HK1157485B - Method and system to negate interference from adjacent transmitters in an electronic article surveillance system - Google Patents

Description

Method and system for eliminating interference of adjacent transmitters in electronic article surveillance system

Cross Reference to Related Applications

The present invention relates to and claims priority FROM U.S. provisional patent application No.61/128,787 entitled "method for canceling interference FROM adjacent transmitters IN AN ELECTRONIC article surveillance SYSTEM," filed on 22/5/2008, entitled "method for adjusting a frequency of a radio frequency signal," the entire contents of which are incorporated herein by reference.

Technical Field

The present invention relates generally to electromagnetic signal transmitters used in electronic article surveillance ("EAS") systems, and more particularly to controlling EAS transmitters to reduce interference.

Background

Electronic article surveillance ("EAS") systems are designed to prevent unauthorized removal of an item from a controlled area. For example, EAS systems are typically implemented at retail stores to deter theft and notify authorized personnel when shoplifting occurs. A typical EAS system may include a monitoring system and one or more security tags. The monitoring system may establish an interrogation zone at an access point of a controlled area, such as an entrance/exit door of a retail store. The security tag may be secured to an article, such as an article of apparel. If a valid tag subsequently enters the interrogation zone, an alarm may be triggered indicating unauthorized removal of the tagged item from the controlled area.

In many environments, there are many different EAS systems that are implemented simultaneously. Examples of such environments include a plurality of small stores arranged in a conventional shopping mall, a single row of commercial establishments, or a shopping mall, for example. As shown in fig. 1, each EAS system generally operates by alternating periods of transmission, reception, and standby or "sleep" times during which the EAS system does not attempt to detect a security tag, but may perform various processing or operational functions. In one known system, the EAS system operates at a frequency 1.5 times the power line frequency, such as 90Hz for a 60Hz line frequency or 75Hz for a 50Hz line frequency, and the start point of the transmit or receive window is timed with the zero crossing points of the power line. During the "transmit" window, the EAS system does not receive, and vice versa. However, the detection capability of the EAS system is greatly reduced due to interfering signals generated by other nearby EAS systems having "out-of-phase" transmitters operating during the "receive" window.

Historically, EAS transmitters in close proximity to each other have been synchronized to avoid these adverse interactions. This compatibility has been achieved using several different levels of synchronization. For example, the carrier oscillator or modulation waveform of the transmitter may be synchronized. In more complex systems, such as those sold by Sensormatic electronics under the trademark "CThe system of (3) may synchronize transmitter configuration sequences between multiple systems.

For example, U.S. patent No.6,201,469, the entire contents of which are incorporated herein by reference, provides for synchronization of transmitter configuration sequences using power line zero crossing functions for which phase is manually adjusted. U.S. patent No.7,212,117, the entire contents of which are incorporated herein by reference, provides a wireless phase-locked loop ("PLL") system for synchronizing the modulation waveform of a transmitter carrier. U.S. patent application serial No.11/729,372, the entire contents of which are incorporated herein by reference, provides a system for synchronization that utilizes a synchronization master signal generated from a global positioning satellite reference signal.

Without such synchronization, EAS systems located within a certain proximity of each other may interfere with each other's receivers, thereby reducing sensitivity, causing false alarms, or even causing system failure. This interference, in turn, can cause service calls to local technicians. The technician then has to go to the site where the system is installed and manually adjust the timing of the system. Persistent or recurring problems cause a large number of repeated service calls, resulting in a large amount of expense and annoyance. Furthermore, the interfering system may not be accessible to service personnel, and therefore it may not even be possible to synchronize the interfering system.

Furthermore, not all currently available EAS systems utilize synchronization, which further exacerbates the problem. To even further complicate the problem, some unsynchronized EAS systems also utilize random transmission of pulses in an aperiodic manner. One such system is described in U.S. patent No.6,750,768. As more and more such systems work in close proximity to each other, the use of unsynchronized EAS systems further increases the likelihood that these systems will interfere with each other and with the synchronized systems.

Therefore, there is a need for a system and method that reduces interference between closely adjacent EAS transmitters without synchronization between the transmitters.

