Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-06-23
2001-07-24
Hofsass, Jeffery (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S010100, C340S010300, C340S572200, C455S132000
Reexamination Certificate
active
06265976
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of radio frequency identification circuits and methodologies.
2. Description of the Prior Art
The antenna of an RFID reader used in transmitting a carrier signal to an RFID tag is operated in a tuned fashion. The antenna, or coil for low frequency inductively coupled systems, is typically operated in such a fashion that the voltage on the coil (and subsequently its magnetic field) is very uniform. The uniformity is important because any fluctuation in the transmitting coil when used as a receiving coil would be detected as a signal from a tag. Even when using a separate receiving coil, it is coupled with the transmitting coil such that slight deviations by the transmitting coil cause amplitude modulation or ON/OFF keying in the receiving coil which are again sensed by the receiving coil as a signal.
With more state-of-the-art RFID systems, the reader communicates with the tag by altering the amplitude of its transmitted signal. In other words, the transmitting coil is changed from a high voltage to a lower voltage and in most cases is usually turned completely off in rapid succession. With a large voltage change to the tuned transmitted coil the recovery time is quite long. Unfortunately, it is typically during this recovery period that a backscattered (or mutually inducted for low frequency H field operation) signal from a tag is sensed. It is important that the change in the voltage on the sensing coil in the reader of RFID tags be kept to a minimum so that the subsequent recovery be minimized.
BRIEF SUMMARY OF THE INVENTION
The invention is an improvement in a reader of RFID tags having an input stage comprising a coil and a first and second channel coupled in parallel to each other between the coil and the input stage of the reader. The first and second channels are characterized by a response and recovery time to signals coupled thereto from the coil. The response and recovery time of the second channel is slower than the response and recovery time of the first channel. The first and second channels each have a node monitored by the input stage. A clamping device is coupled between the first and second channels to clamp signal levels on the monitored node of first and second channels close to each other so that recovery time of the first channel or node is reduced.
The clamping device is a diode, and preferably a Schottky diode. The clamping device clamps the first and second channels to within approximately 0.2 volt of each other.
The reader has a ground and first and second channels have a capacitance coupled between the coil and ground. The capacitance of the first channel is less than the capacitance of the second channel, and is preferably at least 1000 times less than the capacitance of the second channel.
The input stage is a differential amplifier having two differential inputs and the first and second channels are coupled to the two differential inputs. The clamping device couples the monitored nodes of the first and second channel and the monitored nodes are coupled to the two differential inputs.
The invention also comprises a method of operation of the above described circuit. The invention can be better visualized by turning to the following drawings.
REFERENCES:
patent: 3863244 (1975-01-01), Lichtblau
patent: 3986124 (1976-10-01), Mitchell, Jr.
patent: 4748642 (1988-05-01), Bertsche
patent: 4812822 (1989-03-01), Feltz et al.
patent: 5549658 (1996-08-01), Shannon et al.
Dawes, Esq. Daniel L.
Hofsass Jeffery
Myers Dawes & Andras LLP
Nguyen Phung
Single Chip Systems Corporation
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