Amplitude Modulation Questions and Answers

Q: Which statement about the direct-current component at a diode detector's output is correct?

The rectified carrier appears as a direct-current level proportional to the received carrier strength, which is exactly the signal the automatic gain control loop needs. It is therefore extracted deliberately rather than blocked, while a coupling capacitor removes it from the audio path. It does not depend on the index, which sets the alternating component. And a balanced detector for AM is not the arrangement in use; balance belongs to the modulator.

Q: What is the purpose of a de-emphasis network in a receiver?

De-emphasis is an FM technique, correcting the pre-emphasis applied at the transmitter to combat the rising noise spectrum of an FM detector. Amplitude modulation has no such noise slope and uses no pre-emphasis, so no de-emphasis appears in an AM receiver. It has nothing to do with intermediate frequency bandwidth or image rejection, both of which are filtering matters at radio and intermediate frequencies rather than at audio.

Q: Which limitation of AM does frequency modulation address most directly?

Frequency modulation puts the information in the angle rather than the amplitude, so a limiter can strip amplitude noise away before detection - the single biggest practical improvement over AM and the reason FM dominates high-fidelity broadcasting. Its bandwidth is greater, not smaller, which is the price paid. Its receiver is more complex. And it permits class C amplification, so the last option describes a benefit of FM rather than a limitation of AM it addresses.

Q: Which application still favours AM over FM?

Aeronautical voice retains AM because simultaneous transmissions remain mutually audible instead of one capturing the channel, which is a safety property no other scheme offers. Music broadcasting went to FM for its noise performance and bandwidth. Mobile telephony and satellite television are digital, using schemes far removed from either. The aeronautical case is a good reminder that the best modulation depends on what property matters most, not on a general ranking.

Q: What does the term splatter describe?

Splatter is the energy that appears outside the assigned channel when a transmitter is over-modulated or its audio is clipped, and it is heard by listeners tuned to neighbouring channels rather than by the offending station's own audience. Ignition noise is an external interference. Selective fading is a propagation effect. And distortion from a long detector time constant is diagonal clipping, which is a receiver fault confined to that receiver.

Q: Which quantity would be measured to check a transmitter's modulation linearity?

The sides of the trapezoidal display are straight when modulation is linear and curved when it is not, which makes the pattern the standard bench check for linearity as well as for index. Standing wave ratio tests the antenna and feeder. Frequency stability is an oscillator matter. And a receiver's output level depends on its own gain control and tells you nothing reliable about the transmitter's linearity.

Q: Why does AM broadcasting suffer more at night on the medium wave band?

At night the absorbing layer of the ionosphere weakens, so signals from distant stations propagate by reflection and interfere with local ones, producing the familiar night-time fading and heterodyne whistles. Some stations do reduce power at night, but that is a consequence of the propagation change rather than its cause. Receiver sensitivity does not depend on air temperature in any relevant way. And carrier drift is far too small to matter beside a co-channel interferer.

Q: Which is the correct order of increasing power efficiency?

Efficiency rises as unproductive components are removed: full-carrier AM wastes at least two thirds on the carrier, DSB-SC removes that waste, and SSB additionally puts everything into one sideband. So the order is full-carrier AM, DSB-SC, SSB. The reversed sequence inverts it. The remaining options misplace one scheme, and the last in particular puts SSB below DSB-SC, which would require the redundant second sideband to improve efficiency.

91.

A receiver delivers \( 2 \ \mathrm{V} \) of carrier to its detector with an index of \( 0.5 \). The recovered audio amplitude is about

A.0.5 V
B.4 V
C.1 V
D.2 V

Answer: C

The envelope varies about the carrier level by the index times the carrier:

\[ V_{audio} = 0.5 \times 2 = 1 \ \mathrm{V} \]

so about 1 volt peak, ignoring the diode's forward drop and detector efficiency. 2 V is the carrier itself, which appears as a direct-current component rather than as audio. 0.5 V is the index quoted as a voltage. And 4 V exceeds the envelope maximum, which is 3 V here. Note that the recovered audio is proportional to both the index and the carrier level, which is why gain control is needed to keep the output steady.

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92.

Which statement about the direct-current component at a diode detector's output is correct?

A.It is proportional to the modulation index
B.It must be blocked before the gain control loop
C.It is zero for a properly balanced detector
D.It is proportional to the carrier level and drives the gain control

Answer: D

The rectified carrier appears as a direct-current level proportional to the received carrier strength, which is exactly the signal the automatic gain control loop needs. It is therefore extracted deliberately rather than blocked, while a coupling capacitor removes it from the audio path. It does not depend on the index, which sets the alternating component. And a balanced detector for AM is not the arrangement in use; balance belongs to the modulator.

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93.

What is the purpose of a de-emphasis network in a receiver?

