System and method for synchronizing a receiver of an electronic article surveillance system and a transmitter thereof

• US 5,463,376 A
• Filed: 06/16/1993
• Issued: 10/31/1995
• Est. Priority Date: 05/29/1990
• Status: Expired due to Fees

• Current Assignee: Sensormatic Electronics Corporation (Johnson Controls International plc)
• Original Assignee: Sensormatic Electronics Corporation (Johnson Controls International plc)
• Inventors: Stoffer, James F.
• Primary Examiner(s): SWANN III, GLEN R
• Application Number:
  US08/078,457
• Time in Patent Office: 867 Days
• Field of Search: 340/572, 340/551
• US Class Current: 340/572.4
• CPC Class Codes: G08B 13/2414 using inductive tags
• CPC Class Codes: G08B 13/2471 Antenna signal processing by receiver or emitter
• CPC Class Codes: G08B 13/2477 Antenna or antenna activator circuit
• CPC Class Codes: G08B 13/2488 Timing issues, e.g. synchronising measures to avoid signal collision, with multiple emitters or a single emitter and receiver

Abstract

An electronic article surveillance system of type including a transmitter for providing in a preselected area an electromagnetic field periodically swept in frequency over a predetermined range of frequencies for causing tags in the preselected area to generate tag signals containing a frequency within the predetermined range of frequencies and a receiver including a detector for receiving the tag signals and providing output indication of detected tag signals, has a modulator in its transmitter which modulates the electromagnetic field and a modulation responsive circuit in its receiver for synchronizing the detector with the frequency of the electromagnetic field.

Claims

1. An electronic article surveillance system for detecting the presence of a tag in a surveillance zone, comprising:

(1) a transmitter including

(a) first signal generating means responsive to an input signal for generating radio frequency signals at different frequencies,

(b) second signal generating means for generating input signals and applying the generated input signals to said first signal generating means,

(c) modulating means for receiving said radio frequency and providing modulated radio frequency signals and

(d) an antenna driven by the modulated radio frequency signals and establishing a radio frequency field in said surveillance zone; and

(2) a receiver including

(a) an antenna for outputting signals corresponding to said field and the presence of a tag therein,

(b) processing means receiving said antenna output signals for detecting the presence of a tag in said surveillance zone, and

(c) circuit means receiving said antenna output signals and operable, irrespective of the presence or absence of a tag in said surveillance zone, for synchronizing said processing means with said radio frequency signals by discerning the modulation content of said antenna output signals.

2. The system claimed in claim 1, wherein said modulating means is an amplitude-modulating means.

3. A method of providing synchronization between a transmitter of an electronic article surveillance system, adapted for detection of a tag in a surveillance zone in which said transmitter establishes a radio-frequency field, and a receiver of said system, irrespective of the presence or absence of a tag in said field, said method comprising the steps of:

(a) providing a preselected modulation in an output of said transmitter and hence in said radio-frequency field; and

(b) synchronizing said receiver with said transmitter on the basis of detection in said receiver of said preselected modulation in said radio-frequency field.

4. The method claimed in claim 3, wherein said modulation is selected to be an amplitude modulation.

5. An electronic article surveillance system for detecting the presence of a tag in an interrogation zone having other signals therein, comprising:

transmitting means for providing a radio frequency field in said interrogation zone, said radio frequency field exhibiting frequency change with time and amplitude modulation;

detecting means receiving modulated radio frequency signals from the radio frequency modulated field and detecting the amplitude modulation thereof and generating a synchronizing signal exhibiting amplitude change with time correspondingly with the frequency change with time of said radio frequency signal; and

discriminating means coupled to said detecting means for receiving as inputs signals produced by a tag in the interrogation zone and said other signals and for discriminating between said tag signals and said other signals by using the synchronizing signal generated by said detecting means.

6. An electronic article surveillance system as recited in claim 5 wherein said detecting means includes a coupling network.

7. An electronic article surveillance system for detecting the presence of a tag in an interrogation zone, comprising:

transmitting means for providing a radio frequency signal in said interrogation zone, said radio frequency signal exhibiting frequency change with time and amplitude modulation;

receiving means for receiving said modulated radio frequency signal and for receiving signals produced by a tag in the interrogation zone, said receiver means including circuit means for detecting the amplitude modulation in said modulated radio frequency signal and generating therefrom a synchronizing signal for said receiving means;

detecting means responsive to said receiving means for detecting the presence of tag signals and generating an indication of the detection thereof; and

means for preventing the generation of said indication in the absence of generation of said synchronizing signal.

8. In an electronic article surveillance system of type including a transmitter for providing in a preselected area an electromagnetic field periodically swept in frequency over a predetermined range of frequencies for causing tags in said preselected area to generate tag signals containing a frequency within said predetermined range of frequencies and a receiver including detecting means for receiving said tag signals and providing output indication of detected tag signals, the improvement wherein said transmitter includes modulating means for modulating said electromagnetic field and wherein said receiver includes modulation responsive circuit means for synchronizing said detecting means with the frequency of said electromagnetic field.

9. The invention claimed in claim 8 wherein said modulation responsive circuit means includes a modulation detecting circuit.

10. The invention claimed in claim 8 wherein said modulation is an amplitude modulation and wherein said modulation detection circuit is an amplitude modulation detection circuit.

11. The invention claimed in claim 8 wherein said modulating means of said transmitter comprises an amplitude modulating means.

12. The invention claimed in claim 11 wherein said amplitude modulating means comprises a resistance-capacitance coupling network.

13. In a method for detecting article surveillance tags in a predetermined area by providing therein an electromagnetic field periodically swept in frequency over a predetermined range of frequencies for causing tags in said preselected area to generate tag signals containing a frequency within said predetermined range of frequencies and providing a receiver including detecting means for receiving said tag signals and other signals in said predetermined area not tag-generated but within said predetermined range of frequencies and providing output indication of detected tag signals, the improvement comprising the further steps of modulating said electromagnetic field and synchronizing said detecting means with the frequency of said electromagnetic field by detecting modulation therein.