核磁共振射频开关及驱动电路设计.pdf

摘 要 核磁共振射频开关的作用是在系统的发射通道和接收通道之间进行切换，实 现射频激励的发射或磁共振信号的接收；对于发射线圈和接收线圈分离的情况， 使两个线圈不同时处于谐振状态，减小它们之间的耦合。射频开关的速度，影响 射频发射完后发射线圈的失谐速度，进而对死时间的长短有着重要影响。 对于 弛豫时间很短的样品如页岩等，磁共振信号衰减很快，为了能够检测 这些样品的磁共振信号，获得足够的信号强度，需要尽可能缩短死时间。这对食 品与药品检测及油气勘探等领域的核磁共振应用具有重要意义。为此本文提出了 一种快速射频开关设计方案。它采用主动模式开关设计方案，电路由开关单元和 驱动单元构成。通过驱动单元控制开关单元达到主动控制的目的。开关单元的核 心是PIN 管。驱动单元以场效应管为核心，设计中采用了死区控制电路以防止不 同场效应管同时导通；采用光耦电路使驱动电源和控制电路隔离，提高了驱动能 力。本文设计的射频开关具有切换速度快、信号失真小、插入损耗低和隔离度高 等优点，可有效减小发射线圈和接收线圈之间的耦合，缩短死时间，能在短 样 品的磁共振信号检测中发挥重要作用。 关键词：核磁共振，射频开关，PIN 管，驱动电路，死时间 V ABSTRACT The RF switch is a key part of a Nuclear Magnetic Resonance (NMR) system. Its function is to switch between the RF transmitting path and the NMR signal receiving path of the MRI system, and to prevent the transmitting coil and the receiving coil from tuning simultaneously in order to reduce coupling between them. The speed of the RF switch is a vital parameter that has a significant influence on the detuning speed of the transmitting coil, which determines the length of the dead time between transmission and reception in some way. As for samples with extremely short relaxation times such as shale, attenuation of the NMR signal is very quick, which means the dead time must be short enough to receive the signal. To reduce dead time, this thesis presents a RF switch design. This switch is an active type, consisting of a switching unit and a driving unit. The key of the switching unit is the PIN diode which must be driven by the driving unit, while in the switching unit MOSFETs play a central. The RF switch here has features inclu