A.Nothing in AM; it belongs to FM systems
B.To restore the audio balance after AM detection
C.To reduce the intermediate frequency bandwidth
D.To improve the image rejection of the front end

Answer: A

De-emphasis is an FM technique, correcting the pre-emphasis applied at the transmitter to combat the rising noise spectrum of an FM detector. Amplitude modulation has no such noise slope and uses no pre-emphasis, so no de-emphasis appears in an AM receiver. It has nothing to do with intermediate frequency bandwidth or image rejection, both of which are filtering matters at radio and intermediate frequencies rather than at audio.

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94.

Which limitation of AM does frequency modulation address most directly?

A.Its need for a linear power amplifier
B.Its vulnerability to amplitude noise
C.Its bandwidth requirement
D.Its receiver complexity

Answer: B

Frequency modulation puts the information in the angle rather than the amplitude, so a limiter can strip amplitude noise away before detection - the single biggest practical improvement over AM and the reason FM dominates high-fidelity broadcasting. Its bandwidth is greater, not smaller, which is the price paid. Its receiver is more complex. And it permits class C amplification, so the last option describes a benefit of FM rather than a limitation of AM it addresses.

Discuss in Forum

95.

Which application still favours AM over FM?

A.Mobile telephony
B.Satellite television distribution
C.Aeronautical voice communication
D.High-fidelity music broadcasting

Answer: C

Aeronautical voice retains AM because simultaneous transmissions remain mutually audible instead of one capturing the channel, which is a safety property no other scheme offers. Music broadcasting went to FM for its noise performance and bandwidth. Mobile telephony and satellite television are digital, using schemes far removed from either. The aeronautical case is a good reminder that the best modulation depends on what property matters most, not on a general ranking.

Discuss in Forum

96.

What does the term splatter describe?

A.Impulse noise from ignition systems
B.Fading of one sideband relative to the other
C.Distortion produced by a long detector time constant
D.Spectral spreading caused by over-modulation

Answer: D

Splatter is the energy that appears outside the assigned channel when a transmitter is over-modulated or its audio is clipped, and it is heard by listeners tuned to neighbouring channels rather than by the offending station's own audience. Ignition noise is an external interference. Selective fading is a propagation effect. And distortion from a long detector time constant is diagonal clipping, which is a receiver fault confined to that receiver.

Discuss in Forum

97.

Which quantity would be measured to check a transmitter's modulation linearity?

A.The trapezoidal pattern's straightness
B.The antenna standing wave ratio
C.The carrier frequency stability
D.The receiver's audio output level

Answer: A

The sides of the trapezoidal display are straight when modulation is linear and curved when it is not, which makes the pattern the standard bench check for linearity as well as for index. Standing wave ratio tests the antenna and feeder. Frequency stability is an oscillator matter. And a receiver's output level depends on its own gain control and tells you nothing reliable about the transmitter's linearity.

Discuss in Forum

98.

Why does AM broadcasting suffer more at night on the medium wave band?

A.The carrier frequency drifts with temperature
B.Ionospheric propagation brings distant interference
C.Transmitter power is reduced at night by regulation
D.Receivers are less sensitive in cool air

Answer: B

At night the absorbing layer of the ionosphere weakens, so signals from distant stations propagate by reflection and interfere with local ones, producing the familiar night-time fading and heterodyne whistles. Some stations do reduce power at night, but that is a consequence of the propagation change rather than its cause. Receiver sensitivity does not depend on air temperature in any relevant way. And carrier drift is far too small to matter beside a co-channel interferer.

Discuss in Forum

99.

What is selective fading, and why does it trouble AM particularly?

A.Fading that affects only the audio frequencies above 3 kHz
B.Fading caused by the receiver's own gain control
C.Unequal fading of carrier and sidebands, distorting the envelope
D.Fading of the whole signal, reducing the level equally

Answer: C

Selective fading attenuates the carrier and the two sidebands by different amounts, and because AM's envelope depends on their relative magnitudes, the recovered audio distorts badly even when the total signal is strong. Uniform fading merely changes the level and is handled by the gain control. Audio-frequency selectivity is not what fading means. And the gain control responds to fading rather than causing it. Single sideband is less affected, which is one of its practical advantages on long paths.

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100.

Which is the correct order of increasing power efficiency?

A.SSB, DSB-SC, full-carrier AM
B.DSB-SC, full-carrier AM, SSB
C.Full-carrier AM, SSB, DSB-SC
D.Full-carrier AM, DSB-SC, SSB

Answer: D

Efficiency rises as unproductive components are removed: full-carrier AM wastes at least two thirds on the carrier, DSB-SC removes that waste, and SSB additionally puts everything into one sideband. So the order is full-carrier AM, DSB-SC, SSB. The reversed sequence inverts it. The remaining options misplace one scheme, and the last in particular puts SSB below DSB-SC, which would require the redundant second sideband to improve efficiency.

Discuss in Forum

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