Architecture for RFID tag reader/writer

[0001] This application is a national stage application under 35 USC §371(c) of PCT Application No. PCT/SG2005/000047, entitled “ARCHITECTURE FOR RFID READER/WRITER,” filed Feb. 18, 2005, which is hereby incorporated by reference herein.

[0002] The present invention relates to radio frequency identification circuitry, and more particularly, to transceiver architectures for radio frequency identification systems.

[0003] Radio frequency identification (RFID) systems are wireless telemetry systems, whereby a transponder or RFID “tag” communicates information to an interrogation transceiver or RFID “reader.” The information provided by the RFID tag may be location information, the tag's identity, or other data. Continued improvements in circuit miniaturization have resulted in very small RFID tags that can be unobtrusively placed on an item which is to provide the aforementioned telemetry. The RFID tag communicates the information to the RFID reader when interrogated, or otherwise activated to do so.

[0004] FIG. 1 illustrates a simplified block diagram of an RFID system. The system includes an RFID reader 110 and one or more RFID tags 120, between which RFID signals 102 are communicated. The RFID reader 110 operates as a transceiver whereby it transmits an interrogation signal 102a, and receives signal responses 102b from one or more of the RFID tags. The RFID reader 110 is shown with a single transmit/receive antenna, but may include different transmit and receive antennas (or more than one of each) to more effectively communicate with each of the RFID tags 120. The RFID reader 110 may be fixed, or handheld to permit mobile scanning and interrogation of an item. The RFID tags 120 may be battery-powered or passive, in which case the RFID tag obtains power from the transmitted signal 102 for its operation. The RFID tag 120 includes an antenna that may be in the printed form to maintain the tag's small footprint. The system may employ frequency or signal polarization diversity or anti-collision protocols to permit greater communication ability.

[0005] A central component of the RFID system is the transceiver or RFID reader 110. Conventionally, the receiver portion of the RFID reader 110 has been designed using either a homodyne mixing approach or a superheterodyne mixing approach. The homodyne mixing approach provides advantages in that the circuit architecture is relatively simple and generally power efficient. The difficulties with such a direct conversion receiver are well known and include dynamically varying DC offset level, I/Q mismatch, and erroneous baseband signals generated by the cross-modulation of the RF signal (i.e., the received RFID signal) and the LO signal (the internally generated reference signal).

[0006] The superheterodyne mixing approach provides some relief to the aforementioned obstacles, but at significant expense. The superheterodyne design provides relatively good image rejection using an image rejection mixer and corresponding frequency synthesizer. The disadvantage of such a system is that it creates a requirement for an additional frequency synthesizer, as the transmitting and receiving channels typically require their own frequency source in this design. The presence of both frequency synthesizers decreases available chip real estate and drains much needed power.

[0007] Accordingly, what is needed is an improved transceiver architecture which provides good image rejection through the implementation of fewer circuit components.

[0008] The present invention provides an improved RFID transceiver architecture which exhibits good image rejection while using a single frequency source common to the transceiver's transmitting and receiving channels. As a result, the RFID transceiver operates on less power and requires less IC real estate.

[0009] In an exemplary embodiment, an RFID transceiver includes a common frequency source, a receiving channel and a transmitting channel. The common frequency source is configured to provide a predefined intermediate frequency signal f_IF, and a predefined carrier frequency signal f_C. The RFID receiving channel includes a single sideband up-converting mixer having a first input coupled to receive the predefined intermediate frequency signal f_IF, a second input coupled to receive the predefined carrier frequency signal f_C, and an output, the SSB upconverting mixer operable to produce an upconverted signal at a frequency defined by f_C±f_IF. The RFID receiving channel further includes an image rejection downconverting mixer having a first input coupled to receive an RFID signal having a carrier frequency substantially at the predefined carrier frequency f_C, a second input coupled to receive the upconverted signal, and an output, the image rejection mixer operable to produce an output a downconverted signal at substantially the predefined intermediate frequency f_IF. The RFID transmitting channel includes a modulator operable to combine modulation data onto the predefined carrier frequency signal f_C.

[0010] These and other features of the invention will be better understood in view of the drawings and corresponding description as provided below.

