Communications: radio wave antennas – Antennas – With coupling network or impedance in the leadin
Reexamination Certificate
2001-02-28
2004-05-18
Clinger, James (Department: 2821)
Communications: radio wave antennas
Antennas
With coupling network or impedance in the leadin
C340S572400
Reexamination Certificate
active
06738025
ABSTRACT:
TECHNICAL FIELD
The present invention pertains to antenna circuits, and more specifically, to an impedance matching circuit for a modulated backscatter diode-antenna combination particularly adapted for use in radio frequency identification systems.
BACKGROUND OF THE INVENTION
Remote communication utilizing wireless equipment typically relies on radio frequency (RF) technology, which is employed in many industries. One application of RF technology is in locating, identifying, and tracking objects, such as animals, inventory, and vehicles.
RF identification (RFID) tag systems have been developed that facilitate monitoring of remote objects. As shown in
FIG. 1
, a basic RFID system
10
consists of three components, an antenna
12
, a transceiver with decoder
14
, and a transponder (commonly called an RF tag)
16
having its own antenna
24
. In operation, the antenna
12
emits electromagnetic radio signals generated by the transceiver
14
to activate the tag
16
. When the tag
16
is activated, data can be read from or written to the tag.
In some applications, the antenna
12
is a component of the transceiver and decoder
14
to become an interrogator (or reader)
18
, which can be configured either as a hand held or a fixed-mount device. The interrogator
18
emits the radio signals
20
in a range from one inch to one hundred feet or more, depending upon its power output and the radio frequency used. When an RF tag
16
passes through the electromagnetic radio waves
20
, the tag
16
detects the signal
20
and is activated. Data encoded in the tag
16
is then transmitted through reflection by a modulated data signal
22
through an antenna
24
in the tag
16
and to the interrogator
18
for subsequent processing.
An advantage of RFID systems is the non-contact, non-line-of-sight capability of the technology. Tags can be read through a variety of substances such as snow, fog, ice, paint, dirt, and other visually and environmentally challenging conditions where bar codes or other optically-read technologies would be useless. RF tags can also be read at remarkable speeds, in most cases responding in less than one hundred milliseconds.
There are three main categories of RFID tags. These are beam-powered passive tags, battery-powered semi-passive tags, and active tags. Each operates in fundamentally different ways.
The beam-powered RFID tag is often referred to as a passive device because it derives the energy needed for its operation from the radio frequency energy beamed at it. The tag rectifies the field and dynamically changes the reflective characteristics of the tag itself, creating a change in reflectivity that is seen at the interrogator. A battery-powered semi-passive RFID tag operates in a similar fashion, modulating its RF cross section in order to reflect a delta to the interrogator to develop a communication link. Here, the battery is the source of the tag's operational power. Finally, in the active RFID tag, a transmitter is used to create its own radio frequency energy powered by the battery.
Conventional RF tag systems utilize continuous wave backscatter to communicate data from the tag
16
to the interrogator
18
. More specifically, the interrogator
18
transmits a continuous-wave radio signal to the tag
16
, which modulates the signal
20
using modulated backscattering wherein the electrical characteristics of the antenna
24
are altered by a modulating signal from the tag that reflects a modulated signal
22
back to the interrogator
18
. The modulated signal
22
is encoded with information from the tag
16
. The interrogator
18
then demodulates the modulated signal
22
and decodes the information.
Conventional continuous wave backscatter RF tag systems that utilizes passive (no battery) RF tags require adequate power from the signal
20
to power the internal circuitry in the tag
16
used to modulate the signal back to the interrogator
18
. Efficient collection of this energy from the signal
20
is necessary to maximize system performance. Impedance matching of antenna circuit components at the desired frequency is one method to optimize efficiency. However, size and performance constraints of RFID tag systems render existing impedance matching designs unfeasible.
SUMMARY OF THE INVENTION
The disclosed embodiments of the present invention are directed to a diode matching antenna circuit that includes a diode detector circuit having a first terminal coupled to an antenna and a second terminal; and a stub circuit coupled to the second terminal of the diode detecting circuit, the stub circuit structured to match the impedance of the diode detector circuit in combination with the antenna to maximize the performance of the diode detector circuit.
In accordance with another aspect of the present invention, a radio frequency identification tag is provided that includes a circuit for processing radio frequency identification signals in combination with an antenna circuit coupled to the processing circuit and configured from modulated backscatter, the antenna circuit having a diode detector circuit coupled to an antenna and to a stub circuit, the stub circuit configured to match the impedance of the diode detector circuit in combination with the antenna to maximize the performance of the diode detector circuit.
In accordance with another aspect of the invention, the stub circuit is structured to phase shift an output signal in relationship to an input signal. Ideally, the phase shift is about 180° as detected at the interrogator.
In accordance with another aspect of the invention, the stub circuit is structured as a single conductor that functions as an antenna in combination with the diode detector circuit and first antenna. In an integrated form of the diode matching antenna circuit, the stub circuit would comprise a predetermined length and width of metallization.
As will be readily appreciated from the foregoing, the disclosed embodiments of the present invention provide antenna impedance matching for a passive RF tag system that extracts a greater supply voltage from a received signal to achieve enhanced performance, such as a greater transmission range. This increase in the communication distance enables use of passive RF tags for broader applications, such as tracking and identifying inventory in large warehouses, battlefield weaponry, and animals, without increasing the size and cost of such tags.
REFERENCES:
patent: 6046668 (2000-04-01), Forster
patent: 6140924 (2000-10-01), Chia et al.
patent: 6400274 (2002-06-01), Duan et al.
patent: WO 00/67373 (2000-11-01), None
Battelle Memorial Institute K1-53
Clinger James
Seed IP Law Group PLLC
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