Analog Communication Systems MCQs for ESE

In this article, we are going to learn about the Analog communication systems MCQs for ESE.

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Analog Communication Systems MCQs for ESE

Q.1 In phase, modulation, the frequency deviation is

(a) independent of the modulating signal frequency
(b) inversely proportional to the modulating signal frequency
(c) directly proportional to the modulating signal frequency
(d) inversely proportional to the square root of the modulating frequency

Answer: (c) directly proportional to the modulating signal frequency

Q.2 The correct sequence of subsystems in an FM receiver is

(a) mixer, RF amplifier, limiter, IF amplifier, discriminator, audio amplifier
(b) RF amplifier, mixer, IF amplifier, limiter, discriminator, audio amplifier
(c) RF amplifier, mixer, limiter, discriminator, IF amplifier, audio amplifier
(d) mixer, IF amplifier, limiter, audio amplifier, discriminator

Answer: (b) RF amplifier, mixer, IF amplifier, limiter, discriminator, audio amplifier

Q.3 In a superheterodyne receiver, the IF is 455 kHz, if it is tuned to 1200 kHz, the image frequency will be

(a) 1655 kHz
(b) 745 kHz
(c) 2110 kHz
(d) 910 kHz

Answer: (c) 2110 kHz

Explanation: f_si = f_s + 2f_i = 1200 + 2 x 455 = 2110 kHz

Q.4 In an amplitude-modulated system, if the total power is 600 W and the power in a carrier is 400 W, then the modulation index is

(a) 0.5
(b) 0.75
(c) 0.9
(d) 1

Answer: (d) 1

Explanation:

$P_t=P_c \left [ 1 + \frac{m^2}{2} \right ]$ ,

$600=400 \left [ 1 + \frac{m^2}{2} \right ]$ ,

$\frac{m^2}{2}=\frac{6}{4}-1=\frac{1}{2}$ ,

\Rightarrow m =1

Q.5 Which one of the following statements regarding the threshold effect in demodulators is correct?

(a) It is exhibited by all demodulators when the input signal-to-noise ratio is low
(b) It is the rapid fall in the output signal-to-noise ratio when the input signal-to-noise ratio falls below a particular value
(c) It is the property exhibited by all AM-suppressed carrier coherent demodulators
(d) It is the property exhibited by correlation receivers

Answer: (b) It is the rapid fall in the output signal-to-noise ratio when the input signal-to-noise ratio falls below a particular value

Q.6 The essential blocks of a phase lock loop (PLL) are the phase detector, amplifier

(a) high-pass filter and crystal-controlled oscillator
(b) low-pass filter and crystal-controlled oscillator
(c) high-pass filter and voltage-controlled oscillator
(d) low-pass filter and voltage-controlled oscillator

Answer: (d) low-pass filter and voltage-controlled oscillator

Q.7 For an AM wave, the maximum voltage was found to be 10 v and the minimum voltage was found to be 5 v. The modulation index of the wave would be

(a) 0.33
(b) 0.52
(c) 0.40
(d) 0.1

Answer: (a) 0.33

Explanation:

Modulation Index, $m=\frac{V_{max}-V_{min}}{V_{max}+V_{min}}=\frac{10-5}{10 +5}=\frac{5}{15}=0.33$

Q.8 If the radiated power of AM transmitter is 10 kW, the power in the carrier for the modulation index of 0.6 is nearly

(a) 8.24 kW
(b) 8.47 kW
(c) 9.26 kW
(d) 9.6 kW

Answer: (b) 8.47 kW

Explanation:

P_T=P_C \left[ 1 + \frac{m^2}{2}\right ]

10=P_C \left[ 1 + \frac{0.6^2}{2}\right ]

P_C = 8.47 kW

Q.9 In a low-level AM system, the amplifier which follows the modulated stage must be the

(a) Linear device
(b) harmonic device
(c) class-C amplifier
(d) non-linear device

Answer: (a) Linear device

Q.10 MCVF telegraphy uses

(a) SSB techniques
(b) Pulse modulation
(c) out-of-band signaling
(d) FM

Answer: (d) FM

Q.11 In a modulator it is found that the amplitude spectrum consists of a component at f_{c ,}the carrier frequency_, and one component each at f_c – f_s and f_c+f_s, where fs is the modulating signal frequency. Then the modulator used is

