# Wireless & Mobile Communications Questions & Answers – Frequency Modulation Vs. Amplitude Modulation

This set of Wireless & Mobile Communications Questions and Answers for Aptitude test focuses on “Frequency Modulation Vs. Amplitude Modulation”.

1. Which is the process of encoding information from a message source in suitable manner for transmission?
a) Modulation
b) Demodulation
c) Encryption
d) Decryption

Explanation: Modulation is the process of encoding information from a message source in suitable manner for transmission. It translates the baseband message signal to a bandpass signal at frequencies that are very higher compared to the baseband frequency.

2. The bandpass signal is called the modulating signal.
a) True
b) False

Explanation: The bandpass signal is called the modulated signal and the baseband message signal is called the modulating signal. Modulation is done by varying the amplitude, phase or frequency of a high frequency carrier in accordance with the amplitude of the message signal.

3. AM signal have all information in ______ of the carrier.
a) Amplitude
b) Magnitude
c) Frequency
d) Power

Explanation: AM signals have all information in the amplitude of the carrier. Because, AM signal superimpose the exact relative amplitudes of the modulating signal onto the carrier.

4. In frequency modulation, the amplitude of modulated carrier signal is ______and its frequency is ______ by the modulating message signal.
a) Constant, constant
b) Varied, constant
c) Constant, varied
d) Varied, varied

Explanation: In frequency modulation (FM), the amplitude of modulated carrier signal is kept constant while its frequency is varied by the modulating message signal. FM is the most popular analog modulation technique used in mobile communication.

5. FM signals have all their information in ______ of the carrier.
a) Amplitude
b) Magnitude
c) Frequency
d) Power

Explanation: FM signal have all their information in the phase or frequency of the carrier. This provides a nonlinear and very rapid improvement in reception quality once a certain minimum received signal level, called FM threshold is achieved.

6. Amplitude modulation has ______ relationship between the quality of the received signal and the power of the received signal.
a) No
b) Non linear
c) Constant
d) Linear

Explanation: In amplitude modulation schemes, there is a linear relationship between the quality of the received signal and the power of the received signal. AM signals superimpose the exact relative amplitudes of the modulating signal onto the carrier.

7. FM has _______ noise immunity when compared to amplitude modulation.
a) Same
b) Less
c) No
d) Better

Explanation: FM offers many advantages over amplitude modulation which makes it a better choice for many mobile radio applications. Frequency modulation has better noise immunity when compared to amplitude modulation.

8. FM signals are _____ susceptible to atmospheric and impulse noise as compared to AM.
a) Largely
b) Less
c) Not
d) Better

Explanation: FM signals are represented as frequency variations rather than amplitude variations. Therefore, FM signals are less susceptible to atmospheric and impulse noise.

9. Burst noise affects FM systems.
a) True
b) False

Explanation: If the FM received signal is above the FM threshold, burst noise does not affect FM system performance as much as AM systems. It is due to the fact that message amplitude variations do not carry information in FM.

10. AM has a modulation index.
a) True
b) False

Explanation: Unlike AM, FM has a modulation index. Hence, bandwidth occupancy can be varied to obtain greater signal to noise performance. Thus, it is possible to tradeoff bandwidth occupancy for improved noise performance.

11. Which of the following is an advantage of AM systems in comparison to FM sysytems?
a) Occupy less bandwidth
c) Better noise immunity
d) Not susceptible to impulse noise

Explanation: AM signals are able to occupy less bandwidth as compared to FM signals, since the transmission system is linear. Small scale fading causes rapid fluctuations in the received signal. Thus, FM offers superior qualitative performance in fading when compared to AM.

12. An FM signal is a ______ envelope signal.
a) Varied
b) Small
c) Large
d) Constant

Explanation: An FM signal is a constant envelope signal. It is due to the fact that the envelope of the carrier does not change with changes in the modulating signal. Hence, the transmitted power of an FM signal is constant regardless of the amplitude of the message signal.

13. FM uses class______ amplifiers and AM uses class ______ amplifiers.
a) C, C
b) A, C
c) C, A
d) AB, C

Explanation: The constant envelope of the transmitted signal allows efficient class C amplifiers. However in AM, it is critical to maintain linearity between the applied message and amplitude of transmitted signal. Therefore, class A or AB are used for AM systems.

14. AM exhibits capture effect characteristics.
a) True
b) False

Explanation: AM systems do not exhibit capture effect characteristics because all the interferers are received at once and must be discriminated after the modulation process. FM exhibits this characteristic. Capture effect is a direct result of rapid non-linear improvement in received quality for an increase in receives power.

15. Which of the following is a drawback of FM systems?
a) Burst noise
b) Susceptible to atmospheric noise
c) Wider frequency band

Explanation: FM systems have many advantages over AM systems. They also have certain disadvantages. FM systems require a wider frequency band in transmitting media in order to obtain the advantages of reduced noise and capture effect. It is generally several times as large as that needed for AM.

Sanfoundry Global Education & Learning Series – Wireless & Mobile Communications.

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