Frequency Modulation Questions and Answers

Q: What is the main reason two-way radio uses narrowband rather than wideband FM?

Narrowband FM is chosen for channel economy: many users must share a limited allocation, and a sixteen-kilohertz channel serves where 180 would not. Its noise performance is worse than wideband FM's, which is the price paid. Receiver complexity is essentially the same. And it is not compatible with AM receivers, which cannot demodulate any angle modulation. The choice is a spectrum decision, exactly parallel to the choice of SSB on the high-frequency bands.

Q: Which quantity must a receiver's intermediate frequency bandwidth match?

It must pass the occupied bandwidth given by Carson's rule, since clipping the outer sidebands distorts the recovered audio, while admitting more than necessary lets in extra noise and adjacent-channel energy. Matching the deviation alone would be too narrow. The audio bandwidth is far narrower still and belongs after detection. And matching the channel spacing exactly would admit part of the neighbours. Getting this filter right is the single biggest influence on an FM receiver's performance.

Q: What is the effect of an intermediate frequency filter narrower than Carson's bandwidth?

Clipping the sidebands distorts the frequency variation the discriminator reads, so the recovered audio is distorted, worst on loud passages where the deviation is greatest. Capture performance is not improved by mutilating the wanted signal. The carrier sits in the middle of the passband and survives. And nothing a receiver does changes the transmitted deviation. This is the receiver-side counterpart of over-deviation at the transmitter, and both show up as distortion on peaks.

Q: What would you expect of an FM signal received at exactly the threshold level?

At threshold the output is noticeably noisy but still usable; a little above it the noise falls away rapidly and a little below it the output collapses. That sharp transition is what the threshold effect names, and it makes FM coverage boundaries abrupt compared with AM's gradual fade. Perfect reception needs a margin above threshold. Silence comes well below it. And adjacent-channel reception is a selectivity failure rather than a threshold phenomenon.

Q: Which is the best summary of the FM bargain?

FM spends spectrum and buys output signal-to-noise ratio, an option the amplitude family does not offer at all, and it does so while keeping a constant envelope that permits efficient amplification. The other formulations mismatch the currencies: bandwidth is what is spent rather than gained, and while receiver complexity does rise, it is not the principal cost. Naming the bargain in one sentence is the most useful single thing to carry away from the topic.

Q: An engineer must halve an FM channel's width without changing the audio bandwidth. The available move is

Bandwidth is dominated by the deviation term, so reducing the deviation narrows the channel and lowers the index, which costs noise performance - the trade run in reverse. Lowering the audio level does reduce deviation, but only for that programme and not as a system change; the deviation limit is what defines the channel. Carrier frequency does not affect bandwidth. And pre-emphasis and de-emphasis are a matched pair, not alternatives.

91.

What is the main reason two-way radio uses narrowband rather than wideband FM?

A.Simpler receivers than wideband FM needs
B.Compatibility with AM receivers
C.Channel economy in a shared band
D.Better noise performance than wideband FM

Answer: C

Narrowband FM is chosen for channel economy: many users must share a limited allocation, and a sixteen-kilohertz channel serves where 180 would not. Its noise performance is worse than wideband FM's, which is the price paid. Receiver complexity is essentially the same. And it is not compatible with AM receivers, which cannot demodulate any angle modulation. The choice is a spectrum decision, exactly parallel to the choice of SSB on the high-frequency bands.

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92.

Which statement about FM and impulse noise is correct?

A.Impulse noise affects FM more than AM
B.Impulse noise is unaffected by limiting
C.Impulse noise appears only below threshold
D.The limiter removes most of its effect

Answer: D

Impulse interference is largely an amplitude disturbance, so the limiter strips it before detection and FM is far quieter than AM in electrically noisy environments. That is why FM was adopted for mobile use around vehicle ignition systems. Saying it affects FM more inverts the comparison. Limiting is precisely what deals with it. And while everything degrades below threshold, impulse noise is suppressed well above it too.

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93.

Which quantity must a receiver's intermediate frequency bandwidth match?

A.The occupied bandwidth of the transmitted signal
B.The deviation alone
C.The audio bandwidth
D.The channel spacing exactly

Answer: A

It must pass the occupied bandwidth given by Carson's rule, since clipping the outer sidebands distorts the recovered audio, while admitting more than necessary lets in extra noise and adjacent-channel energy. Matching the deviation alone would be too narrow. The audio bandwidth is far narrower still and belongs after detection. And matching the channel spacing exactly would admit part of the neighbours. Getting this filter right is the single biggest influence on an FM receiver's performance.

