Amplitude Modulation Questions and Answers

Q: Consider the following pairs of scheme and bandwidth for a message of highest frequency f m. Full-carrier AM - 2 f m DSB-SC - 2 f m SSB - f m Vestigial sideband - 2 f m How many of the pairs given above are correctly matched?

The first three are right and the fourth is not, so three match. Vestigial sideband occupies a little more than \( f_m \) but distinctly less than \( 2f_m \), which is the whole reason it exists: it captures most of the bandwidth saving of single sideband while keeping the low-frequency content that a sharp filter would destroy. Suppressing the carrier alone saves power without saving any bandwidth, which is the point of the second pair.

Q: Which is the correct expression for the total power of a tone-modulated AM signal in terms of carrier power and index?

The relation is \( P_t = P_c(1 + \mu^{2}/2) \). Doubling the index term would overstate the sideband power fourfold. Omitting the half overstates it twofold and would put the efficiency ceiling at fifty per cent rather than a third. And a linear term in the index has the wrong dependence altogether, predicting sideband power at small index far above the truth. Each sideband holds \( \mu^{2}/4 \) of the carrier power, which is where the half comes from.

Q: A transmitter is modulated by two tones giving indices 0.6 and 0.8. The total power relative to carrier power is

The indices combine in quadrature first:\[ \begin{aligned} \mu_{eff}^{2} &= 0.36 + 0.64 = 1.00 \\ \frac{P_t}{P_c} &= 1 + \frac{1.00}{2} = 1.50 \end{aligned} \]so 1.50 times, exactly the full-modulation figure, and the transmitter is right at its limit. 1.70 adds the indices before squaring. 1.98 squares their sum. 1.25 uses only the larger tone. Note the practical warning: two tones each individually modest can together reach a hundred per cent modulation.

Q: Which property makes AM attractive despite its power inefficiency?

Receiver simplicity is the whole case for it: a diode, a capacitor and a resistor recover the message, needing no local reference and no phase tracking, which made mass broadcasting possible. AM is notably vulnerable to impulse noise rather than immune. Its transmitted power rises with modulation depth. And single sideband is narrower, so AM cannot claim the narrowest bandwidth.

Q: What does a modulation index of zero represent?

An index of zero means no message is applied, so only the carrier is transmitted and the efficiency is zero: all the power and none of the information. It is the reference condition against which the antenna current and total power are compared when the index is measured. Suppressing the carrier is a quite different operation, achieved with a balanced modulator, and leaves the sidebands. Full modulation is an index of one, and folding requires an index above one. The zero case is worth naming because every power formula in the topic reduces to the carrier alone there.

51.

Consider the following statements about full-carrier AM.

1. Bandwidth is twice the highest message frequency

2. At best a third of the power carries information

3. Coherent detection is required at the receiver

Which of the statements given above is or are correct?

A.2 and 3 only
B.1, 2 and 3
C.1 and 2 only
D.1 and 3 only

Answer: C

The first two are correct and the third is the distinguishing point of full-carrier AM: because the carrier is present the envelope follows the message, so a simple diode detector suffices and no coherent reference is needed. That cheapness is the whole commercial argument for the scheme, and it is what suppressed-carrier systems trade away in exchange for power efficiency. Requiring coherent detection is a property of DSB-SC and SSB.

Discuss in Forum

52.

Consider the following pairs of scheme and bandwidth for a message of highest frequency f m.

Full-carrier AM - 2 f m

DSB-SC - 2 f m

SSB - f m

Vestigial sideband - 2 f m

How many of the pairs given above are correctly matched?

A.Only one
B.Only two
C.All four
D.Only three

Answer: D

The first three are right and the fourth is not, so three match. Vestigial sideband occupies a little more than \( f_m \) but distinctly less than \( 2f_m \), which is the whole reason it exists: it captures most of the bandwidth saving of single sideband while keeping the low-frequency content that a sharp filter would destroy. Suppressing the carrier alone saves power without saving any bandwidth, which is the point of the second pair.

Discuss in Forum

53.

Which is the correct expression for the total power of a tone-modulated AM signal in terms of carrier power and index?

A.P c times one plus half mu squared
B.P c times one plus two mu squared
C.P c times one plus mu squared
D.P c times one plus mu over two

Answer: A

The relation is \( P_t = P_c(1 + \mu^{2}/2) \). Doubling the index term would overstate the sideband power fourfold. Omitting the half overstates it twofold and would put the efficiency ceiling at fifty per cent rather than a third. And a linear term in the index has the wrong dependence altogether, predicting sideband power at small index far above the truth. Each sideband holds \( \mu^{2}/4 \) of the carrier power, which is where the half comes from.

Discuss in Forum

54.

