Identification system

Abstract: A dual standard RF-ID protocol system which can receive and recognize HDX and FDX transponder signals comprising an antenna circuit (10,12,20,22) for transmitting an excitation signal to charge-up HDX transponders and immediately initiate FDX transponders to respond. The antenna circuit comprises first (10,12) and second (20,22) pole circuits for receiving an HDX transponder RF frequency response signal having a first frequency and a FDX transponder RF frequency response signal having a second frequency. A mixer/amplifier circuit (24) mixes the HDX transponder RF frequency response signal from the first frequency down to the second frequency. A demodulator circuit (30) is operatively connected to the mixer/amplifier circuit for demodulating the FDX and the HDX frequency signals into a data signal.

Description: TECHNICAL FIELD OF THE INVENTION This invention generally relates to an identification system and particularly to a dual standard RF-ID system for receiving half-duplex and full-duplex identification responses. BACKGROUND OF THE INVENTION Many different communication protocols are used in remote RF identification systems today. Some systems use a full duplex communication protocol wherein the interrogator is continuously transmitting an interrogation signal while the transponder in that field is responding. One advantage of this system is that the transponder can be receiving power from the interrogator continuously while it is transmitting its response signal. Another variation uses frequency or phase modulation. Return signals using frequency modulation or phase modulation are quite different from amplitude modulated signals. Other RF-ID systems use a half-duplex communication protocol wherein the interrogator transmits a powering interrogation signal for a predetermined amount of time and then stops transmitting and listens for a transponder response signal for a predetermined amount of time. Two advantages of this system are that the powering and data transmission phase can be independently optimized and that the interrogation and response signal frequencies can be the same because either the interrogator or the transponder will be transmitting at any given moment, not both. Other advantages are that the transponder signals are stronger and the efficiency is higher which results in less power consumed in the transponder while the interrogation distance is larger. A reader which can read transponders having different communication standard protocols is known from US-A-5,451,958. The reader comprises a first antenna circuit for transmitting an excitation pulse to charge-up half duplex transponders and immediately initiates the full duplex transponders to respond, a second antenna circuit for receiving a transponder RF frequency response signal, a mixer circuit which mixes the full duplex transponder RF frequency signal and the half duplex transponder RF frequency signal with respective LO frequency signals and yields a predetermined IF frequency signal, an amplifier circuit for amplifying the IF frequency signal and comprising a signal-detect circuit which detects the presence or absence of an FDX (FSK) signal being received, an amplitude limiting circuit for amplitude limiting the IF frequency signal, and a demodulator circuit for demodulation of the IF frequency signal into a data signal. It is the object of the present invention to provide a further RF-ID system having one reader i.e., interrogator, which may receive a plurality of transponder response signals, wherein the transponders are operating under different communication protocols. Optimally, the system and reader receive different communication protocols including half and full duplex signaling protocols with the least amount of additional components and with a minimum loss of demodulation speed performance. In accordance with the present invention, a method and apparatus for reading signals from transponders having different communication protocols are provided. The present invention comprises a reader which can read transponders operating in full duplex (FDX) with a frequency shift keyed (FSK), phase shift keyed (PSK) or amplitude shift keyed (ASK) mode or half-duplex (HDX)with a frequency shift keyed (FSK) mode. According to the present invention, a dual standard RF-ID interrogator is provided which can receive and recognize ASK, PSK, and FSK transponder signals. The interrogator comprises the features of claim 1. According to a preferred embodiment, a filter circuit may be operatively connected to the mixer/amplifier circuit for base band filtering the frequency response signals. Also, an amplifier/limiter circuit may be operatively connected to the filter