Presence sensing detector and system for detecting a receiver/transmitter device affixed to an article
US4260983A · kind A · utility
Assignee
Inventors
Key dates
| Filing date | Jan 11, 1979 |
| Grant date | Apr 7, 1981 |
| Priority date | — |
| Expiry date | Jan 11, 1999 |
Classification
- Technology area (CPC G)Physics
- CPC primaryG08B13/2488
- WIPO fieldControl
- WIPO sectorInstruments
Abstract
A presence sensing system comprises a detector having a transmitter for transmitting an interrogation signal to an area to be kept under surveillance and a receiver for receiving a presence indicating signal. A receiver/transmitter device the presence of which is to be detected is adapted to receive the interrogation frequency and to transmit a presence indicating signal in response thereto. A phase locked loop circuit has a first input coupled with the output of the receiver a second input for a reference signal, and an output for providing a phase locked signal which is indicative of phase coherence between the signals on the two inputs. A frequency generator has an output coupled with the second input of the phase lock loop and an alarm is actuable in response to the phase locked signal. The invention also includes the presence sensing detector described in the above system.
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.