Amplitude Modulation Questions and Answers

Answer: C

The envelope varies about the carrier level by the index times the carrier:

\[ V_{audio} = 0.5 \times 2 = 1 \ \mathrm{V} \]

so about 1 volt peak, ignoring the diode's forward drop and detector efficiency. 2 V is the carrier itself, which appears as a direct-current component rather than as audio. 0.5 V is the index quoted as a voltage. And 4 V exceeds the envelope maximum, which is 3 V here. Note that the recovered audio is proportional to both the index and the carrier level, which is why gain control is needed to keep the output steady.

Answer: D

The rectified carrier appears as a direct-current level proportional to the received carrier strength, which is exactly the signal the automatic gain control loop needs. It is therefore extracted deliberately rather than blocked, while a coupling capacitor removes it from the audio path. It does not depend on the index, which sets the alternating component. And a balanced detector for AM is not the arrangement in use; balance belongs to the modulator.

Answer: A

De-emphasis is an FM technique, correcting the pre-emphasis applied at the transmitter to combat the rising noise spectrum of an FM detector. Amplitude modulation has no such noise slope and uses no pre-emphasis, so no de-emphasis appears in an AM receiver. It has nothing to do with intermediate frequency bandwidth or image rejection, both of which are filtering matters at radio and intermediate frequencies rather than at audio.

Answer: B

Frequency modulation puts the information in the angle rather than the amplitude, so a limiter can strip amplitude noise away before detection - the single biggest practical improvement over AM and the reason FM dominates high-fidelity broadcasting. Its bandwidth is greater, not smaller, which is the price paid. Its receiver is more complex. And it permits class C amplification, so the last option describes a benefit of FM rather than a limitation of AM it addresses.

Answer: C

Aeronautical voice retains AM because simultaneous transmissions remain mutually audible instead of one capturing the channel, which is a safety property no other scheme offers. Music broadcasting went to FM for its noise performance and bandwidth. Mobile telephony and satellite television are digital, using schemes far removed from either. The aeronautical case is a good reminder that the best modulation depends on what property matters most, not on a general ranking.

Answer: D

Splatter is the energy that appears outside the assigned channel when a transmitter is over-modulated or its audio is clipped, and it is heard by listeners tuned to neighbouring channels rather than by the offending station's own audience. Ignition noise is an external interference. Selective fading is a propagation effect. And distortion from a long detector time constant is diagonal clipping, which is a receiver fault confined to that receiver.

Answer: A

The sides of the trapezoidal display are straight when modulation is linear and curved when it is not, which makes the pattern the standard bench check for linearity as well as for index. Standing wave ratio tests the antenna and feeder. Frequency stability is an oscillator matter. And a receiver's output level depends on its own gain control and tells you nothing reliable about the transmitter's linearity.

Answer: B

At night the absorbing layer of the ionosphere weakens, so signals from distant stations propagate by reflection and interfere with local ones, producing the familiar night-time fading and heterodyne whistles. Some stations do reduce power at night, but that is a consequence of the propagation change rather than its cause. Receiver sensitivity does not depend on air temperature in any relevant way. And carrier drift is far too small to matter beside a co-channel interferer.

Answer: C

Selective fading attenuates the carrier and the two sidebands by different amounts, and because AM's envelope depends on their relative magnitudes, the recovered audio distorts badly even when the total signal is strong. Uniform fading merely changes the level and is handled by the gain control. Audio-frequency selectivity is not what fading means. And the gain control responds to fading rather than causing it. Single sideband is less affected, which is one of its practical advantages on long paths.

Answer: D

Efficiency rises as unproductive components are removed: full-carrier AM wastes at least two thirds on the carrier, DSB-SC removes that waste, and SSB additionally puts everything into one sideband. So the order is full-carrier AM, DSB-SC, SSB. The reversed sequence inverts it. The remaining options misplace one scheme, and the last in particular puts SSB below DSB-SC, which would require the redundant second sideband to improve efficiency.