High Accuracy Magnetic Measurements with the ASQ-81 Scalar Magnetometer

摘要

The Texas Instruments' ASQ-81 scalar magnetometer provides high resolution totalfield magnetic measurements. It uses an optically pumped metastable helium gas detector that has a range of 23 000 to 75 000 gammas (1 gamma = 1nT) and a 0.016 gamma peak-to-peak noise level. Total magnetic field measurements can be made using the resonance oscillator output of the magnetometer. This output is a frequency modulated signal whose center frequency is Larmor frequency. For sampling rates slower than 0.5 Hz the Larmor frequency can be measured directly from the resonance oscillator using a frequency counter. At higher sampling rates the modulation of the resonance oscillator output can create measurement errors that exceed the instrument's noise level. This paper quantifies this measurement error and describes the circuitry and techniques required to reduce the measurement error to less than the instrument noise level.