circuit for selectively amplifying and limiting the frequency response signals. A LO oscillator circuit may provide a LO frequency signal. Further, a controller may be connected to operate a switching function whereby the mixer/amplifier circuit mixes the HDX(FSK) transponder RF frequency response signal with the LO frequency signal such that the resulting frequency signal is about the second frequency. The demodulator circuit may comprise a zero detector circuit for detecting a number of zero crossings in a predetermined amount of time and a timer circuit for defining the predetermined amount of time for the zero detector circuit. The zero detector circuit generates a train of pulses having different amplitudes depending on the number of detected zero crossings. A monoflop may then be provided for shaping the pulses into well defined pulses. An integrator may be provided for integrating the well-defined pulses and for transforming the pulses into two different D.C. levels. A Schmitt Trigger circuit distinguishes between the two D.C. levels and yields either a high or low data bit for output. Any other FSK, ASK, or PSK demodulator circuit may also be used. According to another aspect of the present invention, in a dual standard RF-ID system, a method for receiving and recognizing PSK and FSK transponder signals may comprise the steps defined in claim 13. The present invention comprises various technical advantages over conventional RF-ID readers. BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which: Fig. 1 is a schematic diagram of a powering portion of an interrogator in a dual standard RF-ID system according to an embodiment of the present invention; Fig. 2 is a schematic diagram of a receiver in a dual standard HDX/FDX transponder system according to one embodiment of the present invention; Fig. 3 is a schematic illustrating the frequency signals used in one embodiment according to the present invention; Fig. 4. is a schematic diagram of a receiver in a dual standard HDX/FDX transponder system according to another embodiment of the present invention; and Fig. 5 is a schematic of a demodulator circuit according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION Half-duplex (HDX) and full duplex (FDX) transponders are typically tuned to and respond to an excitation frequency such as a frequency of 134.2 KHz, for example. HDX and FDX transponders may respond, however, at different frequencies or may respond at the excitation frequency. Full duplex transponders begin transmitting identification code response signals back to an interrogator instantaneously. Half-duplex transponders charge-up during the exciter signal and when the exciter signal has terminated, the half-duplex transponders transmit their own modulated identification code response signals. Full-duplex transponders may retransmit the identification codes at one half of the exciter frequency, for example. For a frequency of about 134.2 KHz, FDX transponders may typically retransmit at about 67.1 KHz. To receive signals from both an FDX transponder and an HDX transponder, which typically retransmits at the exciter frequency, for example, about 134.2 KHz, the present invention provides a receiver for both modes of operation. In a more detailed description of one embodiment of the invention, a power portion of an interrogator transmits an interrogation/exciter power signal of 134.2 KHz to power up the transponders. Components of the power portion that may be used for transmission of the interrogation signal are shown in Fig. 1. A control input triggers RF source 18 to produce a first frequency signal, f1, of abcut 134.2 KHz. The interrogation/exciter power signal at about 134.2 KHz is amplified by amplifier 16 and resonates the antenna resonant circuit comprised of capacitor 12 and coil 10. The interrogation/exciter power signal is transmitted via antenna coil 10. If the resonant circuit 10,12 has a high "Q," upon termination of the exciter signal, a switch 14 may be closed to damp the exciter signal and to immediately stop transmission of the exciter signal. If the resonant circuit 10,12 has a low "Q," the exciter signal may not be damped and therefore switch 14 may be left open or may be omitted. The exciter signal charges-up HDX(FSK) transponders and immediately initiates FDX transponders to respond. The details of the reader portion of the receiver are described in detail below. Components of the interrogator for reception of the response signals from the transponders are shown schematically in Fig. 2. The Fig. 2 reader receiver section is able to receive HDX type transponder identification code response signals as well as FDX type transponder identification code response signals operating at