics
c) have non linear current-voltage characteristics as well as used for Amplitude Modulation
d) used for frequency modulation

View Answer

Answer: c [Reason:] Square law modulators are used for amplitude modulation and they have a non linear current voltage characteristics. They are highly linear in low voltage region.

9. What do you understand by low level AM?
a) Output power is low
b) Modulation is done at high power of carrier and modulating signal
c) Collector Modulation Method is low level AM
d) Output power is high

View Answer

Answer: a [Reason:] In low level AM, modulation is done at low power of carrier and modulating signal therefore output power is low. Collector Modulation Method is high level AM.

10. What do you understand by high level AM?
a) Output power is low
b) Modulation is done at high power of carrier and modulating signal
c) No need to boost the carrier and modulating signal
d) Modulation is done at low power of carrier and modulating signal

View Answer

Answer: b [Reason:] For a high level AM, modulation is done at high power of carrier and modulating signal so power amplifier is used to boost the carrier and modulating signal.

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