Communications MCQ Set 1
1. For Amplitude Modulation, Emitter modulator ________
a) Operates in class C mode
b) Has a low efficiency
c) Output power is high
d) Operates in class B mode
Answer
Answer: b [Reason:] Emitter Modulator operates in class A region. It has a very low efficiency. The output power is low so for modulation at high level, it is not suitable.
2. Why AM is used for broadcasting?
a) More immune to noise
b) Less transmitting power is required
c) It has high fidelity
d) Avoids Receivers Complexity
Answer
Answer: d [Reason:] AM uses square law and envelope detectors for transmission. Since they both have non complex structures so AM is highly recommended for broadcasting.
3. Singletone amplitude modulation ________
a) consists of only one frequency
b) contains a large number of frequency components
c) contains no frequency components
d) contains infinite number of frequency components
Answer
Answer: a [Reason:] Single tone modulation is that in which the baseband signal consists of only one frequency. In other words, modulation is done by a single frequency.
4. AM spectrum consists of ________
a) Carrier frequency
b) Upper sideband
c) Lower sideband
d) Carrier frequency with both upper and lower sideband
Answer
Answer: d [Reason:] Spectrum of Am wave consists of a carrier with two sidebands. It is found that if carrier frequency is 5KHz then the two sidebands produced by it is 5KHz away from the carrier.
5. The minimum channel Bandwidth is used by which modulation technique?
a) VSB
b) SSB
c) DSB-SC
d) FM
Answer
Answer: b [Reason:] A signal has two sidebands which are exactly the mirror images of each other. So we can remove one side band which further reduces its bandwidth.
6. Neper is ________ decibel.
a) Larger than
b) Smaller than
c) Same as
d) Exactly twice of
Answer
Answer: a [Reason:] Neper is a logarithmic unit used for finding ratios of power quantities. Like decibel it is also a dimensionless unit. 1Np = 20/ln10 = 8.686 db.
7. AM broadcast station transmits modulating frequency upto 6KHz. If transmitting frequency is 810KHz, then maximum and lower sidebands are ________
a) 816KHz and 804KHz
b) 826KHz and 804KHz
c) 916KHz and 904KHz
d) Not possible
Answer
Answer: a [Reason:] Maximum frequency = 810 + 6 = 816KHz and Minimum frequency = 810 – 6 = 804KHz. Moreover it has a bandwidth of (816 – 804) = 12KHz.
8. Find lower frequency component in AM wave, given that highest frequency component is 900KHz and bandwidth is 12KHz?
a) 832KHz
b) 600KHz
c) 868KHz
d) 888KHz
Answer
Answer: d [Reason:] Highest frequency component is 900KHz and bandwidth is 12KHz. So lower frequency component is 900 – 12 = 888KHz.
9. Amplitude Modulated wave is ________
a) Sum of carrier and modulating wave
b) Product of carrier and modulating wave
c) Difference of carrier and modulating wave
d) Sum of carrier and its product with modulating wave
Answer
Answer: a [Reason:] The modulation of a wave by varying its amplitude is known as amplitude modulation. It is represented as, s(t) = [A + x(t)] (Cos ωc) t.
10. Envelope Detector is a/an ________
a) Coherent detector
b) Asynchronous Detector
c) Synchronous Detector
d) Product Demodulator
Answer
Answer: b [Reason:] Envelope detectors are asynchronous in nature. The advantage of asynchronous over synchronous is that it is simple, cheap, setup is faster.
Communications MCQ Set 2
1. PDM can be generated by either free running multi-vibrator or monostable multi-vibrator.
a) True
b) False
Answer
Answer: b [Reason:] Pulse density modulation (PDM) is used to represent an analog signal with a binary signal. For generating PDM, only monostable multi-vibrator is used.
2. In case of FM, if the modulating frequency is lowered and the modulating voltage remains constant then the amplitude of distant sidebands first increases and then decreases.
a) True
b) False
Answer
Answer: b [Reason:] If the modulating frequency in FM lowered and the modulating voltage remains constant then the amplitude of distant sidebands only increases.
