Frequency Modulation Questions and Answers

Answer: A

The improvement rises with the modulation index, roughly with its square, which is the whole basis of the bandwidth-for-noise trade: spend more spectrum and get a better output signal-to-noise ratio for the same transmitted power. Carrier frequency is irrelevant to the trade. Wider audio bandwidth reduces the index for a given deviation and so reduces the benefit. And supply voltage affects power rather than the trade. This is the property that has no counterpart at all in amplitude modulation.

Answer: B

Wideband FM does: increasing the deviation widens the spectrum and improves the output signal-to-noise ratio, a trade with no equivalent in the amplitude family. None of the three amplitude schemes offers it, since their output noise depends on the received power and the message bandwidth alone. Recognising which schemes have this degree of freedom is what explains why FM was adopted for high-fidelity broadcasting while SSB was adopted where spectrum was scarce.

Answer: C

The message varies a capacitance or inductance in the oscillator's tuned circuit, so the oscillator frequency follows it: direct FM, simple and capable of large deviation. Supply variation is high-level amplitude modulation. Multiplication produces sidebands rather than frequency deviation. And integrating before a balanced modulator describes indirect FM through a phase modulator. Direct generation trades frequency stability for simplicity, which is why automatic frequency control usually accompanies it.

Answer: D

A varactor is a voltage-variable capacitance, so applying the message across it pulls the oscillator frequency in proportion. It is the standard modern reactance element. Rectification is what a detector diode does. Amplitude limiting is a receiver function. And a diode provides no gain. The varactor's simplicity is why direct FM became cheap, and its temperature sensitivity is why the oscillator needs frequency control around it.

Answer: A

Pulling a free-running oscillator makes its centre frequency drift with temperature and supply, so an automatic frequency control loop or a phase-locked synthesiser is required to hold it on channel. Bandwidth is set by deviation and message frequency, not by the method. Large deviation is direct FM's strength rather than its weakness. And FM needs no linear amplification at all, its envelope being constant. The stability problem is precisely what the indirect method avoids.

Answer: B

Armstrong's method phase modulates a crystal-controlled carrier at low level, then multiplies the frequency to raise the small deviation to the value needed. It gives excellent stability because the crystal is never pulled. Varactor pulling is the direct method it replaces. Amplitude modulation followed by limiting produces a constant-amplitude signal with no frequency deviation. And mixing translates frequency without modulating it.

Answer: C

Multiplication scales the carrier and the deviation by the same factor, while the message frequency is untouched, so the modulation index is multiplied too. That is exactly why the method works: a phase modulator can only produce a small index, and multiplication raises it to the value FM broadcasting needs. Multiplying only one of the two would be physically impossible. And the message frequency is a property of the audio, which no radio-frequency operation can alter.

Answer: D

Because frequency is the derivative of phase, integrating the message before a phase modulator yields frequency modulation, and differentiating before a frequency modulator yields phase modulation. That equivalence is why the two are treated as one family. Amplitude modulation requires a quite different modulator. Uniformly wider phase deviation would need a gain change rather than an integrator. And an integrator attenuates the high frequencies, so it is the opposite of pre-emphasis.

Answer: A

The loop mixes the modulated oscillator down against a crystal reference and feeds the error back to hold the centre frequency, with a time constant long enough not to cancel the wanted modulation. That last detail is the subtle part of the design. Deviation is set by the modulator's sensitivity. Power control is a separate loop. And no transmitter has access to a distant receiver's oscillator. The loop lets a drifting oscillator behave like a crystal-controlled one.

Answer: B

Tuning a resonant circuit off centre puts the signal on the slope of its response, so frequency deviation becomes amplitude variation that an ordinary envelope detector can recover. It is the simplest FM detector and the least linear. Phase comparison between two circuits is the Foster-Seeley discriminator. Zero-crossing counting is a digital-style detector. And locking an oscillator is the phase-locked loop method, which is the most linear of the four.