NOISE REJECTION CIRCUITRY

• US 3,828,337 A
• Filed: 08/20/1973
• Issued: 08/06/1974
• Est. Priority Date: 08/20/1973
• Status: Expired due to Term

• Current Assignee: CHECKPOINT SYSTEMS, INC. (CCL Industries Incorporated)
• Original Assignee: George Jay Lichtblau
• Inventors: Lichtblau, George Jay
• Primary Examiner(s): Trafton, David L.
• Application Number:
  US05/389,728
• Time in Patent Office: 351 Days
• Field of Search: 340/258C,280,152T 343/6.5R,6.5SS
• 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

Abstract

Noise rejection circuitry sutable for detection of a resonant tag in an electronic security system especially adapted for use in retail stores. True signals are distinguished from noise by sensing the absence of one or more pulses in an expected chain of pulses produced by the resonant tag. A parallel noise inhibit function is provided by a staircase generator and comparator which prevent an alarm indication in response to received fast pulse noise.

Claims

1. For use in an electronic security system which includes means for providing in a surveillance zone an electromagnetic field of a frequency which is swept within a predetermined range, and means for detecting the presence of a resonant tag circuit having a frequency within said range, circuitry for discriminating between the presence of a valid tag circuit in said surveillance zone and spurious signals, said circuitry comprising:

means for receiving a train of pulses derived from detection of a tag circuit within said surveillance zone;

means operative upon receipt of a predetermined number of pulses of predetermined periodicity to provide an alarm indication; and

means for resetting said circuitry immediately upon detection of at least one missed pulse in the expected train of pulses.

2. A circuit according to claim 1 including staircase generating means operative in response to the output pulses from said detecting means to geneRate a staircase signal which increases in amplitude;

and means for providing a reset signal to said circuitry upon the exceedance by said staircase signal of a predetermined threshold level.

3. For use in an electronic security system which includes means for providing in a surveillance zone an electromagnetic field of a frequency which is swept within a predetermined range, and means for detecting the presence of a resonant tag circuit having a resonant frequency within said range and for providing pulses in response thereto, noise rejection circuitry for distinguishing between pulses provided by said tag circuit and pulses caused by spurious conditions, said noise rejection circuitry comprising:

a nonretriggerable gating circuit receiving pulses from said detecting means and operative to provide output pulses in response to received pulses of predetermined periodicity;

a retriggerable circuit operative in response to the output pulses from said nonretriggerable gating circuit;

integrator means operative in response to the output pulses from said retriggerable circuit to provide an integrated signal; and

threshold detector means having a predetermined threshold level and operative to provide an output signal for alarm actuation upon exceedance of said threshold level by said integrated signal.

4. Noise rejection circuitry according to claim 3 and including:

staircase generating means operative in response to the output pulses from said detecting means to generate a staircase signal which increases in amplitude in the presence of each of said output pulses and operative to provide a reset signal upon the exceedance by said staircase signal of a predetermined threshold level.

5. Noise rejection circuitry according to claim 3 wherein said retriggerable circuit provides output signal of first or second signal state to said integrator means.

6. Noise rejection circuitry according to claim 3 wherein said retriggerable circuit provides an output signal of first voltage level in response to a continuous train of pulses of predetermined periodicity from said nonretriggerable gating circuit, and provides an output signal of second voltage level in response to the absence of at least one pulse in said train of pulses.

7. Noise rejection circuitry according to claim 6 wherein said integrator means includes means operative in response to said output signal of first voltage level to provide an integrated output signal for alarm actuation;

and means response to said output signal of second voltage level to reset said integrator.

8. Noise rejection circuitry according to claim 7 wherein said first voltage level is greater than said second voltage level.

9. Noise rejection circuitry according to claim 3 wherein said nonretriggerable gating circuit and said retriggerable circuit are each multivibrators.

10. Noise rejection circuitry according to claim 3 wherein said nonretriggerable gating circuit includes an RC circuit;

a threshold detector operative in response to a predetermined voltage on the capacitor of said RC circuit; and

a flip flop operative to receive trigger pulses from said tag circuit and a reset signal from said threshold detector and to provide an output signal to said retriggerable circuit upon receipt of said trigger pulses.

11. Noise rejection circuitry according to claim 3 wherein said retriggerable circuit includes a flip flop;

an RC circuit;

a switch operative to control the charging of the capacitor in said RC circuit; and

a threshold detector operative to provide a reset signal to said flip flop in response to a predetermined voltage level across said capacitor;

means for simultaneously providing from said nonretriggerable gating circuit a set signal to said flip flop and an input signal to said switch;

said switch being operative in response to said input signal to ground said capacitor and thereby prevent said threshold detector from providIng a reset signal to said flip flop for a period of time greater than the minimum interval between pulses received from said nonretriggerable gating circuit.

12. Noise rejection circuitry according to claim 7 wherein said integrator means includes:

an RC circuit driven by the output voltage of said retriggerable circuit;

solid state gating means operative to provide an output signal for alarm actuation in response to a predetermined voltage; and

diode means conductive in response to signals from said retriggerable circuit of said lower voltage level and operative to prevent the voltage across the capacitor in said RC circuit from reaching a voltage required to activate said solid state gating means.

13. An electronic security system comprising transmitter means for providing an electromagnetic field in a surveillance zone at a frequency repetitively swept through a predetermined range;

a multi-resonant tag circuit having a first resonant frequency within said predetermined range of frequencies and a second resonant frequency outside of said predetermined range of frequencies;

receiver means for detecting the presence of said first resonant frequency from a tag circuit present in said surveillance zone;

said receiver means including detector means operative to provide output pulses in response to the presence of the first resonant frequency of said tag circuit in said electromagnetic field;

noise rejection circuitry operative in response to pulses from said detector means for distinguishing between pulses provided by said tag circuit and pulses caused by spurious conditions, said circuitry comprising;

means for receiving a train of pulses derived from detection of a tag circuit within said surveillance zone;

means operative upon receipt of a predetermined number of pulses of predetermined periodicity to provide an alarm indication; and

means for resetting said circuitry immediately upon detection of at least one missed pulse in the expected train of pulses.

14. An electronic security system according to claim 13 wherein said means operative upon receipt of a predetermined number of pulses of predetermined periodicity includes integrator means for providing an integrated output signal in response to said pulses of predetermined periodicity;

and threshold detector means operative to provide an output signal upon receipt of an integrated output signal of predetermined magnitude; and

wherein said resetting means includes a retriggerable circuit operative to receive pulses having a predetermined minimum period and to provide an output voltage of a first voltage level to said integrator means in response to a continuous train of pulses having less than a predetermined period and to provide an output voltage of a second voltage level to said integrator means in response to a missed pulse.