chapter02-1 amplitude modulation

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Chapter 2.1

Continuous Wave Modulation:

Fundamentals and Circuits of Amplitude

Modulation

References:

Communication Electronics: Principles and Applications by Louis E. Frenzel

Communication Systems by S. Haykin

© 2008 The McGraw-Hill Companies

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Topics Covered

AM Concepts

Modulation Index and Percentage of Modulation

Sidebands and the Frequency Domain

AM Power

Classification of AM

Amplitude Modulators

Amplitude Demodulators


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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 amplitude changes in accordance with the amplitude and frequency variations of the modulating signal.

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


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AM Concepts

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


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AM Concepts

Figure : Amplitude modulation. (b) The modulated carrier.


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AM Concepts

Figure: Amplitude modulator showing input and output signals.


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AM Concepts

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

the modulating signal be less than the peak value of

the carrier.

Vm < Vc

Distortion occurs when the amplitude of the

modulating 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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Modulation Index and Percentage

of Modulation

The modulation index (m) is a value that describes

the relationship between the amplitude of the

modulating 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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of Modulation

Overmodulation and Distortion

The modulation index should be a number between 0

and 1.

If the amplitude of the modulating voltage is higher than

the carrier voltage, m will be greater than 1, causing

distortion.

If the distortion is great enough, the intelligence signal

becomes unintelligible.


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of Modulation

Overmodulation 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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of Modulation

Figure : Distortion of the envelope caused by overmodulation where the modulating

signal amplitude Vm is greater than the carrier signal Vc.


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of Modulation

Percentage of Modulation

The modulation index is commonly computed from

measurements 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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of Modulation

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


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Sidebands and

the Frequency Domain

Side frequencies, or sidebands are generated as

part of the modulation process and occur in the

frequency spectrum directly above and below the

carrier frequency.

AM signal described previously

v2= Vc sin2π fct + (Vmsin2π fmt )(sin2π fct)

= Vc sin2π fct + (Vm/Vc)Vc (sin2π fmt )(sin2π fct)

= Vc sin2π fct + mVc (sin2π fmt )(sin2π fct)

= Vc sin2π fct + (mVc/2)cos2πt(fc-fm) - (mVc/2)cos2πt(fc+fm)

Carrier LSB USB


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Sidebands and


Sideband Calculations

Single-frequency sine-wave modulation generates

two sidebands.

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

generates a range of sidebands.

The upper sideband (fUSB) and the lower sideband

(fLSB) are calculated:

fUSB = fc + fm and fLSB = fc − fm


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Sidebands and


Figure : 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 ) Upper

sideband. (e ) Composite AM wave.


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Sidebands and


Frequency-Domain Representation of AM

Observing an AM signal on an oscilloscope, you see only amplitude variations of the carrier with respect to time.

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

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

Bandwidth is the difference between the upper and lower sideband frequencies.

BW = fUSB−fLSB


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Sidebands and


Figure : The relationship between the time and frequency domains.


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Sidebands and


Frequency-Domain Representation of AM

Example:

A standard AM broadcast station is allowed to transmit modulating frequencies up to 5 kHz. If the AM station is transmitting on a frequency of 980 kHz, what are sideband frequencies and total bandwidth?

fUSB = 980 + 5 = 985 kHz

fLSB = 980 – 5 = 975 kHz

BW = fUSB – fLSB = 985 – 975 = 10 kHz


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Sidebands and


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


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Sidebands and



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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 and

sideband 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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AM Power

When the percentage of modulation is less than the

optimum 100, there is much less power in the

sidebands.

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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AM Power

The greater the percentage of modulation, the higher

the sideband power and the higher the total power

transmitted.

Power in each sideband is calculated

PSB = PLSB = PUSB = Pcm2 / 4

Total AM power , PT= Pc(1+m2/2)

Maximum power appears in the sidebands when the

carrier is 100 percent modulated.


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Classification of Amplitude

Modulation

In amplitude modulation, two-thirds of the transmitted

power is in the carrier, which conveys no information.

Double Side Band Suppressed Carrier: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.

In Vestigial Sideband (VSB), one of the sidebands is

partially suppressed and a vestige of the other

sideband is transmitted to compensate for that

suppression.


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DSB Signals

The first step in generating an SSB signal is to suppress

the carrier, leaving the upper and lower sidebands.

