BRIEF DESCRIPTION OF THE FIGURES
Other objects and advantages of the invention will become apparent from a study of the following specification when viewed in the light of the accompanying drawing, in which:
FIG. 1 is a top view of a conventional OFC reflective delay line sensor according to the prior art;
FIG. 2 is a graphical representation of the idealized frequency spectrum of the reflections from one of the reflector arrays of the prior art sensor of FIG. 1;
FIG. 3 is a schematic representation of a stepped tapered transducer delay line device in which all frequency components experience equal delay according to the prior art;
FIG. 4 is a schematic representation of top view of a stepped tapered DFC SAW delay line according to the principles of the invention;
FIG. 5 is a graphical representation of the idealized frequency spectrum of the reflections from the reflector array of a DFC SAW delay line according to the invention;
FIG. 6 is a graphical representation of the idealized frequency spectrum of the reflections from the reflector array of another possible DFC SAW delay line embodiment according to the invention;
FIG. 7 is a schematic representation of top view of a stepped tapered DFC SAW differential delay line with coding in the outer reflectors according to the invention;
FIG. 8 is a schematic representation in plan view of an alternate embodiment of a stepped tapered DFC SAW differential delay line with coding in the outer transducers according to the invention;
FIG. 9 is a schematic representation in plan view of yet another alternate embodiment of a stepped tapered DFC SAW differential delay line with coding in the center transducer according to the invention;
FIG. 10 is a schematic representation in plan view of a continuously tapered DFC SAW delay line with coding in the reflectors according to the invention;
FIG. 11 is a schematic representation in plan view of a continuously tapered DFC SAW differential delay line with coding in the reflectors according to the invention; and
FIG. 12 is a schematic representation in plan view of an alternate embodiment of a continuously tapered DFC SAW differential delay line with coding in the transducer according to the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring first to FIG. 1, there is shown therein an orthogonal frequency coded (OFC) surface acoustic wave (SAW) device 2 according to the prior art. The device includes a piezoelectric substrate 4 on which are mounted a SAW transducers 6 and a bank of SAW reflectors 8. The transducer 6 has interdigital electrode fingers connected to one another by busbars and one of the busbars is connected to an antenna 7 as an input and output means. An input signal is received at the input via the antenna 7. The transducer 6 converts electrical signals to acoustic waves and launches the acoustic waves over a broadband frequency range towards the reflectors 8. The acoustic waves travel as surface acoustic waves along an acoustic track 9, also termed an acoustic channel, to the reflectors 8. The acoustic track 9, the transducer 6 and the reflectors 8 are all aligned along a single axis. In this example, the reflector bank 8 consists of five reflector sections, all on the axis but each at a different distance from the transducer 6 and each with a different center frequency. The round trip time delay of the surface acoustic waves from the transducer 6 to each reflective section of the reflectors 8 and back to the transducer 6 is different. The order in which specific delay times are assigned to the reflective array sections establishes a code in the reflected response. The reflected response may be detected at the output, as indicated by the antenna.
A graphical representation of the idealized frequency spectrum 10 (frequency response S11) of the reflections from the reflective array 8 of an OFC SAW device according to the prior art is shown FIG. 2. The relative frequency of the signal is shown on the horizontal axis and the relative amplitude of the signal is shown on the vertical axis of the graph. The graph shows signals reaching an idealized value of one and centered on an idealized frequency of one. Of course, this may be applied an any number of frequency amplitudes and ranges. Note the five responses (the five peaks) from the five reflective sections, each with different center frequencies and a defined relationship between the nulls and peaks of adjacent reflector sections. Prior art OFC SAW devices, by definition, have the peaks of each reflector or chip frequency response 12 at the first null of the chips that are adjacent in frequency on each side 14. This results in significant spectral overlap, with the side lobes from at least the two reflectors or chips on each side injecting energy within the spectrum of the chip in question 16. This spectral overlap causes codes composed of these chips, which are orthogonal in the time domain, to suffer from significant spectral interference, rendering them functionally non-orthogonal (i.e. codes cannot be differentiated from one another in a multi-sensor environment).
