AM Transmitters and Receivers Questions and Answers

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.

Answer: B

Noise added before most of the gain dominates, so the first stage's noise figure sets the sensitivity and later stages contribute far less. That is why a low-noise preamplifier is placed as early as possible, and why the cascade formula weights the first stage most heavily. Audio gain merely raises both signal and noise. Detector efficiency matters at very low levels but is downstream of the limiting noise. And antenna size affects the signal delivered rather than the receiver's own noise.

Answer: C

The intermediate frequency filter does the work, operating at one fixed frequency where a sharp response can be built and used across the whole tuning range. The antenna circuit contributes broad preselection and image rejection. Tone controls shape audio and cannot separate channels. And the gain control's time constant governs how quickly the receiver adapts to level changes. Concentrating selectivity at a fixed frequency is the superheterodyne's central economy.

Answer: D

As the receiver is tuned, the oscillator must stay a fixed intermediate frequency away from the signal, which a single ganged capacitor cannot do exactly because the required frequency ratio differs from the signal circuit's. Padder and trimmer capacitors correct it at three points across the band. Drift is a stability matter handled by component choice. Antenna matching and audio levelling are separate functions. Tracking is a geometric problem peculiar to ganged tuning.