Amplitude Modulation Questions and Answers

Q: Image rejection in a superheterodyne receiver is provided by

Only filtering before the mixer can reject the image, because after mixing the image and the wanted signal occupy exactly the same intermediate frequency and are indistinguishable. That is the argument for a tuned radio-frequency stage or a preselector, and for choosing a high intermediate frequency where possible. The intermediate frequency amplifier provides adjacent-channel selectivity instead. Gain control and audio filtering act far too late.

Q: Choosing a higher intermediate frequency makes it easier to

A higher intermediate frequency pushes the image further from the wanted signal, so a modest preselector can reject it. The cost is that adjacent-channel selectivity becomes harder, since a filter of given fractional bandwidth is wider in absolute terms - which is exactly the trade the double-conversion receiver resolves by using a high first intermediate frequency and a low second one. Tuning range and noise figure are affected only indirectly.

Q: Why do broadcast AM receivers use an intermediate frequency of about 455 kHz?

The figure is a compromise: high enough that the image falls well outside the band for most tuning positions, low enough that a simple tuned circuit gives the nine or ten kilohertz selectivity a channel needs. Crystal filters are made at far lower and far higher frequencies. Channel spacing is nine or ten kilohertz, nothing like 455. And no regulation prescribes it; the value became a convention because it works.

Q: The ratio of message amplitude to carrier amplitude in AM is called the modulation _____.

It is the modulation index, sometimes expressed as a percentage and then called the depth of modulation. Depth ratio is not standard usage. A figure of merit is a general term used for quite different quantities. And coefficient of coupling belongs to transformer and resonant-circuit theory. Knowing the name matters because every power and efficiency formula in the topic is written in terms of it.

Q: How many sidebands does a single-tone AM signal have?

Two: one above and one below the carrier, at plus and minus the message frequency. A single sideband appears only after one has been deliberately filtered out. Four would arise from two modulating tones. And the message does not ride on the carrier without producing sidebands; the sidebands are precisely how the information is carried, since a pure carrier conveys nothing.

Q: Which quantity stays constant as an AM signal is modulated more deeply?

The carrier power is unchanged; deepening the modulation adds sideband power on top of it, so the total power and the antenna current both rise and the efficiency improves. That is exactly why the antenna current measurement reveals the index. Total power follows \( 1 + \mu^{2}/2 \). Current follows its square root. And efficiency rises towards its ceiling of a third.

Q: Which detector can demodulate a DSB-SC signal?

Only a coherent, or synchronous, detector works, because with the carrier removed the envelope no longer follows the message: it follows the magnitude of the message and reverses phase at every zero crossing. Both of the rectifier options are envelope detectors and would recover a full-wave rectified version of the message. A slope detector is an FM device and is irrelevant here.

Q: What does the envelope of a DSB-SC signal look like for a tone message?

It looks like the rectified tone, so the envelope repeats at twice the message frequency while the carrier phase reverses at each null. That is precisely why an envelope detector fails and why the phase reversal must be tracked by a coherent receiver. The tone itself appears as the envelope only when a large carrier is present. Constant amplitude describes angle modulation. And a square wave would require a square-wave message.

41.

For the same receiver, the image frequency is

A.2110 kHz
B.1655 kHz
C.745 kHz
D.290 kHz

Answer: A

The image sits twice the intermediate frequency above the wanted signal for high-side injection:

\[ f_{image} = 1200 + 2 \times 455 = 2110 \ \mathrm{kHz} \]

so 2110 kHz, and any signal there mixes to the same intermediate frequency and cannot be removed afterwards. That is why radio-frequency selectivity ahead of the mixer matters. 1655 kHz is the oscillator. 745 kHz is the low-side oscillator frequency. And 290 kHz corresponds to no relevant combination.

Discuss in Forum

42.

Image rejection in a superheterodyne receiver is provided by

A.The audio output filter
B.Selectivity ahead of the mixer
C.The intermediate frequency amplifier
D.The automatic gain control loop

Answer: B

Only filtering before the mixer can reject the image, because after mixing the image and the wanted signal occupy exactly the same intermediate frequency and are indistinguishable. That is the argument for a tuned radio-frequency stage or a preselector, and for choosing a high intermediate frequency where possible. The intermediate frequency amplifier provides adjacent-channel selectivity instead. Gain control and audio filtering act far too late.

Discuss in Forum

43.

