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Chapter 3Chapter 3

Amplitude Modulation Fundamentals





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33--1: AM Concepts1: AM Concepts

In the modulation process, the voice, video, or digital

signal modifies another signal called the carrier. In amplitude modulation (AM) the information signal

varies the amplitude of the carrier sine wave.

The instantaneous value of the carrier amplitudechanges in accordance with the amplitude andfrequency variations of the modulating signal.

An imaginary line called the envelope connects thepositive and negative peaks of the carrier waveform.



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Figure 3-1: Amplitude modulation. (a) The modulating or information signal.



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Figure 3-1: Amplitude modulation. (b) The modulated carrier.



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In AM, it is particularly important that the peak value of

the modulating signal be less than the peak value ofthe carrier.

Vm

< Vc

Distortion occurs when the amplitude of themodulating signal is greater than the amplitude of the

carrier.

A modulator is a circuit used to produce AM.

Amplitude modulators compute the product of the

carrier and modulating signals.



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Figure 3-3: Amplitude modulator showing input and output signals.



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33--2: Modulation Index and2: Modulation Index and

Percentage of ModulationPercentage of Modulation The modulation index (m) is a value that describes

the relationship between the amplitude of themodulating signal and the amplitude of the carrier

signal.

m = Vm / Vc

This index is also known as the modulating factor or

coefficient, or the degree of modulation.

Multiplying the modulation index by 100 gives the

percentage of modulation.





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Percentage of ModulationPercentage of ModulationOvermodulation and Distortion

Distortion of voice transmissions produces garbled,harsh, or unnatural sounds in the speaker.

Distortion of video signals produces a scrambled and

inaccurate picture on a TV screen.



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Percentage of ModulationPercentage of Modulation

Figure 3-4: Distortion of the envelope caused by overmodulation where the

modulating signal amplitude Vm

is greater than the carrier signal Vc.



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Percentage of ModulationPercentage of ModulationPercentage of Modulation

The modulation index is commonly computed frommeasurements taken on the composite modulated

waveform.

Using oscilloscope voltage values:

Vm =Vmax −

Vmin

2

The amount, or depth, of AM is then expressed as the

percentage of modulation (100 ×

m) rather than as a

fraction.



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Percentage of ModulationPercentage of Modulation

Figure 3-5: AM wave showing peaks (Vmax) and troughs (Vmin).



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33--3: Sidebands and3: Sidebands and

the Frequency Domainthe Frequency Domain Side frequencies, or sidebands are generated as

part of the modulation process and occur in thefrequency spectrum directly above and below the

carrier frequency.

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the Frequency Domainthe Frequency DomainSideband Calculations

Single-frequency sine-wave modulation generatestwo sidebands.

Complex wave (e.g. voice or video) modulation

generates a range of sidebands. The upper sideband (f USB) and the lower sideband

(f LSB) are calculated:

f USB = f c

+ f m

and f LSB = f c − f

m

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the Frequency Domainthe Frequency Domain

Figure 3-6: The AM wave is the

algebraic sum of the carrier and

upper and lower sideband sine

waves. (a) Intelligence or

modulating signal. (b) Lower

sideband. (c ) Carrier. (d ) Uppersideband. (e ) Composite AM wave.

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the Frequency Domainthe Frequency DomainFrequency-Domain Representation of AM

Observing an AM signal on an oscilloscope, you seeonly amplitude variations of the carrier with respect totime.

A plot of signal amplitude versus frequency is referredto as frequency-domain display.

A spectrum analyzer is used to display the frequencydomain as a signal.

Bandwidth is the difference between the upper andlower sideband frequencies.

BW = f USB−f LSB

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Figure 3-8: The relationship between the time and frequency domains.

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the Frequency Domainthe Frequency DomainFrequency-Domain Representation of AM

Example: A standard AM broadcast station is allowed to transmitmodulating frequencies up to 5 kHz. If the AM station istransmitting on a frequency of 980 kHz, what aresideband frequencies and total bandwidth?

f USB = 980 + 5 = 985 kHz

f LSB = 980 – 5 = 975 kHz

BW = f USB – f LSB = 985 – 975 = 10 kHz

BW = 2 (5 kHz) = 10 kHz

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the Frequency Domainthe Frequency DomainPulse Modulation

When complex signals such as pulses or rectangularwaves modulate a carrier, a broad spectrum ofsidebands is produced.

A modulating square wave will produce sidebandsbased on the fundamental sine wave as well as the third,fifth, seventh, etc. harmonics.

