Detector system for resonant circuit in identity tag - has search coil in null of field coils energised at varying frequency (NL 24.12.80)

摘要

Objects or persons to be identified are labelled with a tag containing a small resonant circuit, tuned to a frequency. In the range 10 to 90kHz. A pair of field coils (1A,1B) are connected in opposition and driven by a signal generator (2). This energises the field coils with a frequency which is swept over the range in 24 steps. A small search coil (3) is placed at the null point between the field coils, all three coils being in one plane. An identity tag in the vicinity distorts the field and produces an output from the search coil when the signal is on its frequency. The output is amplified and rectified (4) and applied, via a threshold (5) to logic processing circuits (6).