Abstract:
SAW devices such as interdigital transducers (IDTs) have been widely used in RADAR applications and as filters. An IDT produces a SAW when excited by a single electrical pulse and can be fabricated to embody a code, which code provides for a passive autocorrelation of a SAW input to the IDT and thereby lends itself to further application as a signal generator in a communication device. However, conventional SAW expanders and compressors typically have insertion losses greater than 20 dB, which can have a profound effect on an RF link budget in a communication system. Insertion losses are drastically reduced by using a single phase unidirectional transducer (SPUDT-type) instead of a conventional IDT as a SAW expander/compressor. A SPUDT-type reflects SAW components which are lost in conventional IDT designs so that a stronger SAW is directed toward a transmission element. In a receiver, a SPUDT-type directs a stronger SAW toward a compressor IDT. A communication system based on SPUDT-type techniques would be low-cost, low-power, small and simple alternative to known short range communications schemes, including for example the Bluetooth™ solution. Operation of a SAW system at a frequency of 2.4 GHz is contemplated.

Description:
REFERENCE TO RELATED PATENT  
       [0001]    This application claims priority from U.S. Ser. No. 60/209,153, filed on Jun. 2, 2000. This prior application, including the entire written description and drawing figures, is hereby incorporated into the present application by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to short range communications using surface acoustic wave (SAW) expanders and compressors.  
           [0004]    2. Description of the Prior Art  
           [0005]    SAW technology is well known for its excellent radio frequency (RF) performance, low cost and small size. SAW is a passive thin film technology that does not require any bias current in order to function. SAW expanders and compressors have been used in RADAR applications for many years.  
           [0006]    The basic “building block” of SAW expanders and compressors is the interdigital transducer (IDT) such as shown in FIG. 1. An IDT  10  is a series of thin metal strips or “fingers”  12  fabricated on a suitable piezoelectric substrate  14 . One set of fingers is connected to an input/output terminal  16 , while the opposite set of fingers is connected to another terminal  18 . In single-ended IDTs, terminal  18  is grounded. For differential input signals however, terminal  18  is a pulse input/output terminal. Spacing “W” between IDT segments is adjusted to conform to the desired chip period of the coded sequence. When excited by a narrow electric pulse at terminal  16 , the IDT generates a coded output SAW signal that propagates in both directions perpendicular to the fingers  12 . If a similarly coded SAW signal impinges on the fingers  12 , then an autocorrelation function is performed and a peak, with associated side lobes, is generated at terminal  16 . These abilities of SAW expanders and compressors are well known in the prior art, having been demonstrated for example in Edmonson, Campbell and Yuen, “Study of SAW Pulse Compression using 5×5 Barker Codes with Quadraphase IDT Geometries”, 1988  Ultrasonics Symposium Proceedings , Vol. 1, Oct. 2-5, 1988, pp. 219-222.  
           [0007]    Thus, the structure shown in FIG. 1 can operate as both a SAW expander, generating a SAW output from a single pulse input, and a SAW compressor, generating a single pulse or peak output from a SAW input. Terminal  16 , as well as terminal  18  in differential IDTs, is both a pulse input terminal and a pulse output terminal. Conversion of an output SAW into an electrical signal for further processing in conventional communications circuits and subsequent transmission through an antenna is accomplished by adding a transmit IDT  24 , aligned with the IDT  22 , as shown in FIG. 2. Both IDTs can be fabricated on the same substrate  14 . A SAW output from IDT  22  is converted into an electrical signal by TX IDT  24 . A SAW receiver would have the same structure as in FIG. 2. A signal input to a receive IDT from receiver processing circuitry would be converted to a SAW which is input to IDT  22 . Like the IDT  22 , the TX IDT  24  may be a differential IDT, wherein the grounded lower terminal would be a pulse output terminal.  
