Amplitude Modulation MCQs

Practice Amplitude Modulation MCQs with four options, answers and explanations for focused Electronics & Telecommunication Engineering revision.

Question 1

Communication SystemsAmplitude Modulation

In conventional amplitude modulation, which carrier parameter is varied according to the message signal?

AAmplitude
BFrequency
CPhase only
DPropagation velocity

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Correct Answer: A. Amplitude

Explanation: In AM, the carrier amplitude varies in accordance with the instantaneous message signal while carrier frequency and phase remain nominally constant.

Reference: Standard communication systems: amplitude modulation

Question 2

Communication SystemsAmplitude Modulation

For a single-tone AM wave, modulation index m is commonly equal to?

AMessage amplitude divided by carrier amplitude
BCarrier amplitude divided by message amplitude
CCarrier frequency divided by message frequency
DMessage frequency multiplied by carrier frequency

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Correct Answer: A. Message amplitude divided by carrier amplitude

Explanation: For standard sinusoidal AM, m=Am/Ac when the modulating and carrier amplitudes are expressed consistently.

Reference: Standard communication systems: amplitude modulation

Question 3

Communication SystemsAmplitude Modulation

An AM signal has carrier amplitude 10 V and modulating-signal amplitude 5 V. Its modulation index is?

A0.25
B0.5
C1.0
D2.0

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Correct Answer: B. 0.5

Explanation: m=Am/Ac=5/10=0.5, corresponding to 50% modulation.

Reference: Standard communication systems: amplitude modulation

Question 4

Communication SystemsAmplitude Modulation

For distortion-free conventional AM with a sinusoidal message, the modulation index should normally not exceed?

A0.1
B0.5
C1
D2

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Correct Answer: C. 1

Explanation: When m exceeds 1, overmodulation causes envelope distortion in conventional AM.

Reference: Standard communication systems: amplitude modulation

Question 5

Communication SystemsAmplitude Modulation

A carrier of 1 MHz is amplitude-modulated by a 5 kHz sinusoid. The upper sideband frequency is?

A995 kHz
B1.000 MHz
C1.005 MHz
D1.500 MHz

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Correct Answer: C. 1.005 MHz

Explanation: The upper sideband is fc+fm=1000 kHz+5 kHz=1005 kHz=1.005 MHz.

Reference: Standard communication systems: amplitude modulation

Question 6

Communication SystemsAmplitude Modulation

For the same 1 MHz carrier modulated by 5 kHz, the lower sideband frequency is?

A995 kHz
B1.005 MHz
C1.050 MHz
D5 kHz

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Correct Answer: A. 995 kHz

Explanation: The lower sideband is fc-fm=1000 kHz-5 kHz=995 kHz.

Reference: Standard communication systems: amplitude modulation

Question 7

Communication SystemsAmplitude Modulation

The bandwidth of a conventional AM signal whose highest modulating frequency is 5 kHz is?

A2.5 kHz
B5 kHz
C10 kHz
D1 MHz

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Correct Answer: C. 10 kHz

Explanation: Conventional AM contains upper and lower sidebands, so bandwidth B=2fm(max)=10 kHz.

Reference: Standard communication systems: amplitude modulation

Question 8

Communication SystemsAmplitude Modulation

In conventional AM, the carrier component itself contains?

AAll of the message information
BNo message information by itself
COnly the highest audio frequency
DOnly digital data

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Correct Answer: B. No message information by itself

Explanation: The unmodulated carrier does not itself contain the message; the information resides in the sidebands.

Reference: Standard communication systems: amplitude modulation

Question 9

Communication SystemsAmplitude Modulation

For a conventional AM signal with modulation index m=1, the total transmitted power relative to carrier power Pc is?

APc
B1.25Pc
C1.5Pc
D2Pc

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Correct Answer: C. 1.5Pc

Explanation: For single-tone AM, Pt=Pc(1+m^2/2). At m=1, Pt=1.5Pc.

Reference: Standard communication systems: amplitude modulation

Question 10

Communication SystemsAmplitude Modulation

In conventional AM with a single-tone message, the maximum transmission efficiency at 100% modulation is approximately?

A25%
B33.3%
C50%
D66.7%

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Correct Answer: B. 33.3%

Explanation: At m=1, total sideband power is Pc/2 and total transmitted power is 1.5Pc, so efficiency=(0.5Pc)/(1.5Pc)=1/3≈33.3%.

Reference: Standard communication systems: AM efficiency

Question 11

Communication SystemsAmplitude Modulation

An AM transmitter has carrier power 400 W and modulation index 0.6. What is the total sideband power?

A36 W
B72 W
C144 W
D472 W

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Correct Answer: B. 72 W

Explanation: For single-tone AM, total sideband power=Pc m^2/2=400×0.36/2=72 W.

Reference: Standard communication systems: AM sideband power numerical

Question 12

Communication SystemsAmplitude Modulation

For the same AM transmitter with Pc=400 W and m=0.6, the total transmitted power is?

A400 W
B436 W
C472 W
D544 W

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Correct Answer: C. 472 W

Explanation: Pt=Pc(1+m^2/2)=400(1+0.18)=472 W.

Reference: Standard communication systems: AM total power numerical

Question 13

Communication SystemsAmplitude Modulation

An AM wave has maximum envelope voltage 15 V and minimum envelope voltage 5 V. Its modulation index is?

A0.25
B0.5
C0.667
D2

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Correct Answer: B. 0.5

Explanation: m=(Vmax-Vmin)/(Vmax+Vmin)=(15-5)/(15+5)=10/20=0.5.

Reference: Standard communication systems: modulation index from envelope