Ultrasonic phased array techniques using switched-mode excitation

Ultrasound is predominantly an ‘active’ sensing modality, with information obtained by transmission of an acoustic wave, followed by analysis of received signals. Transmission
occurs when electrical signals are converted to acoustic signals. This thesis covers the design and application of these electrical signals to an array transducer. As an introduction, the development of a bespoke ultrasound array research platform is discussed. This enabling technology is built around switched-mode excitation: a method of approximating an analogue waveform by selecting between discrete voltage levels. The use of switched-mode methods has led to three major topics of research.

Firstly, a transmit beamformer architecture that provides fine control of excitation sequence timing using embedded-phase locked loops is presented. This enables accurate
implementation of firing sequences or phasing between transducer elements, thus minimizing time-quantization error, and providing an improved representation of the expected pressure field. An introduction to transmit beamforming is given, the impact of timequantization
is discussed, and the transmit beamformer’s performance is demonstrated.

Secondly, a method of arbitrary waveform generation using switched-mode excitation is described. The method encodes width-modulated sequences of three or five discrete
voltage levels, that, once passed through a transducer, give close approximation to the desired arbitrary waveform. Applications include: power control, pulse shaping, and array apodization. Each application is demonstrated by simulation and experimentation. An extension to the method is shown for ‘chirp’ coded imaging, demonstrating the capability for generation of frequency modulated waveforms. The improvement in image quality when compared with conventional square-wave, ‘pseudo-chirp’ excitation signals
is shown.

Lastly, the performance of the width-modulated signals is further extended so as to remove unwanted third-harmonic content whilst still maintaining pulse amplitude control.Removal of the third harmonic reduces harmonic distortion, has benefits in applications such as harmonic imaging, and extends the use of switched-mode operation with wide bandwidth transducers.