[0014] FIG. 2 illustrates a block diagram of a RFID transceiver in accordance with one embodiment of the present invention. The transceiver 200 includes an antenna 210, a circulator 220, an RFID receiving channel 230, an RFID transmitting channel 240, and a common frequency source 250. The RFID receiving channel 230 includes a single sideband upconverting mixer 232, an image rejection down converting mixer 234, and a demodulator 236. The RFID transmitting channel 240 includes a modulator 244, which in a particular embodiment (indicated by the dashed lines) includes the common frequency source.

[0015] The antenna 210 is operable to transmit RFID signal 202, which may be in the form of amplitude shift-keyed (ASK), on-off keyed (OOK), frequency shift-keyed (FSK) formatted signals and to receive RFID signal 202, which may be in the form of amplitude shift-keyed (ASK), on-off keyed (OOK), frequency shift-keyed (FSK) or phase shift-keyed (PSK) formatted signals. Circulator 220 provides isolation between the transmitting and receiving channels 230 and 240 during respective transmit and receive operations. Common frequency source 250 receives a reference frequency signal f_REF, and therefrom provides a predefined intermediate frequency signal f_IF and a predefined carrier frequency signal f_C used in the transmitting and receiving operations. In an alternative embodiment, intermediate frequency signal f_IF is provided from an external source (f_IFext) In particular, the common frequency source includes a single phase-locked loop, an RF oscillator, crystal reference oscillator, reference divider, etc., further described below.

[0016] In a specific embodiment, the receiving channel 230 includes a single sideband upconversion (SSB UC) mixer 232 and an image rejection downconversion (IRM DC) mixer 234. The single sideband up-converting mixer 232 receives the intermediate and carrier frequency signals f_IF and f_C, respectively, and in response, generates an upconverted signal 233 operating at a frequency defined by f_c±f_IF, depending on sideband selection. The single sideband upconversion mixer architecture, as known to the skilled in the art, is operable to attenuate the signal level of the carrier frequency f_C, the intermediate frequency, f_IF, the unselected mixing product, harmonics and mixing products thereof greatly below the signal level of the output mixing product. For example, mixer signal rejection of the aforementioned signals may be on the order of 30-40 dB relative to the selected mixing product. The image rejection downconverting mixer 234 receives the RFID signal 202 (via the receive channel port 220a of the circulator 220) and the upconverted signal 233, and in response produces a frequency translated version of the RFID signal having frequency substantially at f_IF. Downconverted signal 235 is provided to demodulation circuitry 236 (which may be analog or digital) operable to extract the baseband information from the received RFID signal. The demodulator 236 may be an ASK, OOK, FSK, or PSK demodulator.

[0017] The RFID transmitting channel 240 includes a modulator 244, which may be formed as a part of the common frequency source when FSK modulation format is used, or as an independent modulator when ASK or OOK modulation is employed. Each embodiment is further described below.

[0018] Common frequency source 250 includes a first input 250a coupled to receive a reference frequency signal f_REF, a first output 250c for providing the predefined intermediate frequency signal f_IF, and a second output 250d for providing the predefined carrier frequency signal f_C. In a particular embodiment, the common frequency source 250 is a phase locked-loop frequency synthesizer operable to generate the predefined carrier frequency signal f_c using the RF oscillator and supplied reference frequency signal f_REF. In a further specific embodiment, the predefined intermediate frequency signal f_IF is the reference frequency signal itself. The common frequency source 250 may additionally operate as the transmitting channel modulator 244. In such an instance, the common frequency source 250 includes a second input 250b operable to receive modulation data 243 for modulating the frequency of the carrier signal f_C. The FSK modulated carrier signal 245 is supplied to the antenna from which it is transmitted to one or more RFID tags.

[0019] FIG. 3 illustrates a detailed system block diagram of a RFID transceiver in accordance with the present invention, with previously identified features retaining their reference numerals. The RFID receiving channel 230, in addition to the previously mentioned components, includes a front-end amplifier 331 (e.g., a low noise amplifier), a filter 332 (which may be implemented as a discrete component), first and second single pole-triple throw (SP3T) switches 333 and 334, a quadrature phase power divider 335, and a data slicer 336. LNA 331 is operable to amplify the received signal strength of the RFID signal to a desired power level with a minimum of added noise power. Filter 332, which in a particular embodiment is a bandpass filter centered around f_IF, is operable to provide further selectivity by attenuating closely occurring interferors. First and second SP3T switches 333 and 334 are operable to complete the signal path through the RFID receiving channel via one of three demodulators 236, illustrated as an FSK demodulator, an ASK/OOK demodulator, and a PSK demodulator. A quadrature phase power divider 335 (e.g., a polyphase filter) provides 0° and 90° versions of the IF frequency signal f_IF which are used within the demodulator bank to extract the RFID Tag data from the carrier frequency signal f_C. The demodulator output is supplied to the data slicer 336, which recovers the modulation data transmitted from the RFID Tag. The baseband processing circuitry (not shown in FIG. 3) performs further operations (decoding, etc.) to render the RFID Tag modulated data.

