Patent Application
20060274857
The present invention relates generally to radio frequency identification (RFID). The present invention relates more particularly to an RFID receiver that is configured to perform digital down conversion on a radio frequency signal from an RFID tag.
RFID tags for identifying goods are well known. They can be applied to either the goods themselves or to packaging for the goods. RFID tags typically comprise an integrated circuit or chip and an antenna coupled to the chip. Information regarding the goods is stored upon the chip. For example, this information can include identification information, manufacturing information (such as what manufacturing processes have already been performed and/or what manufacturing process is to be performed next), customer information, and/or shipping information. RFID tags can be formed upon adhesive labels to facilitate their application to the goods or packaging.
The antenna typically comprises a plurality of conductive traces formed upon a substrate, such as the label. The antenna facilitates communication between the chip and an RFID reader and/or writer. Information is first programmed onto the chip with an RFID writer. Information is subsequently read from an RFID chip by interrogating the chip with a reader. The reader transmits an interrogation signal that is picked up by the antenna and is then communicated to the chip. The chip subsequently responds by communicating an information signal to the antenna that is then backscattered to the RFID reader.
The information signal can be read by either a hand-held RFID reader or a stationary RFID reader. Hand-held RFID readers can be used in warehouses, for example. In a warehouse, an RFID reader can be used to locate a desired item (having an RFID tag) from among many similar items.
Stationary RFID readers can be used in manufacturing processes. For example, they can be used to determine what manufacturing process is to be performed next on an item passing along a conveyor. Thus, an RFID reader can be used to determine what color an item is to be painted or what accessories are to be added thereto. Such information can be used to determine where in a manufacturing plant the item goes next and thus facilitates the automation of manufacturing processes.
Stationary RFID readers can also be used to verify RFID tags. That is, RFID tags can be interrogated by an RFID verifier to verify their functionality and/or content. Proper functionality may be verified as part of the process for manufacturing RFID tags. Content verification can be performed after an RFID tag has been programmed.
One problem with contemporary RFID readers, whether hand-held or stationary, is that they tend to be costly. One reason that RFID readers tend to be costly is that the receivers thereof use a number of analog components. An analog down converter is one example of such an analog component. Since analog components tend to increase the costs of RFID readers, it is often desirable to replace such components with digital processing devices.
Another problem with RFID applications is that the carrier frequency can be different in other countries. Thus, the frequency to be used by RFID tag verifiers can be country dependent. Knowing the carrier frequency of the tag being read is necessary to insure that regulatory guidelines are not violated by the verification device.
In addition to regulatory compliance concerns, there is also the issue of cross channel separation. In order for an RFID tag that is not being verified not to interfere with another RFID tag's transmitted signals and thus be misinterpreted by other readers in the proximity of the verifications device, the verifier needs to know the exact frequency of the carriers of the existing systems prior to illuminating the verification device's own field. Therefore, it can be important to know the carrier frequency of the tag being read. This is particularly true in situations where nearby tags of a plurality of different frequencies are transmitting at the same time.
This may be the case, for example, in an RFID tag manufacturing facility where there are several adjacent test lines for RFID tag verification. If the channel frequencies of the environment tag is known, then transmissions from other reader/transmitters on adjacent bands and adjacent channel frequencies can be filtered out, and not interfere with the verification reading of the target tag. Thus, it is desirable to be able to determine the carrier frequency of an RFID system environment.
Another problem is being able to remove adjacent bands during downconversion using standard digital filtering methods. Although an antenna has some frequency selectivity associated, it is often not enough rejection to allow for the high signal to noise ration required by a verification system. If ranging or backscatter intensity measurements are desired, it is necessary to maximize the signal to noise ratio. Knowing the specific frequency that is in the “field of view” of the verification antenna, will allow the software to determine which frequencies to filter out. It is generally not enough to only bandpass filter the frequency of interest in an undersampled system.
Systems and methods are disclosed herein to provide a receiver for a radio frequency identification (RFID) reader that uses digital down conversion to facilitate determination of the frequency of a radio frequency signal from an RFID tag and/or to facilitate demodulation of the signal. Undersampling can be used to effect such digital down conversion. By undersampling at pairs of nearby points in a waveform, frequency determination can be effected.
More particularly, in accordance with one embodiment of the present invention, two analog-to-digital converters can be configured so as to effect undersampling of a signal from an RFID tag. Sampling with the two analog-to-digital converters can be clocked such that nearby pairs of points define samples that can be used to determine the frequency of the radio frequency signal. The digital signal resulting from undersampling by one (or optionally both) of the two analog-to-digital-converters defines a frequency down converted signal that can be used for demodulation.
In accordance with one embodiment of the present invention, at least one low noise amplifier receives a radio frequency signal from an RFID tag. Two analog-to-digital converters receive an amplified radio frequency signal from the low noise amplifier(s). A clock provides a timing signal to each of two delays. Each delay provides a delayed clock signal to a dedicated one of the two analog-to-digital converters. The two delayed clock signals are offset in time with respect to one another to facilitate the sampling of pairs of nearby points of the radio frequency signal from the RFID tag.
