Abstract:
A non-propagating magnetic field-based communication system transmits and receives digital data within a limited coverage area environment. The system includes a compact transmitter unit, such as that contained in an ‘tracking’ tag affixed to an object, and a digital detector/demodulator unit. In order to generate and FSK-modulate a non-propagating magnetic field in accordance with modulation signals representative of the digital data, the transmitter unit contains a magnetic field coil and one or more capacitors controllably switched in circuit with the coil in accordance with the data, so as to change the resonant frequency of an inductor-capacitor transmitter resonant circuit. The receiver unit includes a magnetic field-sensing coil in circuit with a capacitor, to form a receiver resonant circuit that resonates at a frequency between the FSK frequencies modulated by the transmitter unit. A digital receiver/demodulator detects whether received frequencies are valid FSK frequencies and derives digital data using differences between valid detected FSK frequencies.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of copending provisional U.S. patent application Ser. No. 60/159,658, filed Oct. 14, 1999, entitled: “Data Communication System Harnessing Frequency Shift Keyed Magnetic Field,” by R. Hash et al., assigned to the assignee of the present application, and the disclosure of which is incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to communication systems and components therefor, and is particularly directed to a non-propagating magnetic field-based communication system having a reduced hardware complexity magnetic field generator and detector arrangement, in combination with a frequency shift keyed (FSK) modulation scheme. The present invention is configured to facilitate the transmission and reception of digital data within a limited coverage area environment, between a compact transmitter unit, such as that contained in an ‘tracking’ tag affixed to an object, and a digital detector/demodulator unit. 
     BACKGROUND OF THE INVENTION 
     Although a variety of communication systems employ propagating magnetic fields, those which used non-propagating magnetic fields generated about a coil are less prevalent. As a non-limiting example, non-propagating magnetic fields may be employed in theft detection systems of the type installed in retail stores. Many of these systems, such as may be installed at the entry/exit of a retail establishment, are designed to convey only a single piece of data—the presence of a ‘tagged’ item. While others, such as ‘smart’ card systems, may convey more than one bit, the amount of information they are capable of transmitting and detecting is still relatively limited. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a non-propagating magnetic field based communication system, that is configured to provide for simplex digital communications without restriction to the amount of data that may be transmitted, via an FSK-modulated non-propagating magnetic field emanating from a modulating source and sensed by an associated demodulating receiver. As a non-limiting example, the invention may be employed in a real time location system for locating and/or identifying transponder-tagged objects. 
     Pursuant to the present invention, the system employs an FSK transmitter unit having an analog section that generates and FSK-modulates the non-propagating magnetic field, and a digital section that converts incoming digital data into switch control signals. The switch control signals controllably switch capacitor components in circuit with a magnetic field coil, thereby modulating or changing the resonant frequency of an inductor-capacitor (LC) tank circuit, to effect FSK-modulation of the magnetic field in accordance with the digital data. 
     The magnetic field coil is small compared to the volumetric extent of its generated magnetic field, so that energy in the magnetic field is not propagated. Under supervisory digital control of a zero-crossing detector, that is coupled in parallel with the resonant LC tank circuit, a pumping switch is periodically operated in a fly-back manner, to provide a DC current boost to the magnetic field coil from its DC power supply, thereby compensating for resistive losses in the tank circuit. The pumping signal has a duration for a small fraction of a cycle of the resonant frequency of the magnetic field, and may be optimized for the intended range of operation of the generated field and the size of the coil. 
     Zero crossing points of the resonant frequency signal are supplied to a microcontroller for control of capacitor insertion switches of a multi-capacitor circuit, producing FSK modulation of the resonant magnetic field. During a calibration mode, vernier adjustment capacitors may be controllably switched in and out of the resonator tank circuit to determine optimum frequency matches for a desired FSK frequency pair. Thereafter, during actual data transmission, calibration-based ‘best match’ capacitors are switchably inserted in parallel with a base capacitor, to precisely define a pair of resonant frequencies associated with the binary states of the digital data. To FSK modulate the magnetic field, a data spreading code, such as a Manchester or other relatively short spreading code used for reduced complexity data communications, may be employed. 
