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, internal dimensions of IDTs are inversely proportional to operating frequency, such that high frequency IDTs present significant manufacturing difficulties. Fabrication of IDTs for high frequency applications is simplified by exploiting a harmonic frequency SAW generated by IDTs. An IDT may therefore be designed according to fundamental frequency internal dimension criteria but can operate at a multiple of the fundamental frequency, thereby providing much higher frequency operation than conventional SAW systems. A communication system based on SAW harmonic 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 second harmonic SAW system at 2.4 GHz based on a fundamental frequency of 1.2 GHz is contemplated.

Description:
REFERENCE TO RELATED PATENT  
       [0001]    This application claims priority from U.S. Serial No. 60/209,152, filed Jun. 2, 2000. 
     
    
     
       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 pulse at terminal  16 , the IDT generates a coded output SAW which propagates in both directions perpendicular to the fingers  12 . If a similarly coded SAW 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, 2-5 October 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. Since wavelength is inversely proportional to frequency of operation, higher frequency IDTs require thinner fingers  12  located in close proximity to each other, which complicates fabrication and reduces fabrication yields. 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 communications system designs sought to overcome these manufacturing difficulties by using lower frequency SAW expanders and compressors having larger and further spaced fingers in conjunction with mixers and local oscillators, as shown in FIG. 4. In the typical prior art communication system  30 , the 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 output from mixer  34  is filtered in high pass filter  38  to produce a 1 GHz signal which is transmitted 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 ′. As discussed above, transmit IDT  20  and receive IDT  20 ′ have similar structure. 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.  
           [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, in reality the technology for mobile communication devices involves undesirable high cost, substantial power consumption and relatively complex hardware.  
         SUMMARY OF THE INVENTION  
         [0011]    It is an object of the present invention to overcome some of the drawbacks of the prior art.  
           [0012]    It is also an object of the present invention to provide a low cost SAW-based communication method and system. As an illustrative example of the cost reduction, SAW devices utilized by the present invention for filtering at near-ISM band frequencies may cost approximately $1.00 each. In contrast, a comparable semiconductor Bluetooth solution may cost greater than $10.00.  
           [0013]    It is a further object of the invention to provide SAW-based transmit and receive units which are easily manufactured. The manufacturing required for the present invention allows for SAW fabrication that utilizes simple, single layer photolithographic techniques.  
           [0014]    Another object of the invention is to provide a low power SAW solution for short range communications. The SAW uses passive thin film technology and requires only a pulse to excite and produce an 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.  
           [0015]    A still further object of the invention is to provide a SAW-based communication arrangement which occupies minimal space. A complete SAW package in accordance with the invention is in the order of 3 mm×3 mm.  
           [0016]    The inventive SAW system reduces manufacturing complexity and cost and increases production yields by exploiting second harmonic components produced by expander/compressor IDTs. This allows the IDTs to be fabricated with larger finger widths than would be required according to known IDT methods and devices.  
           [0017]    In the invention, a wireless communication system comprises an expander/compressor interdigital transducer (IDT) which produces a surface acoustic wave (SAW) output comprising frequency components at a fundamental frequency and a plurality of harmonic frequencies when excited with an electric input signal and produces an electric output signal when excited by a SAW input at the fundamental frequency or one or more of the plurality of harmonic frequencies, a transmit IDT positioned adjacent to the expander/compressor IDT and switchably connected to an antenna, and a receive IDT positioned adjacent to the expander/compressor IDT and switchably connected to the antenna, wherein the transmit IDT and the receive IDT are configured to operate at one of the harmonic frequencies.  
           [0018]    In accordance with another aspect of the invention, a communication system comprises an expander IDT configured to produce a SAW output having a fundamental frequency and a plurality of harmonic frequencies when excited with an electric input signal, a transmit IDT positioned adjacent to the expander IDT and connected to an antenna, a receive IDT connected to the antenna; and a compressor IDT positioned adjacent to the receive IDT and configured to produce an electric output signal when excited by a SAW input comprising the fundamental frequency or one or more the plurality of harmonic frequencies, wherein the transmit IDT and the receive IDT are configured to operate at one of the harmonic frequencies.  
           [0019]    The transmit IDT converts a SAW into an electric signal for transmission via the antenna and the receive IDT converts an electric signal received via the antenna into a SAW. The fundamental frequency may be 1.2 GHz and the transmit IDT and receive IDT operate at the second harmonic frequency of 2.4 GHz. 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 electric input and output signals associated with any of the IDTs may be either unbalanced or differential signals.  