different frequencies. In order to do so, the resonant circuit preferably further comprises a second pole. The second pole may be added with resonance components connected to resonant circuit 10,12. Fig. 2 depicts an embodiment of a resonant circuit having two poles comprising a second coil 20 and a second capacitor 22. Coil 10 and capacitor 12 preferably comprise a first resonant component which operates at a high Q to power in the HDX case. Second coil 20 and second capacitor 22 comprise a second resonant component which operate at the frequency of the response signal from the FDX transponder, for example, at 67.1 KHz frequency. The reader receiver section also comprises a mixer/amplifier circuit 24, a filter circuit 26, an amplifier/limiter circuit 28, a demodulator circuit 30, a controller 32, a switch 34, an exciter signal provider 36, a LO signal provider 38, and an oscillator 40. Filter circuit 26 may comprise an L-C circuit or other device tuned to the frequency of the FDX transponder, f2, of about 67.1 KHz, for example, which is also equal to the converted HDX frequency signal. LO signal provider 38 may provide a third frequency, f3, which is equal to the sum of the exciter frequency, f1, plus the FDX frequency f2. f3 may, for example, then equal about 201.3 KHz as depicted in Fig. 3. Oscillator 40 may provide a signal having a frequency of about 6.442 MHz. LO signal provider 38 and exciter signal provider 36 may be derived from oscillator 40 by using a divider ratio of about 32 and 48, respectively, to derive the 201.3 KHz and 134.2 KHz signals. When an FDX signal is received, the data is provided at about f2 (about 67.1 KHz). Because the second pole of the antenna circuit is tuned to a frequency of f2 (about 67.1 KHz), switch 34 is open under control of controller 32 so that mixer/amplifier circuit 24 operates in a linear amplifier mode. The f2 signal is then amplified and limited by amplifier/limiter circuit 28 and then demodulated by demodulator circuit 30 to extract data. When a HDX signal is received, that data is provided at about f1 (about 134.2 KHz). Therefore, switch 34 is closed by controller 32 and the LO frequency of about f3 (about 201.3 KHz) is mixed with the incoming signal at about f1 (about 134.2 KHz) down to a frequency of about f2 (about 67.1 KHz). The mixed down signal may then be passed through amplifier/limiter circuit 28 and demodulator circuit 30 to extract the data received from HDX transponders. By permitting both signals to pass through mixer/amplifier circuit 24, this circuit can compensate for differences in signal strength of the two different transponder signals. Fig. 4 depicts another embodiment of the present invention. Like references to Fig. 3 represent like elements. According to this embodiment, a signal splitter 42 is provided between the resonant circuit and mixer/amplifier circuit 24. Signal splitter 42 splits incoming signals based on the input frequency. If signal at f2 (67.1 KHz) is received, signal splitter 42 bypasses mixer/amplifier circuit 24 and injects the signal into the circuit prior to filter circuit 26. If a signal at f1 (134.2 KHz) is received, signal splitter 42 allows the signal to pass through to mixer/amplifier circuit 24. Demodulator circuit 30 may comprise a conventional FSK/ASK/PSK demodulator, for example. In order to demodulate the phase modulated signals (PSK signals) with the inherent instantaneous frequency changes, the base band PSK signal may be passed through a PSK conversion circuit. For example, this PSK conversion circuit may be provided by D-type flip flops or bistables. Also, the PSK signal may be switched from a 90° phase shift to a reference signal derived from the exciter during the exciter on-time for FDX. These converted signals may then be passed to a conventional FSK demodulator to produce data. The data may then in turn be provided to controller 32 for further processing. Fig. 5 depicts one embodiment of a circuit of amplifier/limiter circuit 28 and demodulator circuit 30 according to the present invention. This circuit 28, 30 may comprise a PSK conversion circuit 44, a signal log-detect circuit 46, a limiter/comparator 48, a zero detector circuit 50, a timer circuit 52, and a biphase decoder circuit 58 having switches 54 and 56. PSK conversion circuit 44 converts PSK signals prior to FSK demodulation. Signal log-detect circuit 46 selectively amplifies the base-band signals provided by filter circuit 26 and PSK conversion circuit 44. Signal log-detect circuit 46 also provides a carrier signal detect output which facilitates an adaptive receiver in which upon the absence of a signal, the controller can institute much shorter powering pulse durations from the exciter 36. In this