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94.

What is the effect of an intermediate frequency filter narrower than Carson's bandwidth?

A.A rise in the transmitted deviation
B.Distortion of the recovered audio
C.Improved capture performance
D.Loss of the carrier only

Answer: B

Clipping the sidebands distorts the frequency variation the discriminator reads, so the recovered audio is distorted, worst on loud passages where the deviation is greatest. Capture performance is not improved by mutilating the wanted signal. The carrier sits in the middle of the passband and survives. And nothing a receiver does changes the transmitted deviation. This is the receiver-side counterpart of over-deviation at the transmitter, and both show up as distortion on peaks.

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95.

Which pairing correctly matches a scheme to its dominant limitation?

A.FM with receiver simplicity, AM with bandwidth
B.AM with bandwidth, SSB with power efficiency
C.FM with bandwidth, SSB with frequency accuracy
D.FM with frequency accuracy, SSB with bandwidth

Answer: C

FM's cost is bandwidth and SSB's is the demand it places on frequency accuracy, each being the price of that scheme's particular advantage. Reversing them misstates both. FM's receiver is more complex than AM's but that is not its dominant limitation. And SSB's power efficiency is excellent rather than limiting. Matching each scheme to its single binding constraint is a compact way to hold the whole comparison in mind.

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96.

What would you expect of an FM signal received at exactly the threshold level?

A.Perfect reception with no noise
B.Complete silence
C.Reception of the adjacent channel instead
D.Marked noise with the message still discernible

Answer: D

At threshold the output is noticeably noisy but still usable; a little above it the noise falls away rapidly and a little below it the output collapses. That sharp transition is what the threshold effect names, and it makes FM coverage boundaries abrupt compared with AM's gradual fade. Perfect reception needs a margin above threshold. Silence comes well below it. And adjacent-channel reception is a selectivity failure rather than a threshold phenomenon.

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97.

Which is the best summary of the FM bargain?

A.Spend bandwidth to gain noise performance
B.Spend power to gain bandwidth
C.Spend receiver simplicity to gain efficiency
D.Spend frequency accuracy to gain bandwidth

Answer: A

FM spends spectrum and buys output signal-to-noise ratio, an option the amplitude family does not offer at all, and it does so while keeping a constant envelope that permits efficient amplification. The other formulations mismatch the currencies: bandwidth is what is spent rather than gained, and while receiver complexity does rise, it is not the principal cost. Naming the bargain in one sentence is the most useful single thing to carry away from the topic.

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98.

A transmitter deviates \( \pm 40 \ \mathrm{kHz} \) when fed a \( 8 \ \mathrm{kHz} \) test tone. Its deviation ratio against a \( 8 \ \mathrm{kHz} \) audio limit is

A.0.2
B.5
C.40
D.8

Answer: B

Deviation ratio is peak deviation over the highest audio frequency:

\[ D = \frac{40}{8} = 5 \]

giving 5, the same ratio FM broadcasting uses although at a quarter of the deviation and half the audio bandwidth. That coincidence is instructive: the ratio, and therefore the noise advantage, depends on the proportion rather than on the absolute figures, while the occupied bandwidth depends on the absolute deviation. 40 and 8 are the input quantities and 0.2 inverts the ratio.

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99.

Two FM systems share a deviation ratio of five but differ tenfold in deviation. They differ in

A.Neither, since the ratio determines everything
B.Transmitter efficiency but nothing else
C.Occupied bandwidth but not in noise advantage
D.Noise advantage but not in occupied bandwidth

Answer: C

The noise advantage follows the index, so equal ratios give equal noise performance while the absolute deviation sets the bandwidth, which differs tenfold. That is why narrowband FM voice and wideband FM broadcasting can share a deviation ratio and yet occupy channels of sixteen and one hundred and eighty kilohertz respectively. Saying the ratio determines everything overlooks the absolute scale. And efficiency is unaffected either way, the envelope being constant in both.

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100.

An engineer must halve an FM channel's width without changing the audio bandwidth. The available move is

A.Reduce the audio level at the transmitter only
B.Raise the carrier frequency
C.Switch from pre-emphasis to de-emphasis
D.Reduce the deviation, accepting a poorer noise advantage

Answer: D

Bandwidth is dominated by the deviation term, so reducing the deviation narrows the channel and lowers the index, which costs noise performance - the trade run in reverse. Lowering the audio level does reduce deviation, but only for that programme and not as a system change; the deviation limit is what defines the channel. Carrier frequency does not affect bandwidth. And pre-emphasis and de-emphasis are a matched pair, not alternatives.

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