VARIABLE-Q ANTENNA COIL CIRCUIT FOR RADIO FREQUENCY IDENTIFICATION APPLICATIONS

Abstract

A passive radio-frequency identification (RFID) reader is configured to dynamically vary the Q factor of its resonant antenna coil circuit to optimize its performance under different conditions. This RFID reader properly provides optimal performance for both transponder read and write operations, rather than being designed for optimal performance for only one operation or the other, or some fixed compromise between them.

Description

VARIABLE-Q ANTENNA COIL CIRCUIT FOR RADIO FREQUENCY IDENTIFICATION APPLICATIONS i uou FIELD OF INVENTION This description refers to Radio Frequency Identification (REID) systems, and in particular to magnetically coupled passive REID systems. BACKGROUND OF THE INVENTION As shown in FIG. 1, a passive REID system 10 typically includes two main subassemblies: a reader 11 and a transponder 12 that is to be read at some distance from the reader 11. The reader 11 includes an AC voltage source that drives its resonant antenna coil circuit. The reader 11 thus emits an alternating magnetic field from its antenna coil, which is weakly magnetically coupled (represented in FIG. 1 by a dashed double arrow) to a corresponding antenna coil in the transponder 12 that is to be read. Each of these antenna coils is part of a corresponding antenna coil circuit that includes one or more tuning capacitors to make it resonate at a desired frequency, with a Q factor that is limited to some maximum value primarily by the DC resistance of its antenna windings. Ref. 338020 Transponder 12 obtains its operating power from the magnetic field emitted by the reader, and modulates (for example, by means of a switch and a load resistor) the Q factor and / or the resonant frequency of its antenna coil circuit in a pattern corresponding to any information that needs to be sent from transponder 12 to reader 11. This information usually includes an identification number that corresponds exclusively to the individual transponder. BRIEF DESCRIPTION OF THE INVENTION This description describes a reader that has the ability to dynamically adjust the Q factor of its resonant antenna coil circuit in order to improve (e.g., optimize) its performance for reading transponders at long intervals, or for both reading and writing transponders at shorter intervals. To dynamically adjust the Q factor, we add a variable resistor element to the reader's antenna coil circuit, controlled by a controller (e.g., an integrated circuit-based controller) inside the reader. The reader controller can set this variable resistor element to its smallest value (e.g., zero ohms) for optimal read-only performance, or dynamically adjust it to a higher resistance to reduce the Q factor of its resonant antenna coil circuit for writing operations. In general, an innovative aspect of the topic described here can be implemented in circuits for communicating with a REID transponder. The circuits include: an antenna coil circuit having an inductance, capacitance, and resistance that determine a natural resonant frequency and Q factor for the antenna coil circuit; the antenna coil circuit including a variable resistive element; an AC voltage source configured to drive the antenna coil circuit at or near its natural resonant frequency; a receiving subsystem for receiving information from the REID transponder; and a controller communicating with the antenna coil circuit and the AC voltage source. The controller is programmed to modulate the AC voltage source to send information to the REID transponder and to control the variable resistive element. The above and other options may optionally include one or more of the following features, alone or in combination. In particular, one option includes all of the following features in combination. In some implementations, the antenna coil circuit includes one or more inductive elements. The antenna coil circuit may also include one or more capacitive elements. ινΐΛ / a / zuzz / ui ί uoa The controller can be programmed to modulate the AC voltage source by turning the AC voltage source on and off. During circuit operation, the controller can vary the resistance of the variable resistive element to vary the Q factor of the antenna coil circuit. The Q factor can vary between two or more values ​​in a range of 1 to 500 (for example, in a range of 200 to 300). The variable resistive element can be an electronically controlled variable resistive element. The variable resistive element can be a mechanically controlled variable resistive element. The circuit may include a user interface to control the variable resistive element. The controller can be programmed to automatically control the variable resistive element. The variable resistive element can be continuously variable over a range of resistance values. The variable resistive element can vary between two or more discrete resistance values. The variable resistive element can include a resistor with a specified fixed resistance and a switching element configured, in one state, to short-circuit the resistor. The variable resistive element can be electrically connected between two nodes of the antenna circuit that, during circuit operation, experience lower voltage oscillations than other nodes of the antenna circuit with respect to a node of the circuit of interest. The controller can be programmed to vary a resistive variable while the AC voltage source is on and driving the antenna coil circuit. The controller can be programmed to vary a resistive variable while the AC voltage source is off and not conducting the antenna coil circuit. The topic described here can be implemented in specific ways to achieve one or more of the following advantages. The technologies described can allow a single RFID reader to be dynamically reconfigured for different operations at different intervals, such as long-range read-only operations, shorter-range read / write operations, or a compromise between read-only and read / write operations. An RFID reader that does not incorporate the technologies described may sacrifice the maximum read-only range to support read / write operations. Furthermore, an RFID reader