Digital BFO metal detecting device with improved sensitivity at near-zero beat frequencies

摘要

An improved metal detecting device of the beat frequency oscillator type is provided which incorporates a method for determining the side of the zero beat frequency at which a search oscillator is operating. The device includes a search coil conveniently mounted which is part of a search oscillator. A reference oscillator is also provided, the output of which is digitally compared to that of the search oscillator by a digital phase comparator. The digital phase comparator output is applied through a low pass filter to a slope detector circuit. The slope detector output signal is supplied to both an audio output circuit and a filter/comparator. The audio output is sufficient to facilitate location of permeable and conductive substances. The output of the slope detector also contains information which is extracted by the filter/comparator and translated to an audible signal by an oscillator and the audio output circuit. This audible signal serves to facilitate the determination of whether a permeable or a conductive substance is being detected.