           [0008]    The geometry of adjacent IDT fingers  12  is shown in FIG. 3, where Tf is the width of a metallized finger  12  and Ts is the width of the space between the fingers  12 . In typical designs both Tf and Ts are equal to a quarter of a wavelength, λ/4. For example, for a typical SAW system operating in the Industrial, Scientific and Medical (ISM) band at 2.4 GHz the λ/4 dimension could be in the order of 0.425 microns, depending upon the substrate chosen.  
           [0009]    Previous SAW-based communications systems use lower frequency SAW expanders and compressors having larger and further spaced fingers in conjunction with a plurality of components such as mixers and local oscillators, as shown in FIG. 4. In the typical prior art communication system  30 , a lower frequency 266 MHz signal generated by transmit IDT  20  is up-converted in mixer  34 , which receives a 734 MHz signal from local oscillator  36 . The resulting output from mixer  34  is filtered in high pass filter  38  to generate a 1 GHz signal for transmission through antenna  40 . On the receive side, the process is reversed in antenna  42 , mixer  44 , low pass filter  46  and receive compressor IDT  20 ′. The TX and RX IDTs  20  and  20 ′ have the structure shown in FIG. 2. Undesirably, the mixers  34  and  44 , oscillator  36  and filters  38  and  46  from the communications system  30 , result in additional cost, power consumption, occupation in space and a much complex system than is desired for low-cost, low power, short range communication systems. Therefore, there remains a need in the art to reduce the number of components in such a communication system. Previously-reported designs of encoded IDT structures have employed split-electrodes of width λ/8 within each chip segment W to suppress spurious IDT finger reflections (See, for example, M. G. Holland and L. T. Claiborne, “Practical Surface Acoustic Wave Devices”,  Proceedings of the IEEE , Vol. 62, pp. 582-611, May 1974). In contrast, SPUDT-type reflection gratings can be placed judiciously to enhance spurious IDT finger reflections and thereby reduce device insertion loss. These SPUDT techniques have previously been applied to realize low-loss SAW filters, where all of the IDT segments within each section W of the structure have the same polarity (See, for example, 1) C. K. Campbell and C. B. Saw, “Analysis and Design of Low-loss SAW Filters using Single-Phase Unidirectional Transducers”,  IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control , Vol. UFFC-34, pp. 357-367, May 1987; and 2) C. Campbell,  Surface Acoustic Wave Devices and their Signal Processing Applications , Boston: Academic Press, 1989). These SPUDT-type enhancement techniques have not been previously applied to encoded IDTs.  
           [0010]    High-frequency communication techniques involving more conventional non-SAW based circuits and systems also exist. Bluetooth™ wireless technology is one such prior art example. Bluetooth is a de facto standard, as well as a specification for small-form factor, low-cost, short range radio links between mobile PCs, mobile phones and other portable wireless devices. The current Bluetooth short range communications specification operates in the 2.4 GHz (ISM) band; however, the Bluetooth standard in its current infancy undesirably involves high cost, substantial power consumption and relatively complex hardware.  
           [0011]    Although high frequency SAW expanders and compressors offer significant reductions in cost, power consumption, size and complexity over prior SAW-based and non-SAW based communications systems, conventional SAW expanders and compressors typically have insertion losses greater than 20 dB. This may affect the RF link budget of a communication system, as more gain would have to be designed into the system for satisfactory operation. Therefore, there remains a need for a SAW SPUDT-type expander and compressor that would improve the insertion loss and positively impact the RF link budget and complexity of the system and thereby make RF communications systems in which RF signals are generated and processed directly and solely by SAW expanders and compressors feasible.  
         SUMMARY OF THE INVENTION  
         [0012]    An object of the present invention is overcome at least some of the drawbacks of the prior art.  
           [0013]    Advantageously, the use of low loss SPUDT-type SAW devices as described in the present invention offers improved performance compared to conventional SAW devices in respect to generation of coded RF waveforms (expander) and the autocorrelation of coded RF waveforms (compressor) for communication systems.  
