AM Transmitters and Receivers Questions and Answers

Answer: A

Misalignment passes one sideband more than the other, unbalancing them and distorting the recovered envelope - the same effect selective fading produces on the air. The oscillator is unaffected. Gain control still works, though on a distorted signal. And image rejection depends on circuits before the mixer. Recognising that envelope detection assumes symmetric sidebands explains several otherwise puzzling faults.

Answer: B

Band-specific symptoms point to the band-switched components, since everything after the mixer is common to all bands. A failed detector or intermediate stage would affect every band equally, and so would a loudspeaker fault. Using the pattern of which bands or functions fail to localise a fault to the shared or the switched part of the circuit is the standard diagnostic method.

Answer: C

Only a spectrum sweep across the harmonic frequencies will show energy at two, three or four times the carrier. A modulation monitor watches the envelope. An antenna current meter reads total power without distinguishing where it lies in frequency. And an audio analyser measures the modulating signal. Choosing an instrument whose domain matches the question - frequency, time, or audio - is the practical point.

Answer: D

The driver provides the power the final stage needs at its input, bridging the low-level oscillator and buffer to the high-power output. It does not modulate, match or generate. In a high-level modulated transmitter the driver handles an unmodulated carrier and so can also run in class C, which is part of why that architecture is efficient throughout. Each stage's function follows from where it sits in the power progression.

Answer: A

Widening the filter admits more of the neighbouring channels, so selectivity suffers while fidelity improves, since more of the sidebands pass. Sensitivity changes only slightly, through the noise bandwidth. And image rejection depends on circuits before the mixer and is untouched. The filter width is the single control that sets the selectivity-fidelity balance, which is why some receivers make it switchable rather than choosing once for the user.

Answer: B

High-side injection requires the oscillator to cover, say, 1.0 to 2.1 MHz for a 0.55 to 1.6 MHz band, whereas low-side injection would demand a much wider capacitance ratio, which is mechanically harder. Image behaviour depends on tuning position rather than favouring one side everywhere. Tracking correction is needed either way. And the intermediate frequency is a separate choice. Practical constraints on a variable capacitor decided a convention that outlasted the component itself.

Answer: C

The first two are correct. The third is backwards: a higher intermediate frequency helps image rejection but makes adjacent-channel selectivity harder, since a filter of given fractional bandwidth is wider in absolute terms. That opposition is exactly why double conversion exists. Getting the direction of this trade right is what separates understanding the architecture from reciting its block diagram.

Answer: D

The first three are correctly paired and the fourth is not, so three match. Oscillator drift is addressed by oscillator design, thermal compensation or a synthesiser, and de-emphasis is an FM audio network with no bearing on it at all. Mapping each complaint to the one stage that can answer it is the most useful table to carry out of this topic, because it converts a vague symptom into a specific place to look.

Answer: A

Ice changes an antenna's impedance, so a mismatch is reflecting power back into the output stage, which both reduces radiated power and heats the device. Checking the standing wave ratio is the immediate diagnostic. Modulation depth, oscillator frequency and audio level are all unaffected by weather at the antenna. Reasoning from what changed in the environment to the parameter it would move is the quickest route to a cause.

Answer: B

Modulating at high level lets every radio-frequency stage run in class C rather than linearly, which is a large efficiency gain for the same radiated power. Widening the audio bandwidth breaches the channel. Raising the oscillator frequency changes the channel rather than the efficiency. And modulating beyond unity is over-modulation, which is illegal and distorting. Efficiency improvements in transmitters almost always come from changing where the nonlinearity is tolerated.