Chapter 2: Linear Modulation Techniques
Undergraduate Program
School of Electrical and Computer Engineering
Overview
• Introduction
• Conventional AM
• Double-Sideband Suppressed Carrier
• Single Sideband
• Vestigial-sideband
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Modulation Process
• Modulation: transforming an information-bearing signal m(t)
(lowpass) into a narrowband bandpass signal x(t)
• m(t) is also called the modulating signal
• Starting with a sinusoidal signal (carrier)
• Varying A=A(t) accordingly to m(t) – amplitude modulation (AM)
• Varying ϕ = ϕ(t) according to m(t) – phase modulation (PM)
• Varying f=f(t) according to m(t) – frequency modulation (FM)
• FM and PM can be viewed as angle modulation
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Modulation Process
• General form of modulated signal
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Amplitude Modulation (AM)
• Information-bearing signal m(t) is impressed onto the
carrier amplitude
• Four types of AM
1. Conventional
2. Double sideband suppressed carrier (DSB-SC)
3. Single sideband (SSB)
• Can be lower or upper (LSB/USB)
4. Vestigial sideband (VSB)
• Relevant parameters
• Spectral characteristics and bandwidth
• Modulation index
• Power efficiency
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Overview
• Introduction
• Conventional AM
• Double-Sideband Suppressed Carrier
• Single Sideband
• Vestigial-sideband
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Conventional AM
• General form:
• m(t) must be constrained: -1 ≤ m(t)
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Conventional AM - Sinusoidal Modulation
• Modulated signal:
• Minimum and maximum carrier amplitudes
• Modulation index:
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Conventional AM - Sinusoidal Modulation
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Conventional AM - General Case
• General form:
• Modulated signal spectrum
• Measured by spectrum analyzer: no infinite height for delta
function in practice
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Generation of Conventional AM
• Power-law modulator
• Using variable-gain amplifier (modulator)
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Generation of Conventional AM
• Switching modulator
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Demodulation of Conventional AM
• Envelope detector
• RC - lowpass filter
• Attenuates carrier,
passes modulating signal
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Demodulation of Conventional AM
• Product detector
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Advantages/Disadvantages of Conventional AM
• Advantages
• Very simple demodulation (envelope detector)
• “Linear” modulation
• Disadvantages
• Low power efficiency
• Double the baseband bandwidth
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Overview
• Introduction
• Conventional AM
• Double-Sideband Suppressed Carrier
• Single Sideband
• Vestigial-sideband
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Double-Sideband Suppressed Carrier (DSB-SC)
• How to increase power efficiency?
• DSB-SC signal:
• Example: sinusoidal modulation
• Spectrum:
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DSB-SC: General Case
• DSB-SC Signal:
• Spectrum:
• What do you see on spectrum analyzer?
• Bandwidth? Power efficiency? PSD?
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Generation of DSB-SC
• Balanced modulator
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Generation of DSB-SC
• Ring modulator
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Demodulation of DSB-SC
• Why will the envelope detector not work?
• Product detector is usually used for demodulation
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Demodulation of DSB-SC
• Demodulation – Costas loop
V4(t)=K sin2θe
V3(t)=1/2(1/2AOAC)2m2 (t)sin2θe
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Demodulation of DSB-SC
• Product detector + squaring carrier recovery loop
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Demodulation of DSB-SC
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Advantages/Disadvantages of DSB-SC
• Advantages
• High power efficiency
• If message m(t) > 0, envelope detection is possible
• Disadvantages
• Double the baseband bandwidth
• Complex modulation/demodulation (some form of carrier recovery
is required)
• Pilot tone may be required to simplify demodulation
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Summary
• Conventional AM
• Time-domain and frequency-domain representations
• Power efficiency and bandwidth
• Generation (modulation) and demodulation of conventional
AM
• Double sideband suppressed carrier (DSB-SC)
• Spectrum
• Bandwidth
• Generation and demodulation of DSB-SC
• Advantages/disadvantages of conventional & DSB-SC AM
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Overview
• Introduction
• Conventional AM
• Double-Sideband Suppressed Carrier
• Single Sideband
• Vestigial-sideband
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Single Sideband (SSB) AM
• Why SSB-AM?
• Spectral efficiency is of great importance
• Conventional and DSB-SC occupy twice the message
bandwidth
• All the information is contained in either half
• The other is redundant
• Spectral efficiency can be greatly (twice) increased by
transmitting one half
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Generation of SSB: Anaysis
• Hilbert transform can be effectively used
• Start with the message m(t) and show that USB (Upper
SSB) is given by
• Similarly, LSB can be expressed as
• In-phase and quadrature channels are required to generate
SSB
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USB: Frequency-Domain Viewpoint
• Time-domain signal
• Spectra of individual components
• Use multiplication property of FT
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USB: Frequency-Domain Viewpoint
• Combine the two expressions above
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Lower SSB (LSB)
• Analysis method is the same as for USB
• Time-domain signal is
• Its spectrum is
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Example: Sinusoidal Modulating Signal
• Assume that
• Then
• Obviously, this is LSB signal with one spectral component
only at (ωC-Ω)
• Modulated signal
is just a sinusoidal
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Generation of SSB
• Filtering method
• Using balanced modulators
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Demodulation of SSB
• Product detector
• After low-pass filter, only 1st two terms remain
• Coherent demodulation: θ=0
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Overview
• Introduction
• Conventional AM
• Double-Sideband Suppressed Carrier
• Single Sideband
• Vestigial-sideband
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Vestigial-sideband (VSB) AM
• SSB can be simplified by allowing a part of the other
sideband to appear
• A filter implementation is feasible
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VSB spectrum and Filter Response
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Demodulation of VSB
• Multiplier (coherent) demodulator
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Comparison of Conventional AM
• Conventional AM
• Simple to modulate and to demodulate, but low power efficiency
(33-50% max) and double the bandwidth
• DSB-SC
• High power efficiency, but more complex to modulate and
demodulate, doubles the bandwidth
• SSB
• High power efficiency, the same (message) bandwidth, but more
difficult to modulate and demodulate
• VSB
• Lower power efficiency and larger bandwidth but easier to
implement
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