           [0014]    It is therefore an object of the invention to provide a low cost SPUDT-type SAW-based communication method and system. As an illustrative example of the cost reduction resulting from the present invention, SAW devices used for filtering at near-ISM band frequencies may cost approximately $1.00 each. In contrast, a comparable semiconductor Bluetooth solution may cost more than $10.00.  
           [0015]    It is a further object of the invention to provide SPUDT-type SAW-based transmit and receive units that are easily manufactured. The manufacturing required for the present invention allows for SAW fabrication that utilizes simple, single layer photolithographic techniques.  
           [0016]    Another object of the invention is to provide a low power SPUDT-type SAW solution for short range communications. The SPUDT uses passive thin film technology and requires only a pulse to excite and produce a coded RF waveform. Likewise it can perform an autocorrelation function passively. This compares to prior SAW techniques which require frequency conversion circuitry such as mixers, filters and oscillators, and the complex Bluetooth techniques that require separate receive, transmit and processing circuitry. In mobile communication environments, power consumption and size are of primary importance.  
           [0017]    A still further object of the invention is to provide a SPUDT-type SAW-based communication arrangement that occupies minimal space. A complete SAW package in accordance with the invention is in the order of 3 mm×3 mm.  
           [0018]    In an embodiment of the invention, a communication system comprises an expander SPUDT-type IDT which is configured to embody a code and thereby produces a coded SAW output when excited with an electric pulse, a transmit IDT positioned adjacent to the expander IDT and connected to an antenna, a receive IDT connected to the antenna, and a compressor SPUDT-type IDT, positioned adjacent to the receive IDT, which is configured to embody the code and thereby produces an electric pulse output when excited by a coded SAW input. A further aspect of the invention involves the use of a SPUDT-type IDT as the transmit IDT and the receive IDT.  
           [0019]    Wireless communication systems according to the invention may be installed in both a wireless mobile communication device and a wireless earpiece detachable therefrom, to provide for communication between the mobile device and the earpiece. In a further embodiment of the invention, a SAW-based wireless communication system is installed in a wireless mobile communication device, a wireless earpiece detachable therefrom and a holder for the mobile device connected to a personal computer (PC), to provide for communication between the device and the PC through the holder, the device and the earpiece, and the earpiece and the PC through the holder.  
           [0020]    The transmit IDT receives the coded SAW output from the expander IDT and produces a coded electric output signal for transmission via the antenna, and the receive IDT produces the coded SAW input to the compressor IDT from a coded electric signal received via the antenna. The electric input and output signals associated with any of the IDTs may be either unbalanced or differential signals.  
           [0021]    In another inventive aspect, a passive wireless communication system comprises an antenna for converting received communication signals into electric antenna output signals and for converting electric antenna input signals into communication signals, a first IDT connected to the antenna and configured to produce a SAW output when a communication signal is received by the antenna, a second coded SPUDT-type IDT positioned adjacent to the first IDT and configured to produce an electric signal output when excited by the SAW output from the first IDT and to produce a coded SAW output when excited by an electric signal input, and a termination circuit connected across the terminals of the second IDT, wherein the termination circuit causes the second IDT to reflect a coded SAW output toward the first IDT in response to the SAW output produced by the first IDT, and the first IDT produces a coded electric signal as an antenna input signal in response to the reflected coded SAW output from the second IDT. The termination circuit is preferably either a short circuit connection or an open circuit.  
           [0022]    Such a passive wireless communication system may include a SPUDT-type IDT as the first IDT. The first and second IDTs may also have differential electrical signal input and output terminals. Where the code embodied by the second IDT represents identification information or other information relating to an article with which the passive communication system is associated, the passive system may be incorporated into an identification tag, an adhesive label or an equipment nameplate. The passive wireless system preferably receives communication signals from a remote interrogation system, and through operation of the IDTs and termination circuit, automatically and passively responds to the remote interrogation system.  
           [0023]    The code embodied by an IDT may be a Barker code such as a 5-bit or 13-bit Barker code, and may be used for example to represent identification information for an article with which the wireless communication system is associated.  
