Amplitude Modulation Questions and Answers

Q: What is the shape of the AM envelope for a square-wave message?

The envelope follows the message, so a square wave gives a square envelope stepping between the two amplitudes the two message levels imply. That is the basis of on-off keying when the message is a switched carrier. A sinusoidal envelope would require a sinusoidal message. Integration would occur only if the modulator included it. And a constant envelope with phase reversals describes a phase-keyed signal, which is an angle modulation rather than an amplitude one.

Q: Which measurement would confirm that a transmitter is over-modulating?

The flattened trough on an envelope display is the direct evidence: the envelope has hit zero and folded, which is exactly what over-modulation means. Antenna current does rise with modulation depth, but a rise alone cannot distinguish deep legal modulation from illegal over-modulation. Carrier frequency is unaffected. And a falling audio input would reduce the depth rather than reveal an excess of it. Confirming the fault on the waveform rather than inferring it from a meter is what makes the envelope display indispensable.

Q: Full-carrier amplitude modulation is also written as _____ in the standard abbreviations.

It is DSB-FC, double sideband full carrier, with DSB-SC being the suppressed-carrier form, SSB-SC single sideband suppressed carrier, and VSB vestigial sideband. The abbreviations are worth knowing because data sheets and standards use them without expansion, and mistaking DSB-FC for DSB-SC changes both the receiver required and the power budget by a large factor. All four describe amplitude modulations, differing only in what has been removed from the spectrum before transmission.

Q: Which component of an AM receiver sets its adjacent-channel selectivity?

The intermediate frequency filter does, since it operates at a fixed frequency where a sharp response can be built once and used across the tuning range. That concentration of selectivity is the central advantage of the superheterodyne architecture. The antenna and its preselector provide image rejection rather than channel selectivity. De-emphasis belongs to FM. And the gain-control detector measures level rather than discriminating between channels.

81.

A carrier of \( 100 \ \mathrm{V} \) peak is modulated to an index of \( 0.4 \). The envelope maximum is

A.140 V
B.104 V
C.40 V
D.160 V

Answer: A

The envelope reaches the carrier plus the message amplitude:

\[ V_{max} = 100(1+0.4) = 140 \ \mathrm{V} \]

so 140 volts, with a minimum of 60 volts. 104 V would follow from adding four per cent rather than forty. 40 V is the message amplitude alone. 160 V corresponds to an index of 0.6. The peak envelope voltage is what sizes the output devices and the antenna insulation, so it is a design figure rather than a merely academic one, and it grows to twice the carrier at full modulation.

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82.

For an index of unity, the envelope minimum is

A.Twice the carrier amplitude
B.Zero
C.Half the carrier amplitude
D.Equal to the carrier amplitude

Answer: B

At full modulation the message amplitude equals the carrier, so the envelope swings from twice the carrier down to zero, just touching the axis without crossing it. Any deeper modulation would require a negative envelope, which is what folds and distorts. Half the carrier corresponds to an index of a half. Equality with the carrier is the unmodulated case. And twice the carrier is the maximum rather than the minimum. Recognising the just-touching envelope on an oscilloscope is the quickest way to identify a hundred per cent modulation.

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83.

What is the shape of the AM envelope for a square-wave message?

A.A triangular envelope tracking the integral
B.A constant envelope with phase reversals
C.A square envelope switching between two amplitudes
D.A sinusoidal envelope at the fundamental

Answer: C

The envelope follows the message, so a square wave gives a square envelope stepping between the two amplitudes the two message levels imply. That is the basis of on-off keying when the message is a switched carrier. A sinusoidal envelope would require a sinusoidal message. Integration would occur only if the modulator included it. And a constant envelope with phase reversals describes a phase-keyed signal, which is an angle modulation rather than an amplitude one.

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84.

Which of these produces the widest transmitted spectrum for the same audio?