A transmitter is modulated by two tones giving indices 0.6 and 0.8. The total power relative to carrier power is

A.1.25 times
B.1.50 times
C.1.70 times
D.1.98 times

Answer: B

The indices combine in quadrature first:

\[ \begin{aligned} \mu_{eff}^{2} &= 0.36 + 0.64 = 1.00 \\ \frac{P_t}{P_c} &= 1 + \frac{1.00}{2} = 1.50 \end{aligned} \]

so 1.50 times, exactly the full-modulation figure, and the transmitter is right at its limit. 1.70 adds the indices before squaring. 1.98 squares their sum. 1.25 uses only the larger tone. Note the practical warning: two tones each individually modest can together reach a hundred per cent modulation.

Discuss in Forum

55.

Why is a modulation monitor fitted to a broadcast transmitter?

A.To check the antenna's standing wave ratio
B.To log the audio programme for regulatory purposes
C.To keep peaks below over-modulation while maintaining depth
D.To measure the carrier frequency against a standard

Answer: C

The monitor exists to hold the working point in a narrow band: deep enough for efficiency and signal-to-noise ratio, shallow enough that peaks never exceed unity index and splatter into neighbouring channels. Frequency measurement, antenna matching and programme logging are all done by other equipment for other reasons, though a modern transmitter may well include all of them in one rack.

Discuss in Forum

56.

An AM receiver produces distorted audio only on the loudest passages, and the distortion disappears if the volume at the transmitter is reduced.

The most likely cause is

A.A failed automatic gain control
B.An image-frequency interferer
C.Diagonal clipping in the detector
D.Over-modulation at the transmitter

Answer: D

Distortion appearing only on peaks and curable by reducing the transmitter's audio level points to over-modulation: the envelope is being driven past its limit on the loudest passages. A gain control failure would distort at all levels or produce level-dependent noise. An image interferer would appear as a second programme or a whistle rather than as peak distortion. Diagonal clipping is a receiver fault and would not respond to the transmitter's audio level in this way.

Discuss in Forum

57.

What is the effect on AM of a channel that attenuates one sideband more than the other?

A.Envelope distortion, since the sidebands no longer balance
B.A shift in the carrier frequency
C.A reduction in bandwidth with no other effect
D.Complete loss of the recovered message

Answer: A

Selective fading of one sideband unbalances the pair, so the envelope no longer reproduces the message faithfully and the recovered audio is distorted - a familiar problem on medium and short wave, and one reason single sideband is preferred for long-distance work. The carrier frequency is unaffected. Bandwidth is reduced only in the trivial sense that part of the signal is missing. And the message is degraded rather than lost entirely.

Discuss in Forum

58.

Which property makes AM attractive despite its power inefficiency?

A.The narrowest bandwidth of any analogue scheme
B.Receiver simplicity
C.Immunity to impulse noise
D.Constant transmitted power regardless of modulation

Answer: B

Receiver simplicity is the whole case for it: a diode, a capacitor and a resistor recover the message, needing no local reference and no phase tracking, which made mass broadcasting possible. AM is notably vulnerable to impulse noise rather than immune. Its transmitted power rises with modulation depth. And single sideband is narrower, so AM cannot claim the narrowest bandwidth.

Discuss in Forum

59.

What does a modulation index of zero represent?

A.A fully modulated signal at its efficiency limit
B.An over-modulated signal whose envelope has folded
C.An unmodulated carrier
D.A suppressed carrier with both sidebands

Answer: C

An index of zero means no message is applied, so only the carrier is transmitted and the efficiency is zero: all the power and none of the information. It is the reference condition against which the antenna current and total power are compared when the index is measured. Suppressing the carrier is a quite different operation, achieved with a balanced modulator, and leaves the sidebands. Full modulation is an index of one, and folding requires an index above one. The zero case is worth naming because every power formula in the topic reduces to the carrier alone there.

Discuss in Forum

60.

An AM transmitter's carrier power is \( 400 \ \mathrm{W} \) and each sideband carries \( 50 \ \mathrm{W} \). The modulation index is

A.0.50
B.0.25
C.1.00
D.0.71

Answer: D

Each sideband holds \( \mu^{2}/4 \) of the carrier power, so

\[ \begin{aligned} \frac{\mu^{2}}{4} &= \frac{50}{400} = 0.125 \\ \mu &= \sqrt{0.5} = 0.71 \end{aligned} \]

giving about 0.71, and a total power of 500 watts. 0.50 would put 25 watts in each sideband. 0.25 is the ratio of sideband to carrier power taken as the index. 1.00 would put 100 watts in each. Working from measured sideband power is how a spectrum analyser reading is converted into a modulation figure, and the factor of four is the step most often dropped.

Discuss in Forum

Loading…

Topics

Other Subjects

Tags: Amplitude Modulation Questions and Answers || Amplitude Modulation MCQ Questions and Answers || Amplitude Modulation GK Questions and Answers || Amplitude Modulation GK MCQ Questions || Amplitude Modulation Multiple Choice Questions and Answers || Amplitude Modulation GK || GK on Amplitude Modulation || Analog Communication Questions and Answers || Analog Communication MCQ Questions and Answers || Analog Communication GK Questions and Answers || GK on Analog Communication

Amplitude Modulation Questions and Answers

Topics

Other Subjects

About Us

Stay informed about all important Dates & Notifications