EE AND ECE IMPORTANT MCQ-COMMUNICATION SYSTEMS PART THREE

1) Radiation resistance of an antenna is a
A [ ]) dc resistance
B [v]) ac resistance
C [ ]) either (a) or (b)
D [ ]) none of the above

2) The input to a coherent detector is DSB - SC signal plus noise. The noise at the detector output is
A [v]) in phase component
B [ ]) quadrature component
C [ ]) zero
D [ ]) the envelope

3) All of the output pulses are at full transmitter power for a strong signal in all of the following except
A [ ]) PWM
B [v]) PAM
C [ ]) PFM
D [ ]) PCM

4) In a telephone system, the grade of service means
A [ ]) the quality of sound reception at the receiver telephone
B [v]) the probability that a called for connection is established
C [ ]) the type of wires cables used for the system
D [ ]) none of the above

5) AGC(Automatic gain control) voltage is applied to the stages which are
A [v]) before the detector stage
B [ ]) after the detector stage
C [ ]) either before or after the detector stage
D [ ]) none of the above

6) Which of the following stage is not present in AM receiver as compared to FM receiver?
A [ ]) AM amplifier
B [ ]) Demodulator
C [v]) Amplitude limiter
D [ ]) AF amplitude

7) The function of duplexer in a RADAR is
A [v]) to permit the use of same antenna for transmission and reception
B [ ]) to provide gain to the transmitted power
C [ ]) to amplify the received signal
D [ ]) both (b) and (c)

8) In FM, as the modulating frequency decreases and the modulating voltage amplitude
A [ ]) increases, the modulation index increases
B [ ]) decrease, the modulation index increases
C [v]) remains constant, the modulation index increases
D [ ]) remains constant, the modulation index decreases
HINTS-modulation index=frequency deviation /modulating frequency

9) Modulation system used for video-modulation in television broadcast is
A [ ]) SSB AM with carrier
B [ ]) SSB with carrier suppressed
C [ ]) DSB AM with carrier
D [v]) a modified form of SSB with carrier called vestigial sideband

10) The highest modulation frequencies typically used in FM broadcast is
A [ ]) 5 kHz
B [v]) 15 kHz
C [ ]) 25 kHz
D [ ]) 10 kHz

11) For a short dipole having an effective length less than half the wavelength the radiation resistance is
A [v]) proportional to length
B [ ]) proportional to square root of length
C [ ]) proportional to square of length
D [ ]) none of the above

12) A narrow band noise show
A [ ]) AM only
B [v]) AM and FM

C [ ]) FM
D [ ]) PCM

13) The modulation index of a narrow band FM signal is
A [v]) nearly equal to 0.3
B [ ]) much less than unity
C [ ]) nearly 0.5
D [ ]) much more than unity

14) Which is a digital coding technique?
A [ ]) PCM
B [ ]) DM
C [ ]) DPCM
D [v]) All

15) For the transmission of normal speech, the PCM channel needs a bandwidth of__________ kHz.
A [ ]) 2
B [v]) 4
C [ ]) 16
D [ ]) 32

16) Neper is
A [v]) larger than a decibel
B [ ]) smaller than decibel
C [ ]) equal to 1 decibel
D [ ]) none of the above

17) Which of following digital modulation technique?
A [ ]) ASK
B [ ]) FSK
C [ ]) PSK
D [v]) All

18) The standard intermediate frequency for FM ratio is
A [v]) 10.7 MHz
B [ ]) 4.5 MHz
C [ ]) 75 kHz
D [ ]) 455 kHz

19) In an end fire array, the radiation pattern
A [v]) is in the plane of array
B [ ]) is at right angles to array
C [ ]) is at an angle to the plane of array
D [ ]) either (b) or (c)

20) Armstrong FM transmitter uses
A [ ]) LC oscillator
B [v]) crystal oscillator
C [ ]) RC oscillator
D [ ]) either (a) or (c)

21) An antenna having a length of λ⁄4 and grounded at one end is called
A [v]) Marconi antenna
B [ ]) Yagi antenna
C [ ]) Turnstile antenna
D [ ]) None of the above

22) Pre-emphasis circuit is used
A [ ]) after modulation
B [v]) before modulation
C [ ]) before detection
D [ ]) after detection

23) In which frequency range is the noise figure in a transistor minimum
A [v]) intermediate frequencies
B [ ]) low frequencies
C [ ]) high frequencies
D [ ]) very high frequencies

24) In commercial FM Broadcasting, A radio station transmitting the signal at 98.3 MHz, it is
A [v]) carrier frequency
B [ ]) modulation frequency
C [ ]) message frequency
D [ ]) channel frequency

25) The message carrying efficiency is best in
A [ ]) AM
B [ ]) FM
C [v]) AM-SC
D [ ]) Phase modulation

