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Modulasi AmplitudoPertemuan 3

Matakuliah : H0122 / Dasar Telekomunikasi

Tahun : 2008

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Mahasiswa dapat menjelaskan tujuan modulasi, prosesdan 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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Introduction Amplitude Modulation is the process of changing the Amplitude of arelatively high frequency carrier signal in accordance with theamplitude of the modulating signal (Information). AM Modulators are nonlinear devices with 2 inputs

A single, high frequency of carrier signal of constant-amplitudecarrier signal The low frequency information signal.

The Output of AM Modulator is called Modulated Wave and the shape ofthe Modulated Wave is called AM Envelope. It is often used for commercial broadcasting of both audio and videosignals.

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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 consistsof: 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 carriervoltage)/(unmodulated carrier voltage), dengan 0

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The AM waveform reaches maximum value when the modulatingsignal 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 ofthe modulating signal, and the shape of the envelope is identicalto 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-fcmaxand fcis called lower side band

(LSB)

Frequencies between fcand fc+fmmaxis called upper side band

(USB)

The Bandwidth of AM DBSFC is FB = 2fm(max)

AM signal does not contain modulating signal frequency

Amplitude

Frequency

Upper side

frequencies

Lower side band

Lower side

frequencies

Upper side band

Carrier

fc-f

m(max)

fc fc+fm(max)

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Modulation Index Modulation index is an indicator to describe the amount of amplitudechange (modulation) present in an AM waveform (depth ofmodulation), 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 outputwaveform 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) sinct Bandwidth : BT= 2 fm

e(t)

1/2T Tt

t

e(t)

fm

f

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 + , where < fm

Bandwidth DSB, SSB, and VSB: BT(SSB)

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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 sectionBandpass

filter

AM

detector

Bandpass

filter

Audio

section

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Telah dipelajari karakteristik modulasi amplitudo Telah dipelajari peran sidebands.

Summary

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