(a) AM
(b) AM and or narrow band FM with depth < 0.5
(c) FM
(d) PAM

Answer: (b) AM and or narrow band FM with depth < 0.5

Q.12 The modulation index of an AM wave is changed from 0 to 1. The trasnmitted power is

(a) Unchanged
(b) halved
(c) Increased by 50%
(d) Quadrupled

Answer: (c) Increased by 50%

Q.13 In an SSB transmitter one is most likely to find

(a) Class-C audio amplifier
(b) Tuned modulator
(c) Class-B RF amplifier
(d) Class-AB power amplifier

Answer: (c) Class-B RF amplifier

Q.14 A public broadcast system using amplitude modulation with a double sideband should invariably have which one of the following features at the transmitter

(a) Band limit the signal to 5 kHz
(b) use a high-frequency carrier oscillator
(c) use a stable crystal oscillator
(d) use an all-round radiating transmitting antenna

Answer: (a) Band limit the signal to 5 kHz

Q.15 which one of the following statements is correct? In a ratio detector

(a) Linearity is worse than that of a phase discriminator
(b) stabilization is provided against signal strength variations
(c) the output is twice that obtainable from a similar phase discriminator
(d) the circuit is the same as that in a discriminator, except that the diode connections are reversed

Answer: (b) stabilization is provided against signal strength variations

Q.16 A composite signal x_c(t) is expressed as:

$x_c(t)=A_c\cos w_ct-A_m\cos (w_c-w_m)t + A_m \cos (w_c+w_m)t$

which one of the following methods can be employed to retrieve the sinusoidal components wm from x_c(t).

(a) An envelop detector, square low detector
(b) only a discriminator
(c) Only a square low detector
(d) Only an envelop detector

Answer: (b) only a discriminator

Q.17 A circuit produces an output y(t) = a + bx(t)² where x(t) is its input. This circuit can produce which one of the following?

(a) Rectified output
(b) Pulse modulation
(c) Amplitude modulation
(d) Frequency modulation

Answer: (c) Amplitude modulation

Q.18 Which one of the following blocks is not common in both AM and FM receivers?

(a) RF amplifier
(b) Mixer
(c) IF amplifier
(d) Slope detector

Answer: (d) Slope detector

Q.19 In the FDM system used for the telephone, which modulation scheme is adopted?

(a) AM
(b) DSB-SC
(c) SSB-SC
(d) FM

Answer: (c) SSB-SC

Q.20 FM broadcast standards specify a maximum deviation of frequency to be equal to 75 kHz and a maximum modulating the frequency of 15 kHz. What is the modulation index for FM waves?

(a) 1/5
(b) 5
(c) 60
(d) 1125

Answer: (b) 5

Explanation:

Modulation Index, $m_f=\frac{maximum\;deviation\;of\;frequency (\delta)}{Maximum\;modulating\;frequency(f_m)}$

Given that, $\delta=75 kHz \; and \; f_m = 15 kHz$

\therefore m_f =\frac{75}{15}= 5

Q.21 Why does an FM radio station perform better than an Am station radiating the same total power?

(a) FM is immune to noise
(b) AMhasonly two sidebands while FM has more
(c) FM uses larger bandwidth for large modulation depth
(d) Capture effect appears in FM

Answer: (a) FM is immune to noise

Q.22 On modulating a carrier of frequency f_{c by an audio signal fs, the following components} have resulted: f_c, f_c+f_s, and f_c-f_s. What is this type of modulation likely to be?

(a) Amplitude modulation DSB
(b) Single sideband modulation
(c) Frequency modulation only
(d) AMplitude modulation or frequency modulation

Answer: (d) AMplitude modulation or frequency modulation

Q.23 The modulation index of an AM is changed from 0 to 1. how does the transmitted power change?

(a) Gets halved
(b) Gets doubled
(c) Gets increased by 50 percent
(d) Remains unchanged

Answer: (c) Gets increased by 50 percent

Q.24 Which one of the following is an indirect way of generating FM

(a) Reactance FET modulator
(b) Varactor diode modulator
(c) Armstrong modulator
(d) Reactance tube modulator

Answer: (c) Armstrong modulator

Q.25 A balanced modulator, is used in the generation of which of the following?

(a) DSB-SC signal
(b) FM signal
(c) PM signal
(d) PAM signal

Answer: (a) DSB-SC signal

Q.26 An amplitude-modulated signal occupied a frequency range from 395 kHz to 405 kHz. it can be demodulated by which one of the following?

