Muting Circuit for an Analog Filter in a Radio Frequency Identification (RFID) System

Abstract

An apparatus is provided that enables a radio frequency identification (RFID) reader to quickly recover from transient inputs to its receive subsystem when transitioning from writing to an RFID transponder to reading the RFID transponder's response. Specifically, the apparatus comprises a muting circuit that attenuates transients in the receive subsystem while writing to the transponder and helps the receive subsystem to quickly settle after encountering such a transient.

Description

Technical Field

[0001] This specification relates to radio frequency identification (RFID) systems, and more particularly to inductively coupled passive RFID systems.

Background Art

[0002] As shown in FIG. 1, a passive RFID system generally includes two main subassemblies: an RFID reader and an RFID transponder that is read at some distance from the RFID reader. The RFID reader includes an AC voltage source that drives the resonant antenna coil circuit of the RFID reader. In this way, the RFID reader emits an alternating magnetic field from its antenna coil that is weakly inductively coupled (represented by the dotted double arrow in FIG. 1) to the corresponding antenna coil in the transponder being read. These antenna coils are each part of a corresponding antenna coil circuit that includes one or more tuning capacitors to resonate it at a desired frequency.

[0003] The transponder obtains its operating power from the emitted magnetic field of the RFID reader and modulates the Q value and / or resonant frequency of its antenna coil circuit (using, for example, a switch and a load resistor) into a pattern corresponding to any information to be sent from the transponder to the RFID reader. This information typically includes an identification number that uniquely corresponds to an individual transponder.

[0004] Due to the electromagnetic coupling between the RFID reader and the transponder, the modulation of the transponder appears as fluctuations in the current and voltage present in the antenna coil circuit of the RFID reader. The RFID reader can then detect and demodulate these fluctuations using a receiving subsystem (represented by RX in Figure 1) to extract any information sent by the transponder. This receiving subsystem generally includes an amplitude modulation detector and one or more filtering stages and / or gain stages. The maximum reading range is generally achieved when both the antenna coil circuit of the RFID reader and the antenna coil circuit of the transponder are tuned to resonate at the frequency of the AC voltage source of the RFID reader, when both antenna coils are oriented so as to enable optimal electromagnetic coupling, when both antenna coil circuits have the highest achievable Q value, and when the transponder modulates its Q value as deeply as possible while receiving sufficient power from the RFID reader for its circuit to operate.

[0005] The information present in the transponder is generally stored in some form of non-volatile memory. This memory can include a combination of factory-programmed memory locations and / or field-programmable memory locations. Some transponders can also generate and / or store dynamic information such as temperature transducer readings. Generally, when activated by a magnetic field emitted by a nearby RFID reader, the transponder automatically sends out a subset of its stored information.

[0006] Some transponders are not only readable but also writable. A compatible RFID reader can modulate its own emitted alternating magnetic field (e.g., by turning an AC voltage source that drives its resonant antenna coil circuit on and off) to send commands and / or data to one of these transponders. This function is called "writing" in contrast to the "reading" function described earlier. Devices that implement both the reading and writing functions are still generally referred to as "RFID readers". Transponder writing can be used to initially program the transponder during manufacturing, to program field-programmable memory locations, or to activate special transponder functions, for purposes such as instructing the transponder to send a different set of information than it automatically sends by default. Summary of the Invention Means for Solving the Problems

[0007] When writing to an RFID transponder, it is often necessary for the RFID reader to send a command to the transponder and then immediately listen for a response from the transponder. While the RFID reader is modulating its emission magnetic field to send a command to the transponder, the demodulator portion of the RFID reader observes this modulation as an extremely strong signal, which is much stronger than the signal normally received from the transponder. This strong signal can saturate the demodulation circuit of the RFID reader. Even worse, the filter in the demodulation circuit may require a significant amount of time (i.e., compared to the time it takes to send a receive transmission) to recover from this saturation, and this recovery may not be completed in time for the RFID reader to successfully demodulate the response from the transponder. This problem becomes more troublesome when higher filter performance is desired to improve the transponder reading range of the RFID reader. This situation forces a compromise between the transponder reading performance of the RFID reader and its ability to write commands and data to the transponder.

