DSB and SSB Modulation Questions and Answers

Answer: A

Its role is entirely to serve as a reference large enough that the envelope follows the message, which is what lets a diode recover the audio without any local oscillator. It carries no message energy at all. Propagation is indifferent to whether a carrier is present. And bandwidth is set by the message frequency, since the sidebands sit at the message frequency either side of the carrier regardless of the carrier's amplitude. Understanding the carrier as a receiver aid rather than a transmission necessity is the key to the whole family.

Answer: B

The name refers to the symmetry that makes the carrier components cancel while the sidebands add, which is what suppresses the carrier without any filtering. In practice the balance is trimmed with a potentiometer and its accuracy sets the carrier suppression figure. Impedance matching is a separate design matter. Sideband amplitude balance follows automatically from a real message rather than from the circuit's name. And supply current symmetry is incidental.

Answer: C

Unbalancing lets a carrier component through, and with enough of it the signal becomes ordinary full-carrier AM - which is one practical way to generate AM from a balanced modulator, and also the fault mode that shows up as poor carrier suppression. Sideband suppression needs a filter or a quadrature network. The output does not vanish; the sidebands are unaffected. And message polarity is not what balance controls.

Answer: D

Moving the carrier oscillator from one side of a fixed filter to the other switches the transmitted sideband with no change of filter, which is the standard economical arrangement in transceivers. The phasing method can switch by reversing a phase, which is also simple but requires accurate networks in the first place. Weaver's method is more complex still. And no design fixes the sideband irrevocably, since every practical transceiver offers both.

Answer: A

Only the bandwidth stays the same, since both sidebands remain in place either side of where the carrier was. Total power falls by at least two thirds. Receiver complexity rises sharply, because a coherent reference must now be generated locally. And efficiency improves from at best a third to effectively all of it. Isolating the one invariant makes the trade-off easy to remember: power and receiver cost move, bandwidth does not.

Answer: B

The gap between the two sidebands is twice the lowest message frequency, and that gap is all the room the filter has to make its transition from passband to stopband. Speech starting at 300 Hz gives a 600 Hz window, which is achievable at a low intermediate frequency and impossible at a high carrier. Video reaching towards zero gives no window at all, which is why television uses vestigial sideband. Suppression depth and operating frequency are separate parameters.

Answer: C

Independent sideband doubles the traffic on a channel by putting a different message on each sideband, which suits point-to-point telephony where both ends have capable equipment. Broadcasting needs simple receivers. Television video needs the low-frequency content that vestigial sideband preserves. And aeronautical voice deliberately keeps full-carrier AM for its non-capture property, where two simultaneous transmissions are both heard.

Answer: D

Selecting the opposite sideband mirrors the recovered spectrum about the reinsertion frequency, so the lowest message components come out highest and speech becomes unintelligible noise. It is a qualitatively different fault from a frequency error, which shifts everything by a constant amount and leaves speech recognisable. A delay would be inaudible. A uniform amplitude change is a level matter. Distinguishing inversion from offset by ear is a useful diagnostic skill.

Answer: A

Complexity rises as the transmitter sheds components: full-carrier AM needs a diode, DSB-SC needs a coherent detector with carrier recovery, and SSB needs that plus accurate frequency setting since it has no symmetry to lock to. So the order is full-carrier AM, DSB-SC, SSB. The reversed order inverts it, and the remaining options misplace one scheme. Notice that the ranking is the exact mirror of the transmitter efficiency ranking, which is the central trade of the topic.

Answer: B

Television video is vestigial sideband with a full carrier, so an envelope detector recovers the picture without any local reference - a deliberate choice that kept receivers cheap. The video is amplitude modulated, not frequency modulated; it is the sound that uses FM. The vestigial filter shapes the spectrum rather than removing the need for a carrier. And the colour burst provides the phase reference for the suppressed colour subcarrier, which is a separate signal within the same channel.