AM Transmitters and Receivers Questions and Answers

Answer: A

A series padder and a parallel trimmer, adjusted at the low and high ends of the band respectively, bring the oscillator onto the required curve at three points, leaving small errors in between. Neutralising capacitors cancel feedback in an amplifier. De-emphasis belongs to FM. And supply bypassing is a decoupling measure. Each small adjustable component in a receiver has one specific job, and the alignment procedure sets them in a defined order.

Answer: B

The loop rectifies the detected carrier level and reduces the gain of the stages ahead of the detector, holding its input roughly constant over an enormous range of received strengths. The listener's volume control stays under manual control after the detector, deliberately. Oscillator amplitude is set by its own circuit. And the detector's load is a fixed component. Feeding a measured output back to an earlier gain is the general shape of every automatic level system.

Answer: C

Delaying the action lets weak signals enjoy full gain and applies reduction only once the signal exceeds a threshold, which improves sensitivity on weak stations without sacrificing the protection strong ones need. A long time constant is a separate design parameter concerning how fast the loop responds. Audio-only control would not protect the detector. And the oscillator carries no information about signal strength. The refinement is a good example of shaping a control law to the operating range that matters.

Answer: D

A diode envelope detector suffices because the transmitted carrier makes the envelope follow the message, and its cheapness is the whole commercial argument for full-carrier AM. A product detector is needed for suppressed-carrier signals. A phase-locked loop and a ratio detector are both angle-modulation devices. Matching the detector to what the modulation puts the information in is the quickest way to read any receiver diagram.

Answer: A

The rectified carrier gives a direct voltage proportional to received strength, which is exactly the signal the automatic gain control loop needs, and it also drives a tuning meter where one is fitted. The audio is the alternating component, separated by a coupling capacitor. The oscillator's bias is independent. And the intermediate frequency is fixed by the filters. One detector output thus serves two purposes, which is why it is split rather than simply coupled onward.

Answer: B

Squelch silences the receiver until the signal exceeds a threshold, so the operator is not subjected to noise between transmissions. It is essential in any receiver left monitoring a quiet channel. Modulation depth is a transmitter matter. Image rejection is provided by preselection. And oscillator stability comes from the circuit design or a control loop. Squelch is a convenience rather than a performance feature, but its absence makes a radio unusable for monitoring.

Answer: C

The intermediate frequency bandwidth caps the audio bandwidth, since the sidebands must pass to be detected, and narrowing it for selectivity is exactly what dulls the sound. That opposition between selectivity and fidelity is why some receivers offer a switchable bandwidth. Antenna efficiency affects signal strength. Oscillator phase noise is negligible at these bandwidths. And the gain control threshold governs weak-signal behaviour. Every one of the three figures of merit traces back to a filter choice.

Answer: D

The bandwidth must accommodate both sidebands, so

\[ f_{max} = \frac{8}{2} = 4 \ \mathrm{kHz} \]

giving 4 kHz, which is why AM broadcasting sounds bandwidth-limited. 8 kHz treats the whole passband as if it were one sideband. 16 kHz doubles rather than halves. And 2 kHz would follow from dividing twice. The factor of two between channel bandwidth and audio bandwidth runs through every AM calculation and is worth checking in both directions.

Answer: A

Narrowing the filter rejects the neighbours better and cuts the outer sidebands that carry the treble, so interference falls and the sound becomes muffled. That is the selectivity-against-fidelity trade in one control. Fidelity certainly does not improve. Sensitivity changes only slightly, through the reduced noise bandwidth. And describing the change as merely a lower noise floor omits the loss of audio content, which is what the listener notices first.

Answer: B

The chain runs 1, 2, 3, 4, 5, and every alternative breaks a dependency: the mixer cannot precede the antenna, the intermediate frequency amplifier cannot act before the translation that creates that frequency, and the detector cannot precede the filtering that defines the channel. Reading a block diagram against this order is the fastest way to locate a fault, since each stage's failure has a characteristic symptom.