Referring next to FIG. 3, there is shown therein a stepped tapered transducer SAW delay line device 20 according to the prior art in which all frequency components experience an equal delay. The device includes a piezoelectric substrate 22 on which are mounted stepped SAW transducers 24 and 26. The stepped transducer 24 launches acoustic waves along an acoustic track over a broadband frequency range towards the stepped transducer 26. Each transducer is constructed in a stepped tapered fashion, with discrete sub-transducers 28 that are spatially distributed across the overall acoustic aperture of the device. In this example, the transducers 24 and 26 are shown with four sub-transducer sections 28 transverse to the acoustic wave propagation, each with a different center frequency (f1, f2, f3, and f4). The physical centers of each of the sub-transducer sections 28 lie along a line transverse to the wave propagation direction so that each sub-transducer section 28 of a transducer is the same acoustic distance from its counterpart sub-transducer section 28 of the other transducer. These sections define four acoustic channels or tracks between the respective sub-transducer sections 28. The wave generated in each acoustic channel is defined by the electrode spacing and structure within the sub-transducer 28 in that track. A wideband excitation will simultaneously excite all of the sub-transducers 26, causing a wave to propagate in each acoustic channel. The acoustic time delay from the transducer 24 to the sub-transducer 26 is equal for all acoustic channels, as a result of the equal length of the acoustic tracks from sub-transducer center to sub-transducer center. It should be noted that a similar delay line function can be implemented as a one-port device using a single stepped tapered transducer 24 and having the transducer element 26 implemented as a stepped tapered reflector. In that case, the device delay would be equal to the round trip delay of the acoustic wave from transducer 24 to the reflector 26 and back.
Referring next to FIG. 4, there is shown therein a stepped tapered discrete frequency coded (DFC) SAW device 30 according to the principles of the present invention. The device includes a piezoelectric substrate 32 on which are mounted a stepped tapered SAW transducer 34 and a stepped tapered SAW reflector 36. The transducer 34 is constructed much like the transducer 24 shown in FIG. 3. The transducer 34 is connected to an input/output device 37, which may be an antenna, electrical interface, or signal processing system, for example. The transducer 34 launches acoustic waves over a broadband frequency range towards the reflector 36. Each transducer and reflector element is constructed in a stepped tapered fashion, with discrete frequency sub-transducers 35 in the transducer 34 and discrete frequency sub-reflectors 38 in the reflector 36 spatially distributed across the overall acoustic aperture of the device. In this example, the transducers are again shown with four sections disposed adjacent one another transverse to the acoustic wave propagation, each with a different center frequency. These sections 35 and 38 define four acoustic channels, although other numbers of acoustic channels may of course be provided by providing more or fewer sub-transducer sections. The present apparatus differs from the device of FIG. 3 by the fact that the acoustic tracks or channels are of different lengths for each sub-transducer section. The sub-transducer sections 38 of the reflector 36 are connected to one another by busbars with connections shared between adjacent sections, although separate connections may be provided in an alternate embodiment.
The surface acoustic wave generated in each acoustic channel is defined by the electrode spacing and structure within the sub-transducer in that track. A wideband excitation will simultaneously excite all sub-transducers, causing a wave to propagate in each acoustic channel. The round trip acoustic time delay from the transducer 34 to each reflective section of the transducer 36 and back is different for each acoustic channel as a result of its different length. The order in which specific delay times are assigned to the reflective array sections establishes a code in the reflected response.
The spatial diversity of this structure introduces substantially more freedom in the design process than is existent in OFC device design according to the prior art. Individual reflector sections, or “chips” are no longer constrained to be short, defined time lengths. Since each chip occupies its own acoustic channel, it can be of almost arbitrarily long time extent. This allows the designer to utilize many more reflective electrodes, increasing the overall reflection efficiency for the chip, and reducing insertion loss. Since the acoustic channels are narrowband, this increased chip time length does not degrade the channel bandwidth. Also, methods that are well known in the art for sculpting the passband response of each channel can be used. Conventional apodization, withdrawal weighting, sub-transducer or block weighting, and other known methods can be applied to generate chip responses that are as close to “brick wall” filter responses in the frequency domain as desired. This substantially reduces or eliminates inter-chip interference, allowing codes to work together in a manner that is functionally more orthogonal in the frequency domain than prior art devices.
An additional degree of freedom is provided by using a structure that spatially distributes the chips transversely across the die. Specifically, in the present invention, the chips can be located with time delays that are arbitrarily determined, unlike OFC devices according to the prior art, which are constrained to have specific time relationships to one another by their physical layout in a single acoustic track. These added degrees of freedom available to the device designer in the present invention make DFC SAW devices much more flexible in design, and capable of enhanced performance over the prior art.
In embodiments where the input/output element 37 is an antenna, a corresponding antenna 37a is provided on an interrogation device 37b that is powered by a power supply or battery 37c. Alternately, the input/output element 37 can be directly connected to the interrogation device 37b.
