Phase Modulation Questions and Answers

Q: Which family does phase modulation belong to?

PM belongs to angle modulation, alongside FM, because it varies an argument of the carrier's cosine rather than its multiplier. Amplitude modulation varies the multiplier. Pulse modulation encodes samples in the properties of pulses. And spread spectrum deliberately widens a signal far beyond its information bandwidth. The family classification predicts the constant envelope, the Bessel spectrum and the limiter's usefulness all at once.

Q: What is the unit of phase modulation sensitivity?

Sensitivity is quoted in radians per volt, so multiplying by the peak message amplitude gives the index directly. Hertz per volt is the frequency modulator's figure. Volts per radian would describe a detector's output. And radians per second is an angular frequency rather than a sensitivity. Unit checking catches most errors in this topic before any arithmetic is attempted, because each family's parameters have distinct dimensions.

Q: Which quantity is the same for FM and PM at a given index and message frequency?

The occupied bandwidth is identical, because both schemes share the Bessel spectrum and Carson's rule; the difference between them lies entirely upstream, in how the index arises. Sensitivities are measured in different units. Sideband power distribution follows the same Bessel amplitudes and so is also shared, but the option as worded excludes the carrier and is therefore an incomplete statement. And the index calculation is precisely what differs.

Q: What is the effect on a PM spectrum of raising the message amplitude?

Raising the amplitude raises the index, so energy spreads into higher-order sidebands and more pairs matter, while their spacing stays at the message frequency throughout. The carrier weakens rather than strengthens, since total power is fixed. And the bandwidth remains proportional to message frequency however large the index. Separating what the index controls, namely amplitudes, from what the message frequency controls, namely positions, is the key to reading any angle-modulated spectrum.

Q: Which practical difficulty does a large PM index create at the modulator?

A practical modulator's phase shift is linear only over a fraction of a radian, so a large index distorts unless the index is built up by frequency multiplication afterwards. That constraint is exactly why the Armstrong transmitter exists in the form it does. Carrier amplitude is held constant by design. Bandwidth always depends on the index. And limiting at the receiver is unaffected by the transmitter's index. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

Q: Why does frequency multiplication raise the modulation index?

Multiplying the signal multiplies its instantaneous phase, and hence the phase deviation, by the same factor as the carrier frequency, so a small index at low frequency becomes a usable one at the operating frequency. Bandwidth grows rather than narrows, in proportion. The message is untouched, being already embedded in the carrier. And the carrier survives multiplication. This is the mechanism that makes indirect generation practical, and it applies to both angle schemes.

Q: Which is a disadvantage of the indirect method of angle-modulation generation?

The chain of multipliers, each followed by filtering to reject unwanted harmonics, makes the transmitter long and complex, which is the price of the crystal stability the method delivers. Instability is the direct method's weakness, not this one's. Large deviation is precisely what multiplication achieves. And no angle modulation needs linear amplification. Weighing complexity against stability is the actual decision a transmitter designer faces.

Q: What does a phase-locked loop provide when used as a PM detector?

Used with a wide enough loop bandwidth, the phase error inside the loop tracks the modulation and is the recovered message, which makes the loop a natural PM detector as well as an FM one, the difference being which node is tapped. An envelope is constant here. A frequency count would give the derivative. And rectification belongs to amplitude detection. Recognising that one circuit can serve both angle schemes, depending on where the output is taken, is a genuinely useful piece of practical knowledge.

Q: Which node of a phase-locked loop gives the FM message rather than the PM message?

The oscillator control voltage follows the instantaneous frequency and so delivers the FM message, while the phase detector's residual error follows the phase and delivers the PM message. That one loop serves both, read at two different points, is a neat consequence of frequency being the derivative of phase. The filter input and the reference are not signal outputs in this sense. Knowing which node carries which quantity is exactly the kind of detail that separates understanding a block diagram from memorising it.

Q: Which statement about the two angle modulations is false?

The false statement is the first: PM's bandwidth is directly proportional to the message frequency, since its index is what stays constant. The other three are all true and are the properties that make the two schemes one family. This is the same index-versus-bandwidth confusion that recurs throughout the topic, and it is worth testing yourself on the distinction until the two statements cannot be interchanged. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

71.

Which family does phase modulation belong to?

A.Pulse modulation
B.Spread spectrum
C.Angle modulation
D.Amplitude modulation

Answer: C

PM belongs to angle modulation, alongside FM, because it varies an argument of the carrier's cosine rather than its multiplier. Amplitude modulation varies the multiplier. Pulse modulation encodes samples in the properties of pulses. And spread spectrum deliberately widens a signal far beyond its information bandwidth. The family classification predicts the constant envelope, the Bessel spectrum and the limiter's usefulness all at once.

Discuss in Forum

72.

What is the unit of phase modulation sensitivity?

A.Hertz per volt
B.Volts per radian
C.Radians per second
D.Radians per volt

Answer: D

Sensitivity is quoted in radians per volt, so multiplying by the peak message amplitude gives the index directly. Hertz per volt is the frequency modulator's figure. Volts per radian would describe a detector's output. And radians per second is an angular frequency rather than a sensitivity. Unit checking catches most errors in this topic before any arithmetic is attempted, because each family's parameters have distinct dimensions.

Discuss in Forum

73.

