DSB and SSB Modulation Questions and Answers

Q: In which part of a suppressed-carrier signal does the information reside?

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.

Q: Which scheme requires the most capable receiver?

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.

Q: Which statement about the carrier in full-carrier AM is correct?

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.

Q: Why is a balanced modulator so called?

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.

Q: What happens if a balanced modulator is deliberately unbalanced?

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.

Q: Which quantity is unchanged when a full-carrier AM signal is converted to DSB-SC?

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.

Q: What is the role of the message's lowest frequency in SSB filter design?

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.

81.

In which part of a suppressed-carrier signal does the information reside?

A.In the envelope's mean level rather than its variation
B.Entirely in the sidebands
C.Entirely in the carrier
D.Split evenly between carrier and sidebands

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.

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82.

Which scheme requires the most capable receiver?

A.On-off keying
B.Vestigial sideband television video
C.SSB
D.Full-carrier AM

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.

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83.

What is the bandwidth of a DSB-SC signal carrying a 5 kHz message?

A.5 kHz
B.2.5 kHz
C.20 kHz
D.10 kHz

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.

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84.

Which statement about the carrier in full-carrier AM is correct?

A.It provides the reference that makes envelope detection work
B.It carries half the message energy
C.It is required by the propagation medium
D.It sets the bandwidth of the transmitted signal

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.

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85.

Why is a balanced modulator so called?

A.It draws equal current on both halves of the message cycle
B.Its symmetry cancels the carrier at the output
C.Its input and output impedances are equal
D.It balances the two sidebands in amplitude

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.

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86.

What happens if a balanced modulator is deliberately unbalanced?

A.The output falls to zero
B.The message is inverted in polarity
C.A carrier appears, giving full-carrier AM
D.One sideband is suppressed

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.

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87.

Which method of SSB generation is easiest to switch between sidebands?

A.The phasing method with a fixed network
B.The Weaver method
C.None, since the sideband is fixed by design
D.The filter method with a movable carrier oscillator

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.

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88.

Which quantity is unchanged when a full-carrier AM signal is converted to DSB-SC?

A.The occupied bandwidth
B.The total radiated power
C.The receiver complexity required
D.The efficiency of transmission

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.

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89.

What is the role of the message's lowest frequency in SSB filter design?

A.It has no bearing on the filter at all
B.It sets the transition width the filter must achieve
C.It fixes the sideband suppression achievable
D.It determines the operating frequency

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.

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90.

Which application would suit independent sideband transmission?

A.Television video distribution
B.Aeronautical voice communication
C.Two voice circuits over one high-frequency channel
D.Broadcasting music to consumer receivers

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.

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