Automatic field-frequency lock in an nmr spectrometer

Abstract

The field-frequency lock system of an impulse NMR spectrometer operates in an automatic bidirectional search mode characterized by relatively low pulse repetition rate, and relatively high power excitation. Searching is accomplished by monitoring the frequency of an absorption mode lock signal with a bidirectional counter. The sense input of the bidirectional counter is derived from a phase comparison of the absorption mode signal with the concomitant dispersion mode signal. The resulting counter output is converted to analog form yielding a search mode control signal. Transitions in the sign of the phase comparison signal initiate a transition to the relatively low power, high repetition rate excitation of the lock mode. In the search mode, facility is provided to maintain the control signal output although certain transient conditions may reduce the amplitude of the lock signal below minimum levels for operation.