Frequency Modulation Questions and Answers

Answer: C

Each sideband amplitude is \( J_n(\beta) \), a Bessel function of the first kind, evaluated at the modulation index, which is why FM spectra are tabulated rather than computed by hand. Simple powers of the index describe only the narrowband approximation. Square-wave coefficients arise from a different problem entirely. And Legendre polynomials belong to spherical geometry. The Bessel behaviour also explains the carrier nulls, which occur at particular indices and are used to calibrate deviation.

Answer: D

The Bessel function \( J_0 \) has zeros, the first near an index of 2.405, so a carrier null observed on a spectrum analyser fixes the deviation exactly for a known modulating frequency. It is the classic deviation measurement. The null is a spectral accident rather than an efficiency technique, and FM power is constant regardless. Bandwidth is unaffected. And a limiter is needed for noise rejection whatever the index. This is a good example of theory yielding a practical measurement.

Answer: A

FM power is constant, since the amplitude is constant and only the frequency varies: modulation redistributes power among the carrier and sidebands without changing the total. That is the sharpest contrast with AM, whose total power rises with modulation depth, and it is why an FM transmitter can use an efficient class C output stage. Neither the index nor the message frequency alters the total. Holding this fact firmly prevents a great many errors in FM power questions.

Answer: B

The constant envelope means amplitude distortion carries no penalty, so the output stage can run in the efficient class C mode that a varying-envelope signal like SSB forbids. That efficiency is one of FM's practical advantages alongside its noise performance. Bandwidth is wide rather than narrow. The carrier is present throughout. And the deviation being small relative to the carrier is true but irrelevant to amplifier class. Constant envelope is the property to remember, since it also underlies the limiter's usefulness.

Answer: C

Because the information is in the frequency, amplitude variation can be removed entirely, and with it most of the additive noise. That is the single largest reason FM outperforms AM on a noisy channel. It does not restrict deviation, which is set at the transmitter. Discriminator saturation is not the concern; unlimited amplitude variation would appear as noise at the output. And audio level is handled after detection. The limiter is what makes the constant-envelope property pay.

Answer: D

A frequency discriminator differentiates phase, so its output noise rises in proportion to frequency, giving the characteristic triangular spectrum and making the top of the audio band the noisiest part. That is exactly what pre-emphasis and de-emphasis are designed to correct. A flat spectrum is what an AM detector delivers. A falling spectrum would need the opposite processing. And nothing concentrates at the deviation frequency. The triangular shape is the reason FM broadcasting uses pre-emphasis at all.

Answer: A

Pre-emphasis boosts the treble before modulation, and matching de-emphasis at the receiver restores the balance while attenuating the noise that rose with frequency. The net effect is a useful improvement in signal-to-noise ratio at the top of the band. It raises rather than reduces high-frequency deviation, which is why it must be limited to avoid over-deviation on bright material. Compression is a separate loudness technique. And removing the direct component is unrelated.

Answer: B

Europe uses 50 microseconds and North America 75, a difference that matters because a receiver with the wrong de-emphasis sounds either dull or shrill. The corresponding turnover frequencies are about 3.2 kHz and 2.1 kHz respectively. 25 and 150 microseconds are not standard values for this application. The fact that two standards exist at all is a reminder that these figures are conventions agreed for interoperability rather than results derived from physics.

Answer: C

The limiter and discriminator together let the stronger signal dominate and the weaker vanish once the difference exceeds a few decibels, which gives clean reception in the presence of interference. Hearing both equally is the AM behaviour, and it is precisely why aeronautical communication retains AM: a second transmission must be noticed. Preferential amplification of the weaker signal happens in no receiver. And arrival order plays no part. Capture is a benefit for broadcasting and a hazard for air traffic control.

Answer: D

Above threshold FM gives a large signal-to-noise improvement; below it the output degrades abruptly rather than gracefully, which is why an FM link is either good or useless with little middle ground. AM degrades smoothly by comparison. Minimum deviation is a system design choice. Limiter action begins well below threshold. And over-deviation from pre-emphasis is a transmitter matter. The sharpness of the threshold is what makes FM link budgets less forgiving than AM ones.