           [0024]    According to a further aspect of the invention, a coded SPUDT-type IDT comprises a pair of substantially parallel electrically conductive rails, one or more groups of interdigital elements, each group comprising a plurality of interdigital elements, and one or more SAW reflectors. Each interdigital element is connected to one of the rails and extends substantially perpendicular thereto toward the other rail, and the particular code embodied by such an IDT is determined by a connection pattern of the interdigital elements in each group. A coded SPUDT-type compressor IDT performs a passive autocorrelation function on the coded SAW input based on the code to thereby produce the electric pulse output. A reflector in a SPUDT-type IDT may comprise a plurality of reflector grating elements, wherein the reflector grating elements of a reflector may be either open circuited or are connected to the other grating elements in the same reflector.  
           [0025]    A SPUDT-type SAW system according to the invention may be employed in the design of virtually any new short range wireless communication system, for example to enable communication between an earpiece unit and an associated mobile communications device. The inventive systems may also replace RF signal generation circuitry in existing short range communications system, including for example “Bluetooth” systems. A further system in accordance with the invention may be employed in “smart” identification tag systems and remote interrogation systems such as inventory systems and meter reading/telemetry systems.  
           [0026]    Further features of the invention will be described or will become apparent in the course of the following detailed description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    In order that the invention may be more clearly understood, preferred embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:  
         [0028]    [0028]FIG. 1 shows an IDT;  
         [0029]    [0029]FIG. 2 is a block diagram of a conventional SAW-based transmit IDT;  
         [0030]    [0030]FIG. 3 is an illustration of typical finger geometry in an IDT;  
         [0031]    [0031]FIG. 4 is a block diagram of a prior art SAW-based communication system;  
         [0032]    [0032]FIG. 5 is a SPUDT-type expander/compressor;  
         [0033]    [0033]FIG. 6 illustrates a first embodiment of the invention;  
         [0034]    [0034]FIG. 7 illustrates an autocorrelation function of a 5 bit Barker code;  
         [0035]    [0035]FIG. 8 is a differential implementation of the system of FIG. 6;  
         [0036]    [0036]FIG. 9 shows a variation of the first embodiment;  
         [0037]    [0037]FIG. 10 is a differential implementation of the system of FIG. 9;  
         [0038]    [0038]FIG. 11 represents a system in which the invention could be implemented;  
         [0039]    [0039]FIG. 12 illustrates a second embodiment of the invention;  
         [0040]    [0040]FIG. 13 is a differential implementation of the second embodiment;  
         [0041]    [0041]FIG. 14 is a variation of the second embodiment;  
         [0042]    [0042]FIG. 15 is a differential implementation of the system of FIG. 14; and  
         [0043]    [0043]FIG. 16 represents a block diagram of a system in which the second embodiment could be employed.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0044]    The configuration shown in FIG. 5 illustrates the layout geometry of an IDT using reflector gratings  52  to form a SPUDT  50 . The dimension for each finger  12 , space and reflector  52  is λ/4 (see for example C. K. CAMPBELL,  Surface Acoustic Wave Devices for Mobile and Wireless Communications , Boston, Mass.: Academic Press, 1998, Chapter 12). In FIG. 5 and subsequent drawings, the substrate  14  has been omitted for clarity, but it is to be understood that IDT structures may be fabricated on a common substrate.  
         [0045]    In comparison to the IDT  10  shown in FIG. 1, the SPUDT  50  operates in a similar fashion to produce a SAW in response to a pulse at the pulse input/output terminal. Each set of fingers  12  produces a SAW that propagates in both directions perpendicular to the fingers. However, the reflector gratings  52  in SPUDT  50  reflect any SAWs impinging thereon and propagating in a direction from the right hand side to the left hand side of FIG. 5 in the opposite direction, from left to right. The spacing of reflector gratings  52  and fingers  12  prevents destructive interference between the reflected waves and waves produced by the fingers  12  which propagate in the desired direction. For optimum performance, the SPUDT reflector gratings are placed judiciously with respect to adjacent IDTs. This placement selection is dependent on whether the reflection gratings are open or short circuited, as well as on the piezoelectric substrate type involved.  