3. Skip distance depends on time of day.
a) True
b) False
Answer
Answer: b [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.
4. Leak type bias is used in plate modulated amplifier to ________
a) increase bandwidth
b) decrease bandwidth
c) prevent tuning
d) prevent excessive grid current
Answer
Answer: d [Reason:] Leak bias is mainly used to prevent excessive grid current in plate modulated amplifiers.
5. Full duplex operation allows transmission in only one direction at the same time.
a) True
b) False
Answer
Answer: b [Reason:] Full duplex operation allows transmission in both directions at the same time.
6. A woofer is connected from the input through a ________
a) high pass filter
b) low pass filter
c) band pass filter
d) band stop filter
Answer
Answer: b [Reason:] A woofer is connected from the input through a low pass filter to restrict the high frequencies.
7. Which of the following is used for attenuating high frequencies?
a) resistance
b) inductance
c) series capacitance
d) shunt capacitance
Answer
Answer: d [Reason:] Shunt capacitance is used to improve the power factor in the network. It also improves voltage stability. It is used in communication system for attenuating high frequencies.
8. If the length of antenna is equal to whole wavelength the radiation is zero in all direction?
a) True
b) False
Answer
Answer: b [Reason:] If the length of antenna is equal to whole wavelength only the radiations which is at right angles is zero.
9. For getting zero radiation at right angles, length of antenna should be __________
a) ʎ
b) 2 ʎ
c) ʎ⁄2
d) ʎ⁄4
Answer
Answer: a [Reason:] For getting zero radiation at right angles, length of antenna should be equal to the whole wavelength.
10. What is the commercial frequency deviation of frequency modulation?
a) 70 KHz
b) 75 KHz
c) 80 KHz
d) 85 KHz
Answer
Answer: b [Reason:] Frequencies lying nearly about 75KHz is considered as commercial frequency deviation for frequency modulation.
Communications MCQ Set 3
1. FM stands for ________
a) Frequency Modulation
b) Frequency Moderator
c) Frequent Moderator
d) Frequency Demodulation
Answer
Answer: a [Reason:] FM stands for Frequency Modulation. It is the encoding of information on a carrier wave by varying the instantaneous frequency of the wave.
2. Why a sinusoidal signal is considered analog?
a) It moves in both positive and negative direction
b) It is positive for one half cycle
c) It is negative for one half cycle
d) It has infinite amount of steps between a maximum and minimum value
Answer
Answer: d [Reason:] A sinusoidal wave is an analog signal. An analog electrical signal is a signal with an infinite amount of ‘steps’ between a minimum and maximum value. Say a signal has 0V as the minimum and 4.2V as the maximum voltage, that means the signal could be 3.3V or 3.345464634546….V.
3. The minimum height of antenna required for transmission in terms of ʎ is ________
a) 3ʎ⁄2
b) ʎ⁄4
c) 2ʎ
d) ʎ
Answer
Answer: b [Reason:] For effective transmission length of antenna should be one forth of ʎ. (L = ʎ⁄4 where ʎ = c⁄f). Actually the reason for maintaining antenna height to ʎ⁄4 is for matching impedance.
4. What do you understand by the term analog communication?
a) A continuous signal with varying phase or amplitude
b) A discrete signal
c) A numerical coded signal
d) A suitable method for long distance communication
Answer
Answer: a [Reason:] Analog communication means that information is transmitted in the form of a continuous signal. Coding is applied in case of digital communication. Moreover for a long distance communication, it suffers from many losses such as distortion and interference.
5. What is Demodulation?
a) Process of varying one or more properties of a periodic waveform
b) Recovering information from modulated signal
c) Process of mixing a signal with a sinusoid to produce a new signal
d) Involvement of noise
Answer
Answer: b [Reason:] Demodulation is the process of recovering information from a modulated carrier wave while it is modulation in which properties of a periodic waveform is varied. A modulator is a device that performs modulation while demodulator performs demodulation. A modem can perform both functions.
