基于FPGA的核磁共振测井仪控制逻辑设计

Design of An FPGA-Based Control Logic in NMR Logging Tool

核磁共振(NMR)测井仪以CPMG脉冲序列为测量基础,按照CPMG脉冲序列的时序要求完成大功率射频脉冲的发射和微弱回波信号的接收,在脉冲发射完成后快速泄放天线中储存的能量.该文介绍一种基于现场可编程门阵列(FPGA)的NMR测井仪控制逻辑和典型脉冲序列,详细说明在CPMG脉冲序列下FPGA的工作时序和流程;讨论NMR测井仪发射电路、Q-转换电路和隔离电路的控制原理和时序要求,给出各电路的控制逻辑仿真结果;最后,利用所设计的控制逻辑和自制NMR测井仪探头,在实验室条件下对水溶液进行了测量,得到满意的结果.

Using a CPMG pulse sequence, NMR logging tool transmits high-power radiofrequency （RF） pulses, receives weak echo signals following timing schedule of the CPMG pulse sequence, and dissipates energy stored in the antenna after transmitting the RF pulse. In this article, the control logic of the NMR logging tool is introduced. Field programmable gate array （FPGA） was used to control the timing of the CPMG pulse. The principles and timing requirement of transmitter, Q-switch and de-coupler were given. Simulation results were provided. The control logic introduced was used in a prototype NMR logging tool successfully.