| 摘要 |
<p>In an anti-shoplifting surveillance system, first and second receiver means are employed on either side of a guarded portal ("SIDE ONE" "SIDE TWO" of Fig.1). These receiver means detect perturbations in an electromagnetic field produced by the presence of a high-permeability, low-coercivity marker. From the outputs (A, B or A', B' of Fig. 1) of those receivers a sum signal and a diference signal are produced in a pre-amp (1). The high-order harmonics of the sum signal are emphasized in an amp/filter (2) and the low-order harmonics of the difference signal are emphasized in an inhibit amp (3). The ratio of high-order to low-order harmonics is determined in logic and processing circuitry (9) to test for the presence of a marker. To enhance sensitivity to various orientations of a marker, a phase control (5) and a phase driver (6) periodically reverse the phase of one of the receiver means. To improve noise rejection, a timing generator (4) inhibits reception during the energization of the system's transmission coils and further limits processing of received signals to the first quarter of each cycle (when perturbations resulting from a marker are most likely).</p> |
