DSB and SSB Modulation Questions and Answers

Q: Which describes the spectrum of a quadrature-modulated colour subcarrier?

The colour information is carried as two DSB-SC signals on the same suppressed subcarrier, ninety degrees apart in phase, which is how two colour-difference signals fit into one channel. Separating them needs two coherent detectors and hence the transmitted burst. Adjacent SSB carriers would need more spectrum. A single full-carrier AM signal could carry only one message. And frequency modulation is used for the sound rather than the chrominance.

Q: Which control on an SSB transmitter limits the peak envelope power?

Automatic level control reduces gain as the envelope approaches the amplifier's linear limit, holding the peak envelope power below the point where intermodulation becomes unacceptable. The beat frequency oscillator belongs to the receiver, supplying the reinserted carrier. The sideband selector chooses which sideband is sent. And the receiver's gain control is at the far end of the link, with no influence on what the transmitter radiates.

Q: A DSB-SC transmitter radiates \( 60 \ \mathrm{W} \). Converting to SSB with the same message power in the retained sideband radiates

Half the power was in each sideband, so keeping one gives\[ P = \frac{60}{2} = 30 \ \mathrm{W} \]which is 30 watts for the same information, since the discarded sideband was redundant. 60 W would mean nothing was saved. 120 W doubles rather than halves. And 15 W would discard half of the retained sideband as well. The saving is smaller than the step from full-carrier AM, where the carrier dwarfs both sidebands, which is worth keeping in proportion.

Q: Which is the most demanding specification of an SSB filter?

The filter must fall from passband to stopband within twice the message's lowest frequency, a few hundred hertz, and achieving that at a usable centre frequency is what makes crystal, ceramic or mechanical construction necessary. Insertion loss is easily made up with gain. Impedance is a matching matter. Temperature coefficient matters for frequency accuracy but is a far smaller design difficulty than the transition width, which is why the filter method is defined by its filter.

Q: Which best describes vestigial sideband in relation to the other schemes?

Vestigial sideband keeps one full sideband and a small part of the other, retaining the low-frequency content that a sharp filter would destroy while saving most of the bandwidth SSB would save. It is emphatically an amplitude scheme rather than an angle one. Its carrier is retained rather than suppressed, so envelope detection works. And it is not single sideband with a pilot, which would be a different compromise addressing the carrier rather than the filter problem.

91.

Which describes the spectrum of a quadrature-modulated colour subcarrier?

A.One full-carrier AM signal
B.A frequency-modulated subcarrier
C.Two DSB-SC signals sharing one suppressed carrier
D.Two SSB signals on adjacent carriers

Answer: C

The colour information is carried as two DSB-SC signals on the same suppressed subcarrier, ninety degrees apart in phase, which is how two colour-difference signals fit into one channel. Separating them needs two coherent detectors and hence the transmitted burst. Adjacent SSB carriers would need more spectrum. A single full-carrier AM signal could carry only one message. And frequency modulation is used for the sound rather than the chrominance.

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92.

What is the effect of transmitting SSB with too much drive?

A.The sideband is suppressed further
B.The carrier reappears at full strength
C.The bandwidth narrows as the amplifier compresses
D.Intermodulation products spread outside the channel

Answer: D

Over-driving pushes the linear amplifier into compression, and the resulting nonlinearity generates intermodulation products that fall beside the wanted signal and interfere with neighbouring channels. It is the commonest operating fault on the amateur and marine bands and the reason automatic level control is fitted. Sideband suppression is a filter property. The carrier residue is set by modulator balance. And bandwidth grows rather than narrows, which is the whole problem.

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93.

Which control on an SSB transmitter limits the peak envelope power?

A.Automatic level control
B.The beat frequency oscillator
C.The sideband selector
D.The receiver's gain control

Answer: A

Automatic level control reduces gain as the envelope approaches the amplifier's linear limit, holding the peak envelope power below the point where intermodulation becomes unacceptable. The beat frequency oscillator belongs to the receiver, supplying the reinserted carrier. The sideband selector chooses which sideband is sent. And the receiver's gain control is at the far end of the link, with no influence on what the transmitter radiates.

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94.