incorporating the described technologies can dynamically adjust the Q-factor used for read / write operations to optimize its performance based on external conditions. For example, it can reduce this Q-factor if only very short-range read / write operations are required (e.g., at a range of a few centimeters), or increase it to support a longer read / write range. This dynamic adjustment can be performed while the reader and transponder are communicating with each other. Details of one or more embodiments of the subject matter described herein are set forth in the accompanying Figures and in the following description. Other features, aspects, and advantages of the subject matter will become apparent from the description, the figures, and the claims. BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a schematic diagram of a conventional passive RFID system. Figures 2A-2E show signals that highlight the effect of changing the Q factor of the reader's resonant antenna coil circuit according to the technologies described. Figures 3A-3D show signals that highlight the effect of an intermediate Q factor value on a transponder's ability to demodulate commands and data written to it from the reader. Figures 4A-4D show signals that highlight the effect of a high Q factor on a transponder's ability to demodulate commands and data written to it from the reader. Figures 5A-5D show signals that highlight the effect of a low Q factor on a transponder's ability to demodulate commands and data written to it from the reader. FIG. 6 is a schematic diagram of a passive RFID reader incorporating a variable Q antenna coil circuit with a first example of a variable resistor element. FIG. 7 is a schematic diagram of a passive RFID reader incorporating a variable Q antenna coil circuit with a second example of a variable resistor element. Similar reference numbers and designations in the various Figures indicate similar elements. DETAILED DESCRIPTION OF THE INVENTION This description outlines the technologies for improving (e.g., optimizing) the performance of a reader RFID for long-interval read-only operation, shorter-interval read / write operation, or a compromise between the two. An RFID reader drives its resonant antenna coil circuit with an alternating voltage source, which produces an alternating voltage waveform, such as a sine or square wave. This results in a predominantly sinusoidal current waveform in the reader's antenna coil, which creates an alternating magnetic field in a volume of space around the reader's antenna coil. Due to magnetic coupling between a reader and a transponder, transponder modulation appears as variations in the electrical currents and voltages present in the reader's antenna coil circuit. The reader can then use a receiving subsystem (represented by RX in FIG. 1) to detect and demodulate these variations in order to recover the information transmitted by the transponder. This receiving subsystem typically includes an amplitude modulation detector and one or more filtering and / or gain stages.The maximum read range is achieved when both the reader's antenna coil circuit and the transponder's antenna coil circuit are tuned to resonate at the reader's AC voltage source frequency, both antenna coils are oriented for optimal magnetic coupling, both antenna coil circuits have the highest Q factors that are practical, and the transponder modulates its Q factor as deeply as is practical, while still receiving enough power from the reader for its circuitry to function. The information contained in a transponder is typically stored in some type of non-volatile memory. This memory may include a combination of factory-programmed and / or field-programmable memory locations. Some transponders can also generate and / or store dynamic information, such as a temperature transducer reading. Typically, the transponder will automatically transmit a subset of its stored information when activated by the magnetic field emitted by a nearby reader. Some transponders can be not only read but also written to. A compatible reader can modulate its own emitted alternating magnetic field (for example, by switching the AC voltage source that powers its resonant antenna coil circuit on and off) to send commands and / or data to one of these transponders. This function is called writing, as opposed to the reading function described earlier. A device that performs both reading and writing functions is still generically called a reader. Writing to a transponder can be used for purposes such as instructing the transponder to send a different set of information than it automatically sends by default, for initial transponder programming at the time of manufacture, for programming field-programmable memory locations, or for activating special transponder functions. When the reader switches the AC voltage source that drives its resonant antenna coil circuit on or off, it takes time for the area of ​​the alternating current in the coil (and therefore the magnitude of the emitted magnetic field) to grow or shrink accordingly. The time constant for this growth or shrink is determined by the circuit's Q factor. A higher Q factor is generally desirable in practice to obtain the best read interval, but this can make writing to the transponder impossible when the time constant is long relative to the number of bit periods to be written. This is illustrated in Figures 2A to 2E. Figure 2B shows an example of an AC voltage source signal, where the AC voltage source is switched on and off by the modulation signal in Figure 2A. This example has a bit period of 32 cycles, but other bit periods are generally possible. Figures 2C to 2E show the corresponding antenna coil currents (and therefore the strengths of the emitted alternating magnetic fields) with three different Q factors of the resonant antenna coil circuit. These three Q factors, as well as all other specific Q factors described herein, are illustrative examples; in practice, Q factors outside this range may be used. The low Q factor of 1 in FIG. 2C results in a well-defined area with a low peak current magnitude. In other words, the modulated area of ​​the emitted alternating magnetic field is very