Phase Modulation Questions and Answers

Q: What is the effect of a phase modulator's non-linearity on the transmitted spectrum?

Non-linear phase shift distorts the modulation, and distortion products appear as additional sidebands outside the predicted set, spilling into neighbouring channels. It is the reason the linear range of the modulator caps the usable index. The carrier weakens and returns according to the Bessel functions rather than vanishing permanently. Bandwidth grows rather than narrows. And the envelope stays constant, since a phase modulator does not touch amplitude.

Q: Which stage would you add to make a frequency discriminator serve a PM receiver?

A discriminator recovers the derivative of phase, so an integrator after it restores the phase and hence the PM message. Differentiating would compound the error. A limiter is needed in any angle receiver for amplitude noise but performs no conversion. And de-emphasis would apply a correction PM does not need, since its detector noise is already flat. Adding the inverse of whatever the detector does is the general recipe for adapting one angle detector to the other scheme.

Q: What distinguishes narrowband PM from AM despite their similar spectra?

The spectra look alike in magnitude, but the sideband phases differ by a quarter cycle, which is what keeps the PM envelope constant while AM's varies. That single difference changes the detector required, the amplifier class permitted and the noise behaviour. Sideband count and spacing are the same in the narrowband case. And the power comparison depends on how the two are normalised rather than being intrinsically lower. Phase is what separates them, which is a reminder that a magnitude spectrum is an incomplete description.

Q: Which measurement would fail to distinguish narrowband PM from AM?

A magnitude spectrum shows three lines in both cases and hides the phase relationship that separates them. The envelope trace settles it at once, since AM's varies and PM's does not. A power measurement distinguishes them too, because AM's total rises with modulation while PM's does not. And limiting destroys an AM signal while leaving PM intact. Choosing a measurement that responds to the actual difference is the point, and it is why magnitude-only displays can mislead.

Q: What is the practical consequence of PM's flat noise spectrum for system design?

Because the noise does not rise with frequency, neither pre-emphasis nor de-emphasis is required, which removes a matched pair of networks and the standards question of which time constant to use. Deviation is set by the index wanted rather than by the noise shape. A limiter is still needed for amplitude noise. And no scheme can occupy less than the message bandwidth. The saving is real but modest, which is part of why FM kept its place in broadcasting.

Q: Which European and North American difference affects FM but not PM?

The pre-emphasis time constant differs, fifty microseconds in Europe against seventy-five in North America, and it is meaningless for PM, which uses no pre-emphasis at all. Channel spacing, deviation limits and audio bandwidth are regulatory choices that would apply to either scheme. The example is a useful reminder that some standards exist to correct a property of one modulation and simply do not arise for another.

Q: Which scheme's index can be read directly off a phase measurement?

For PM the index is the peak phase deviation, so measuring the phase excursion gives the index immediately. FM's index requires the deviation and the message frequency together. AM's index is an amplitude ratio. And SSB has no index in this sense. The directness of the PM measurement is a small practical advantage and a useful mnemonic for what the index actually means in each family.

Q: What would a vector or constellation display show for a PM signal?

Constant amplitude with varying phase traces an arc at fixed radius, which is the visual signature of angle modulation and of every phase keying scheme. Movement along a radius at fixed angle is amplitude modulation. A stationary point is an unmodulated carrier. And a figure through the origin would imply the amplitude passing through zero, which suppressed-carrier AM does. Reading a constellation display this way identifies the modulation family at a glance.

91.

What is the effect of a phase modulator's non-linearity on the transmitted spectrum?

A.The bandwidth narrows below Carson's estimate
B.The envelope begins to vary with the message
C.Extra sidebands appear beyond those the index predicts
D.The carrier vanishes permanently

Answer: C

Non-linear phase shift distorts the modulation, and distortion products appear as additional sidebands outside the predicted set, spilling into neighbouring channels. It is the reason the linear range of the modulator caps the usable index. The carrier weakens and returns according to the Bessel functions rather than vanishing permanently. Bandwidth grows rather than narrows. And the envelope stays constant, since a phase modulator does not touch amplitude.

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92.

Which stage would you add to make a frequency discriminator serve a PM receiver?

A.A differentiator after the detector
B.A limiter before the detector
C.A de-emphasis network after the detector
D.An integrator after the detector

Answer: D

A discriminator recovers the derivative of phase, so an integrator after it restores the phase and hence the PM message. Differentiating would compound the error. A limiter is needed in any angle receiver for amplitude noise but performs no conversion. And de-emphasis would apply a correction PM does not need, since its detector noise is already flat. Adding the inverse of whatever the detector does is the general recipe for adapting one angle detector to the other scheme.

