High accuracy magnetic measurements with the ASQ-81 scalar magnetometer

The Texas Instruments' ASQ-81 scalar magnetometer provides high-resolution total field magnetic measurements. It uses an optically pumped metastable helium gas detector that has a range of 23,000 to 75,000 gammas (1 gamma = 1 nT) and a 0.016 gamma peak-to-peak noise level. The authors first discuss the theory of operation for the ASQ-81 helium, optically pumped, controlled-oscillator type scalar magnetometer. Following this discussion, the measurement error induced by high sampling rates for a controlled oscillator magnetometer is quantized. A scheme for making frequency measurements that are free of this induced error by synchronizing the measurements with the modulation frequency of the controlled oscillator is presented. The authors describe the circuitry and techniques required to reduce the measurement error to less than the instrument noise level.