clean, with sharp edges, but it is not very strong. This can lead to reliable write operations at very short intervals, but little or no ability to read or write transponders at longer intervals. The much higher Q factor of 100 in FIG. 2E results in a much higher coil current magnitude, with a slow rise and fall. In this example, the rise and fall time constants are much longer than the bit period. In other words, this will allow reading a transponder at long intervals, but generally prevents write operations. The Q factor shown in FIG. 2E is an illustrative example; in any given practical implementation, a designer may choose to use a lower or higher Q factor based on a variety of factors. ΜΛ / a / ZUZZ / U 11 UOU factors. An intermediate Q factor of 10 in FIG. 2D is a compromise that provides a stronger emitted alternating magnetic field than that of FIG. 2C, while still providing modulation clean enough for write operations to succeed within a certain range. Neither read nor write will succeed within the maximum read range supported in FIG. 2E, but both read and write can succeed within a longer range than that supported in FIG. 2C. FIGS. 2A-2E are explained in more detail below. Although the intermediate Q factor of 10 in FIG. 2D is a compromise, it is neither the only possible nor the best for all conditions. To understand this, consider how a transponder will demodulate the alternating magnetic field corresponding to the current signal in FIG. 2D at different reader intervals. This is illustrated in FIGS. 3A to 3D. FIG. 3A shows the same modulation signal as FIG. 2A, and FIG. 3B shows the same reader coil current signal as FIG. 2D. A transponder located very close to the reader will couple much more strongly to the reader's antenna coil circuit than one located farther away. This corresponds to a large variation in the magnitude of the current induced in the transponder coil by the alternating magnetic field emitted by the reader. Naturally, there is a minimum threshold of alternating magnetic field strength present at the transponder, below which the transponder can no longer detect the presence of the alternating magnetic field emitted by the reader. This is represented in Figure 3B by two horizontal lines. The dotted line represents the minimum magnitude of the reader's antenna coil current detectable by a transponder very close to the reader, while the dashed line represents the minimum magnitude of the reader's antenna coil current detectable by a transponder located much farther from the reader. The signal in FIG. 3C illustrates how a transponder very close to the reader could demodulate the emitted magnetic field corresponding to the reader coil current signal in FIG. 3B. Being so close to the reader, the transponder detects both the alternating magnetic field cycles generated while the modulation signal is high, and the additional cycles corresponding to the resonant coil circuit of the reader's antenna coil after the modulation signal drops. This results in a distorted reproduction of the original modulation signal in FIG. 3A, with longer high periods and shorter low periods. The signal in FIG. 3D represents how a transponder located far from the reader might demodulate the emitted magnetic field corresponding to the reader coil current signal in FIG. 3B. Being far from the reader, the transponder does not detect the first few cycles of the alternating magnetic field generated while the modulation signal is high, and it does not detect the triggering of the reader's resonant antenna coil circuit after the modulation signal drops. This results in a distorted reproduction of the original modulation signal in FIG. 3A, with shorter high periods and longer low periods. Although a well-designed transponder should be able to tolerate some degree of distortion of the demodulated signal it detects from the reader, any particular Q factor could cause enough distortion of the demodulated signal to prevent proper demodulation by a transponder that is too close or too far from the reader. To further illustrate how changing the Q factor of the reader's resonant antenna coil circuit can affect a transponder's ability to demodulate commands and data sent to it from the reader, FIGS. 4A to 4D and FIGS. 5A to 5D apply the analysis and thresholds of FIGS. 3A to 3D to the higher and lower Q factors of FIGS. 2E and 2C, respectively. FIG. 4A shows the same modulation signal as FIG. 2A and 3A. FIG. 4B shows the high Q current waveform from FIG. 2E with the thresholds of FIG. Figures 4C and 4D show that in either of the two intervals selected for this analysis, the transponder is unable to adequately reconstruct the original modulation signal. Clearly, there might be some greater distance at which a transponder could correctly reconstruct the original modulation signal, but there would be an area close to the reader where writing to the transponder would fail. Figure 5A shows the same modulation signal as Figures 2A and 3A. Figure 5B shows the low-Q current waveform from Figure 2C with the thresholds from Figure 3B added. Figure 5C shows that a nearby transponder can reconstruct the original modulation signal with very little distortion, while a more distant transponder does not detect the reader's alternating magnetic field at all. Therefore, if a reader needs to be able to read transponders at the longest practical intervals, but also needs to write to transponders at necessarily shorter intervals, then the reader must be able to adjust the Q factor of its resonant antenna coil circuit to configure it for either long-range read-only operations or shorter-range read / write operations. Furthermore, a single lower Q factor value may not be suitable for all conditions; the reader might need to dynamically adjust its Q factor. A reader can adjust the Q factor value it uses for read / write operations based on some user input, such as a user-modifiable configuration setting. Or, it can adjust it automatically in a dynamic way, such as by testing different Q factor values ​​in rapid succession until a transponder responds correctly to a command sent by the reader.Ideally, the reader should be able to dynamically adjust its Q factor