Amplitude Modulation Questions and Answers

Answer: A

Amplitude modulation varies the carrier amplitude in step with the message while its frequency and phase stay fixed, so the message appears as the envelope of the transmitted waveform. Varying the frequency instead gives frequency modulation, and varying the phase gives phase modulation; both are angle modulations and behave quite differently in bandwidth and in noise. Varying two parameters at once is not a standard scheme and would make demodulation ambiguous.

Answer: B

The envelope reproduces the message, which is exactly why a simple diode detector recovers it: rectify, smooth, and the message appears. That simplicity is AM's great advantage and the reason broadcast receivers could be made cheaply for a century. The carrier frequency is the rate at which the waveform oscillates inside the envelope. The sideband frequencies are properties of the spectrum. And no local oscillator is involved in envelope detection at all.

Answer: C

The index is \( \mu = A_m/A_c \), the ratio of message amplitude to carrier amplitude, and it fixes how deeply the envelope is driven. Inverting the ratio gives a number greater than one for normal modulation, which is not the convention. A ratio of frequencies is the deviation ratio in angle modulation, a different quantity entirely. And the power ratio is \( 1 + \mu^{2}/2 \), which contains the index but is not it.

Answer: D

From the envelope extremes:

\[ \begin{aligned} \mu &= \frac{V_{max}-V_{min}}{V_{max}+V_{min}} \\ &= \frac{10}{20} = 0.5 \end{aligned} \]

so 0.5, or fifty per cent modulation. 0.33 is the ratio of the minimum to the maximum. 3.0 is the ratio of maximum to minimum, which is greater than one and so cannot be an index for an undistorted signal. 0.67 comes from dividing the difference by the maximum alone. Reading the index off an oscilloscope trace this way is the standard bench measurement.

Answer: A

Beyond an index of one the envelope would have to go negative, so it folds instead: the carrier phase reverses and the recovered message is badly distorted, with splatter into adjacent channels. That is why broadcast transmitters are limited to just under a hundred per cent modulation and why a modulation monitor is a standard fitting. An index of a half is perfectly normal. Two would be heavily over-modulated. And ten per cent is merely shallow, wasting power but distorting nothing.

Answer: B

The envelope cannot go below zero, so the troughs clip flat and the detector recovers a distorted waveform rich in harmonics that also spread the transmitted spectrum well beyond the nominal channel. The carrier frequency is unaffected. Bandwidth increases rather than reduces, which is precisely the interference problem. And the phase reversal that occurs is exactly what a diode envelope detector cannot follow, so the message certainly is changed.

Answer: C

The sidebands sit at the sum and difference frequencies:

\[ f_c \pm f_m = 1000 \pm 5 \ \mathrm{kHz} \]

so 995 and 1005 kHz, with the carrier still present between them. The second option gives only the upper sideband and the carrier. 990 and 1010 kHz would follow from a ten kilohertz tone. And the last option lists the message and carrier frequencies rather than the sidebands, which is a confusion between the baseband and passband descriptions.

Answer: D

Both sidebands are present and each sits \( f_m \) from the carrier, so the occupied bandwidth is \( 2f_m \). That factor of two is what single sideband removes, and it is the first entry in every comparison of the modulation schemes. A bandwidth of \( f_m \) would describe single sideband. Half of \( f_m \) is less than the message needs and is impossible. Four times would follow from a wideband angle modulation rather than from AM.

Answer: A

Bandwidth is twice the highest message frequency:

\[ B = 2 \times 3.4 = 6.8 \ \mathrm{kHz} \]

so 6.8 kHz, which is why AM broadcast channels are spaced at nine or ten kilohertz. 3.4 kHz is the message's own upper limit. 3.1 kHz is the message bandwidth, from 300 Hz to 3.4 kHz, which is not what sets the transmitted width because the sidebands are measured from the carrier. 6.2 kHz doubles the message bandwidth instead of the highest frequency, a subtle and common slip.

Answer: B

The spectrum holds the carrier and both sidebands, which is what the name full-carrier double-sideband describes. Suppressing the carrier gives DSB-SC, and suppressing one sideband as well gives SSB; both save power or bandwidth at the cost of a more complex receiver. A carrier alone with information in its phase is phase modulation, which is not an amplitude scheme at all.