| 摘要 |
The present invention relates to methods and apparatus for detecting the presence of a soft magnetic marker in an interrogation zone. A low frequency electromagnetic field is produced utilizing dual frequency generators causing the magnetic marker to produce a phase shift of the modulation of transmission harmonics. These signals are readily distinguished from ferrous alloys and noise. This invention utilizes a modulated transmitter carrier signal generated in an interrogation zone. Modulated harmonics of the transmitter carrier signal are detected when a marker is present in the interrogation zone. The modulation detected can be at the same frequency as the modulation of the transmitter signal or a harmonic of the modulation of the transmitter signal. Detection of the modulation allows higher signal to noise ratios and therefore reduces false detections.
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