     An alternate embodiment of the transmitter unit eliminates the multi-capacitor circuit and employs a microcontroller to generate and control pulse timing and duration used to pump the field coil. This approach requires accurate values of inductance and capacitance in the resonant circuit, but offers the advantage of reduced parts count, allowing its use in compact, portable applications. Since the microcontroller pumps the circuit every cycle, frequency error due to resonant circuit tolerance is pulled into correction on a cycle by cycle basis. 
     The voltage supplied to the fly-back configuration also offers a suitable power control mechanism. This approach is favored for large changes in power, as it allows the pulse width of the pump to be maintained at the proper width for high efficiency. A variable voltage regulator may be employed to effect this change. Also, the use of the variable voltage regulator affords inclusion of a power control loop by monitoring the voltage produced in the resonant circuit and adjusting the supply voltage to maintain it at a constant level. This provides constant communication performance when large metal objects such as automobiles or forklifts move in close proximity to the transmitting unit. 
     The receiver unit includes an LC tank detector circuit that includes a magnetic field-sensing coil in parallel with an associated capacitor. The LC tank circuit resonates at a frequency between the two FSK frequencies employed by the transmitter unit. The resonant detector circuit is coupled to a sense amplifier, which amplifies the voltage produced by the tank circuit for the desired receiver sensitivity and buffers the detected voltage to the appropriate logic level for use by a digital receiver/demodulator. 
     The digital receiver is referenced to a clock frequency that corresponds to the difference between the FSK frequencies of the selected modulation pair. The digital receiver contains two signal buffer paths, that operate on alternate sample periods, corresponding to one-half the period of the received data spread code, so that at least one of the two buffer paths will not be sampling data during transitions in the received FSK frequency. The output of the sense amplifier is coupled to the clock input of a frequency counter, whose contents are coupled to data inputs of first and second selectively enabled alternate sample latches. The count value in the frequency counter is cleared upon active reset, or when its sample enable input is not active. When enabled, the frequency counter is incremented by the rising edge of the change in the output of amplifier. At the end of the sample time, the contents of the frequency counter are clocked into one of the two latches, whose contents are clocked into the other latch. 
     Since the contents of a respective latch indicate the number of successive rising edges of the received signal within a prescribed measurement interval (sample time), they are representative of the frequency of the latched data. This count value is coupled to the digital demodulator and compared with each of two stored counts associated with the two valid FSK frequencies. If the latched count is representative of a valid frequency, it is transferred to the other latch for subsequent comparison with the next frequency-associated count. The difference between the two latched count values is coupled to a state machine, which demodulates the spreading code of the data. The demodulated data is buffered, so that it may be clocked out for validation of parameters such as preamble, cyclic redundancy check (CRC) code sequence and message length. 
     The state machine demodulates the data by comparing successive FSK tones with a predefined start-of-message sequence. Upon detecting this sequence, the state machine initializes the data demodulation circuitry, so that the data may be clocked out as it is detected and demodulated. As is customary in FSK-modulation systems, data values may be represented by respectively different sequences of the two FSK tones. Similar to detecting the start of a message, the state machine may detect the end of a message by comparing successively received FSK tones with a predefined end-of-message sequence. Upon detecting a valid end-of message sequence, the state machine returns the receiver&#39;s demodulation circuitry to its idle state. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 diagrammatically illustrates the overall system configuration of a non-limiting, but preferred embodiment of the frequency shift keyed non-propagating magnetic field-based communication system of the present invention; 
     FIG. 2 shows a first embodiment of a transmitter unit that may be employed in the system of FIG. 1; 
     FIG. 3 shows an alternative embodiment of a transmitter unit that may be employed in the system of FIG. 1; 
     FIG. 4 diagrammatically illustrates the receiver unit of the system of FIG. 1; and 
     FIG. 5 diagrammatically illustrates the configuration of the digital receiver portion of the receiver of FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     Before describing in detail the frequency shift keyed non-propagating magnetic field-based communication system of the present invention, it should be observed that the invention resides primarily in prescribed modular arrangements of conventional magnetic field generation and sensing components, in combination with digital communication circuits and associated digital signal processing components and attendant supervisory control circuitry therefor, that controls the operations of such circuits and components. In a practical implementation, these modular arrangements may be readily implemented using relatively compact analog field coils and associated capacitors, that are coupled with application specific integrated circuit (ASIC) chip sets, programmable digital signal processors, or general purpose processors. 
