A new field-tracking NMR magnetometer system

An automatic NMR magnetometer covering the magnetic field range 0.1-3.2 T is described. Instead of continuous tracking of the magnetic field by a conventional feedback loop technique, a new approach to triangular frequency modulation of the marginal oscillator is used in this instrument, ensuring symmetrical frequency deviation about the resonance position. Direct digital display of the magnetic field value is obtained by digital counting of the swept frequency with a suitable counting gate time. The particular advantage of this system is that the restrictions characteristic of automatic frequency control feedback loop systems, as well as for adiabatic passage conditions, can be omitted. The magnetometer is a data-sampling system with a sampling frequency of 43 Hz and a typical tracking rate of about 0.12 T s -1 . The maximum search time is 0.5 s. Because of the good dynamic properties and measurement accuracy approaching 10 -6 T and lack of modulation coils in the probe, the instrument is particularly suited for the measurement and calibration of magnetic fields in ESR and NMR spectrometers.