DSB and SSB Modulation Questions and Answers

Q: Replacing that AM transmitter with SSB carrying the same intelligence saves

SSB radiates only the one sideband:\[ 600 - 100 = 500 \ \mathrm{W} \ \mathrm{saved} \]a saving of 500 watts, or about 83 per cent, since the carrier and the redundant sideband are both discarded. 200 W would be the saving from suppressing one sideband alone. 100 W is the power actually transmitted. And the last option misses the point entirely: the message power is unchanged, which is exactly why everything else can be thrown away. Both economies are independent, which is why the three schemes form a ladder rather than a pair.

Q: Which comparison of the three amplitude schemes is correct?

SSB wins on both counts: the message bandwidth alone, and every watt in the information. DSB-SC matches its efficiency but needs twice the bandwidth. Full-carrier AM is worst on both, and its only advantage is receiver simplicity. Saying all three share a bandwidth is true only of the two double-sideband forms, and it is precisely the distinction single sideband exists to exploit.

Q: Peak envelope power is the quantity specified for an SSB transmitter because

The output devices must handle the peaks without distortion, so peak envelope power is what determines linearity and the power rating, while the average power of speech is only a fraction of it. Average power is if anything easier to measure with a simple wattmeter. Regulators specify both in various contexts. And the average power of speech is small but certainly not zero, which is why a licence limit in peak envelope power is not the same as one in average power.

Q: Why does SSB require linear amplification?

The information is carried in a varying envelope, so a nonlinear stage would distort it and generate intermodulation products that spread outside the channel. That is the practical cost of the scheme, since linear amplifiers are less efficient than the class C stages an AM or FM transmitter can use. There is no carrier to preserve. Bandwidth grows only as a symptom of the distortion. And phase linearity matters but is not the reason amplitude linearity is required.

Q: Intermodulation in an SSB transmitter shows up as

Nonlinearity mixes the message components, producing third and fifth order products that fall close to the wanted signal and cannot be filtered away, which is why the two-tone test and the intermodulation distortion figure are headline specifications. The operating frequency is unaffected. The wanted sideband is distorted rather than lost. And the power rating is a design parameter that distortion does not change, though exceeding it certainly causes distortion.

Q: A vestigial sideband signal is demodulated in a television receiver by

Because the carrier is deliberately retained, a simple envelope detector works, which is what kept television receivers affordable for decades. The vestigial shaping is arranged so that the sum of the wanted sideband and the vestige gives a flat overall response. Costas and squaring loops are suppressed-carrier techniques. And a limiter with a discriminator is the FM sound detector, which sits in the same receiver but on a different carrier.

21.

Replacing that AM transmitter with SSB carrying the same intelligence saves

A.500 W of the 600 W radiated
B.200 W of the 600 W radiated
C.100 W of the 600 W radiated
D.Nothing, since the message power is unchanged

Answer: A

SSB radiates only the one sideband:

\[ 600 - 100 = 500 \ \mathrm{W} \ \mathrm{saved} \]

a saving of 500 watts, or about 83 per cent, since the carrier and the redundant sideband are both discarded. 200 W would be the saving from suppressing one sideband alone. 100 W is the power actually transmitted. And the last option misses the point entirely: the message power is unchanged, which is exactly why everything else can be thrown away. Both economies are independent, which is why the three schemes form a ladder rather than a pair.

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22.

Which comparison of the three amplitude schemes is correct?

A.All three occupy the same bandwidth
B.SSB has the least bandwidth and the best power efficiency
C.DSB-SC has the least bandwidth of the three
D.Full-carrier AM has the best power efficiency

Answer: B

SSB wins on both counts: the message bandwidth alone, and every watt in the information. DSB-SC matches its efficiency but needs twice the bandwidth. Full-carrier AM is worst on both, and its only advantage is receiver simplicity. Saying all three share a bandwidth is true only of the two double-sideband forms, and it is precisely the distinction single sideband exists to exploit.

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23.

For a two-tone SSB test signal of equal tones, the peak envelope power is

A.Four times the average power
B.Half the average power
C.Twice the average power
D.Equal to the average power

Answer: C

Two equal tones add to an envelope peaking at twice either amplitude, so

\[ \frac{P_{PEP}}{P_{avg}} = \frac{(2A)^{2}/2}{A^{2}} = 2 \]

giving twice the average, or 3 dB. Equality would describe a single tone, whose envelope is constant. Four times would follow from comparing peak power with the power of one tone alone. And half inverts the relation. The two-tone test is the standard SSB linearity measurement precisely because its peak-to-average ratio is known exactly.

