Abstract:
A SAW RFID tag or sensor has an antenna for receiving and propagating an RF signal, an input/output IDT electrically connected to the antenna, and a dual track reflective IDT having a first track and a second track located adjacent and acoustically coupled to the input/output IDT. An RF signal received from the antenna by the input/output IDT is transformed by the input/output IDT into an acoustic way which is propagated to the dual track IDT. One track of the dual track IDT is in-phase with the phase reference of the input/output IDT and the other track is in quadrature phase with said phase reference. The input/output IDT receives two orthogonal complex modulated acoustic waves reflected from the first and second tracks and transforms them to a modulated orthogonal complex RF signal which is propagated from the antenna.

Description:
RELATED APPLICATION 
   This application claims priority from U.S. Provisional Patent Application No. 60/370,207 filed Apr. 8, 2002. 

   FIELD OF INVENTION 
   This invention relates to surface acoustic wave radio frequency identification devices or sensors. 
   BACKGROUND OF INVENTION 
   Surface acoustic wave (SAW) devices have been regularly used for the passive retrieval of information via a wireless retransmission of interrogative radio frequency (RF) signals. This information could be in the form of a modulated code embossed into the geometry of the device for use as identification device (RFID) tags or information pertaining to a sensor that is attached as a load impedance. These thin film devices require no battery for certain operations as they selectively reflect an interrogating RF pulse back to an interrogating transceiver with the information adapted into the RF signal. 
   A SAW RFID tag or sensor typically has a suitable antenna attached to the input/output inter-digital transducer (IDT) of the SAW device and receives an RF pulse sent from the interrogation transceiver located either line-of-sight or non-line-of-sight a few metres away. This RF pulse is transformed by the IDT into an acoustic wave and propagates towards other IDTs adjacent to the originating IDT placed on the same piezoelectric substrate. The other adjacent reflective IDT is then configured either as a modulated structure for an RFID application or has a measuring load attached for an application as a sensor. 
   SAW RFID tags are ideally suited for people or asset tracking. The tags are inconspicuously small, require no battery for certain operations and can be easily read by interrogating circuitry at distances of a few metres in either a line-of-sight or non-line-of-sight path. Generally 32, 64 or even 96 bits are encoded into the geometries of these devices to provide unique identification for up to billions of items. Low cost is one of the major parameters associated with RFID tags, as most of them will be disposable and non-recoverable. 
   SAW sensors have a wide range of applications which include automotive (torque, proximity radar and tire pressure), medical (biosensors) and industrial (temperature, pressure, humidity mass and vapour) applications. 
   SUMMARY OF INVENTION 
   According to the present invention, at least one of the adjacent reflective IDTs is divided into two separate adjacent tracks with the positioning of the IDT geometry such that the reflected signals from these two tracks are offset by 90°. This permits the two reflected signals to be orthogonal or in quadrature with each other. Signals that are in quadrature do not interfere with each other, as they are two independent components of a composite signal. 
   An advantage of this invention is that each track can therefore have independent information associated with it. The two independent tracks allow for a either a reference or second measuring signal to be orthogonally sent along with the first measuring signal to be independently separated at the interrogating transceiver. The reference signal is used to negate anomalies of a first measuring sensor. A second sensor could be totally independent or be configured as a differential pair, measuring the same parameter, with the first sensor. 
   For RFID tag applications, a complex 32-bit code can be derived by combining the 16 bits of data from the in-phase track with the orthogonal 16 bits of data from the quadrature-phase track. This would reduce the total length and cost of the device yet improve the read times. For sensor applications, a measuring sensor could be attached as a load to one track and either a second measuring sensor or reference load be attached to the other track. This would assist in tracking anomalies such as temperature drift or wireless propagation related errors. The second sensor could be configured to measure a differential signal with respect to the first sensor. 
   SAW RFID tags used for people and asset tracking are one of the numerous techniques that this invention will assist. The SAW RFID tag may replace the optical scanning bar code for the identification of items. Two independent tracks sending modulated coded orthogonal complex signals back to the interrogation transceiver simultaneously would improve the total capacity of the system such as having two 16-bit codes in the same IDT length as a single 16-bit code. The speed of the bits arriving back to the interrogation transceiver would be twice as fast as the latter, thereby improving read times for longer codes. 
   Similarly, SAW sensors for the wireless monitoring of various sensor applications will have the option of either a reference load or a second measuring signal be attached to the second track. These two sensors attached to the same SAW device on different signal orthogonal tracks would greatly reduce the cost of the monitoring project. 
   Such dual track SAW RFID tags and sensors may be typically used in but not limited to the frequency bands of 400 MHz to &gt;5 GHz depending on the application of harmonic techniques. They are very well suited for the newer Ultra Wide Band (UWB) frequency allocations. 