         [0046]    Thus, the SPUDT  50  outputs a much higher power SAW propagating in the direction indicated by the arrow in FIG. 5. The “right to left” SAW produced by the leftmost set of fingers  12  in FIG. 5 is the only SAW component which does not contribute to the SAW output in the indicated direction. In the transmitter of FIG. 2, roughly half of the total signal power generated in the expander/compressor  22  propagates toward the TX IDT  24 . The upper terminal in FIG. 5 may be either grounded, in single-ended designs, or connected as a pulse input/output terminal in differential designs.  
         [0047]    In the SPUDT  50  shown in FIG. 5, each of the three groups of fingers includes four fingers. Starting at the left hand side of the Figure, the first finger of each of the first two sets is attached to the top rail, but in the third set, the first finger is attached to the bottom rail. This indicates a 180° phase shift as what is derived from a ++− configuration.  
         [0048]    According to a first embodiment of the invention as shown in FIG. 6, a SAW-based communication system  60  has a SPUDT-type expander  50   a , a transmit (TX) IDT  56 , a SPUDT-type compressor  50   b  and a receive (RX) IDT  64 . These structures are in-line with each other as shown in FIG. 6. As discussed above in relation to FIG. 2, these structures may be placed on a suitable piezoelectric substrate using thin film lithographic procedures.  
         [0049]    With the SPUDT-type expander  50   a , a directional acoustic wave propagates towards the TX IDT  56 . This will improve the insertion loss of the device, as any acoustic waves generated by the centre and rightmost groups of fingers propagating away from TX IDT  56 , which normally would be lost, are reflected back towards the TX IDT  56  by the reflection gratings in expander  50   a.    
         [0050]    A narrow pulse which represents digital data and can be generated by using simple digital circuitry or an existing data source is injected into the SPUDT  50   a  of FIG. 6 through pulse input and output terminal  54   a  to activate a piezoelectric effect that converts electrical to mechanical (acoustic wave) motion. The acoustic waves are coded depending on the geometry of the SPUDT  50   a . These acoustic waves then propagate within the substrate to the TX IDT  56 . The coded acoustic waves are then transformed to an electrical coded RF signal within the proximity of the TX IDT  56 . When the TX IDT  56  is attached to a suitable antenna  62  through the band pass filter  61 , the coded RF signal can propagate throughout the air. Feedback of the transmit signal to the RX IDT  64  through its antenna connection does not interfere with the SPUDT-type expander  50   a , since the RX IDT  64  is not aligned therewith. Pulse output  54   b  is not read during signal transmission to prevent erroneous data detection.  
         [0051]    A coded electrical signal that enters the RX IDT  64  via antenna  62  and band pass filter  61  generates an acoustic wave that propagates towards the SPUDT-type compressor  50   b . An autocorrelation function is passively performed in the SPUDT  50   b  and if the coded waveform from the RX IDT  64  matches with the code on the SPUDT  50   b , a peak is generated at the pulse output terminal  54   b . The reflection gratings in compressor  50   b  enhance conversion of the SAW from RX IDT  64  into electrical energy. Although a signal received at antenna  62  is split between the TX IDT  56  and the RX IDT  64 , the SAW generated at TX IDT  56  causes no interference with the receive compressor  50   b . Any pulse output on terminal  54   a  during a receive operation is ignored.  
         [0052]    The peak produced by the SPUDT-type compressor  50   b  can represent digital data. For example, in accordance with an on-off keying technique, following an intialization or synchronization sequence, the presence of a peak within a bit period may be interpreted as a ‘1’ data bit, whereas the absence of a peak would represent a ‘0’ bit.  