[0008] This specification describes a demodulator filter chain having high gain and narrow bandwidth for demodulating weak signals from transponders at the extreme range, yet which can quickly recover from large transient inputs encountered by the demodulator filter chain while the RFID reader is modulating its emission magnetic field to write commands and data to the transponder.

[0009] In general, one innovative aspect of the subject matter described in this specification can be embodied as a circuit for communicating with a radio frequency identification (RFID) transponder. The circuit includes an antenna circuit and a receiving subsystem coupled to the antenna circuit. The receiving subsystem includes a detector circuit configured to detect a voltage present in the antenna circuit, a filter stage with a feedback loop, the filter stage being configured to output fluctuations in the detected voltage caused by transmissions to and from the RFID transponder, a microcontroller configured to demodulate the output fluctuations as digital data received from the RFID transponder as part of the communication, and a first switch disposed between the detector circuit and the filter stage, the first switch being configured to decouple the filter stage from the detector circuit when the first switch is closed and to couple the filter stage to the detector circuit when the first switch is open. The microcontroller is configured to maintain the first switch in a closed state during transmission to the RFID transponder and to open the first switch after a first time interval after transmission to the RFID transponder has ended, the first time interval corresponding to the settling time of the detector circuit.

[0010] Each of the foregoing embodiments and other embodiments can optionally include one or more of the following features, either alone or in combination. In particular, one embodiment includes all of the following features in combination.

[0011] In some implementations, the first switch includes a pair of cascaded transistors.

[0012] The microcontroller can be configured to (i) monitor an input of the filter stage after transmission to the RFID transponder has ended to determine whether the detector circuit has settled, and (ii) establish the length of the first time interval based on the result of the determination of whether the detector circuit has settled.

[0013] The receiving subsystem can further include a second switch disposed as part of a feedback loop of the filter stage, the second switch reducing the resistance of the feedback loop when closed and increasing the resistance of the feedback loop when open, and the microcontroller is configured to maintain the second switch in a closed state during transmission to the RFID transponder and to open the second switch after a first time interval has elapsed and after a second time interval, the second time interval corresponding to the settling time of the filter stage.

[0014] The filter stage can include an operational amplifier, and the second switch is connected to short-circuit the feedback loop of the operational amplifier when the second switch is closed. The second switch can include a pair of cascaded transistors. The microcontroller can be configured to monitor the input of the filter stage after transmission to the RFID transponder has ended to determine whether the detector circuit has settled, establish the length of the first time interval based on the result of the determination of whether the detector circuit has settled, monitor the output of the filter stage after the first interval has ended to determine whether the filter stage has settled, and establish the length of the second time interval based on the result of the determination of whether the filter stage has settled.

[0015] In some embodiments, the filter stage is a first filter stage, and the receiving subsystem further includes a second filter stage disposed between the first filter stage and the microcontroller, and a third switch disposed as part of the feedback loop of the second filter stage. The microcontroller is configured to open the third switch after a third time interval after the end of the second time interval, and the third time interval corresponds to the settling time of the second filter stage. The second filter stage can include an operational amplifier, and the third switch is connected so as to short-circuit the feedback loop of the operational amplifier when the third switch is closed. The third switch can include a pair of cascaded transistors. In some embodiments, after the transmission to the RFID transponder is completed, the microcontroller monitors the input part of the filter stage to determine whether the detector circuit has settled, and establishes the length of the first time interval based on the result of the determination of whether the detector circuit has settled. After the first interval ends, the microcontroller monitors the output part of the first filter stage to determine whether the first filter stage has settled, and establishes the length of the second time interval based on the result of the determination of whether the first filter stage has settled. After the second interval ends, the microcontroller monitors the output part of the second filter stage to determine whether the second filter stage has settled, and establishes the length of the third time interval based on the result of the determination of whether the second filter stage has settled.

