Phase Modulation Questions and Answers

Answer: A

DSB-SC reverses phase as the message crosses zero, so its locus passes through the origin where the envelope vanishes. PM stays on a circle of constant radius. A stationary point is an unmodulated carrier. And confinement to one quadrant would describe a restricted phase excursion rather than any standard scheme. The three loci - radius, circle and line through the origin - cover AM, angle modulation and suppressed-carrier AM respectively.

Answer: B

If the locus is a circle then the radius carries nothing and can be forced to a constant without losing information, which is precisely what a limiter does - and any noise that perturbed the radius is discarded with it. Noise certainly reaches the receiver; the point is that part of it can be thrown away. Limiting does not add power. And the radius carrying the message would describe amplitude modulation, where limiting would be destructive.

Answer: C

Limiting removes the radial perturbation, leaving the tangential component, which displaces the phase and therefore does appear at the detector output. That residual is what sets the ultimate noise performance and what the threshold effect governs. Claiming none remains would imply noise-free reception at any signal level, which the threshold phenomenon plainly contradicts. The amplitude component is what was discarded. And the frequency of the noise relative to the carrier is not the distinction being drawn.

Answer: D

FM and PM share a limiter, a constant-envelope input and an angle detector, differing only in whether an integration is applied to the recovered signal, so their receivers are nearly the same equipment. AM's diode detector has nothing in common with either. And SSB needs an accurately reinserted carrier and a product detector, which is a different architecture again. Grouping schemes by receiver architecture rather than by name is often the more useful classification in practice.

Answer: A

Everything else follows from it: because FM's index carries a division by the message frequency and PM's does not, their bandwidths scale differently, their detector noise spectra differ, and pre-emphasis becomes necessary for one and pointless for the other. Both have constant envelopes. Both have infinitely many sidebands. And FM is routinely generated indirectly, by the Armstrong method. Tracing a family of differences back to one root is the most compact way to hold this topic.