EAS system with requency hopping

• US 5,349,332 A
• Filed: 10/13/1992
• Issued: 09/20/1994
• Est. Priority Date: 10/13/1992
• Status: Expired due to Term

• Current Assignee: Sensormatic Electronics LLC (Johnson Controls International plc)
• Original Assignee: Sensormatic Electronics Corporation (Johnson Controls International plc)
• Inventors: Ferguson, David B., Booker, LeRoy A., Szklany, Craig R.
• Primary Examiner(s): Swann, Glen
• Application Number:
  US07/959,685
• Time in Patent Office: 707 Days
• Field of Search: 340/572, 340/551
• US Class Current: 340/572.2
• CPC Class Codes: G08B 13/2414 using inductive tags
• CPC Class Codes: G08B 13/2477 Antenna or antenna activator circuit
• CPC Class Codes: G08B 13/248 EAS system combined with another detection technology, e.g. dual EAS and video or other presence detection system
• 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 EAS system in which a transmitter transmits an RF transmitter signal into an interrogation zone and a receiver receives RF signals from the interrogation zone. The received RF signals include any RF tag signals generated by tags situated in the zone and adapted to respond to the RF transmitter signal. In order to reduce interference effects, the RF carrier frequency of the transmitter signal is adapted to take on a plurality of different frequency values during different ones of a plurality of finite dwell time periods of the RF transmitter signal.

Claims

1. An EAS system for use with a tag, said EAS system comprising:

transmitting means for transmitting an RF transmitter signal into an interrogation zone, said RF transmitter signal having a RF carrier which is controlled by said transmitting means to have a plurality of different values each occurring over a different one of a plurality of finite dwell time periods of said RF transmitter signal, each finite dwell time period being spaced by a finite time interval from the preceding finite dwell time period;

and receiving means adapted to be responsive to RF signals for making a determination and providing an indication that an RF tag signal has been received, said RF tag signal being produced by said tag in response to said RF transmitter signal and having a RF carrier frequency whose value is related to the value of the RF carrier of said RF transmitter signal.

2. An EAS system in accordance with claim 1 wherein:

each finite dwell time period has an extent which is equal to the extent of each of the other finite dwell time periods.

3. An EAS system in accordance with claim 1 wherein:

each finite dwell time period has an extent which is greater than the extent of the preceding finite dwell time period.

4. An EAS system in accordance with claim 1 wherein:

each finite dwell time period has an extent which is less than the extent of the preceding dwell time period.

5. An EAS system in accordance with claim 1 wherein:

said transmitter means determines as to whether the extent of a particular finite dwell time period is equal to, greater than or less than the preceding finite dwell time period one of fixedly and pseudorandomly.

6. An EAS system in accordance with claim 1 wherein:

said RF transmitter signal is controlled to be at a reduced amplitude level during each of said finite time intervals relative to the amplitude level of said RF transmitter signal during each of said finite dwell time periods.

7. An EAS system in accordance with claim 6 wherein:

said reduced amplitude level is at or less than the level allowed by governmental regulations for out-of-band signals.

8. An EAS system in accordance with claim 6 wherein:

said plurality of different values of said RF carrier are within a predetermined RF frequency band;

and said receiving means is responsive to signals within said RF frequency band and when said receiving means receives a signal during a finite time interval said receiving means identifies the presence of interference.

9. An EAS system in accordance with claim 1 wherein:

said transmitter means controls said RF transmitter signal such that each of said plurality of different values of said RF carrier frequency of said RF transmitter signal is one of;

selected to be greater than the preceding value in accordance with a predetermined fixed sequence;

selected to be less than the preceding value in accordance with a predetermined fixed sequence; and

selected pseudorandomly.

10. An EAS system in accordance with claim 1 wherein:

said transmitter means further transmits an IF transmitter signal at an IF carrier frequency into said interrogation zone;

said tag signal is related to said IF carrier frequency, said tag including first means for mixing said RF transmitter signal and said IF transmitter signal to develop said tag signal;

and said receiving means includes second means for mixing any received RF signals with the RF carrier frequency of said RF transmitter signal to extract and "first" signal content in a band including said IF carrier frequency.

11. An EAS system in accordance with claim 10 wherein:

said IF carrier frequency is modulated based upon a modulation frequency;

said tag signal is related to said modulation frequency;

and said receiving means includes;

means for detecting in said "first" signal content and "second" signal content in a band including said modulation frequency; and

means for comparing said detected "second" signal content with a signal at said modulation frequency.

12. An EAS system in accordance with claim 10 wherein:

the RF carrier frequency of said RF transmitter signal is in the 902-928 MHz frequency range;

and said IF carrier frequency of said IF transmitter signal is in the 40-150 kHz frequency range.

13. An EAS system in accordance with claim 10 wherein:

said RF carrier frequency of said RF transmitter signal is in the microwave frequency range (i.e., greater than about 900 MHz).

