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C H A P T E R 4

AMPLITUDE MODULATIONS AND DEMODULATIONS Fundamental of Communication Systems ELCT332 FALL2011

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) Fundamental of Communication Systems ELCT332 FALL2011 2

DSB-SC Modulation and Demodulation

Modulation Demodulation Double-sideband, suppressed carrier (DSB-SC) modulation and demodulation.

Fundamental of Communication Systems ELCT332 FALL2011 3

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.

Fundamental of Communication Systems ELCT332 FALL2011 Nonlinear DSB-SC modulator.

Switching modulators

: multiplying m(t) by any periodic signal of the fundamental radian frequency ω c 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

Fundamental of Communication Systems ELCT332 FALL2011 Ring modulation.

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.

Condition for envelope detection: A+m(t)≥0 for all t Fundamental of Communication Systems ELCT332 FALL2011 AM signal and its envelope.

Tone Modulation

Message Signals m(t) with Zero Offset m min =-m max

Modulation index Tone-modulated AM: (a)

= 0.5; (b)

= 1.

Message Signals m(t) with Zero Offset m min ≠-m max

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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

(f )

and

−

(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. Fundamental of Communication Systems ELCT332 FALL2011 19

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.

𝑥 1 𝑡 = 2𝜑 𝑄𝐴𝑀 𝑡 𝑐𝑜𝑠𝜔 𝑐 𝑡 = 𝑚 1 𝑡 + 𝑚 1 𝑡 𝑐𝑜𝑠2𝜔 𝑐 𝑡 + 𝑚 2 𝑡 𝑠𝑖𝑛2𝜔 𝑐 𝑡 𝑥 2 𝑡 = 2𝜑 𝑄𝐴𝑀 𝑡 𝑠𝑖𝑛𝜔 𝑐 𝑡 = 𝑚 2 𝑡 − 𝑚 2 𝑡 𝑐𝑜𝑠2𝜔 𝑐 𝑡 + 𝑚 1 𝑡 𝑠𝑖𝑛2𝜔 𝑐 𝑡

Cochannel Interference

𝑥 1 𝑡 = 2𝜑 𝑄𝐴𝑀 𝑡 cos(𝜔 𝑐 𝑡 + 𝜃) = 𝑚 1 𝑡 𝑐𝑜𝑠𝜃 − 𝑚 2 𝑡 𝑠𝑖𝑛𝜃 + 𝑚 1 𝑡 (𝑐𝑜𝑠2𝜔 𝑐 𝑡 + 𝜃) + 𝑚 2 𝑡 sin⁡ 𝑐 𝑡 + 𝜃) Fundamental of Communication Systems ELCT332 FALL2011 21

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 f c =20kHz, and the baseband signal bandwidth is 6kHz. The VSB shaping filter H i (f) at the input, which cuts off the lower sideband gradually over 2kHz, is shown in (a). Find the output filter H o (f) required for distortionless reception.

VSB modulator and receiver filters.

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VSB Television System

Transmitter filter

H T (f)

, receiver front-end filter

H R (f )

, and the receiver output low-pass filter

H o (f )

in VSB Television systems .

Television signal spectra: (a) DSB video signal plus audio; (b) signal transmitted.

VSB bandwidth: 6MHz Fundamental of Communication Systems ELCT332 FALL2011 25

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. Fundamental of Communication Systems ELCT332 FALL2011 27

Carrier Acquisition in DSB-SC Signal-Squaring Method

𝑥 𝑡 = 𝑚 𝑡 𝑐𝑜𝑠𝜔 𝑐 𝑡 2 = 1 2 𝑚 2 𝑡 + 1 2 𝑚 2 𝑡 𝑐𝑜𝑠2𝜔 𝑐 𝑡 1 2 𝑚 2 𝑡 = 𝑘 + 𝜙(𝑡) 𝑥 𝑡 = 1 2 𝑚 2 𝑡 + 𝑘𝑐𝑜𝑠2𝜔 𝑐 𝑡 + 𝜙(𝑡)𝑐𝑜𝑠2𝜔 𝑐 𝑡 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. Fundamental of Communication Systems ELCT332 FALL2011 29

Figure 4.31

Effect of the number of picture elements on resolution. (a) Poor resolution (b) Better resolution.