Research & Design On Transceiver Of Magnetic Field Lock System For NMR Spectrometer

With the wide application of nuclear magnetic resonance (NMR) technology in the research of molecular chemistry and bio-molecular, people's requirements on the experimental apparatus design are become more and more. For high-frequency NMR spectrometer, in order to complete the precise frequency and phase lock function, magnetic field lock system for maintaining the conditions of nuclear magnetic resonance should be re-designed. Based on this requirement, this thesis aims at the RF digitalizing technology being applied to the transceiver of magnetic field lock system for NMR spectrometer and related research.The main research of this thesis is to design a new NMR spectrometer magnetic field lock system transceiver. With the principle and structure of traditional NMR spectrometer magnetic field lock system transceiver being analyzed detailed, the principle of direct digital frequency synthesizer and IF digital quadrature demodulation will be used for designing new NMR spectrometer lock system transceiver .These technologies which will be applied to the new transceiver unit are discussed and based on the discussion, hardware circuit schematic and control program will be designed. By using the direct digital frequency synthesizer technology, the transmitter of the lock channel will have ability of fast switch of frequency ,phase and amplitude ,it also can provide accurate high speed pulsed output .Field programmable gate array(FPGA) block will be used as communication interface which will communicate between the system and digital direct synthesis(DDS) chip ,utilizing embedded blocks such as ram and the rich resources of logic gate of FPGA , it will have the ability of controlling DDS chip to generate desired modulated RF signal . As the digital down conversion and direct IF signal sampling technology, analog filter of former receiver will be omitted, system commissioning will be simpler and system will run more stably. The data gained from the receiver will be directly transmitted to FPGA and processed there. This has simplized the complexity of the design and increased the reliability. As the control core of the magnetic field locking control unit, FPGA chip has increased flexibility of the system and shortened the time of circuit designing and commissioning.The schematics of the transceiver of magnetic field lock system and FPGA program of control block are all presented in the paper, and it has verified by simulation and measurement results. Compared with traditional lock system transceiver, the new one which utilizes RF digitalizing technology has obvious advantages on measuring speed and accuracy .RF system digitalizing technology will be widely used in modern nuclear magnetic resonance spectrometer,and it must be one of the trends in development of NMR spectrometer in the future.