(a) Using an envelope detector and filter
(b) Multiplying with a 395 kHz local signal
(c) Multiplying with a 405 kHz local signal
(d) Low pass filtering with a cut-off at 400 kHz

Answer: (a) Using an envelope detector and filter

Q.27 Which one of the following statements is correct?

(a) AGC in radio receivers will keep the total signal output fairly constant but the noise component will be reduced; with a result, the S/N ratio will improve
(b) AGC in radio receivers is a linear operation with respect to both signals and noise
(c) Sudden changes in the output while tuning, which may cause damage to the components avoidable features of AGC
(d) AGC operation is independent of the filter components used in the AGC circuit

Answer: (c) Sudden changes in the output while tuning, which may cause damage to the components avoidable features of AGC

Q.28 In a PLL

(a) Capture range = Lock range $\neq$
(b) Capture range = Lock range = Free running Frequency
(c) Capture range > Lock range
(d) Capture range < Lock range

Answer: (d) Capture range < Lock range

Q.29 What bandwidth is needed for an FM signal that has a peak deviation of $\pm 3 kHz$ and handles audio signals from 200 Hz and 5 kHz?

(a) 6 kHz
(b) 16 kHz
(c) 10 kHz
(d) 9.6 kHz

Answer: (b) 16 kHz

Explanation: $\Delta f = 3 kHz \;and\; f_m = 5 kHz$

So bandwidth of FM signal = $2(\Delta f + f_m)= 2(3+5) = 16 kHz$

Q.30 The threshold effect in demodulators is

(a) the rapid fall of output SNR when the input SNR falls below a particular value
(b) exhibited by all the demodulators when the input SNR is low
(c) exhibited by all AM-suppressed carrier coherent demodulator
(d) exhibited by correlation receivers

Answer: (a) the rapid fall of output SNR when the input SNR falls below a particular value

Q.31 What is the ratio of modulating power to the total power at 100% AM modulation?

(a) 1 : 3
(b) 1 : 2
(c) 2 : 3
(d) 1 : 4

Answer: (a) 1 : 3

Explanation:

Total Power, $P_T=P_C \left( 1 + \frac{m^2}{2}\right )$

Ratio of modulating power to total power = $\frac{P_T \frac{m^2}{2}}{P_C \left( 1 + \frac{m^2}{2}\right )}$

at m = 1, ratio = \frac{1}{3} = 1 : 3

Q.32 One of the main functions of the RF amplifiers in a superheterodyne receiver is to

(a) Provide improved tracking
(b) permit better adjacent channel rejection
(c) Increase the tuning range of the receiver
(d) improve the rejection of the image frequency

Answer: (d) improve the rejection of the image frequency

Q.32 Amplitude modulation is used for broadcasting because

(a) it is more noise immune than other modulation systems
(b) compared with other systems it requires less transmitting power
(c) its use avoids receiver complexity
(d) no other modulation system can provide the necessary BW for high fidelity

Answer: (c) its use avoids receiver complexity

Q.33 Boosting of higher frequency at the transmitter is done by using

(a) De-emphasis
(B) AGC circuit
(c) Pre-emphasis
(d) Armstrong method

Answer: (c) Pre-emphasis

Q.34 In the process of modulation

(a) Some characteristics of a high-frequency sine wave are varied in accordance with the instantaneous value of a low-frequency signal
(b) Parameters of carrier wave are held constant
(c) For proper and efficient radiation, the receiving antennas should have heights comparable to half-wavelength of the signal received
(d) The signal is converted first within the range of 10 Hz to 20 Hz

Answer: (a) Some characteristics of a high-frequency sine wave are varied in accordance with the instantaneous value of a low-frequency signal

Q.35 In communication systems, modulation is the process of

(a) Improving frequency stability of the transmitter
(b) Combining signal and radio frequency waves
(c) Generating constant frequency radio waves
(d) Reducing distortion in RF waves

Answer: (b) Combining signal and radio frequency waves

Q.36 The phase detector circuit in the phase-locked loop demodulators recognizes

(a) voltage changes between the input and VCO signals
(b) frequency changes between the input and VCO signals
(c) impedance changes between the input and VCO signals
(d) resistance changes between the input and VCO signals

Answer: (b) frequency changes between the input and VCO signals

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Analog Communication Systems MCQs for ESE

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