26) An SSB-AM signal cannot be demodulated by
A [v]) Balanced modulator
B [ ]) Complete phase shift generator
C [ ]) Diode balanced modulator
D [ ]) Product detector

27) The modulation index of wide band FM signal is
A [ ]) less than unity
B [ ]) unity
C [v]) greater than unity
D [ ]) nearly 0.67

28) The IF frequencies in a TV receiver
A [v]) 38.9 MHz for video and 33.4 MHz for audio
B [ ]) 38.9 MHz for both video and audio
C [ ]) 33.4 MHz for both video and audio
D [ ]) 33.4 MHz for video and 38.9 MHz for audio

29) The most noise resistant modulation system is
A [v]) PCM
B [ ]) SSB suppressed carrier
C [ ]) FM
D [ ]) pulse position modulation

30) Which of the following is the digital system?
A [ ]) PWM
B [ ]) PAM
C [ ]) PPM
D [v]) PCM

31) In FDM systems used for telephone, which modulation scheme is adopted?
A [ ]) AM
B [ ]) DSB-SC
C [ ]) SSB-SC
D [v]) FM

32) A pre-emphasis circuit
A [ ]) boosts base frequencies
B [ ]) pre amplifies the complete audio band
C [v]) amplifies higher audio frequencies
D [ ]) converts FM into phase modulation

33) In orthogonal BFSK, the energy of information bearing term is
A [ ]) the same as total transmitted energy
B [v]) one-half of total transmitted energy
C [ ]) one-fourth of total transmitted energy
D [ ]) none of the above

34) Which of the following is a false statement about Armstrong modulation system?
A [ ]) The system is basically phase, not frequency, modulation
B [ ]) AFC is not needed, as a crystal oscillator is used
C [v]) Equalization is unnecessary
D [ ]) Frequency multiplication must be used

35) In telephone, the dial tone has a frequency of
A [ ]) 5 Hz
B [ ]) 15 Hz
C [v]) 33 Hz
D [ ]) 233 Hz

36) Companding is used
A [ ]) to overcome quantising noise in PCM
B [v]) in PWM receivers to reduce impulse noise
C [ ]) to protect small signals in PCM from quantising noise
D [ ]) none of the above

37) An ungrounded antenna near the ground
A [ ]) is horizontally polarized
B [ ]) acts as an antenna array
C [v]) acts as a single antenna of twice the height
D [ ]) is not likely to need an earth mat

38) Which of the following circuits cannot be demodulate SSB?
A [ ]) Product detector
B [ ]) Balanced modulator
C [v]) Phase discriminator
D [ ]) BFO

39) PWM can be generated
A [ ]) by a free running multivibrator
B [ ]) by integrating the signal
C [v]) by a monostable multivibrator
D [ ]) by differentiating pulse position modulation

40) Which of the following analog modulation scheme requires the minimum transmitted power and minimum channel bandwidth?
A [ ]) VSB
B [v]) DSB-SC
C [ ]) SSB
D [ ]) AM

41) Which of the following cannot be used to demodulate SSB?
A [ ]) Complete phase shift generator
B [ ]) Product detector
C [ ]) Diode balanced modulator
D [v]) Bipolar transistor balanced modulator

42) For F2 layer the critical frequency is about
A [ ]) 500 MHz
B [ ]) 100 MHz
C [ ]) 25 MHz
D [v]) 10 MHz

43) Probability density function of thermal noise is
A [v]) Poisson
B [ ]) Binomial
C [ ]) Gaussian
D [ ]) None of the above

44) The main advantage of TDM over FDM is that it
A [ ]) needs less power
B [ ]) needs less bandwidth
C [v]) needs simple circuitry
D [ ]) gives better S/N ratio

45) Which of the following is very useful as multiband HF receiving antenna?
A [ ]) Square loop
B [ ]) Conical horn
C [ ]) Folded dipole
D [v]) Log-periodic

46) If Vs is signal voltage, Vn is noise voltage, signal to noise ratio S/N is defined as
A [ ]) S/N=Vs/Vn
B [v]) S/N=(Vs/Vn)²
C [ ]) S/N=√Vs/Vn
D [ ]) none of the above

47) In shot noise
A [ ]) the current rises around mean value
B [ ]) the current rises to peak value, decreases to zero and then again rises
C [ ]) the current may become negative
D [v]) the current is always less than the mean value

48) The output VR of the ratio detector is related with the output VF of similar Foster-Seeley
discriminator as follows.
A [ ]) VR = VF
B [ ]) VR < VF
C [v]) VR = 2VF
D [ ]) VF = 2VR
49) In grid bias modulation system the power required for modulating amplifier
A [ ]) is negligibly low since grid is negative
B [ ]) is large as compared to that for plate modulation
C [ ]) is almost the same as compared with plate modulation
D [v]) is small as compared to that for plate modulation
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