Abstract: A method for the operation and for the evaluation of signals of an eddy cent probe wherein a transmitting coil is supplied with alternating current with preferably three frequency components. The receiving coil signal corresponding to an a.c. voltage is broken down into corresponding frequency components and by phase-control rectification projection signals are formed corresponding to a real part or an imaginary part of the coil signal. From at least two, optionally individually amplified projection signals of the same type (real or imaginary signal) differential signals are formed. Finally, by the combination of at least two differential signals a combination signal is formed and evaluated. In particular, quotients and amount or value sums of differential signals are formed. The method offers decisive advantages in seeking mines in connection with the suppression of disturbing ground or soil signals and in the identification and classification of mines on the basis of signals from an eddy current probe.

Abstract: A wire loop reference transmitter is coupled with the excitation winding and the receiver winding of metal detection apparatus and which can be alternately opened and closed through a switch. In another version the separation of the reference phase transmitter reference signal from other signals is accomplished by periodically turning the reference signal off and on at a predetermined switching frequency. According to a further version this separation is achieved by a phase control demodulator controlled by a switched-on reference signal of a first controllable phase and by a switched-off reference signal with a control signal to obtain a 180 degree second control phase. Still further, control signals of a phase controlled rectifier can be at 90 degrees to the reference signal of the reference phase transmitter.