14. An EAS system in accordance with claim 10 wherein:

said RF carrier of said RF transmitter signal is frequency modulated.

15. An EAS system in accordance with claim 10 wherein:

said transmitting means controls said IF carrier frequency to have a plurality of different values each occurring over a different one of a plurality further finite dwell periods of said IF transmitter signal.

16. An EAS system in accordance with claim 15 wherein:

each further finite dwell period is spaced by a further finite time interval from the preceding further finite dwell period;

and said IF transmitter signal is controlled to be at a reduced amplitude level during each of said further finite time intervals relative to the amplitude level of said IF transmitter signal during each of said further finite dwell periods.

17. An EAS system in accordance with claim 1 wherein:

said RF carrier frequency of said RF transmitter signal is at a microwave frequency (i.e., greater than about 900 MHz).

18. An EAS system in accordance with claim 1 further comprising:

said tag.

19. A method of operating an EAS system for use with a tag, said method comprising:

transmitting an RF transmitter signal into an interrogation zone, said RF transmitter signal having a RF carrier frequency which is controlled to have a plurality of different values each occurring over a different one of a plurality of finite dwell time periods of said RF transmitter signal, each finite dwell time period being spaced by a finite time interval from the preceding finite dwell time period;

receiving RF signals;

determining whether an RF tag signal is included in said received RF signals, said RF tag signal being produced by said tag in response to said RF transmitter signal and having an RF carrier frequency whose value is related to the value of the RF transmitter signal; and

generating an indication that a RF tag signal has been received.

20. A method in accordance with claim 19 wherein:

each finite dwell time period has an extent which is equal to the extent of each of the other finite dwell time periods.

21. A method in accordance with claim 19 wherein:

each finite dwell time period has an extent which is greater than the extent of the preceding finite dwell time period.

22. A method in accordance with claim 19 wherein:

each finite dwell time period has an extent which is less than the extent of the preceding dwell time period.

23. A method in accordance with claim 19 wherein:

the determination as to whether the extent of a particular finite dwell time period is equal to, greater than or less than the preceding finite dwell time period is made one of fixedly and pseudorandomly.

24. An EAS system in accordance with claim 19 wherein:

said RF transmitter signal is at a reduced amplitude level during each of said finite time intervals relative to the amplitude level of said RF transmitter signal during each of said finite dwell time periods.

25. A method in accordance with claim 24 wherein:

said reduced amplitude level is at or less than the level allowed by governmental regulations for out-of-band signals.

26. A method in accordance with claim 19 wherein:

said plurality of different values of said RF carrier frequency are within a predetermined RF frequency band;

and said method further includes identifying the presence of interference in said system when RF signals within said RF frequency band are received during one of said finite time intervals.

27. A method in accordance with claim 19 wherein:

each of said plurality of different values of said RF carrier frequency of said RF transmitter signal is one of;

selected to be greater than the preceding value in accordance with a predetermined fixed sequence;

selected to be less than the preceding value in accordance with a predetermined fixed sequence; and

selected pseudorandomly.

28. A method in accordance with claim 19 further comprising:

transmitting an IF transmitter signal at an IF carrier frequency into said interrogation zone;

said tag signal being related to said IF carrier frequency, said tag producing said tag signal by mixing said RF transmitter signal and said IF transmitter signal;

and said step of receiving includes mixing any received RF signals with the RF carrier frequency of said RF transmitter signal to extract first signal content in a band including said IF carrier frequency.

29. A method in accordance with claim 28 wherein:

said IF carrier frequency is modulated based upon a modulation frequency;

said tag signal is related to said modulation frequency;

and said step of receiving further includes;

detecting from said "first" signal content and "second" signal content in a band including said modulation frequency;

and comparing said detected "second" signal content with a signal at said modulation frequency.

30. A method in accordance with claim 28 wherein:

the RF carrier frequency of said RF transmitter signal is in the MHz frequency range;

and said IF carrier frequency of said IF transmitter signal is in the kHz frequency range.

31. A method in accordance with claim 28 wherein:

said RF carrier of said RF transmitter signal is in the microwave frequency range.

32. A method in accordance with claim 28 wherein:

said RF carrier frequency of said RF transmitter signal is frequency modulated.

33. A method in accordance with claim 28 wherein:

said IF carrier frequency has a plurality of different values each occurring over a different one of a plurality further finite dwell periods of said IF transmitter signal.

34. A method in accordance with claim 33 wherein:

each further finite dwell period is spaced by a further finite time interval from the preceding further finite dwell period;

and said IF transmitter signal is at a reduced amplitude level during each of said further finite time intervals relative to the amplitude level of said IF transmitter signal during each of said further finite dwell periods.

35. A method in accordance with claim 19 wherein:

said RF carrier frequency of said RF transmitter signal is at a microwave frequency.