Magnetic surveillance system with odd-even harmonic and phase discrimination

摘要

Apparatus for detecting the presence within a surveillance zone of a body of high permeability material creates a surveillance field varying sinusoidally at a fundamental frequency. Parallel filter channels select the second and third harmonic components of the perturbations created by said material when present in the field and the phase of the second harmonic relative to a reference signal is compared. The signal level of each of the second and third harmonic components as well as the aforesaid phase congruency cooperatively control an alarm circuit. Rhomboid shaped transmitter coils surrounding figure "8" receiver coils improve coupling to the high permeability body.