A.Full-carrier AM with a limited audio peak
B.Single sideband with the same audio
C.DSB-SC with the same audio
D.Full-carrier AM with a hard-clipped audio peak

Answer: D

Hard clipping generates harmonics of the audio, and each harmonic produces its own sideband pair further from the carrier, so the transmitted spectrum spreads well beyond the nominal channel. That splatter is exactly what a peak limiter is fitted to prevent, and why limiting is preferred to clipping. Properly limited AM and DSB-SC both occupy twice the audio bandwidth, and single sideband occupies only the audio bandwidth, so all three of the alternatives are narrower.

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85.

Why is a peak limiter preferred to a clipper in a broadcast audio chain?

A.It controls peaks without generating harmonics
B.It raises the average modulation depth further
C.It removes the need for a modulation monitor
D.It allows the index to exceed unity safely

Answer: A

A limiter reduces gain smoothly as the peak approaches the limit, so it holds the index below unity without the harmonic generation a hard clipper produces and therefore without splatter into adjacent channels. A clipper achieves the same peak control but at the cost of distortion. Both raise the average depth; that is not the distinguishing point. A monitor is still needed. And nothing makes an index above unity safe in full-carrier AM.

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86.

Which measurement would confirm that a transmitter is over-modulating?

A.A fall in the audio input level
B.An envelope display showing a flattened trough
C.A rise in the antenna current alone
D.A drop in the carrier frequency

Answer: B

The flattened trough on an envelope display is the direct evidence: the envelope has hit zero and folded, which is exactly what over-modulation means. Antenna current does rise with modulation depth, but a rise alone cannot distinguish deep legal modulation from illegal over-modulation. Carrier frequency is unaffected. And a falling audio input would reduce the depth rather than reveal an excess of it. Confirming the fault on the waveform rather than inferring it from a meter is what makes the envelope display indispensable.

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87.

Full-carrier amplitude modulation is also written as _____ in the standard abbreviations.

A.SSB-SC
B.VSB
C.DSB-FC
D.DSB-SC

Answer: C

It is DSB-FC, double sideband full carrier, with DSB-SC being the suppressed-carrier form, SSB-SC single sideband suppressed carrier, and VSB vestigial sideband. The abbreviations are worth knowing because data sheets and standards use them without expansion, and mistaking DSB-FC for DSB-SC changes both the receiver required and the power budget by a large factor. All four describe amplitude modulations, differing only in what has been removed from the spectrum before transmission.

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88.

Which component of an AM receiver sets its adjacent-channel selectivity?

A.The antenna
B.The audio de-emphasis network
C.The automatic gain control detector
D.The intermediate frequency filter

Answer: D

The intermediate frequency filter does, since it operates at a fixed frequency where a sharp response can be built once and used across the tuning range. That concentration of selectivity is the central advantage of the superheterodyne architecture. The antenna and its preselector provide image rejection rather than channel selectivity. De-emphasis belongs to FM. And the gain-control detector measures level rather than discriminating between channels.

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89.

What is the consequence of an intermediate frequency filter that is too narrow?

A.Loss of the higher audio frequencies
B.Poor image rejection
C.Increased impulse noise
D.A rise in the transmitted bandwidth

Answer: A

Narrowing the filter cuts the outer parts of the sidebands, which carry the highest audio frequencies, so the recovered sound loses its treble - the familiar muffled quality of a receiver switched to its narrow setting for interference rejection. Image rejection depends on filtering before the mixer. Impulse noise is if anything reduced by a narrower bandwidth. And the transmitted bandwidth is a property of the transmitter, entirely unaffected by the receiver's choices.

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90.

Which is true of a diode detector's efficiency at very low signal levels?

A.It becomes exactly linear below the diode's forward drop
B.It falls, and the detector behaves as a square-law device
C.It rises, because the diode is more linear near zero
D.It is independent of level by design

Answer: B

Well below the diode's forward voltage the characteristic is curved, so the detector behaves as a square-law device with the distortion that implies, and its output falls faster than the input. That is why substantial intermediate frequency gain precedes the detector and why the automatic gain control keeps the level high. The diode is less linear near zero, not more. Level independence would be desirable but is not what a junction gives. And exact linearity below the forward drop is the opposite of the truth.

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