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Pulse Width Modulation

Pulse Width Modulation. Professor: Dr. Miguel Alonso Jr. Outline. Principle Types Transmission Bandwidth Technique for Generation Other Applications Schematics of the project PWM circuit (foundation for group projects). Principle.

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Pulse Width Modulation

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  1. Pulse Width Modulation Professor: Dr. Miguel Alonso Jr.

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

  3. 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

  4. 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

  5. Transmission Bandwidth • Much more complex than PAM tranmission bandwidth • Recall BT = K1/τ • This is a course approximation • A fine approximation is needed for PWM • BT = 0.5/trise

  6. 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.

  7. 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

  8. 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

  9. Schematics for PWM circuit

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