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Due to the simplicity of circuit design using the principle of beating metal type BFO (Beat Frequency Oscillator) widely distribution. The basis of these devices is supposed phenomenon of the formation of the beating occur when mixing two close in frequency signals (Fig. 1.2).

Fig. 1.2. A simplified block diagram of the detector working on the principle beating

In BFO metal detectors use two oscillators that are configured on the same frequency. In this case, the frequency of the reference (reference, model) generator unchanged. The coil circuit of the measuring oscillator is at the same time search or measuring coil.

When the search coil excited by an electromagnetic field appears metal object, the frequency of the measuring oscillator is changed. Signal the modified frequency is fed to the mixer where it is mixed with the reference signal frequency. As a result, the output of the mixer is allocated a signal with a beat frequency, which is supplied to the display unit.

As an indicator of the detectors on the beat is usually used acoustic devices, however, there are designs with a needle and led indicators. When using acoustic indicators the change in the sign of the beats is quite simple to determine the kind of metal, color or black, is made the object located in the zone of action of the device.

It should be noted that the beat frequency in the BFO detectors lies in the low-frequency range, closer to the lower limit of sound perception the human ear. This allows to considerably simplify the design of the unit indication, since the beat frequency of the measurement, search engine) and auxiliary (reference) generator can be analyzed by ear. However the sensitivity of the detectors, operating on the principle of beating leaves a lot to be desired. However, the characteristics of these devices are completely satisfied unassuming users.

Author: M. V. Adamenko