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93.

In a PM system, what does doubling the modulator sensitivity achieve?

A.It doubles the index for the same message
B.It doubles the bandwidth exactly
C.It halves the occupied bandwidth
D.It leaves the index unchanged

Answer: A

Sensitivity and message amplitude multiply to give the index, so doubling either one doubles the index. The bandwidth grows but by less than a factor of two, because Carson's expression carries a unity term alongside the index. Halving the bandwidth would require the index to fall. And leaving the index unchanged would make the sensitivity meaningless. Distinguishing what scales exactly from what scales approximately is worth doing explicitly in every bandwidth question.

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94.

Which is true of narrowband PM's spectrum?

A.Its sidebands are spaced at twice the message frequency
B.It resembles an AM spectrum with a phase difference
C.It contains no sidebands at all
D.It is identical to a wideband PM spectrum

Answer: B

At small index only the first sideband pair matters, giving a spectrum that looks like AM's three lines but with the sidebands in phase quadrature rather than in phase. That similarity is why narrowband angle modulation occupies a channel comparable with AM's. Sidebands are certainly present. A wideband spectrum has many more significant pairs. And the spacing is the message frequency in every case.

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95.

What distinguishes narrowband PM from AM despite their similar spectra?

A.The spacing of the sidebands
B.The total transmitted power being lower
C.The sideband phases and the constant envelope
D.The number of sidebands present

Answer: C

The spectra look alike in magnitude, but the sideband phases differ by a quarter cycle, which is what keeps the PM envelope constant while AM's varies. That single difference changes the detector required, the amplifier class permitted and the noise behaviour. Sideband count and spacing are the same in the narrowband case. And the power comparison depends on how the two are normalised rather than being intrinsically lower. Phase is what separates them, which is a reminder that a magnitude spectrum is an incomplete description.

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96.

Which measurement would fail to distinguish narrowband PM from AM?

A.An oscilloscope trace of the envelope
B.A power measurement with and without modulation
C.A limiter test followed by detection
D.A magnitude spectrum alone

Answer: D

A magnitude spectrum shows three lines in both cases and hides the phase relationship that separates them. The envelope trace settles it at once, since AM's varies and PM's does not. A power measurement distinguishes them too, because AM's total rises with modulation while PM's does not. And limiting destroys an AM signal while leaving PM intact. Choosing a measurement that responds to the actual difference is the point, and it is why magnitude-only displays can mislead.

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97.

What is the practical consequence of PM's flat noise spectrum for system design?

A.No audio shaping networks are needed at either end
B.The transmitter can use a smaller deviation
C.The receiver needs no limiter
D.The bandwidth can be reduced below the message bandwidth

Answer: A

Because the noise does not rise with frequency, neither pre-emphasis nor de-emphasis is required, which removes a matched pair of networks and the standards question of which time constant to use. Deviation is set by the index wanted rather than by the noise shape. A limiter is still needed for amplitude noise. And no scheme can occupy less than the message bandwidth. The saving is real but modest, which is part of why FM kept its place in broadcasting.

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98.

Which European and North American difference affects FM but not PM?

A.The audio bandwidth transmitted
B.The pre-emphasis time constant
C.The channel spacing
D.The maximum deviation permitted

Answer: B

The pre-emphasis time constant differs, fifty microseconds in Europe against seventy-five in North America, and it is meaningless for PM, which uses no pre-emphasis at all. Channel spacing, deviation limits and audio bandwidth are regulatory choices that would apply to either scheme. The example is a useful reminder that some standards exist to correct a property of one modulation and simply do not arise for another.

Discuss in Forum

99.

Which scheme's index can be read directly off a phase measurement?

A.AM
B.SSB
C.PM
D.FM

Answer: C

For PM the index is the peak phase deviation, so measuring the phase excursion gives the index immediately. FM's index requires the deviation and the message frequency together. AM's index is an amplitude ratio. And SSB has no index in this sense. The directness of the PM measurement is a small practical advantage and a useful mnemonic for what the index actually means in each family.

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100.

What would a vector or constellation display show for a PM signal?

A.A point moving along a radius at constant angle
B.A stationary point
C.A figure of eight through the origin
D.A point moving along an arc at constant radius

Answer: D

Constant amplitude with varying phase traces an arc at fixed radius, which is the visual signature of angle modulation and of every phase keying scheme. Movement along a radius at fixed angle is amplitude modulation. A stationary point is an unmodulated carrier. And a figure through the origin would imply the amplitude passing through zero, which suppressed-carrier AM does. Reading a constellation display this way identifies the modulation family at a glance.

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