Metal detector system with undesirable target and mineralized ground

Abstract

A synchronous demodulator subsystem produces a ground balance pulse and a discriminate pulse each time a detector head housing the receive coil and an associated transmit coil passes over a metallic object (e.g., a target) buried in the ground. In one form, a pair of synchronous demodulators produce V.sub.X and V.sub.R signals, each of which includes a pulse each time the detector head passes over a target. The V.sub.X and V.sub.R signals are combined to produce the ground balance and discriminate pulses. Alternatively, a pair of synchronous demodulators are used to directly produce the ground balance and discriminate pulses. The polarity of the discriminate pulse is dependent upon on whether the target is a desirable or an undesirable target. Contrariwise, the polarity of the ground balance pulse is always the same. Further the ground balance pulse signal contains very little ground mineralization information whereas the discriminate pulse signal may contain a significant amount of ground mineralization information when the target is buried in mineralized ground. The ground balance and discriminate pulses are filtered to remove ground mineralization information and converted into in-phase or 180 degrees out-of-phase ringing signals. The ground balance ringing signal is modulated in a triangle modulator and the result divided into the discriminate ringing signal. The result of the division is a series of pulses on one or the other of two lines depending upon whether the target is a desirable or undesirable target. The division pulses control the polarity and magnitude of a DC voltage produced by an integrator, which in turn controls a corresponding increase or decrease in the level of an audio signal.