Frequency Modulation Questions and Answers

Q: How does the FM spectrum differ from the PM spectrum for the same message amplitude?

In FM the index is deviation over message frequency, so it falls as the note rises; in PM the index is the phase deviation and is independent of message frequency. The consequence is that PM spreads more at high audio frequencies and less at low ones than FM does, which is exactly the difference pre-emphasis exploits. Both have sidebands. Neither has a constant bandwidth. And they are certainly not identical, though they are closely related through differentiation and integration.

Q: Which detector requires a limiter ahead of it for good performance?

The Foster-Seeley discriminator responds to amplitude as well as frequency, so any residual amplitude variation appears as noise at its output and a limiter is essential. The ratio detector provides its own limiting through the ratio it forms. A phase-locked loop is inherently insensitive to amplitude once it has captured. And a zero-crossing counter works on the limited waveform by construction. Knowing which detectors need help with amplitude is a practical distinction rather than a theoretical one.

Q: What is the main reason FM was adopted for high-fidelity broadcasting?

FM buys a large improvement in output signal-to-noise ratio by spending bandwidth, which is exactly the right trade at very high frequencies where spectrum is comparatively plentiful, and it is why FM broadcasting sounds so much cleaner than AM. Its receiver is more complex than AM's. Its channel is far wider. And while it tolerates tuning error better than SSB, that is not why it was chosen. The trade is the answer, and it is unavailable in the amplitude family.

Q: What varies in frequency modulation?

The carrier frequency varies while its amplitude stays constant, which is the definition and the source of every property FM has: constant power, class C amplification, limiter immunity to amplitude noise, and an infinite Bessel spectrum. Amplitude variation is AM. Varying both would be a hybrid nobody uses. And the message frequency is a property of the audio that no modulator alters. Starting from the definition rather than a formula makes the whole topic coherent.

Q: Which unit is frequency deviation quoted in?

Deviation is a frequency, so it is quoted in kilohertz, typically as plus or minus so many. Radians measure phase deviation, which is the corresponding parameter in phase modulation and is numerically the index there. Decibels express ratios. And volts measure the message amplitude that causes the deviation. Keeping the units straight prevents the common error of quoting an index as a frequency or a deviation as a bare number.

Q: What is the modulation index of an unmodulated FM carrier?

With no message there is no deviation, so the index is zero and the spectrum reduces to the carrier alone. It is the reference condition against which the Bessel amplitudes are tabulated. An index of one is a specific modulated case. An infinite index would need infinite deviation. And the quantity is perfectly well defined at zero, which is the point: the carrier holds all the power when nothing is being sent, and modulation redistributes it without changing the total.

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.

51.

How does the FM spectrum differ from the PM spectrum for the same message amplitude?

A.They are identical in every respect
B.FM's index falls with rising message frequency, PM's does not
C.PM has no sidebands at all
D.FM has a constant bandwidth and PM a varying one

Answer: B

In FM the index is deviation over message frequency, so it falls as the note rises; in PM the index is the phase deviation and is independent of message frequency. The consequence is that PM spreads more at high audio frequencies and less at low ones than FM does, which is exactly the difference pre-emphasis exploits. Both have sidebands. Neither has a constant bandwidth. And they are certainly not identical, though they are closely related through differentiation and integration.

Discuss in Forum

52.

Which detector requires a limiter ahead of it for good performance?

A.The phase-locked loop
B.The zero-crossing counter
C.The Foster-Seeley discriminator
D.The ratio detector

Answer: C

The Foster-Seeley discriminator responds to amplitude as well as frequency, so any residual amplitude variation appears as noise at its output and a limiter is essential. The ratio detector provides its own limiting through the ratio it forms. A phase-locked loop is inherently insensitive to amplitude once it has captured. And a zero-crossing counter works on the limited waveform by construction. Knowing which detectors need help with amplitude is a practical distinction rather than a theoretical one.

Discuss in Forum

53.

What is the main reason FM was adopted for high-fidelity broadcasting?

A.Its receiver simplicity
B.Its narrow channel requirement
C.Its tolerance of frequency error at the receiver
D.Its noise performance at the cost of bandwidth

Answer: D

FM buys a large improvement in output signal-to-noise ratio by spending bandwidth, which is exactly the right trade at very high frequencies where spectrum is comparatively plentiful, and it is why FM broadcasting sounds so much cleaner than AM. Its receiver is more complex than AM's. Its channel is far wider. And while it tolerates tuning error better than SSB, that is not why it was chosen. The trade is the answer, and it is unavailable in the amplitude family.

Discuss in Forum

54.

What varies in frequency modulation?

A.The carrier frequency
B.The carrier amplitude
C.The carrier amplitude and its phase together
D.The message frequency, in proportion to the carrier

Answer: A

The carrier frequency varies while its amplitude stays constant, which is the definition and the source of every property FM has: constant power, class C amplification, limiter immunity to amplitude noise, and an infinite Bessel spectrum. Amplitude variation is AM. Varying both would be a hybrid nobody uses. And the message frequency is a property of the audio that no modulator alters. Starting from the definition rather than a formula makes the whole topic coherent.

Discuss in Forum

55.

Which unit is frequency deviation quoted in?

A.Volts
B.Kilohertz
C.Radians
D.Decibels

Answer: B

Deviation is a frequency, so it is quoted in kilohertz, typically as plus or minus so many. Radians measure phase deviation, which is the corresponding parameter in phase modulation and is numerically the index there. Decibels express ratios. And volts measure the message amplitude that causes the deviation. Keeping the units straight prevents the common error of quoting an index as a frequency or a deviation as a bare number.

Discuss in Forum

56.

What is the modulation index of an unmodulated FM carrier?

A.Infinite
B.Undefined
C.Zero
D.One

Answer: C

With no message there is no deviation, so the index is zero and the spectrum reduces to the carrier alone. It is the reference condition against which the Bessel amplitudes are tabulated. An index of one is a specific modulated case. An infinite index would need infinite deviation. And the quantity is perfectly well defined at zero, which is the point: the carrier holds all the power when nothing is being sent, and modulation redistributes it without changing the total.

Discuss in Forum

57.

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.

Discuss in Forum

58.

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

59.

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

60.

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

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