DSB and SSB Modulation Questions and Answers

Answer: C

For a real message the sidebands are conjugate images about the suppressed carrier, which is the frequency-domain statement that the time signal is real. That symmetry is what allows either sideband alone to carry the whole message and hence makes SSB possible. Identical phase across the band would not give a real signal. Quadrature describes the relationship between the two channels of a quadrature-modulated pair, not between sidebands. And nothing here is random.

Answer: D

Quadrature modulation puts two independent messages on one carrier, ninety degrees apart in phase, and a pair of coherent detectors separates them; the television colour subcarrier is the classic analogue example. Duplication would waste the capacity. A single suppressed-carrier message is DSB-SC. And two separate carriers is simply two channels, which needs twice the spectrum rather than sharing it.

Answer: A

Phase error causes crosstalk: part of each message appears in the other channel, which is why the television colour burst is transmitted and why the phase reference is guarded so carefully. Level errors affect amplitude rather than separation. Bandwidth is fixed by the messages. And envelope detection plays no part in a quadrature system, which is coherent by necessity.

Answer: B

SSB is the most demanding, because a reinsertion error of even a few tens of hertz shifts every recovered component by that amount and degrades speech noticeably. Full-carrier AM tolerates tuning errors of hundreds of hertz, being envelope detected. Wideband FM is comparatively forgiving. And on-off keying only needs the tone to fall inside the filter. That sensitivity is the practical reason SSB waited for stable synthesised oscillators to become common.

Answer: C

The unwanted sideband is removed by the filter, so its residue is set by the filter's stopband attenuation, typically sixty decibels or better in a good crystal design. Modulator balance limits carrier suppression, which is a separate specification measured at a different frequency. Oscillator stability affects frequency accuracy. And output linearity governs intermodulation rather than sideband suppression.

Answer: D

Everything follows from one observation: the carrier carries no information and the second sideband duplicates the first, so both can be discarded, saving power and bandwidth respectively at the cost of a more capable receiver. Envelope detection is perfectly clean when a carrier is present. Full-carrier AM carries speech well, as a century of broadcasting shows. And no regulation forbids a carrier; several services require one.

Answer: A

It stands for double sideband suppressed carrier: both sidebands are transmitted and the carrier is removed by the balancing action of the modulator. The companion abbreviations are DSB-FC for the full-carrier form, SSB-SC for single sideband suppressed carrier and VSB for vestigial sideband. Standards and data sheets use them without expansion, and mistaking one for another changes both the receiver required and the power budget substantially, so the four are worth learning as a set rather than individually.

Answer: B

All the information is in the sidebands, which is precisely why the carrier can be discarded without loss: it is a fixed-amplitude, fixed-frequency component that conveys nothing. What it does provide is a phase reference, and removing it transfers that burden to the receiver. Splitting the information would make suppression impossible. And the envelope's mean level tells you the carrier strength rather than the message, which is why it drives the gain control and not the audio output.

Answer: C

SSB demands the most: a stable, accurately tuned oscillator to reinsert the carrier, since an error of tens of hertz is audible and an error of hundreds makes speech unintelligible. Full-carrier AM needs a diode. On-off keying needs only a filter and a detector. Vestigial sideband television retains its carrier deliberately so that envelope detection suffices, which is what kept receivers affordable. Ranking the schemes by receiver burden is the mirror image of ranking them by transmitter efficiency.

Answer: D

Both sidebands are present and each extends 5 kHz from the suppressed carrier, so the occupied width is 10 kHz, exactly as full-carrier AM would need. 5 kHz is the single sideband figure, available only after one sideband has been removed. 2.5 kHz is narrower than the message and impossible. And 20 kHz would correspond to a 10 kHz message. The point to hold is that suppressing the carrier changes the power distribution and nothing about the width.