Modulasi AmplitudoPertemuan 3

Matakuliah : H0122 / Dasar TelekomunikasiTahun : 2008

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Mahasiswa dapat menjelaskan tujuan modulasi, proses dan jenis

modulasi amplitudo.

Learning Outcomes

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• Pengertian modulasi• Tujuan Modulasi• Proses Modulasi Amplitudo• Klasifikasi Modulasi

Amplitudo

Outline Materi

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•Modulasi ialah proses untuk mengkombinasikan suatu sinyal yang berisi informasi dengan sinyal lain.

•Persamaan matematikanya:– ec(t) = Ec(t) cos (ωct + Φ(t))

dengan•ωc = 2f c•ec = sinyal termodulasi (gelombang

carrier )•Ec(t) = envelope dari ec(t)•Φ(t) = deviasi fasa dari ec(t)• f c = frekuensi carrier

Pengertian

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Introduction

• Amplitude Modulation is the process of changing the Amplitude of a relatively high frequency carrier signal in accordance with the amplitude of the modulating signal (Information).

• AM Modulators are nonlinear devices with 2 inputs• A single, high frequency of carrier signal of

constant-amplitude carrier signal• The low frequency information signal.

• The Output of AM Modulator is called Modulated Wave and the shape of the Modulated Wave is called AM Envelope.

• It is often used for commercial broadcasting of both audio and video signals.

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AM Waveform

•Generally.• Carrier Signal => VCsin(2fct)• Modulating Signal => Vmsin(2fmt)• Modulated Wave => Vam(t)

•For Double sideband full carrier (DSBFC) AM waveform consists of:

• DC voltage• The carrier frequency fc• Lower side frequency (fc - fm)• Upper side frequency (fc + fm) Known as AM envelope

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•Indeks Modulasi :M = (modulated peak voltage-unmodulated carrier

voltage)/(unmodulated carrier voltage), dengan 0<m<1

fc-fm fc fc + fmf

E

EcmEc

2mEc

2

fc-fm fc fc + fmf

E

EcmEc

2mEc

2

Indeks Modulasi

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• The AM waveform reaches maximum value when the modulating signal amplitude is at maximum value.

• The AM waveform reaches minimum value when the modulating signal amplitude is at maximum negative.

• The repetition rate of the envelope is equal to the frequency of the modulating signal, and the shape of the envelope is identical to the shape of the modulating signal.

AM Waveform

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AM Waveform

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Frequency Spectrum

• The Frequency Spectrum of AM DBSFC is shown below:• Frequencies between fc-fcmax and fc is called lower side band

(LSB) • Frequencies between fc and fc+fmmax is called upper side band

(USB) • The Bandwidth of AM DBSFC is FB = 2fm (max)• AM signal does not contain modulating signal frequency

Am

plitu

de

Frequency

Upper side frequencies

Lower side band

Lower side frequencies

Upper side band

Carrier

fc-fm(max) fc fc+fm(max)

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Modulation Index

•Modulation index is an indicator to describe the amount of amplitude change (modulation) present in an AM waveform (depth of modulation), basically stated in form of percentage.

•It is defined mathematically as :m = Em / Ec

• m = modulation coefficient (unitless)• Em = peak change in the amplitude of the

output waveform voltage• Ec = peak change in the amplitude of the

unmodulated carrier voltage• Percent modulation:

M= m x 100%

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• Indeks modulasi dan daya sinyal :Pt = Pc [1+ {m2/2}]

• dengan– Pt = daya total – Pc = daya carrier

Indeks Modulasi

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0 < m < 1 m = 1 m > 1mod 100 % (over-modulation)

Indeks Modulasi

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• Klasifikasi modulasi amplitudo linier:– Double side band (DSB)

•ec(DSB) = emEc(t) sin ωct •Bandwidth : BT = 2 fm

e(t)

1/2T Tt

t

e(t)

fmf

LSB USBfc

fc + fmfc - fm

t

BT=2 fm

fc

Klasifikasi AM

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•Single side band (SSB)– Bandwidth : BT = fm

•Vestigial side band (VSB)– Bandwidth : BT = fm + α, where α <

fm•Bandwidth DSB, SSB, and VSB:

– BT(SSB) < BT(VSB) < BT(DSB)

=30%

ModulasiDSB

Filter VSB

SSB USB

=30% fmfw

fc

2

fc + fc + fm

=30%

ModulasiDSB

Filter VSB

SSB USB

=30% fmfw

fc

2

fc +

=30%

ModulasiDSB

Filter VSB

SSB USB

=30% fmfw

fc

2

=30%

ModulasiDSB

Filter VSB

SSB USB

=30% fmfw

=30%

ModulasiDSB

Filter VSB

SSB USB

=30% fm

ModulasiDSB

Filter VSB

ModulasiDSB

Filter VSB

ModulasiDSB

Filter VSB

SSB USB

=30% fmfw

fc

2

fc + fc + fm

Klasifikasi AM

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AM Transmitter Diagram

RF carrier oscillator

Carrier driver

Modulating signal source

AM modulator and output

power amplifier

Antenna

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AM Receiver Diagram

RF section

Bandpass Filter

Mixer/ converter section

Bandpass filter

IF section Bandpass filter

AM detector

Bandpass filter

Audio section

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• Telah dipelajari karakteristik modulasi amplitudo

• Telah dipelajari peran sidebands.

Summary