[0016] Generally, another innovative aspect of the subject matter described herein can be embodied as a method for communicating with a radio frequency identification (RFID) transponder. The method includes transmitting an RFID reader RF signal from an RFID reader, receiving, in an antenna circuit of the RFID reader, a transponder RF signal transmitted by the RFID transponder in response to the RFID reader RF signal, detecting, using a detector circuit of the RFID reader, a voltage present in the antenna circuit between the transmitting and the receiving, coupling a filter stage to the detector circuit after a first time interval has elapsed since the transmitting of the RFID reader RF signal ended, where the filter stage is decoupled from the detector circuit during the transmitting, using the filter stage to filter the detected voltage in the antenna circuit to provide an output variation after the coupling of the filter stage to the detector circuit, and demodulating the output variation to extract digital data received from the RFID transponder and transmitted within the transponder RF signal.

[0017] The foregoing embodiments and other embodiments can each optionally include one or more of the following features, either alone or in combination. In particular, one embodiment includes all of the following features in combination.

[0018] For example, the first time interval can correspond to a settling time of the detector circuit.

[0019] The coupling and decoupling of the filter stage can be implemented using a switch between the filter stage and the detector circuit. The switch can be closed during the transmitting of the RFID reader RF signal. The switch can be opened after the first time interval.

[0020] The method can include varying the resistance of a feedback loop of a filter stage during transmission from an RFID reader so as to reduce the resistance of the feedback loop. The resistance of the feedback loop can be varied by opening and closing a second switch, which is within the feedback loop. In some embodiments, varying the resistance of the feedback loop includes increasing the resistance of the feedback loop after a second time interval following the end of a first time interval, and the second time interval corresponds to the settling time of the filter stage.

[0021] The method can include monitoring an input of a filter stage after an RFID reader emits an RF signal to determine whether a detector circuit has settled, and establishing a length of a first time interval based on a result of the determination of whether the detector circuit has settled. The method can include monitoring an output of the filter stage after the first interval has ended to determine whether the filter stage has settled, and establishing a length of a second time interval based on a result of the determination of whether the filter stage has settled. The filter stage can be a first filter stage and can be coupled to a second filter stage, and the method can include monitoring an output of the second filter stage after the second interval has ended to determine whether the second filter stage has settled, and establishing a length of a third time interval based on a result of the determination of whether the second filter stage has settled.

[0022] The RFID reader RF signal can be a signal for writing information to an RFID transponder.

[0023] The subject matter described herein can, in certain embodiments, be implemented to realize one or more of the following advantages. Achieving a long maximum read range will, of course, require the designer to implement filter elements with high gain and high Q values within the receive subsystem of the RFID reader. As a result, inevitably, the filter elements will require some finite non-zero time to recover from a very large transient input. The receive subsystem of a possible RFID reader for read / write operations must be able to recover from transient events caused by writing quickly enough to process the response from an associated transponder. This requirement generally limits the maximum achievable read-only range. The techniques disclosed herein enable the RFID reader to reduce or eliminate that compromise by reducing the magnitude of the transient events caused by writing that the receive subsystem encounters. Further, the techniques disclosed herein reduce the recovery time of the filter elements of the RFID reader's receive subsystem by controlling the progression of the transient events caused by writing through cascaded filter elements.

[0024] Details of one or more embodiments of the subject matter of this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

Brief Description of the Drawings

[0025]

Figure 1

Figure 2

Figure 3A

Figure 3B

Figure 3C

Figure 4

Figure 5A

Figure 5B

Figure 5C

Best Mode for Carrying Out the Invention

[0026] Like reference numerals and names in the various drawings indicate like elements.

[0027] In this specification, a demodulator filter chain having a high gain and a narrow bandwidth for demodulating weak signals from transponders in the extreme range, and yet capable of quickly recovering from large transient inputs encountered by the demodulator filter chain while the RFID reader modulates its emitted magnetic field to write commands and data to the transponder, will be described.

[0028] FIG. 1 shows a schematic diagram of an RFID reader 110 and an RFID transponder 160. The RFID reader 110 includes an AC voltage source 112 that drives a tuned antenna coil circuit including one or more capacitors 113 and an antenna coil 115 at or near the resonant frequency of the tuned antenna coil circuit. Variations in voltage and / or current present in the antenna coil circuit of the RFID reader 110 caused by the coupled transponder 160 are detected and demodulated by the receiving subsystem 120. The RFID transponder 160 includes an antenna coil 165 and a capacitor 163, which together form a tuned antenna coil circuit. The RFID transponder 160 also includes a switching element 169 and a resistor 167, which enable the impedance presented by the RFID transponder 160 to the RFID reader 110 to be modulated by the RFID transponder 160. The tuned antenna coil circuits corresponding to each of the RFID reader 110 and the RFID transponder 160 are coupled to each other by weak electromagnetic coupling 157.

