Abstract:
A method for controlling electronic article surveillance (EAS) transmissions is described. The method includes calculating system parameters associated with one or more of a desired frequency a desired duty cycle, and a desired phase difference between antennas for a transmitter, and initializing a counter with a value based on the system parameters. The method also includes comparing a count from the counter to the system parameters, and modulating EAS transmission signals based on the comparison between the count and the system parameters. An EAS transmitter and an EAS system are also described.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application relates to and claims priority from Provisional Application Ser. No. 60/570,030, filed May 11, 2004, titled “Arbitrary Antenna Phasing in an Electronic Article Surveillance System” and U.S. application Ser. No. 11/121,1898, filed May 4, 2005, “Methods and Apparatus for Arbitrary Antenna Phasing in an Electronic Article Surveillance System” the entire disclosure of which is hereby incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to the processing of electronic article surveillance (EAS) tag signals, and more particularly to a system and method of using phase shifting of a plurality of transmitter oscillators in a transmitter used in an EAS system. 
         [0004]    2. Description of the Related Art 
         [0005]    In acoustomagnetic or magnetomechanical electronic article surveillance, or “EAS,” a detection system may excite an EAS tag by transmitting an electromagnetic burst at a resonance frequency of the tag. When the tag is present within an interrogation zone defined by the electromagnetic field generated by the burst transmitter, the tag resonates with an acoustomagnetic or magnetomechanical response frequency that is detectable by a receiver in the detection system. 
         [0006]    The typical default mode of operation of these EAS systems in most countries that do not adhere to the standards promulgated by the European Telecommunications Standards Institute (“ETSI”) uses phase flipping on the transmitter to produce various electromagnetic field patterns that provide for excitation of the tags in various orientations. However, the emissions standards in some countries (notably those adhering to ETSI standards) prevent the system from transmitting in certain antenna configurations with any significant current levels. 
         [0007]    For example, a figure eight antenna configuration produces an electromagnetic field that meets ETSI standards, but tags located in certain positions and orientations within the interrogation zone may not get excited by the figure eight antenna configuration because these tags are located in “nulls” within the resultant electromagnetic field. An aiding antenna configuration produces fewer nulls, but particular current levels may result in electromagnetic field levels that do not meet the ETSI standards. Another issue is that due to mismatches in the antenna tuning, there may be phase shifts between the two antenna elements. These mismatches result in an imperfect electromagnetic field, for example, decreased power efficiency in the interrogation zone and increased emission levels in figure eight antenna configurations. Decreased power efficiency makes the excitation and subsequent detection of EAS tags within the interrogation zone more difficult. Increased emission levels may not meet ETSI standards. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    A method for controlling electronic article surveillance (EAS) transmissions is provided that may comprise calculating system parameters associated with one or more of a desired frequency a desired duty cycle, and a desired phase difference between antennas for a transmitter. The method may further comprise initializing a counter with a value based on the system parameters, comparing a count from the counter to the system parameters, and modulating EAS transmission signals based on the comparison between the count and the system parameters. 
         [0009]    A transmitter for an EAS system is also provided. The EAS system may include a plurality of antennas, and the transmitter may comprise a plurality of amplifiers, each antenna configured to transmit a signal originating from a corresponding one of the amplifiers, and a processor configurable to adjust a phase shift between the outputs of the amplifiers based on a received value. 
         [0010]    An EAS system is provided that may comprise at least one EAS tag, a plurality of antennas, at least one receiver configured to utilize the antennas to receive emissions from the tag, and at least one transmitter. The transmitter may be configured to transmit signals from the antennas to cause the tag to resonate when the tag is in a vicinity of the transmitter. Each transmitter may comprise a plurality of antennas, each of which may be configured to transmit a signal originating from a corresponding amplifier. The transmitter may be configurable to adjust a phase between outputs of the amplifiers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    For a better understanding of the invention, together with other objects, features and advantages, reference should be made to the following detailed description which should be read in conjunction with the following figures wherein like numerals represent like parts. 
           [0012]      FIG. 1  is a block diagram of an electronic article surveillance (EAS) system. 