     Consequently, the configuration of such arrangements of circuits and components have, for the most part, been illustrated in the drawings by readily understandable block diagrams, which show only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with details which will be readily apparent to those skilled in the art having the benefit of the description herein. Thus, the block diagram illustrations are primarily intended to show the major components of the invention in a convenient functional grouping, whereby the present invention may be more readily understood. 
     FIG. 1 diagrammatically illustrates the overall system configuration of a non-limiting, but preferred embodiment of the frequency shift keyed non-propagating magnetic field-based communication system of the present invention, as comprising a transmitter unit  1  and a receiver unit  2 , which are linked together by means of a non-propagating magnetic field  3  generated and FSK-modulated by the transmitter unit  1  and detected and demodulated by the receiver unit  2 . 
     The transmitter unit  1  is operative to generate and FSK-modulate the non-propagating magnetic field  3  in accordance with modulation signals representative of digital data to be transmitted to the receiver unit. For this purpose, as shown in FIG. 2, the transmitter unit  1  comprises an analog section  10 , which is configured to generate and FSK-modulate the non-propagating magnetic field, and a digital section  20  that is operative to convert an incoming digital data stream (DATA_IN) supplied to a transmitter input port  21  into switch control signals. These switch control signals are used to controllably switch the resonant frequency of magnetic coil—capacitor components of the analog section between first and second precisely calibrated or pre-tuned frequency values, and thereby effect FSK-modulation of the magnetic field in accordance with the digital data. 
     The analog section includes a magnetic field coil  11 , relatively large amplitude energizing current for which is supplied by a DC power supply or battery  12 , that is coupled to the coil by way of a ‘pumping’ switch  13 . The magnetic field coil  11  is small compared to the volumetric extent of its generated magnetic field, so that the energy in the magnetic field is not propagated, but is physically confined about the axis of the coil, as in a typical solenoid architecture. Under supervisory digital control of a zero-crossing detector  22  coupled in parallel with a resonant (‘tank’ or ‘ringing’) circuit  14  formed by the field coil  11  and one or more capacitors  15 - 0 - 15 -N, and  15 -FSK of a capacitor circuit  15 , the pumping switch  13  is periodically closed and opened in a fly-back manner, to provide a DC current boost to the coil  1  from the battery  12 , in order to compensate for resistive losses in the ringing circuit  14 . 
     The pumping signal generated by the zero crossing detector  22  provides for switch closure at or near the point at which the resonating current signal in the tank circuit crosses zero. This pumping signal has a duration for a small fraction of a cycle of the resonant frequency of the magnetic field, and may be optimized for the intended range of operation of the generated field and the size of coil  11 . The zero crossing points of the resonant frequency are supplied to a supervisory microcontroller  23 , for control of capacitor insertion switches of the capacitor circuit  15  and thereby FSK modulation of the resonant magnetic field. 
     More particularly, within the capacitor circuit  15 , a first base frequency-defining capacitor  15 - 0  is hardwired electrically in parallel with the coil  11 , while the remaining capacitors  15 -FSK and  15 - 1 - 15 -N are selectively connected in parallel with the coil  11  by the selective closure of respective series connected switches  16 -FSK and  16 - 1 - 16 -N. The switches  16  may be implemented as digitally controllable electronic switch devices, such as, but not limited to field effect transistors (FETs), bipolar transistors, and the like. The selective closure of one or more of the switches  16  by the supervisory microcontroller  23  (e.g., in accordance with respective binary states of the digital data stream applied to input port  21 ) places one or more of the capacitors  15  in parallel with the base capacitor  15 - 0 , so as to controllably lower or tune the resonant frequency of generated magnetic field. 