FIG. 5 shows a graphical representation of the idealized frequency spectrum 40 (frequency response S11) of the reflections from the reflective array 36 of a DFC SAW delay line device according to the invention. Note the five responses from the five reflective sections, each with different center frequencies. Unlike prior art OFC SAW devices, the individual chip frequency responses are designed to be steep and fairly flat over the passband. Each chip frequency response 42 rolls off with steep skirts or sides 44, such that the chip frequency response 42 has fallen off substantially at the first null of the chips that are adjacent in frequency on each side 46. Such a design approach minimizes the amount of inter-chip interference by effectively eliminating side lobe interactions, i.e. no (or very little) energy from the side lobes on one chip occurs in the spectral passband of another chip. While the chip passbands shown in FIG. 5 are somewhat rounded, the passband shape can be controlled by the designer to produce as flat a passband and as steep skirts as desired. The individual chip frequencies can be separated as far as necessary to reduce or eliminate interchip interference. Use of discrete frequency bands as a basis on which to build a spatially distributed (multi-channel) acoustic device means that the chips are nearly mutually orthogonal, i.e., the response of any chip will not be detected by the transducer of any other chip and significantly, this allows realization of codes composed of these chips which are functionally orthogonal (i.e. codes can be differentiated from one another in a multi-sensor environment). This is a substantial improvement over prior art OFC devices, in which the orthogonality conditions result in chip frequency responses that overlap and interfere with one another significantly.
FIG. 6 shows a graphical representation of the idealized frequency spectrum 50 (frequency response S11) of the reflections from the reflective array 36 of a DFC SAW delay line device according to an alternate embodiment of the invention. Note the five responses from the five reflective sections are still evident. In this embodiment, however, the reflective sections have been designed so that their frequency responses cross at an arbitrarily selected amplitude level, such as the 6 dB point 52. Such a design approach substantially reduces the amount of inter-chip interference over the prior art, as evidenced by the small spectral overlap areas 54, while maintaining spectral energy efficiency by avoiding gaps in the overall device passband.
Referring next to FIG. 7, there is shown therein a stepped tapered discrete frequency coded (DFC) differential delay line SAW device 60 according to another embodiment of the present invention. The device includes a piezoelectric substrate 62 on which are mounted a stepped tapered SAW transducer 64 and two stepped tapered SAW reflectors 66. The transducer 64 is connected to an input/output device 65, such as described above. The reflectors 66 are mirror images of one another, and are spaced different distances away from transducer 64. The transducer 64 has its sub-transducer sections aligned with one another along the transverse to the wave propagation direction, but the reflectors 66 have their sub-transducer sections out of alignment with one another relative to the transverse of the propagation direction so that the physical center if each section is at a different distance from the transducer 64. It is also foreseen that two or more of the sections could be aligned with one another while others of the sections are out of alignment relative to one another with reference to the transverse to the wave propagation direction.
The transducer 64 launches acoustic waves over a broadband frequency range towards reflectors 66. Each transducer and reflector element is constructed in a stepped tapered fashion, with discrete sub-transducers or sub-reflectors 68 spatially distributed across the overall acoustic aperture of the device. In this example, the transducer is again shown with four sections transverse to the acoustic wave propagation, each with a different center frequency. These sections define acoustic channels. The wave generated in each acoustic channel is defined by the electrode spacing and structure within the sub-transducer in that track. A wideband excitation will simultaneously excite all sub-transducers, causing a wave to propagate in each acoustic channel. The round trip acoustic time delay from the transducer 64 to each reflective section of the reflector 66 and back is different for each acoustic channel. The order in which specific delay times are assigned to the reflective array sections establishes a code in the reflected response.
In a device as shown in FIG. 7, the coded response reflected back from the left reflector bank 66b will have a shorter delay than the coded response reflected back from the right reflector bank. This is due to the greater separation between the transducer 64 and the right reflector bank 66a, which results in an increased acoustic wave propagation time and hence an increased delay as compared to the separation between the transducer 64 and the left reflector 66b. Once again, in the present invention, the reflective chips can be located with time delays that are arbitrarily determined, and can have arbitrary time lengths as selected by the designer to achieve performance goals. These device parameters are not constrained by conditions of orthogonality as in prior art OFC devices. The signal propagating from the transducer 64 to the left to reflector 66b and back to the transducer 64 will add linearly to the signal propagating to the right reflector 66a and back to the transducer 64. The combined signal will be correlated in the interrogator by the matched filter response of the code of reflector, resulting in two compressed pulses in the time domain. The separation of these two pulses can be used by the interrogator to determine the quantity of interest to be sensed. This may be by actually measuring the time delay between the pulses or by using the frequency response generated by these two taps in a transversal filter.
