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/2D NMR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 amplitude, phase and frequency of RF pulses. Two cascading numerically controlled oscillators (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 cascading NCOs, whereas the amplitude is set by the cascading NCOs and digital step attenuators. It was demonstrated that the basic electrical specifications and function of the transmitter can meet the needs of routine 1D/2D NMR experiments.
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