[0029] Figure 2 shows a schematic diagram of a first example of the RFID reader 210 including details related to its receiving subsystem 220. Here, the receiving subsystem 220 is implemented in a conventional configuration. The modulation of the RFID transponder 160 is detected by a detector 230 including a bridge rectifier 13, a smoothing capacitor 14, and a discharge resistor 15. The AC component of the detected signal is coupled through a DC blocking capacitor 16 to one active filter stage 240a or two or more cascaded active filter stages 240a, 240b, etc. Each active filter stage 240a / b includes an operational amplifier 18, as well as appropriate passive elements at its input and a feedback circuit for providing appropriate transfer characteristics. The detected and filtered signal is passed from the active filter stage to a microcontroller 250 for demodulating and (e.g., using an electronic display) displaying the information sent by the RFID transponder, e.g., 160. Generally, the microcontroller 250 is further configured to control the RFID reader 210.

[0030] Figures 3A - 3C show some of the main waveforms within the RFID reader 210.

[0031] Figure 3A shows an example of a carrier keying waveform 322 representing the modulation of the emitted alternating magnetic field 157 of the RFID reader 210. Here, the low level corresponds to the AC voltage source 112 being turned off, while the high level corresponds to the voltage output by the AC voltage source 112 when the AC voltage source 112 is on. In some implementations, the modulation of the emitted alternating magnetic field 157 of the RFID reader 210 is within a frequency range of 100 - 400 kHz. For example, the frequency of the modulation of the RFID reader 210 is f cIt can be set to 134.2 kHz. The modulated interval 325 of the carrier keying waveform corresponds to the RFID reader 210 sending a command to an RFID transponder, such as 160. The RFID reader 210 then keeps its carrier constantly on, and during that time, the RFID reader 210 listens for a response from the RFID transponder, such as 160.

[0032] FIG. 3B shows an example of the input waveform 323 of the microcontroller 250, which represents the output voltage from the last active filter stage, which is the active filter stage 240b in FIG. 2. FIG. 3C shows the demodulated bit waveform 324 corresponding to the input waveform 323 being converted by the microcontroller 250 into binary values before clock and data extraction. Digital data can be extracted from this demodulated bit waveform 324. In some embodiments, the amplitude and phase of the output waveform resulting from demodulation (see, e.g., FIG. 3C) are stored and processed as floating-point values before final decoding.

[0033] Referring now to FIGS. 3A - 3C, as a result of the modulation 325 of the carrier keying waveform, the active filter stages 240a, 240b saturate, which can be clearly seen within the saturated interval 326 of the demodulator input waveform. After the RFID reader 210 stops modulating its emitted carrier, the active filter stages 240a, 240b gradually recover, resulting in filter ringing within the ringing interval 327 of the demodulator input waveform and corresponding incorrect transitions and distorted bits within the corrupted interval 328 of the demodulated bit waveform. Finally, as shown by the normal interval 329 of the demodulator input waveform and the cleanly demodulated waveform within the properly decoded interval 330 of the demodulated bit waveform, the RFID reader 210 can clearly receive a response from the RFID transponder, such as 160. The RFID transponder, such as 160, sends its response to the RFID reader 210 at the frequency f of the modulation of the RFID reader 210.c One-Nth of the frequency f T ,

[0034]

Number

[0035] It should be noted that it is transmitted as being modulated by. Here, N can be 2, 8, 10, 32, 64, or other integer values.

[0036] Generally, the information stored in the transponder can be any type of data stored in digital form (e.g., words, numbers, and other alphanumeric strings). During transmission, the transponder modulates the carrier signal to encode this digital data into an RF waveform. The reader demodulates the received waveform to extract this digital data and retrieve the information stored in the transponder. Since this data was stored in digital form on the transponder, it is considered digital data even if it is not restored to its original digital form during or after the demodulation process.

