具有快速调制功能NMR发射机的设计与实现

A Transmitter Capable of Fast Modulation for NMR Spectrometer-Design and Implementation

介绍了一种具有快速幅度、相位和频率调制功能的核磁共振射频发射机.数字中频子系统中采用了两个级联的数控振荡器,前一级数控振荡器在FPGA内实现,用于产生基带调制信号;后一级则采用具有正交调制功能的DDS芯片,由它来完成基带信号的采样率转换、中频产生、调制和输出.射频脉冲的相位和频率偏置的设置直接由级联NCO实现,而幅度设置则由步进衰减器和中频幅度调制共同实现.该发射机的基本性能和功能满足常规1D/2DNMR实验需求.

This paper presents the design and implementation of a radio frequency （RF） transmitter for NMR spectrometer, which is capable of fast modulation of the ampli- tude, phase and frequency of RF pulses. Two cascading numerically controlled oscilla- tors （NCOs） were used in the digital IF subsystem. The first NCO was implemented in a field programmable gate array （FPGA）, generating baseband modulating signal. The second one was a direct digital frequency synthesizer （DDS）, which is responsible for sampling rate conversion, IF generation and modulation, and transmission of modulated IF signal. The phase and frequency offset of a RF pulse can be set directly by the casca ding NCOs, whereas the amplitude is set by the cascading NCOs and digital step attenu- ators. It was demonstrated that the basic electrical specifications and function of the transmitter can meet the needs of routine 1D/2D NMR experiments.