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Department of Computer Engineering
University of San Carlos - Technological Center
Principles of Communication
Amplitude Modulation
Fundamentals and Concepts
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University of San Carlos - Technological Center
Department of Computer Engineering
Amplitude Modulation
Information signal varies the amplitude of thecarrier sine wave
Inefficient, but widely used because of its simplicity
Ex. AM radio, CB radio, Aircraft towercommunication, Garage openers, Keyless car
door entry
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Amplitude Modulation
Increase or decrease in amplitude of themodulating signal causes correspondingincrease or decrease in amplitude of carrier
signal
Carrier frequency remains constant
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Carrier peak is the zero reference for the modulating
signal
Amplitude Modulation
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Envelope
An imaginary line connecting positive andnegative peaks of the carrier
Shape of the modulating signal
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Mathematically,
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Note that
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Mathematically,
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Department of Computer Engineering
Mathematically,
carrier waveform carrier waveform * modulating waveform
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Department of Computer Engineering
Modulator
Circuit used for producing AM
Also known as
Analog multipliers
Mixers
Converters
Product detectors
Phase detectors
Opposite: demodulators or detectors
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Department of Computer Engineering
Modulator
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Department of Computer Engineering
Modulation Index
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Department of Computer Engineering
Overmodulation
Results in distortion of information
Prevention: requires that modulators somehowadjust the modulating voltage VmCompressor circuits
Amplify lower-level signals while suppressing higher-
level signals Higher average power output level without
overmodulation
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University of San Carlos - Technological Center
Department of Computer Engineering
Distortion
Generally unwanted alteration of the originalshape of a signal, causing loss of information
Ex. Garbled, harsh or unnatural sounds in voice
transmission Ex. Scrambled or inaccurate TV picture
Produces nonsinusoidal signal, results inadjacent channel interference
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Department of Computer Engineering
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Department of Computer Engineering
Percentage of Modulation
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SidebandsGenerated as part of the modulation process Occur in frequency spectrum directly above and below
carrier frequency
Single-frequency sine wave modulating signal Generates only two sidebands
Complex wave modulating signal
Generates a whole range of sidebands
Ex. Voice signals consist of many sine wave componentsthat produce a range of sidebands
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Sideband Calculations
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Sideband Calculations
carrier lower sideband upper sideband
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Sideband Calculation
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D f C E i i
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D t t f C t E i i
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Department of Computer Engineering
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Department of Computer Engineering
Bandwidth
Department of Computer Engineering
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Department of Computer Engineering
Bandwidth
Department of Computer Engineering
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AM broadcast stations
Bandwidth of 10kHz
Spectrum from 540 to 1600kHz
Total of 107 10kHz-wide channels for AM radiostations
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Pulse Modulation
Complex signals
Pulse or rectangular waves
Square waves, triangular waves
Sawtooth waves
Distorted sine waves
Fourier theory
A complex wave is made up of a fundamental sinewave and harmonics at different amplitudes
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Harmonics
Department of Computer Engineering
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Pulses generate extremely wide-bandwidth signals
All significant sidebands must be transmitted,otherwise distortion occurs
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Pulse Modulation
Amplitude-shift keying (ASK) Amplitude modulation by square waves or binary
pulses
Used to transmit binary data ON/OFF keying (OOK)
Modulation by turning the carrier on and off
Only the carrier is transmitted
Sidebands result from frequency of pulses plus theirharmonics
Ex. Transmitting Morse code
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Amplitude-Shift Keying (ASK)
Department of Computer Engineering
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ON/OFF Keying (OOK)
Department of Computer Engineering
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Splatter
Harmonic sideband interference
Distortion by overmodulation generatesharmonics that also modulate the carrier,
producing more sidebands
Remember that an AM signal is the sum of themodulated carrier and sideband frequencies
Department of Computer Engineering
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Effect of Overmodulation on AM bandwidth
a) Sine wave of 500 Hz modulating a 1MHz carrier
b) Distorted 500Hz sine wave with significant second, third and fourth
harmonics
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AM Power
Department of Computer Engineering
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AM Power Formula
Department of Computer Engineering
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Single-Sideband Modulation
Improve AM efficiency by suppressing the carrierand eliminating one sideband
DSB Signals
SSB Signals
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