FM Transmitters and Receivers Questions and Answers

Q: What is the effect of transmitting the pilot at too low a level?

The pilot's job is to be detected, so too low a level leaves receivers unable to recognise a stereo transmission and they stay in mono. The difference signal is still transmitted, and indeed still occupies its deviation, so the power is spent for no benefit. Bandwidth is unaffected by the pilot's level. And the sum signal is independent. Faults that waste transmitted capacity without producing an audible error are the ones most easily overlooked in monitoring.

Q: Which measurement would verify a stereo transmission end to end?

Channel separation, measured by feeding one channel only and checking how little appears in the other, tests the whole chain from matrix through subcarrier to decoder. Antenna current does not change with FM modulation at all. Standing wave ratio tests the antenna. And modulator input level tests only the audio front end. An end-to-end measurement must exercise every stage in between, which separation does and none of the alternatives do.

Q: Which is the best single summary of an FM receiver's advantage over an AM one?

Because the information is in the angle, everything the noise does to the amplitude can be thrown away by a limiter, which is the mechanism behind both the noise advantage and the capture effect. FM occupies far more spectrum, not less. Its detector is more complex than a diode. And tuning tolerance is comparable rather than better. Stating an advantage together with the mechanism that produces it is what distinguishes understanding from recitation.

Q: What does a transmitter's exciter produce?

The exciter delivers a finished modulated signal at a few watts, which the power amplifier then raises to the transmitted level. Separating the two makes maintenance and redundancy practical, since a spare exciter can be switched in without touching the power stage. An unmodulated carrier is what the oscillator alone gives. The composite is the exciter's input. And bias is a supply matter. Equipment vocabulary repays learning because it maps onto how systems are actually maintained.

Q: Why is a spare exciter often fitted to a broadcast transmitter?

Redundancy at the low-power end is cheap and covers the most complex part of the chain, so a standby exciter restores service in seconds. Running two in parallel would not add power, since they feed one amplifier. The frequency reference is internal to each. And two programmes would need two transmitters and two channels. Placing redundancy where complexity is high and cost is low is a general principle of broadcast engineering.

Q: Which part of an FM receiver sets the ultimate signal-to-noise ratio on a strong signal?

On a strong signal the noise is dominated by the demodulation gain, which the deviation sets, together with the improvement de-emphasis contributes. The audio amplifier's noise is negligible by comparison. Antenna aperture affects the signal delivered rather than the ultimate ratio once the signal is ample. And extra filters improve selectivity without changing the noise floor. Which quantity binds depends on the signal level, which is why FM specifications quote sensitivity and ultimate ratio separately.

Q: What happens to an FM signal passed through a hard limiter and then a narrow filter?

Limiting flattens amplitude and filtering removes the harmonics that flattening created, leaving a clean constant-envelope signal carrying the same modulation. That is exactly why nonlinear amplification is harmless to angle modulation and why class C stages can be cascaded. Conversion to amplitude modulation is what a slope detector does deliberately. Deviation is unchanged. And the signal survives precisely because the information is not in the amplitude.

Q: Which fault would make a receiver's audio distort only on loud passages?

Loud passages deviate furthest, so a filter that clips the outer sidebands distorts them while quiet passages pass cleanly. That level dependence is the signature of a bandwidth problem, at the receiver or in the transmitter's deviation. A pilot fault gives mono. A muting fault gives silence or noise. And excessive audio gain would clip at the output, which is also level dependent but would not respond to retuning the station. Noting whether a symptom follows level, frequency or tuning is what separates the possibilities.

81.

Which change at the transmitter would improve stereo coverage?

A.A wider audio bandwidth
B.A longer de-emphasis time constant
C.A lower pilot level
D.More transmitted power

Answer: D

Since stereo is limited by signal-to-noise ratio in the difference channel, more power extends the area where that channel is usable. A wider audio bandwidth would push the difference sidebands even higher into the noisy region. De-emphasis is set by standard and belongs to the receiver anyway. And a lower pilot would make locking harder. Improving a coverage limit means addressing whichever quantity actually binds, which here is received signal strength.

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82.

What is the effect of transmitting the pilot at too low a level?

A.Receivers fail to switch to stereo
B.The difference signal is lost entirely at the transmitter
C.The occupied bandwidth exceeds the channel
D.The sum signal is attenuated in the receiver

Answer: A

The pilot's job is to be detected, so too low a level leaves receivers unable to recognise a stereo transmission and they stay in mono. The difference signal is still transmitted, and indeed still occupies its deviation, so the power is spent for no benefit. Bandwidth is unaffected by the pilot's level. And the sum signal is independent. Faults that waste transmitted capacity without producing an audible error are the ones most easily overlooked in monitoring.

Discuss in Forum

83.

Which measurement would verify a stereo transmission end to end?