   
     DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: 
       FIG. 1  is a schematic view of a prior art QPSK combining circuit, 
       FIG. 2  is a similar view of a prior art SAW sensor, 
       FIG. 3  is a similar view of a SAW RFID tag in accordance with one embodiment of the invention, 
       FIG. 4  is similar to  FIG. 3  but additionally depicts the 45° offset within the split finger placement of the adjacent in-phase (I) and quadrature-phase (Q) tracks, 
       FIG. 5  is a similar view of a prior art SAW tag, 
       FIG. 6  is a similar view of a SAW (RFID) tag in accordance with the invention, 
       FIG. 7  is a similar view showing how a signal from a SAW device in accordance with the invention can be demodulated at the sensor, 
       FIG. 8  is a similar view showing how an offset in time can be created between the I and Q information streams, and 
       FIG. 9  shows how the principle shown in  FIG. 8  can be incorporated in a SAW device in accordance with the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In communication systems, quaternary phase shift keying (QPSK) techniques have been used to encode two independent bits of information onto a single output signal.  FIG. 1  outlines a typical known QPSK combining circuit. The in-phase (I) information  107  acts as an input to a balanced modulator  112  which also receives a carrier from a local oscillator  120 . The output of the balanced modulator  112  is connected as an input to a linear summer  130 . The quadrature-phase (Q) information  105  acts as an input to a separate balanced modulator  110  which receives a 90 degree phase shifted carrier  125 . The output from the balanced modulator  110  is connected as a second input to the linear summer  130 . This process permits the two signals to be orthogonal, (i.e. in quadrature) with each other. Signals that are in quadrature do not interfere with each other, as they are two independent components of the composite signal  140 . The main advantage of I/Q modulation is the combining of the two signals and then later at the interrogation transceiver the splitting of the composite signal into its original two independent components. 
   Information from a measuring transducer can be wirelessly sent back to an interrogation transceiver by means of a known SAW sensor shown in  FIG. 2. A  SAW IDT structure is lithographically deposited on a suitable substrate  200 . The antenna  210  attached to an input/output IDT  220  receives an RF pulse  265  sent from an interrogation transceiver  260 . The RF pulse  265  is transformed by the input/output IDT  220  to an acoustic wave which propagates toward the adjacent reflective IDT  230 . This second IDT is then configured such that the measuring transducer acts as a load admittance  250  to the IDT. The external transducer&#39;s admittance  250  will change with respect to what particular parameter (temperature, pressure, etc.) being measured. A change in admittance will affect the acoustic reflection  240  characteristics (p) of the IDT  230 . 
   SAW IDT structures in their simplest form have alternating fingers attached to either a “+ve” bus bar or a “−ve” bus bar. The fingers are normally a quarter acoustic wavelength in width at the IDT design frequency with an equal space between each alternate finger. More complex IDTs which result in improved signal characteristics use split-finger geometry. Each finger width and adjacent space is approximately an eighth-wavelength in width. The P-matrix notation for the reflection of a split finger transducer, which is terminated by a load admittance Y L  is, 
                 P   11     ⁡     (     Y   L     )       =       P     11   ,   sc       +       2   ⁢     P   13   2           P   33     +     Y   L                   (   1   )             
 
   For a split finger transducer which is connected to a load Y L , when Y L =0 (open circuit) then IDT achieves maximum reflection and when Y L =∞ (short circuit) then a minimum reflection occurs within the IDT. 
   The I/Q technique shown previously in  FIG. 1  can be applied to a dual track SAW sensor in accordance with one embodiment of the invention as illustrated in  FIG. 3. A  SAW IDT structure is lithographically constructed on to a suitable substrate  300 . The antenna  310  which is attached to the input/output IDT  320  receives an RF pulse  265  from the interrogation transceiver  260 . The RF pulse  265  is transformed by the input/output IDT  320  to an acoustic wave which propagates toward a dual track reflective IDT  330 . The lower track functions in a similar fashion to the SAW sensor device of  FIG. 2  in that the measuring transducer acts as a load admittance Y L    350  to the IDT. The external transducer&#39;s admittance will change with respect to which particular parameter (temperature, pressure, etc.) being measured. A change in admittance will affect the acoustic wave reflection  345  characteristics (pQ) of the lower track reflective IDT. The upper track also functions in a similar manner to the SAW sensor device of  FIG. 2  in that a reference or second load admittance  355  will affect the acoustic wave reflection  340  characteristics (pI) of the upper track reflective IDT. The composite signal reflected back to the input/output IDT would then be, 
                 P     11   ⁢   comp       ⁡     (       Y   L     ,     Y   ref       )       =       P   ⁢           ⁢     Q     11   ,   sc         +       2   ⁢   P   ⁢           ⁢     Q   13   2           P   ⁢           ⁢     Q   33       +     Y   L         +     P   ⁢           ⁢     I     11   ,   sc         +       2   ⁢   P   ⁢           ⁢     I   13   2           P   ⁢           ⁢     I   33       +     Y   ref                   (   2   )             
 
where the I channel (upper track is terminated in a load admittance Y ref  and the Q channel (lower track) is terminated with a load admittance Y L .
 