between two or more values ​​while its resonant antenna coil circuit is running, so that it can quickly make adjustments without waiting for the oscillations of the antenna coil circuit to decay. Figure 6 is a schematic diagram of a passive RFID reader 100 incorporating a variable-Q antenna coil circuit with a first example of a variable resistor element 110. The passive RFID reader 100 includes an AC voltage source 120, which drives a resonant antenna coil circuit at or near its natural resonant frequency. The resonant antenna coil circuit includes one or more capacitors 130 and the antenna coil 150, and has a maximum natural Q factor determined primarily by the DC resistance of the antenna coil 150. In use, the antenna coil 150 is magnetically coupled to a similar resonant antenna coil circuit in a passive RFID transponder to be read, for example, as shown in FIG. 1. The passive RFID transponder includes the ability to vary its coupling with the reader, typically by varying the Q factor and / or resonant frequency of its antenna coil circuit, in order to send information from the transponder to the reader. A typical method comprises a switching element and a load resistor, which allow the transponder to modulate the Q factor of its own resonant antenna coil circuit. This modulation causes variations in the voltages and currents present in the reader's resonant antenna coil circuit, which are detected by the reader's receiving subsystem 140.The output of the receiving subsystem 140 is interpreted by a controller 160 (also called a microcontroller) for display and / or interfacing with other devices. The variable resistor 110 allows the controller 160 to vary the Q factor of the reader's resonant antenna coil circuit as needed. When reading data from a transponder at the maximum interval, the controller 160 sets the variable resistor 110 to its minimum value. However, when the reader needs to write data to a transponder, the controller 160 increases the resistance of the variable resistor 110 to reduce the Q factor of the reader's resonant antenna coil circuit from its natural maximum value, and then switches the AC voltage source 120 on and off to modulate the alternating magnetic field emitted by the coil 150. The reader can also read from the transponder in this configuration, with a reduced maximum interval. In some implementations, the variable resistance element 110 includes a continuously variable resistance element with a resistance range. Here, the controller 160 is configured to set any appropriate resistance value within that resistance range. In some implementations, the variable resistance element 110 includes a resistor that is adjustable to any one of a group of discrete resistance values. Here, the controller 160 is configured to set any one of the groups of discrete resistance values. An example of this type, in which the resistor is adjustable to either of the two resistance values, is described below. Figure 7 is a schematic diagram of a passive RFID reader 200 incorporating a variable-Q resonant antenna coil circuit with a second example of a variable resistor element 210. Except for the variable resistor element 210, the passive RFID reader 200 has the same components as the passive RFID reader 100. Here, the variable resistor element 210 includes a fixed resistor element 212 and a switching element 214 connected in parallel with the fixed resistor element 212. Opening or closing the switching element 214 allows the variable resistor element 212 to present either of two resistance values ​​to the resonant antenna coil circuit of the reader 200, resulting in either of two selectable Q factors for the resonant antenna coil circuit of the reader 200.For example, controller 160 can close switching element 214, thereby short-circuiting the fixed resistance element 212 of the resonant antenna coil circuit and selecting a high Q factor setting. Alternatively, controller 160 can open switching element 214, thereby allowing the fixed resistance element 212 to reduce the Q factor of the resonant antenna coil circuit. In some embodiments, the switching element 214 can be implemented as one or more transistors. In some implementations, the switching element 214 can be implemented as an opto-isolator. In implementations using a series resonant antenna coil circuit, one or both nodes of the antenna coil circuit may experience large voltage swings with respect to some other reference node, such as system ground. These voltage swings typically exceed 1,000 volts with respect to system ground in practical reader designs. The variable resistor element can have the same effect on the Q factor of the resonant antenna circuit if placed between any two consecutive nodes of the series resonant circuit. However, it can be beneficial to place it between two consecutive nodes that each experience smaller voltage swings, such as between the AC voltage source and one of the reactive elements of the antenna circuit. This can reduce (or minimize) the voltage stresses across the variable resistor element and its control circuitry. In view of the two example circuits described above, it is instructive to review Figures 2A–2E, which show signals that highlight the effect of changing the Q factor of the antenna coil circuit according to the technologies described. Again, Figure 2A shows a 121kHz digital timing signal that switches the 120V AC voltage source on and off. Figure 2B shows an example of a 121V voltage signal produced by the 120V AC voltage source. In this example, the 120V AC voltage source generates a 134.2kHz sinusoidal signal while being switched on by the 121kHz modulation signal, and the 121kHz modulation signal has high and low bit periods equal to N=32 cycles of the 120V AC voltage source. 