     In order to generate the FSK modulation switch control signals for application to the switches  16  of the magnetic field generator, the microcontroller  23  of the digital section  20  requires a clock signal as a modulation reference. For this purpose, a base frequency as defined by the coil  11  and the base capacitor  15 - 0  of the tank circuit  14  may be employed; alternatively, a separate, independent clock source, such as a crystal oscillator  24 , may be used. In this latter case, the digital control section  20  may also adjust the resonant frequency to account for tolerances due to component variation or proximity of ferrous metals to the field coil  11 . 
     For this purpose, capacitors  15 - 1 , . . . ,  15 -N constitute additional, small valued capacitors that may be selectively coupled in parallel with the base capacitor  15 - 0  and/or an FSK capacitor  15 -FSK by associated by-pass switches  16 - 1 , . . . ,  16 -N, to provide for vernier tuning of the resonant frequency of the tank circuit  14 . The independent clock source acts as a reference for the supervisory microcontroller  23  to measure the resonant frequency established by the tank circuit  14 , at transmitter initialization and periodically thereafter. This ensures that the two resonant frequencies, namely, a first FSK resonant frequency F 1  defined by field coil  1  and base capacitor  15 - 0  (plus any fine tuning by one of more of capacitors  15 - 1  and  15 -N), and a second FSK resonant frequency F 2  defined by field coil  1  and FSK capacitor  15 -FSK (plus any fine tuning by one of more of capacitors  15 - 1  and  15 -N) are within spec. 
     During a calibration mode, the vernier adjustment capacitors  15 - 1 , . . . ,  15 -N may be controllably switched in and out of the resonator circuit  14  to determine the optimum frequency matches for the desired frequency pair. Thereafter, during data transmission, these ‘best match’ capacitors are switchably inserted in parallel with the base capacitor  15 - 0  and capacitor  15 -FSK, as necessary, to define the resonant frequencies associated with the binary states of the digital data. As a non-limiting example of using the invention with a real time location system transponder tag, the pair of FSK frequencies F 1  and F 2  may correspond to F 1 =114.7 kHz and F 2 =147.5 kHz. These frequencies provide for low power and low cost receiver and demodulator components to be used. 
     FIG. 3 shows a reduced hardware complexity embodiment of the transmitter unit, where microcontroller  23  of the digital section generates and controls the pulse timing and duration used to pump the field coil  11 . The transmitter unit of FIG. 3 eliminates some of the analog circuitry at the cost of requiring accurate, temperature-stable components in the resonant LC network. This embodiment is preferred in small battery-operated and portable applications. 
     The analog portion  10  of the transmitter unit of FIG. 3 places a relatively low inductance auxiliary coil  11 A in a transformer-coupled configuration with the high inductance field coil  11 . The auxiliary coil  11 A is coupled to the battery  12  through the pumping switch  13 , selective closure of which is controlled directly by the microcontroller  23 . Because the field generating tank circuit is now DC-isolated from the pumping switch, a relatively simple switch can be used. 
     Both transmitter embodiments, when employed in heavy industrial applications, benefit from a power control loop. This allows for correction of the magnetic field level, thereby maintaining communication performance, when the system is affected by the proximity of metal such as a passing automobile or forklift. For this, the power source  12  may be appropriately adjusted by a control signal generated by monitoring the level of the voltage present in the resonant LC circuit. The power source  12  may be controllably varied by means of an adjustable regulator  12 R, wherein the detected resonant circuit voltage is fed back via a feed back link  12 FB to the adjustment portion of the regulator  12 R, to form a closed control loop. 
     The receiver unit of the system of FIG. 1 is illustrated diagrammatically in FIG. 4 as comprising a resonant (LC tank) detector circuit  30  that includes a magnetic field-sensing coil  31  coupled in parallel with an associated capacitor  32 . The inductance-capacitance parameters of coil  31  and capacitor  32 , respectively, are such that the tank circuit  30  resonates at a frequency between the two FSK frequencies employed by the transmitter unit. For the non-limiting example using frequencies of F 1 =114.7 kHz and F 2 =147.5 kHz, described above, the resonant frequency of the receiver tank circuit  30  may be 121 kHz. 
