Metal detectors BFO (Beat Frequency Oscillator), theory. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors
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Due to the simplicity of circuit solutions, metal detectors such as BFO (Beat Frequency Oscillator) using the beat principle have become widespread. These devices are based on the phenomenon of the formation of beats that occur when two signals close in frequency are mixed (Fig. 1.2).
Rice. 1.2. Simplified block diagram of a beating metal detector
BFO metal detectors use two oscillators tuned to the same frequency. In this case, the frequency of the reference (reference, exemplary) generator is unchanged. The measuring generator circuit coil is also a search or measuring coil.
When a metal object appears in the zone of the electromagnetic field excited by the search coil, the frequency of the measuring generator changes. The changed frequency signal is fed to the mixer, where it is mixed with the reference frequency signal. As a result, a signal with a beat frequency is allocated at the mixer output, which is fed to the display unit.
As an indicator in beat detectors, acoustic devices are usually used, however, there are designs with arrow and LED indicators. When using acoustic indicators, by changing the sign of the beats, it is quite easy to determine what metal, non-ferrous or black, is made of an object located in the area of the device.
It should be noted that the beat frequency in BFO metal detectors lies in the low-frequency range, closer to the lower limit of the sound perception of the human ear. This makes it possible to significantly simplify the design of the display unit, since the frequency beat of the main (measuring, search) and auxiliary (reference) oscillator can be analyzed by ear. However, the sensitivity of metal detectors operating on the principle of beats leaves much to be desired. Nevertheless, the characteristics of these devices will fully satisfy unpretentious users.
Author: Adamenko M.V.
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