         [0053]    The coding of the SPUDT-type expander  50   a  and compressor  50   b  and the associated autocorrelation function performed by the SPUDT as discussed above are determined by the finger geometry of the SPUDT. A preferred coding scheme is a Barker code. Barker codes are particularly useful for IDT coding, since they minimize the energy in side lobes associated with a compressed pulse generated by the autocorrelation function performed on a SAW input to an expander/compressor IDT. The SPUDTs  50   a  and  50   b  as shown in FIG. 6 are coded with a ++− Barker code.  
         [0054]    [0054]FIG. 7 shows an example of the autocorrelation function relating to a 5 bit Barker code (+++−+) performed by an expander/compressor SPUDT or IDT when a received signal is converted to a SAW by a receive IDT such as RX IDT  64 . The autocorrelation function is mathematically equivalent to a series of shift and add operations as shown in FIG. 7 and generates the peak and associated side lobes shown at the bottom of FIG. 7 if the received signal was generated with an identically-coded SPUDT or IDT. The amplitude of the autocorrelation peak is proportional to the code length N, which is  5  in the example shown in FIG. 7, whereas the side lobes are amplitude  1 .  
         [0055]    In the system of FIG. 6, only the SPUDTs  50   a  and  50   b  must be coded. As discussed above, Barker codes are preferred. Since the amplitude of the autocorrelation peak generated when a received signal is compressed by a Barker-coded expander/compressor IDT is dependent on the length N of the Barker code, higher-length codes are most preferred. For example, the maximum length known Barker code with N=13 (+++++−−++−+−+) will generate an autocorrelation waveform similar to that shown in FIG. 7, but having a peak of amplitude  13  and additional side lobes with amplitude  1 .  
         [0056]    [0056]FIG. 8 shows a differential design of the system of FIG. 6. In a differential system, both the upper and lower sets of fingers in the TX IDT  56 ′ and RX IDT  64 ′ are connected to antenna  62 ′ and band pass filter  61 ′. As indicated by the multiple connections in FIG. 8, the filter  61 ′ and antenna  62 ′ must also be differential components. Expander SPUDT  50   a ′ and compressor SPUDT  50   b ′ may be single-ended, with terminals  55   a ′ and  55   b ′ grounded as shown in FIG. 6, or differential, wherein terminal  55   a ′ is a pulse input terminal and terminal  55   b ′ is a pulse output terminal. The differential system in FIG. 8 operates similarly to the system of FIG. 6, as will be apparent to those skilled in the art.  
         [0057]    Major advantages of a SPUDT over an IDT in a SAW-based communication system include the direction of significantly more of the acoustic waves generated by an expander toward the transmit element, TX IDT  56  for example, as well as improved conversion of input SAW mechanical energy into electrical energy. The SPUDT design technique can effect a significant reduction in device insertion loss and thereby enhance signal-processing performance. This same principle may also be exploited on the transmit and receive sides of a communication system, as shown in FIG. 9. In the system of FIG. 9, the transmit IDT and the receive IDT are SPUDT-type IDTs  72  and  74 , which include reflection gratings. TX SPUDT  72  improves conversion of the SAW output by expander SPUDT  50   a  to an electrical signal. Also, when a pulse is received through antenna  62  and filter  61 , the RX SPUDT  74  transmits a stronger SAW toward SPUDT-type compressor  50   b . FIG. 10 shows a differential implementation of the SPUDT-type system of FIG. 9.  
         [0058]    The inventive arrangements disclosed above can reduce the cost, power consumption, size and complexity of virtually any short range communications system. This SAW based technology will allow communication devices to be placed in power sensitive applications such as a wireless earpiece to allow the user a longer “talk-time” over Bluetooth devices.  
         [0059]    SPUDT-type SAW-based communication systems according to the first embodiment of the invention may be incorporated into short range communication systems, including any situation for which Bluetooth was designed.  
