Communications: electrical – Selective – Interrogation response
Reexamination Certificate
2001-03-09
2004-07-20
Horabik, Michael (Department: 2635)
Communications: electrical
Selective
Interrogation response
Reexamination Certificate
active
06765476
ABSTRACT:
TECHNICAL FIELD
The present invention relates to RF identification (RFID) tag systems, and particularly to RFID tags that communicate using frequency modulation.
BACKGROUND INFORMATION
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
includes two components: an interrogator or reader
12
, and a transponder (commonly called an RF tag)
14
. The interrogator
12
and RF tag
14
include respective antennas
16
,
18
. In operation, the interrogator
12
transmits through its antenna
16
a radio frequency interrogation signal
20
to the antenna
18
of the RF tag
14
. In response to receiving the interrogation signal
20
, the RF tag
14
produces an amplitude-modulated response signal
22
that is modulated back to the interrogator
12
through the tag antenna
18
by a process known as backscatter.
The conventional RF tag
14
includes an amplitude modulator
24
with a switch
26
, tag
14
is activated by the interrogation signal
20
, a driver (not shown) creates a modulating on/off signal
27
based on an information code, typically an identification code, stored in a non-volatile memory (not shown) of the RF tag
14
. The modulating signal
27
is applied to a control terminal of the switch
26
, which causes the switch
26
to alternately open and close. When the switch
26
is open, the tag antenna
18
reflects a portion of the interrogation signal
20
back to the interrogator
12
as a portion
28
of the response signal
22
. When the switch
26
is closed, the interrogation signal
20
travels through the switch
26
to ground, without being reflected, thereby creating a null portion
29
of the response signal
22
. In other words, the interrogation signal
20
is amplitude-modulated to produce the response signal
22
by alternately reflecting and absorbing the interrogation signal
20
according to the modulating signal
27
, which is characteristic of the stored information code. The RF tag
14
could also be modified so that the interrogation signal is reflected when the switch
26
is closed and absorbed when the switch
26
is open. Upon receiving the response signal
22
, the interrogator
12
demodulates the response signal
22
to decode the information code represented by the response signal. The conventional RFID systems thus operate with an oscillator or clock in which the RF tag
14
modulates a RF carrier frequency to provide an indication to the interrogator
12
that the RF tag
14
is present.
The substantial advantage of RFID systems is the non-contact, non-line-of-sight capability of the technology. The interrogator
12
emits the interrogation signal
20
with a range from one inch to one hundred feet or more, depending upon its power output and the radio frequency used. Tags can be read through a variety of substances such as odor, 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.
A typical RF tag system
10
often contains a number of RF tags
14
and the interrogator
12
. RF tags are divided into three main categories. These categories are beam-powered passive tags, battery-powered semi-passive tags, and active tags. Each operates in fundamentally different ways.
The beam-powered RF tag is often referred to as a passive device because it derives the energy needed for its operation from the interrogation signal beamed at it. The tag rectifies the field and changes the reflective characteristics of the tag itself, creating a change in reflectivity that is seen at the interrogator. A battery-powered semi-passive RF 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 RF tag, a transmitter is used to create its own radio frequency energy powered by the battery.
The range of communication for such tags varies according to the transmission power of the interrogator
12
and the RF tag
14
. Battery-powered tags operating at 2,450 MHz have traditionally been limited to less than ten meters in range. However, devices with sufficient power can reach up to 200 meters in range, depending on the frequency and environmental characteristics.
Conventional continuous wave backscatter RF tag systems utilizing passive (no battery) RF tags require adequate power from the interrogation signal
20
to power the internal circuitry in the RF tag
14
used to amplitude-modulate the response signal
22
back to the interrogator
12
. While this is successful for tags that are located in close proximity to an interrogator
12
, for example less than three meters, this may be insufficient range for some applications, for example, which require greater than 100 meters.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a radio frequency identification system having a radio frequency transceiver for generating a continuous wave RF interrogation signal that impinges upon a RF identification tag. An oscillation circuit in the RF identification tag modulates the interrogation signal with a subcarrier of a predetermined frequency and modulates the frequency-modulated signal back to the transmitting interrogator. The interrogator recovers and analyzes the subcarrier signal and determines its frequency. According to one aspect of the invention, the interrogator generates an output indicative of the frequency of the subcarrier frequency, thereby identifying the responding RF identification tag as one of a “class” of RF identification tags configured to respond with a subcarrier signal of a predetermined frequency.
According to various aspects of the invention, the RF identification tag includes a RF antenna configured to receive the interrogation signal, a rectifier having an input coupled to the radio frequency antenna and an output, and a tag oscillator having an input coupled to the output of the rectifier and an output coupled to an input of the RF antenna. The tag oscillator generates an output signal modulating the RF interrogation signal with the subcarrier RF signal.
According to one aspect of the invention, the RF identification tag oscillator further is either a single-transistor oscillator or a uni-junction transistor oscillator.
According to still other aspects of the invention, the tag oscillator in the RF identification tag is activated by the RF interrogation signal received at the tag antenna. According to various aspects of the invention, the RF tag system is a “passive” system, wherein the tag oscillator is powered by a radio frequency signal received at the antenna. Alternatively, the RF identification tag is either a battery-powered semi-passive tag, or an active tag.
According to another aspect of the invention, the RF transceiver includes a radio frequency oscillator generating a RF interrogation signal, an antenna coupled to the radio frequency oscillator, and a detector coupled between the RF oscillator and the antenna for recovering a subcarrier RF signal from the interrogation signal.
According to various other aspects of the invention, the interrogator includes a RF antenna configured to transmit and receive RF signals with a RF oscillator coupled to the antenna by a strip line. The RF oscillator is configured to generate a continuous wave RF interrogation signal, and means are included for recovering the subcarrier frequency generated by the tag oscillator from the frequency-modulated interrogation signal. According to one aspect of the invention, the subcarrier recovery means
Anderson Gordon A.
Gilbert Ronald W.
Steele Kerry D.
Bangachon William
Battelle Memorial Institute Kl-53
Horabik Michael
Seed IP Law Group PLLC
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