           [0021]    An expander/compressor IDT, or an expander IDT and a compressor IDT are preferably configured to embody a code and thereby produce a coded SAW output when excited with an input electric pulse and an output electric pulse when excited by a coded SAW input. The code embodied by these IDTs may be a Barker code such as a 5-bit Barker code, and may be used for example to represent identification information for an article with which the wireless communication system is associated.  
           [0022]    According to a further aspect of the invention, a passive wireless communication system comprises an antenna for receiving communication signals and converting the received communication signals into electric antenna output signals and converting electric antenna input signals into output communication signals and transmitting the output communication signals, a first IDT connected to the antenna and configured to produce first SAW outputs in response to the electric antenna output signals and to produce the antenna input signals in response to first SAW inputs, a second IDT positioned adjacent to the first IDT and configured to produce a second SAW output comprising frequency components at a fundamental frequency and a plurality of harmonic frequencies when excited with an electric signal and to produce an electric signal output when excited by a second SAW input at the fundamental frequency or one or more of the plurality of harmonic frequencies, and a termination circuit connected to the second IDT, wherein the first IDT is configured to operate at one of the harmonic frequencies, the termination circuit causes the second IDT to reflect a second SAW output toward the first IDT in response to each first SAW output produced by the first IDT, and the first IDT produces an antenna input signal in response to each reflected second SAW output from the second IDT. The second IDT in such a passive system may be configured to embody a code.  
           [0023]    In a particular embodiment of this aspect of the invention, a passive wireless communication system further comprises a third IDT which is positioned between the first IDT and the second IDT and reflects a third SAW output toward the first IDT in response to the first SAW output produced by the first IDT, wherein the first IDT produces a second antenna input signal in response to the reflected second SAW output from the third IDT. The passive wireless communication system may also include a fourth IDT which is positioned adjacent to the second IDT on a side of the second IDT opposite to the third IDT and reflects a third SAW output toward the first IDT in response to the first SAW output produced by the first IDT, wherein the first IDT produces a third antenna input signal in response to the reflected third SAW output from the fourth IDT. When the passive wireless communication system includes a third IDT and/or fourth IDT, these IDTs are preferably configured to operate at one of the harmonic frequencies, which may be the same as the harmonic frequency at which the first IDT operates.  
           [0024]    The third and fourth IDTs, like the second IDT, may be connected to a respective termination circuit. A termination circuit is preferably either an open circuit, a short circuit or a sensor circuit. 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.  
           [0025]    An IDT which may be used in embodiments of the invention preferably comprises a pair of substantially parallel electrically conductive rails and one or more groups of interdigital elements, each group comprising a plurality of interdigital elements. Each interdigital element is connected to one of the rails and extends substantially perpendicular thereto toward the other rail. Any such IDT may be configured to embody a particular code as determined by a connection pattern of the interdigital elements in each group. A coded IDT performs a passive autocorrelation function on a SAW input based on the code to thereby produce an electric pulse output when the SAW input is similarly coded.  
           [0026]    A 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, as described briefly above and in further detail below. 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.  
           [0027]    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  
       [0028]    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:  
         [0029]    [0029]FIG. 1 shows an IDT;  
         [0030]    [0030]FIG. 2 is a block diagram of a conventional SAW-based transmit IDT;  
         [0031]    [0031]FIG. 3 is an illustration of typical finger geometry in an IDT;  
         [0032]    [0032]FIG. 4 is a block diagram of a prior art SAW-based communication system;  
         [0033]    [0033]FIG. 5 is an IDT adapted for second harmonic operation;  
         [0034]    [0034]FIG. 6 shows a representation of a first embodiment of the invention;  
         [0035]    [0035]FIG. 7 is a differential implementation of the first embodiment;  
         [0036]    [0036]FIG. 8 illustrates an autocorrelation function of a 5 bit Barker code;  
         [0037]    [0037]FIG. 9 is a second embodiment of the invention;  
         [0038]    [0038]FIG. 10 is a differential implementation of the second embodiment;  
         [0039]    [0039]FIG. 11 represents a system in which the invention could be implemented;  
         [0040]    [0040]FIG. 12 illustrates a third embodiment of the invention;  
         [0041]    [0041]FIG. 13 is a differential implementation of the third embodiment;  
         [0042]    [0042]FIG. 14 shows a variation of the third embodiment;  
         [0043]    [0043]FIG. 15 is a differential implementation of the system of FIG. 14; and  
         [0044]    [0044]FIG. 16 is a block diagram of a system in which the third embodiment could be employed. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0045]    As discussed above, the lithographic process to produce SAW devices at higher frequencies is difficult due to the very small finger width. At 2.4 GHz, the wavelength would be approximately 1.7 microns, requiring a finger width of 0.425 microns depending on the substrate chosen. This very small width will affect the overall yield of the fabrication process and will impact on the price of the devices.  
