DSB and SSB Modulation Questions and Answers

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.

Q: What is the practical consequence of SSB's need for linear amplification?

Linear class AB operation is markedly less efficient than the class C stages an unmodulated or angle-modulated carrier permits, so an SSB transmitter runs hotter and draws more supply power for a given output. That is the price of putting information in the envelope. Bandwidth is narrower than DSB-SC, not wider. Its receiver uses a product detector rather than a diode. And solid-state linear amplifiers are entirely standard.

Q: Which scheme puts the greatest demand on transmitter frequency accuracy?

With no transmitted carrier and no symmetry to lock to, an SSB receiver can only set its reinserted carrier from its own dial, so any transmitter frequency error appears directly as an audio pitch shift. Both ends must therefore be accurate. Full-carrier AM is envelope detected and tolerant. DSB-SC provides a symmetric spectrum that a Costas loop can lock to, which relieves the requirement. And vestigial sideband retains a carrier for the receiver to follow.

Q: Which single change would convert a DSB-SC transmitter into a full-carrier AM transmitter?

Adding back a carrier large enough that the envelope follows the message turns DSB-SC into full-carrier AM, which is exactly what deliberately unbalancing the modulator achieves. Removing a sideband would give SSB instead. A quadrature phase shift is a step in the phasing method of SSB generation. And raising the message amplitude without a carrier present does not create one; it merely scales the sidebands.

101.

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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102.

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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103.

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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104.

What is the practical consequence of SSB's need for linear amplification?

A.Lower transmitter efficiency than AM or FM can achieve
B.A wider occupied bandwidth than DSB-SC
C.A requirement for a diode detector at the receiver
D.An inability to use solid-state output devices

Answer: A

Linear class AB operation is markedly less efficient than the class C stages an unmodulated or angle-modulated carrier permits, so an SSB transmitter runs hotter and draws more supply power for a given output. That is the price of putting information in the envelope. Bandwidth is narrower than DSB-SC, not wider. Its receiver uses a product detector rather than a diode. And solid-state linear amplifiers are entirely standard.

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105.

Which scheme puts the greatest demand on transmitter frequency accuracy?

A.Vestigial sideband, because of the filter shaping
B.SSB, because the receiver has no carrier to track
C.Full-carrier AM, because the carrier must be exact
D.DSB-SC, because both sidebands must align

Answer: B

With no transmitted carrier and no symmetry to lock to, an SSB receiver can only set its reinserted carrier from its own dial, so any transmitter frequency error appears directly as an audio pitch shift. Both ends must therefore be accurate. Full-carrier AM is envelope detected and tolerant. DSB-SC provides a symmetric spectrum that a Costas loop can lock to, which relieves the requirement. And vestigial sideband retains a carrier for the receiver to follow.

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106.

Which single change would convert a DSB-SC transmitter into a full-carrier AM transmitter?

A.Insert a ninety-degree phase shift in the message path
B.Increase the message amplitude beyond the carrier level
C.Add a carrier component at the output
D.Remove one sideband with a filter

Answer: C

Adding back a carrier large enough that the envelope follows the message turns DSB-SC into full-carrier AM, which is exactly what deliberately unbalancing the modulator achieves. Removing a sideband would give SSB instead. A quadrature phase shift is a step in the phasing method of SSB generation. And raising the message amplitude without a carrier present does not create one; it merely scales the sidebands.

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DSB and SSB Modulation Questions and Answers

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DSB and SSB Modulation Questions and Answers

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