Measuring harmonics of low-level electrical signals: application to magnetic measurements

A system for the measurement of the Fourier components of low-level electrical signal is presented. The system has been applied to the measurement of the harmonics of the dynamic hysteresis loop of an amorphous material, finding a good correlation between the hysteresis loop obtained by conventional methods and the one reconstructed from the harmonics measurement. The theoretical limit of the system sensibility in magnetic flux measurement, assuming that the noise from the sense coil is sufficiently small to allow the most sensitive range of the lock-in amplifier used, is 1 nWb at 20 Hz.