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24.

Peak envelope power is the quantity specified for an SSB transmitter because

A.It is easier to measure than average power
B.Regulators have no interest in average power
C.The average power of speech is zero
D.It sets the linearity and the device ratings

Answer: D

The output devices must handle the peaks without distortion, so peak envelope power is what determines linearity and the power rating, while the average power of speech is only a fraction of it. Average power is if anything easier to measure with a simple wattmeter. Regulators specify both in various contexts. And the average power of speech is small but certainly not zero, which is why a licence limit in peak envelope power is not the same as one in average power.

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25.

Why does SSB require linear amplification?

A.Its envelope varies with the message
B.Its carrier must be preserved exactly
C.Its bandwidth would otherwise double
D.Its phase must be held constant across the band

Answer: A

The information is carried in a varying envelope, so a nonlinear stage would distort it and generate intermodulation products that spread outside the channel. That is the practical cost of the scheme, since linear amplifiers are less efficient than the class C stages an AM or FM transmitter can use. There is no carrier to preserve. Bandwidth grows only as a symptom of the distortion. And phase linearity matters but is not the reason amplitude linearity is required.

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26.

Intermodulation in an SSB transmitter shows up as

A.An increase in the peak envelope power rating
B.Products falling beside the wanted channel
C.A shift of the operating frequency
D.A loss of the wanted sideband only

Answer: B

Nonlinearity mixes the message components, producing third and fifth order products that fall close to the wanted signal and cannot be filtered away, which is why the two-tone test and the intermodulation distortion figure are headline specifications. The operating frequency is unaffected. The wanted sideband is distorted rather than lost. And the power rating is a design parameter that distortion does not change, though exceeding it certainly causes distortion.

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27.

Vestigial sideband is used for television video because

A.Colour information requires two sidebands
B.Envelope detection is impossible with single sideband
C.The video spectrum reaches down towards direct current
D.It halves the transmitter power needed

Answer: C

Video content extends almost to direct current, so a filter sharp enough to remove one sideband cleanly would destroy the low-frequency content that carries the picture's large-area brightness. Leaving a vestige of the unwanted sideband solves it while still saving most of the bandwidth. Power saving is incidental. Colour is carried on a subcarrier within the same channel. And envelope detection of SSB is poor, but the driving reason here is the spectral shape of the message.

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28.

A vestigial sideband signal is demodulated in a television receiver by

A.A Costas loop
B.A squaring loop
C.A limiter followed by a discriminator stage
D.Envelope detection, the carrier being retained

Answer: D

Because the carrier is deliberately retained, a simple envelope detector works, which is what kept television receivers affordable for decades. The vestigial shaping is arranged so that the sum of the wanted sideband and the vestige gives a flat overall response. Costas and squaring loops are suppressed-carrier techniques. And a limiter with a discriminator is the FM sound detector, which sits in the same receiver but on a different carrier.

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29.

Independent sideband transmission places

A.A different message on each sideband
B.The same message on both sidebands, for redundancy against fading
C.A pilot carrier in each sideband
D.The message on one sideband and its inverse on the other

Answer: A

Independent sideband uses the two halves of a double-sideband channel to carry two separate messages, doubling the traffic in the same bandwidth. It was widely used on high-frequency point-to-point circuits. Duplicating one message is what ordinary DSB does and wastes half the capacity. A pilot in each sideband is a possible refinement rather than the definition. And transmitting an inverse would convey nothing extra.

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30.

Companded single sideband was introduced to

A.Permit class C amplification of the signal
B.Improve the effective signal-to-noise ratio on fading paths
C.Reduce the occupied bandwidth below the message bandwidth
D.Remove the need for a reinserted carrier

Answer: B

Compression at the transmitter and matching expansion at the receiver lift weak passages above the noise and restore the dynamics afterwards, which improved high-frequency voice circuits materially. Bandwidth cannot fall below the message bandwidth by any such means. A carrier must still be reinserted, and indeed a pilot is often sent to help. And the envelope still varies, so linear amplification remains necessary. The three schemes differ on exactly two axes, power and bandwidth, and every question on the topic reduces to one of them.

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