07-05-2010, 01:47 PM
bfo metal detector
BFO ( beat frequency oscillator ) metal detectors use two oscillators, each of which produces a radio frequency.The one called the the search loop oscillators uses a coil of wire. The second oscillator called the reference oscillator uses a much smaller coil of wire, and is usually inside the control box . By adjusting the oscillators so their frequencies are very nearly the same, the difference between them is made audible as a beat note, this beat note changes slightly when the search loop is moved over or near to a piece of metal. This is the principle of operation of this circuit. Usually, the search oscillator is kept fixed say at 100khz and to arrange for the reference oscillator to be adjustable 100khz plus or minus 250hz. a beat note of 250hz to 0 to 250hz is provided by this setting. This type of detector is most sensitive when the beat note is close to zero, about 5hz .
The circuit basically consists of two balanced oscillator. One acts as the detector element, the other provides the reference signal.The reference oscillator can be constructed using various circuit topology: inductor-capacitor (LC), resistor-capacitor (RC), or even a crystal (quartz) oscillator. the detector oscillator always use inductor-capacitor topology as the change in inductance as the coil nears a metal object is used as the working principle of the circuit. the detector oscillator is tuned to have same frequency as the reference oscillator in the absense of the metal object near it.
The circuit diagram and the details on construction of the circuit are given in this page:
http://schematicwiringsimple-electronic-applications/metal-detector-circuit-schematic-using-beat-frequency-oscillator-bfo/
BFO ( beat frequency oscillator ) metal detectors use two oscillators, each of which produces a radio frequency.The one called the the search loop oscillators uses a coil of wire. The second oscillator called the reference oscillator uses a much smaller coil of wire, and is usually inside the control box . By adjusting the oscillators so their frequencies are very nearly the same, the difference between them is made audible as a beat note, this beat note changes slightly when the search loop is moved over or near to a piece of metal. This is the principle of operation of this circuit. Usually, the search oscillator is kept fixed say at 100khz and to arrange for the reference oscillator to be adjustable 100khz plus or minus 250hz. a beat note of 250hz to 0 to 250hz is provided by this setting. This type of detector is most sensitive when the beat note is close to zero, about 5hz .
The circuit basically consists of two balanced oscillator. One acts as the detector element, the other provides the reference signal.The reference oscillator can be constructed using various circuit topology: inductor-capacitor (LC), resistor-capacitor (RC), or even a crystal (quartz) oscillator. the detector oscillator always use inductor-capacitor topology as the change in inductance as the coil nears a metal object is used as the working principle of the circuit. the detector oscillator is tuned to have same frequency as the reference oscillator in the absense of the metal object near it.
The circuit diagram and the details on construction of the circuit are given in this page:
http://schematicwiringsimple-electronic-applications/metal-detector-circuit-schematic-using-beat-frequency-oscillator-bfo/
