Spread Spectrum PWM Signal Generator for Fully Digital Audio Amplifier

A generation scheme to make a low noise low distortion PWM signal spreading its spectrum for the fully digital audio amplifier is proposed. The carrier frequency of the PWM is fluctuated randomly to spread the spectrum of the carrier component. The noise-shaping filter in the delta-sigma modulator is implemented so that the filter works with a changing sampling period preserving the designed frequency characteristics. Such a filter is designed in continuous-time manner considering the distortion due to the PWM and is realized as a digital filter to compensate the effect of the sampling period fluctuation with feedback manner. A S/(N+D) ratio of over 120 dB is achieved for the generated PWM signal.