Abstract: The target signal produced by the receive coil of an induction-balance metal detector is demodulated and filtered in matched V.sub.X and V.sub.R demodulator and filters (11, 13) driven by quadrature versions (TA/TB) of the signal that drives the transmit coil of the detector. The resultant phase quadrature signals--V.sub.X and V.sub.R --are applied to a ramp computer (15) that internally produces a ramp signal whose slope is a function of the magnitude of the sum of the V.sub.X and V.sub.R signals. The ramp computer (15) inludes a comparator that combines the ramp signal with a fixed signal whose magnitude is a function of the magnitude of the V.sub.X signal. The result is a pulse whose width is a function of the phase angle and, thus, the identity of the target that created the V.sub.X and V.sub.R signals. The pulse is integrated (17) and the results stored at a TI voltage whose magnitude is functionally related to the identity of the target.

Abstract: Synchronous demodulators demodulate the signal developed in a receive coil coupled to an oscillator driven transmit coil and produce signals that include target pulses when the detector head housing the transmit and receive coils passes over a metallic object (e.g., a target) buried in the ground. The signals are processed to remove ground mineralization information and to determine the "phase angle" of the target, which angle uniquely identifies the detected target. The tangent of the phase angle is determined using either implicit division (sometimes referred to as steepest descent division) or direct division. Regardless of which division technique is used, as the signals are processed, they are continuously filtered to remove ground mineralization information. In addition, the target component of the signals is converted from pulse form into ringing signal form.

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.

Abstract: A metal detector uses a transmitting search coil inductively coupled to a receiving coil for detecting the presence of metal objects near the surface of the ground within the field of the coils. An oscillator generates a signal transmitted by the transmit coil, and the signals detected by the receive coil are coupled to the signal inputs of two synchronous demodulators. The output of the oscillator also is applied at different phases to the reference signal inputs of the two synchronous demodulators. The outputs of these demodulators then are passed through low pass and bandpass filters having a low cutoff frequency which is higher than the highest frequency components generated in the synchronous demodulators due to ground effects.

Abstract: A metal detector for detecting and distinguishing between ferrous and non-ferrous objects in an area which may contain a ferrous, mineralized soil background is disclosed. The hysteresis effect is used to distinguish between ferrous and non-ferrous objects by generating groups of transmit current pulses, each group including a polarizing pulse of one polarity followed by a pair of read pulses of the opposite polarity and less magnitude. The transmit pulses are applied to a transmit coil to induce corresponding receive voltage pulses in a receive coil. A pair of receive pulses corresponding to the read transmit pulses are fed through a low pass filter and thereafter sampled to produce a sample output signal corresponding to the difference between the sampled voltage levels of the pair of receive pulses.