Phase Dithered Modulation in Switched Mode Excitation for Improved Amplitude Control

Multi-level switched mode transmitters provide a simplified method of exciting ultrasound transducers compared to linear amplifiers. Excitation waveforms are created which switch between typically 3 or 5 high voltage supplies with high drive current. Previously, Harmonic Reduction Pulse Width Modulation (HRPWM) has shown how control of amplitude and instantaneous frequency or phase can be achieved. Arbitrary excitation can be achieved using Arbitrary Harmonic Reduction Pulse Width Modulation (AHRPWM). However, the synthesis of exact tones frequencies remains problematic, especially where high frequencies, low amplitudes, or integer divisions of the transmitter sampling frequency occur. This paper presents a method of improving the amplitude control of modulated waveforms through the application of phase dithering. The addition of random noise to phase during the calculation of the modulated waveforms has the benefit of disturbing periodicity which occurs where the synthesized frequency is an integer division of the sampling frequency. An experimental investigation demonstrates the improvement of amplitude control from 11% to 3% standard deviation for a 10 MHz switched waveform sampled at 80 MHz.