A fast-response field tracking proton magnetometer for field programming and plotting

A commercially available Robinson NMR oscillator has been adapted to provide a high-speed field tracking proton magnetometer covering the range 2.4-4.7 kG. Particular features of the instrument are an inherent search characteristic with search rates as high as 350 G s-1, and a 2 ms response time to field changes using 217 Hz field modulation of only 1.8 G peak to peak. Extremely high locking accuracy is achieved with the oscillator frequency deviation less than 1.3 Hz RMS in a constant field. This performance is made possible by using a new type of detector which measures each resonance position independently, and which is inherently free of zero drifts and systematic offsets. The instrument is designed as a sampled-data system, and it can achieve the forward transfer function G(z)=1/(z-1) giving complete error correction in one half cycle of the modulation. Measured and theoretical responses to a step function of magnetic field are compared. A frequency to voltage converter is incorporated which enables high-resolution ESR spectra to be plotted as a function of field. The instrument also serves as a very flexible field programmer and has proved valuable in locking the magnetic field to the address ramp of a signal accumulation over long periods.