         [0060]    An illustrative example of a system into which the inventive wireless communication system could be incorporated is shown in FIG. 11, wherein  112  denotes an earpiece,  114  is a mobile wireless communication device and  116  is a holder or cradle for holding the device  114  and coupling device  114  to a personal computer (PC)  120 . In system  110 , the earpiece  112 , device  114  and cradle  116  incorporate a SAW communication device as disclosed above. This allows a user to communicate audibly between the wireless communication device  114 , which may for example be carried on their belt or person, and the wireless earpiece  112  with a built-in microphone, as indicated at  118   a  in FIG. 11. This system could be then expanded to include communication between the earpiece  112  and the personal computer  120 , as indicated by  118   b , when a SAW system in cradle  116  is attached to the PC via a bus connection. This system may then be further expanded to include network communications designated  118   c  in FIG. 11 between the wireless device  114  on the belt or person with the PC  120  to incorporate connectivity via small pico-cell networks. A further extension of the communication systems according to the first and second embodiments could be a personal area network (PAN) based on SAW technology rather than the more excessive Bluetooth strategy.  
         [0061]    In a second embodiment of the invention, the SPUDT design techniques discussed above are applied to passive SAW RF systems. In such systems, SAW devices usually perform only as RF expanders. As shown in FIG. 12, such a passive system  130  comprises two IDTs  122  and  124 . A pulse that has been sent out by a local requesting unit is received at the antenna  128  and excites IDT  122  to produce an acoustic wave. This wave then propagates to a SPUDT-type coded IDT  124  that has a suitable termination  126  connected across its terminals  132  and  134  to produce a reflection coefficient of magnitude  1 . Termination  126  could be an open or short circuit termination, which will re-excite the coded SPUDT-type IDT  124  to produce a coded acoustic wave back to the IDT  122  that is connected to the antenna  128 . The result is that an impulse sent out by a local requesting unit excites a coded SPUDT which then returns back to the requesting unit a coded RF waveform.  
         [0062]    At the requesting unit, autocorrelation of the coded waveform returned from the device  130  would preferably be performed by a DSP or other conventional signal processing circuitry, such that different codes can be used for different SPUDT-type IDTs such as IDT  124 . In order for the requesting unit to passively perform the autocorrelation, a separate coded IDT must be provided in the requesting unit for each different code embodied in all devices  130  with which communication is desired. This would severely limit the number of devices  130  that could be deployed.  
         [0063]    The size of the complete SAW device  130 , as discussed above, could be on the order of 3 mm square. This would allow the device to be incorporated into labels such as shipping or address labels, equipment nameplates, adhesive stickers such as vehicle license plate stickers and other forms of identification tags. The code embodied in the SPUDT  124  could for example be a code that provides information about an item to which the device  130  is attached. Device  130  could therefore be implemented in an identification or location system for example.  
         [0064]    Although IDT  122  in FIG. 12 is a single-ended IDT, a differential design is also contemplated, as shown in FIG. 13. In addition, the IDT connected to the antenna  128  could also be a SPUDT-type IDT as in FIGS. 14 and 15.  
         [0065]    [0065]FIG. 16 shows a system into which passive SAW RF devices according to the third embodiment of the invention could be implemented. A requesting unit  150 , which may for example be a hand-held unit with a display or part of a larger interrogation and tracking system, sends an RF pulse  144  to a label, tag or the like generally indicated at  160 . The tag  160  includes a SAW device  130 ,  130 ′,  140  or  140 ′ and may be attached to or placed on or inside an item. The coded return signal  146  generated by the SAW device in tag  160 , is received at the requesting device and processed to determine tag information. The information thus determined may for example be displayed to a user or operator of the requesting device  150 , forwarded from the requesting unit  150  to an information, tracking or billing system for further processing, or both.  
         [0066]    It will be appreciated that the above description relates to preferred embodiments by way of example only. Many variations on the invention will be obvious to those knowledgeable in the field, and such obvious variations are within the scope of the invention as described and claimed, whether or not expressly described.