Research On Measurement Methods Of Broadband NMR Signals

Nuclear Magnetic Resonance(NMR)technology is a method for analyzing substances from a microscopic level.Since its discovery,it has been highly valued in domestic and overseas,and has been widely used in many fields.Compared with other logging technologies,NMR logging technology not only selects the observation range,but also has the characteristics of high reduction,high resolution,and high precision.The nuclear magnetic resonance logging technology abroad is relatively mature,and it is still in the development stage in domestic,with great research value and development potential.In this thesis,based on the NMR,according to its working principle,the investigation of NMR technology at home and abroad,design a set of NMR measurement circuit that can measure echo signals in a wide frequency band,divided into NMR electronic circuit and probe.The electronic circuit module is composed of a power amplifier circuit,an isolation circuit and a preamplifier.It has a low-cost power amplifier that excites the antenna to generate an alternating magnetic field to cause nuclear magnetic resonance of protons in the sample.At the same time,the field effect tube is composed of an isolation circuit with a wide frequency band to prevent its large energy signal from entering the preamplifier and causing circuit damage.A preamplifier composed of a low-noise operational amplifier receives the original NMR signal from the probe;The probe with "inside-out" structure is divided into two parts: permanent magnet and antenna.The middle part of the permanent magnet generates a uniform static magnetic field of 2MHz,and extends to both sides to generate a certain gradient of decaying magnetic field.The helically wound antenna transmits the signal from the power amplifier and receives the original nuclear magnetic resonance signal.In order to eliminate the need for antenna tuning under wide-band conditions,this thesis does some research and calculations on the untuned NMR technology,which lays the foundation for further research on the tuning-free NMR measurement circuit under wide-band conditions.The whole measuring circuit has a simple structural design,low cost and small volume,which can realize the miniaturization of low-field nuclear magnetic resonance instruments,and provides conditions for experimental research and verification of low magnetic field nuclear magnetic resonance technology.