DSB and SSB Modulation Questions and Answers

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.

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.

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.

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.

Answer: C

A balanced modulator multiplies the carrier with the modulating signal, and its output contains the sum and difference frequencies, which are the two sidebands, with the carrier suppressed. It is therefore essentially a product modulator, and it is used to generate DSB-SC and, with a filter, SSB. An oscillator generates a signal rather than processing one, a mixer is a related product device but is meant for frequency conversion, and an amplifier only increases the level. The answer is a product modulator.