ce- chapter2 am

8/3/2019 CE- Chapter2 AM

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AMPLITUDEMODULATION


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MODULATION PRINCIPLES

The process of having the information to betransmitted alter a high-frequency signal forthe purpose of transmitting the information

somewhere in the electromagnetic spectrumvia radio, wire, or fiber optic.

Information signal (modulating signal): voice,

video, binary data Carrier: - higher frequency signal

- Sine wave


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AMPLITUDE MODULATION with SINEWAVES

Amplitude Modulation is the simplest andearliest form of transmitters. (535 1605 kHz)

AM applications include broadcasting in

medium- and high-frequency applications, CBradio, and aircraft communications.


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during the modulation process: carrierfrequency remain constant but its amplitudevaries accordance with the modulating signal.

Envelope showing it is same as themodulating signal.

Time domain signal variation of voltage over

time. Peak value of the modulating signal be less

than the peak value of the carrier.

AMPLITUDE MODULATION with SIGNWAVES (Cont.)


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Sine wave carrier can be expressed as below:


tfVv ccc 2sin

Where

vc = instantaneous value of the sine voltage

Vc = the peak value of the sine wave asmeasured between zero and themax.amplitude.

fc= frequency of the carrier sine wave.

t = some particular points in time during the accycle.

*for modulating signal, signal can be expressed with similarexpression


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Complete modulated wave can be expressedas:

carriermodulation

tftfV

carrier

tfVvtfvv

envelopevVv

cmmcc

c

mc

)2(sin2sin2sin

2sin

)(?

2

12

1

modulator

vm v2

Vc



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AMPLITUDE MODULATION WITH

DIGITAL SIGNAL

Binary 1= produce maximum carrier amplitude

Binary 0= produce a lower-value carrier.

Amplitude Shift Keying (ASK):

The carrier is switched between twodifferent carrier levels.

On-Off Keying (OOK):

The binary 1 turns the carrier on and thebinary 0 level turns the carrier off.


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MODULATION INDEX

Modulation Index - The ratio between theamplitudes between the amplitudes of themodulating signal and carrier, expressed by

the equation:

Should be between 0 and 1. Ideal condition: m=1

If more than 1: severe distortion of the

modulated waveform or overmodulation.

c

m

V

Vm =


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Modulation index can be computed from Vmaxand Vmin.

It can be expressed as:

The amount or depth of AM is normallyexpressed as percent modulation rather thanas a fractional.

Percent of modulation is 100 x m.

MODULATION INDEX

minmax

minmax=VV

VVm


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OVERMODULATION

The waveform is flattened near the zero line. A sine wave whose negative peaks have

been clipped off.


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SIDEBANDS AND THE FREQ.

DOMAIN

Sidebands is a modulating signal that containsmultiple frequencies such as voice, video ordigital signal.

Time domain information can be obtained usingan oscilloscope.

Frequency domain information can becalculated using Fouriermethods, but

trigonometric methods are simpler and valid.


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Sidebands are calculated using the formulasat below:

From the equation for an AM signal v2:

From trigonometric identity:

sin A sin B= [cos (A-B)/2 ] [cos (A+B)/2]

SIDEBANDS

mcLSB

mcUSB

fff

fff

)2(sin2sin2sin2 tftfVtfVv cmmcc


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By substituting the identity into the expression,the instantaneous amplitude of the signalsbecomes:

This means, an AM wave contains carrier and

sidebands frequencies. When modulation occurs, sidebands are

produced.

SIDEBANDS

USB

mc

m

LSB

mc

m

carrier

cc fftV

fftV

tfVe )(2cos

2

)(2cos

2

2sin2


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FREQUENCY DOMAIN

Plot the carrier and sideband amplitudes withrespect to frequency.

http://www.rfcafe.com/references/electrical/NEETS%20Modules/NEETS-Module-12-1-41-1-50.htm


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BANDWIDTH

Signal bandwidth is an important characteristic ofany modulation scheme.

The difference between the upper and lowersidebands freq.

In general, a narrow bandwidth is desirable.

Bandwidth is calculated by:

B=2fm

Example:If the carrier frequency is 2.8MHz and the maximummodulating frequency for voice signals is 3kHz.Calculate the upper, lower sidebands and also thebandwidth.

(ans: fUSB= 2803kHz, fLSB=2797kHz and BW= 6kHz)


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COMMON AM APPLICATION

AM radio broadcasting

TV picture (video)

Two- way radio (aircraft, amateur radio (SSB),

military)

Digital data transmissions

Computer modems


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AM POWER DISTRIBUTION

Power in a transmitter is important, but themost important power measurement is that ofthe portion that transmits the information.