Disclosure of Invention

The present invention advantageously provides a method and system for reducing interference from adjacent transmitters in an electronic article surveillance ("EAS") system. In general, embodiments of the present invention determine the transmission pattern and/or energy level of the received signal and prevent an EAS system from using the received signal to detect EAS tags or perform noise calculations during times when neighboring EAS systems are transmitting.

In accordance with one aspect of the present invention, a method for reducing interference in an EAS system is provided. The EAS system includes a detection zone. At least one reference pattern of transmit windows is provided for an interfering EAS system. The reference pattern indicates a sequence of time slots in which the interfering EAS system is transmitting. A sample pattern of a signal is received. Each signal having a respective amplitude. The received sample pattern is compared to at least one reference pattern. In response to determining that the received pattern of samples matches at least one reference pattern, the at least one reference pattern is used to adjust samples received during a receive window corresponding to a time slot in which the interfering EAS system is transmitting.

According to another aspect of the invention, another method for reducing interference in an EAS system is provided. The EAS system includes a detection zone. A plurality of signals is received. Each signal having a respective amplitude. If the amplitude of at least one of the received signals exceeds a predetermined threshold, the received signal having an amplitude exceeding the predetermined threshold is discarded, and a determination is made as to whether an EAS tag is present in the detection zone by considering only the non-discarded signals.

In accordance with yet another aspect of the invention, an EAS system includes a transmitter, a receiver, a memory, and a controller. The transmitter is operable to transmit an interrogation signal to excite an EAS tag within the detection zone. The receiver is operable to receive a pattern of samples of a signal. Each signal having a respective amplitude. The memory includes at least one reference pattern for interfering with a transmission window of the EAS system. The reference pattern indicates a sequence of time slots in which the interfering EAS system is transmitting. The controller is electrically connected to the transmitter, the receiver, and the memory. The controller is operable to compare the received sample pattern to a reference pattern and, if the received sample pattern matches the reference pattern, to use the reference pattern to adjust samples received during a receive window corresponding to a time slot in which the interfering EAS system is transmitting.

Drawings

A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a graph illustrating a transmit/receive sequence of a prior art electronic article surveillance ("EAS") system synchronized to a power cycle of an AC power line;

FIG. 2 is a block diagram of an exemplary EAS system constructed in accordance with the principles of the present invention;

FIG. 3 illustrates a plurality of EAS systems operating in close proximity to one another in accordance with the principles of the present invention;

FIG. 4 is a flow diagram of an exemplary out-of-phase transmitter sequence detection and adjustment process in accordance with the principles of the present invention;

FIG. 5 is a flow diagram of an exemplary frame pattern detection process in accordance with the principles of the present invention; and

fig. 6 is a flow chart of an exemplary excess energy detection process according to the principles of the present invention.

Detailed Description

Before describing in detail exemplary embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of apparatus components and processing steps related to implementing a system and method for reducing interference between closely adjacent EAS transmitters without having to synchronize the various transmitters with one another.

Accordingly, the system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

As used herein, relational terms, such as "first" and "second," and "top" and "bottom," and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements.

One embodiment of the present invention advantageously provides a method and system for eliminating or reducing interference generated by adjacent EAS transmitters located adjacent or in close proximity to an EAS system. The method and system reduces the interference by identifying the transmission pattern of the interfering system and ignoring any signals received during the interfering system transmission time.

Referring now to the drawings in which like reference designators refer to like elements, there is shown in fig. 2 an exemplary EAS system provided in accordance with the principles of the present invention and designated generally as "10". The EAS system 10 includes an electronic controller circuit 12 electrically connected to both the receiver circuit 14 and the transmitter circuit 16, the electronic controller circuit 12 may include a microprocessor. The transmitter circuit 16 transmits an interrogation signal within the interrogation zone to excite an EAS tag causing the EAS tag to generate a response signal. The receiver circuit 14 receives a response signal from an EAS tag to detect the EAS tag within the interrogation zone. The receiver circuit 14 and the transmitter circuit 16 are electrically connected to an antenna assembly 18. The antenna assembly 18 may include two separate antenna coils, an upper coil 20 and a lower coil 22, either or both of which may be used to transmit and receive signals. The antenna assembly 18 may have one or more coils 20, 22 that function as receive antennas and one or more coils 20, 22 that function as transmit antennas. Alternatively, the coil assembly 18 may include one or more coils 20, 22 that function as both a receive antenna and a transmit antenna.

