Phase Modulation Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.