A DSB-SC transmitter radiates \( 60 \ \mathrm{W} \). Converting to SSB with the same message power in the retained sideband radiates

A.15 W
B.30 W
C.60 W
D.120 W

Answer: B

Half the power was in each sideband, so keeping one gives

\[ P = \frac{60}{2} = 30 \ \mathrm{W} \]

which is 30 watts for the same information, since the discarded sideband was redundant. 60 W would mean nothing was saved. 120 W doubles rather than halves. And 15 W would discard half of the retained sideband as well. The saving is smaller than the step from full-carrier AM, where the carrier dwarfs both sidebands, which is worth keeping in proportion.

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95.

Which statement about power saving is correct for the amplitude family?

A.The two savings are equal in magnitude
B.Neither suppression saves any power
C.Most of the saving comes from suppressing the carrier
D.Most of the saving comes from suppressing one sideband

Answer: C

At full modulation the carrier holds two thirds of the total and each sideband a sixth, so suppressing the carrier is much the larger economy; removing the second sideband then halves what remains. The proportions are worth carrying because they explain why DSB-SC captures most of the power benefit while SSB adds the bandwidth benefit on top. Claiming equality misreads the power relation, and claiming no saving contradicts it entirely.

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96.

What must a receiver supply that a full-carrier AM transmitter provides?

A.Audio amplification
B.Automatic gain control
C.A band-pass filter at the intermediate frequency
D.A carrier-frequency reference

Answer: D

For suppressed-carrier signals the receiver must generate the carrier-frequency reference that the transmitter no longer sends, either by locking a loop to the modulation or by setting an oscillator accurately. The other three items are needed in every receiver whatever the scheme, so none of them distinguishes the suppressed-carrier case. Framing the difference this way - what has moved from transmitter to receiver - is the clearest way to see the trade being made.

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97.

Which is true of the envelope of a single sideband signal for a tone message?

A.It is constant, since one sideband is a single sinusoid
B.It follows the message exactly
C.It follows the rectified message
D.It is zero except at the instants of the message peaks

Answer: A

A single tone gives a single sideband component, which is one sinusoid, so its envelope is constant and carries no information at all - the reason envelope detection is useless for SSB. For a real message of many components the envelope does vary, but it still bears no simple relation to the message. Following the message exactly is the full-carrier AM case, and following the rectified message is DSB-SC. This single-tone case is the sharpest illustration of why coherent detection is unavoidable.

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98.

Why is the two-tone test preferred to a single tone for SSB linearity measurement?

A.Two tones halve the peak envelope power
B.A single tone gives a constant envelope and reveals no nonlinearity
C.A single tone would over-drive the transmitter output stage badly
D.Two tones are easier to generate accurately

Answer: B

One tone produces a constant envelope, so the amplifier is never asked to follow a varying level and compression and intermodulation simply do not appear. Two tones give a known two-to-one peak-to-average ratio and a well-defined set of products to measure. A single tone is no more likely to over-drive. Generating two accurate tones is harder rather than easier. And two tones raise the peak-to-average ratio rather than halving the peak power.

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99.

Which is the most demanding specification of an SSB filter?

A.Its input impedance
B.Its temperature coefficient of centre frequency
C.Its transition width against the lowest message frequency
D.Its insertion loss in the passband

Answer: C

The filter must fall from passband to stopband within twice the message's lowest frequency, a few hundred hertz, and achieving that at a usable centre frequency is what makes crystal, ceramic or mechanical construction necessary. Insertion loss is easily made up with gain. Impedance is a matching matter. Temperature coefficient matters for frequency accuracy but is a far smaller design difficulty than the transition width, which is why the filter method is defined by its filter.

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100.

Which best describes vestigial sideband in relation to the other schemes?

A.A form of frequency modulation
B.Double sideband with a suppressed carrier
C.Single sideband with a pilot carrier added
D.A compromise keeping most of SSB's bandwidth saving

Answer: D

Vestigial sideband keeps one full sideband and a small part of the other, retaining the low-frequency content that a sharp filter would destroy while saving most of the bandwidth SSB would save. It is emphatically an amplitude scheme rather than an angle one. Its carrier is retained rather than suppressed, so envelope detection works. And it is not single sideband with a pilot, which would be a different compromise addressing the carrier rather than the filter problem.

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DSB and SSB Modulation Questions and Answers

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