核磁共振测井仪天线及模拟电路设计

Design of the Antenna and Simulative Circuit in the Nuclear Magnetic Resonance Logging Tool

核磁共振测井系统的主要功能就是利用产生的CPMG脉冲序列,经过功率放大器放大后,通过天线激发样品核磁共振回波信号,Q-sw itch电路消除天线储存能量后,天线开始接收回波信号经过双工器电路滤波后,再由前置放大器和二级放大器电路将回波信号放大。调谐电路对系统共振频率的漂移进行实时调谐,针对以上天线和模拟电路的功能,对天线和模拟电路进行了设计。由于核磁共振测井仪在实际现场测井过程中处于高温、高压状态,致使回波信号受到很大干扰,非常微弱,其幅值在几百纳伏到几个微伏之间。所以,在设计中采用了耐高温元件以及电磁兼容技术,有效地减少了噪声和干扰的影响,提高了信号的幅度和稳定性,整体调试结果表明天线、模拟电路的设计、制作有效可行。

NMRL produces CPMG pulse and the transmitter translates the low level RF pulses into high voltage pulse at the antenna. The antenna is used for both transmitting and receiving function, and the Q-switch is used to dissipate energy stored in the antenna after a transmit pulse. The duplexer acts as a very broad band pass filter during receiving mode and the prearnp is a low noise wide band-width amplifer that is used to amplify the echo signals. On the other side , the tuning is used to add capacitance to the antenna to adjust its resonance frequency. According to the function of NMRL, we have designed the antenna and the simulative circuit of NMRL and the signal level of NMRL is the order of hundreds of nanovolts, up to a few millivolts due to the antenna is under the high temperature and pressure in log operation site, so we adopt high temperature component and electronics-magnetic compatible technology to improve the amplitude and sensitivity of signal The experiments show that the new antenna and simulative circuit are reliable.