Phase Modulation Questions and Answers

Q: Which of these does phase modulation share with frequency modulation?

Both angle schemes have a constant envelope, which is what allows saturated amplification and makes limiting effective against amplitude noise. The frequency-dependent index, the triangular noise spectrum and the consequent need for pre-emphasis all belong to FM alone. Sorting the shared properties from the distinguishing ones is the most efficient way to hold the pair in mind, since the shared list explains why they are one family and the distinguishing list explains why both exist.

Q: What happens to a PM signal if the message contains a direct-current component?

A constant message gives a constant phase offset, which is unobservable at the receiver and therefore carries no information - one practical reason PM is not used where a message's mean value matters. In FM by contrast a constant message gives a permanent frequency offset, which is why FM cannot tolerate a direct component either, but for a different reason. The index stays finite and the envelope stays constant. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Q: A designer needs an angle-modulated link whose noise performance does not deteriorate at the top of the audio band, and prefers to avoid audio shaping networks. Which scheme suits?

Phase modulation has a flat detector noise spectrum, so it delivers even performance across the band with no shaping at all. Plain FM would be noisiest exactly where the designer cares. FM with pre-emphasis achieves a similar result but requires the very networks being avoided, along with matched de-emphasis at every receiver. And amplitude modulation offers no bandwidth-for-noise trade whatever. The requirement as stated points at one scheme uniquely.

Q: Which statement about PM and FM bandwidths at a fixed index is correct?

At a given index and message frequency the two bandwidths are computed identically, because the Bessel structure is shared; the schemes differ in how the index itself is arrived at. Neither is uniformly larger once the index is fixed. And PM bandwidth certainly does depend on message frequency, in direct proportion. The distinction to hold is that the difference lies in the index, not in the formula that turns an index into a bandwidth. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Q: Why is PM rarely used for analogue broadcasting?

In practice pre-emphasised FM matches PM's noise behaviour while keeping the simpler discriminator and the established infrastructure, so there was never a strong case for switching. PM carries music perfectly well in principle. Its bandwidth at a comparable index is similar rather than far greater. And high-power PM transmitters present no special difficulty. The answer is one of engineering history and installed base as much as physics. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Q: What is the practical role of phase modulation in analogue systems?

Its main analogue role is inside the Armstrong transmitter, where a phase modulator and an integrator together produce crystal-stable FM. It did not replace AM in broadcasting, aeronautical voice deliberately retains AM for its non-capture property, and medium wave uses AM for receiver simplicity. PM's own moment came with digital radio, where phase shift keying is ubiquitous. Knowing where each scheme actually appears prevents a good deal of confusion.

Q: Consider the following pairs of property and the scheme it describes. Constant envelope - Both FM and PM Flat detector noise - PM Index falls with message frequency - FM Requires linear amplification - Both FM and PM How many of the pairs given above are correctly matched?

The first three are right and the fourth is not, so three match. Neither angle scheme requires linear amplification, and that is one of their principal advantages: a constant envelope means the output stage can be driven into saturation. Linear amplification is demanded by the amplitude schemes, single sideband most acutely. The three correct pairs together are a compact summary of the whole topic.

31.

Which of these does phase modulation share with frequency modulation?

A.A triangular detector noise spectrum
B.A need for pre-emphasis
C.A constant envelope
D.A frequency-dependent index

Answer: C

Both angle schemes have a constant envelope, which is what allows saturated amplification and makes limiting effective against amplitude noise. The frequency-dependent index, the triangular noise spectrum and the consequent need for pre-emphasis all belong to FM alone. Sorting the shared properties from the distinguishing ones is the most efficient way to hold the pair in mind, since the shared list explains why they are one family and the distinguishing list explains why both exist.

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32.

What happens to a PM signal if the message contains a direct-current component?

A.The carrier frequency shifts permanently
B.The index becomes infinite
C.The envelope acquires a constant offset
D.A constant phase offset appears

Answer: D

A constant message gives a constant phase offset, which is unobservable at the receiver and therefore carries no information - one practical reason PM is not used where a message's mean value matters. In FM by contrast a constant message gives a permanent frequency offset, which is why FM cannot tolerate a direct component either, but for a different reason. The index stays finite and the envelope stays constant. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

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33.

In FM, what does a direct-current component in the message produce?

