Abstract:
A radio frequency interface circuit ( 11 ) for a radio frequency identification tag comprising—at least two input terminals (RF+, RF−) for connecting the circuit ( 10 ) with an antenna structure of the radio frequency identification tag, —one or more variable resistive loads ( 14 ) coupled across pairs of the input terminals (RF+, RF−)—one or more rectifiers ( 15 ) each connected on its input side to a pair of input terminals (RF+, RF−) and on its output side to a parallel connection of voltage control means ( 16 ) and modulation control means ( 17 ), wherein combiner means ( 18 ) are provided which are adapted to receive an output signal ( 19, 20 ) from the voltage control means ( 16 ) and the modulation control means ( 17 ), respectively, and to generate a control signal ( 21 ) for controlling each variable resistive load ( 14 ) depending on the received signals ( 19, 20 ) in such a way that each variable resistive load ( 14 ) serves as a modulation and voltage regulation circuit, and wherein each variable resistive load is adapted to serve as an electro static discharge protection circuit.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a radio frequency interface circuit for a radio frequency identification tag. 
       BACKGROUND OF THE INVENTION 
       [0002]    A radio frequency identification (RFID) tag usually comprises ESD (electro static discharge) structures on its antenna terminals in order to avoid over voltage conditions which could damage an integrated circuit (IC) of the RFID tag. The ESD structure is a voltage clamp which limits the voltage between two terminals to a predefined maximum voltage. For example, such a voltage clamp comprises diode structures which short-circuit two terminals if the voltage between the two terminals exceeds a predefined maximum voltage. Thus, electric current flows over the diode structures from one terminal to the other terminal avoiding a damage of the IC due to a high current or voltage. 
         [0003]      FIG. 4  shows a typical radio frequency interface circuit for a RFID tag in detail. An ESD structure  22  serves as a voltage clamp and couples two input terminals RF+ and RF− for connecting the shown circuit with an antenna structure (not shown) of the RFID tag. If the voltage between the two terminals RF+ and RF− exceeds a predefined clamp voltage, the ESD structure  22  conducts such that electric current flows from terminal RF+ to RF− or vice versa. A rectifier  15  connected on its input side to the two terminals RF+ and RF− is protected by the ESD structure  22  from high electrostatic discharging voltages. A limiting transistor  23  and a modulation transistor  24  are connected in parallel to the outputs of the rectifier  15  and serve as modulation and voltage regulation circuit, respectively. The limiting transistor  23  is controlled by voltage control means  16  which are supplied by the rectifier  15 . The modulation transistor  24  is controlled by modulation control means  17  which are also supplied by the rectifier  15 . This known circuitry has the drawback that the ESD structures usually applied for such circuitry has a large parasitic capacitance. A further drawback is that the rectifier  15  must be designed to deliver a high current for the limiting transistor  23 . 
         [0004]    U.S. Pat. No. 5,815,355 discloses a modulation compensated clamp circuit for a RFID tag which comprises a shunt transistor connected between two input terminals for connecting with an antenna of the RFID tag. A further load transistor is connected in parallel to the shunt transistor between the two terminals and used to vary the Q factor of a tank circuit depending on data read from a memory. Both transistors are placed before a rectifier for supplying the electronic circuitry of the RFID tag with electric power. However, the two transistors connected in parallel between both input terminals still cause a high input capacitance which is not desired since it degrades the radio frequency performance. Particularly in the UHF (ultra high frequency) band, which is often used for RFID transponder, a low input capacitance plays a significant role in order to detect also low signals from a RFID reader. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0005]    It is an object of the invention to provide a radio frequency interface circuit for a radio frequency identification tag which comprises an over voltage or ESD protection and has a lower input capacitance than the above mentioned circuits. 
         [0006]    In order to achieve the object defined above, with a radio frequency interface circuit for a radio frequency identification tag according to the invention characteristic features are provided so that a circuit according to the invention can be characterized in the way defined below, that is: 
         [0007]    at least two input terminals for connecting the circuit with an antenna structure of the radio frequency identification tag, 
         [0008]    one or more variable resistive loads coupled across pairs of the input terminals, 
         [0009]    one or more rectifiers each connected on its input side to a pair of input terminals and on its output side to a parallel connection of voltage control means and modulation control means, 
         [0010]    wherein combiner means are provided which are adapted to receive an output signal from the voltage control means and the modulation control means, respectively, and to generate a control signal for controlling each variable resistive load depending on the received signals in such a way that each variable resistive load serves as a modulation and voltage regulation circuit, and 
         [0011]    wherein each variable resistive load is adapted to serve as an electro static discharge protection circuit. 
