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Matakuliah : H0122 / Dasar Telekomunikasi
Tahun
: 2008
Modulasi Amplitudo
Pertemuan 3
Learning Outcomes
Mahasiswa dapat menjelaskan tujuan modulasi, proses
dan jenis modulasi amplitudo.
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Outline Materi
•
•
•
•
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Pengertian modulasi
Tujuan Modulasi
Proses Modulasi Amplitudo
Klasifikasi Modulasi Amplitudo
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Pengertian
• 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 = 2f c
• ec = sinyal termodulasi (gelombang carrier )
• Ec(t) = envelope dari ec(t)
• Φ(t) = deviasi fasa dari ec(t)
• f c = frekuensi carrier
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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(2fct)
• Modulating Signal => Vmsin(2fmt)
• 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
• Indeks Modulasi :
M = (modulated peak voltage-unmodulated carrier
voltage)/(unmodulated carrier voltage), dengan 0<m<1
E
mEc
2
Ec
mEc
2
f
fc-fm
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fc
fc + fm
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AM Waveform
• 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.
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AM Waveform
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Frequency Spectrum
Amplitude
Carrier
Lower side band
Upper side band
Lower side
frequencies
fc-fm(max)
Upper side
frequencies
fc
fc+fm(max) Frequency
• 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
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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
• Indeks modulasi dan daya sinyal :
Pt = Pc [1+ {m2/2}]
• dengan
– Pt = daya total
– Pc = daya carrier
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Indeks Modulasi
0<m<1
m=1
m>1
mod 100 %
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(over-modulation)
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Klasifikasi AM
• Klasifikasi modulasi amplitudo linier:
– Double side band (DSB)
• ec(DSB) = emEc(t) sin ωct
• Bandwidth : BT = 2 fm
e(t)
e(t)
1/2T
T
t
f
fm

t
t
LSB
fc
fc - fm
fc
USB
fc + fm
BT=2 fm
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Klasifikasi AM
• 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)
SSB
=30%
Modulasi
DSB
USB
fm
Filter
VSB
=30%
fw
 fc
fc + 
fc + fm
2
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AM Transmitter Diagram
Antenna
Modulating
signal
source
AM modulator
and output
power amplifier
Carrier driver
RF carrier
oscillator
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AM Receiver Diagram
RF
section
IF section
Bandpass
Filter
Bandpass
filter
Mixer/
converter
section
AM
detector
Bandpass
filter
Bandpass
filter
Audio
section
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Summary
• Telah dipelajari karakteristik modulasi amplitudo
• Telah dipelajari peran sidebands.
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