Frequency Modulation Questions and Answers

Q: Which parameter does the message amplitude control in FM?

Message amplitude sets the deviation, and hence the index and the bandwidth. It does not touch the message frequency, which is a property of the sound itself, nor the carrier amplitude, which stays constant throughout. Channel spacing is a regulatory allocation the transmitter must fit inside. This single mapping - amplitude to deviation, frequency to rate - is the first thing to fix in mind about angle modulation.

Q: What does a limiter remove from a received FM signal?

It removes amplitude variation, which in FM carries only noise and fading, leaving the frequency information untouched. That is the mechanism behind FM's noise advantage. Removing frequency deviation would remove the message. Phase information is preserved, being the very thing the discriminator reads. And de-emphasis rather than limiting undoes pre-emphasis. The limiter is the one stage with no counterpart in an AM receiver.

Q: Where is de-emphasis applied?

De-emphasis follows the discriminator, restoring the audio balance and attenuating the noise that the discriminator's triangular spectrum piled up at high frequencies. Its partner, pre-emphasis, sits before the modulator at the transmitter. Placing it at intermediate frequency would filter the modulated signal rather than the audio. And the antenna is far from the audio path. The pairing is symmetrical and each half must match the other's time constant.

Q: Which effect makes FM sound clean when two stations share a channel?

The capture effect lets the stronger signal dominate and the weaker disappear, so the listener hears one programme cleanly instead of two mixed together. It is a benefit in broadcasting and a hazard in air traffic control, which is why aeronautical voice stays with AM. Threshold describes the collapse of quality at low signal. Pre-emphasis is an audio-shaping technique. And diagonal clipping is an AM detector fault.

Q: What is the FM broadcast channel spacing in most of the world?

FM broadcast channels are spaced 200 kHz apart, which accommodates Carson's 180 kHz plus a guard band. Nine kilohertz is medium-wave AM spacing. Twenty-five kilohertz is typical of narrowband FM land mobile channels. And a megahertz is far wider than any broadcast allocation needs. The figures are worth carrying because they anchor the abstract bandwidth arithmetic to real allocations.

Q: What deviation does FM broadcasting use?

The standard is plus or minus 75 kHz, giving a deviation ratio of five against a 15 kHz audio limit. Five kilohertz is narrowband FM for voice. Two hundred kilohertz is the channel spacing rather than the deviation. And fifteen kilohertz is the audio bandwidth. Confusing deviation with channel spacing is easy and consequential, since the two differ by more than a factor of two once Carson's rule is applied.

Q: Which detector is inherently amplitude-insensitive once locked?

A phase-locked loop tracks frequency and its output depends on the oscillator's control voltage rather than on input amplitude, so it is largely immune once captured. The three alternatives all convert frequency to amplitude at some stage and are therefore sensitive to it, needing a limiter ahead of them. That practical difference, along with its integrability, is why the loop dominates modern FM receivers.

Q: What does the deviation ratio describe?

It is the index evaluated at the highest message frequency, which is the worst case for bandwidth and therefore the figure a system is designed around. The lowest message frequency gives the largest index but the narrowest spectrum in Carson terms, so it is not the design case. Channel spacing is a regulatory quantity. And FM power ratios are not what the term describes. It is a system parameter where the modulation index is a per-tone one.

Q: Which is constant in an FM signal?

Only the amplitude is constant. The instantaneous frequency varies with the message, which is the definition. The bandwidth varies with deviation and hence with modulation depth. And the index varies with both deviation and message frequency, so it differs for every tone in a complex message. Identifying the one invariant is the quickest route to answering questions about power, amplifier class and limiter action.

Q: Which modulation is produced by differentiating the message before a frequency modulator?

Differentiating before an FM modulator gives phase modulation, since frequency is the derivative of phase and the two operations cancel. The dual arrangement, integrating before a phase modulator, gives FM. Amplitude modulation needs an entirely different modulator. Single sideband is an amplitude scheme. And no pre-processing simply doubles the index. The pair of equivalences is why the two schemes are studied as one family.

71.

Which parameter does the message amplitude control in FM?

A.Message frequency
B.Carrier amplitude
C.Occupied channel spacing
D.Deviation

Answer: D

Message amplitude sets the deviation, and hence the index and the bandwidth. It does not touch the message frequency, which is a property of the sound itself, nor the carrier amplitude, which stays constant throughout. Channel spacing is a regulatory allocation the transmitter must fit inside. This single mapping - amplitude to deviation, frequency to rate - is the first thing to fix in mind about angle modulation.

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72.

What does a limiter remove from a received FM signal?

