Frequency Modulation Questions and Answers

Answer: A

Counting crossings over a short interval gives the instantaneous frequency, which is the FM message. It works on a limited waveform and is inherently digital, which makes it attractive in integrated receivers, though it is noisier than a phase-locked loop at low signal levels. Envelope amplitude is deliberately discarded by the limiter. Phase deviation is not counted directly. And the index is a derived quantity rather than a measurement. The method illustrates that frequency can be recovered by timing as well as by tuned circuits.

Answer: B

Sensitivity is deviation per volt, usually in kilohertz per volt, and it is what converts an audio level into a frequency swing. Power is constant in FM, so power per volt is zero. Bandwidth follows from deviation rather than being specified directly. And index per hertz inverts the relationship, since a higher message frequency lowers the index for fixed deviation. Knowing the sensitivity is what lets a designer set the audio level for legal deviation.

Answer: C

Energy moves outward: successive Bessel orders rise, more sideband pairs carry appreciable power, and the carrier falls, passing through nulls. The spacing stays fixed at the message frequency throughout, since it is set by the modulating frequency rather than by the index. The carrier growing would violate constant total power. And a fixed number of pairs is the narrowband approximation rather than the general behaviour. Watching a spectrum analyser as deviation is increased makes the Bessel behaviour immediately concrete.

Answer: D

Sidebands sit at multiples of the message frequency either side of the carrier, so their spacing is that frequency regardless of the index. The index controls their amplitudes rather than their positions, which is a distinction the Bessel expansion makes precise. Deviation sets how far out significant energy extends. Half the message frequency and the index-scaled figure appear in no standard result. Positions from the message, amplitudes from the index, is the pairing to remember.

Answer: A

Without limiting, amplitude variation reaches the discriminator and the receiver loses both its noise immunity and its capture behaviour, sounding much like a poor AM set. Failed de-emphasis makes the audio shrill but leaves capture intact. An open antenna gives no signal at all rather than degraded behaviour. And a drifting oscillator mistunes the station, producing distortion as the signal slides off the discriminator's linear range. Reasoning from symptom to stage is the practical skill here.

Answer: B

Without de-emphasis the transmitter's pre-emphasis remains uncorrected, so the audio is too bright and the triangular noise spectrum is left unattenuated at the top of the band, giving a characteristic hiss. Muffled audio would follow from de-emphasis applied twice or from a lost treble path. General distortion points to the discriminator or mistuning. And silence indicates a broken signal path rather than a shaping network. The symmetry of the pre-emphasis and de-emphasis pair is what makes the symptom so characteristic.

Answer: C

The chain runs mixer, intermediate frequency amplifier, limiter, discriminator, de-emphasis: translate, amplify and filter, strip amplitude, convert frequency to voltage, then shape the audio. Every alternative order breaks a dependency - limiting before amplification leaves too little signal, discriminating before limiting admits amplitude noise, and de-emphasis before detection would shape a modulated signal rather than audio. Reading a block diagram against this order identifies a fault quickly.

Answer: D

Limiting requires a signal large enough to drive the limiter into its constant-output region, which is why the gain comes first; a weak signal would pass through with its amplitude variation intact. The limiter does not damage anything upstream of it. The amplifier handles constant-amplitude signals perfectly well, as the stages after limiting show. And the discriminator needs a strong, limited input. The ordering follows from the physics of the limiter rather than from convention.

Answer: A

The modulation index is the discriminator between the two regimes: much less than one is narrowband, giving a spectrum like AM's, while several or more is wideband, giving the Bessel spectrum and the noise advantage. Carrier frequency, power and detector type are all independent choices. Because the index depends on both deviation and message frequency, one system can be narrowband at its highest audio frequency and wideband at its lowest, which is why the deviation ratio is defined at the worst case.

Answer: B

FM's cost is bandwidth, several times what AM needs for the same audio, which is why it could not be accommodated on the medium-wave band. Its transmitter efficiency is higher, since class C is available. It is markedly less vulnerable to amplitude noise, which is its chief advantage. And it needs no linear amplification, unlike SSB. Listing the one genuine disadvantage against three advantages is a fair summary of why FM displaced AM wherever spectrum allowed.