[0020] The RFID transmitting channel 240 includes a modulator 244, a preamplifier 342, and a power amplifier 344. Modulator 244 is configured to operate as an OOK/ASK modulator, whereby the on-off/amplitude state of the switches 244 changes responsive to the supplied modulation signal 243. Alternatively, the modulator 244 is implemented as a PLL (an exemplary embodiment of which is further detailed below) in direct-modulation for generating an FSK modulated signal, or further alternatively as a sigma-delta modulator based fractional-N PLL synthesizer to produce an FSK modulated signal. In both of the aforementioned cases the modulation data is supplied to the PLL synthesizer, which in turn produces an FSK modulated carrier signal responsive to the supplied modulation data.

[0021] The illustrated embodiment of the common frequency source includes a phase locked loop frequency synthesizer 350 having a reference divider 356, a phase/frequency detector of PLL 351, a loop filter 352, a voltage-controlled oscillator (VCO) 353 and a reference crystal oscillator 354. A reference frequency signal f_REF is generated by the reference crystal oscillator 354 and is supplied to the reference divider 356. The reference divider 356 is operable to reduce the frequency (divide) the supplied reference frequency by any desired factor, including no reduction, i.e., a division factor of 1. The reference divider 356 is operable to output a first divided reference signal to the phase/frequency detector (PFD) of PLL 351, a second divided reference signal to the SSB up-conversion mixer 232, and a third divided reference signal to the 0°/90° divider 335. In a specific embodiment of the invention, the first, second and third divided reference signals are at the same frequency, although in an alternative embodiment the first, second and/or third divided reference signals operate at different frequencies. The VCO control voltage output from PFD of PLL 351 is filtered in loop filter 352 which can be inside or outside the chip. The filtered VCO control signal is used as control voltage input to VCO 353 completing the loop. In a particular embodiment, the second and third divided reference signals supplying the SSB mixer 232 and the 0°/90° divider 335 operate at a predefined intermediate frequency signal f_IF. Alternatively, the predefined intermediate frequency signal f_IF is supplied from an external source (f_IFext).

[0022] The loop filter 352 receives the VCO control voltage and provides a filtered version thereof to the VCO 353. The VCO generates a predefined carrier signal f_C, which is substantially the frequency of the received RFID signal 202. The VCO also produces a phase/frequency feedback signal 355 (referred to as an error signal herein) for indicating the phase or frequency of the carrier reference signal f_C produced. The error signal 355 is supplied to the PFD in the PLL 351 which compares it with the reference signal, and adjusts the generated output voltage accordingly. A buffer amplifier 360 is operable to receive and amplify the carrier frequency signal f_C, as well as to provide load isolation. A differential to single-ended stage 362 (e.g., a balun or active differential to single-ended converter) is used to convert between the differential output of the buffer amplifier 360 and the single-ended input of the modulator 244.

[0023] In a specific embodiment, the modulator 244 includes the VCO 353. In such an instance, the VCO 353 includes an input for receiving FSK modulation data 243, whereby the frequency of the generated carrier frequency signal f_C is modulated. In such an embodiment, modulator 244 is disabled or omitted from the design. The RFID transceiver, as shown, further includes a modulation controller 364 for routing the modulation data 243 to either the VCO 353 (when FSK modulation is employed), or to the modulator 244 (when ASK or OOK modulation is used). Clock, data, and latch enable signals are carried via a three wire bus 366 to the phase locked loop, reference divider 356 and are operable to define the carrier frequency signal f_C in a particular embodiment.

[0024] While the above is a detailed description of the present invention, it is only exemplary and various modifications, alterations and equivalents may be employed in various apparati and processes described herein. Accordingly, the scope of the present invention is hereby defined by the metes and bounds of the following claims.