The signal from the receive antenna is amplified, filtered and detected by receiver circuitry 14, which receiver circuitry 14 provides both amplitude and frequency information to the controller 12. Based on design constraints, which may include program instructions in firmware, the controller 12 is capable of transmitting signals of various frequencies to the system 10 environment at specific times and for specific durations through transmitter circuitry 16 electrically connected to a transmit antenna 18.

The controller 12 is in communication with a memory 24, the memory 24 including a sequence detector 26, a threshold amplitude 27, a set of reference patterns 28 for other EAS systems, and a current pattern 30 of signals received by the receiver 14. The sequence detector 26 determines the current pattern 30 of interfering signals by retaining only those signals above the threshold amplitude 27 and instructs the controller 12 to ignore any signals received when the interfering system is transmitting accordingly. In one embodiment, the reference pattern 28 and the current pattern 30 may each be represented as a series of bits, where each bit represents a window. The bit may be set to "1" if the transmitter is operating during that window, or may be set to "0" if the transmitter is not operating in the corresponding window. The set of reference patterns 28 may include both a full reference sequence (i.e., patterns using both the auxiliary configuration and the figure-8 configuration) and an auxiliary sequence (i.e., patterns using only the auxiliary configuration). The operation of sequence detector 26 is discussed in more detail below.

Fig. 3 illustrates an exemplary multiple EAS system 30 that may be used in embodiments of the invention. Fig. 3 shows the antenna assembly 18 from several independent EAS systems 10, 32, 34, 36, and 38. Each of the three 10, 32 and 34 of the system are separated by a distance no greater than the limiting distance d 1. The two systems 36 and 38 are also spaced apart from each other by a distance no greater than the limiting distance d 1. Systems 34 and 36 are separated by a distance d2 that is greater than limit distance d 1. Each of these independent systems follows the same predetermined transmit and receive spacing pattern, including various permutations of transmit frequencies and antenna phases. The receive antenna may detect signals from other transmit antennas within a radius of up to 500 ft.

Many EAS systems do not typically start transmitting immediately upon power up, but rather perform a synchronization process to ensure that they do not transmit while another receiver of another system is "listening" for a response signal from an EAS tag. In such systems, multiple transmitters are synchronized to be "in phase" with each other to avoid such interference. As is known in the art, this phase alignment may need to be adjusted from time to time if the transmitters become "out of phase" with each other.

Even if these systems remain in phase, if other unsynchronized systems and systems using non-periodic transmit pulses are operating in the vicinity of a synchronized EAS system, one or more of these other EAS systems will likely transmit while the receiver of the other system is listening, resulting in undesirable interference and the risk of false alarms and other system problems. However, in most cases, the amplitude and energy of the signal provided by the "out of phase" transmitter will be higher than the amplitude of the response signal from the EAS tag. Therefore, amplitude discrimination techniques can be employed to ignore these interfering transmitter signals. A signal amplitude (or energy) threshold value that is higher than the expected amplitude of the response signal from the EAS tag may be set by the hardware and/or software of electronic controller circuit 12. If the amplitude of the detected signal is above the threshold, the detector will ignore that particular received sample and not use it for detection statistics.

In some cases, the jamming transmitter may transmit a repeating pattern using a combination of supplemental transmit pulses ("0-shaped") and 8-shaped transmit pulses, where the two coils 20, 22 that make up the system transmitter antenna 18 alternately flip their phase relationship between 0 ° (also referred to as "in-phase") and 180 ° (also referred to as "substantially out-of-phase") operation. The 8-pattern amplitude may be much lower than the amplitude of the response signal from the tag (and thus below a predetermined amplitude threshold), so the system may not ignore these received samples and performance may be degraded. However, if the interfering transmission patterns are repetitive, pattern recognition techniques may be used to identify these signals.

