Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-03-14
2001-05-08
Hofsass, Jeffery (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S572200, C340S572700, C455S078000
Reexamination Certificate
active
06229442
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to communication devices, and more particularly to radio frequency identification (RFID) devices having displacement current control.
BACKGROUND OF THE INVENTION
In general, an RFID system consists of one or more tags, a tag reader, and a host computer system. Tags are devices that can come in many sizes and form factors, but are usually small and lightweight. Tags are commonly used as portable data devices that wirelessly communicate with RFID readers at distances ranging from a few millimeters to several meters. The information stored in a tag can be used, for example, to identify an individual or object carrying the tag.
RFID technology is used in a variety of applications because of its convenience and flexibility. An example application for RFID technology is a building security system. As part of a building security system, RFID systems are used to grant access only to individuals carrying authorized tags (or cards). When an individual places their card in the vicinity of the reader, the reader interrogates the card and obtains identification information stored in the card. After further processing, the reader communicates the individual's identification (“ID”) code to a host computer in the security system. If the ID code received by the host computer system is authorized, the door is unlocked to permit access to the building.
RFID systems are also used to detect specific items and link those items with other information and events. RFID systems can be used, for example, to track products being built in a factory, to trigger manufacturing steps to occur, to assist in inventory control, etc. Read-only tags are ones in which the data is programmed once, and the tag only sends the stored information to the reader. Read-write tags have the ability to be reprogrammed to suit the needs of the application. Therefore, read-write tags can be used as portable databases, eliminating the need for central databases.
Most RFID tags contain an integrated circuit (“IC”) to store and process data, and to perform communication functions. RFID tags also contain an electrode, which is used as the radio frequency interface with the reader. The IC requires power to operate, which can be supplied by a battery. Most applications, however, require tags to be small and inexpensive, so batteryless, or “passive”, tags are in very wide use. Passive tags receive energy from the radio frequency (“RF”) field generated by a reader, and the IC converts the RF to direct current (“DC”) operating power for itself. Once operating, the IC communicates with the reader, which has an electrode system for transmission and reception of signals. Power and data are transferred between tag and reader through one or more electrodes in each device.
Some tag-reader systems communicate via magnetic fields, while other types of systems communicate via electric fields. Electric field tags offer advantages in cost, size, weight and flexibility compared with magnetic field tags. Many applications demand small, compact and inexpensive readers, as well. Shrinking the size of electric field RFID readers, however, presents unique design challenges. Without addressing these challenges, reader performance is significantly impaired.
FIG. 1A
is an example of an electric field RFID system
10
having a compact RFID device
12
. The RFID device
12
is composed of two basic elements, an exciter electrode (e.g., antenna, plate, etc.)
14
and electronic circuitry
16
. The RFID device
12
may be any part of an RFID reader system containing tag excitation circuitry, such as, a tag reader, a tag writer, a tag reader/writer, a tag excitation device (in which the circuitry that performs the tag reading function is located in a separate unit), or any combination thereof. The exciter electrode
14
is a sheet of electrically conductive material. The electronic circuitry
16
contains all of the functional circuitry required to drive the exciter electrode
14
, communicate information between a tag
20
and the RFID device
12
, and exchange information with a host computer system
22
via an input/output (“I/O”) cable
18
. Power is provided to the RFID device
12
by the host computer system
22
via the I/O cable
18
. The electronic circuitry
16
is commonly assembled on a substrate
17
comprised of a dielectric material, such as, epoxy glass printed circuit board (PCB). Alternatively, the substrate
17
may be made of a wide variety of materials, such as, polymer sheets or films, paper or cardboard, ceramic, etc. Components used in electronic circuitry
16
are interconnected by conductors on substrate
17
. The conductors are formed of metals, metal foil, metal film, electrically conductive inks or paints, etc., and may be constructed using any suitable means, such as deposition and etching.
FIG. 1B
is a side pictorial view/schematic diagram of the RFID system of
FIG. 1A
, which represents a monopole electric field RFID system. An exciter voltage source
30
generates a high alternating current (“AC”) voltage that is connected to the exciter electrode
14
. The exciter electrode
14
, driven by the exciter voltage source
30
, causes an AC electric field to be radiated outward toward the tag
20
. When the tag
20
is close enough to the exciter electrode
14
, and sufficient energy is coupled, the tag
20
then begins to function. This causes a small displacement current
32
to flow into the tag
20
. Displacement current is that which flows through a dielectric when a time-varying potential exists across the dielectric. Current
32
flows through the tag
20
, a common impedance path
34
(e.g., earth ground), and an RFID device reference connection
41
, ultimately returning to the exciter voltage source
30
at the exciter voltage source return node
42
. Therefore, current
32
provides operating energy for the tag
20
. Relatively high voltage levels are required on the exciter electrode
14
in order to produce an adequate magnitude of current
32
when the tag
20
is at long distances from the exciter electrode
14
. A receive electrode (not shown) is often located on or near the exciter electrode
14
for the purpose of receiving signals from tags.
It should be noted that
FIG. 1B
is not drawn to scale, that is, the tag
20
is typically positioned at a much greater distance from the exciter electrode
14
than is the electronic circuitry
16
. Parasitic displacement current
76
flows from the exciter electrode
14
to other impedances that are broadly distributed in the environment surrounding the RFID device
12
and common to the RFID device reference connection
41
. Parasitic displacement current
76
is due to stray capacitance, and is generally very small in magnitude. In
FIG. 1B
, the sum of current
32
and current
76
is shown as current
92
as it flows back to RFID device
12
through common impedance path
34
and RFID device reference connection
41
.
In
FIG. 1B
, trace
36
, trace
40
and sensitive component
38
(collectively referred to as “circuit elements”) provide a simple representation of circuitry on the electronic circuitry
16
. Because of their close proximity, significant capacitance exists between the exciter electrode
14
and the circuitry disposed on the electronic circuitry
16
. Smaller separations between the exciter electrode
14
and the electronic circuitry
16
increase this capacitance. The exciter voltage source
14
is also located on the electronic circuitry
16
, and the exciter voltage source return node
42
is common with many elements on the electronic circuitry
16
(e.g., circuit ground). Because of the AC potential difference between the exciter electrode
14
and circuit elements
36
,
38
and
40
, displacement current flows through the dielectric space between the exciter electrode
14
and the electronic circuitry
16
. For illustration purposes, this displacement current is represented by lumped currents
44
,
46
,
48
,
50
, and
52
, which flow respectively through lumped capacitances
Kakkanad Sebastian T.
Quaderer James G.
Rolin John Howard
Hofsass Jeffery
Hughes Terri S.
La Anh
Motorola Inc
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