High efficiency class-D RF amplifiers and mobile NMR systems

Abstract

This thesis details the development of a hand held mobile NMR system. The new system addressed the drawbacks on most of the existing NMR systems of being physically large and heavy with poor power inefficiency. In order to achieve the goal of producing a compact and power efficient NMR system, several high efficiency Class D RF amplifiers were developed which showed efficiencies of over 80%. The single cycle controlled PWM amplifier working at 2 MHz was a novel design and no other PWM Class D amplifiers has been reported to operate at high frequency with output power greater than 100 W. Development of the 20 MHz amplifier with 100 W constant amplitude output showed that the Class D amplifiers are suitable for NMR applications even at high frequency and further development of this amplifier to work at 17 MHz with amplitude modulation was successful. It paved the way for the development of a full mobile system with the newly available 17 MHz Mini-MOUSE sensor. The fully integrated mobile system was developed using SoC FPGA technologies. The use of a 400 Msps DDS device for RF pulse generation results in a broadband system. A duplexer was developed to enable sensor tuning and matching with the receiver amplifiers on the duplexer output stage provided 72 dB gain. The web based control program running on an iPad has shown great portability and it allows the system to be operated outdoors, even in remote areas. Experiments on industrial applications has shown successful T2eff measurements on polymer composites and rubber samples and showed the potential for such a system to be used in industrial applications. The fully integrated system prototype fits in a case measuring 140x120x100 mm (LxWxH) and weighs 800 grams and fulfil all the requirements for a mobile NMR system.