Amplitude modulation by square waves or rectangular

pulses is referred to as amplitude shift keying (ASK). ASK is used in some types of data communications.

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Figure 3-11: Frequency spectrum of an AM signal modulated by a square wave.

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Figure 3-12: Amplitude modulation of a sine wave carrier by a pulse or rectangularwave is called amplitude-shift keying. (a) Fifty percent modulation. (b) One hundred

percent modulation.

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the Frequency Domainthe Frequency DomainPulse Modulation

Continuous-wave (CW) transmission can be achievedby turning the carrier off and on, as in Morse code

transmission.

Continuous wave (CW) transmission is sometimesreferred to as On-Off keying (OOK).

Splatter is a term used to describe harmonic sideband

interference.

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33--4: AM Power4: AM Power

In radio transmission, the AM signal is amplified by a

power amplifier. A radio antenna has a characteristic impedance that is

ideally almost pure resistance.

The AM signal is a composite of the carrier andsideband signal voltages.

Each signal produces power in the antenna.

Total transmitted power (PT) is the sum of carrier

power (Pc

) and power of the two sidebands (PUSB and

PLSB).

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When the percentage of modulation is less than the

optimum 100, there is much less power in thesidebands.

Output power can be calculated by using the formula

PT = (IT)2R

where IT

is measured RF current and R is antenna

impedance

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The greater the percentage of modulation, the higher

the sideband power and the higher the total powertransmitted.

Power in each sideband is calculated

PSB = PLSB = PUSB = Pcm2 / 4

Maximum power appears in the sidebands when the

carrier is 100 percent modulated.

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33--5: Single5: Single--Sideband ModulationSideband Modulation

In amplitude modulation, two-thirds of the transmitted

power is in the carrier, which conveys no information.

Signal information is contained within the sidebands.

Single-sideband (SSB) is a form of AM where the

carrier is suppressed and one sideband is eliminated.

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DSB Signals

The first step in generating an SSB signal is to suppressthe carrier, leaving the upper and lower sidebands.

This type of signal is called a double-sidebandsuppressed carrier (DSSC) signal. No power is wastedon the carrier.

A balanced modulator is a circuit used to produce thesum and difference frequencies of a DSSC signal but to

cancel or balance out the carrier. DSB is not widely used because the signal is difficult to

demodulate (recover) at the receiver.



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SSB Signals

One sideband is all that is necessary to conveyinformation in a signal.

A single-sideband suppressed carrier (SSSC) signal

is generated by suppressing the carrier and onesideband.

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SSB Signals

SSB signals offer four major benefits:1. Spectrum space is conserved and allows moresignals to be transmitted in the same frequencyrange.

2. All power is channeled into a single sideband. Thisproduces a stronger signal that will carry fartherand will be more reliably received at greater

distances.3. Occupied bandwidth space is narrower and noise in

the signal is reduced.

4. There is less selective fading over long distances.

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Disadvantages of DSB and SSB

Single and double-sideband are not widely usedbecause the signals are difficult to recover (i.e.

demodulate) at the receiver.

A low power, pilot carrier is sometimes transmittedalong with sidebands in order to more easily recover the

signal at the receiver.

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Signal Power Considerations

In SSB, the transmitter output is expressed in terms ofpeak envelope power (PEP), the maximum power

produced on voice amplitude peaks.

Applications of DSB and SSB

A vestigial sideband signal (VSB) is produced by

partially suppressing the lower sideband. This kind ofsignal is used in TV transmission.

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33--6: Classification of6: Classification of

Radio EmissionsRadio Emissions A code is used to designate the types of signals that

can be transmitted by radio and wire. The code is made up of a capital letter and a number.

Lowercase subscript letters are used for more specific

definition. Examples of codes:

DSB two sidebands, full carrier = A3

DSB two sidebands, suppressed carrier = A3b

OOK and ASK = A1

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l ifi i f




Radio EmissionsRadio Emissions The International Telecommunications Union (ITU),

a standards organization, uses a code to describesignals.

Examples are:

A3F amplitude-modulated analog TV J3E SSB voice

F2D FSK data

G7E phase-modulated voice, multiple signals

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3 6 Cl ifi i f




Radio EmissionsRadio Emissions

Figure 3-19: Radio emission code designations.

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3 l ifi i f6 Cl ifi i f




Radio EmissionsRadio Emissions

Figure 3-20: ITU emissions designations.

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