High efficiency class-D RF amplifiers and mobile NMR systems

<p>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.</p>

High efficiency class-D RF amplifiers and mobile NMR systems

<p>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.</p>

A 3-bit load-pulling digital power amplifier

A radio frequency (RF) 3-bit digital power amplifier (DPA) is described in this paper. It consists of three RF amplifiers connected at their outputs with a transmission line (TL) network. The three amplifiers are designed for different output powers (POUT). The TL network allows them to load-pull one other to achieve eight different amplitude states by alternatively enabling and disabling the amplifiers via their gate bias. A prototype was designed in the National Instruments' Microwave Office (MWO) for 500 MHz with the aid of a genetic algorithm to optimize the TL network for all seven active (on-) states. The optimizer efficiencies goals were based on data derived from load-pull simulation. The POUT goals were based on a 1 Vrms step-size. In simulation, ≥50% efficiency was achieved at all on-states with 29.7 dBm peak POUT. A practical prototype based on the simulation achieved an efficiency of ≥40% over all seven on-states. A peak POUT of 28.9 dBm was achieved, with the lowest state at 22.4 dBm.