   The two orthogonal acoustic wave reflections  340  and  345  recombine at the input/output IDT  320  to form a composite signal. A conversion from acoustic energy to RF energy takes place in the input/output IDT  320  to radiate out through the antenna  310  in the form of a modified RF pulse back to the interrogation transceiver  260 . 
     FIG. 4  shows in more detail the 45° offset  435  within the split finger placement of the adjacent I and Q tracks which produces the overall 90° difference  448  between the two signals, placing them in phase quadrature with respect to each other. 
   A prior art single track SAW RFID tag is illustrated in  FIG. 5. A  SAW IDT structure is again lithographically constructed on a suitable substrate  500 . The antenna  510  which is attached to the input/output IDT  520  receives an RF pulse  265  sent from an interrogation transceiver  560 . The RF pulse  265  is transformed by the input/output IDT  520  to an acoustic wave which propagates toward a coded reflecting IDT  530 . Since the load Y L =0 ( 550 ) there will be a maximum reflection from equation (1). The reflected acoustic wave  540  returning from the coded reflecting IDT  530  will have the code associated with it. For this example, the binary code could be 11011. 
   A dual track SAW RFID tag in accordance with another embodiment of the invention is illustrated in  FIG. 6. A  SAW IDT structure is again lithographically constructed on a suitable substrate  600 . The antenna  610  which is attached to the input/output IDT  620  receives an RF pulse  265  sent from the interrogation transceiver  260 . This RF pulse  265  is transformed by the input/output IDT  620  to an acoustic wave which propagates toward a dual tack coded reflective IDT  630  and  635 . Since the load Y L =0 ( 650 ) for the lower track reflective IDT  630  there will be a maximum reflection from equation (1). Similarly, the load Y L =0 ( 655 ) for the upper track reflective IDT  635  and there will also be a maximum reflection from equation (1). The reflected acoustic waves  640  and  645  returning from their coded reflecting IDTs  635  and  630  respectively will each have unique codes independently associated with them. For this example the in-phase code of the upper track  635  may be 1010 and the quadrature-phase code of the lower track  630  may be 1101. 
   The interrogation transceiver may consist, in part, of a quadrature demodulator illustrated in FIG.  7 . An orthogonal complex signal returning  700  from the interrogated SAW device, being either an RFID tag or sensor, would enter the receiver&#39;s front end  705  where, once arrived, it would be suitably filtered and amplified. From the front-end, it would be split  710  and equal portions of the signal would proceed to the two balanced modulators  730  and  740 . For the lower branch of  FIG. 7 , a carrier recovery source  720  is multiplied with the split orthogonal complex signal within the balanced modulator  740 . This signal is then filtered  745  to produce demodulated in-phase ( 1 ) information  760 . Similarly, for the upper branch of  FIG. 7 , a carrier recovery source  720  is passed through a 90° phase shifter  725  and then multiplied with the other split orthogonal complex signal within the balanced modulator  730 . This signal is then filtered  735  to produce demodulated quadrature-phase (Q) information  750 . 
   Another aspect of this invention is the ability to create an offset in time between the I and Q information streams. Offset QPSK (OQPSK) is a modified version of QPSK shown previously in  FIG. 1 , where the I and Q bits are offset or shifted in phase by one-half of a bit.  FIG. 8  illustrates where half-bit delay  807  is placed in one of the data arms. All other components (with the exception of the delay) are identical with those of FIG.  1 . 
   A similar technique can then be applied to a SAW RFID tag or SAW sensor employing a dual track second IDT in accordance with the invention.  FIG. 9  schematically outlines how this “half-bit” offset can be implemented into the SAW structure. Note the similarity between the offset IDT of FIG.  9  and previous  FIG. 4  where there is a 45° offset in finger placement between adjacent tracks of the second IDT  435  to achieve an overall reflective displacement of 90° between the two tracks  448 . The offset of  FIG. 9   935  incorporates both the 45° finger offset along with a nλ offset to effect an offset ranging from 0 when n=0 to a half bit when n=one-half of the equivalent integer acoustic wavelengths which constitute the length of the interrogation pulse of one bit duration. This offset would be beneficial to avoid phase ambiguity problems, as a predetermined protocol would place the I information ahead of the offset Q information for more precise detection schemes. 
   Other embodiments and advantages of the invention will now be readily apparent to a person skilled in the art from the foregoing description, a scope of the invention being defined in the appended claims.