Communication Engineering Set 3 MCQs

Q1 | Frequency modulation index defines the relationship between the and bandwidth of transmitted signal.

frequency of message signal
amplitude of message signal
amplitude of carrier signal
frequency of carrier signal

Answer: amplitude of message signal

Explaination:
the frequency modulation

Q2 | Which of the following are two methods for generating FM signal?

coherent method, noncoherent method
product detector, envelope detector
direct method, indirect method
slope detector, zero crossing detector

Answer: direct method, indirect method

Explaination:
direct method and indirect method are the methods used for generating fm signals. these methods are differentiated by the variation of the carrier frequency.

Q3 | In indirect method, the carrier frequency is directly varied in accordance with the input modulating signal.

true
false

Answer: false

Explaination:
the above is the case for direct method. in the indirect method, a narrowband fm signal is generated using a balanced

Q4 | Which of the following is used to vary the frequency of the carrier frequency in accordance with the baseband signal amplitude variations in direct method of FM generation?

integrator
envelope detector
multivibrator
voltage controlled oscillators

Answer: voltage controlled oscillators

Explaination:
in direct method, vcos are used to vary the frequency of the carrier signal in accordance with the baseband signal amplitude variations. these oscillators use devices with reactance that can be varied by the application of a voltage.

Q5 | Frequency demodulator is a frequency to amplitude converter circuit.

true
false

Answer: true

Explaination:
frequency demodulator produces an output voltage with instantaneous amplitude that is directly proportional to the instantaneous frequency of the input fm signal. thus, frequency demodulator is a frequency to amplitude converter circuit.

Q6 | Which of the following is not a technique for FM demodulation?

slope detection
zero crossing detection
product detector
phase locked discriminator

Answer: product detector

Explaination:
various techniques such as slope detection, zero crossing detection, phase locked discrimination and quadrature

Q7 | Which of the following FM demodulator is sometimes known as pulse averaging discriminator?

slope detection
zero crossing detection
quadrature detection
phase locked discriminator

Answer: zero crossing detection

Explaination:
zero crossing detector is sometimes known as pulse averaging discriminator. the rationale behind this technique is to use the output of the zero crossing detector to generate a pulse train with an average value that is proportional to frequency of the input signal.

Q8 | PLL in FM detection stands for

phase locked loop
programmable logic loop
phase locked logic
programmable locked loop

Answer: phase locked loop

Explaination:
pll stands for phase locked loop. the pll is a closed loop control system which can track the variations in the received signal phase and frequency.

Q9 | In angle modulation, signal to noise ratio before detection is a function of

modulation index
input signal to noise ratio
maximum frequency of the message
if filter bandwidth

Answer: if filter bandwidth

Explaination:
in angle modulation systems, the signal to noise ratio before detection is the function of the receiver if filter bandwidth, received carrier power, and received interference. however, signal to noise ratio after detection is a function of maximum

Q10 | FM can improve the receiver performance through adjustment of transmitted power.

true
false

Answer: false

Explaination:
fm can improve receiver performance through adjustment of the modulation index at the transmitter, and not the transmitted power. this is not the case in am since linear modulation techniques do not trade bandwidth for snr.

Q11 | De-emphasis circuit is used

before detection
after detection
before encoding
after encoding

Answer: after detection

Explaination:
de-emphasis means attenuation of those frequencies by the amount by which they are boosted. it is done at the receiver end

Q12 | Why frequency fogging is used in a carrier system?

to reduce noise
to reduce cross talk
to converge frequencies
to reduce distortion

Answer: to reduce cross talk

Explaination:
the interchanging of the frequencies of carrier channels to accomplish specific purposes. it is used to prevent feedback and oscillation. it is also used to reduce cross-talk and also to correct for a high frequency response slope in the transmission line.

Q13 | For a phase modulated signal, the frequency deviation is proportional to

frequency only
amplitude only
only width
phase only

Answer: amplitude only

Explaination:
for a phase modulated system, it is amplitude which is directly proportional to the deviation.

Q14 | The frequency deviation is proportional to frequency in phase modulated signal.

true
false

Answer: false

Explaination:
in phase modulation, the phase of the carrier signal is varied with respect to the amplitude of the message signal. for a phase modulated signal, the frequency deviation is proportional to amplitude.

Q15 | Which of the following is not necessarily an advantage of FM over AM?

less modulating power is required
better noise immunity is provided
higher bandwidth is required
carrier is of any shape

Answer: higher bandwidth is required

Explaination:
in frequency modulation, frequency of carrier gets varied with respect to the wave being propagated. fm has many advantages over am but requirement of higher bandwidth is not an efficient condition.

Q16 | What is number of possible outputs if there is 7 line digital input?

64
32
16
128

Answer: 128

Explaination:
total possible outputs will be 27 which is equal to 128.

Q17 | What is the frequency of the stereo sub carrier signal in FM broadcasting?

19 khz
45 khz
55 khz
38 khz

Answer: 38 khz

Explaination:
stereo broadcasting is made possible by using a subcarrier on fm radio stations, which takes the left channel and “subtracts” the right channel from it. a

Q18 | What is the bandwidth required in SSB signal?

fm
2fm
> 2fm
< 2fm

Answer: fm

Explaination:
in an am modulated system, total bandwidth required is from fc + fm to fc

Q19 | One of the advantage of using a high frequency carrier wave is that it dissipates very small power.

true
false

Answer: true

Explaination:
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. and also it allows less noise interference and enables multiplexing. this is the reason for sending the audio signals at high frequency carrier signals for communication purpose.

Q20 | The antenna current is 10A. Find the percentage of modulation when the antenna current increases to 10.4A?

50%
30%
28.5%
23%

Answer: 28.5%

Q21 | Find the total power, if the carrier of an AM transmitter is 800W and it is modulated to 50%?

100w
800w
500w
900w

Answer: 900w

Explaination:
p =p (1+u2 ), according to

Q22 | Spread spectrum is used for

encrypting signal
hiding signal
encrypting & hiding signal
none of the mentioned

Answer: encrypting & hiding signal

Explaination:
spread spectrum is used for hiding and encrypting signals.

Q23 | Which is a quantization process?

rounding
truncation
rounding & truncation
none of the mentioned

Answer: rounding & truncation

Explaination:
rounding and truncation are examples of quantization process.

Q24 | Quantization is a process.

few to few mapping
few to many mapping
many to few mapping
many to many mapping

Answer: many to few mapping

Explaination:
quantization is a many to few mapping process.

Q25 | Which conveys more information?

high probability event
low probability event
high & low probability event
none of the mentioned

Answer: low probability event

Explaination:
high probability event conveys less information than a low probability event.