AM Transmitters and Receivers Questions and Answers

Answer: A

Volume sits after the detector and is deliberately manual, while the automatic gain control adjusts the radio and intermediate frequency stages to hold the detector's input steady. Handing the earlier gains to the listener would risk overloading the detector. Detector bias is fixed by design. The division between what the loop controls and what the user controls is a deliberate design decision rather than an accident of history.

Answer: B

The detector output carries a direct voltage proportional to carrier strength, which must be kept out of the audio path while still being available to the gain control. A series capacitor does exactly that. De-emphasis is an FM network. Selectivity is set long before the detector. And loudspeaker matching is the output transformer's job. One component separating two uses of the same node is a small but characteristic piece of receiver design.

Answer: C

The rectified carrier level is proportional to received signal strength, so the detector's direct output drives the meter and peaks when the station is correctly tuned. Audio level depends on the modulation as well as the signal. Oscillator amplitude is constant. And loudspeaker current follows the programme material. Using the same node for gain control and for a strength indication is an economy that also explains why the meter reads signal rather than loudness.

Answer: D

With the feedback path broken the stages run at full gain regardless of signal strength, so the detector saturates and the meter pins on anything. A failed loudspeaker gives silence with a normal meter. An open antenna gives low readings. And a padder fault spoils tracking, giving mistuning rather than saturation. Predicting the symptom from the broken loop, rather than working backwards from the symptom, is often the faster route.

Answer: A

A licence specifies frequency, maximum power and occupied bandwidth - the quantities that affect other users. How the transmitter achieves them, including its amplifier class, stage count and modulator type, is the designer's business entirely. That division between regulated outcome and unregulated implementation is what allows technology to change without renegotiating every licence, and it is worth understanding when reading any specification.

Answer: B

Overload distorts the strongest signals while leaving weak ones clean, which is the characteristic level-dependent symptom and usually points to a gain control that is not reducing gain enough. Silence indicates a broken path. A tuning whistle indicates interference or an image. And a single-band fault points at switched components. Noting whether a symptom depends on signal strength narrows the search immediately.

Answer: C

A short antenna presents a large reactance and radiates inefficiently, so much of the power is lost in the matching network and in conductor resistance. That is the practical problem at long wavelengths and the reason medium-wave broadcast masts are so tall. The frequency is set by the oscillator. Modulation depth is a transmitter parameter. And harmonics come from nonlinear stages rather than from a passive antenna. Antenna size is a physics constraint, not an engineering choice.

Answer: D

At medium-wave wavelengths of hundreds of metres, an efficient radiator must be a substantial fraction of a wavelength tall, which is why the masts are so large and why the antenna is often the most expensive part of the installation. Aircraft interference, monitor mounting and measurement convenience are all incidental. The link between wavelength and antenna size is the single most consequential fact in choosing a broadcast band.

Answer: A

Because total power rises as \( 1 + \mu^{2}/2 \), deeper modulation draws more average current from the supply, which is why a broadcast transmitter's supply is sized on programme statistics rather than on the carrier alone. Voltage regulation matters but is a design requirement rather than something the index changes. Oscillator stability is isolated by buffering. And harmonic content depends on the amplifier's linearity. Following the power relation through to the supply is the practical end of the theory.

Answer: B

It sits after the output stage, on the way to the antenna, because that is where the harmonics are generated and the only place they can be stopped before radiation. Filtering earlier would achieve nothing, since the class C output stage creates its own. The audio chain works far below the carrier. And supply filtering deals with ripple rather than radio-frequency harmonics. Placing each filter where its unwanted content arises is the general rule.