           [0013]      FIG. 2  is a front view of an antenna pedestal for an EAS system illustrating an aiding current flow through the antenna elements therein, and a portion of an electromagnetic field resulting from the aiding current flows. 
           [0014]      FIG. 3  is a side view of the antenna pedestal of  FIG. 2  illustrating another portion of the electromagnetic field resulting from the aiding current flows. 
           [0015]      FIG. 4  is a front view of an antenna pedestal for an EAS system illustrating a figure eight current flow through the antenna elements therein, and a portion of an electromagnetic field resulting from the figure eight current flows. 
           [0016]      FIG. 5  is a side view of the antenna pedestal of  FIG. 4  illustrating another portion of the electromagnetic field resulting from the figure eight current flow. 
           [0017]      FIG. 6  is a block diagram of a portion of a transmitter for an EAS system. 
           [0018]      FIG. 7  is a flowchart illustrating operation of a portion of the transmitter of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    For simplicity and ease of explanation, the invention will be described herein in connection with various embodiments thereof. Those skilled in the art will recognize, however, that the features and advantages of the invention may be implemented in a variety of configurations. It is to be understood, therefore, that the embodiments described herein are presented by way of illustration, not of limitation. 
         [0020]      FIG. 1  illustrates an EAS system  10  that may include a first antenna pedestal  12  and a second antenna pedestal  14 . The antenna pedestals  12  and  14  may be connected to a control unit  16  that includes a transmitter  18  and a receiver  20 . The control unit  16  may be configured for communication with an external device, for example, a computer system controlling or monitoring operation of a number of EAS systems. In addition, the control unit  16  may be configured to control transmissions from transmitter  18  and receptions at receiver  20  such that the antenna pedestals  12  and  14  can be utilized for both transmission of signals for reception by an EAS tag  30  and reception of signals generated by the excitation of EAS tag  30 . Specifically such receptions typically occur when the EAS tags  30  are within an interrogation zone  32 , which is generally between antenna pedestals  12  and  14 . System  10  is representative of many EAS system embodiments and is provided as an example only. For example, in an alternative embodiment, control unit  16  may be located within one of the antenna pedestals  12  and  14 . In still another embodiment additional antennas that only receive signals from the EAS tags  30  may be utilized as part of the EAS system. Also a single control unit  16 , either within a pedestal or located separately may be configured to control multiple sets of antenna pedestals. 
         [0021]    In one embodiment, antenna pedestals  12  and  14  each include two antenna elements.  FIG. 2  is an illustration of an antenna pedestal, for example antenna pedestal  12  that may include two antenna elements  40  and  42  therein. In the illustrated embodiment, antenna elements  40  and  42  may be provided within antenna pedestal  12  in a loop configuration. In this configuration, and as illustrated, each antenna loop  50  and  52  may be substantially rectangular. Antenna pedestal  12  includes a central member  56  through which a portion  60  of antenna loop  50  may pass. A portion  62  of antenna loop  52  may also pass through central member  56 . As such, portion  60  and portion  62  can be located near enough to one another that an electromagnetic field caused by current passing through antenna loop  50  is affected by an electromagnetic field caused by current passing through antenna loop  52 . Current arrows  70  for antenna loop  50  and current arrows  72  for antenna loop  52  illustrate that antenna pedestal  12  may be configured in a configuration that is commonly referred to as an aiding configuration. 
         [0022]    In the aiding configuration, the current through antenna loops  50  and  52  is generally traveling in the same direction, except for portions  60  and  62  as shown. In the aiding configuration, the currents flowing through antenna loops  50  and  52  are typically considered to be in phase. An aiding configuration current flow through antenna loops  50  and  52  results in a vertical component of electromagnetic field  80  having a general shape and nulls  82  as is shown in  FIG. 2 . 
         [0023]      FIG. 3  is a side view of the antenna pedestal  12  illustrating the horizontal component of the electromagnetic field  80  that extends from antenna pedestal  12  when operating in an aiding configuration. As illustrated, the horizontal component includes no nulls from a top to bottom of antenna pedestal  12 . This horizontal component is representative of an electromagnetic field that may not meet ETSI standards. 
