Dissertation

Microwave Oscillator Ultrasound Receivers

Robbins, Tom

2020

Overview

Ultrasound refers simply to those sound waves which are above the range of human hearing. Since the discovery of the piezoelectric effect in the late 19th Century, the generation and reception of ultrasound has become an integral (though understated) part of modern life in fields as diverse as cardiac imaging and automotive range-finding. For the vast majority of applications, high-frequency ultrasound is generated and received by means of some piezoelectric material, most often derived from lead zirconate titanate (PZT), a technology which has changed little since the 1970s. In recent decades, there has been a push to overcome some limitations posed by PZT and related technologies. Two prominent technologies which have emerged as alternatives are micro-electromechanical systems (MEMS) such as capacitivie micromachined ultrasound transducers (CMUT), and optical Fabry-Perot hydrophones. We examined the possibility of creating a purely electronic analogue of the optical Fabry-Perot hydrophone. The device we invented is called a microwave oscillator ultrasound receiver (MOUR). It is implemented by a compact electromagnetic resonator on a piece of high-permittivity circuit board covered by a thin superstrate layer. When the superstrate layer deforms (as a result of incoming ultrasound), the transfer function of the resonator shifts, and that shift can be detected by reading out the IQ modulation of a carrier transmitted through the resonator at or near its resonant frequency. To multiplex the device, an array of resonators, each with a different resonant frequency, can be produced. Each element is thereby associated with its own microwave channel, since each element will modulate only the carrier near its electromagnetic resonance. In this way, frequency division multiplexing (FDM) is implemented, allowing fast readout of each element by tuning the receiver circuitry to the relevant microwave frequency. This has the potential to greatly simplify the design of ultrasound receiver arrays, as a single electrical pathway can be used to read out multiple elements, without the need for digitisation or analogue switching. We designed, manufactured and tested a proof-of-concept device comprising a single electromagnetic resonator implemented as a defected microstrip structure (DMS) in the shape of a hairpin on a piece of high-permittivity circuit board. The board was covered with a layer of black latex paint to form a low-Young’s modulus superstrate layer and characterised with a conventional ultrasound source. We present these results, along with a detailed description of the device’s operating principle, its applications, and design considerations.

• Publisher: ProQuest Dissertations & Theses
• Subject: Ultrasonic imaging