In one embodiment, the system 10 may only evaluate signal amplitudes above a threshold. Once the pattern is identified, all interference samples received (both below and above the threshold) can be ignored. Thus, for the case of an "out-of-phase" transmitter consisting of 0-and 8-shaped components, depending on the distance, only the auxiliary signal will exceed the threshold. Once the pattern of the auxiliary signal is identified, the 8-word component can be automatically ignored even if the 8-word component alone cannot be identified by the threshold test. In addition, an adaptive scheme may be introduced to automatically set the limit depending on the threshold of the received signal.

Referring now to fig. 4, an exemplary operational flow diagram is provided that describes steps performed by the sequence detector 26 for deciding when to terminate data acquisition from the serial connection and begin RF transmission in accordance with the principles of the present invention. The threshold amplitude 27 for the detection signal, i.e., AMP _ THR, is initially set to a minimum level where the amplitude of the signal received from the EAS tag is not adjusted (step S102). The receiver 14 receives a signal during a receive window (step S104). The sequence detector 26 determines whether the amplitude of the received signal RX _ AMP is greater than the threshold amplitude 27 (step S106). If the amplitude of the received signal is less than the threshold amplitude, the EAS system is not experiencing interference during the current receive window and the sequence detector 26 cycles back to prepare to receive the signal during the next receive window. However, if the received signal is greater than the threshold amplitude 27, the sequence detector 26 updates the currently received pattern 29 with the received signal (step S108). The sequence detector 26 compares the current pattern 29 of the received signal with at least one reference pattern 28 of the auxiliary transmission window (step S110). If the currently received pattern 29 matches the reference pattern 28 (step S112), the sequence detector 26 uses the reference pattern 28 to predict when the interfering system will transmit and to adjust the samples obtained during those periods in the processing stage (step S114), thereby effectively preventing those erroneous signals from being considered as signals or noise received from EAS tags. The reference pattern 28 may include only the auxiliary pattern or a combination of the auxiliary pattern and the figure-8 pattern. The sequence detector 26 may use the reference pattern, which only includes the auxiliary pattern, to adjust for all interfering signals, i.e. the auxiliary signal and the figure-8 signal. Likewise, any signal that is not adjusted but is during the noise window is used to calculate noise statistics. The algorithm causes the performance improvement to be due in large part to the fact that the adjusted window does not affect the noise calculation and therefore does not increase the noise by mistake.

Referring now to fig. 5, an exemplary operational flow diagram is provided that describes steps performed by the sequence detector 26 to identify a transmission pattern received from an interfering EAS system. The process begins by setting the threshold amplitude 27 to a level above which the signal received from the EAS tag is to be conditioned (step S116), ensuring that the true tag signal is not lost. This level may be determined experimentally as the maximum amplitude that may be received from the EAS. The receiver 14 receives the signal during the receive window (step S117) and determines the amplitude of the received signal RX _ AMP. The sequence detector 26 determines whether the amplitude of the received signal is greater than the threshold amplitude 27 (step S118). If the amplitude of the received signal is less than the threshold amplitude, sequence detector 26 clears the bit SAMPLED _ SEQ corresponding to the present receive window in current pattern 29 and shifts the number of received receive windows to the left (step S120). However, if the received signal is greater than the threshold amplitude 27 (step S118), the sequence detector 26 sets the bit corresponding to the present receive window in the current pattern 29 and shifts the number of receive windows to the left (step S122).

Next, the sequence detector 26 determines whether the number of processed reception windows RX _ WIN _ COUNT is equal to the maximum number of reception windows used to generate the sample sequence (step S124). If not, the number of receive windows processed is increased by 1 (step S126) and the sequence detector 26 loops back to prepare to receive signals during the next receive window. However, if the maximum number of receive windows has been reached (step S124), the sequence detector 26 compares the complete current received pattern 29 with the complete pattern reference sequence FULL _ REF _ SEQ (step S128). If the patterns match, the sequence detector 26 performs an adjustment process on the samples obtained during those time periods in which the complete reference pattern is matched (step S130). If the patterns do not match (step S128), the sequence detector 26 compares the full currently received pattern 29 with an auxiliary pattern reference sequence AID _ REF _ SEQ, which is associated with the full reference pattern (step S130). If these patterns match, the sequence detector 26 adjusts the samples obtained during those time periods in the processing stage during which the complete reference pattern corresponding to the auxiliary reference pattern is matched (step S130). If the patterns do not match (step S132), the sequence detector 26 performs a logical left shift (step S134) on both the complete reference pattern and the corresponding auxiliary reference pattern to include all possible variations due to system timing.