         [0024]      FIG. 4  is an illustration of an antenna pedestal, for example antenna pedestal  12 , that also may include two antenna elements  40  and  42  therein and configured as described above. Specifically the two antenna elements  40  and  42  are configured as antenna loops  50  and  52 . More specifically current arrows  90  for antenna loop  50  and current arrows  92  for antenna loop  52  illustrate that antenna pedestal  12  may be configured in a configuration that is commonly referred to as a figure eight configuration. In the figure eight configuration, the current through antenna loops  50  and  52  is generally traveling in the opposite directions, except for portions  60  and  62  as shown. In the figure eight configuration, the currents passing through antenna loops  50  and  52  are typically considered to be 180 degrees out of phase. A figure eight configuration current flows through antenna loops  50  and  52  results in a electromagnetic field  100  whose general shape is shown in  FIG. 4  and that includes nulls  102  as shown in  FIG. 4 . 
         [0025]      FIG. 5  is a side view of the antenna pedestal  12  illustrating the horizontal component of the electromagnetic field  100  that extends from antenna pedestal  12  when operating in a figure eight configuration. As shown, the horizontal component may include a null approximate a center of antenna pedestal  12 . 
         [0026]    Switching the current flow through antenna loops  50  and  52  back and forth from an aiding configuration to a figure eight configuration is sometimes referred to as phase flipping. Phase flipping is utilized to produce changes to the electromagnetic field such that EAS tag  30  (shown in  FIG. 1 ) is excited regardless of its physical orientation. 
         [0027]    However, as described above, emissions standards in countries adhering to the European Telecommunications Standards Institute (“ETSI”) standards prevent the antenna pedestal  12  from transmitting in an aiding configuration with any significant current levels. Therefore, the electromagnetic field (e.g., electromagnetic field  80  shown in  FIGS. 2 and 3 ) may not be strong enough to excite EAS tags  30  in certain orientations within the interrogation zone  32 . Further, while a figure eight configuration meets ETSI standards, some EAS tag  30  positions and orientations within the interrogation zone  32  may not be excited by the electromagnetic field  100  because these EAS tags  30  may pass through nulls  102  in the electromagnetic field  100  present within the interrogation zone  32 . There also may be undesirable phase shifts between the antenna loops  50  and  52 . These phase shifts may be due to mismatches in antenna tuning between the two antenna loops  50  and  52 , which results in deviations from the desired electromagnetic fields  80  and  100 . Such mismatches may also result in a significant loss of symmetry between the fields generated by the antenna loops  50  and  52 , resulting in increased emissions that may not meet ETSI standards. 
         [0028]      FIG. 6  is a block diagram of a portion of a transmitter  110  for an EAS system such as EAS system  10 . The transmitter  110  may include a digital signal processor  111  having a pulse width modulator (PWM)  112  to provide signals to amplifiers  114  and  116 . These signals may be then transmitted through antenna elements  40  and  42 , respectively. It is to be understood that the embodiments described herein may also be accomplished utilizing a DSP that interfaces to a PWM module that is external to the DSP. 
         [0029]    PWM  112 , and thus transmitter  110  may be configured, as further described below, to improve the detection of surveillance tags (e.g., EAS tags  30  shown in  FIG. 1 ), which may be located in “nulls” in the electromagnetic fields generated by, for example, EAS system  10 . In addition, PWM  112  may be configured to compensate for mismatches in the tuning of antenna elements  40  and  42  that may result in phase shifts between the various antenna elements  40  and  42 , which can result in an imperfect electromagnetic fields and decreased power efficiency within the interrogation zone  32  (shown in  FIG. 1 ). Further, transmitter  110  is capable of operation under the ETSI standards described above. 
         [0030]    As shown in  FIG. 6 , PWM  112  includes a plurality of control oscillators  130  and  132  that may be configurable such that antenna elements  40  and  42  embody for example, a figure eight configuration, an aiding configuration, or other arbitrary phase configuration. These various configurations can result in an electromagnetic field emanating from antenna elements  40  and  42  that is applicable for different EAS system installations. Arbitrary phase configurations are desirable, for example, to address impedance differences and transmission cable lengths that are installation dependent and to reduce the occurrences of nulls within an interrogation zone. 
