在核磁共振（NMR）领域，射频脉冲信号的质量、形状对NMR性能及应用有着重要影响.本文基于现场可编程门阵列（FPGA）和直接数字频率合成（DDS）芯片AD9910设计了一种硬件结构更为简单的NMR射频脉冲发生器，实现了射频脉冲各项参数的数字化调制.其频率、相位、振幅的控制精度分别达到了32位、16位和14位，脉冲调制的时间精度为0.01 μs，可灵活生成持续时间不小于0.1 μs、载波频率不高于400 MHz的各类软脉冲和硬脉冲.同时，针对脉冲序列的特点建立了"脉冲+延时"的基础模型，提出了一种通用性更强的列表式脉冲序列控制方案，精简了对上级控制单元的控制需求.此外，对射频脉冲信号的频谱特性进行了理论分析，并采用Hanning窗对软脉冲的包络波形进行了优化处理，仿真和实验结果表明，Hanning窗可以有效抑制软脉冲的频谱泄漏问题.
朱云峰
,
何为
,
何传红
,
王怡
,
齐天昊
,
陈柏冰
,
徐征
. 基于数字调制技术的核磁共振射频脉冲发生器[J]. 波谱学杂志, 2018
, 35(3)
: 318
-327
.
DOI: 10.11938/cjmr20172607
Radiofrequency (RF) pulse transmitter is an important part of a nuclear magnetic resonance (NMR) spectrometer. In this paper, a simple NMR RF pulse transmitter was designed based on field programmable gate array (FPGA) and direct digital synthesis (DDS) chip AD9910. Digital modulation of various parameters was achieved. The control accuracy of frequency, phase and amplitude reached 32 bits, 16 bits and 14 bits, respectively. The time accuracy of pulse modulation was 0.01 μs, capable of generating soft/hard pulses with duration longer than 0.1 μs and frequency less than 400 MHz. A "pulse + delay" model was established, and a generic list-type sequence controller was presented, as such the requirements for higher-level control units were reduced. The enveloping waveform of soft pulses was optimized using Hanning window, instead of rectangular window. Simulation and experimental results demonstrated that the use of Hanning window could effectively reduce frequency sidebands of the soft pulses.
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