CHAPTER 4
AMPLITUDE MODULATIONS AND
DEMODULATIONS
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Baseband Communications: Message signals are directly transmitted without any
modification.
Carrier Communications: Uses modulation to shift the frequency spectrum of a
signal
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation
(PM)
Modulation
Amplitude modulation: Amplitude A(t) is proportional to m(t)
Frequency modulation: Frequency is proportional to m(t)
Phase modulation: Phase
is proportional to m(t)
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DSB-SC Modulation and Demodulation
Modulation
=
Demodulation
Double-sideband, suppressed carrier (DSB-SC) modulation and demodulation.
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Example
For a baseband signal
, find the DSB-SC signal, and identify USB/LSB
Example of DSB-SC modulation.
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Modulators
Multiplier modulators: Output is proportional to the product of two input signals.
Nonlinear modulators: Achieved by nonlinear devices.
Nonlinear DSB-SC modulator.
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Switching modulators: multiplying m(t) by any periodic signal of the fundamental
radian frequency ωc
Switching modulator for DSB-SC.
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Diode Bridge Modulator
Multiplication of a signal by a square pulse
train is in reality a switching operation
(a) Diode-bridge electronic switch. (b) Series-bridge diode modulator. (c) Shunt-bridge diode modulator.
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Ring Modulator
Double Balanced Modulator
Ring modulation.
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Example: Heterodyning Frequency mixing/conversion
Super-heterodyning
Sub-heterodyning
Frequency mixer or converter.
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Amplitude Modulation (AM)
DSB-SC demodulation requires the receiver to possess a
carrier signal that is synchronized with the incoming carrier.
AM signal and its envelope.
Condition for envelope detection:
A+m(t)≥0 for all t
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Tone Modulation
Message Signals m(t) with Zero Offset
mmin=-mmax
Modulation index
Tone-modulated AM: (a) µ = 0.5; (b) µ = 1.
Message Signals m(t) with Zero Offset
mmin≠-mmax
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Sideband and Carrier Power
Demodulation of AM Signals
Rectifier
Rectifier detector for AM.
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Envelope Detector
Envelope detector for AM.
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Bandwidth-Efficient Amplitude Modulations
For a baseband signal m(t) with bandwidth B Hz, DSB
modulations require twice the RF bandwidth of transmit
(a) Original message spectrum and (b) the redundant bandwidth consumption in DSB modulations.
To Improve the spectral efficiency of AM
• Single-side (SSB) modulation
• Remove either LSB or USB
• Quadrature amplitude modulation (QAM)
• Sending two messages over the same bandwidth of 2B Hz.
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Amplitude Modulation: Single Sideband (SSB)
Bandpass filtering
SSB spectra from suppressing one DSB sideband.
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Hilbert Transform
Transfer function of an ideal π/2 phase shifter (Hilbert transformer).
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Representation of SSB Signals
Expressing SSB spectra in terms of M+(f ) and M−(f ).
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Tone Modulation: SSB
SSB spectra for tone modulation.
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SSB Modulation Systems
Phase shift method
Using the phase-shift method to generate SSB.
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SSB Modulation Systems
Selective filtering method
Weaver’s method
Two step modulation
•
Smaller carrier frequency f first to
widen the SSB signal
(a) Relative power spectrum of speech signal and (b) the corresponding USB spectrum.
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Quadrature Ampltude Modulation (QAM)
QAM offers an attractive to SSB-SC. It can be exactly generated without requiring sharp
cutoff bandpass filters.
