Amplitude Modulation MCQ Quiz - Objective Question with Answer for Amplitude Modulation - Download Free PDF

Concept:

The total transmitted power for an AM system is given by:

Pc = Carrier Power

μ = Modulation Index

The above expression can be expanded to get:

The total power is the sum of the carrier power and the sideband power, i.e.

Calculation:

Given: Pc = 100 W and μ = 1

We can write:

The total sideband power = 50 W

power in upper sideband + power in lower side band = 50 W

power in upper sideband = power in lower sideband = 25 W

The correct answer is: 3) Pulse Duration Modulation (PDM)

Explanation:

In communication engineering, Pulse Width Modulation (PWM) is also known as:

• Pulse Duration Modulation (PDM)

• Pulse Length Modulation (PLM)

Key Characteristics of PWM/PDM:

• Definition: The width (duration) of pulses varies based on the modulating signal, while the amplitude remains constant.

• Applications:

  • Motor speed control

  • LED dimming

  • Power delivery (DC-DC converters)

In communication system engineering, DSB-SC stands for Double Sideband Suppressed Carrier.

Concept:

In a balanced modulator, 2 AM modulators are connected in a way that the resultant signal does not contain the carrier spectrum, i.e. to generate a Double Side-band suppressed carrier (DSB-SC).

Analysis:

The circuit diagram of a balanced modulator is as shown:

The resultant DSB signal is the difference of the two, i.e.

Hence,

The frequency spectrum of the DSB-SC signal:

The standard DSB-SC equation is given as:

Assuming a single tone message signal as:

The DSB-SC signal can be written as:

∴ The output of balanced modulator contains the modulation frequency, LSB and USB.

The correct answer is: 2) Amplitude Modulation (AM)

Explanation:

DSB-SC, VSB, and SSB are all variants of Amplitude Modulation (AM), where the amplitude of the carrier signal is varied in proportion to the message signal. Here’s how they differ:

1. DSB-SC (Double Sideband Suppressed Carrier)

   • Transmits both sidebands but suppresses the carrier.

   • More power-efficient than standard AM but requires coherent detection.

2. VSB (Vestigial Sideband)

   • A compromise between DSB and SSB, transmitting one full sideband + part of the other.

   • Used in TV broadcasting (e.g., analog NTSC/PAL signals).

3. SSB (Single Sideband)

   • Transmits only one sideband (either USB or LSB) and suppresses the carrier.

   • Maximizes bandwidth and power efficiency (used in HF radio, military comms).

Concept:

A single tone modulated signal is given as:

Maximum envelope Amax : Ac (1+μ)

Minimum envelope Amin : Ac (1-μ)

The above can be written as:

Calculation:

Given:  Amax = 3 V and Amin = 1 V

The modulation index will be:

μ = 0.5

The general expression of a DSB-SC signal is given as:

sDSB-SC(T) = x(t) cosωct

For a single-tone message signal A_m cosω_mt, the DSB-SC signal will be:

sDSB-SC(T) = Amcosωct cosωmt

ωm = Frequency of the message signal

ωc = Carrier signal frequency

The spectrum as represented:

We observe the carrier is suppressed in DSB-SC modulation.

In TV Transmission the use of FM is made for Audio transmission and AM for Video transmission.

Vestigial Sideband Modulation (VSB) is used for video modulation in TV transmission due to the following reasons :

• Video signal exhibits a large bandwidth and significant low-frequency content which suggests the use of VSB.  
• VSB (vestigial sideband) transmission transmits one sideband fully and the other sideband partially thus, reducing the bandwidth requirement.
• The circuitry for demodulation in the receiver should be simple and therefore cheap. VSB demodulation uses simple envelope detection.

The spectrum of a vestigial sideband is as shown:

In TV Transmission the use of FM is made for Audio transmission and AM for Video transmission.

