Design And Implementation Of NQR Excitation Pulse Sequence And Probe Circuit

Nuclear Quadrupole Resonance(NQR)technology is a non-contact solid-state radio frequency spectrum analysis technology that can be widely used in explosive detection,mineral exploration and other fields.However,due to the inherent low signal-to-noise ratio,radio frequency interference and spurious signals,the practical application of NQR technology is restricted.In order to improve its practicability and expand application fields,many foreign research institutions are devoted to suppressing noise and radio frequency interference in NQR detection technology and improving the signal-to-noise ratio of NQR detection,which has become the focus and hotspot of research in this field in recent years.From the false signal concept,it can be seen that the ringing tail signal related to radio frequency can be designed to reduce the ringing tail suppression circuit,so that the recovery time of the antenna coil that is placed in the same place is effectively shortened,but its suppression effect is limited.Therefore,on the basis of in-depth research on the form of excitation pulse sequence and strong false signal characteristics,consider starting with the design of composite multi-pulse excitation sequence to improve the signal-to-noise ratio from the NQR signal generation level.Using the phase relationship between the false signal and the radio frequency pulse,a new composite multi-pulse excitation sequence can be constructed by alternately changing the pulse phase to effectively suppress the false signal with a long duration.This article focuses on the nuclear quadrupole resonance excitation pulse generator,and the design and improvement of the ringing tail suppression circuit.At present,there are empirical and specific settings for the parameters of the excitation pulse sequence,and the parameters cannot be flexibly configured in different detection environments.This paper adopts the method of combining PXI system composed of arbitrary signal generator PXI-5421,counter / timer PXIe-6612 and oscilloscope PXIe-5114 with LabVIEW software platform to generate NQR excitation pulse sequence with controllable parameters.The parameters of the excitation pulse sequence are designed through LabVIEW programming.PXI-5421 and PXIe-5114 are synchronously triggered by the single pulse signal output from PXIe-6612.The NQR excitation pulse sequence is generated by any signal generator PXI-5421.On this basis,a new type of Q-value switching circuit is designed,which has a high Q-value in the working state and a low Q-value in the process of transmission and reception switching.The designed circuit is simulated and analyzed to verify the ringing tail time Relationship with Q-value.The experimental results show that the designed ring signal suppression circuit can reduce the probe’s ring recovery time from 390.152 us to 7.955 us,which can effectively suppress the ringing tail time.Finally,the performance of the excitation pulse generator is tested and analyzed.The pulse generator can output FID sequence,SE sequence and parameter-defined sequence.The output frequency is adjustable from 500 kHz to 6 MHz,and the initial phase is 0 ° to 360 ° The time is adjustable,and the minimum output pulse width is 80 ns.The test results show that it basically meets the expected requirements.