A.The audio level at the modulator input
B.Separation between the two channels
C.The antenna current with modulation applied
D.The standing wave ratio at the feed point

Answer: B

Channel separation, measured by feeding one channel only and checking how little appears in the other, tests the whole chain from matrix through subcarrier to decoder. Antenna current does not change with FM modulation at all. Standing wave ratio tests the antenna. And modulator input level tests only the audio front end. An end-to-end measurement must exercise every stage in between, which separation does and none of the alternatives do.

Discuss in Forum

84.

Which is the best single summary of an FM receiver's advantage over an AM one?

A.Its detector is simpler and cheaper to manufacture
B.It tolerates larger frequency errors in tuning
C.Amplitude noise can be discarded before detection
D.It occupies less spectrum per channel

Answer: C

Because the information is in the angle, everything the noise does to the amplitude can be thrown away by a limiter, which is the mechanism behind both the noise advantage and the capture effect. FM occupies far more spectrum, not less. Its detector is more complex than a diode. And tuning tolerance is comparable rather than better. Stating an advantage together with the mechanism that produces it is what distinguishes understanding from recitation.

Discuss in Forum

85.

Which instrument measures a receiver's capture ratio?

A.A single generator swept across the channel width
B.A spectrum analyser connected to the antenna terminals
C.An audio distortion meter at the loudspeaker output
D.Two signal generators on one frequency

Answer: D

Capture is a contest between two signals, so the measurement needs two generators on the same frequency, one level adjusted until the other is suppressed. A single generator cannot create the contest. A spectrum analyser observes rather than tests. And a distortion meter measures the result without providing the stimulus. Any specification describing behaviour between two signals requires two sources, which is a useful check on whether a proposed test can measure what it claims.

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86.

What does a transmitter's exciter produce?

A.A complete modulated signal at low power
B.An unmodulated carrier for the modulator to act upon
C.The audio composite before it reaches the modulator
D.The direct voltage that biases the output stage

Answer: A

The exciter delivers a finished modulated signal at a few watts, which the power amplifier then raises to the transmitted level. Separating the two makes maintenance and redundancy practical, since a spare exciter can be switched in without touching the power stage. An unmodulated carrier is what the oscillator alone gives. The composite is the exciter's input. And bias is a supply matter. Equipment vocabulary repays learning because it maps onto how systems are actually maintained.

Discuss in Forum

87.

Why is a spare exciter often fitted to a broadcast transmitter?

A.It allows two programmes to be broadcast at once
B.It can be switched in without interrupting service
C.It doubles the transmitted power when both run together
D.It provides the reference for the automatic frequency control

Answer: B

Redundancy at the low-power end is cheap and covers the most complex part of the chain, so a standby exciter restores service in seconds. Running two in parallel would not add power, since they feed one amplifier. The frequency reference is internal to each. And two programmes would need two transmitters and two channels. Placing redundancy where complexity is high and cost is low is a general principle of broadcast engineering.

Discuss in Forum

88.

Which part of an FM receiver sets the ultimate signal-to-noise ratio on a strong signal?

A.The antenna's physical aperture at the receiving site
B.The number of ceramic filters in the intermediate chain
C.The deviation and the de-emphasis together
D.The audio amplifier's own noise floor

Answer: C

On a strong signal the noise is dominated by the demodulation gain, which the deviation sets, together with the improvement de-emphasis contributes. The audio amplifier's noise is negligible by comparison. Antenna aperture affects the signal delivered rather than the ultimate ratio once the signal is ample. And extra filters improve selectivity without changing the noise floor. Which quantity binds depends on the signal level, which is why FM specifications quote sensitivity and ultimate ratio separately.

Discuss in Forum

89.

What happens to an FM signal passed through a hard limiter and then a narrow filter?

A.The frequency modulation is converted to amplitude modulation
B.The deviation is halved along with the bandwidth
C.The signal is destroyed because limiting is nonlinear
D.The constant envelope is restored with the modulation intact

Answer: D

Limiting flattens amplitude and filtering removes the harmonics that flattening created, leaving a clean constant-envelope signal carrying the same modulation. That is exactly why nonlinear amplification is harmless to angle modulation and why class C stages can be cascaded. Conversion to amplitude modulation is what a slope detector does deliberately. Deviation is unchanged. And the signal survives precisely because the information is not in the amplitude.

Discuss in Forum

90.

Which fault would make a receiver's audio distort only on loud passages?

A.An intermediate filter narrower than the deviation requires
B.A failed pilot detector in the stereo decoder
C.An open-circuit muting transistor in the audio path
D.Excessive gain in the audio output stage

Answer: A

Loud passages deviate furthest, so a filter that clips the outer sidebands distorts them while quiet passages pass cleanly. That level dependence is the signature of a bandwidth problem, at the receiver or in the transmitter's deviation. A pilot fault gives mono. A muting fault gives silence or noise. And excessive audio gain would clip at the output, which is also level dependent but would not respond to retuning the station. Noting whether a symptom follows level, frequency or tuning is what separates the possibilities.

Discuss in Forum

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