Phase Modulation Questions and Answers

Q: For the same peak deviation and the same message, PM differs from FM in that

The distinguishing feature is that the PM index is independent of message frequency, so its bandwidth grows in proportion to that frequency while FM's grows only through the additive term. Both schemes hold their power constant, both have constant envelopes, and both therefore permit efficient saturated amplification. Those three shared properties are exactly what makes them one family; the index behaviour is what separates them.

Q: Why does PM need no pre-emphasis?

Pre-emphasis exists to counter a rising noise spectrum, and PM does not have one: its detector output noise is flat, so lifting the treble at the transmitter would buy nothing and cost bandwidth. Bandwidth comparisons depend on the index and the message frequency rather than favouring PM generally. The PM index is constant with frequency rather than larger at the top. And detector noise figures are comparable; it is the spectral shape that differs.

Q: A simple phase modulator can be built from

Placing a voltage-variable capacitance in a resonant circuit through which the carrier passes shifts its phase in proportion to the message, giving PM over a small linear range. That small range is exactly why the Armstrong transmitter multiplies afterwards. A balanced modulator with a common input produces a squared signal rather than a phase shift. A limiter and integrator perform no modulation. And a square-law detector is a demodulator.

Q: Why is the linear range of a practical phase modulator limited?

A resonant circuit's phase response is approximately linear only near resonance, so large phase excursions distort, which caps the index a direct phase modulator can produce at a fraction of a radian. Frequency multiplication then raises it. Carrier amplitude does vary a little but is restored by limiting. Integration is a choice about which scheme you want rather than a requirement of the modulator. And the limit is one of phase angle rather than of message frequency.

Q: The phase ambiguity in PM arises because

There is no absolute phase; only differences are observable, so a receiver that recovers a carrier cannot tell which of several equivalent phases it has locked to. Differential encoding solves it in digital systems, and a transmitted reference solves it in analogue ones. The unit of the index is incidental. The infinite spectrum is a bandwidth matter. And sensitivity drift changes the index rather than creating an ambiguity of reference.

Q: How can an FM detector be used to demodulate PM?

An FM detector recovers the derivative of phase, so integrating its output restores the phase itself and hence the PM message. Integrating before the detector would alter the signal rather than the recovered message. Differentiating afterwards would double the error. And a limiter is needed in any case for amplitude noise but performs no conversion. The pairing of integrators and differentiators around the detector mirrors exactly what happens around the modulator. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

11.

For the same peak deviation and the same message, PM differs from FM in that

A.Its envelope is not constant
B.It requires linear amplification
C.Its index does not fall as the message frequency rises
D.Its total power varies with modulation

Answer: C

The distinguishing feature is that the PM index is independent of message frequency, so its bandwidth grows in proportion to that frequency while FM's grows only through the additive term. Both schemes hold their power constant, both have constant envelopes, and both therefore permit efficient saturated amplification. Those three shared properties are exactly what makes them one family; the index behaviour is what separates them.

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12.

The output noise spectrum of an ideal phase detector is

A.Triangular, rising with frequency
B.Falling with frequency
C.Concentrated near the carrier
D.Flat across the message band

Answer: D

A phase detector recovers phase directly, so its output noise is flat, unlike the frequency discriminator's triangular spectrum which rises with frequency. That difference is the whole reason FM broadcasting needs pre-emphasis and PM does not. A falling spectrum would require the opposite processing. And nothing concentrates near the carrier at the detector output, which is a baseband signal. The shape of the noise spectrum is the practical consequence of which angle quantity is being measured.

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13.

Why does PM need no pre-emphasis?

A.Its output noise is already flat across the band
B.Its bandwidth is narrower than FM's
C.Its index is larger at high frequencies
D.Its detector is inherently quieter than a discriminator

Answer: A

Pre-emphasis exists to counter a rising noise spectrum, and PM does not have one: its detector output noise is flat, so lifting the treble at the transmitter would buy nothing and cost bandwidth. Bandwidth comparisons depend on the index and the message frequency rather than favouring PM generally. The PM index is constant with frequency rather than larger at the top. And detector noise figures are comparable; it is the spectral shape that differs.

