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Abstract

Metal detector (1) including an oscillator (10) generates an RF field in the vicinity of the head (2). Two reference signals (13, 14) are generated, one signal (14) being in phase with the oscillator (10) and the other signal (13) being shifted from the oscillator signal by 90.degree.. The signal (8,9) from the receive coils (6,7) is amplified and quadrature demodulated into two signal channels, each channel giving the orthogonal component of the original signal (8,9). The signals of each channel are digitized and read by a microprocessor (37) that controls the amplitude of the balance signals (43, 49). One signal (43) is in phase with the oscillator signal (11) and the second signal (49) is shifted 90.degree. from the oscillator signal (11). Signals (43,49) return to detection circuitry (6,7), maintaining balance on each channel. By monitoring the magnitude of the phase shifted and unshifted signals (49,43), the microprocessor (37) determines the characteristics of the product. In a second version, metal detector (62) includes a digital phase shifting circuit (77) but only one phase sensitive detector (84). The circuit (77) is manipulated by microprocessor (89) to maximize RF signal (87). The balance circuits (92, 100) remove signal (87). By monitoring the control signals (79) used for balancing, the microprocessor (89) determines characteristics of a product passing in the vicinity of the head assembly (63).

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First Claim