The sequence detector 26 determines whether all possible variations of the reference sequence 28 have been compared, i.e. whether the sequence COUNT SEQ _ COUNT is MAX _ SEQ _ COUNT (step S136). If not, the number of sequence counts is incremented by 1 (step S138) and the sequence detector 26 loops back to decision block S128 to compare the currently received pattern to the reference pattern 28. When all the variations of the reference pattern have been compared, then the sequence detector 26 loops to begin receiving a new set of signals during the next receive window.

Referring now to fig. 6, an exemplary operational flow diagram is provided that describes steps performed by the sequence detector 26 to identify interfering transmissions without determining the actual transmission pattern. This process is useful when the interfering signal is generated by an asynchronous EAS system in which no transmit pattern is provided. Unlike the previous process, this process focuses on the occurrence of excess energy to determine whether to perform adjustments to the signal in the processing stage. In other words, the received signal having more energy than would normally be detected from an EAS tag is not used for tag detection or background noise calculation. Alternatively, the process described in fig. 6 may be used with the processes of fig. 4 and/or 5 to condition a signal having excess energy when no corresponding pattern is to be determined.

The process begins by setting the threshold amplitude 27 to a level such that: the signal received from the EAS tag below this level is conditioned (step S140) to ensure that the actual tag signal is not lost. The receiver 14 receives the signal during the receive window (step S142) and determines the amplitude of the received signal RX _ AMP. The sequence detector 26 determines whether the amplitude of the received signal is greater than the threshold amplitude 27 (step S144). If the amplitude of the received signal is less than the threshold amplitude, the EAS system is not subject to interference during the present receive window, and the sequence detector 26 cycles back to prepare to receive the signal during the next receive window. However, if the amplitude of the received signal is greater than the threshold amplitude 27 (step S144), the sequence detector 26 adjusts the samples received in the processing stage (step S146). Thus, the EAS system uses only samples that can reasonably be inferred to necessarily be produced by an EAS tag to detect the tag. In other words, the EAS system only considers the signals that are not discarded to determine whether an EAS tag is present in the detection zone. Likewise, any signal that is not adjusted but is a noise window is used to calculate noise statistics.

The present invention can be realized in hardware, software, or a combination of hardware and software. Any kind of computing system, or other apparatus adapted for carrying out the methods described herein, is suited to perform the functions described herein.

A typical combination of hardware and software could be a computer system having one or more processing elements and a computer program stored on a storage medium that, when being loaded and executed, controls the computer system such that it carries out the methods described herein. The present invention can also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which-when loaded in a computing system-is able to carry out these methods. Storage medium refers to any volatile or non-volatile memory device.

Computer program or application in the context of the present invention refers to any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduced in different physical forms.

In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. It will be evident that the invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and accordingly, reference should be had to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.

Claims (19)

1. A method for reducing interference between a plurality of Electronic Article Surveillance (EAS) systems, each EAS system including a corresponding detection zone, the method comprising:

providing at least one reference pattern for a transmission window of an interfering EAS system, the at least one reference pattern indicating a sequence of time slots in which the interfering EAS system is transmitting;

receiving a sample pattern of signals, each signal having a respective amplitude;

comparing the received sample pattern with the at least one reference pattern; and

in response to determining that the received sample pattern matches the at least one reference pattern, using the at least one reference pattern, adjusting samples received during a receive window corresponding to a time slot in which the interfering EAS systems are transmitting to reduce interference between each EAS system.

2. The method of claim 1, further comprising determining whether an EAS tag is present in the corresponding detection zone without regard to the conditioned sample.

3. The method of claim 1, wherein the sample pattern of signals comprises a sequence of bits, each bit representing a receive window, the method further comprising: it is determined whether the amplitude of the received signal exceeds a predetermined threshold.

4. The method of claim 3, wherein the predetermined threshold is at least equal to a maximum amplitude received from an EAS tag.