𝑥1 𝑡 = 2𝜑𝑄𝐴𝑀 𝑡 𝑐𝑜𝑠𝜔𝑐 𝑡 = 𝑚1 𝑡 + 𝑚1 𝑡 𝑐𝑜𝑠2𝜔𝑐 𝑡 + 𝑚2 𝑡 𝑠𝑖𝑛2𝜔𝑐 𝑡
𝑥2 𝑡 = 2𝜑𝑄𝐴𝑀 𝑡 𝑠𝑖𝑛𝜔𝑐 𝑡 = 𝑚2 𝑡 − 𝑚2 𝑡 𝑐𝑜𝑠2𝜔𝑐 𝑡 + 𝑚1 𝑡 𝑠𝑖𝑛2𝜔𝑐 𝑡
Cochannel Interference
𝑥1 𝑡 = 2𝜑𝑄𝐴𝑀 𝑡 cos(𝜔𝑐 𝑡 + 𝜃)
= 𝑚1 𝑡 𝑐𝑜𝑠𝜃 − 𝑚2 𝑡 𝑠𝑖𝑛𝜃 + 𝑚1 𝑡 (𝑐𝑜𝑠2𝜔𝑐 𝑡 + 𝜃) + 𝑚2 𝑡 sin⁡
(2𝜔𝑐 𝑡 + 𝜃)
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Vestigial Sideband (VSB)
VSB modulation system, also called asymmetric sideband, is a comprise between DSB and
SSB. VSB signals are relatively easy to generate, and their bandwidth is some what greater
than that of SSB signals.
Spectra of the modulating signal and corresponding DSB, SSB, and VSB signals.
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Vestigial Sideband (VSB): Modulator and Demodulator
VSB modulator and demodulator.
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Example:
The carrier frequency of a certain VSB signal is fc=20kHz, and the baseband signal bandwidth
is 6kHz. The VSB shaping filter Hi(f) at the input, which cuts off the lower sideband gradually
over 2kHz, is shown in (a). Find the output filter Ho(f) required for distortionless reception.
VSB modulator and receiver filters.
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VSB Television System
Transmitter filter HT (f), receiver front-end filter HR(f ),
and the receiver output low-pass filter Ho(f ) in VSB Television systems.
Television signal spectra: (a) DSB video signal plus audio; (b) signal transmitted.
VSB bandwidth: 6MHz
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Frequency Division Multiplexing (FDM)
Several signals share the band of a channel, each signal is modulated by different
carrier frequency. The various carriers are separated to avoid overlap. These
carriers are referred as subcarriers.
L-carrier hierarchical long-haul analog telephone frequency division multiplexing system.
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Phase-Locked Loop (PLL)
A device typically used to track the
phase and the frequency of the carrier
component of an incoming signal.
Basic PLL Operation
•
•
•
VCO is a an oscillator whose
frequency can be linearly controlled
by an input voltage
The multiplier output is further filtered
by the loop filter and then applied to
the input of the VCO
The voltage changes the frequency
of the VCO and keeps the loop
locked by forcing the VCO output to
track the face of the input signal
Phase-locked loop and its equivalent circuit.
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Carrier Acquisition in DSB-SC
Signal-Squaring Method
𝑥 𝑡 = 𝑚 𝑡 𝑐𝑜𝑠𝜔𝑐 𝑡
2
1
1
= 𝑚2 𝑡 + 𝑚2 𝑡 𝑐𝑜𝑠2𝜔𝑐 𝑡
2
2
1 2
𝑚 𝑡 = 𝑘 + 𝜙(𝑡)
2
1
𝑥 𝑡 = 𝑚2 𝑡 + 𝑘𝑐𝑜𝑠2𝜔𝑐 𝑡 + 𝜙(𝑡)𝑐𝑜𝑠2𝜔𝑐 𝑡
2
Generation of coherent demodulation carrier using signal squaring.
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Carrier Acquisition in DSB-SC
Costas Loop
Costas phase-locked loop for the generation of a coherent demodulation carrier.
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Figure 4.31 Effect of the number of picture elements on resolution. (a) Poor resolution (b) Better resolution.
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Figure 4.32 Scanning pattern (raster).
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Figure 4.33 (a) Horizontal deflection signal. (b) Vertical deflection signal.
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Figure 4.34 Television video signal.
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Figure 4.35 (a) Television transmitter.
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Figure 4.35 (b) Television receiver.
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