The Effect of Word Length on the Harmonic Content of Microprocessor-Based PWM Waveform Generators

In a microprocessor-based pulsewidth modulated (PWM) waveform generator, the quality of the waveform depends on the accuracy with which the position of the switching edges of the waveform can be determined. This in turn depends on the length of the binary word or digital counter used to control the edge positions. The quantization effects experienced in microprocessor-based PWM waveform generators are analyzed for various word lengths. It is demonstrated that good waveform quality can be achieved by an 8-b microprocessor for a wide range of applications, including those in which the carrier frequency is in the ultrasonic range. The microprocessor employs a fast method of voltage control based on addition. Furthermore, an increase in the line-to-line output voltage is obtained without output distortion by adding a measure of third harmonic to the phase waveform.