         [0012]    The characteristic features according to the invention provide the advantage that several functions such as modulation, voltage regulation and electro static discharge protection are combined in one variable resistive load. This allows to implement the circuit according to the invention at lower costs than the circuits discussed in the introductory part. Finally, the circuit according to the invention has a relatively low input capacity since merely the variable resistive load and the rectifier connected in parallel to the antenna form this capacitance. Thus, with the circuit according to the invention, a high radio frequency performance particularly in the UHF range around 800 MHz may be achieved. Furthermore, the rectifier may be designed for lower currents since the voltage limiting structures are placed before the rectifier. This also may support a reduction of the input capacitance of the circuit. 
         [0013]    The combiner means may be adapted to detect an over voltage condition from the output signal received from the voltage control means and to generate the control signal such that each variable resistive load is set to a value limiting the input voltage of each rectifier. 
         [0014]    The combiner means may further be adapted to modulate the control signal in accordance with the output signal received from the modulation control means such that the resistance of each variable resistive load is modulated for transmitting data over the antenna structure of the radio frequency identification tag. 
         [0015]    In order to achieve a high radio frequency performance, the circuit may be adapted to be operated in the UHF range of about 800 MHz. However, it should be noted that the circuit may also be operated in a frequency range up to about 2.5 GHz. 
         [0016]    Preferably, a variable resistive load is a MOSFET, coupled across a pair of input terminals (RF+, RF−), wherein the control signal is coupled to the gate of the MOSFET. 
         [0017]    The MOSFET should be designed to clamp typical electrostatic discharging voltages. For example, it may be designed similar to a NFET such as it is described in the publication “ESD protection for the deep sub micron regime—a challenge for design methodology”, H. Gossner, Proceedings of the 17 th  International Conference on VLSI Design (VLSID&#39;04), 2004 IEEE. 
         [0018]    According to a further aspect, the invention relates to an integrated circuit for a frequency identification tag comprising a radio frequency interface circuit as described above. 
         [0019]    According to a further aspect, the invention relates to a radio frequency identification tag comprising a radio frequency interface circuit as described above. 
         [0020]    The aspects defined above and further aspects of the invention are apparent from the exemplary embodiments to be described hereinafter and are explained with reference to these exemplary embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention will be described in more detail hereinafter with reference to exemplary embodiments. However, the invention is not limited to these exemplary embodiments. 
           [0022]      FIG. 1  shows a schematic block circuit diagram of a RFID system. 
           [0023]      FIG. 2  shows a schematic block circuit diagram of a RFID tag. 
           [0024]      FIG. 3  shows a first embodiment of a radio frequency interface circuit for a RFID tag according to the invention. 
           [0025]      FIG. 4  shows a radio frequency interface circuit for a RFID tag according to prior art. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]    Identical, similar, and functional identical or similar elements can be denoted with the same reference numerals in the following description. For the description of  FIG. 4  refer to the introductory part of this description. 