This type of signal is called a double-sideband suppressed carrier (DSSC) signal. No power is wasted on the carrier.

A balanced modulator is a circuit used to produce the sum and difference frequencies of a DSSC signal but to cancel or balance out the carrier.

DSB is not widely used because the signal takes large

BW and is difficult to demodulate (recover) at the

receiver.


Modulation


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Figure : A frequency-domain display of DSB signal.


Modulation


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SSB Signals

One sideband is all that is necessary to convey

information in a signal.

A single-sideband suppressed carrier (SSSC) signal

is generated by suppressing the carrier and one

sideband.


Modulation


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Modulation

SSB Signals

SSB signals offer four major benefits:

1. Spectrum space is conserved and allows more signals to be transmitted in the same frequency range.

2. All power is channeled into a single sideband. This produces a stronger signal that will carry farther and 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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Modulation

A vestigial sideband signal (VSB) is produced by

partially suppressing the lower sideband. This kind of

signal is used in TV transmission.

In commercial TV transmission, the upper side band,

25% of the lower side band and the picture carrier are

transmitted.

Video information typically contains frequencies as high

as 4.2MHz. To reduce the BW to the 6MHz maximum

allowed TV signal, a portion of LSB is suppressed

leaving only a small vestige of the LSB. Video signals

above 0.75MHz are suppressed in LSB and all video

frequencies are transmitted in USB.


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Modulation


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There are two types of amplitude modulators. They are low-

level and high-level modulators.

Low-level modulators generate AM with small signals and

must be amplified before transmission.

High-level modulators produce AM at high power levels,

usually in the final amplifier stage of a transmitter. In high-

level modulation, the modulator varies the voltage and power

in the final RF amplifier stage of the transmitter. The result is

high efficiency in the RF amplifier and overall high-quality

performance.


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Low-Level AM: Diode Modulator

Diode modulation consists of a resistive mixing network, a diode rectifier, and an LC tuned circuit.

The carrier is applied to one input resistor and the modulating signal to another input resistor.

This resistive network causes the two signals to be linearly mixed (i.e. algebraically added).

A diode passes half cycles when forward biased.

The coil and capacitor repeatedly exchange energy, causing an oscillation or ringing at the resonant frequency.


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Figure : Amplitude modulation with a diode.


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High-Level AM: Collector Modulator

The collector modulator is a linear power amplifier that takes the low-level modulating signals and amplifies them to a high-power level.

A modulating output signal is coupled through a

modulation transformer to a class C amplifier.

The secondary winding of the modulation transformer is

connected in series with the collector supply voltage of

the class C amplifier.


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Figure : A high-level collector modulator.


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Demodulators, or detectors, are circuits that accept

modulated signals and recover the original modulating

information.

Figure : A diode detector AM demodulator.


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Diode Detector

On positive alternations of the AM signal, the capacitor charges quickly to the peak value of pulses passed by the diode.

When the pulse voltage drops to zero, the capacitor discharges into the resistor.

The time constant of the capacitor and resistor is long compared to the period of the carrier.

The capacitor discharges only slightly when the diode is not conducting.

The resulting waveform across the capacitor is a close approximation to the original modulating signal.


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Diode Detector

Because the diode detector recovers the envelope of

the AM (modulating) signal, the circuit is sometimes

called an envelope detector.

If the RC time constant in a diode detector is too long,

the capacitor discharge will be too slow to follow the

faster changes in the modulating signal.

This is referred to as diagonal distortion.


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Modulators for DSSC and SSB

For DSSC signal:

Modulator: Balanced modulator

For SSB signal:

Modulator: Balanced modulator with filter.


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Demodulators for DSSC and SSB

DSB and SSB Demodulation

To recover the intelligence in a DSB or SSB signal, the

carrier that was suppressed at the receiver must be

reinserted.

A product detector is a balanced modulator used in a

receiver to recover the modulating signal.

Any balanced modulator can be used as a product

detector to demodulate SSB signals.


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Demodulators for DSSC and SSB

Figure : A balanced modulator used as a product detector to demodulate an SSB

signal.


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Questions?

Confusions!

Thank you.

chapter02-1 amplitude modulation

Documents

modulating signal amplitude

carrier amplitude changes

concepts modulation

carrier signal vc

modulation process

degree of modulation

carrier voltage

information signal