5. The method of claim 3, further comprising: in response to determining that the amplitude of the received signal exceeds the predetermined threshold, setting a corresponding bit of the sequence of bits.

6. The method of claim 3, further comprising: clearing a corresponding bit in the bit sequence in response to determining that the amplitude of the received signal does not exceed the predetermined threshold.

7. The method of claim 1, wherein the interfering EAS system may transmit in an aiding configuration and a figure-8 configuration, the reference pattern comprising aiding samples and figure-8 samples.

8. The method of claim 1, wherein the interfering EAS system may transmit in an aiding configuration and a figure-8 configuration, and wherein the reference pattern includes only aiding samples, the method further comprising:

associating the reference graph with a complete reference graph, the complete reference graph comprising an auxiliary sample and a glyph sample; and

adjusting samples received during a receive window corresponding to a time slot in which the interfering EAS system transmits in the aiding configuration and in the figure-8 configuration.

9. The method of claim 8, further comprising:

determining whether an EAS tag is present in the corresponding detection zone without regard to the conditioned sample.

10. The method of claim 1, wherein in response to determining that the received sample pattern does not match the at least one reference pattern, the method further comprises:

determining that the amplitude of at least one received signal exceeds a predetermined threshold;

discarding the at least one received signal determined to have an amplitude exceeding the predetermined threshold; and

determining whether an EAS tag is present in the corresponding detection zone without regard to the at least one signal that is discarded.

11. An Electronic Article Surveillance (EAS) system, comprising:

a transmitter operable to transmit an interrogation signal to excite an EAS tag within a detection zone;

a receiver operable to receive a pattern of samples of signals, each signal having a respective amplitude;

a memory comprising at least one reference pattern for a transmission window of an independent interfering EAS system, the at least one reference pattern indicating a sequence of time slots in which the interfering EAS system transmits; and

a controller electrically connected to the transmitter, the receiver, and the memory, the controller operable to:

comparing the received sample pattern with the at least one reference pattern; and

in response to determining that the received pattern of samples matches the at least one reference pattern, using the at least one reference pattern, adjusting the samples received during a receive window corresponding to a time slot in which the interfering EAS systems are transmitting to reduce interference between each EAS system.

12. The electronic article surveillance system of claim 11, wherein the controller is further operable to determine whether an EAS tag is present in the detection zone without regard to the adjusted samples.

13. The electronic article surveillance system of claim 11, wherein the sample pattern of signals further comprises a sequence of bits, each bit representing a receive window, the controller further operable to determine whether an amplitude of the received signal exceeds a predetermined threshold.

14. The electronic article surveillance system of claim 13, wherein the predetermined threshold is greater than or equal to a maximum amplitude received from an EAS tag.

15. The electronic article surveillance system of claim 13, wherein the controller is further operable to set a corresponding bit of the sequence of bits in response to determining that the amplitude of the received signal exceeds the predetermined threshold.

16. The electronic article surveillance system of claim 13, wherein in response to determining that the amplitude of the received signal does not exceed the predetermined threshold, the controller is further operable to clear a corresponding bit in the sequence of bits.

17. The electronic article surveillance system of claim 11, wherein the interfering EAS system may transmit in an aiding configuration and a figure-8 configuration, the reference pattern comprising aiding samples and figure-8 samples.

18. The electronic article surveillance system of claim 11, wherein the interfering EAS system may transmit in an aiding configuration and a figure-8 configuration, and wherein the reference pattern includes only aiding samples, the controller further operable to:

associating the reference graph with a complete reference graph, the complete reference graph comprising an auxiliary sample and a glyph sample; and

adjusting samples received during a receive window corresponding to a time slot in which the interfering EAS system transmits in a helper configuration and in a figure-8 configuration.

19. The electronic article surveillance system of claim 11, wherein in response to determining that the received sample pattern does not match the at least one reference pattern, the controller is further operable to:

determining that the amplitude of at least one received signal exceeds a predetermined threshold;

discarding the at least one received signal determined to have an amplitude exceeding the predetermined threshold; and

only the signals that are not discarded are considered to determine whether an EAS tag is present in the detection zone.