[0037] FIG. 4 shows a schematic diagram of a second example of an RFID reader 410 including details related to its receiving subsystem 420. Here, the receiving subsystem 420 is implemented in accordance with the technology disclosed herein. In addition to the receiving subsystem 420, the RFID reader 410 includes components 112, 113, and 115 described above in connection with the RFID reader 210.

[0038] In addition to components 230, 16, and 250 that it shares with receiving subsystem 220, receiving subsystem 420 also includes detector muting switch 425, which is closed while writing to an RFID transponder, such as 160, and open while reading from an RFID transponder, such as 160. Detector muting switch 425 attenuates strong signals from detector 230 that are present while writing to an RFID transponder, such as 160, and enables the charge state of DC blocking capacitor 16 to settle more quickly after transitioning from transponder write back to transponder read. Here, the settling time is related to the transient response of a capacitor (or other output device) to a sudden change in input. For example, the settling time can refer to the time elapsed until the output of a capacitor (or other output device) remains within a specific error band from an ideal instantaneous step input.

[0039] In addition to the foregoing components, the RFID reader 410 includes cascaded active filters 440a, 440b implemented in accordance with the techniques disclosed herein. Each active filter 440a / b includes an operational amplifier 18, as well as appropriate passive elements at its input and a feedback circuit such as the feedback circuit of the corresponding filter stage 240a, 240b. Here, corresponding filter stage muting switches 445a / b are connected to the feedback network of each active filter stage 440a / b. The filter stage muting switches 445a / b are closed during writing and open during reading. The corresponding filter stage muting switches 445a / b allow each filter stage 440a / b to recover more quickly after transitioning back from transponder writing to transponder reading. The microcontroller 250 optionally monitors intermediate circuit nodes, such as nodes A, B, C, within the cascaded active filters 440a, 440b. All muting switches 425, 445a, 445b are closed during transponder writing and then opened in a carefully timed sequence that proceeds through the successive active filter stages 440a, 440b starting from the detector 230, achieving an optimal recovery time. Each switch opening event occurs substantially simultaneously with the output of the preceding detector stage 230 or the output of the preceding filter stage 440a, 440b settling to its quiescent state, such that the perturbation of each filter stage 440a / b is minimized and is carefully timed. In some implementations, the optimal timing for opening each switching element 425, 445a, 445b is determined analytically through circuit analysis. In some implementations, the optimal timing for opening each switching element 425, 445a, 445b is determined empirically through measurement of the circuit's response to a large transient input.In an implementation such as that shown in FIG. 4, the optimal timing for opening each of the switching elements 425, 445a, 445b is determined as part of a closed-loop system in which the microcontroller 250 monitors intermediate circuit nodes, such as nodes A, B, C, to determine when to open each of the switching elements 425, 445a, 445b.

[0040] FIGS. 5A-5C show their improved behavior when signals corresponding to the signals shown in FIGS. 3A-3C are applied to the RFID reader 410. Here, the carrier keying waveform 322 shown in FIG. 5A is the same as the carrier keying waveform 322 shown in FIG. 3A. FIG. 5B shows an example of the input waveform 523 of the microprocessor 250, which represents the output voltage from the final active filter stage, which is the active filter stage 440b in FIG. 4. FIG. 5C shows the demodulated bit waveform 524 corresponding to the input waveform 523 being converted by the microprocessor 250 into binary values before clock and data extraction. Note that the modulated interval 325 of the carrier keying waveform shown in FIG. 5A does not cause saturation of the active filters 440a, 440b, as indicated by the flat portions of the waveforms 523, 524 in FIGS. 5B-5C. FIGS. 5B-5C also show that the ringing interval 527 of the demodulator input waveform is much shorter and the corrupted interval 528 of the demodulated bit waveform is correspondingly shorter compared to the intervals corresponding to each of the waveforms 323, 324. One or more additional properly decoded data bits can now be seen within the normal interval 529 of the demodulator input waveform and within the properly decoded interval 530 of the demodulated bit waveform.

[0041] In summary, the present specification describes a demodulator filter chain having high gain and narrow bandwidth for demodulating weak signals from transponders in an extreme range, yet capable of quickly recovering from large transient inputs encountered by the demodulator filter chain while the RFID reader modulates its emitted magnetic field to write commands and data to the transponder.

