Abstract
A Robinson-type oscillator with an automatic frequency control circuit suitable for the measurement of internal magnetic fields, is described. An AFC circuit ensures that the tuning of the tank circuit is such that the first harmonic of the modulation frequency is kept at a minimum in the detector output. Cascade amplifier states permit operation at higher frequencies with a better signal-to-noise ratio and larger sweep amplitudes than with other types of amplifiers. The use of the equipment is demonstrated by measuring the 57Fe NMR signal in iron and iron-cobalt alloys.