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

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

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

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

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.

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

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

Schematics for PWM circuit