An RF Transmit Circuit Research For Miniature NMR

Nuclear magnetic resonance (NMR) is a kind of resonance phenomenon caused by the interaction of electromagnetic wave and nucleus with both angular momentum and magnetic moment in the outside magnetic field. The nature of this phenomenon is the energy level jump of nucleus after absorbing the energy from the electromagnetic wave. With the development of modern quantum physics, it has been able to take advantage of nuclear magnetic resonance technology to study material’s microstructure and then solid state nuclear magnetic resonance (MAS NMR), liquid nuclear magnetic resonance and magnetic resonance imaging (MRI) appeared.NMR radio frequency (RF) circuit is the base of resonance phenomenon; it can provide RF signals of desired electromagnetic wave. In modern more applications, it requires RF signals with the characters that high frequency stability, low phase noise, high frequency resolution and RF circuit with the characters that high integration level, small size, low cost. The purpose of this topic is to design and realize the digital RF circuit, generate high-resolution and low-phase-noise NMR RF signal, provide the main hardware for digital micro-NMR system.In practice, NMR RF signal includes a high-frequency excitation signal and a layer-election envelope signal. In this topic, the used method is direct digital frequency synthesis (DDS), and this task is divided into three parts to complete. First, we make use of microcontroller to configure the digital DDS devices AD9854 provided by Analog company to generate orthogonal high-frequency signal. Second, we combine microcontroller, waveform memory ROM, DAC converters to produce layer-election envelope signal. Third, we get NMR RF signal after mixing the two signals.The frequency of high-frequency excitation signal from AD985 is high-resolution, switches quickly and phase difference of output quadrature signal is accurate. The method of getting layer-election envelope signal is flexible, it can adapt to different application. The mixer circuit is simple, it reduce the volume of RF circuit. From all of those, the research of this subject conforms to digitized and miniaturized NMR techniques trends.Through designed RF circuits, we get the simple MRI RF signal in this topic, and compared with simulation signal, the quality of signal need further continuous improvement from circuit design and component selection.