2C shows a first signal 151L corresponding to the current in the reader's antenna coil circuit 100 or 200 with a low Q factor equal to 1. FIG.Figure 2D shows a second signal, 151M, corresponding to the current in the antenna coil circuit of reader 100 or 200 with an intermediate Q factor of 10. Figure 2E shows a third signal, 151H, corresponding to the current in the antenna coil circuit of reader 100 or 200 with a high Q factor of 100. Note that each of the signals, 151L, 151M, and 151H, has a different vertical scale. Referring now to the example of the antenna coil circuit with Q=1, the antenna coil current 151L shows a clean on / off modulation, but its magnitude (and therefore the magnitude of the emitted magnetic field) is small, in this case around 1 mA, as shown in Figure 2C.Referring now to the example of the antenna coil circuit with Q=100, the magnitude of the current in antenna coil 151H is much higher, in this case about 60 mA, but it builds up slowly and continues to run for a long time after the AC drive 121 is removed, as shown in FIG. 2E. Referring now to the example of the antenna coil circuit with Q=10, the current in antenna coil 151M represents a compromise between the magnitude and distortion of the modulated envelope, as shown in FIG. 2D. Here, the magnitude of the current in antenna coil 151M is approximately 10 mA. ooh In summary, this description describes a passive RFID reader configured to dynamically vary the Q factor of its resonant antenna coil circuit in order to optimize its performance for transponder read and write operations at shorter intervals, transponder read-only operations at longer intervals, or some compromise between the two, rather than being designed for a single compromise between read-only operations versus read / write operations. The modalities of the subject matter and the functional operations described herein can be implemented in digital electronic circuits, in tangibly embedded computer programs or firmware, in computer hardware, including the structures described herein and their structural equivalents, or in combinations of one or more of these. The modalities of the subject matter described herein can be implemented as one or more computer programs, that is, one or more instruction modules of computer programs encoded in a tangible, non-transient storage medium for execution by, or to control the operation of, data processing devices.The computer storage medium can be a machine-readable storage device, a machine-readable storage substrate, a random-access or serial memory device, or a combination of one or more of these. Alternatively or additionally, the program instructions can be encoded in an artificially generated propagated signal, for example, an electrical, optical, or electromagnetic signal generated by the machine to encode information for transmission to a suitable receiving device for execution by a data processing device. The term controller refers to electronic control and / or data processing hardware and encompasses all types of devices and machines for data processing, including, for example, a programmable processor, a computer, or multiple processors or computers. The controller may also be, or include, special-purpose logic circuits, such as an FPGA (field-programmable gate array) or an ASIC (application-specific integrated circuit). Optionally, in addition to the hardware, the controller may include code that creates a runtime environment for computer programs, such as code that constitutes the processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of these. A computer program, which may also be called or described as a program, software, software application, application, module, software module, script, or code, may be written in any form of programming language, including compiled or interpreted languages, or declarative or procedural languages; and may be deployed in any form, including as a standalone program or as a module, component, subroutine, or other unit suitable for use in a computer environment. A program may, but does not necessarily, correspond to a file in a file system.A program can be stored in a portion of a file that contains other programs or data—for example, one or more scripts stored in a markup language document—in a single file dedicated to the program in question, or in multiple coordinated files—for example, files that store one or more modules, subprograms, or code snippets. A computer program can be deployed to run on a single computer or on multiple computers located at one site or distributed across multiple sites and interconnected by a data communication network. The processes and logical flows described here can be implemented by one or more processing units executing one or more computer programs to perform functions by operating on input data and generating results. These processes and logical flows can also be implemented by special-purpose logic circuits, such as an FPGA or an ASIC, or by a combination of special-purpose logic circuits and one or more programmed computers. Computer-readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media, and memory devices, including, by way of example, semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. To provide user interaction, the modalities described herein can be implemented using a computer with a display device, such as an OLED (organic light-emitting diode) or LCD (liquid crystal display), to show information to the user, and a keyboard and pointing device, such as a mouse or touchpad, through which the user can provide input to the computer. Other types of devices can also be used to facilitate user interaction; for example, the information provided to the user can be any form of sensory information, such as visual, auditory, or tactile information; and input from the user can be received in any form, including acoustic, verbal, or tactile information.In addition, a computer can interact with a user by sending and receiving documents from a device used by the user; for example, by sending web pages to a web browser on a user's device in response to requests received from the web browser. Furthermore, a computer can interact with a user by sending text messages or other forms of messaging to a personal device, such as a smartphone running a messaging application, and receiving reply messages from the user in return. Other modalities are found in the following claims. It is hereby stated that, as of this date, the best method known to the applicant for putting the aforementioned invention into practice is the one that is clear from the present description of the invention.