         [0027]      FIG. 1  shows a schematic block circuit diagram of an RFID system comprising an RFID reader  1  and a number of RFID tags  2   a ,  2   b , wherein, for the sake of clearness, only two RFID tags are depicted. RFID reader  1  communicates with the RFID tags  2   a ,  2   b  in a contact-less manner via modulated electromagnetic signals, provided the RFID tags  2   a ,  2   b  are within the transmission and receiving range of the RFID reader  1 . The RFID reader  1  comprises control means  3 , like a microprocessor or micro-controller, which control means  3  communicate via a data bus with program storage means  4 . The program storage means  4  is adapted to store an operating system OS for basic operation of the control means  3  and application program code SW to be processed by the control means  3 . In one embodiment the program storage means  4  may be configured as a non-volatile memory, like a PROM, EPROM, EEPROM or the like. In another embodiment the program storage means  4  may be configured as a user definable ASIC, PAL or the like. Further, the control means  3  and the program storage means  4  may be integrated into a single chip. It should be observed that the application program code SW and the operating system OS may be integrated. The control means  3  further communicate with a random access memory  5 . The control means  3 , when processing the program code SW, cooperate with input/output means  8 , which e.g. can be configured as a link interface to a computer. The control means  3  further communicate with radio frequency communication means  6  which are connected to an antenna  7  for transmitting electromagnetic signals SS to the RFID tags  2   a ,  2   b . These electromagnetic signals SS may be used for both transmitting data to the RFID tags  2   a ,  2   b  and energizing the RFID tags  2   a ,  2   b  if they are configured as passive tags. The RFID tags  2   a ,  2   b  respond to the RFID reader with response signals RS 1 , RS 2 . Data exchange between the RFID reader  1  and the RFID tags  2   a ,  2   b  may be accomplished by standard data transmission protocols and standard modulation methods. For instance, the electromagnetic signal SS sent from the RFID reader  1  to the RFID tags  2   a ,  2   b  is configured as a pulse-width modulated signal. The response signals RS 1 , RS 2  from the RFID tags  2   a ,  2   b  to the RFID reader are e.g. load modulated signals, wherein a carrier signal or subcarrier signal contained in the electromagnetic signal SS is modulated by switching a load impedance connected to the antennas of the RFID tags  2   a ,  2   b , so that varying energy is drawn from the carrier signal or subcarrier signal. Switching the load impedances at the RFID tags causes a change of the impedance of the antenna  7  of the RFID reader  1  and hence a varying amplitude of the voltage at the antenna  7  of the RFID reader  1 , which varying voltage amplitude represents an input signal IS to the radio frequency communication means  6 . For recovery of data contained in the input signal IS the input signal IS is rectified or demodulated by radio frequency communication means  6 , yielding data stream signal DS. The control means  3  extract the data coded in the data stream signal DS, e.g. by comparing it with defined bit levels. 
         [0028]      FIG. 2  shows a schematic block circuit diagram of an exemplary embodiment of the RFID tags  2   a ,  2   b . Each RFID tag  2   a ,  2   b  is configured as a passive tag and comprises an antenna  10 , an analogue radio frequency interface  11  that is connected to the antenna  10 , a digital control unit  12  that is connected to the analogue radio frequency interface  11 , and a memory  13  that is connected to the digital control unit  12 . The memory  13  is a non-volatile memory, like an EEPROM, so that data that are written into the memory  13  during communication with the RFID reader  1  remain stored even when the RFID tag  2   a ,  2   b  is switched off, e.g. because it leaves the transmitting range of the RFID reader  1  and is therefore not longer energized by the RFID reader  1 . Memory  13  may also contain program code for operating the digital control unit  12  and a unique identification number. Antenna  10  receives the electromagnetic signals SS from the RFID reader  1  and passes them to the analogue radio frequency interface  11 . In general, the analogue radio frequency interface  11  comprises a rectifier RECT and a voltage regulator VREG with integrated energy storage element, like a capacitor, to derive from the received electromagnetic signals SS the necessary operating voltage VDD for the digital control unit  12  and the memory  13 . Further, analogue radio frequency interface  11  comprises a demodulator DEMOD to extract data DIN from the electromagnetic signals SS and to pass them to the digital control unit  12 . Digital control unit  12  processes the received data DIN and may respond to the RFID reader  1  by creating output data DOUT and passing them to the analogue radio frequency interface  11 . Analogue radio frequency interface  11  comprises a modulator MOD that modulates the output data DOUT and transmits the modulated signals as response signals RS 1 , RS 2  via antenna  10 . 
         [0029]      FIG. 3  shows an embodiment of the analogue radio frequency interface circuit  11  according to the invention. The circuit  11  comprises two input terminal RF+ and RF− for connecting with an antenna (not shown) of the RFID tag, such as the antenna  10  of  FIG. 2 . Electromagnetic signals received by the antenna are passed to a rectifier  15  which rectifies the alternating current into an direct current which is supplied to voltage control means  16  and modulation control means  17  connected in parallel with the output side of the rectifier  15 . In order to achieve a stable and defined operating voltage VDD as necessary for supplying the electronic circuits of the RFID tag such as the digital control unit  12  and the memory  13  (as shown in  FIG. 2 ), the voltage regulation means  16  measure the output voltage of the rectifier  15  and compare it with a reference voltage. In case of deviations of the rectifier output voltage from the reference voltage, the voltage control means  16  generate an output signal  19  which is supplied to combiner means  18  for further processing. Also, the modulation control means  17  generate an output signal  20  depending on data DATA, for example received from the digital control unit  12 . The data may be the output data DOUT read out from the memory  13  by the digital control unit  12 . The output signal  20  generated by the modulation control means  17  is also supplied to the combiner means  18  for further processing. 