Choosing a higher intermediate frequency makes it easier to

A.Reduce the local oscillator tuning range
B.Lower the receiver's noise figure
C.Reject the image
D.Achieve adjacent-channel selectivity

Answer: C

A higher intermediate frequency pushes the image further from the wanted signal, so a modest preselector can reject it. The cost is that adjacent-channel selectivity becomes harder, since a filter of given fractional bandwidth is wider in absolute terms - which is exactly the trade the double-conversion receiver resolves by using a high first intermediate frequency and a low second one. Tuning range and noise figure are affected only indirectly.

Discuss in Forum

44.

Why do broadcast AM receivers use an intermediate frequency of about 455 kHz?

A.It is the lowest frequency at which a crystal filter can be made
B.It equals the channel spacing used in broadcasting
C.It is required by international regulation
D.It balances image rejection against achievable selectivity

Answer: D

The figure is a compromise: high enough that the image falls well outside the band for most tuning positions, low enough that a simple tuned circuit gives the nine or ten kilohertz selectivity a channel needs. Crystal filters are made at far lower and far higher frequencies. Channel spacing is nine or ten kilohertz, nothing like 455. And no regulation prescribes it; the value became a convention because it works.

Discuss in Forum

45.

The ratio of message amplitude to carrier amplitude in AM is called the modulation _____.

A.Index
B.Depth ratio
C.Factor of merit
D.Coefficient of coupling

Answer: A

It is the modulation index, sometimes expressed as a percentage and then called the depth of modulation. Depth ratio is not standard usage. A figure of merit is a general term used for quite different quantities. And coefficient of coupling belongs to transformer and resonant-circuit theory. Knowing the name matters because every power and efficiency formula in the topic is written in terms of it.

Discuss in Forum

46.

How many sidebands does a single-tone AM signal have?

A.None, since the message rides on the carrier
B.Two
C.One
D.Four

Answer: B

Two: one above and one below the carrier, at plus and minus the message frequency. A single sideband appears only after one has been deliberately filtered out. Four would arise from two modulating tones. And the message does not ride on the carrier without producing sidebands; the sidebands are precisely how the information is carried, since a pure carrier conveys nothing.

Discuss in Forum

47.

What is the maximum theoretical efficiency of full-carrier AM?

A.About 67 per cent
B.About 100 per cent
C.About 33 per cent
D.About 50 per cent

Answer: C

At an index of unity the sidebands carry a third of the total, so the efficiency ceiling is about 33 per cent and any practical figure is lower still. Fifty per cent would require the carrier and sidebands to be equal in power, which the relation does not permit. Two thirds is the fraction the carrier takes, not the sidebands. And a hundred per cent would mean no carrier at all, which is DSB-SC rather than full-carrier AM.

Discuss in Forum

48.

Which quantity stays constant as an AM signal is modulated more deeply?

A.The total power
B.The antenna current
C.The transmission efficiency
D.The carrier power

Answer: D

The carrier power is unchanged; deepening the modulation adds sideband power on top of it, so the total power and the antenna current both rise and the efficiency improves. That is exactly why the antenna current measurement reveals the index. Total power follows \( 1 + \mu^{2}/2 \). Current follows its square root. And efficiency rises towards its ceiling of a third.

Discuss in Forum

49.

Which detector can demodulate a DSB-SC signal?

A.A synchronous detector
B.A diode envelope detector
C.A simple half-wave rectifier and smoothing capacitor
D.A slope detector tuned to one side of the carrier

Answer: A

Only a coherent, or synchronous, detector works, because with the carrier removed the envelope no longer follows the message: it follows the magnitude of the message and reverses phase at every zero crossing. Both of the rectifier options are envelope detectors and would recover a full-wave rectified version of the message. A slope detector is an FM device and is irrelevant here.

Discuss in Forum

50.

What does the envelope of a DSB-SC signal look like for a tone message?

A.A square wave at the message frequency
B.The magnitude of the tone, at twice its frequency
C.The tone itself, exactly as sent
D.A constant amplitude with the information in the phase

Answer: B

It looks like the rectified tone, so the envelope repeats at twice the message frequency while the carrier phase reverses at each null. That is precisely why an envelope detector fails and why the phase reversal must be tracked by a coherent receiver. The tone itself appears as the envelope only when a large carrier is present. Constant amplitude describes angle modulation. And a square wave would require a square-wave message.

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