Which quantity is the same for FM and PM at a given index and message frequency?

A.The occupied bandwidth
B.The modulator sensitivity
C.The transmitted power in the sidebands only
D.The way the index is calculated

Answer: A

The occupied bandwidth is identical, because both schemes share the Bessel spectrum and Carson's rule; the difference between them lies entirely upstream, in how the index arises. Sensitivities are measured in different units. Sideband power distribution follows the same Bessel amplitudes and so is also shared, but the option as worded excludes the carrier and is therefore an incomplete statement. And the index calculation is precisely what differs.

Discuss in Forum

74.

What is the effect on a PM spectrum of raising the message amplitude?

A.The bandwidth becomes independent of message frequency
B.More sideband pairs become significant
C.The sidebands move further apart
D.The carrier grows stronger

Answer: B

Raising the amplitude raises the index, so energy spreads into higher-order sidebands and more pairs matter, while their spacing stays at the message frequency throughout. The carrier weakens rather than strengthens, since total power is fixed. And the bandwidth remains proportional to message frequency however large the index. Separating what the index controls, namely amplitudes, from what the message frequency controls, namely positions, is the key to reading any angle-modulated spectrum.

Discuss in Forum

75.

Which practical difficulty does a large PM index create at the modulator?

A.The bandwidth becomes independent of the index
B.The receiver can no longer limit the signal
C.The phase shift ceases to be linear with the message
D.The carrier amplitude collapses

Answer: C

A practical modulator's phase shift is linear only over a fraction of a radian, so a large index distorts unless the index is built up by frequency multiplication afterwards. That constraint is exactly why the Armstrong transmitter exists in the form it does. Carrier amplitude is held constant by design. Bandwidth always depends on the index. And limiting at the receiver is unaffected by the transmitter's index. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

Discuss in Forum

76.

Why does frequency multiplication raise the modulation index?

A.It narrows the bandwidth
B.It amplifies the message before modulation
C.It removes the carrier
D.It scales the phase deviation along with the frequency

Answer: D

Multiplying the signal multiplies its instantaneous phase, and hence the phase deviation, by the same factor as the carrier frequency, so a small index at low frequency becomes a usable one at the operating frequency. Bandwidth grows rather than narrows, in proportion. The message is untouched, being already embedded in the carrier. And the carrier survives multiplication. This is the mechanism that makes indirect generation practical, and it applies to both angle schemes.

Discuss in Forum

77.

Which is a disadvantage of the indirect method of angle-modulation generation?

A.A long chain of multipliers and filters is needed
B.The carrier frequency is unstable
C.Large deviation is unattainable in principle
D.Linear amplification is required

Answer: A

The chain of multipliers, each followed by filtering to reject unwanted harmonics, makes the transmitter long and complex, which is the price of the crystal stability the method delivers. Instability is the direct method's weakness, not this one's. Large deviation is precisely what multiplication achieves. And no angle modulation needs linear amplification. Weighing complexity against stability is the actual decision a transmitter designer faces.

Discuss in Forum

78.

What does a phase-locked loop provide when used as a PM detector?

A.A rectified version of the message
B.A phase error signal proportional to the message
C.An envelope proportional to the message
D.A frequency count proportional to the message

Answer: B

Used with a wide enough loop bandwidth, the phase error inside the loop tracks the modulation and is the recovered message, which makes the loop a natural PM detector as well as an FM one, the difference being which node is tapped. An envelope is constant here. A frequency count would give the derivative. And rectification belongs to amplitude detection. Recognising that one circuit can serve both angle schemes, depending on where the output is taken, is a genuinely useful piece of practical knowledge.

Discuss in Forum

79.

Which node of a phase-locked loop gives the FM message rather than the PM message?

A.The loop filter input
B.The reference input
C.The oscillator control voltage
D.The phase detector output

Answer: C

The oscillator control voltage follows the instantaneous frequency and so delivers the FM message, while the phase detector's residual error follows the phase and delivers the PM message. That one loop serves both, read at two different points, is a neat consequence of frequency being the derivative of phase. The filter input and the reference are not signal outputs in this sense. Knowing which node carries which quantity is exactly the kind of detail that separates understanding a block diagram from memorising it.

Discuss in Forum

80.

Which statement about the two angle modulations is false?

A.Both have constant envelopes
B.Both share the Bessel spectrum
C.Both permit saturated amplification of the transmitted signal
D.PM's bandwidth is independent of the message frequency

Answer: D

The false statement is the first: PM's bandwidth is directly proportional to the message frequency, since its index is what stays constant. The other three are all true and are the properties that make the two schemes one family. This is the same index-versus-bandwidth confusion that recurs throughout the topic, and it is worth testing yourself on the distinction until the two statements cannot be interchanged. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

Loading…

Topics

Other Subjects

Tags: Phase Modulation Questions and Answers || Phase Modulation MCQ Questions and Answers || Phase Modulation GK Questions and Answers || Phase Modulation GK MCQ Questions || Phase Modulation Multiple Choice Questions and Answers || Phase Modulation GK || GK on Phase Modulation || Analog Communication Questions and Answers || Analog Communication MCQ Questions and Answers || Analog Communication GK Questions and Answers || GK on Analog Communication

Phase Modulation Questions and Answers

Topics

Other Subjects

About Us

Stay informed about all important Dates & Notifications