Zhou, Lixia; (2005) Passive phase shift modulation for high-speed data transmission in implantable closed-loop neuroprostheses. Doctoral thesis , UCL (University College London).

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Abstract

In designing a closed-loop implantable neuroprosthesis, naturally-occurring nerve signals will be used to control the stimulation. A telemetry system, capable of transmitting power to the implant and relaying two raw electroneurograms out of the body at high speed, yet having simple circuitry and working over a wide range of coupling coefficient with low power loss, is required. However, at the start of this project, no such device was available. This thesis describes a novel method. Passive Phase Shift Modulation (PPSM), developed by the author, for high-speed passive reverse signalling via an inductively coupled link. A telemetry system based on PPSM is also devised by the author and presented in the thesis. In this system, power is provided to the implant and signals are conveyed out of the body continuously via the same radio-frequency (RF) channel. Unlike conventional Passive Impedance Modulation, it synchronously shorts the implant power-receiving coil for half the carrier cycle when a digital binary bit '1' is transmitted. This stores the energy in the coil and then releases it back to the circuit in time to generate a transient current surge in the transmitting coil, indicating a modulation. The scheme transmits phase shift modulation, but results in amplitude modulation. The transient responses under PPSM have been explored in great depth by two approaches: 1) mathematically deducing the analytical solutions and 2) a semi-numerical method using modem computer aided tools. The results are comparable. The influence of the circuit parameters has been analysed, showing that the signal link does not disturb the optimised efficient power transfer link. The telemetry system implementing PPSM is designed and built on the bench. It consists of a transmitter, an implant circuit, an extemal circuit and two coils. The digital logic is implemented by programmable gate arrays, bringing the electronic design up to date with modem technology. The performance is evaluated, and agrees with that predicted by theory and simulation. PPSM has advantages in speed, energy efficiency, and simple circuitry with comparable working range compared to previous methods. It enables transmission of signals with large bandwidth without necessarily increasing the frequency of a carrier. It is, therefore, a satisfactory method for designing a practical feed-back controlled neuroprosthesis.

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