manner, ID sequence duration is minimized with maximum efficiency. The amplified signals from signal log-detect circuit 46 are then amplitude limited by limiter/comparators circuit 48. After amplification and limitation, the signal may be passed to zero detector circuit 50 combined with a timer circuit 52. Zero detector circuit 50 and timer circuit 52 function to demodulate the baseband signals to determine whether the low or high bit FSK frequency is being sent. Timer circuit 52 may provide a signal every predetermined amount of time to zero detector circuit 50. For example, timer circuit 52 may provide a signal every microsecond or millisecond. An easily integratable ASIC type receiver may be used to perform the demodulation. The demodulation may be performed by determining the number of zero crossings in a given time duration via zero detector circuit 50 wherein the time elapsed between one zero crossing to another zero crossing are different for different frequencies, i.e., about 62.1 KHz and about 67.1 KHz, and therefore the number of zero crossings in the same amount of time are different for different frequencies. Zero detector circuit 50 then outputs a train of pulses of constant width and various periods, depending on how many zero crossings were detected, which subsequently is shaped by a monoflop into a well defined pulse. This well defined pulse is then integrated through an integrator which yields different D.C. levels. A Schmitt Trigger can be used to distinguish between the two D.C. levels and yield either the high or low FSK bit. The demodulation can also be performed by any conventional demodulator including a Phase-Lock Loop (PLL) or product detector. Further, digital signal processing sampling or envelope detection techniques may also be used. DSP is simple and feasible because the signals are provided at low IF which results in a much lower cost of implementation. Further, a bi-phase modulation circuit 58 comprising switches 54 and 56 may be provided because most FDX signals use a biphase encoded data signal. Bi-phase modulation circuit 58 operates to decode biphase signal to non-return to zero (NRZ) signals. Switches 52 and 54 switch from the bi-phase modulation circuit path 58 to the non-return to zero path upon the termination of the exciter signal which allows HDX data signals to pass straight through the NRZ path to the data output port. As discussed above, when the HDX signal is received at a frequency of about f1 (about 134.2 KHz), it is mixed down to a frequency of about f2 (about 67.1 KHz) for further processing. To do so, controller 32 closes switch 34 and LO signal provider 38 provides a signal to mixer 24 which heterodynes the HDX(FSK) signal, which has a frequency of f1, down to a frequency of f2 for further processing. In order to determine when a HDX signal is being received, signal log-detect circuit 46 may detect the absence of an FDX signal and thus the presence of the HDX signal. The detection signal of signal log-detect circuit 46 may be directly passed to controller 30 so that controller 30 may close switch 34 such that mixer/amplifier circuit 24 mixes the incoming HDX transponder frequency response signal with the LO oscillator frequency signal from f1 down to f2. Further, upon the signal-log detector detecting the absence of the FDX signal, the receiver is ready to receive the HDX transponder signal. During the duration that the exciter frequency is being transmitted, the half-duplex transponders are charging up, preparing to respond to the termination of the exciter pulse. At the end of the exciter duration, switch 14 (Fig. 1) is closed such that the residual resonance of the antenna circuit 10,12 is greatly damped. The significant drop in power level of the exciter signal is detected by the half duplex transponders and therefore, they begin to transmit and the full duplex transponders cease transmitting. The HDX transponders store the energy from the exciter signal to power the generation of a new carrier signal at the same frequency as the exciter signal, for example, about f1 (about 134.2 KHz). A second FSK frequency is generated such as at about 124.2 KHz, for example such that the transponder identification code is FSK modulated between about 134.2 KHz and about 124.2 KHz. The HDX(FSK) signals are then processed as described above. Then the controller opens switch 14 again to initiate another cycle of interrogation. According to the present invention, an integrated FDX/HDX receiving system has been provided without use of parallel receivers and hardware intensive solutions. As such, a low cost, high functionality system without duplication of receiver circuits has been provided which may be used with existing systems.