         [0031]    In the illustrated embodiment, each oscillator  130  and  132  may be incorporated within the PWM  112  or similar processing circuitry that includes a period register  140  and a compare register  142  for receiving a frequency control signal  144  and a pulse width control signal  146 , respectively. The frequency control signal  144  and the pulse width control signal  146  may be generated within the DSP  111 , for example, using program control algorithms contained within a processing portion  150  of the DSP  111  and are sometimes referred to as system parameters. The PWM  112  may also include a counter  152 , Which receives phase control signals  154  from the processing portion  150  of the DSP  111 . 
         [0032]    In one embodiment period register  140  and frequency control signal  144  may be utilized to generate an average frequency for the modulated transmissions from PWM  112 . More specifically a desired transmission frequency may not be an exact multiple of a master clock  156  within the DSP  111  that is supplied to the period register  140 , the compare register  142 , and the counter  152  of both oscillators  130  and  132 . Therefore, to achieve the desired frequency on average, the frequency control signal  144  may be configured to dither a value within the period register  140 , for example, utilizing software within the DSP processing portion  150 . As used herein, the term “dither” is understood to mean switching back and forth between two or more values. By dithering the values within the period register  140 , the frequency output by the period register  140  changes. These frequency outputs are multiples of the frequency of the master clock  156 . When these frequency outputs are averaged, the average is equal to the desired transmission frequency. 
         [0033]    As an example, in order to achieve a desired transmission frequency that is equivalent of 2500.6 master clock cycles, the period register  140  may be dithered back and forth between 2500 master clock cycles two times and 2501 clock cycles three times. For the 2500 master clock cycle portion of the example, once the counter  152  has counted 2500 clock cycles, compare logic  160 , which monitors the output of the counter  152  and the period register  140  output, outputs a signal  162 . Signal  162  may be used to reset the counter  152  and may also be applied to PWM output logic  164 . Pulse width control signal  146  and compare register  142  are configured to control a duty cycle of the PWM output  166 . 
         [0034]    To control the duty cycle, the output of the counter  152  and output of compare register  142  may be compared by compare logic  168 . The output  170  of the compare logic  168  may also be input to PWM output logic  164  as a set and clear signal. Continuing with the above example, for a 25% duty cycle PWM output, the pulse width control signal could set the compare register  142  such that after 625 clock cycles, output  170  of compare logic  168  changes state (setting PWM output logic  164 ) and remain in that changed state until counter  152  is reset (clearing PWM output logic  164 ). In other words, the width of the power amplifier drive signal (output  166 ) may be controlled by adjusting the compare register  142 . 
         [0035]    To provide the arbitrary phase antenna pattern between antenna elements  40  and  42  the counters (e.g., counter  152 ) in each of the oscillators  130  and  132  may be initialized with an offset relative to one another. For example, if the period of the oscillator  130  is to be 1000 cycles of master clock  156 , then implementing a phase shift of 45 degrees would require that one of the oscillators be initialized with a counter value of zero, while the other oscillator be initialized with a counter value of 125. The 125 value is the period divided by the fraction of 360 degrees or 1000×(360/45)=125. The offset value of 125 may be reduced or increased based on mismatches in the tuning between antenna elements  40  and  42  and variances in the lengths between the amplifiers  114  and  116  and the corresponding antenna elements  40  and  42 . 
         [0036]    Based on the offset value, the output signals  162  from the compare logic of each oscillator  130  and  132  may be offset from one another. Likewise, the output signals  170  from the compare logic  168  of each oscillator  130  and  132  may be offset. These output signals  162  and  170  may be utilized within oscillator  130  and  132 , respectively, to control the pulse width modulator output logic  164 . Therefore, the oscillators  130  and  132  generate corresponding offset pulse modulated signal bursts. The offset pulse modulated signal bursts generated by each oscillator  130  and  132  may then be amplified by the respective amplifiers  114  and  116  to drive each corresponding antenna element  40  and  42 . 
