Phase Modulation Questions and Answers

Answer: A

PM's equivalent deviation is index times message frequency, so it grows with the audio frequency while FM's deviation is fixed by amplitude alone. At the top of the band PM therefore deviates more and occupies more spectrum. Equality holds only at one particular frequency. And deviation is certainly not fixed in PM, which is the whole point of the comparison. This is the quantitative form of the qualitative difference between the two indices.

Answer: B

A rising boost of six decibels per octave is a differentiation, and differentiating the message before an FM modulator gives PM. That is exactly the shape that broadcast pre-emphasis approximates over part of the band, which is why pre-emphasised FM and PM converge. De-emphasis is the inverse and would move FM further from PM. A flat response leaves FM as FM. And a notch would simply remove content.

Answer: C

A first-order boost rises at six decibels per octave above its turnover frequency, which is the frequency response of a differentiator, so pre-emphasis differentiates the upper part of the audio band and leaves the lower part alone. That partial differentiation is why pre-emphasised FM sits between plain FM and PM in behaviour. It is certainly not integration, and it does not invert phase. Seeing shaping networks as approximate calculus operations makes their effect on modulation easy to predict.

Answer: D

The first two hold for both schemes. The third does not: only FM has the rising detector noise spectrum that pre-emphasis corrects, PM's being flat. Attributing pre-emphasis to the whole family is a natural but wrong generalisation, and it obscures the one genuine asymmetry in their noise behaviour. Sorting shared properties from scheme-specific ones is the most useful discipline in this topic.

Answer: A

Sweeping the message frequency and watching the bandwidth is decisive: PM scales in proportion, FM barely widens. Sideband spacing equals the message frequency in both. Carrier nulls occur in both, at indices determined differently. And total power is constant in both. Choosing a measurement that responds to the one asymmetry between the schemes, rather than to their many shared properties, is the essence of a good diagnostic. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

Answer: B

Because the amplitude carries nothing, distorting or clipping it destroys no information, so a saturated amplifier is entirely acceptable and the efficiency gain is free. Wide bandwidth is a cost rather than an enabler. Carrier strength varies with index and is irrelevant. And a large index is common but not necessary; narrowband FM tolerates nonlinearity just as well. This single property is why angle modulation dominates power-limited links.

Answer: C

A frequency-independent index belongs to PM alone; FM's index falls as the message frequency rises. The other three are shared and are what make the two schemes one family. Framing the question as what is not shared is a useful check on whether the family resemblance has been learned along with the one distinction, since most errors on this topic come from over-generalising in one direction or the other. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Answer: D

The Armstrong arrangement is the answer: integrate the audio, phase modulate a crystal oscillator at low level, then multiply up to reach the deviation wanted, so the crystal is never pulled and the centre frequency is as stable as the crystal itself. A reactance modulator gives large deviation directly but drifts. A balanced modulator with a filter produces single sideband. And collector modulation is high-level AM. This is the one place phase modulation earns its keep in analogue transmitters.

Answer: A

Each stage needs filtering to reject unwanted harmonics, and any phase noise or spurious content is multiplied along with the wanted signal, so a long chain becomes both bulky and noisy. That is the practical ceiling on the indirect method. Crystal stability improves the result rather than limiting the chain. Audio bandwidth plays no part in the radio-frequency chain. And the receiver's architecture is irrelevant to a transmitter design decision.

Answer: B

A doubler doubles the instantaneous phase, so the carrier frequency and the phase deviation both double while the audio rate is untouched. The index therefore doubles too. Scaling only one of the two would be physically impossible, since they are parts of one waveform. And the message frequency is a property of the modulating signal that no radio-frequency operation can alter. Being clear about what scales and what does not is the whole of understanding indirect generation.