DSB and SSB Modulation Questions and Answers

Answer: C

It is the level of the unwanted sideband relative to the wanted one, typically forty to sixty decibels down in a good transmitter, and it is limited by filter shape in the filter method or by network accuracy in the phasing method. Peak to average is the crest factor. Carrier residual is carrier suppression, a separate specification. And intermodulation is a linearity measure, also separate.

Answer: D

Any mismatch in the diodes or transistors, or in the transformer windings, leaves a carrier residue, so carrier suppression is a matter of balance and is usually trimmed with a potentiometer. Message amplitude affects the sideband level rather than the residue. Frequency matters through stray reactances but is not the chief limit. And the following filter can help only if the carrier falls outside its passband, which in a double-sideband output it does not.

Answer: A

Two tones through a nonlinearity produce third and fifth order products spaced beyond the pair, and their level below the tones is the intermodulation figure. It is the standard linearity measurement. Frequency accuracy needs a counter. Sideband suppression is measured with a single tone, where the unwanted sideband appears as a distinct line. And average power alone would tell nothing about linearity. Keeping the carrier is a decision about the receiver, and keeping the second sideband a decision about the channel.

Answer: B

The combination of every watt in the information and half the bandwidth is worth many decibels of effective improvement on a crowded, fading band, which is decisive when propagation is marginal. Its receivers are more complex than AM's, needing an accurate reinserted carrier. It is less troubled by selective fading than double sideband but not immune. And it demands better frequency stability than any other analogue voice scheme. That is the trade the suppressed-carrier family exists to make explicit.

Answer: C

A pitch error corrected by fine tuning is a carrier reinsertion frequency error: every component was shifted by the same number of hertz, deepening the voice. Selecting the wrong sideband gives spectrally inverted, entirely unintelligible audio that fine tuning cannot fix. Over-modulation would distort rather than shift pitch. And a saturated gain control would flatten the dynamics without changing the pitch at all. Which of the two economies applies is worth naming before any arithmetic is attempted.

Answer: D

Spectrally inverted, unintelligible audio is the classic signature of opposite sideband selection: the recovered spectrum is mirrored, so low frequencies become high and speech becomes noise. Over-driving distorts but leaves speech recognisable. An equal frequency error on both would cancel rather than garble. And propagation does not invert a spectrum, which is why the sideband convention exists at all. The pattern repeats across the family: what the transmitter sheds, the receiver must supply.

Answer: A

The detector follows the magnitude, so it delivers a full-wave rectified version of the message, rich in harmonics and quite unlike the original. Something is certainly recovered, which is what makes the fault confusing in practice: the audio is present but wrong. Perfect recovery needs a coherent detector. And a simple polarity reversal would be harmless by comparison with what actually happens.

Answer: B

With every watt in the information and the receiver bandwidth reduced to the message bandwidth, SSB gives the best output signal-to-noise ratio per watt transmitted of the amplitude family. Full-carrier AM is worst, wasting two thirds of its power. A large pilot pushes DSB-SC back towards AM's inefficiency. And vestigial sideband with a full carrier is closer to AM than to SSB on this measure, being chosen for spectral reasons rather than power ones. Placing the scheme on the power-and-bandwidth grid settles most questions of this kind at once.

Answer: C

The first two are correct. The third is the persistent misconception: removing the carrier removes a spectral line, not any bandwidth, since the two sidebands still sit either side of where it was. Bandwidth is saved only by removing a sideband. Keeping that distinction clear is what makes the three-way comparison of AM, DSB-SC and SSB straightforward rather than confusing.

Answer: D

The first three are right and the fourth is not, so three match. Vestigial sideband retains its carrier precisely so that envelope detection works, which is what made television receivers cheap. The pattern across the four is worth holding: keep the carrier and the receiver is simple, remove it and the receiver must supply a reference, and the bandwidth saving is a separate axis governed by how many sidebands are sent.