         [0037]    These various embodiments provide significant advantages to the operation of EAS transmitters in that arbitrary phase shifts between multiple transmit channels driving, for example, antenna elements  40  and  42  of an antenna pedestal may be provided. One implementation allows for phase shifts between the antenna elements  40  and  42  ranging from about zero degrees to about 180 degrees. A phase difference of about 180 degrees between antenna elements  40  and  42  is effective for reducing emissions, but results in a particular set of nulls in the electromagnetic field that emanates from antenna elements  40  and  42 . A phase difference of about zero degrees between antenna elements  40  and  42  results in a spatially different and generally smaller set of nulls, however emissions are higher. Therefore selection of a phase shift between antenna elements  40  and  42  somewhere between zero degrees and 180 degrees may result in a null set smaller than the nulls produced with a 180 phase shift, while still having an emission level within ETSI standards. 
         [0038]    With a phase shift of less than 180 degrees, performance of the EAS transmitter  110  may be increased because excitation of EAS tags  30  becomes less dependent on a correlation between the electromagnetic fields generated and orientations of the EAS tags  30 . In other words, an arbitrary phase difference between antenna elements  40  and  42  may be utilized to eliminate, or at least reduce nulls in the generated electromagnetic fields. One embodiment of an EAS transmitter that may be implemented is a quadrature transmitter that has a 90 degree phase shift between antenna elements  40  and  42 . Such an embodiment may eliminate the need to phase flip the transmissions (switching back and forth between aiding and figure eight configurations) as is performed in some known applications. Eliminating phase flipping of EAS transmitters also reduces memory requirements of a controller of the EAS transmitter. 
         [0039]      FIG. 7  is a flowchart  200  illustrating processes embodied within transmitter  110  that achieve the above described arbitrary phase shifting within the EAS transmitter. First, at  202 , period registers  140  of each oscillator  130  and  132  in the PWM  112  may be set using a system parameter that corresponds to a desired frequency. Setting the period registers  140  with system parameters that result in the desired frequency output from the PWM  112  may include determining the number of cycles of master clock  156  to be counted within the compare logic  160 . If the number of cycles of master clock  156  is not an exact multiple of the master clock frequency setting the period registers  140  may include dithering the values set within the period registers  140  such that an average frequency output of the PWM  112  is at the desired frequency. Once the count of master clock  156  cycles is equal to the set value, a counter within each oscillator  130  and  132  may be reset, and the counter  152  may begin again to count to the set value, which may be the same as previously set or which has been dithered to a new value as described above. 
         [0040]    At  204 , compare registers  142  within the oscillators  130  and  132  may be configured with a value such that an output of the PWM is at a desired duty cycle. The configuration may be based on the number of clock cycles in the desired PWM frequency. For example, for a 50% duty cycle, the compare registers  142  are configured at  204  with a count value that is one-half of the count value set at  202  within the period registers. 
         [0041]    At  206 , counters may be initialized within the oscillators  130  and  132  and counts may be output, at  208 , to both the period registers  140  and the compare registers  142  of each corresponding oscillator  130  and  132 . To shift a phase of the transmissions between the respective antennas, the counters may be initialized at  206  with different values as above described. The counter  152  may then be started. 
         [0042]    The embodiments described herein provide arbitrary phase shifts between EAS transmitter antennas by using two or more independent transmitter oscillators for the different transmitter channels. The independent transmitter oscillators allow arbitrary phase shifts between the channels while still operating, and transmitting, at the same frequency. As the period registers are also programmable, the transmitter oscillators are also configurable to allow arbitrary frequency shifts between the transmitter channels. 
         [0043]    In the above described exemplary embodiments, the transmitter oscillators may be digitally implemented numerically controlled oscillators (NCOs) that are included as part of the pulse width modulator control circuitry that is contained within certain digital signal processors. As described above, a phase shift may be implemented by initializing the count registers of the two separate oscillators with an offset relative to one another. Transmit frequencies may also be programmed for each oscillator by changing the period registers of the oscillators. Also, while described in terms of a digital signal processor, the above described embodiments may also be implemented in other programmable devices and in discrete circuits. 
         [0044]    It is to be understood that variations and modifications of the present invention can be made without departing from the scope of the invention. It is also to be understood that the scope of the invention is not to be interpreted as limited to the specific embodiments disclosed herein, but only in accordance with the appended claims when read in light of the forgoing disclosure.