         [0046]    A solution to this problem would be to fabricate the device to operate at 1.2 GHz to produce a wavelength of 3.4 microns using unique finger geometry and then take advantage of the second harmonic that the device will support. This will allow for a more relaxed lithographic process and increase production yield, as the lines are not as thin and are spaced farther from each other.  
         [0047]    The use of second harmonic IDT geometries has been well know for several years, see for example CAMPBELL and EDMONSON, “Conductance Measurements on a Leaky SAW Harmonic One-Port Resonator”,  IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control,  Vol. 47, No. 1, January 2000, pp. 111-116, but has never been applied to expanders or compressors. FIG. 5 illustrates an example of the finger configuration for a second harmonic SAW device with 2 chips (+ and −). 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.  
         [0048]    As shown in FIG. 5, a so-called “three-finger” IDT, in which each of the four groups of fingers includes three fingers, is required for second harmonic operation. Corresponding fingers of each group are separated by a distance ‘a’ equal to fundamental wavelength λ 0 . Each finger and space in this three-finger IDT therefore has a width ‘b’ of λ 0 /6.  
         [0049]    The two double fingers in each group start out at the left hand side of the IDT attached to the top rail, but beyond the centre line they are attached to the bottom rail. This indicates a 180° phase shift as what is derived from a + and − configuration. As stated above, the finger and space width of the second harmonic IDT is λ 0 /6. For a 2.4 GHz second harmonic output, the fundamental frequency is 1.2 GHz, corresponding to a wavelength λ 0 =3.4 microns. The required finger width will be λ 0 /6=0.567 microns instead of the 0.425 microns finger width for a 2.4 GHz IDT. FIG. 5 shows a single-ended IDT with a grounded lower terminal, but a differential IDT design could also be employed.  
         [0050]    According to a first preferred embodiment of the invention, with an IDT arrangement which can directly produce a high frequency output signal, a SAW-based communications system could comprise an expander/compressor IDT  52 , a transmit (TX) IDT  56  and a receive (RX) IDT  60 . 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.  
         [0051]    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 middle IDT  52  of FIG. 6 through pulse input and output terminal  54  to activate a piezoelectric effect that converts electrical to mechanical (acoustic wave) motion. The acoustic waves can be coded depending on the geometry of the IDT  52 . 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  58  through the switch  62  and band pass filter  57 , the coded RF signal can propagate throughout the air.  
         [0052]    The same device can then perform in a similar reciprocal fashion. A coded electrical signal that enters the RX IDT  60  via the antenna  58 , band pass filter  57  and switch  62  generates an acoustic wave that propagates towards the middle expander/compressor IDT  52 . An autocorrelation function is passively performed in the IDT  52  and if the coded waveform from the RX IDT  60  matches with the code on the expander/compressor IDT  52 , a peak is generated at the pulse input and output terminal  54 .  
         [0053]    As discussed above, any of the IDTs shown in FIG. 6 could be implemented as differential IDTs. A fully differential system is shown in FIG. 7. In comparison with the system of FIG. 6, all of the grounded terminals in FIG. 7 are pulse input and output terminals in FIG. 7. Although two switches  58  and  58 ′ are shown, a single differential switching arrangement may be used. As indicated by the multiple connections in FIG. 7, the filter  61 ′ and antenna  62 ′ must also be differential components. Expander/compressor IDT  52 ′ may be single-ended, with terminal  55 ′ grounded as shown in FIG. 6, or differential, wherein terminal  55 ′ is a pulse input and output terminal. The differential system in FIG. 7 operates similarly to the system of FIG. 6, as will be apparent to those skilled in the art.  
         [0054]    The peak produced by an expander/compressor IDT such as  52  or  52 ′ 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.  
         [0055]    The coding of the expander/compressor IDTs  52 ,  52 ′ and the associated autocorrelation function performed by the IDTs as discussed above are determined by the finger geometry of the IDT. A preferred IDT coding scheme is a Barker code. Barker codes are particularly useful for IDT coding, since they minimize the energy in the side lobes associated with a compressed pulse generated by the autocorrelation function performed on a SAW input to an expander/compressor IDT. In FIG. 6 for example, the expander/compressor IDT  52  embodies a 5 bit +++−+ Barker code.  
