AM Transmitters and Receivers Questions and Answers

Q: What is the consequence of a badly matched antenna for the output device?

A mismatch reflects power back into the stage, raising the voltage and current it must withstand and heating it, which is why protection circuits monitor the standing wave ratio and reduce power. The oscillator is buffered from the load, so no drift follows. Modulation depth is set upstream. And the filter continues to work, though the reflected energy passes through it twice. Following a fault through to the component that suffers is the practical value of the concept.

Q: Which stage determines an AM receiver's ultimate audio quality?

The filter width caps the audio bandwidth, so it sets the ceiling on quality no matter how good everything after it is. A better loudspeaker or a larger amplifier cannot restore sidebands the filter removed. The gain control's timing affects how level changes are handled rather than the bandwidth. This is why AM broadcasting sounds as it does even on excellent equipment, and why the switchable-bandwidth receiver was invented.

Q: What does a receiver's dial calibration depend on?

The dial reads the signal frequency, which is the oscillator frequency less the intermediate frequency, so correct tracking is what makes the calibration true across the band. Padder and trimmer adjustments exist precisely to align it at three points. Audio gain, detector characteristics and loudspeaker impedance have no bearing on which frequency the dial indicates. Alignment is the process of making a mechanical scale agree with an electrical relationship.

Q: Which receiver measurement requires a calibrated signal generator?

Sensitivity is quoted as the input level needed for a stated output signal-to-noise ratio, so a generator of known output is essential. Audio power needs a load and a voltmeter. Loudspeaker impedance needs a bridge. And idle current needs only an ammeter. Measurements that reference an input level always require a calibrated source, which is why the signal generator is the central instrument on a receiver bench.

Q: Which term describes a receiver's ability to reproduce the modulation faithfully?

Fidelity is the term, and it is limited chiefly by the intermediate filter width and by the detector's linearity. Selectivity is the rejection of other channels, sensitivity the ability to use weak signals, and image rejection the suppression of one particular interfering frequency. The three classical figures of merit pull against one another, which is why a receiver specification is a set of compromises rather than a list of maxima.

101.

Which change would reduce a transmitter's occupied bandwidth?

A.Retuning the antenna coupling network for a better match
B.Limiting the audio bandwidth further
C.Raising the modulation index towards unity
D.Increasing the carrier power at the output stage

Answer: B

Occupied bandwidth is twice the highest audio frequency, so filtering the audio more tightly is the only one of these that narrows it. Raising the index increases power and, if taken too far, spreads the spectrum through distortion. More carrier power widens nothing but wastes efficiency. And antenna matching affects radiated power rather than spectral width. Knowing which parameter controls bandwidth in each modulation family prevents a great many wrong answers.

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102.

What is the consequence of a badly matched antenna for the output device?

A.The modulation index falls in proportion
B.The harmonic filter becomes ineffective
C.Reflected power raises its dissipation
D.The carrier frequency drifts upward with load

Answer: C

A mismatch reflects power back into the stage, raising the voltage and current it must withstand and heating it, which is why protection circuits monitor the standing wave ratio and reduce power. The oscillator is buffered from the load, so no drift follows. Modulation depth is set upstream. And the filter continues to work, though the reflected energy passes through it twice. Following a fault through to the component that suffers is the practical value of the concept.

Discuss in Forum

103.

Which stage determines an AM receiver's ultimate audio quality?

A.The loudspeaker's frequency response curve
B.The audio amplifier's power output rating
C.The automatic gain control's attack time
D.The intermediate frequency filter width

Answer: D

The filter width caps the audio bandwidth, so it sets the ceiling on quality no matter how good everything after it is. A better loudspeaker or a larger amplifier cannot restore sidebands the filter removed. The gain control's timing affects how level changes are handled rather than the bandwidth. This is why AM broadcasting sounds as it does even on excellent equipment, and why the switchable-bandwidth receiver was invented.

Discuss in Forum

104.

What does a receiver's dial calibration depend on?

A.Correct oscillator tracking across the band
B.The audio amplifier's gain at mid-band
C.The detector diode's forward voltage drop
D.The loudspeaker's impedance at low frequency

Answer: A

The dial reads the signal frequency, which is the oscillator frequency less the intermediate frequency, so correct tracking is what makes the calibration true across the band. Padder and trimmer adjustments exist precisely to align it at three points. Audio gain, detector characteristics and loudspeaker impedance have no bearing on which frequency the dial indicates. Alignment is the process of making a mechanical scale agree with an electrical relationship.

Discuss in Forum

105.

Which receiver measurement requires a calibrated signal generator?

A.Supply current at idle with no signal present
B.Sensitivity in microvolts
C.Audio output power into a resistive load
D.Loudspeaker impedance at the crossover point

Answer: B

Sensitivity is quoted as the input level needed for a stated output signal-to-noise ratio, so a generator of known output is essential. Audio power needs a load and a voltmeter. Loudspeaker impedance needs a bridge. And idle current needs only an ammeter. Measurements that reference an input level always require a calibrated source, which is why the signal generator is the central instrument on a receiver bench.

Discuss in Forum

106.

Which term describes a receiver's ability to reproduce the modulation faithfully?

A.Sensitivity
B.Image rejection
C.Fidelity
D.Selectivity

Answer: C

Fidelity is the term, and it is limited chiefly by the intermediate filter width and by the detector's linearity. Selectivity is the rejection of other channels, sensitivity the ability to use weak signals, and image rejection the suppression of one particular interfering frequency. The three classical figures of merit pull against one another, which is why a receiver specification is a set of compromises rather than a list of maxima.

Discuss in Forum

AM Transmitters and Receivers Questions and Answers

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AM Transmitters and Receivers Questions and Answers

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