Claims

1. Circuits for communication with a radio frequency identification (RFID) transponder, characterized in that they comprise: an antenna coil circuit having an inductance, a capacitance, and a resistance that determines a natural resonant frequency and a Q factor for the antenna coil circuit, the antenna coil circuit comprising a variable resistive element; an AC voltage source configured to drive the antenna coil circuit at or near the natural resonant frequency of the antenna coil circuit; a receiving subsystem for receiving information from the RFID transponder; and a controller in communication with the antenna coil circuit and the AC voltage source, the controller being programmed to modulate the AC voltage source to send information to the RFID transponder and to control the variable resistive element.

2. Circuits according to claim 1, characterized in that the antenna coil circuit comprises one or more inductive elements.

3. Circuits according to claim 1 or 2, characterized in that the antenna coil circuit comprises one or more capacitive elements.

4. Circuits in accordance with any of the preceding claims, characterized in that the controller is programmed to modulate the alternating voltage source by switching the alternating voltage source on and off.

5. Circuits in accordance with any of the preceding claims, characterized in that during the operation of the circuit the controller varies a resistance of the variable resistive element to vary a Q factor of the antenna coil circuit.

6. Circuits in accordance with any of the preceding claims, characterized in that the Q factor varies between two or more values ​​in a range of 1 to 500.

7. Circuits in accordance with any of the preceding claims, characterized in that the variable resistive element is an electronically controlled variable resistive element.

8. Circuits in accordance with any of the preceding claims, characterized in that the variable resistive element is a mechanically controlled variable resistive element.

9. Circuits in accordance with any of the preceding claims, characterized in that they further comprise a user interface for controlling the variable resistive element.

10. Circuits in accordance with any of the preceding claims, characterized in that the controller is programmed to automatically control the variable resistive element.

11. Circuits in accordance with any of the preceding claims, characterized in that the variable resistive element is continuously variable over a range of resistance values.

12. Circuits in accordance with any of the preceding claims, characterized in that the variable resistive element is variable between two or more discrete resistance values.

13. Circuits according to claim 12, characterized in that the variable resistive element comprises a resistor having a specified fixed resistance and a switching element configured, in one state, to short-circuit the resistor.

14. Circuits in accordance with any of the preceding claims, characterized in that the variable resistive element is electrically connected between two nodes of the antenna circuit which, during operation of the circuit, experience lower voltage oscillations than other nodes of the antenna circuit with respect to a node of the circuit of interest.

15. Circuits in accordance with any of the preceding claims, characterized in that the controller is programmed to vary a resistive variable while the AC voltage source is switched on and drives the antenna coil circuit.