         [0030]    In one embodiment a summing circuit designed for summing the received output signals  19  and  20  may implement the combiner means  18 . In another embodiment a more complex circuit being designed for processing the received output signals  19  and  20  according to a certain algorithm may realize the combiner means  18 . The combiner means  18  generate a control signal  21  for controlling a variable resistive load coupling the two terminals RF+ and RF−. The variable resistive load may be implemented by a transistor such as a MOSFET  14  as shown in  FIG. 3 . The combination of the variable resistive load implemented by the MOSFET  14  and the combiner means  18  serves as a modulation, voltage regulation and electro static discharge protection circuit. In order to accomplish these tasks, the combiner means  18  control the MOSFET  14  via the control signal  21  as explained in the following. 
         [0031]    If an over voltage condition occurs at the terminals RF+ and RF−, for example due to an electrostatic discharging voltage, the conductivity of the variable resistive load of the MOSFET  14  between the terminals RF+ and RF− is decreased due to the overvoltage and an electric current generated by the electrostatic discharging voltage flows over the variable resistive load of the MOSFET  14  from terminal RF+ to terminal RF− or vice versa. In this case, the MOSFET  14  serves as electro static discharge protection circuit which clamps overvoltages on the terminals RF+ and RF−. 
         [0032]    A further functionality of the combiner means  18  is the voltage regulation and limitation in combination with the voltage control means  16 . If the voltage control means  16  detect a deviation of the output voltage of the rectifier  15  from the reference voltage, for example due to high filed strength of the electromagnetic signals received via the antenna of the RFID tag, they generate an output signal  19  indicating this rectifier output voltage deviation to the combiner means  18 . The combiner means  18  control the MOSFET  14  in that they regulate the input voltage of the rectifier  15  via regulation of the variable resistive load of the MOSFET  14  between the terminals RF+ and RF− in such a way that the rectifier output voltage approximates the reference voltage. Thus, input voltage fluctuations of the rectifier may be regulated by controlling input voltage of the rectifier  15  via the variable resistive load of the MOSFET  14 . In this case, the MOSFET  14  under the control of the combiner means  18  serves as voltage regulation and limitation circuit. 
         [0033]    Finally, the combiner means  18  in combination with MOSFET  14  serve as modulation circuit. The modulation control means  17  receive a DATA signal containing data which may be read out from the memory  13  by the digital control unit  12  and which should be transferred over the radio frequency interface of the RFID tag. The data are sent out as load modulated signals, wherein a carrier signal or subcarrier signal contained in the electromagnetic signal is modulated by switching the MOSFET  14 , which forms a load impedance connected to the antenna of the RFID tag. The MOSFET  14  is switched under control of the control signal  21  generated by the combiner means  18  corresponding to the data contained in the DATA signal. Thus, varying energy is drawn from the carrier signal or subcarrier signal. Switching the MOSFET  14  causes a change of the impedance of the antenna of a RFID reader and hence a varying amplitude of the voltage at the antenna of the RFID reader. This varying voltage amplitude represents an input signal to radio frequency communication means of the RFID reader. 
         [0034]    The invention has the advantage that the input capacitance is reduced significantly in contrast to known radio frequency interface circuits for RFID tags. Furthermore, the silicon area required for the circuit according to the invention is smaller than the silicon area necessary for the integration of the known circuits since only a few devices, the variable resistive load in combination with the combiner means fulfill several functions, namely the modulation, voltage regulation and electro static discharge protection. Finally, the rectifier may be designed for smaller currents since the variable resistive load, which causes an electric power reduction, is placed before the rectifier. Thus, the rectifier may be implemented by smaller devices of an integrated circuit which causes less parasitics, particularly capacitances between the device and the substrate.