Claims: A dual standard RF-ID interrogator which can receive and recognize transponder signals comprising: an antenna circuit (10, 12, 20, 22) for transmitting an excitation signal to charge-up half duplex (HDX) transponders and immediately initiate full duplex (FDX) transponders to respond and for receiving a frequency response signal from the half duplex transponders having a first frequency and a frequency response signal from the full duplex transponders having a second frequency; a mixer/amplifier circuit (24) operatively connected to the antenna circuit for mixing the frequency response signal received from the half duplex transponders down to the second frequency; and a demodulator circuit (30) operatively connected to the mixer/amplifier circuit (24) for demodulating the frequency response signals received from the full duplex transponders and the mixer/amplifier circuit (24) into a data signal. The interrogator of Claim 1 further comprising: a detector circuit (46) operatively connected to the mixer/amplifier circuit (24) for detecting the full duplex signal to place the mixer/amplifier circuit (24) in linear amplification mode. The interrogator of Claim 2 further comprising: an oscillator circuit (38, 40) for providing an LO frequency signal; switching means (34) connected between the oscillator circuit (38, 40) and the mixer circuit (24); and a controller (32) operatively connected to the detector circuit (46) for actuating the switching means (34) such that the mixer/amplifier circuit (24) mixes the frequency response signal of the half duplex transponder from the first frequency down to the second frequency when a full duplex signal is not detected by the detector circuit (46). The interrogator of any preceding Claim further comprising: a splitter circuit (42) operatively connected to the antenna circuit and the mixer circuit (24), for passing the frequency response signals from the half duplex transponder to the mixer circuit (24) and the frequency response signals from the full duplex transponders to the demodulator circuit (30). The interrogator of any preceding claim, wherein the demodulator circuit (30) is operable to produce a train of pulses having different amplitudes and constant widths and comprises: a zero detector circuit (50) for detecting a number of zero crossings in a predetermined amount of time; a timer circuit (52) for defining the predetermined amount of time for the zero detector circuit (50); and wherein the zero detector circuit (50) produces a train of pulses having constant widths and different amplitudes depending on the number of detected zero crossings. The interrogator of Claim 5, wherein the demodulator circuit (30) further comprises: a monoflop for shaping the pulses into well defined pulses; an integrator for integrating the well-defined pulses and for transforming the pulses into two different D.C. levels; and a Schmitt Trigger circuit for distinguishing between the two D.C. levels and yielding either a high or low bit. The interrogator of any preceding claim wherein the demodulator circuit (30) comprises a biphase decoder circuit (58). The interrogator of any preceding claim, wherein the second frequency is equal to one half of the first frequency. The interrogator of any preceding claim, wherein a third frequency is equal to the sum of the first frequency and the second frequency. The interrogator of any preceding claim further comprising: a filter circuit (26) operatively connected to the mixer/amplifier circuit (24) for base band filtering the frequency response signals. The interrogator of any preceding claim further comprising: an amplifier/limiter circuit (28) operatively connected between the mixer/amplifier circuit (24) and the demodulator circuit (30) for selectively amplifying and limiting the frequency response signals. The interrogator of any preceding claim further comprising: an exciter source (36, 40); a controller (32) for initiating the excitation signal from the exciter source; and a signal log-detect circuit (46) operatively connected to the mixer/amplifier circuit (24) for providing a carrier detect output to the controller (32) such that the controller (32) may initiate shorter powering pulse durations from the exciter (36). A method for operating a dual standard RF-ID interrogator comprising the steps of: transmitting an excitation signal to charge-up half duplex transponders and immediately initiate full duplex transponders to respond; receiving a frequency response signal from the half duplex transponder having a first frequency and a frequency response signal from the full duplex transponder having a second frequency; detecting the frequency response signal received from the full duplex transponder; mixing the frequency response signal from the half duplex transponder down to the second frequency if the response signal from the full duplex transponder is not detected; and demodulating the frequency response signals from the full duplex transponder and the mixing step into a data signal. The method of Claim 13 further comprising the step of amplifying the frequency response signal from the full duplex transponder. The method of Claim 13 or Claim 14, further comprising the step of base band filtering the frequency response signals. The method of any of Claims 13 to 15 further comprising the step of selectively amplifying and limiting the frequency response signals.

Inventors: ASLANIDIS KONSTANTIN, SCHUERMANN JOSEF H, ASLANIDIS, KONSTANTIN, SCHUERMANN, JOSEF H., Schürmann, Josef H.
Applicants: TEXAS INSTRUMENTS DEUTSCHLAND, TEXAS INSTRUMENTS DEUTSCHLAND GMBH
Application year: 1997
Main classification: G06K7
IPC codes: G06K19, G06K7