Analog Modulation.

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Analog Modulation

• CEC

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Contents

• Electromagnetic Spectrum.
• Communication System Block Diagram.
• Definition of Modulation.
• Need for Modulation.
• Types of Analog Modulation.
• Modulation Index.
• Expressions for Modulated Waves.
• Spectrum of Modulated Waves.

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Wavelength
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Frequency Range Uses
Very Low Frequency (VLF) 10 30 KHz Telegraphy
Low Frequency (LF) 30 300 KHz Maritime Navigation
Medium Frequency (MF) 300 3000 KHz Broadcasting
High Frequency (HF) 3 30 MHz Long Distance
Very High Frequency (VHF) 30 300 MHz TV, Radar
Ultra High Frequency (UHF) 300 3000 MHz Short Distance
Super High Frequency (SHF) 3 30 GHz Satellite
Extra High Frequency (EHF) 30 300 GHz Government
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Radio Broadcast
Purpose MW/SW Range
AM Broadcast Medium Wave 535 1605 KHz
AM Broadcast Short Wave 3 30 MHz
FM Broadcast - 88 108 MHz

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Television Broadcast
Frequency Band Range
VHF (Lower) I 47 68 MHz
VHF (Upper) II 174 230 MHz
VHF (Lower) III 470 598 MHz
VHF (Upper) IV 606 870 MHz

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Satellite Communication
Frequency Range
UHF 300 MHz 1 GHz
L Band 1 2 GHz
S Band 2 4 GHz
C Band 4 8 GHz
X Band 8 12 GHz
Ku Band 12 18 GHz
K Band 18 27 GHz
Ka Band 27 40 GHz

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Communication System Blocks

Free Space
Transmitter
Channel
Receiver
Reception, Amplification, Demodulation and
Reproduction
Intelligence Pickup, Amplification, Modulation
and Broadcasting
Noise
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Modulation

• Modulating Signal of low frequencies contains
  intelligence, base band signal.
• Carrier a high frequency signal, contains no
  intelligence.
• Modulation means superimposing intelligence onto
  a carrier.
• Resultant signal called modulated signal.
• Analog Modulation Modulating signal is an
  analog (time varying) signal (eg speech, music
  etc.).

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Modulation

• Digital Modulation Modulating signal is a
  digital signal such as square pulses.
• Carrier Parameter varied continuously in Analog
  Modulation.
• Carrier Parameter varied in discrete steps in
  Digital Modulation.
• Carrier Parameter varied in Amplitude, Frequency
  or Phase.

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Why Modulation?

• Reception at longer distances.
• To reduce antenna size.
• Simultaneous transmissions.
• Multiplexing sharing a common medium.
• Reduction in noise and interference.
• Improved signal to noise ratio.

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Atmospheric Attenuation

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Envelope
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Analog Modulation

• Modulating signal alters one of the
  characteristics of the carrier wave.
• Amplitude Modulation amplitude of the carrier
  varied in accordance with the instantaneous
  amplitude of the modulating signal.
• Frequency Modulation frequency of the carrier
  varied in accordance with the instantaneous
  amplitude of the modulating signal.

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Analog Modulation

• Phase Modulation phase of the carrier varied in
  accordance with the instantaneous amplitude of
  the modulating signal.
• Modulator A circuit which produces modulated
  output.

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Modulation Index

• Also called modulation depth.
• Defined differently in each modulation scheme.
• Describes by how much the modulated variable of
  the carrier signal varies around its unmodulated
  level.
• Measure based on the ratio of modulation
  excursions of the RF signal to the level of the
  unmodulated carrier.

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vm Vmsin2fmt
Vm lt Vc

vc Vcsin2fct
v2 v1sin2fct
Envelope of modulated signal v1 Vc vm Vc
Vmsin2fmt
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Expression for Amplitude Modulated Wave

• Sine wave carrier vc Vcsin2fct
• vc - instantaneous value of carrier signal.
• Vc - peak amplitude of carrier signal.
• fc - frequency of carrier signal.
• Sine wave modulating signal vm Vmsin2fmt
• vm - instantaneous value of information
• signal.
• Vm - peak amplitude of information signal.
• fm - frequency of modulating signal.
• Assume Vmlt Vc.

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Expression for Amplitude Modulated Wave

• Instantaneous value of the envelope
• v1 Vc vm Vc Vmsin2fmt
• Peak value of carrier voltage
• v2 v1sin2fct
• Substituting for v1, modulated signal
• v2 (Vc Vmsin2fmt)sin2fct
• Vcsin2fct Vmsin2fmtsin2fct
• Second term is the carrier waveform multiplied
  by the modulating signal.

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Expression for Amplitude Modulated Wave

• Expression for AM signal
• Since

Upper Side Band
Lower Side Band
Carrier
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Frequency Spectrum of AM Wave
Total Power in an AM Wave
Pc Carrier Power
Carrier
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Amplitude Modulation
A low frequency signal rides over a high
frequency signal.
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Modulating signal will be the envelope of the
carrier
Envelope contains intelligence
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m lt 1
m 1
m gt 1
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Expression for FM Wave

• Instantaneous frequency fi(t) varied linearly
  with message signal m(t)
• kf is the frequency sensitivity of the modulator
  in hertz per volt.
• Instantaneous angle
• Frequency modulated signal

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Spectrum of FM Wave

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Phase Modulation

• Phase angle varied linearly/sinusoidally with the
  message signal m(t)
• kp is the phase sensitivity of the modulator in
  radians per volt.
• Phase modulated signal

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Bandwidth

• Difference between highest and lowest frequencies
  of interest.
• For AM, bandwidth is the difference between two
  side band frequencies.
• Bandwidth in AM (fc fm) (fc fm) 2fm.
• FM and PM can have infinite number of sideband
  components.
• Bandwidth requirement is higher for FM and PM.

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Thank You