16. Circuits in accordance with any of the preceding claims, characterized in that the controller is programmed to vary a resistive variable while the AC voltage source is off and does not conduct the antenna coil circuit.

17. A method for communicating with a radio frequency identification (RFID) transponder using a REID circuit comprising a resonant antenna coil circuit having a Q factor, characterized in that, during a read-only mode of an RFID circuit, receiving in the RFID circuit a signal from the transponder while operating the resonant antenna coil circuit at a first Q factor value, varying the Q factor from the first value to a second value lower than the first value; and during a read / write mode of the RFID circuit, receiving in the RFID circuit a signal from the transponder and transmitting from the RFID circuit a signal to the transponder while operating the resonant antenna coil circuit at the second Q factor value.

18. The method according to claim 17, characterized in that the resonant antenna coil circuit comprises an inductance, a capacitance, and a resistance that determine the Q factor for the antenna coil circuit, and the Q factor is varied from the first value to the second value by varying a resistance of the resonant antenna coil circuit.

19. The method according to claim 18, characterized in that the resonant antenna coil circuit comprises a variable resistor and the resistance of the resonant antenna coil circuit is varied by varying a resistance of the variable resistor.

20. The method according to claim 18 or 19, characterized in that the resistance is varied electronically.

21. The method according to claim 18 or 19, characterized in that the resistance is varied mechanically.

22. The method according to any of claims 17-21, characterized in that it further comprises positioning the RFID circuit closer to the RFID transponder while operating the RFID circuit in read / write mode than when operating the RFID circuit in read-only mode.

23. The method according to any of claims 17-22, characterized in that the RFID circuit automatically adjusts the Q factor of the resonant antenna coil circuit from the first value to the second value.

24. The method according to claim 23, characterized in that the RFID circuit transmits command signals to a plurality of different values ​​for the Q factor of the resonant antenna circuit and selects the second value based on a response of the RFID transponder to the command signals.

25. The method according to any of claims 17-24, characterized in that the RFID circuit adjusts the Q factor of the resonant antenna coil circuit in response to a user's input to the RFID circuit.

26. The method in accordance with any of claims 17-25, characterized in that the Q factor is adjusted while driving the resonant antenna circuit.

27. The method according to any of claims 17-26, characterized in that the resonant antenna circuit is driven by an alternating voltage source.

28. The method according to any of claims 17-27, characterized in that the transponder signal received in the RFID circuit while operating the resonant antenna coil circuit at the first Q factor value comprises information stored in the non-volatile memory of the transponder.

29. The method according to claim 28, characterized in that the information stored in the non-volatile memory of the transponder comprises an identification number that corresponds exclusively to the transponder.

30. The method according to any of claims 17-29, characterized in that the signal transmitted from the REID circuit while operating the resonant antenna coil circuit at the second value for the Q factor comprises selected information from the group consisting of information instructing the transponder to send information different from that which the transponder automatically sends by default, information for initial programming of the transponder, information for programming field-programmable memory locations of the transponder, and information for activating a function of the transponder.

31. The method according to any of claims 17-30, characterized in that the transmission from the REID circuit of a signal to the transponder comprises driving the resonant antenna coil at or close to the natural resonant frequency of the resonant antenna coil.

32. The method according to claim 31, characterized in that an alternating voltage source that drives the coil of the resonant antenna is switched off when the Q factor is varied.

33. The method according to claim 31, characterized in that an alternating voltage source that drives the coil of the resonant antenna is switched on when the Q factor is varied. 10 34. The method according to one of claims 17-33, characterized in that the Q factor is continuously varied from the first value to the second.

35. The method according to any of claims 17-34, characterized in that the second value 15 for the Q factor is selected such that a time constant of a magnetic field envelope associated with the signal transmitted from the RFID circuit is shorter than a bit period associated with the signal data.

36. A computer-readable medium characterized in that it contains program instructions for communicating with a radio-frequency identification (RFID) transponder using an RFID circuit comprising a resonant antenna coil circuit having a Q factor, wherein the execution of the program instructions by one or more processors of a computer system causes the one or more processors to perform the steps of: during a read-only mode of a REID circuit, receiving at the REID circuit a signal from the transponder while operating the resonant antenna coil circuit to a first value for the Q factor; varying the Q factor from the first value to a second value lower than the first value;and during a REID circuit read / write mode, receive a signal from the transponder in the REID circuit and transmit a signal from the RFID circuit to the transponder while operating the resonant antenna coil circuit at the second Q factor value;

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