     The resonant detector circuit  30  is coupled to a sense amplifier  35 , which amplifies the voltage produced by the receiver detector circuit for the desired receiver sensitivity and buffers the detected voltage to the appropriate logic level for use by a digital receiver  40 , the output of which is coupled to a state machine-based demodulator  45 . The digital receiver  40  is referenced to a prescribed receiver clock frequency F RCLK , as may be supplied by a crystal clock  42 . For the present example, the receiver clock is set to a frequency corresponding to the difference between the FSK frequencies of the selected modulation pair. For the current example of employing transmitter frequencies of 114.7 kHz and 147.5 kHz, the receiver clock F RCLK  may be set at F RCLK =32.8 kHz. This reduced clock frequency maintains very low power consumption at low cost. The use of such a relatively low clock frequency in the receiver requires a slower data rate, since one clock cycle of the receiver clock represents only 3.4-3.8 FSK clock cycles. 
     As diagrammatically illustrated in FIG. 5, the digital receiver contains two signal buffer paths  50  and  60 , that operate on alternate sample periods that are one-half the period of the received data spread code. This ensures that at least one of the two buffer paths will not be sampling data during transitions in the received FSK frequency. As a non-limiting example, the data spreading code may comprise a Manchester (two-chip spreading) or other relatively short spreading code used for reduced complexity data communications. The receiver integration time is sufficiently long to provide for counting the number of rising edges in a received FSK signal, and readily differentiate between the two valid FSK frequencies (here, F 1 =114.7 kHz and F 2 =147.5 kHz), determine when a frequency change occurs, and reject other FSK signals and/or noise. 
     To this end, the output of the receiver unit&#39;s sense amplifier  35  is coupled over line  36  to clock inputs of each of a frequency counter  51  and  61 , the contents of which are coupled to data (D) inputs of sample shift registers or A latches  52  and  62 , that are respectively coupled in cascade with associated B latches  53  and  54 . The contents of the frequency counters  51  and  61  are cleared or reset to zero, upon an active reset being applied from prescribed bit stages of a latch control counter  70  to their respective clear inputs CLR from prescribed bit stages. As a non-limiting example, latch control counter  70  may comprise an eight bit counter. As long as they are enabled, the contents of frequency counters  51  and  61  are modified (e.g., incremented) by the rising edge of the change in the output signal from sense amplifier  35 . 
     The (eight-bit) latch control counter  70  is continuously clocked by the receiver clock F RCLK =32.8 kHz, so that its contents are sequentially changed (e.g., incremented) and roll over. In the course of the contents of the latch control counter  70  being successively incremented by the receiver clock, the logical states of its respective Q=3 and Q=7 bits will eventually change (e.g., form a logical ‘0’ to a logical ‘1’), so that clear or reset signals are periodically applied to the CLR inputs of frequency counters  51  and  61 . For the non-limiting example of implementing the latch control counter by means of an eight-bit counter, its Q=3 and Q=7 bit stages to supply reset to the frequency counters provides for the above-referenced alternate sampling intervals for the two buffer paths  50  and  60 . 
     The Q=3 and Q=7 bit stages of latch control counter are applied to the clock inputs of the various latches of the buffer paths  50  and  60 . As a consequence, at the end of their alternate sample times, the contents of the frequency counters  51  and  61  are respectively transferred or clocked into their associated A latches  52  and  62 ; also, the current contents of the A latches  52  and  62  are clocked into the cascaded B latch  53  and  63 , respectively. 
     Since the contents of A latches  52  and  62  indicate the number of successive rising edges of the received signal within a prescribed measurement interval (sample time), they are representative of the frequency of the latched data. These count values are coupled to respective A inputs of subtraction units  54  and  64 , whose B inputs are coupled to the outputs of latches  53  and  63 . The difference outputs DIFF provided by subtraction units  54  and  64  are coupled to the D inputs of respective difference latches  55  and  65 . The difference latches  55  and  65 , whose contents are coupled to a state machine within the demodulator  45 , are clocked by the latch control counter  70 , as described above. 
