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

Explanation: fsi = fs + 2fi = 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

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

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

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

Q.10 MCVF telegraphy uses

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

Q.11 In a modulator it is found that the amplitude spectrum consists of a component at fc , the carrier frequency, and one component each at fc – fs and fc+fs, 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%

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

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 xc(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 xc(t).

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

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

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

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

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

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 fc by an audio signal fs, the following components have resulted: fc, fc+fs, and fc-fs. 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

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

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

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

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

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

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

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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