A.A permanent shift of the carrier frequency
B.A constant phase offset
C.No effect at all
D.An amplitude change proportional to the offset

Answer: A

A constant message deviates the frequency constantly, so the carrier sits off its nominal channel for as long as the offset persists, which is why FM audio chains are always alternating-coupled. The constant-phase result belongs to PM, where it is harmless but useless. Saying there is no effect ignores the definition of frequency deviation. And amplitude is unaffected in any angle modulation.

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34.

A designer needs an angle-modulated link whose noise performance does not deteriorate at the top of the audio band, and prefers to avoid audio shaping networks.

Which scheme suits?

A.Amplitude modulation
B.Phase modulation
C.Frequency modulation without pre-emphasis
D.Frequency modulation with pre-emphasis

Answer: B

Phase modulation has a flat detector noise spectrum, so it delivers even performance across the band with no shaping at all. Plain FM would be noisiest exactly where the designer cares. FM with pre-emphasis achieves a similar result but requires the very networks being avoided, along with matched de-emphasis at every receiver. And amplitude modulation offers no bandwidth-for-noise trade whatever. The requirement as stated points at one scheme uniquely.

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35.

An engineer measures an angle-modulated transmitter and finds its occupied bandwidth doubles when the test tone frequency doubles, the tone amplitude being unchanged.

Which scheme is it?

A.Amplitude modulation
B.Single sideband
C.Phase modulation
D.Frequency modulation

Answer: C

Strict proportionality between bandwidth and message frequency identifies phase modulation, whose index is frequency-independent so that the whole Carson expression scales. FM would widen only through the additive term, roughly from 24 to 28 kHz in a typical case rather than doubling. AM's bandwidth doubles too, but AM is not an angle modulation and would fail the constant-envelope test. And SSB would simply follow the message bandwidth. The measurement is a clean diagnostic.

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36.

Which statement about PM and FM bandwidths at a fixed index is correct?

A.PM's is always the larger
B.FM's is always the larger
C.PM's is independent of the message frequency
D.They are given by the same expression

Answer: D

At a given index and message frequency the two bandwidths are computed identically, because the Bessel structure is shared; the schemes differ in how the index itself is arrived at. Neither is uniformly larger once the index is fixed. And PM bandwidth certainly does depend on message frequency, in direct proportion. The distinction to hold is that the difference lies in the index, not in the formula that turns an index into a bandwidth. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

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37.

Why is PM rarely used for analogue broadcasting?

A.FM with pre-emphasis achieves the same result with simpler receivers
B.PM cannot carry music
C.PM occupies far more bandwidth
D.PM transmitters cannot be built at the power levels broadcasting requires

Answer: A

In practice pre-emphasised FM matches PM's noise behaviour while keeping the simpler discriminator and the established infrastructure, so there was never a strong case for switching. PM carries music perfectly well in principle. Its bandwidth at a comparable index is similar rather than far greater. And high-power PM transmitters present no special difficulty. The answer is one of engineering history and installed base as much as physics. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

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38.

What is the practical role of phase modulation in analogue systems?

A.The modulation used for medium-wave transmission
B.An intermediate step in generating FM
C.A replacement for AM in broadcasting
D.The standard scheme for aeronautical voice

Answer: B

Its main analogue role is inside the Armstrong transmitter, where a phase modulator and an integrator together produce crystal-stable FM. It did not replace AM in broadcasting, aeronautical voice deliberately retains AM for its non-capture property, and medium wave uses AM for receiver simplicity. PM's own moment came with digital radio, where phase shift keying is ubiquitous. Knowing where each scheme actually appears prevents a good deal of confusion.

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39.

Consider the following statements about phase modulation.

1. Its index is the peak phase deviation in radians

2. Its detector output noise is flat with frequency

3. Its bandwidth is independent of the message frequency

Which of the statements given above is or are correct?

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

Answer: C

The first two are correct. The third is wrong, and instructively so: it is the index that is independent of message frequency, while the bandwidth is directly proportional to it. Conflating the two is the commonest error on the topic, because the independence of the index is so often quoted without the consequence being drawn. Working one numerical example at two message frequencies fixes it permanently.

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40.

Consider the following pairs of property and the scheme it describes.

Constant envelope - Both FM and PM

Flat detector noise - PM

Index falls with message frequency - FM

Requires linear amplification - Both FM and PM

How many of the pairs given above are correctly matched?

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

Answer: D

The first three are right and the fourth is not, so three match. Neither angle scheme requires linear amplification, and that is one of their principal advantages: a constant envelope means the output stage can be driven into saturation. Linear amplification is demanded by the amplitude schemes, single sideband most acutely. The three correct pairs together are a compact summary of the whole topic.

Discuss in Forum

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