     Thus, as received frequency or tone-representative data output on line  36  from amplifier  35  is applied to counters  51  and  52 , it successively increments their contents. Then, in the course of latch control counter  70  being sequentially incremented by the receiver clock, as the respective Q=3 and Q=7 bits of latch control counter  70  change to a logical ‘1’, the latches of the respective signal buffer paths  50  and  60  are updated. 
     In particular, the contents of frequency counters  51  and  61 , respectively, are loaded into the A latches  52  and  62 , and the contents of the A latches  52  and  62 , respectively are transferred to B latches  53  and  63 , for comparison with the next tone (frequency) clocked into the A latches  52  and  62 . Also, the difference latches  55  and  65  are clocked. The differences between the contents of latches  52  and  53 , and between the contents of latches  62  and  63 , as output by subtraction units  54  and  64  are coupled to the state machine, which demodulates the spreading code of the data. The demodulated data is then buffered, so that it may be clocked out for validation of parameters such as preamble, cyclic redundancy check (CRC) code sequence and message length. 
     As a non-limiting demodulation scheme, the state machine compares a received sequence of FSK tones with a predefined start-of-message sequence (corresponding to a start synchronization code) . As a non-limiting example, the start-of-message sequence may comprise a plurality of successive samples at one FSK frequency or tone (such as three spreading chip periods at the higher of the two FSK tones), followed by a plurality of successive samples at the second FSK frequency (e.g., three spreading chip periods at the lower of the two FSK tones). For the example of three successive samples of one tone followed by three successive samples of the other tone, the difference between the contents of the respective A and B latches  52 / 62  and  53 / 63  would be the numerical sequence (0, 0,−N, 0, 0). Upon detecting this sequence, the state machine initializes the data demodulation circuitry, so that the data may be clocked out as it is detected and demodulated. 
     As is customary in FSK-based modulation systems, data values of ‘1’ and ‘0’ are represented by respectively difference sequences of the two FSK tones. As a non-limiting example, a logical ‘one’ may correspond to one spreading chip period at the higher FSK tone (147.5 KhZ) followed by one spreading chip period at the lower FSK tone (114.7 kHz); a logical ‘zero’ may correspond to one spreading chip period at the lower FSK tone (114.7 kHz), followed by one spreading chip period at the higher FSK tone (147.5 KhZ). For this example, the data bit sequence (00) would result in latch differences of (−N,+N); the data bit sequence (01) would result in latch differences of (0,+N); the data bit sequence (10) would result in latch differences of (0,−N); and the data bit sequence (11) would result in latch differences of (+N,−N). This allows a determination of the logic level provided at the output at the end of each sample period to be clocked out. It also provides for detection of any errors in format that may indicate corruption of the data. 
     Similar to detecting the start of a message, the state machine may detect the end of a message by comparing a received sequence of FSK tones with a predefined end-of-message sequence. As a non-limiting example, the end-of-message sequence may be complementary to the start-of-message sequence, described above. Namely, in the present example, an and-of-message sequence may comprise a three spreading chip periods at the lower of the two FSK tones), followed by three spreading chip periods at the higher of the two FSK tones) . In this case, the difference between the contents of the A and B latches would be the numerical sequence (0, 0, +N, 0, 0). Upon detecting a valid end-of message sequence, the state machine returns the receiver&#39;s demodulation circuitry to its idle state. 
     As will be appreciated from the foregoing description, the present invention provides a relatively compact, and reduced complexity communication system that FSK-modulates a non-propagating magnetic field for simplex digital communications, without restriction to the amount of data transmitted between a modulating source and an associated demodulating receiver. This makes the invention readily suited for real time location systems for locating and/or identifying transponder-tagged objects. 
     While we have shown and described several embodiments in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art. We therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.