Metal detector for earth or walls - uses analogue multiplier to compare sinusoidal signal of frequency determined by oscillator with probe coil with set frequency

摘要

An LC oscillator is tuned by the probe coil to the metal detector. The sinusoidal frequency generated by the oscillator from a f.m. demodulator is compared with a stipulated frequency. The comparison is carried out by an analogue multiplier. The output signal serves as the d.c. voltage delivered by the f.m. demodulator as the result of the frequency comparison. The d.c. voltage is amplified by an operational amplifier (3) and displayed by a voltmeter, LED line or other indicator (4). The f.m. receiving circuit (2) includes a limiting amplifier, quadrature multiplier and a l.f. amplifier. ADVANTAGE - Simple, single seeking coil. Simple circuit, battery-powered. Highly sensitive, easy to build and simple to calibrate. Light and small.