Frequency down conversion facilitates the use of a field programmable gate array (FPGA) and/or a digital signal processor (DSP) for determination of the frequency and for demodulation, thus eliminating costly analog components while increasing the flexibility of the receiver.
The use of digital down conversion eliminates analog components. It also increases the flexibility of the receiver by more readily facilitating frequency determination and/or demodulation by digital circuitry that can be re-programmed or otherwise re-configured to accommodate different or addition desired functionality. For example, such digital circuitry can readily accommodate changes in the modulation method used.
This invention will be more fully understood in conjunction with the following detailed description taken together with the following drawings.
Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.
An RFID receiver uses direct digital down conversion to facilitate determination of the frequency of a radio frequency signal from an RFID tag and to facilitate demodulation of the signal. The receiver can be used in either an RFID tag verifier or an RFID reader. RFID tag verifiers are used to check the operability and programming of RFID tags, typically as part of the manufacturing process thereof. RFID readers are used to scan products, such as in retail stores and warehouses.
According to one embodiment of the present invention, an analog-to-digital converter is configured so as to undersample a radio frequency signal from an RFID tag. Undersampling, as discussed in detail below, results in frequency down conversion of the RFID signal. As those skilled in the art will appreciate, down conversion provides a lower frequency signal that can more readily be processed.
According to one embodiment of the present invention, two analog-to-digital converters can be configured such that both undersample a signal from the RFID tag and such that samples are taken by both analog-to-digital converters at approximately the same, but different, times. The resulting pairs of adjacent samples can be used to determine the frequency of the radio frequency signal. The digital signal resulting from undersampling by one (or optionally both) of the two analog-to-digital converters defines a frequency down converted signal that can be used for demodulation.
Referring now to
According to one aspect of the present invention, down conversion of the carrier frequency is accomplished by undersampling a radio frequency signal from an RFID tag. Such undersampling may be performed at a frequency of approximately 40 MHz, for example. Due to the use of undersampling, samples 13 are not taken frequently enough to accurately define the sampled radio frequency RFID tag signal. Rather, the samples are taken at well below the Nyquist rate and thus result in aliasing.
This aliasing provides a down converted representation of the RFID tag signal that is suitable for demodulation. The down converted representation is a lower frequency signal or down converted signal 14. More particularly, down converted signal 14 can be demodulated using OOK demodulation techniques to obtain digital data therefrom. Down converted signal 14 can similarly be demodulated using other demodulation techniques, such as BPSK, if desired.
Referring now to
It is sometimes necessary to determine the frequency of an RFID tag's output. This may be the case, for example, when RFID tag verifiers have been provided to RFID tag manufacturers in a plurality of different countries, wherein the output frequencies of the tags is different in each country. Rather than rely upon an operator to know and correctly enter the RFID tag frequency, it is advantageous to automatically sense the frequency of the RFID tag's output signal.
According to one aspect of the present invention, the frequency of an RFID tag signal 10 can be determined by repeatedly undersampling pairs 21 of nearby points on the carrier waveform that are very close to one another in time. The sample points of such sample pairs 21 can be between approximately 5 pSec and 100 pSec apart from one another for a carrier frequency of approximately 900 MHz, with higher resolution typically being used at higher frequencies, such as 30 pSec for 2.4 GHz and 5 pSec at 24 GHz, for example.
The slope of a line joining the two points of at least one of such pairs 21 can be used to determine the frequency of the carrier of an RFID tag. The slope of the line having the greatest slope of all such lines is proportional to the frequency of the carrier signal. For example, line 22 is the line between adjacent sample points of
Referring now to
Two exemplary embodiments of the present invention are illustrated in
Referring now to
A clock provides a timing signal to each of two different delays 43 and 44. Delays 43 and 44, in combination with the inherent propagation delays of other circuitry though which the clock signals are communicated, provide delays that result in the phase difference shown in
The output of analog-to-digital converter 45 is a series of digital signals corresponding to amplitudes of the RFID signal at periodic points in time, as best indicated by sample points 13 in
The down converted signal 14 can be used for demodulation of an OOK modulated carrier since the down converted signal 14 has at least some amplitude in the same places that the carrier signal 10 has amplitude (at the pulses) and goes to zero in the same places as does the carrier signal 10. Thus, where there are pulses 11 in the carrier signal 10, there will be pulses in the lower frequency down converted signal 14 and where there are non-pulse regions 12 in the carrier signal, there will be non-pulse areas in the down converted signal 14.
A field programmable gate array (FPGA), a digital signal processor (DSP), or a combination thereof 47 can be used to perform demodulation and/or frequency determination. The use of such digital processing provides enhanced flexibility. For example, the use of such digital processing makes conversion to a different modulation method simpler. Thus, demodulation by BPSK instead of OOK can be accomplished simply by the reprogramming or reconfiguration of the FPGA and/or DSP, rather than necessitating the replacement of components of the receiver.
Referring now to
Thus, according to at least one aspect of the present invention, the use of costly analog components is mitigated by using digital down conversion and/or frequency determination. Further, the size and weight of readers utilizing a receiver according to the present invention is reduced. The use of a field programmable gate array (FPGA) and/or a digital signal processor (DSP) for determination of the frequency and demodulation increases the flexibility of the receiver.
Embodiments described above illustrate, but do not limit, the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.
