The Digital Field-Frequency Lock System of High-Resolution NMR Spectrometer

To obtain a high-resolution NMR spectrum, the NMR spectrometer needs ultra-high magnetic field which is very hard to be stable. The temporal variation of magnetic field causes the change of nuclear resonance frequency from one transient to another, thus worsens the accumulative signal, and the high-resolution spectrum cannot be obtained. This paper gives the design of the digital field-frequency lock system which uses PID to implement feedback control and the way of combining internal and external loops to lock the resonance frequency, based on the phase-locked theory. The digital field-frequency lock technique can make the resonance frequency and RF frequency maintain very good relative stability, thus avoids the lines moving and eliminates the effect of the unstable magnetic field. The system can effectively improve the long-term stability of the ultra-high magnetic field and obtain high-resolution spectra.