A.Amplitude variation
B.Frequency deviation
C.Phase information
D.The pre-emphasis applied at the transmitter

Answer: A

It removes amplitude variation, which in FM carries only noise and fading, leaving the frequency information untouched. That is the mechanism behind FM's noise advantage. Removing frequency deviation would remove the message. Phase information is preserved, being the very thing the discriminator reads. And de-emphasis rather than limiting undoes pre-emphasis. The limiter is the one stage with no counterpart in an AM receiver.

Discuss in Forum

73.

Where is de-emphasis applied?

A.Across the antenna terminals
B.After the discriminator
C.Before the modulator
D.In the intermediate frequency amplifier

Answer: B

De-emphasis follows the discriminator, restoring the audio balance and attenuating the noise that the discriminator's triangular spectrum piled up at high frequencies. Its partner, pre-emphasis, sits before the modulator at the transmitter. Placing it at intermediate frequency would filter the modulated signal rather than the audio. And the antenna is far from the audio path. The pairing is symmetrical and each half must match the other's time constant.

Discuss in Forum

74.

Which effect makes FM sound clean when two stations share a channel?

A.Pre-emphasis
B.Diagonal clipping
C.Capture
D.Threshold

Answer: C

The capture effect lets the stronger signal dominate and the weaker disappear, so the listener hears one programme cleanly instead of two mixed together. It is a benefit in broadcasting and a hazard in air traffic control, which is why aeronautical voice stays with AM. Threshold describes the collapse of quality at low signal. Pre-emphasis is an audio-shaping technique. And diagonal clipping is an AM detector fault.

Discuss in Forum

75.

What is the FM broadcast channel spacing in most of the world?

A.9 kHz
B.25 kHz
C.1 MHz
D.200 kHz

Answer: D

FM broadcast channels are spaced 200 kHz apart, which accommodates Carson's 180 kHz plus a guard band. Nine kilohertz is medium-wave AM spacing. Twenty-five kilohertz is typical of narrowband FM land mobile channels. And a megahertz is far wider than any broadcast allocation needs. The figures are worth carrying because they anchor the abstract bandwidth arithmetic to real allocations.

Discuss in Forum

76.

What deviation does FM broadcasting use?

A.Plus or minus 75 kHz
B.Plus or minus 5 kHz
C.Plus or minus 200 kHz
D.Plus or minus 15 kHz

Answer: A

The standard is plus or minus 75 kHz, giving a deviation ratio of five against a 15 kHz audio limit. Five kilohertz is narrowband FM for voice. Two hundred kilohertz is the channel spacing rather than the deviation. And fifteen kilohertz is the audio bandwidth. Confusing deviation with channel spacing is easy and consequential, since the two differ by more than a factor of two once Carson's rule is applied.

Discuss in Forum

77.

Which detector is inherently amplitude-insensitive once locked?

A.The balanced slope detector
B.The phase-locked loop
C.The slope detector
D.The Foster-Seeley discriminator

Answer: B

A phase-locked loop tracks frequency and its output depends on the oscillator's control voltage rather than on input amplitude, so it is largely immune once captured. The three alternatives all convert frequency to amplitude at some stage and are therefore sensitive to it, needing a limiter ahead of them. That practical difference, along with its integrability, is why the loop dominates modern FM receivers.

Discuss in Forum

78.

What does the deviation ratio describe?

A.The ratio of deviation to channel spacing
B.The ratio of carrier power to sideband power
C.The index at the highest message frequency
D.The index at the lowest message frequency

Answer: C

It is the index evaluated at the highest message frequency, which is the worst case for bandwidth and therefore the figure a system is designed around. The lowest message frequency gives the largest index but the narrowest spectrum in Carson terms, so it is not the design case. Channel spacing is a regulatory quantity. And FM power ratios are not what the term describes. It is a system parameter where the modulation index is a per-tone one.

Discuss in Forum

79.

Which is constant in an FM signal?

A.Instantaneous frequency
B.Occupied bandwidth
C.Modulation index
D.Amplitude

Answer: D

Only the amplitude is constant. The instantaneous frequency varies with the message, which is the definition. The bandwidth varies with deviation and hence with modulation depth. And the index varies with both deviation and message frequency, so it differs for every tone in a complex message. Identifying the one invariant is the quickest route to answering questions about power, amplifier class and limiter action.

Discuss in Forum

80.

Which modulation is produced by differentiating the message before a frequency modulator?

A.Phase modulation
B.Amplitude modulation
C.Single sideband
D.Wideband frequency modulation of double the index

Answer: A

Differentiating before an FM modulator gives phase modulation, since frequency is the derivative of phase and the two operations cancel. The dual arrangement, integrating before a phase modulator, gives FM. Amplitude modulation needs an entirely different modulator. Single sideband is an amplitude scheme. And no pre-processing simply doubles the index. The pair of equivalences is why the two schemes are studied as one family.

Discuss in Forum

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