[0042] Embodiments of the subject matter and the functional operations described in this specification can be implemented as a digital electronic circuit, as tangible computer software or firmware, as computer hardware including the structures disclosed in this specification and their structural equivalents thereof, or as one or a combination of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer programs, i.e., as one or more modules of computer program instructions, encoded on a tangible non-transitory storage medium for execution by, or to control the operation of, a data processing apparatus. The computer storage medium can be a machine-readable storage device, a machine-readable storage substrate, a random access or serial access memory device, or a combination of one or more of them. Alternatively or in addition, the program instructions can be encoded on an artificially generated propagated signal, e.g., an electrical, optical, or electromagnetic signal generated by a machine, generated to encode the information for execution by an appropriate receiver device by a data processing apparatus.

[0043] The term "microcontroller" refers to data processing hardware and encompasses all kinds of devices, apparatuses, and machines for processing data, including, by way of example, programmable processors. A microcontroller can be a dedicated logic circuit, such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit), and can further include a dedicated logic circuit, such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit). Optionally, in addition to hardware, a microcontroller can also include code that creates an execution environment for a computer program, such as processor firmware, a protocol stack, a database management system, an operating system, or code constituting a combination of one or more of them.

[0044] A computer program, also referred to as or described as a program, software, software application, app, module, software module, script, or code, can be written in any form of programming language, including compiled languages or interpreted languages, or declarative languages or procedural languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. The program may, although not necessarily, correspond to a file in a file system. The program can be stored in a part of a file that holds other programs or data, such as one or more scripts stored within a markup language document, in a single file dedicated to the program, or in multiple related files, such as files that hold one or more modules, subprograms, or portions of the code. A computer program can be deployed to execute on one computer or on multiple computers located at one site or distributed across multiple sites and interconnected by a data communication network.

[0045] The processes and logical flows described herein can be implemented by one or more programmable computers executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logical flows can also be implemented by dedicated logic circuitry, such as an FPGA or ASIC, or by a combination of dedicated logic circuitry and one or more programmed computers.

[0046] Suitable computer-readable media for storing computer program instructions and data include, by way of example, semiconductor memory devices such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks or removable disks; magneto-optical disks; and all forms of non-volatile memory, media, and memory devices including CD-ROM disks and DVD-ROM disks.

[0047] To enable interaction with a user, embodiments of the subject matter described herein can be implemented on a computer having a display device for displaying information to the user, such as an LCD (liquid crystal display) monitor or an organic light emitting diode (OLED) monitor, and a keyboard and a pointing device by which the user can input to the computer, such as a mouse or a trackball. Other kinds of devices can be used to enable interaction with the user as well, for example, feedback provided to the user can be any form of sensory feedback such as visual feedback, auditory feedback, or tactile feedback, and input received from the user can be received in any form including acoustic input, voice input, or tactile input. Additionally, the computer can interact with the user by sending documents to and receiving documents from the device the user is using, for example, by sending a web page in response to a request received from a web browser on the user's device. Also, the computer can interact with the user by sending a text message or other form of message to a personal device, such as a smartphone, running a messaging application and receiving a response message as a reply from the user.

[0048] Other embodiments are described in the appended claims.

Description of the Reference Numerals

[0049] 13 Bridge rectifier 14 Smoothing capacitor 15 Discharge resistor 16 DC blocking capacitor, component 18 Operational amplifier 110 RFID reader 112 AC voltage source, component 113 Capacitor, component 115 Antenna coil, component 120 Receiver subsystem 157 Capacitive electromagnetic coupling, radiating alternating magnetic field 160 RFID transponder 163 Capacitor 165 Antenna coil 167 Resistor 169 Switching element 210 RFID reader 230 Detector, component, detector stage 240a Active filter stage 240b Active filter stage 250 Microcontroller, component, microprocessor 322 Carrier keying waveform 323 Input waveform 324 Demodulated bit waveform 325 Modulated interval of carrier keying waveform, modulation of carrier keying waveform 326 Saturated interval of demodulator input waveform 327 Ringing interval of demodulator input waveform 328 Damaged interval of demodulated bit waveform 329 Normal interval of demodulator input waveform 330 Properly decoded interval of demodulated bit waveform 410 RFID reader 420 Receiver subsystem 425 Detector muting switch, switching element 440a Active filter, active filter stage 440b Active Filter, Active Filter Stage 445a Filter Stage Muting Switch, Switching Element 445b Filter Stage Muting Switch, Switching Element 523 Input Waveform 524 Demodulated Bit Waveform 527 Ringing Interval of Demodulator Input Waveform 528 Damaged Interval of Demodulated Bit Waveform 529 Normal Interval of Demodulator Input Waveform 530 Properly Decoded Interval of Demodulated Bit Waveform Node A Node B Node C

