C H A P T E R 4

AMPLITUDE MODULATIONS AND DEMODULATIONS

Fundamental of Communication Systems ELCT332 FALL2011

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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)

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

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DSB-SC Modulation and Demodulation

=

Modulation

Demodulation

Example of DSB-SC modulation.

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Example

For a baseband signal , find the DSB-SC signal, and identify USB/LSB

Nonlinear DSB-SC modulator.

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Modulators

Multiplier modulators: Output is proportional to the product of two input signals.

Nonlinear modulators: Achieved by nonlinear devices.

Switching modulator for DSB-SC.

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Switching modulators: multiplying m(t) by any periodic signal of the fundamental radian frequency ωc

(a) Diode-bridge electronic switch. (b) Series-bridge diode modulator. (c) Shunt-bridge diode modulator.

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Diode Bridge Modulator

Multiplication of a signal by a square pulse train is in reality a switching operation

Ring modulation.

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Ring Modulator

Double Balanced Modulator

Frequency mixer or converter.

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Example: Heterodyning Frequency mixing/conversion

Sub-heterodyning

Super-heterodyning

AM signal and its envelope.

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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

Tone-modulated AM: (a) µ = 0.5; (b) µ = 1.

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Tone Modulation

Message Signals m(t) with Zero Offsetmmin=-mmax

Modulation index

Message Signals m(t) with Zero Offsetmmin≠-mmax

Rectifier detector for AM.Fundamental of Communication Systems ELCT332 FALL2011 12

Sideband and Carrier PowerDemodulation of AM Signals

Rectifier

Envelope detector for AM.

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Envelope Detector

(a) Original message spectrum and (b) the redundant bandwidth consumption in DSB modulations.

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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

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.

SSB spectra from suppressing one DSB sideband.

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Amplitude Modulation: Single Sideband (SSB)

Bandpass filtering

Transfer function of an ideal π/2 phase shifter (Hilbert transformer).

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Hilbert Transform

Expressing SSB spectra in terms of M+(f ) and M−(f ).

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Representation of SSB Signals

SSB spectra for tone modulation.

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Tone Modulation: SSB

Using the phase-shift method to generate SSB.

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SSB Modulation Systems

Phase shift method

(a) Relative power spectrum of speech signal and (b) the corresponding USB spectrum.

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Selective filtering method

SSB Modulation Systems

Weaver’s method

Two step modulation•Smaller carrier frequency f first to widen the SSB signal

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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.

𝑥1ሺ𝑡ሻ= 2𝜑𝑄𝐴𝑀ሺ𝑡ሻ𝑐𝑜𝑠𝜔𝑐𝑡 = 𝑚1ሺ𝑡ሻ+𝑚1ሺ𝑡ሻ𝑐𝑜𝑠2𝜔𝑐𝑡+𝑚2ሺ𝑡ሻ𝑠𝑖𝑛2𝜔𝑐𝑡

𝑥2ሺ𝑡ሻ= 2𝜑𝑄𝐴𝑀ሺ𝑡ሻ𝑠𝑖𝑛𝜔𝑐𝑡= 𝑚2ሺ𝑡ሻ−𝑚2ሺ𝑡ሻ𝑐𝑜𝑠2𝜔𝑐𝑡+𝑚1ሺ𝑡ሻ𝑠𝑖𝑛2𝜔𝑐𝑡

𝑥1ሺ𝑡ሻ= 2𝜑𝑄𝐴𝑀ሺ𝑡ሻcos(𝜔𝑐𝑡+𝜃)= 𝑚1ሺ𝑡ሻ𝑐𝑜𝑠𝜃−𝑚2ሺ𝑡ሻ𝑠𝑖𝑛𝜃+𝑚1ሺ𝑡ሻ(𝑐𝑜𝑠2𝜔𝑐𝑡+𝜃) + 𝑚2ሺ𝑡ሻsin(2𝜔𝑐𝑡+𝜃)

Cochannel Interference

Spectra of the modulating signal and corresponding DSB, SSB, and VSB signals.

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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.

VSB modulator and demodulator.

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Vestigial Sideband (VSB): Modulator and Demodulator

VSB modulator and receiver filters.

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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.

Transmitter filter HT (f), receiver front-end filter HR(f ),and the receiver output low-pass filter Ho(f ) in VSB Television systems.

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VSB Television System

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

VSB bandwidth: 6MHz

L-carrier hierarchical long-haul analog telephone frequency division multiplexing system.

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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.

Phase-locked loop and its equivalent circuit.

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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

Generation of coherent demodulation carrier using signal squaring.

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Carrier Acquisition in DSB-SC

Signal-Squaring Method

𝑥ሺ𝑡ሻ= ሾ𝑚ሺ𝑡ሻ𝑐𝑜𝑠𝜔𝑐𝑡ሿ2 = 12𝑚2ሺ𝑡ሻ+ 12𝑚2ሺ𝑡ሻ𝑐𝑜𝑠2𝜔𝑐𝑡

12𝑚2ሺ𝑡ሻ= 𝑘+𝜙(𝑡)

𝑥ሺ𝑡ሻ= 12𝑚2ሺ𝑡ሻ+ 𝑘𝑐𝑜𝑠2𝜔𝑐𝑡+𝜙(𝑡)𝑐𝑜𝑠2𝜔𝑐𝑡

Costas phase-locked loop for the generation of a coherent demodulation carrier.

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Carrier Acquisition in DSB-SC

Costas Loop

Figure 4.31 Effect of the number of picture elements on resolution. (a) Poor resolution (b) Better resolution.

Fundamental of Communication Systems ELCT332 FALL2011

Figure 4.32 Scanning pattern (raster).

Fundamental of Communication Systems ELCT332 FALL2011

Figure 4.33 (a) Horizontal deflection signal. (b) Vertical deflection signal.

Fundamental of Communication Systems ELCT332 FALL2011

Figure 4.34 Television video signal.

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Figure 4.35 (a) Television transmitter.

Fundamental of Communication Systems ELCT332 FALL2011

Figure 4.35 (b) Television receiver.

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