Schematic drawing of an instrumentation system which measures a magnetic field fluctuation using a reciprocal frequency counter.
Schematic drawing of an external lock system for solid-state NMR; it stabilizes magnetic field fluctuations using a microcoil and a reciprocal frequency counter.
The train of 7Li microcoil NMR signals at the crystal filter output measured by a spectrum analyzer. The horizontal axis shows the time, and the vertical axis is the signal power dBm (zero-span mode). Three cycles of 10 ms NMR acquisition are plotted as an upper line and noise signals, which mean no NMR signal, are also shown as a lower line. There is more than 20 dB difference between amplified NMR signals and noise signals.
The working principle used to stabilize the central magnetic field using the frequency counter. The difference between the magnetic field at the magnet center and the microcoil position, generated by the compensation coil, is corrected by calculation. This calculation is conducted in each feed back control step to keep the central magnetic field equal to the target magnetic field.
Results of an evaluation test of the frequency counter system to measure a triangle magnetic field wave (5 Hz) generated by the sweep mode operation of a room temperature z0 shim coil. (a) A small amplitude magnetic field fluctuation such as 0.1 ppm. (b) A large amplitude magnetic field fluctuation such as 100 ppm. The frequency counter system accurately follows both types of magnetic field fluctuations.
The 7Li NMR spectrum measured by a microcoil, used in an external field-frequency lock system. The half height line width is 20.48 Hz; it is not feasible to improve the line width using room temperature shim coils, as a sample for the microcoil is too small to allow this. Magnetic susceptibility matching is much more important.
Results of an evaluation test of the external lock system with 7Li microcoil NMR. After the external lock was established, a triangle magnetic field wave (0.02 Hz) was generated by the sweep mode operation of a room temperature z0 shim coil. The external lock was kept in operation and solid-state NMR measurements shown in (a) and (b) as open circles were conducted during the 0.2 ppm/s swept magnetic field. The fluctuation of the triangle field wave was found from the compensation coil current shown in (b).
The external field-frequency lock system stabilizes the magnetic field fluctuation generated by the NMR magnet operated in external current mode. The solid-state NMR measurements shown in (a) and (b) were repeated every 3 minutes. The solid line in (b) shows the original magnetic field fluctuation vs. time for the NMR magnet.
Article metrics loading...
Full text loading...