Phase Modulation Questions and Answers

Answer: A

With no message there is no deviation, so it is zero and the spectrum is the carrier alone, holding all the power. One radian and ninety degrees are particular modulated cases. And the quantity is well defined at zero, which is the reference condition for the Bessel tables. Starting from the unmodulated case and asking what modulation does to it is a reliable way to reason about any of the three families. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

Impulse interference is largely an amplitude disturbance, so the limiter strips it before detection and both FM and PM are far quieter than AM in electrically noisy surroundings. Claiming the reverse inverts the comparison. It certainly can be reduced, and limiting is the standard means. And the index affects the spectrum rather than the noise mechanism. This immunity is a large part of why angle modulation displaced AM for mobile use.

Answer: C

The approximation depends on the index being small, so that only the first sideband pair carries appreciable power. The message frequency being far below the carrier is assumed throughout and is not the criterion. Deviation exceeding message frequency is the wideband condition in FM, the opposite case. And bandwidth never equals deviation. Because the criterion is stated on the index, it applies to both schemes even though each computes its index differently.

Answer: D

Integrating the message turns a phase modulator into a frequency modulator, and differentiating turns a frequency modulator into a phase modulator. That pair of operations is the whole relationship between the schemes, and it is why the Armstrong transmitter can produce FM from a phase modulator. Limiting and amplification affect amplitude. Filtering and mixing translate and select frequencies. And squaring is a nonlinear operation used in carrier recovery. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: A

Insensitivity to level with strict proportionality to frequency points to phase modulation at small index, where the bandwidth is close to twice the message frequency and the index term contributes little. A large-index FM signal would respond strongly to level, since level sets deviation. AM's bandwidth follows the highest message frequency but would also be insensitive to level, so the constant-envelope check is needed to separate them. And SSB would follow the message bandwidth.

Answer: B

A constant envelope settles it immediately: amplitude schemes vary their envelope with the message and angle schemes do not. Sidebands appear in every modulated signal and so distinguish nothing. A rise in power with modulation would indicate AM, so observing no rise is informative, though the envelope test is more direct. And sideband counting depends on the index rather than on the family. One glance at an envelope is worth several calculations here.

Answer: C

The instantaneous phase is the carrier's phase plus a deviation proportional to the message amplitude, which is the definition of PM. Adding a term in the message's integral gives frequency modulation written as a phase, the dual expression. Multiplication would produce an unusable signal. And dividing by the index is meaningless. Writing the phase expression down correctly is the starting point for differentiating it to obtain the instantaneous frequency, which is how the equivalent deviation is derived.

Answer: D

An FM discriminator differentiates, so its output noise spectrum rises with frequency and the top of the audio band needs help; a PM detector's noise is flat and needs none. FM's index falls rather than rises with frequency. PM's bandwidth is not generally narrower. And PM tolerates shaping perfectly well; it simply has no reason for it. The whole pre-emphasis story follows from one property of one detector, which is worth knowing precisely.

Answer: A

PM needs no pre-emphasis, so applying de-emphasis alone would roll off the treble that was never boosted, giving dull audio. Noise would fall too, but at the cost of signal, so there is no free gain. Demodulation would still occur, since de-emphasis sits after the detector. And the index is a transmitter parameter unaffected by receiver shaping. The pair must be matched end to end, which is exactly why two regional pre-emphasis standards cause audible problems when mixed.

Answer: B

It is the peak phase deviation, and in PM that quantity is also the modulation index, which is why the two terms are used interchangeably here in a way they never are for FM. Peak frequency deviation is the FM parameter. Modulation depth is the AM index expressed as a percentage. And the deviation ratio is an FM system parameter defined at the highest message frequency. Each family has its own vocabulary and mixing them is the most reliable way to reach a wrong answer.