Pulse Width Modulation

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Pulse Width Modulation

• Professor Dr. Miguel Alonso Jr.

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Outline

• Principle
• Types
• Transmission Bandwidth
• Technique for Generation
• Other Applications
• Schematics of the project PWM circuit (foundation
  for group projects)

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Principle

• Pulse width modulation uses a square wave and
  varies the duty cycle to convey the information
• The duty cycle is directly proportional to the
  amplitude of the message signal

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Types

• Three types of pulse-width modulation (PWM) are
  possible.
• The pulse center may be fixed in the center of
  the time window and both edges of the pulse moved
  to compress or expand the width
• The lead edge can be held at the lead edge of the
  window and the tail edge modulated.
• The tail edge can be fixed and the lead edge
  modulated

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Transmission Bandwidth

• Much more complex than PAM tranmission bandwidth
• Recall BT K1/t
• This is a course approximation
• A fine approximation is needed for PWM
• BT 0.5/trise

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Example

• A Baseband signal with a bandwidth of 10kHz is to
  be sampled and converted into a PWM signal. Rise
  time cannot exceed 1 of the sampling time
  interval. Determine the approximate transmission
  bandwidth. Assume the minimum Nyquist sampling
  rate.

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Techniques for Generation

• Intersective
• Digital (using a microcontroller and a counter to
  count the pulse width)
• In intersective generation of a PWM, the message
  signal is compared to a sawtooth waveform.
• When the message signal is greater than the
  sawtooth, the out goes high, otherwise, it goes
  low

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Other Applications

• Telecommunications the width of the pulse can
  correspond to specific data values encoded at the
  transmitter ( we will talk about encoding schemes
  later on)
• Motor Control the average power delivered to
  the motor is proportional to the pulse width

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Schematics for PWM circuit