Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1990-03-16
1991-09-24
Swann, III, Glen R.
Communications: electrical
Condition responsive indicating system
Specific condition
G08B 1318
Patent
active
050517279
ABSTRACT:
The invention relates to a shoplifting detection system of the transmission type, suitable in particular for the use of high-frequency interrogating signals, in which system an electronic label provided with a resonance circuit can effect an electromagnetic coupling between at least two antenna coils, at least one of which is a transmitting antenna coil which, in operation, is fed with an A.C. interrogating signal from a transmitter circuit, and at least one other of which is a receiving antenna coil which supplies a received signal to a receiver circuit. According to the invention said receiver circuit comprises a phase-sensitive synchronous detector to which the received signal is supplied, and to which a reference signal is supplied of such a phase that a component in the received signal, caused by an electronic label, provides a maximum output signal of said synchronous detector, and a signal phase-shifted through 90.degree. relatively to said component provides a minimum output signal of said synchronous detector.
REFERENCES:
patent: 4260990 (1981-04-01), Lichtblau
patent: 4274090 (1981-06-01), Cooper
patent: 4308530 (1981-12-01), Kip et al.
patent: 4724426 (1988-02-01), Lundberg
N.V. Nederlandsche Apparatenfabriek Nedap
Swann III Glen R.
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