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14.

FM with pre-emphasis and PM become similar because

A.Both then occupy exactly the same bandwidth
B.Pre-emphasis makes FM's index frequency-independent at the top of the band
C.Pre-emphasis removes FM's sidebands
D.PM acquires a triangular noise spectrum when pre-emphasised

Answer: B

Boosting the high audio frequencies raises the deviation there, which counteracts the fall in FM index with frequency and pushes FM towards PM's behaviour over the pre-emphasised part of the band. The two schemes converge for that reason, which is a neat illustration that the difference between them is a matter of message shaping. Pre-emphasis does not remove sidebands, PM is not pre-emphasised, and the bandwidths remain different.

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15.

Which scheme gives better output signal-to-noise ratio at high modulating frequencies, before any shaping?

A.Both equally
B.Neither, since the comparison is undefined
C.PM
D.FM

Answer: C

PM does, because its flat noise spectrum does not penalise the top of the band the way FM's triangular one does. At low modulating frequencies FM has the advantage instead, which is the mirror image of the same fact. The comparison is perfectly well defined for a given deviation and message, and it is precisely what motivates pre-emphasis in FM broadcasting. Neither scheme is uniformly better, which is why the answer depends on where in the band you look.

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16.

A simple phase modulator can be built from

A.A balanced modulator with the message on both inputs
B.A limiter followed by an integrator
C.A square-law detector with a low-pass output
D.A varactor across a tuned circuit in the carrier path

Answer: D

Placing a voltage-variable capacitance in a resonant circuit through which the carrier passes shifts its phase in proportion to the message, giving PM over a small linear range. That small range is exactly why the Armstrong transmitter multiplies afterwards. A balanced modulator with a common input produces a squared signal rather than a phase shift. A limiter and integrator perform no modulation. And a square-law detector is a demodulator.

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17.

Why is the linear range of a practical phase modulator limited?

A.The phase shift of a tuned circuit is linear only over a small angle
B.The carrier amplitude falls as phase is shifted
C.The message must be integrated first
D.The modulator saturates once the message frequency rises past its passband

Answer: A

A resonant circuit's phase response is approximately linear only near resonance, so large phase excursions distort, which caps the index a direct phase modulator can produce at a fraction of a radian. Frequency multiplication then raises it. Carrier amplitude does vary a little but is restored by limiting. Integration is a choice about which scheme you want rather than a requirement of the modulator. And the limit is one of phase angle rather than of message frequency.

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18.

Detection of PM requires

A.A slope-tuned circuit and a diode
B.A coherent phase reference
C.An envelope detector
D.A limiter alone

Answer: B

Phase is meaningful only against a reference, so PM demodulation needs a coherent carrier the receiver can compare against, supplied by a phase-locked loop or recovered from a pilot. An envelope detector reads amplitude, which carries nothing here. A limiter removes amplitude noise but recovers no message by itself. And a slope detector plus diode is an FM arrangement, which will demodulate PM only after differentiation or with an acknowledged frequency-dependent error.

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19.

The phase ambiguity in PM arises because

A.The spectrum contains infinitely many sidebands
B.The modulator's sensitivity drifts with temperature
C.Absolute phase cannot be known without a reference
D.The index is measured in radians

Answer: C

There is no absolute phase; only differences are observable, so a receiver that recovers a carrier cannot tell which of several equivalent phases it has locked to. Differential encoding solves it in digital systems, and a transmitted reference solves it in analogue ones. The unit of the index is incidental. The infinite spectrum is a bandwidth matter. And sensitivity drift changes the index rather than creating an ambiguity of reference.

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20.

How can an FM detector be used to demodulate PM?

A.Precede it with an integrator
B.Follow it with a differentiator
C.Precede it with a limiter only
D.Follow it with an integrator

Answer: D

An FM detector recovers the derivative of phase, so integrating its output restores the phase itself and hence the PM message. Integrating before the detector would alter the signal rather than the recovered message. Differentiating afterwards would double the error. And a limiter is needed in any case for amplitude noise but performs no conversion. The pairing of integrators and differentiators around the detector mirrors exactly what happens around the modulator. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Discuss in Forum

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