Amplitude Modulation Questions and Answers

Answer: A

The image sits twice the intermediate frequency above the wanted signal for high-side injection:

\[ f_{image} = 1200 + 2 \times 455 = 2110 \ \mathrm{kHz} \]

so 2110 kHz, and any signal there mixes to the same intermediate frequency and cannot be removed afterwards. That is why radio-frequency selectivity ahead of the mixer matters. 1655 kHz is the oscillator. 745 kHz is the low-side oscillator frequency. And 290 kHz corresponds to no relevant combination.

Answer: B

Only filtering before the mixer can reject the image, because after mixing the image and the wanted signal occupy exactly the same intermediate frequency and are indistinguishable. That is the argument for a tuned radio-frequency stage or a preselector, and for choosing a high intermediate frequency where possible. The intermediate frequency amplifier provides adjacent-channel selectivity instead. Gain control and audio filtering act far too late.

Answer: C

A higher intermediate frequency pushes the image further from the wanted signal, so a modest preselector can reject it. The cost is that adjacent-channel selectivity becomes harder, since a filter of given fractional bandwidth is wider in absolute terms - which is exactly the trade the double-conversion receiver resolves by using a high first intermediate frequency and a low second one. Tuning range and noise figure are affected only indirectly.

Answer: D

The figure is a compromise: high enough that the image falls well outside the band for most tuning positions, low enough that a simple tuned circuit gives the nine or ten kilohertz selectivity a channel needs. Crystal filters are made at far lower and far higher frequencies. Channel spacing is nine or ten kilohertz, nothing like 455. And no regulation prescribes it; the value became a convention because it works.

Answer: A

It is the modulation index, sometimes expressed as a percentage and then called the depth of modulation. Depth ratio is not standard usage. A figure of merit is a general term used for quite different quantities. And coefficient of coupling belongs to transformer and resonant-circuit theory. Knowing the name matters because every power and efficiency formula in the topic is written in terms of it.

Answer: B

Two: one above and one below the carrier, at plus and minus the message frequency. A single sideband appears only after one has been deliberately filtered out. Four would arise from two modulating tones. And the message does not ride on the carrier without producing sidebands; the sidebands are precisely how the information is carried, since a pure carrier conveys nothing.

Answer: C

At an index of unity the sidebands carry a third of the total, so the efficiency ceiling is about 33 per cent and any practical figure is lower still. Fifty per cent would require the carrier and sidebands to be equal in power, which the relation does not permit. Two thirds is the fraction the carrier takes, not the sidebands. And a hundred per cent would mean no carrier at all, which is DSB-SC rather than full-carrier AM.

Answer: D

The carrier power is unchanged; deepening the modulation adds sideband power on top of it, so the total power and the antenna current both rise and the efficiency improves. That is exactly why the antenna current measurement reveals the index. Total power follows \( 1 + \mu^{2}/2 \). Current follows its square root. And efficiency rises towards its ceiling of a third.

Answer: A

Only a coherent, or synchronous, detector works, because with the carrier removed the envelope no longer follows the message: it follows the magnitude of the message and reverses phase at every zero crossing. Both of the rectifier options are envelope detectors and would recover a full-wave rectified version of the message. A slope detector is an FM device and is irrelevant here.

Answer: B

It looks like the rectified tone, so the envelope repeats at twice the message frequency while the carrier phase reverses at each null. That is precisely why an envelope detector fails and why the phase reversal must be tracked by a coherent receiver. The tone itself appears as the envelope only when a large carrier is present. Constant amplitude describes angle modulation. And a square wave would require a square-wave message.