A computer‐controlled all‐pass phase shift filter for a variable frequency two‐phase ac magnetic susceptometer

A well‐established method for the accurate measurement of small changes in ac magnetic susceptibility is to initially null and then monitor the output of the susceptometer as various parameters are modified. Use of a computer‐controlled phase shift filter has allowed us to build a system that will introduce virtually orthogonal signals to compensate for the in‐phase and out‐of‐phase components of the output. The system described is designed for use over the frequency range 50 Hz–4 kHz. Extended frequency ranges are possible with only slight modifications.