DSB and SSB Modulation Questions and Answers

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.

Q: In the vestigial sideband amplitude modulation (AM-VSB), the transmitted signal has a bandwidth of ..............., where B is bandwidth of original analog signal.

The transmitted bandwidth of VSB is B + Δ.Vestigial sideband modulation sends one sideband in full (bandwidth B) plus a small vestige of the other sideband, of width Δ. The total bandwidth is therefore B + Δ, more than single sideband (B) but much less than double sideband (2B). Television broadcasting used VSB for this reason.B − Δ would be less than one full sideband, and the ± forms do not describe a single bandwidth.

101.

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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102.

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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103.

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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104.

In the vestigial sideband amplitude modulation (AM-VSB), the transmitted signal has a bandwidth of ..............., where B is bandwidth of original analog signal.

A.B − Δ
B.B + Δ
C.B ± Δ
D.B ± 2Δ

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

The transmitted bandwidth of VSB is B + Δ.

Vestigial sideband modulation sends one sideband in full (bandwidth B) plus a small vestige of the other sideband, of width Δ. The total bandwidth is therefore B + Δ, more than single sideband (B) but much less than double sideband (2B). Television broadcasting used VSB for this reason.

B − Δ would be less than one full sideband, and the ± forms do not describe a single bandwidth.

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DSB and SSB Modulation Questions and Answers

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