NMR field frequency lock system

An NMR (nuclear magnetic resonance) spectrometer has a permanent magnet provided with a field coil. A lock system cyclically pulses a deuterium lock sample to produce a FID (free induction decay) signal that is analyzed by measuring the time interval during which the FID signal crosses the zero axis a predetermined number of times. The measured interval is compared to a predetermined interval or set point, and the current through the field coil is adjusted in the direction tending to correct the field strength so as to maintain the measured interval equal to the preset interval. Such measurement, comparison and adjustment are done several times each second. When the sample is removed, the lock system retains the last field setting achieved with the sample.