Amplitude Modulation Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

Answer: D

The two spectra are the same pair of sidebands, with AM adding a carrier line between them. That is why the bandwidths are equal and why the only saving is in power. Halving the bandwidth requires removing a sideband, which is single sideband. AM has two sidebands, not one. And the sidebands of both schemes sit at the message frequency either side of the carrier, so the last option misplaces them by a factor of two.