AM Transmitters and Receivers Questions and Answers

Answer: D

Feedback through the device's internal capacitance can make a tuned amplifier oscillate at its own tuned frequency, and neutralisation cancels that path with an equal and opposite one. It is essential in valve stages and in some transistor designs. Harmonic radiation is dealt with by output filtering. Oscillator pulling is prevented by buffering. And overheating is a thermal design matter. Each unwanted behaviour has its own specific countermeasure, and confusing them is a common examination error.

Answer: A

It transforms the antenna's impedance to the value the output stage needs for maximum power transfer, and usually filters harmonics on the way. Without it the stage would be mismatched, delivering less power and risking damage. Modulation happens upstream. Harmonic generation is deliberate only in a multiplier chain, which an AM transmitter does not use. And metering rectifiers are small separate circuits. Matching and filtering in one network is a standard economy of transmitter design.

Answer: B

A dummy load presents the correct resistive impedance and turns the output into heat instead of radiation, so a transmitter can be tuned and measured without interfering with other users or breaching its licence. It does not increase the power measured; if anything a good load makes the measurement honest. It replaces the antenna rather than the modulator. And it is deliberately resistive, with no reactive part. Testing into a known load is what makes transmitter measurements repeatable.

Answer: C

The mixer combines the incoming signal with the local oscillator and produces the difference, so every station arrives at one fixed intermediate frequency where the gain and selectivity can be provided once. Amplification at the signal frequency is the preselector's small contribution. Audio recovery is the detector's job. And sideband selection belongs to single sideband receivers. The translation is the single idea that made the architecture universal.

Answer: D

High-side injection puts the oscillator above the signal:

\[ f_{LO} = 800 + 455 = 1255 \ \mathrm{kHz} \]

giving 1255 kHz. The 345 kHz option is low-side injection, which works but needs a wider oscillator tuning ratio across the band. 455 kHz is the intermediate frequency and 800 kHz the signal. The choice of side also fixes where the image falls, which is the next thing any receiver designer checks.

Answer: A

The image sits a further intermediate frequency above the oscillator:

\[ f_{image} = 800 + 2 \times 455 = 1710 \ \mathrm{kHz} \]

so 1710 kHz, and a signal there would mix to the same intermediate frequency and be inseparable from the wanted one. 1255 kHz is the oscillator itself. 345 kHz is the low-side alternative. And 910 kHz is twice the signal frequency. Because the image cannot be removed after mixing, everything depends on rejecting it before.

Answer: B

Only filtering ahead of the mixer can help, because after mixing the image and the wanted signal occupy the same frequency and no filter can distinguish them. That is the argument for a tuned radio-frequency stage, and for choosing an intermediate frequency high enough to put the image far from the wanted signal. Every later stage is too late. This irreversibility is the same principle as anti-alias filtering before a sampler.

Answer: C

A higher intermediate frequency pushes the image further from the wanted signal, so a simple 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. That opposition is exactly why double-conversion receivers use a high first intermediate frequency and a low second one. Oscillator drift and detector distortion are not driven by the choice.

Answer: D

The first, high intermediate frequency places the image far away, and the second, low one provides the sharp adjacent-channel selectivity, resolving a conflict that a single conversion cannot. Gain is a separate matter and easily obtained. Receiving two stations at once needs two receivers. And a preselector is still fitted, though its job is easier. Splitting a requirement between two stages, each optimised for one half, is a recurring engineering pattern.

Answer: A

The classical trio is sensitivity, how weak a signal can be used; selectivity, how well neighbours are rejected; and fidelity, how faithfully the audio is reproduced - and they pull against one another, since narrowing the bandwidth improves selectivity while harming fidelity. Gain, bandwidth and impedance are circuit parameters. Power and efficiency belong to transmitters. And the last three are modulation parameters. Naming the trade-off explicitly is what distinguishes receiver design from mere amplification.