6. Data transmitted for a given amount of time is called ________
a) Noise
b) Power
c) Frequency
d) Bandwidth
Answer
Answer: d [Reason:] Bandwidth is defined as a range within a band of frequencies or wavelengths. It is also defined as the amount of data that can be transmitted in a fixed amount of time.
7. If Output can be represented as linear combination of input then ________
a) The system is linear
b) The system is causal
c) The system is non causal
d) The system is time invariant
Answer
Answer: a [Reason:] The input signal is broken into many components, each component is processed individually, and the result is finally reunited . It also follows superposition property.
8. Amplitude Modulation suffers from ________
a) Modulation
b) Intra Modulation
c) Cross Modulation
d) Frequency Modulation
Answer
Answer: c [Reason:] Cross modulation is generally occurred in receivers receiving an AM signal in the presence of other strong AM signal. Actually the modulation from the strong signal was cross modulated and appeared on the weaker signal being received.
9. Medium which sends information from source to receiver is called ________
a) Transmitter
b) Transducer
c) Receiver
d) Channel
Answer
Answer: d [Reason:] Channel is a medium through which a message is transmitted. Transmitter is used process the electrical signal from different aspects. Transducer is used to convert one form of energy into another form. Now when the signal is received at the output of the channel, it consists noise. So the role of receiver is to estimate the original message signal.
10. Telephones send information through wires in form of ________
a) radio signals
b) electrical signal
c) electromagnetic waves
d) microwaves
Answer
Answer: b [Reason:] A telephone converts sound into electronic signals which is suitable for transmission over long distances, where it is converted to sound again. The essential components of telephone are a microphone which acts as a transmitter and an earphone which acts as a receiver.
11. Cell phones sent information in form of ________
a) microwaves
b) electrical signals
c) radio signals
d) electromagnetic waves
Answer
Answer: d [Reason:] Cell phones send electromagnetic waves from one base station to other. The waves are sent through antennas. This is called wireless communication.
12. Ability of receivers to select the wanted signals among various incoming signal is called ________
a) Selectivity
b) Stability
c) Sensitivity
d) Modulation
Answer
Answer: a [Reason:] Ability of receiver to select wanted signal from various incoming signal is called selectivity. It rejects the other signals at closely lying frequencies. Sensitivity is the minimum magnitude of input signal required to produced a specified output. While modulation is the process of varying one or more properties of carrier signal.
13. Which device is used for tuning the receiver according to incoming signal (especially in TV)?
a) Low pass filter
b) High pass filter
c) Zener diode
d) Varacter diode
Answer
Answer: d [Reason:] Varacter diode is a tuning diode having variable capacitance. Tuning or Varacter diodes are operated in reverse bias and therefore no current flows through it. Varacter diodes are mainly used in Voltage Controlled Oscillators (VCOs) and RF Filters.
Communications MCQ Set 4
1. Video signals are transmitted through ________
a) Frequency Modulation
b) Amplitude Modulation
c) Pulse Modulation
d) Either frequency or amplitude modulation
Answer
Answer: b [Reason:] Video signals required a large transmission bandwidth to transmit, So modulation of video signals is possible only by amplitude modulation.
2. What you understand by the term discone antenna?
a) combination of disc and cone
b) combination of disc and cone with a spacing of ʎ/4
c) combination of disc and cone with a spacing of ʎ/2
d) same as a simple antenna
Answer
Answer: a [Reason:] Discone antenna is a combination of disc and cone. It is usually vertically mounted, with disc at top and the cone beneath.
3. A system has a voltage of 15V and it produces a total current of 1300μA. Find the power of system?
a) 0.025W
b) 0.035W
c) 0.0195W
d) 0.045W
Answer
Answer: c [Reason:] Power, P = 15 x 1300 x 10-6 =0.0195W.
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
Answer: a [Reason:] Total resistance, 
5. Modem is combination of both modulator and demodulator.
a) True
b) False
Answer
Answer: a [Reason:] A modem (modulator-demodulator) is used to modulate one or more carrier wave as well as to demodulate them for getting information.