Claims

1. A circuit for communicating with a radio frequency identification (RFID) transponder, comprising: an antenna circuit; a receiving subsystem coupled to the antenna circuit; wherein the receiving subsystem includes: a detector circuit configured to detect a voltage present in the antenna circuit; a filter stage having a feedback loop and configured to output fluctuations of the detected voltage caused by transmission to and from the RFID transponder; a microcontroller configured to demodulate the output fluctuations as digital data received from the RFID transponder as part of the communication; and a first switch disposed between the detector circuit and the filter stage, the first switch configured to decouple the filter stage from the detector circuit when the first switch is closed and to couple the filter stage to the detector circuit when the first switch is open; wherein: the microcontroller is configured to: maintain the first switch in a closed state during the transmission to the RFID transponder; open the first switch after a first time interval after the transmission to the RFID transponder has ended, wherein the first time interval corresponds to a settling time of the detector circuit.

2. The circuit according to claim 1, wherein the first switch comprises a pair of cascaded transistors.

3. The circuit according to claim 1, wherein the microcontroller is configured to: monitor an input of the filter stage after the transmission to the RFID transponder has ended to determine whether the detector circuit has settled; establish a length of the first time interval based on a result of the determination as to whether the detector circuit has settled.

4. The receiving subsystem further comprises a second switch disposed as part of the feedback loop of the filter stage, the second switch configured to reduce a resistance of the feedback loop when the second switch is closed and to increase the resistance of the feedback loop when the second switch is open; wherein the microcontroller is configured to: During the transmission to the RFID transponder, maintain the second switch in a closed state, and open the second switch after a second time interval from the end of the first time interval configured as such, wherein the second time interval corresponds to the settling time of the filter stage, the circuit according to claim 1. **Claim 5** wherein the filter stage comprises an operational amplifier, and the second switch is connected so as to short-circuit the feedback loop of the operational amplifier when the second switch is closed, the circuit according to claim 4. **Claim 6** and the second switch comprises a pair of cascaded transistors, the circuit according to claim 4. **Claim 7** wherein the microcontroller after the transmission to the RFID transponder is completed, monitors the input of the filter stage to determine whether the detector circuit has settled, and establishes the length of the first time interval based on the result of the determination as to whether the detector circuit has settled; and after the first time interval is completed, monitors the output of the filter stage to determine whether the filter stage has settled, and establishes the length of the second time interval based on the result of the determination as to whether the filter stage has settled configured to perform, the circuit according to claim 4. **Claim 8** wherein the filter stage is a first filter stage, and the receiving subsystem further comprises a second filter stage disposed between the first filter stage and the microcontroller, and a third switch disposed as part of the feedback loop of the second filter stage and the microcontroller is configured to open the third switch after a third time interval from the end of the second time interval, wherein the third time interval corresponds to the settling time of the second filter stage, the circuit according to claim 4. **Claim 9** wherein the second filter stage comprises an operational amplifier, and the third switch is connected so as to short-circuit the feedback loop of the operational amplifier when the third switch is closed, the circuit according to claim 8. **Claim 10** The circuit according to claim 8, wherein the third switch comprises a pair of cascaded transistors.

11. After the microcontroller After the transmission to the RFID transponder is completed, monitor the input of the filter stage to determine whether the detector circuit has settled, and establish the length of the first time interval based on the result of the determination as to whether the detector circuit has settled; After the first time interval has ended, monitor the output of the first filter stage to determine whether the first filter stage has settled, and establish the length of the second time interval based on the result of the determination as to whether the first filter stage has settled; After the second time interval has ended, monitor the output of the second filter stage to determine whether the second filter stage has settled, and establish the length of the third time interval based on the result of the determination as to whether the second filter stage has settled The circuit according to claim 8, which is configured to perform the above.

