Design Of NMR RF Switch And Driver Circuit

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 T2relaxation 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 including high speed, low loss, small signal distortion and high switch isolation. With these merits, it can effectively decrease coupling between the transmitting coil and receiving coil and meanwhile shorten dead time, which is very helpful in detecting the NMR signal of samples with short T2relaxation times.