Muting circuit for analog filters in radio frequency identification (RFID) systems
An apparatus, which allows a radio frequency identification (RFID) reader to recover quickly from transient input to its receiving subsystem when transitioning from writing to an RFID transponder to reading its response. In particular, this apparatus is comprised of muting circuits, which both attenuate transients in its receiving subsystem while writing to a transponder, and help the receiving subsystem settle quickly after experiencing such transients.
1 . Circuitry for communicating with a radio frequency identification (RFID) transponder, the circuitry comprising:
an antenna circuit; and
a receiving subsystem coupled with the antenna circuit, wherein the receiving subsystem comprises:
a detector circuit configured to detect voltages present in the antenna circuit,
a filter stage comprising a feedback loop, the filter stage configured to output variations of the detected voltages that are caused by transmissions to and from the RFID transponder,
a microcontroller configured to demodulate the output variations as digital data received from the RFID transponder as part of the communications, and
a detector muting switch, that includes a pair of cascaded transistors, and that is disposed between the detector circuit and the filter stage to decouple the filter stage from the detector circuit when the detector muting switch is closed, and couple the filter stage with the detector circuit when the detector muting switch is open,
wherein the microcontroller is configured to:
maintain the detector muting switch closed during transmissions to the RFID transponder,
monitor, after an end of transmissions to the RFID transponder, an input of the filter stage to determine whether the detector circuit has settled, and establish a length of a first time interval based on a result of the determination whether the detector circuit has settled, and
monitor, after the end of the first time interval, an output of the filter stage to determine whether the filter stage has settled, and establish a length of a second time interval based on a result of the determination whether the filter stage has settled.
2 . The circuitry of claim 1 , further comprising:
a second switch, that includes a pair of cascaded transistors, and that is disposed as part of a feedback loop of the filter stage to decrease 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.
3 . The circuitry of claim 2 , wherein the microcontroller is further configured to:
maintain the second switch closed during the transmissions to the RFID transponder,
open the detector muting switch after the first time interval since the end of the transmissions to the RFID transponder, the first time interval corresponding to a settling time of the detector circuit, and
open the second switch after the second time interval since the end of the first time interval, the second time interval corresponding to a settling time of the filter stage.
4 . The circuitry of claim 3 , wherein the microcontroller is configured to open the detector muting switch after a first time interval based on an optimum timing determined by analytical circuit analysis.
5 . The circuitry of claim 4 , wherein opening the detector muting switch based on the optimum timing determined by the analytical circuit analysis comprises performing real-time circuit analysis.
6 . The circuitry of claim 1 , wherein the filter stage comprises an operational amplifier.
7 . The circuitry of claim 2 , wherein the second switch is connected to short the feedback loop of an operational amplifier, when the second switch is closed.
8 . The circuitry of claim 1 , wherein the filter stage comprises a filter stage-muting switch.
9 . The circuitry of claim 2 , wherein the microcontroller is configured to open the detector muting switch and the second switch sequentially.
10 . The circuitry of claim 2 , 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,
a third switch that includes a pair of cascaded transistors, and that is disposed as part of the feedback loop of the second filter stage.
11 . The circuitry of claim 10 , wherein the microcontroller is further configured to open the third switch after a third time interval since the end of the second time interval, the third time interval corresponding to a settling time of the second filter stage.
12 . The circuitry of claim 10 , wherein the second filter stage comprises an operational amplifier, and the third switch is connected to short the feedback loop of the operational amplifier, when the third switch is closed.
13 . A method for communicating with a radio frequency identification (RFID) transponder, the method comprising:
transmitting an RFID reader RF signal from an RFID reader;
receiving, at 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, voltages present in the antenna circuit during the transmitting and the receiving;
after a first time interval since an end of transmitting the RFID reader RF signal, coupling a filter stage to the detector circuit, the filter stage being decoupled from the detector circuit during the transmitting;
filtering, using the filter stage upon coupling the filter stage to the detector circuit, the detected voltages in the antenna circuit to provide output variations;
varying a resistance of a feedback loop of the filter stage to decrease resistance of the feedback loop during transmissions from the RFID reader and increase resistance of the feedback loop after a second time interval since the end of the first time interval, the second time interval corresponding to a settling time of the filter stage;
demodulating the output variations to provide digital data received from the RFID transponder transmitted in the transponder RF signal; and
monitoring, after emitting the RFID reader RF signal, an input 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 whether the detector circuit has settled.
14 . The method of claim 13 , wherein the first time interval corresponds to a settling time of the detector circuit.
15 . The method of claim 13 , wherein the coupling and decoupling of the filter stage is performed with a switch between the filter stage and the detector circuit.
16 . The method of claim 15 , wherein the switch is closed during transmitting the RFID reader RF signal.
17 . The method of claim 15 , wherein the switch is opened after the first time interval.
18 . The method of claim 13 , wherein the RFID reader RF signal is a signal for writing information to the RFID transponder.