Frequency Modulation Questions and Answers

Answer: A

Channel economy decides it: with deviation of a few kilohertz a voice channel needs only twelve to sixteen kilohertz, so many users share a band. The noise improvement is modest, since it grows with the index, and that is the price paid. Envelope detection is impossible for any FM signal. And a limiter is still needed. Narrowband FM is a compromise chosen for spectrum efficiency rather than for fidelity.

Answer: B

Excess deviation widens the spectrum, so energy appears in the neighbouring channel and interferes with it, which is why deviation limiters are fitted ahead of the modulator. The carrier nulls at particular indices but returns; it does not vanish permanently. A receiver's limiter removes amplitude variation and can do nothing about excessive bandwidth. And the recovered audio level rises rather than falls, up to the point where the receiver's filter starts to clip the spectrum.

Answer: C

It limits the peak deviation, holding the occupied bandwidth inside the channel however loud the programme becomes, and it is placed after pre-emphasis because pre-emphasis itself increases deviation on bright material. Audio bandwidth is set by a separate low-pass filter. Carrier power is fixed by the output stage. And the pre-emphasis time constant is a design constant. Understanding the order of the audio chain - pre-emphasis, then limiting, then modulation - explains several exam answers at once.

Answer: D

The stronger signal suppressing the weaker is the capture effect, a direct consequence of limiting and frequency discrimination, and it is what makes FM sound clean in the presence of interference while making it unsuitable where simultaneous transmissions must both be heard. Threshold describes the abrupt collapse of quality at low signal strength. Aliasing belongs to sampling. And diagonal clipping is an AM envelope-detector fault.

Answer: A

The abrupt transition from excellent to unusable is the threshold effect: above it FM delivers its full noise advantage, below it the output collapses. AM by contrast degrades smoothly, which makes it more forgiving at the edge of coverage. Capture concerns two competing signals. Selective fading distorts rather than producing a sharp cliff. And pre-emphasis overload would distort loud treble regardless of range.

Answer: B

Single sideband needs the message bandwidth, AM twice it, narrowband FM a little more than twice, and wideband FM many times more, so the order is SSB, AM, narrowband FM, wideband FM. The reversed sequence inverts it and the others misplace a scheme. The ladder is worth memorising because it maps directly onto the noise performance ladder in the opposite direction, which is the central trade of analogue communication.

Answer: C

The first three are right and the fourth is not, so three match. It is AM whose bandwidth is independent of modulation depth, being always twice the highest message frequency; FM's bandwidth grows with deviation and hence with modulation depth, which is the property that makes its bandwidth-for-noise trade possible. Getting this one pairing the wrong way round is among the most common confusions between the two families.

Answer: D

The first two are correct. The third is true only as a narrowband approximation: the exact FM spectrum contains infinitely many sideband pairs with Bessel amplitudes, which is why bandwidth is defined by a convention such as Carson's rule rather than counted exactly. Treating the single-pair result as general is the standard mistake, and it leads to bandwidth estimates that are badly wrong for any wideband signal.

Answer: A

In phase modulation the corresponding parameter is peak phase deviation, measured in radians, and it is the modulation index directly. That is the neatest distinction between the two angle modulations: in PM the index depends on message amplitude alone, while in FM it also depends inversely on message frequency. Amplitude deviation belongs to AM. The deviation ratio and Carson's rule are FM system quantities rather than PM parameters.

Answer: B

In FM the index is deviation over message frequency, so it falls as the note rises; in PM the index is the phase deviation and is independent of message frequency. The consequence is that PM spreads more at high audio frequencies and less at low ones than FM does, which is exactly the difference pre-emphasis exploits. Both have sidebands. Neither has a constant bandwidth. And they are certainly not identical, though they are closely related through differentiation and integration.