Frequency Modulation Questions and Answers

Q: Consider the following pairs of property and the scheme it belongs to. Constant transmitted power - FM Capture effect - FM Envelope carries the message - AM Bandwidth independent of modulation depth - FM How many of the pairs given above are correctly matched?

The first three are right and the fourth is not, so three match. It is AM whose bandwidth is independent of modulation depth, being always twice the highest message frequency; FM's bandwidth grows with deviation and hence with modulation depth, which is the property that makes its bandwidth-for-noise trade possible. Getting this one pairing the wrong way round is among the most common confusions between the two families.

Q: What is the phase modulation counterpart of frequency deviation?

In phase modulation the corresponding parameter is peak phase deviation, measured in radians, and it is the modulation index directly. That is the neatest distinction between the two angle modulations: in PM the index depends on message amplitude alone, while in FM it also depends inversely on message frequency. Amplitude deviation belongs to AM. The deviation ratio and Carson's rule are FM system quantities rather than PM parameters.

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.

61.

Which is the correct order of increasing bandwidth for the same message?

A.SSB, narrowband FM, AM, wideband FM
B.SSB, AM, narrowband FM, wideband FM
C.Wideband FM, narrowband FM, AM, SSB
D.AM, SSB, wideband FM, narrowband FM

Answer: B

Single sideband needs the message bandwidth, AM twice it, narrowband FM a little more than twice, and wideband FM many times more, so the order is SSB, AM, narrowband FM, wideband FM. The reversed sequence inverts it and the others misplace a scheme. The ladder is worth memorising because it maps directly onto the noise performance ladder in the opposite direction, which is the central trade of analogue communication.

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62.

Consider the following pairs of property and the scheme it belongs to.

Constant transmitted power - FM

Capture effect - FM

Envelope carries the message - AM

Bandwidth independent of modulation depth - FM

How many of the pairs given above are correctly matched?

A.Only two
B.All four
C.Only three
D.Only one

Answer: C

The first three are right and the fourth is not, so three match. It is AM whose bandwidth is independent of modulation depth, being always twice the highest message frequency; FM's bandwidth grows with deviation and hence with modulation depth, which is the property that makes its bandwidth-for-noise trade possible. Getting this one pairing the wrong way round is among the most common confusions between the two families.

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63.

Consider the following statements about FM.

1. Its transmitted power is independent of the modulation

2. Its bandwidth grows with the modulation index

3. Its sidebands are limited to one pair either side of the carrier

Which of the statements given above is or are correct?

A.1 and 3 only
B.2 and 3 only
C.1, 2 and 3
D.1 and 2 only

Answer: D

The first two are correct. The third is true only as a narrowband approximation: the exact FM spectrum contains infinitely many sideband pairs with Bessel amplitudes, which is why bandwidth is defined by a convention such as Carson's rule rather than counted exactly. Treating the single-pair result as general is the standard mistake, and it leads to bandwidth estimates that are badly wrong for any wideband signal.

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64.

What is the phase modulation counterpart of frequency deviation?

A.Peak phase deviation
B.Peak amplitude deviation
C.The deviation ratio
D.Carson's bandwidth

Answer: A

In phase modulation the corresponding parameter is peak phase deviation, measured in radians, and it is the modulation index directly. That is the neatest distinction between the two angle modulations: in PM the index depends on message amplitude alone, while in FM it also depends inversely on message frequency. Amplitude deviation belongs to AM. The deviation ratio and Carson's rule are FM system quantities rather than PM parameters.

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65.

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

66.

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

67.

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.

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68.

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.

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69.

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.

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70.

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.

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