Vestigial Sideband modulation (VSB) is used for video modulation in TV transmission due to the following reasons :

• Video signal exhibits a large bandwidth and significant low-frequency content which suggests the use of VSB.  
• VSB (vestigial side band) transmission transmits one side band fully and the other side band partially thus, reducing the bandwidth requirement.
• The circuitry for demodulation in the receiver should be simple and therefore cheap. VSB demodulation uses a simple envelope detection.

Concept:

The total transmitted power for an AM system is given by:

Pc = Carrier Power

μ = Modulation Index

Analysis:

When the carrier is not modulated, i.e. modulation index = 0, the transmitted power is the carrier power only, i.e.

P_t = P_C

When modulated with a modulation index of 40%, the total power is calculated as:

The percentage increase in power will be:

 = 8 %

In a balanced modulator, 2 AM-modulators are connected in a way that the resultant signal does not contain the carrier spectrum, i.e. to generate a Double Side-band suppressed carrier (DSB-SC).

The circuit diagram of a balanced modulator is as shown:

The resultant DSB signal is the difference between the two, i.e.

Hence,

1) Square law modulator is used for the generation of conventional AM signal which includes the carrier frequency component as well.

2) An Armstrong modulator is used to generate an FM signal.

3) Envelop detector is used for AM demodulation.

Amplitude Modulation (AM) is preferred for picture transmission in TV because of the following reasons:

• The distortion which arises due to interference between multiple signals is more in FM than AM because the frequency of the FM signal continuously changes.
• Steady production of the picture is affected because of this.
• If AM were used, the ghost image, if produced is steady.
• Also, the circuit complexity and bandwidth requirements are much less in AM than in FM.

On the other hand, FM is preferred for sound because of the following reasons:

• The bandwidth assigned to the FM sound signal is about 200 kHz, of which not more than 100 kHz is occupied by significant side bands.
• This is only 1.4 % of the total channel bandwidth of 7 MHz. This results in efficient utilization of the channel.

A wave has 3 parameters Amplitude, Phase, and Frequency. Thus there are 3 types of modulation techniques.

Amplitude Modulation: The amplitude of the carrier is varied according to the amplitude of the message signal.

Frequency Modulation: The frequency of the carrier is varied according to the amplitude of the message signal.

Phase Modulation: The Phase of the carrier is varied according to the amplitude of the message signal.

Concept:

The generalized AM expression is represented as:

s(t) = Ac [1 + μa mn (t)] cos ωc t

The total transmitted power for an AM system is given by:

Pc = Carrier Power

μ = Modulation Index

The above expression can be expanded to get:

The total power is the sum of the carrier power and the sideband power, i.e.

Analysis:

Total sideband power is calculated as:

% of sideband power is given as:

⇒ 

⇒ 

putting μ = 0.4, we get

⇒ 

= 7.4 %

Concept:

The generalized AM expression is represented as:

s(t) = Ac [1 + μa mn (t)] cos ωc t

The total transmitted power for an AM system is given by:

Pc = Carrier Power

μ = Modulation Index

The above expression can be expanded to get:

The total power is the sum of the carrier power and the sideband power, i.e.

The power in a single sideband will be:

With , the above can be written as:

  ---(1)

Power Saved = Pc in DSB - SC

Analysis:

When μ = 1, the transmitted power will be:

As μ = 1

Power Saved = 66 %

Concept:

AM: In amplitude modulation, the amplitude of the carrier signal varies in accordance with the instantaneous amplitude of the modulating signal.

The time-domain representation of an amplitude-modulated signal is given as:

s(t) = Ac [1 + ka m(t)] cos 2πfct

s(t) = Ac cos 2πfct + Ac ka m(t) cos 2πfct

Where the carrier Ac cos ωct is modulated in amplitude and ka is the amplitude sensitivity of the modulator.

The frequency-domain representation of an amplitude-modulated signal is given as:

∴ We can see that the bandwidth of an AM signal is twice that of the maximum frequency present in the message signal.

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