12. A method for communicating with a radio frequency identification (RFID) transponder, comprising: Transmitting an RFID reader RF signal from an RFID reader; Receiving, in an antenna circuit of the RFID reader, a transponder RF signal transmitted by the RFID transponder in response to the RFID reader RF signal; During the transmitting step and the receiving step, using a detector circuit of the RFID reader to detect a voltage present in the antenna circuit; A step of coupling a filter stage to the detector circuit after a first time interval after the step of transmitting the RFID reader RF signal has ended, wherein during the transmitting step, the filter stage is decoupled from the detector circuit, and the first time interval corresponds to the settling time of the detector circuit; After coupling the filter stage to the detector circuit, using the filter stage to filter the detected voltage in the antenna circuit to provide an output variation. demodulating the output variation to provide digital data received from the RFID transponder transmitted within the transponder RF signal A method comprising: **Claim 13** The method of claim 12, wherein the coupling and decoupling of the filter stage are performed using a switch between the filter stage and the detector circuit. **Claim 14** The method of claim 13, wherein the switch is closed during the step of transmitting the RFID reader RF signal. **Claim 15** The method of claim 14, wherein the switch is opened after the first time interval. **Claim 16** A method for communicating with a radio frequency identification (RFID) transponder, comprising: transmitting an RFID reader RF signal from an RFID reader; receiving, in an antenna circuit of the RFID reader, a transponder RF signal transmitted by the RFID transponder in response to the RFID reader RF signal; detecting, using a detector circuit of the RFID reader, a voltage present in the antenna circuit between the step of transmitting and the step of receiving; coupling a filter stage to the detector circuit after a first time interval after the step of transmitting the RFID reader RF signal has ended, wherein the filter stage is decoupled from the detector circuit during the step of transmitting; after coupling the filter stage to the detector circuit, using the filter stage to filter the detected voltage in the antenna circuit to provide an output variation; demodulating the output variation to provide digital data received from the RFID transponder transmitted within the transponder RF signal; changing a resistance of a feedback loop of the filter stage to reduce the resistance of the feedback loop during transmission from the RFID reader A method comprising: **Claim 17** wherein the resistance of the feedback loop is changed by opening and closing a second switch, The method of claim 16, wherein the second switch is within the feedback loop. **Claim 18** The step of changing the resistance of the feedback loop includes increasing the resistance of the feedback loop after a second time interval after the first time interval ends, The method according to claim 16, wherein the second time interval corresponds to the settling time of the filter stage. **Claim 19** A method for communicating with a radio frequency identification (RFID) transponder, comprising: Transmitting an RFID reader RF signal from an RFID reader; Receiving, in an antenna circuit of the RFID reader, a transponder RF signal transmitted by the RFID transponder in response to the RFID reader RF signal; Detecting a voltage present in the antenna circuit using a detector circuit of the RFID reader during the transmitting step and the receiving step; Coupling a filter stage to the detector circuit after a first time interval after the step of transmitting the RFID reader RF signal ends, wherein the filter stage is decoupled from the detector circuit during the transmitting step; After coupling the filter stage to the detector circuit, filtering the detected voltage in the antenna circuit using the filter stage to provide an output variation; Demodulating the output variation to provide digital data received from the RFID transponder transmitted in the transponder RF signal; After emitting the RFID reader RF signal, monitoring an input portion of the filter stage to determine whether the detector circuit has settled, and establishing a length of the first time interval based on a result of the determination of whether the detector circuit has settled; A method comprising: **Claim 20** The method according to claim 19, further comprising monitoring an output portion of the filter stage after the first time interval ends to determine whether the filter stage has settled, and establishing a length of a second time interval based on a result of the determination of whether the filter stage has settled. **Claim 21** The filter stage is a first filter stage and is coupled to a second filter stage. The method according to claim 20, comprising the step of monitoring an output part of the second filter stage after the second time interval has ended, determining whether the second filter stage has been tuned, and establishing a length of a third time interval based on a result of the determination as to whether the second filter stage has been tuned.

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