Amplitude Modulation Questions and Answers

Q: What is the main advantage of single sideband for long-distance voice work?

SSB gives the message bandwidth only and puts every watt into information, which together are worth many decibels of effective improvement over full-carrier AM on a crowded band. The receiver is more complex, needing a reinserted carrier of accurate frequency. It is less troubled by selective fading than double sideband but not immune. And its envelope varies, so linear amplification is required, which is one of its practical costs.

Q: Why does an SSB receiver need an accurate carrier reinsertion frequency?

Reinserting the carrier at the wrong frequency translates the whole recovered spectrum by the error, so every component shifts by the same number of hertz rather than in proportion, which makes speech sound unnatural and renders music unusable. A hundred hertz is tolerable for speech intelligibility; much more is not. Envelope detection is not used in SSB at all. The sidebands cannot overlap, there being one. And gain control settles regardless of the frequency error.

Q: Which statement about AM in the presence of noise is correct?

Amplitude noise adds to the envelope and so appears directly in the recovered message, which is why AM sounds noisy on a weak signal and why impulse interference from ignition systems and switchgear is so intrusive. The carrier does not suppress noise; it merely provides the reference the detector needs. Noise falls across the whole received band, carrier and sidebands alike. And a limiter would remove the message along with the noise.

Q: What is the effect of transmitting AM with a reduced carrier?

Reducing the carrier to a pilot level recovers most of the power efficiency while leaving a reference the receiver can lock to, which is the compromise used in some point-to-point and broadcast systems. The receiver must amplify that pilot and reinsert a full carrier, so it is more complex, not simpler. Performance clearly differs from full-carrier AM in both efficiency and receiver requirements. And the bandwidth is unchanged, since both sidebands remain.

Q: Which pair of quantities does the AM power relation connect?

The relation \( P_t = P_c(1 + \mu^{2}/2) \) connects total power to the index, with the carrier power as the reference. Bandwidth depends on the message frequency and not at all on the index, which is one of the sharpest contrasts with frequency modulation, where the index is what sets the bandwidth. The carrier and message frequencies determine where the sidebands fall rather than how much power they hold. And antenna current relates to power, not to frequency.

Q: Which is the strongest argument for retaining AM in aeronautical voice communication?

The decisive property is that two AM transmissions on one channel are both heard, each interfering with but not silencing the other, whereas an FM receiver captures the stronger and the weaker vanishes. In air traffic control, hearing that two aircraft transmitted at once matters more than audio quality. AM's noise performance is poor. Single sideband is narrower. And frequency stability is still required, though less critically than for SSB.

Q: What does the term depth of modulation refer to?

Depth of modulation is simply the index written as a percentage, so an index of 0.8 is eighty per cent modulation. The sideband power fraction is the efficiency, a related quantity computed from the index but not the same number. Bandwidth as a fraction of the channel is an occupancy measure. And the number of sidebands depends on how many tones the message contains, not on the depth.

Q: An AM signal shows envelope maxima of \( 12 \ \mathrm{V} \) and minima of \( 4 \ \mathrm{V} \). The carrier amplitude is

The carrier is the mean of the envelope extremes:\[ A_c = \frac{12+4}{2} = 8 \ \mathrm{V} \]so 8 volts, with a message amplitude of 4 volts and therefore an index of 0.5. Reading the maximum as the carrier ignores the modulation entirely. The minimum is the carrier less the message. And 16 volts is the sum of the extremes, which is twice the carrier. Both the carrier and the index can be read from one oscilloscope trace this way, which is why the envelope display remains a standard bench tool.

Q: Which is a disadvantage of high-level modulation?

Modulating the final stage means the audio modulator itself must deliver power comparable to the carrier, which for a large transmitter is a substantial amplifier in its own right, historically a large audio stage or a pulse-width modulated supply. The reward is class C efficiency in the radio-frequency chain, so the second option inverts the benefit. Crystal control of the carrier is unaffected. And sideband symmetry is preserved by a properly designed modulator.

Q: What limits the audio bandwidth of an AM broadcast in practice?

The channel allocation sets it: with nine or ten kilohertz available and double-sideband transmission, the audio must be limited to four or five kilohertz. That is a regulatory and physical limit rather than an equipment one, which is why AM broadcasting cannot match FM for fidelity however good the equipment. Audio processing affects loudness and dynamics. Carrier stability is far tighter than the audio bandwidth. And receiver audio stages are easily capable of more than the channel can deliver.

71.

What is the main advantage of single sideband for long-distance voice work?

A.A constant envelope that permits class C amplification
B.Half the bandwidth and all the power in the message
C.A simpler receiver than AM requires
D.Immunity to selective fading of the carrier

Answer: B

SSB gives the message bandwidth only and puts every watt into information, which together are worth many decibels of effective improvement over full-carrier AM on a crowded band. The receiver is more complex, needing a reinserted carrier of accurate frequency. It is less troubled by selective fading than double sideband but not immune. And its envelope varies, so linear amplification is required, which is one of its practical costs.

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72.

Why does an SSB receiver need an accurate carrier reinsertion frequency?

A.The sidebands would otherwise overlap
B.The automatic gain control would fail to settle
C.A frequency error shifts every recovered component equally
D.The envelope cannot otherwise be detected

Answer: C

Reinserting the carrier at the wrong frequency translates the whole recovered spectrum by the error, so every component shifts by the same number of hertz rather than in proportion, which makes speech sound unnatural and renders music unusable. A hundred hertz is tolerable for speech intelligibility; much more is not. Envelope detection is not used in SSB at all. The sidebands cannot overlap, there being one. And gain control settles regardless of the frequency error.