         [0056]    [0056]FIG. 8 shows an example of the autocorrelation function performed by the expander/compressor IDT  52  of FIG. 6 when a signal received through the antenna  58  and switch  62  is converted to a SAW by RX IDT  60 . The autocorrelation function is mathematically equivalent to a series of shift and add operations as shown in FIG. 8 and generates the peak and associated side lobes shown at the bottom of FIG. 8. The amplitude of the autocorrelation peak is proportional to the code length N, which is 5 in the example shown in FIG. 8, whereas the side lobes are amplitude 1. This passive autocorrelation decodes received signals that were generated with an identically-coded IDT.  
         [0057]    In the system of FIGS. 6 and 7, only the expander/compressor IDTs  52 ,  52 ′ 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.  
         [0058]    Also evident from FIGS. 6 and 7 are the relative lengths of the RX IDTs  60 ,  60 ′, the expander/compressor IDTs  52 ,  52 ′ and TX IDTs  56 ,  56 ′. By far the longest IDTs, expander/compressor IDTs  52 ,  52 ′, are fabricated with a finger width of 0.567 microns to facilitate second harmonic operation at 2.4 GHz. Only the shorter IDTs  56 ,  56 ′,  60  and  60 ′ must be fabricated for 2.4 GHz operation with the smaller finger width of 0.425 microns. Therefore, the more stringent manufacturing requirements apply only to the shorter elements, which will increase production yields. Fabrication of the shorter elements with thinner fingers is considerably less difficult than fabrication of the much longer expander/compressor IDT with the same finger width. Furthermore, the representations shown in the drawings are simplified views of expander/compressor IDTs. In reality, the IDTs  52 ,  52 ′ will often comprise more than the single set of fingers shown in FIGS. 6 and 7 per code bit.  
         [0059]    The antenna switch  62  in FIG. 6 and switches  62  and  62 ′ in FIG. 7 are required to prevent feedback of a transmission signal from the TX IDTs  56  and  56 ′ to the RX IDTs  60 ,  60 ′, which would occur if both the TX and RX IDTs were connected to the antennas  58 ,  58 ′. Such feedback would cause the RX IDTs  60 ,  60 ′ to convert the fed back signal to a SAW, which in turn would propagate through IDT  52 ,  52 ′ and cause interference. Switches  62 ,  62 ′ similarly prevent a received signal from feeding back through the TX IDT  56 ,  56 ′. However, small-scale switches of the type normally employed in such arrangements tend to be prone to failure. The switch and associated complex control circuits also occupy space and consume power. Such problems are critical concerns in highly integrated device designs and mobile communications equipment in which SAW systems according to the instant invention could be employed.  
         [0060]    A second embodiment of the invention as shown in FIG. 9 eliminates the antenna switches and the problems associated therewith. According to the second embodiment, the SAW-based communication system  70  has an expander IDT  52   a  and a compressor IDT  52   b . A pulse representing data input at terminal  54  is converted to a coded SAW by expander IDT  52   a . Transmit IDT  56  then converts the resultant coded SAW into an electrical signal for transmission via band pass filter  57  and antenna  58 . Feedback of the transmit signal to the RX IDT  60  does not interfere with the IDT  52   a  in the transmit module  80   a . Pulse output  54   b  is not read during signal transmission to prevent erroneous data detection. A signal received at antenna  58  is filtered by band pass filter  57 , input to RX IDT  60 , converted to a SAW and decoded by autocorrelation in compressor IDT  52   b  provided the received signal code corresponds to the coding of IDT  52   b . The autocorrelation peak is output at terminal  54   b . Although the received signal is split between the TX IDT  56  and the RX IDT  60 , the SAW generated at TX IDT  56  causes no interference with the receive module  80   b . Any pulse output on terminal  54   a  during a receive operation is ignored.  
         [0061]    The IDTs shown in FIG. 9 are similar in construction to those in FIG. 6. The expander IDT  52   a  and compressor IDT  52   b  are fabricated for second harmonic operation at 2.4 GHz and coded in the same way as IDT  52  of FIG. 6. TX IDT  56  and RX IDT  60  operate at a fundamental frequency of 2.4 GHz.  
         [0062]    Any of the IDTs in FIG. 9 may be differential IDTs, as shown in FIG. 10. In the differential arrangement, terminals of the TX IDT  56  and RX IDT  60  shown as grounded in FIG. 9 are pulse input or output terminals in FIG. 10. Although single-ended IDTs are preferred for the expander IDT  52   a ′ and compressor IDT  52   b ′, these IDTs may also be differential IDTs, in which case terminals  55   a ′ and  55   b ′ are connected as pulse input and output terminals, respectively, instead of to ground.  
         [0063]    Although the problems associated with the antenna switches  62  and  62 ′ of the first embodiment are eliminated in the second embodiment, transmit and receive signal splitting at the antenna result in signal power losses within the system. Any choice between the first and second embodiments trades off the relatively higher failure rates, control circuit complexity, size and power consumption of the first embodiment against the signal power losses of the second embodiment.  
