Amplitude Modulation Questions and Answers

Answer: A

Squaring the input produces the message together with a term proportional to the message squared, which is second-harmonic distortion; it is small only when the modulation index is small. That is why the square-law detector is used for weak signals and in measurement rather than in broadcast receivers. The carrier is removed by the output filter. Components at twice the carrier are filtered out. And the whole point is that baseband output does appear.

Answer: B

Coherent detection multiplies by a locally generated carrier matched in frequency and phase, and errors in either degrade or destroy the output. It is mandatory for suppressed-carrier signals and gives a small noise advantage for full-carrier AM, but the extra complexity is why envelope detection dominates broadcasting. A diode characteristic is an envelope-detector matter. A limiter belongs to FM. And it is envelope detection that needs no reference.

Answer: C

A frequency error makes the effective phase drift, so the recovered output is multiplied by a slow beat at the difference frequency, heard as a rise and fall or, for speech, as an unnatural warble. That sensitivity is the practical price of coherent detection and the reason a pilot tone or a phase-locked loop is used to derive the reference. The amplitude is certainly affected. Polarity inversion is what an exact half-cycle phase error would give. And the output does not vanish, except momentarily as the phase passes through quadrature.

Answer: D

Displaying the modulated envelope against the modulating signal gives a trapezoid whose end dimensions yield the index, while any curvature of its sides reveals modulation non-linearity. It is a standard transmitter bench check and is more informative than the plain envelope display. Carrier frequency needs a counter. Harmonic distortion is measured with a spectrum analyser or distortion meter. And noise figure is a receiver measurement requiring a noise source.

Answer: A

Suppressing the carrier saves the two thirds of power it consumed, at the cost of requiring a coherent receiver because there is no longer an envelope that follows the message. Bandwidth is unchanged at twice the message frequency, since both sidebands remain. It uses less power, not more. And removing a sideband as well is single sideband, which is a further step.

Answer: B

Single sideband occupies just the message bandwidth, half that of any double-sideband form, which is why it dominates long-distance voice radio. Full-carrier and suppressed-carrier double sideband both need twice the message bandwidth. Vestigial sideband sits between the two and is used for television video, where the message extends to very low frequencies and a clean sideband filter is impossible.

Answer: C

Video content reaches down towards direct current, so a filter sharp enough to remove one sideband cleanly without damaging the low-frequency components cannot be built; leaving a vestige of the unwanted sideband solves it. Power saving is a secondary benefit. Envelope detection of SSB is indeed poor, but the driving reason here is the spectral shape of the message. And the carrier is retained precisely so that a simple envelope detector can be used in the receiver.

Answer: D

Over-modulation on peaks produces envelope clipping and spectral splatter during those peaks, which is heard as distortion and appears as interference in adjacent channels; the average index is irrelevant to whether the peaks are legal. That is why broadcasters run a peak limiter rather than trusting average levels. Carrier power is set by the transmitter and does not sag. Nothing shifts the carrier frequency. And an acceptable average does not licence peaks above unity.

Answer: A

Signals arriving at a receiver differ by many tens of decibels between a local and a distant station and fade with propagation, so gain must be adjusted automatically to keep the detector working in its linear region and the output level steady. The index is a transmitter matter. Frequency drift is handled by oscillator design and by the tuning system. And the detector is reasonably linear once the signal is well above its threshold, which is exactly what the gain control ensures.

Answer: B

Inverting the power relation:

\[ \begin{aligned} 1 + \frac{\mu^{2}}{2} &= \frac{650}{500} = 1.30 \\ \mu &= \sqrt{0.60} = 0.77 \end{aligned} \]

so about 0.77. 0.30 is the excess power ratio taken as the index directly. 0.55 halves the excess before rooting it. 1.30 is the power ratio itself, which would imply over-modulation and shows how easily the ratio and the index are confused. Working backwards from measured powers like this is the standard way a transmitter's modulation is verified.