AM Transmitters and Receivers Questions and Answers

Answer: C

It varies the drive level rather than the supply, which needs far less audio power than collector or plate modulation but gives poorer linearity and lower efficiency. Supply variation is the high-level method proper. The coupling network is passive and cannot be modulated at audio rates. And modulating the oscillator would produce frequency modulation. Where the audio is injected sets both the power the modulator must supply and the linearity achieved.

Answer: D

Its linearity is inferior, so distortion appears at deeper modulation, which is why it suits low-power and non-broadcast applications where the audio power saving matters more than fidelity. It needs a smaller audio amplifier, not a larger one, which is its whole attraction. Full modulation is achievable with care. And the carrier is retained throughout. Every modulation point trades audio power against linearity in this way.

Answer: A

The tank circuit stores energy and reconstructs a clean sinusoid from the short conduction pulses, which is what makes class C usable at all. De-emphasis is an FM audio network. Harmonic generation is deliberate only in a multiplier chain, which AM does not use. And oscillator isolation is the buffer's task. The tuned circuit is doing filtering and impedance transformation at once, which is why its Q matters to both efficiency and bandwidth.

Answer: B

Good selectivity with poor sensitivity points to the front end, since the filter is evidently working while something before the gain is adding noise or losing signal. The filter's shape governs selectivity, which is fine. The gain control's timing affects how the receiver responds to level changes rather than how weak a signal it can use. And the audio transformer is far downstream. Using one good figure of merit to exonerate a stage is as useful as using a bad one to accuse it.

Answer: C

Its very high Q gives adjacent-channel selectivity no simple tuned circuit can match, which is why it is fitted at the intermediate frequency where one fixed response serves the whole band. Image rejection is a preselector function at the signal frequency. The detector's direct component is blocked by a capacitor. And crystals do stabilise oscillators, but that is a different application of the same component. Matching component to function rather than to name is the point.

Answer: D

Shape factor, the ratio of the bandwidth at heavy attenuation to that at light attenuation, measures how quickly the response falls away and therefore how well adjacent channels are rejected. Insertion loss is the signal lost in the passband. Centre frequency places the response. And terminating impedance is a matching requirement. Filter specifications each answer a different question, and shape factor is the one that corresponds to what a listener perceives as selectivity.

Answer: A

They deal with different problems: the preselector rejects the image, two intermediate frequencies away, and the intermediate filter rejects the adjacent channel a few kilohertz away. Neither can do the other's job, which is why both are fitted. Gain comes mostly from the intermediate amplifier. Signal level is managed by the gain control. And audio bandwidth follows from the intermediate filter rather than the preselector. Two filters at two frequencies for two offsets is the architecture in one sentence.

Answer: B

Occupied bandwidth is twice the highest audio frequency, so filtering the audio more tightly is the only one of these that narrows it. Raising the index increases power and, if taken too far, spreads the spectrum through distortion. More carrier power widens nothing but wastes efficiency. And antenna matching affects radiated power rather than spectral width. Knowing which parameter controls bandwidth in each modulation family prevents a great many wrong answers.

Answer: C

A mismatch reflects power back into the stage, raising the voltage and current it must withstand and heating it, which is why protection circuits monitor the standing wave ratio and reduce power. The oscillator is buffered from the load, so no drift follows. Modulation depth is set upstream. And the filter continues to work, though the reflected energy passes through it twice. Following a fault through to the component that suffers is the practical value of the concept.

Answer: D

The filter width caps the audio bandwidth, so it sets the ceiling on quality no matter how good everything after it is. A better loudspeaker or a larger amplifier cannot restore sidebands the filter removed. The gain control's timing affects how level changes are handled rather than the bandwidth. This is why AM broadcasting sounds as it does even on excellent equipment, and why the switchable-bandwidth receiver was invented.