6. Any signal and its Hilbert transform are mutually orthogonal.
a) True
b) False
Answer
Answer: a [Reason:] Consider f(t) be any signal and fh(t) is its Hilbert transform. If we integrate both of them under proper boundary conditions, we get 0 which shows that both are mutually perpendicular i.e. 
7. What Squelch circuit does?
a) helps to receive signals that are useful for the system
b) helps to make channel noise free
c) helps to receive two or more than two signals at once
d) helps to suppress the audio output in the absence of sufficiently strong desired signal
Answer
Answer: d [Reason:] A Squelch circuit blocks the output in the absence of sufficiently strong desired signal. It is widely used in two-way radios to suppress the annoying sounds.
8. Mostly used radio receivers are?
a) pulsed receivers
b) CW receivers
c) TRF receivers
d) super heterodyne receivers
Answer
Answer: d [Reason:] Super heterodyne are radio receivers that uses frequency mixing to convert received signal to a fixed intermediate frequency that can be processed more conveniently. It is also the mostly used receivers.
9. Find the step size of a signal with peak to peak amplitude of 4V and is quantized into 64 levels?
a) 32.3 x 10-3
b) 30.23 x 10-3
c) 62.5 x 10-3
d) 53.3 x 10-3
Answer
Answer: c [Reason:] Step size, 
10. If the step size of a system is 0.0625, find its quantization power?
a) 3.25 x 10-3
b) 3.25 x 10-4
c) 3.25 x 10-2
d) 3.25 x 10-1
Answer
Answer: b [Reason:] 
Communications MCQ Set 5
1. Which of the following statement is true about frequency modulation?
a) noise gets decrease if we decrease deviation
b) noise gets decrease if we increase deviation
c) noise gets decrease by maintaining deviation constant
d) noise do not depends on deviation
Answer
Answer: b [Reason:] In frequency modulated system, if we increase deviation then the noise in system gets decrease.
2. Aspect ratio is the ratio of ________
a) width to height
b) height to width
c) power to width
d) width to power
Answer
Answer: a [Reason:] Aspect ratio is the ratio of width to the height of image (screen).
3. Colpitts oscillator is used in frequency modulation.
a) True
b) False
Answer
Answer: b [Reason:] Crystal oscillator is used in frequency modulation.
4. Carrier frequency is given by the formula ________
Answer
Answer: a [Reason:] Carrier frequency is given by the formula, 
where L is inductance and C is Capacitance.
5. The difference between phase and frequency modulation ________
a) lies in the poorer radio response of phase modulation index
b) lies in the different definition of modulation index
c) is too great to make the two systems compatible
d) is purely theoretical
Answer
Answer: b [Reason:] It is the difference in definition of modulation index which makes it different between phase and frequency modulated systems.
6. The frequency deviation is proportional to both amplitude and frequency in phase modulated signal.
a) True
b) False
Answer
Answer: b [Reason:] For a phase modulated signal, the frequency deviation is proportional to amplitude only.
7. In a frequency modulated signal, the power _________ as the modulation index increases.
a) remains constant
b) increase
c) decrease
d) can be increase or decrease
Answer
Answer: a [Reason:] In a frequency modulated system, if we increase the modulation index the power remains constant.
8. Frequency fogging is used to reduce noise in a carrier system.
a) True
b) False
Answer
Answer: b [Reason:] Frequency fogging is used to cross talk in a carrier system.
9. Pre-emphasis circuit is used ________
a) before detection
b) after detection
c) before modulation
d) after modulation
Answer
Answer: c [Reason:] Pre-emphasis is used in frequency modulated or phase modulated transmitters to equalize the drive power of transmitting signal in terms of deviation ratio. It is done at the transmitter. Its circuit is used before modulation.
10. De-emphasis circuit is used in circuit before detection.
a) True
b) False
Answer
Answer: b [Reason:] De-emphasis means attenuating those frequencies by the amount by which they are boosted. De-emphasis is done at the receiver end i.e. it is used after detection.
