Design Of Digital Down Conversion Based On FPGA For Nuclear Magnetic Resonance Systems

Miniature nuclear magnetic resonance systems always have particular needs of complexity,expansion and cost.We want to study and design high-performance NMR instruments which are also flexible,compact and low-cost.NMR transceiver is performed in digital domain instead of traditional analog domain gradually in order to reduce circuitry volume,signal distortion and noise aliasing.Digital signal processing chips are applied to achieve receivers,and direct digital synthesizers chips are used to generate RF signals in digital NMR systems.The structure above can meet the needs of modern NMR signal measurement,but it is complex and cumbersome because of the use of multiple chips or circuit boards,and function update is hard.NMR receiver usually uses commercial digital down convert chips,which is always lack of adequate dynamic range and flexibility.FPGA has become a signal engine and design platform because of its reconfigurability and parallelism.FPGA-based DDC system can be updated and expanded flexibly,and develop portable NMR instruments which have high performance and smaller volume.Digital receiver module is the core of NMR system.In this paper,demodulation,DDC and filters of single channel NMR digital receiver are realized by FPGA chips.We compare two methods of achieving numerically controlled oscillator(NCO):look-up table and CORDIC algorithm,finally adopt CORDIC to realize NCO which can control phase,amplitude and frequency flexibly.The compensation filter is used to improve passband characteristics of CIC filter.The half-band filter and low-pass decimation filter can decimate signals and filter noise further.At last the signal splitter gets I and Q signals and sends them for subsequent signal transmission.FPGA-based design tool DSP Builder is used to design NCO,mixer and decimation filters,which can reduce design cost and realize complicated algorithm.After simulation,synthesizition and debugging,the system is combined with NMR console for practical testing and gets qualified data.The system proposed can configure parameters flexibly according to the requirements,and have advantages of compact,reconfigurability and high speed.This system is suitable for study and application.