AM carrier power remain unchanged. So, it iswasted matter.

Power in an AM transmitter is calculatedaccording to the formula at the right:

PT= PC + PLSB + PUSB or

PT= PC (1+m2/2)


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SIDEBANDS POWERS

Depends upon the value of the modulationindex:

The greater the percentage of mod, the higher the

sideband power. The power in each sideband Ps is expressed

by:

The power in each sideband is 25% of thecarrier power if 100% modulation.

4

)(2

mPPPP

C

USBLSBS


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POWER CALCULATIONS

Total antenna current is given by:

2/1 2mIICT

The total AM power the is:RIP

TT

2

Modulation percentage then is given by:

1/2 2 CTIIm


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SINGLE SIDEBANDS

The two sidebands of an AM signal are mirrorimages of one another

As a result, one of the sidebands is redundant

Using single-sideband suppressed-carriertransmission results in reduced bandwidthand therefore twice as many signals may betransmitted in the same frequency range.

Typically, a 3dB improvement in signal-to-noise ratio is achieved as a result of SSBSC.

Sidebands generated only during modulationprocess (ex: when someone speaks into mic).


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BENEFITS OF SINGLESIDEBANDS

The spectrum space occupied by the SSBsignal is only half that of AM and DSB signals.

All the power previously devoted to the carrier

and other sideband can be channeled into thesignal sideband.

Less noise on the signal because the SSBsignal has less BW than an AM or a DSB

signal. Less fading than an AM signal. The signal

alternately increases and decreases instrength as it is picked up by the receiver.


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TRANSMITTED SIGNAL OF AM

AND SSB

http://www.rfcafe.com/references/electrical/NEETS%20Modules/NEETS-Module-17-2-11-2-20.htm


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The power advantage of SSB over AM is 3:1.

Peak Envelope Power (PEP) is expressed by:

PEP=V2/R

where V= root mean square (rms) output voltage

R= load resistance

OR

PEP=VsImaxwhere Vs= the amplifier supply voltage

Imax= the current peak

SSB POWER


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SSB

Two-way radio used in the military, CB radioand radio amateurs.

DSB

FM and TV broadcasting to transmit two-channel stereo signals.

Used in some types of phase-shift keying totransmit binary data.

DSB AND SSB APPLICATIONS


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AM CIRCUITS

Two basic ways to produce amplitudemodulation:

1- Multiply the carrier by a gain or attenuation

factor that varies with the modulating signal.2- Linearly mix or algebraically add the carrierand modulating signals and then apply the

composite signals to a nonlinear device orcircuit.

LOW LEVEL MODULATION


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LOW LEVEL MODULATIONSYSTEM

Low-level modulation systems use linear poweramplifiers


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LOW-LEVEL AMPLITUDE

MODULATOR

Differential Amplifier Modulator

DIFFERENTIAL AMPLIFIER


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DIFFERENTIAL AMPLIFIERMOD

The differential amplifier makes an excellentamplitude modulator.

Has high gain

Good linearity

Can be modulated 100%

Better frequency than this circuit is PIN Diode

Modulator.


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HIGH-LEVEL AMPLITUDE

MODULATION

High-level means the signals are generatedare relatively high voltage and poweramplitudes.

The modulator varies the voltage and power inthe final RF amplifier stage of the transmitter.

One of the HLAM is the collector modulator.

G


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Collectormodulator

HIGH-LEVEL AMPLITUDE

MODULATION


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AMPLITUDE DEMODULATOR

A circuit that accepts a modulated signal andrecovers the origin modulating information.

Available in any radio receivers.

The simplest and most widely used amplitudedemodulator is the diode detector.


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BALANCED MODULATOR

Generates a DSB signal.

The inputs are the carrier and a modulatingsignal and the output are the upper and lower

sidebands. The output can be further processed by filters

or phase-shifting circuitry to eliminate one ofthe sidebands to get SSB.

Examples of balanced modulator:1- Diode ring or lattice modulator

2- IC balanced modulator (1496/1596)


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AN SSB TRANSMITTER

Two primary method of generating SSBsignals:

1- Filter 2- Phasing

An SSB transmitter using the filter

method


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FILTER METHOD

A crystal oscillator provides the carrier signalto be applied to the balanced modulator.

The DSB signal is passed to the band-pass

filter to selects either the upper or the lowersideband.

The filter are usually designed with a

bandwidth of approximately 2.5 to 3kHz.


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The phasing method of SSB signalgeneration

AN SSB TRANSMITTER

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