CHAPTER 2: AMPLITUDE MODULATION

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AM RADIO/RECEIVER

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TOPICS

Need for Modulation Principles of AM Modulation Index and Signal Power AM Circuits Single Sideband Suppressed Carrier

(SSBSC) SSB Circuits

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NEED FOR MODULATION It is because modulation makes the information

signal more compatible with the medium.

Modulation = Imposing information at low frequency onto a higher frequency signal.

A technique for transmitting information efficiently from one place to another.

Simplest form of modulation is the amplitude modulation.

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PRINCIPLES OF AMPLITUDE MODULATION

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PRINCIPLES OF AM AM is defined as:

Amplitude of carrier frequency change proportionately to the value of the modulation signal.

Advantages: Simple modulator circuits Cheap :low-quality form of modulation used for

commercial broadcasting of audio & video signal.

Disadvantages: Poor performance due to noise Inefficient use of transmitter power.

Application: 2 way radio communications, broadcasting, aircraft

comm. & citizen band (CB) radio.6

AM modulators are nonlinear devices 2 input and 1 output:

modulating signal and carrier signal.

Several types of amplitude modulation AM DSBFC DSB-SC SSB

AM generation is shown in Figure 2.1

Modulated wave = AM envelope as shown in Figure 2.2

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Figure 2.1: Block diagram of Amplitude Modulation

Information, Vm(t)

Carrier, Vc(t)

ModulatorVAM (t)

8Figure 2.2 AM signal with the envelope

AM IN ACTION

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AM begins with carrier vc , a sine wave with frequency c & amplitude Vc:

Modulating signal:

Then AM is:

DERIVATION OF AM EQUATION

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Where m (modulation index) is defined as Vm/Vc, hence:

The voltage resulting AM wave envelope at any instant is:

tfmVVV

VVmtfVVV

vVV

mccenv

cmmmcenv

mcenv

2sin

]/[2sin

)2sin1( tfmVV mcenv

tftfmV

tfVv

cmc

cenv

2sin)2sin1(

2sin

This yield, the upper and lower sidebands – frequency & amplitude. 11

tffV

tffV

tfVv mcm

mcm

ccAM )(2cos2

)(2cos2

2sin

Carrier LSB USB

tffVm

tffVm

tfVv

mcc

mcc

cc

)(2cos2

)(2cos2

2sin

)cos()cos(2/1))(sin(sin bababa

Using Trigo ID

AM FREQUENCY SPECTRUM & BANDWIDTH

AM modulators are non-linear device => non-linear mixing occurs.

Output envelope is complex wave made up of DC voltage, carrier frequency, the sum (fm + fc) & difference (fc – fm) frequencies.

AM spectrum contains frequency component spaced fm Hz on either side of the carrier.

Figure 2.3 shows the frequency spectrum of AM wave.

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Figure 2.3: Frequency spectrum for AM wave

fc fc + fmfc - fm

Bandwidth = 2fm

Vc

Vm /2Vm /2

SPECTRUM PARAMETERS

Center frequency = Carrier frequency =

Upper sideband freq. = carrier freq. + modulating freq.

Lower sideband freq. = carrier freq. - modulating freq.

mcUSB fff

mcLSB fff

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Center frequency peak amplitude:

Upper and lower sideband voltages:

Bandwidth = Maximum freq. - minimum freq.

2m

LSBUSB

VVV

cV

m

mcmc

lsbusb

f

ffff

ff

ffBW

2

)()(

minmax

cf

EXAMPLE 2.1

Q. Modulating signal fm =3 kHz frequency and a carrier frequency fc =1 MHz. What is the upper & lower sideband frequency? Then find the bandwidth of the modulated signal.

A. 997 kHz, 1003 kHz, 6 kHz.

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EXAMPLE 2.2

Q. A 1.4 MHz carrier is modulated by a signal with frequencies from 20Hz & 10KHz. Determine the range of frequencies generated for the upper and lower sidebands?

A. USB = 1.400020Hz, 1.410000Hz, LSB = 1.390000Hz, 1.399980Hz

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