Active transmitter ringdown for switching power amplifier

• US 7,444,120 B2
• Filed: 05/04/2005
• Issued: 10/28/2008
• Est. Priority Date: 05/11/2004
• Status: Active Grant

• Current Assignee: Sensormatic Electronics LLC (Johnson Controls International plc)
• Original Assignee: Sensormatic Electronics Corporation (Johnson Controls International plc)
• Inventors: Frederick, Thomas J., Herring, Richard L., Oakes, Jeffrey T.
• Primary Examiner(s): VO, NGUYEN THANH
• Application Number:
  US11/121,899
• Publication Number:
  US 20050253721A1
• Time in Patent Office: 1,273 Days
• Field of Search: 455/41.1, 455/41.2, 455/41.3, 455/91, 455/115.1, 455/127.1, 455/127.2, 340/10.1
• US Class Current: 455/41.2
• CPC Class Codes: G08B 13/2408 using ferromagnetic 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

Abstract

A method for controlling signal decay of a transmitted signal within a transmitter is described. The method includes measuring an amount of current induced back into the transmitter by the decaying signal, and using the current measurement to control the decay of the signal after the signal is transmitted from the load. A transmitter for an electronic article surveillance (EAS) system is also described which includes a current sensing circuit configured to at least sense an amount of current induced back into the transmitter by the load after transmission of the signal, and a transmitter control circuit configured to utilize the sensed current to determine an amount and a polarity of current to be applied to the load to reduce the induced current to a desired value.

Claims

1. A method for controlling signal decay of an electro-magnetic transmission from a transmitter, said method comprising:

measuring an amount of current induced into the transmitter by a decaying field remaining after the electro-magnetic transmission; and

using the current measurement to control a decay rate of the decaying field.

2. A method according to claim 1 wherein using the current measurement to control the decay rate comprises applying a voltage of opposite polarity as the polarity of the measured current.

3. A method according to claim 1 further comprising:

measuring an amount of current output by the transmitter during a transmission burst; and

using the current measurements to control a burst control algorithm component configured to control generation of the transmitted signal during a transmission time of the transmitter.

4. A method according to claim 1 further comprising:

determining completion of a first electromagnetic transmission; and

initiating a second electromagnetic transmission having an opposite polarity as the first electro-magnetic transmission.

5. A method according to claim 1 further comprising:

determining when the current induced into the transmitter has decayed to a value; and

applying a detuning circuit to the transmitter.

6. A method according to claim 1 wherein using the current measurement comprises using the current measurement to determine an amount of opposite polarity current to be output by the transmitter.

7. A method according to claim 1 wherein using the current measurement comprises:

determining a magnitude of the current induced into the transmitter from in-phase and quadrature components of the current measurement; and

comparing the magnitude of the current measurement against a desired transmitter current to set a current output level for the transmitter.

8. A transmitter for an electronic article surveillance (EAS) system, said transmitter configured to output a transmission signal to an external load, said transmitter comprising:

a current sensing circuit configured to at least sense an amount of current induced back into said transmitter by the load after transmission of the signal; and

a transmitter control circuit configured to utilize the sensed current to determine an amount and a polarity of current to be applied to the load to reduce the induced current to a desired value.

9. A transmitter according to claim 8 wherein said transmitter comprises a modulator configured to output the transmission signal, said transmitter control circuit configured to reverse polarity of the transmission signal after completion of a transmission period.

10. A transmitter according to claim 8 wherein said current sensing circuit comprises an analog-to-digital converter.

11. A transmitter according to claim 8 wherein said current sensing circuit is further configured to sense an amount of current applied to the load during a signal transmission, and wherein said transmitter control circuit comprises an end-of burst transition control algorithm programmed with the transmission periods of said transmitter, said end-of burst transition control algorithm configured to switch the sensed current signals from a burst control algorithm to a ringdown control algorithm after completion of a transmission period for said transmitter.

12. A transmitter according to claim 8 further comprising a detuning circuit and wherein said transmitter control circuit comprises an end-of ringdown transition control algorithm programmed to switch said detuning circuit onto the load upon determining that an amount of current being applied to the load after completion of a transmission period is below a threshold.

13. A transmitter according to claim 8 wherein said transmitter control circuit comprises a burst control algorithm configured to receive the sensed current during a transmission period for said transmitter, said burst control algorithm comprising a controller programmed to:

compare an amount of current applied to the load with a desired load current resulting in an error signal; and

utilize the error signal to adjust an amount of current being applied to the load.

14. A transmitter according to claim 8 wherein said transmitter control circuit comprises a ringdown control algorithm configured to receive the sensed current induced into said transmitter by the load, said ringdown control algorithm comprising a controller programmed to:

compare an amount of current induced back into said transmitter by the load with a desired current amount resulting in an error signal; and

utilize the error signal to determine an amount and a polarity for a current to be applied to the load.

15. A transmitter according to claim 8 wherein said transmitter control circuit comprises a proportional, integral, derivative controller.

16. A transmitter according to claim 8 wherein said transmitter control circuit comprises a ringdown control algorithm configured to receive the sensed current during a non-transmission period for said transmitter, said ringdown control algorithm comprising a controller programmed to:

compare an amount of current induced back into said transmitter by the load with a desired current amount resulting in an error signal; and

apply the error signal to a closed loop controller configured to control an amount and a polarity of current being applied to the load.

17. An electronic article surveillance (EAS) system comprising:

a receiver configured to receive signals generated by EAS tags; and

a transmitter configured to apply a signal to a load and further configured to transmit a signal at a resonant frequency of the EAS tag, said transmitter further configured to sense both an amount of current applied to the load during transmission periods and an amount of current induced by the load back into said transmitter during non-transmission periods, said transmitter configured to utilize the sensed currents to control an amount and a polarity of current applied to the load during both transmission periods and non-transmission periods.

18. An EAS system according to claim 17 wherein said transmitter comprises:

a modulator applying the current to the load; and

a transmitter control circuit configured to reverse a polarity of a signal output by said modulator after completion of a transmission period.

19. An EAS system according to claim 17 wherein said transmitter comprises an end-of burst transition control algorithm configured with the transmission periods of said transmitter, said end-of burst transition control algorithm configured to switch the sensed current signals from a burst control algorithm to a ringdown control algorithm after completion of a transmission period for said transmitter.

20. An EAS system according to claim 17 wherein said transmitter comprises:

a detuning circuit; and

an end-of ringdown transition control algorithm programmed to switch said detuning circuit onto said load upon determining that an amount of current being applied to the load is below a threshold.

21. An EAS system according to claim 17 wherein said transmitter comprises a ringdown control algorithm configured to receive the sensed current induced back into said transmitter during a non-transmission period for said transmitter, said ringdown control algorithm comprising a controller programmed to:

compare an amount of current induced into said transmitter by the load with a desired current amount resulting in an error signal; and

utilize the error signal to determine an amount and a polarity for a current to be applied to the load.