Phase Modulation Questions and Answers

Answer: C

The distinguishing feature is that the PM index is independent of message frequency, so its bandwidth grows in proportion to that frequency while FM's grows only through the additive term. Both schemes hold their power constant, both have constant envelopes, and both therefore permit efficient saturated amplification. Those three shared properties are exactly what makes them one family; the index behaviour is what separates them.

Answer: D

A phase detector recovers phase directly, so its output noise is flat, unlike the frequency discriminator's triangular spectrum which rises with frequency. That difference is the whole reason FM broadcasting needs pre-emphasis and PM does not. A falling spectrum would require the opposite processing. And nothing concentrates near the carrier at the detector output, which is a baseband signal. The shape of the noise spectrum is the practical consequence of which angle quantity is being measured.

Answer: A

Pre-emphasis exists to counter a rising noise spectrum, and PM does not have one: its detector output noise is flat, so lifting the treble at the transmitter would buy nothing and cost bandwidth. Bandwidth comparisons depend on the index and the message frequency rather than favouring PM generally. The PM index is constant with frequency rather than larger at the top. And detector noise figures are comparable; it is the spectral shape that differs.

Answer: B

Boosting the high audio frequencies raises the deviation there, which counteracts the fall in FM index with frequency and pushes FM towards PM's behaviour over the pre-emphasised part of the band. The two schemes converge for that reason, which is a neat illustration that the difference between them is a matter of message shaping. Pre-emphasis does not remove sidebands, PM is not pre-emphasised, and the bandwidths remain different.

Answer: C

PM does, because its flat noise spectrum does not penalise the top of the band the way FM's triangular one does. At low modulating frequencies FM has the advantage instead, which is the mirror image of the same fact. The comparison is perfectly well defined for a given deviation and message, and it is precisely what motivates pre-emphasis in FM broadcasting. Neither scheme is uniformly better, which is why the answer depends on where in the band you look.

Answer: D

Placing a voltage-variable capacitance in a resonant circuit through which the carrier passes shifts its phase in proportion to the message, giving PM over a small linear range. That small range is exactly why the Armstrong transmitter multiplies afterwards. A balanced modulator with a common input produces a squared signal rather than a phase shift. A limiter and integrator perform no modulation. And a square-law detector is a demodulator.

Answer: A

A resonant circuit's phase response is approximately linear only near resonance, so large phase excursions distort, which caps the index a direct phase modulator can produce at a fraction of a radian. Frequency multiplication then raises it. Carrier amplitude does vary a little but is restored by limiting. Integration is a choice about which scheme you want rather than a requirement of the modulator. And the limit is one of phase angle rather than of message frequency.

Answer: B

Phase is meaningful only against a reference, so PM demodulation needs a coherent carrier the receiver can compare against, supplied by a phase-locked loop or recovered from a pilot. An envelope detector reads amplitude, which carries nothing here. A limiter removes amplitude noise but recovers no message by itself. And a slope detector plus diode is an FM arrangement, which will demodulate PM only after differentiation or with an acknowledged frequency-dependent error.

Answer: C

There is no absolute phase; only differences are observable, so a receiver that recovers a carrier cannot tell which of several equivalent phases it has locked to. Differential encoding solves it in digital systems, and a transmitted reference solves it in analogue ones. The unit of the index is incidental. The infinite spectrum is a bandwidth matter. And sensitivity drift changes the index rather than creating an ambiguity of reference.

Answer: D

An FM detector recovers the derivative of phase, so integrating its output restores the phase itself and hence the PM message. Integrating before the detector would alter the signal rather than the recovered message. Differentiating afterwards would double the error. And a limiter is needed in any case for amplitude noise but performs no conversion. The pairing of integrators and differentiators around the detector mirrors exactly what happens around the modulator. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.