         [0064]    The 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.  
         [0065]    This invention may be incorporated into any situation for which Bluetooth was designed. An illustrative example of a system into which a system in accordance with the first or second embodiment could be incorporated is shown in FIG. 11.  
         [0066]    One contemplated application of the invention is illustrated in FIG. 11, wherein  102  denotes an earpiece,  104  is a mobile wireless communication device and  106  is a holder or cradle for holding the device  104  and coupling device  104  to a personal computer (PC)  110 . In system  100 , the earpiece  102 , device  104  and cradle  106  incorporate a SAW communication device as disclosed above. This allows a user to communicate audibly between the wireless communication device  104 , which may for example be carried on their belt or person, and the wireless earpiece  102  with a built-in microphone, as indicated at  108   a  in FIG. 11. This system could be then expanded to include communication between the earpiece  102  and the personal computer  110 , as indicated by  108   b , when a SAW system in cradle  106  is attached to the PC via a bus connection. This system may then be further expanded to include network communications (designated  108   c  in FIG. 11) between the wireless device  104  on the belt or person with the PC  110  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.  
         [0067]    In a third embodiment of the invention, the second harmonic 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  120  may comprise two IDTs  122  and  124 . IDT  124  is fabricated according to fundamental frequency criteria, whereas IDT  122  operates at a harmonic of the fundamental, as discussed above. 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 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 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 IDT which then returns back to the requesting unit a coded RF waveform.  
         [0068]    At the requesting unit, autocorrelation of the coded waveform returned from the device  120  would preferably be performed by a DSP or other conventional signal processing circuitry, such that different codes can be used for different 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  120  with which communication is desired. This would severely limit the number of devices  120  that could be deployed.  
         [0069]    The size of the complete SAW device  120 , 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 name plates, adhesive stickers such as vehicle license plate stickers and other forms of identification tags. The code embodied in the IDT  124  could for example be a code that provides information about an item to which the device  120  is attached. Device  120  could therefore be implemented in an identification or location system for example.  
         [0070]    Although IDT  122  in FIG. 12 is a single-ended IDT, a differential design is also contemplated, as shown in FIG. 13.  
         [0071]    In FIGS. 12 and 13, IDT  124  is shown as a coded IDT that produces a coded reflected SAW that can provide information to the requesting unit. However, in the systems of FIGS. 14 and 15, the IDTs are not coded. As shown in FIG. 14 for example, the passive communication system includes four IDTs,  122 ,  136 ,  138  and  142 , of which IDTs  136 ,  138  and  142  are fabricated as fundamental frequency components. IDT  122  is fabricated for operation at a harmonic frequency of fundamental. The terminals of IDT  136  are either open circuited as in FIG. 14 or short circuited such that a SAW produced by IDT  122  in response to a pulse received from a requesting unit by antenna  128  is reflected back toward the IDT  122  by IDT  136 . A return RF signal is therefore transmitted to the requesting unit as discussed above in relation to FIG. 12, although the return signal generated by device  130  is not coded. The terminals of IDT  142  are also either open or shorted, to thereby generate a second return signal to the requesting unit.  
         [0072]    The middle IDT  138  is connected to a sensor  144 , which may for example be a load impedance which changes according to a sensed characteristic or property such as moisture or temperature. A further reflected SAW, the magnitude and phase of which is dependent upon the impedance of the sensor  144 , is generated by IDT  138  and results in a third RF return signal. The reflection characteristics and thus the magnitude and phase of the RF return signal generated by the so-called reference IDTs  136  and  142  are known, depending upon the open or short circuiting of the terminals. These reference return signals can be compared to the return signal generated by the IDT  138  to determine the state of sensor  144  and thereby the value of the measured characteristic or property.  
         [0073]    The device  130 ′ shown in FIG. 15 is a fully differential realization of the device  130 .  
         [0074]    [0074]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  152  to a label, tag or the like generally indicated at  160 . The tag  160  includes a SAW device  120 ,  120 ′,  130  or  130 ′ and may be attached to or placed on or inside an item or at a location where measurement by sensor  144  is to be made. The return signals  154  generated by the SAW device in tag  160 , are received at the requesting device. For a device  120  or  120 ′, which produces a coded return signal  154 , the return signal is processed to determine tag information. For sensor applications in which reference return signals and a sensor return signal are generated, the signals are compared to determine sensor information. The tag or sensor 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.  
         [0075]    It will be appreciated that the above description relates to the preferred embodiment 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.