Discuss in Forum

73.

Which statement about AM in the presence of noise is correct?

A.Noise is suppressed by the carrier's presence
B.Noise affects only the carrier and not the sidebands
C.Noise can be removed entirely by a limiter
D.Noise adds directly to the recovered envelope

Answer: D

Amplitude noise adds to the envelope and so appears directly in the recovered message, which is why AM sounds noisy on a weak signal and why impulse interference from ignition systems and switchgear is so intrusive. The carrier does not suppress noise; it merely provides the reference the detector needs. Noise falls across the whole received band, carrier and sidebands alike. And a limiter would remove the message along with the noise.

Discuss in Forum

74.

What is the effect of transmitting AM with a reduced carrier?

A.Better efficiency but a receiver that must reinsert the carrier
B.Worse efficiency and a simpler receiver
C.Identical performance to full-carrier AM
D.A halving of the occupied bandwidth

Answer: A

Reducing the carrier to a pilot level recovers most of the power efficiency while leaving a reference the receiver can lock to, which is the compromise used in some point-to-point and broadcast systems. The receiver must amplify that pilot and reinsert a full carrier, so it is more complex, not simpler. Performance clearly differs from full-carrier AM in both efficiency and receiver requirements. And the bandwidth is unchanged, since both sidebands remain.

Discuss in Forum

75.

Which pair of quantities does the AM power relation connect?

A.Antenna current and carrier frequency
B.Total power and modulation index
C.Bandwidth and modulation index
D.Carrier frequency and message frequency

Answer: B

The relation \( P_t = P_c(1 + \mu^{2}/2) \) connects total power to the index, with the carrier power as the reference. Bandwidth depends on the message frequency and not at all on the index, which is one of the sharpest contrasts with frequency modulation, where the index is what sets the bandwidth. The carrier and message frequencies determine where the sidebands fall rather than how much power they hold. And antenna current relates to power, not to frequency.

Discuss in Forum

76.

Which is the strongest argument for retaining AM in aeronautical voice communication?

A.It uses the least bandwidth of any scheme
B.It requires no frequency stability at the transmitter
C.Simultaneous transmissions remain intelligible rather than capturing
D.It offers the best noise performance available

Answer: C

The decisive property is that two AM transmissions on one channel are both heard, each interfering with but not silencing the other, whereas an FM receiver captures the stronger and the weaker vanishes. In air traffic control, hearing that two aircraft transmitted at once matters more than audio quality. AM's noise performance is poor. Single sideband is narrower. And frequency stability is still required, though less critically than for SSB.

Discuss in Forum

77.

What does the term depth of modulation refer to?

A.The bandwidth occupied as a fraction of the channel
B.The ratio of sideband power to total power
C.The number of sidebands present in the spectrum
D.The modulation index expressed as a percentage

Answer: D

Depth of modulation is simply the index written as a percentage, so an index of 0.8 is eighty per cent modulation. The sideband power fraction is the efficiency, a related quantity computed from the index but not the same number. Bandwidth as a fraction of the channel is an occupancy measure. And the number of sidebands depends on how many tones the message contains, not on the depth.

Discuss in Forum

78.

An AM signal shows envelope maxima of \( 12 \ \mathrm{V} \) and minima of \( 4 \ \mathrm{V} \). The carrier amplitude is

A.8 V
B.12 V
C.4 V
D.16 V

Answer: A

The carrier is the mean of the envelope extremes:

\[ A_c = \frac{12+4}{2} = 8 \ \mathrm{V} \]

so 8 volts, with a message amplitude of 4 volts and therefore an index of 0.5. Reading the maximum as the carrier ignores the modulation entirely. The minimum is the carrier less the message. And 16 volts is the sum of the extremes, which is twice the carrier. Both the carrier and the index can be read from one oscilloscope trace this way, which is why the envelope display remains a standard bench tool.

Discuss in Forum

79.

Which is a disadvantage of high-level modulation?

A.Both sidebands are attenuated unequally
B.The modulator must handle high audio power
C.The transmitter must run in class A throughout
D.The carrier cannot be crystal controlled

Answer: B

Modulating the final stage means the audio modulator itself must deliver power comparable to the carrier, which for a large transmitter is a substantial amplifier in its own right, historically a large audio stage or a pulse-width modulated supply. The reward is class C efficiency in the radio-frequency chain, so the second option inverts the benefit. Crystal control of the carrier is unaffected. And sideband symmetry is preserved by a properly designed modulator.

Discuss in Forum

80.

What limits the audio bandwidth of an AM broadcast in practice?

A.The transmitter's carrier stability
B.The receiver's audio amplifier
C.The channel allocation
D.The dynamic range of the audio processor

Answer: C

The channel allocation sets it: with nine or ten kilohertz available and double-sideband transmission, the audio must be limited to four or five kilohertz. That is a regulatory and physical limit rather than an equipment one, which is why AM broadcasting cannot match FM for fidelity however good the equipment. Audio processing affects loudness and dynamics. Carrier stability is far tighter than the audio bandwidth. And receiver audio stages are easily capable of more than the channel can deliver.

Discuss in Forum

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