# Physics Questions and Answers – Communication Systems – Amplitude Modulation

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Communication Systems – Amplitude Modulation”.

1. Identify the type of modulation where the frequency of the modulated wave is equal to that of the carrier wave.
a) Frequency modulation
b) Amplitude modulation
c) Carrier modulation
d) Phase modulation

Explanation: In Amplitude modulation, the modulating wave is superimposed on a high-frequency carrier wave such that the amplitude of the modulated wave varies as the amplitude of the modulating wave.

2. What does AGC stand for?
a) Automatic gain control
b) Automatic gear control
c) Amplitude gain control
d) Automotive gear center

Explanation: AGC stands for automatic gain control. Automatic gain control changes (AGC) the overall gain of a receiver automatically so that the strength of the received signal remains almost constant.

3. Why is AM used for broadcasting in the communication systems?
a) Its use avoids receiver complexity
b) It is more immune to other modulation systems
c) It requires less transmitting power
d) No noise disturbances

Explanation: Amplitude modulation (AM) is used for broadcasting because it avoids receiver complexity. Moreover, only a diode and a capacitor are sufficient to separate the audio signal from the amplitude-modulated wave.
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4. Of the following which is the preferred modulation scheme for digital communication?
a) Pulse code modulation
b) Pulse amplitude modulation
c) Pulse position modulation
d) Pulse width modulation

Explanation: Pulse code modulation is the one preferred for digital communication. Pulse codes are used to convert an analog signal to a digital one. It is the standard form of digital audio in computers, compact discs, etc.

5. A carrier of peak voltage 15 V is used to transmit a message signal. If the modulation index is 70%, then what will be the peak voltage of the modulating signal?
a) 25 V
b) 11 V
c) 10.5 V
d) 30 V

Explanation: μ = $$\frac {A_m}{A_c}$$.
Am = μ × Ac.
Am = $$( \frac {70}{100})$$ × 15 V
Am = 10.5 V.
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6. PAM stands for pulse attenuation modulation.
a) True
b) False

Explanation: PAM stands for pulse amplitude modulation. It is defined as the process in which amplitudes of the regularly spaced rectangular pulses vary in direct proportion to the instantaneous values of the continuous message signal.

7. ‘X’ for an AM is the ratio of change in amplitude of the carrier wave to the amplitude of the original carrier wave. Identify ‘X’.
a) Automatic gain control
b) Modulation index
c) Phase index
d) Amplitude index

Explanation: The modulation index for an AM wave is defined as the ratio of change in the amplitude of the carrier wave to the amplitude of the original carrier wave.
μ = $$\frac {(Change \, in \, the \, amplitude \, of \, the \, carrier \, wave)}{(Amplitude \, of \, the \, original \, carrier \, wave)}$$.

8. How can the noise be reduced in AM signal?
a) Increasing amplitude
b) Increasing wavelength
c) Increasing bandwidth
d) Increasing frequency deviation

Explanation: Noise is a form of amplitude variations in the transmitted signal due to several factors such as the atmosphere, industries, etc. This noise can be reduced by increasing the deviation of the frequency of the signal.

9. Suggest a possible communication channel for the transmission of a message signal that has a bandwidth of 200kHz.
a) TV transmission
b) Optical fiber

Explanation: The communication channel for the transmission of a message signal that has a bandwidth of 200kHz is FM radio. AM radio is the channel required for a message signal having a bandwidth of 5 MHz. Other options are not valid.

10. Identify the type of modulation where the modulating wave is superimposed on a high-frequency carrier wave.
a) Phase modulation
b) Amplitude modulation
c) Frequency modulation
d) Wavelength modulation

Explanation: In Amplitude modulation, the modulating wave is superimposed on a high-frequency carrier wave such that the amplitude of the modulated wave varies as the amplitude of the modulating wave. The frequency of the modulated wave is equal to the frequency of the carrier wave.

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