FIG. 8 shows another stepped tapered discrete frequency coded (DFC) differential delay line SAW device 70 according to yet another embodiment of the present invention. The device includes a piezoelectric substrate 72 on which are mounted a stepped tapered SAW transducer 74 and two stepped tapered SAW transducers 76. The difference between this embodiment and the embodiment of FIG. 7 is that the left and right elements 76a and 76b are transducers with output connections 75 and 77 rather than reflectors which reflect the signal back to the center element as in FIG. 7. The center element 74 is connected only to an input 73. The transducers 76a and 76b are mirror images of one another, and are spaced different distances away from transducer 74. The transducer 74 launches acoustic waves over a broadband frequency range towards the transducers 76a and 76b. Each transducer element is constructed in a stepped tapered fashion, with discrete sub-transducers (within the transducers 74 and 76) spatially distributed across the overall acoustic aperture of the device. In this example, the transducer is again shown with four sections transverse to the acoustic wave propagation, each with a different center frequency. These sections define acoustic channels. The wave generated in each acoustic channel is defined by the electrode spacing and structure within the sub-transducer in that track. A wideband excitation will simultaneously excite all sub-transducers, causing a wave to propagate in each acoustic channel. In this embodiment, the external transducers 76a and 76b will receive the acoustic wave after an acoustic time delay corresponding to the spacing from the transducer 74 to each sub-transducer of 76, delays that will once again be different for each acoustic channel. The order in which specific delay times are assigned to the outer transducer sub-transducer sections establishes a code in the response.
In a device as shown in FIG. 8, the coded response from the left transducer 76b will have a shorter delay than the coded response from the right transducer 76a. This is due to the greater separation between the transducer 74 and the right transducer 76a, which results in an increased acoustic wave propagation time and hence an increased delay as compared to the separation between transducer 74 and the left transducer 76b. The outer transducers 76a and 76b will convert the acoustic wave back into two radio frequency (RF) electric signals, which after being received at the outputs 75 and 77 can be measured or transmitted to a receiver.
This SAW device can be used for communication purposes or as a sensor. When used as a sensor, this device can be used in one of two ways. The three transducers can all be connected in parallel and connected to an antenna for communication to the interrogator, or the transducers 76 can be used as reflectors in which case only the transducer 74 is connected to the antenna. In the latter case the matching conditions on the transducers 76 can be used to vary the reflectivity of these transducers. They can be electrically shorted, or left open circuited, or connected to an external impedance element which is in itself a passive sensing element. This is an interesting case because the change in impedance from the external sensing element changes the reflectivity of the corresponding transducer which changes the amplitude of one pulse of the differential pair of pulses. This can be measured by an interrogator circuit, and so the SAW device is acting as a link between the passive sensor and an interrogator, while also providing the function of an RFID tag. Once again, the reflective chips can be located with time delays that are arbitrarily determined, and can have arbitrary time lengths as selected by the designer to achieve performance goals.
FIG. 9 shows yet another stepped tapered discrete frequency coded (DFC) differential delay line SAW device 80 according to the present invention. The device includes a piezoelectric substrate 82 on which are mounted a stepped tapered SAW transducer 84 and two stepped tapered SAW reflectors 86. The reflectors 86 are composed of sub-reflectors aligned so that the centerlines of the sub-reflectors are coincident, i.e. relative to the transverse of the propagation direction. The reflectors 86 are also spaced different distances away from transducer 84. The transducer 84 launches acoustic waves over a broadband frequency range towards the reflectors 86. Each transducer and reflector element is constructed in a stepped tapered fashion, with discrete sub-transducers (within the transducer 84) and sub-reflectors 88 spatially distributed across the overall acoustic aperture of the device.
In this example, the transducer is again shown with four sections transverse to the acoustic wave propagation, each with a different center frequency. These sections define acoustic channels. The wave generated in each acoustic channel is defined by the electrode spacing and structure within the sub-transducer in that track. A wideband excitation will simultaneously excite all sub-transducers, causing a wave to propagate in each acoustic channel. In this embodiment, the sub-transducers of the transducer 84 are distributed such that the acoustic wave launched in each channel will experience a different delay when propagating to one of the reflectors and back to the transducer 84, where the wave will be transduced back into an RF signal. This once again implements a code.
It should be noted that in this embodiment, the coded signal contained in the reflected response from the wave that reflects off of the rightmost reflector bank will be a time delayed, time reversed version of the coded signal contained in the wave reflecting off of the leftmost reflector bank. Availability of both a coded signal and its time reversal can be useful in certain applications. It should be evident that this functionality can also be achieved using another embodiment that has transducers in place of the reflectors 86. As before, the transducers and/or reflectors are connected to input and/or output devices or circuits.
FIGS. 4, 7, 8, and 9 all show stepped tapered implementations of the present invention. Another series of preferred embodiments use continuously tapered transducer and reflector elements. FIGS. 10, 11, and 12 show embodiments similar in nature to FIGS. 4, 7 and 9, respectively, but implemented using continuously tapered transducers rather than stepped tapered transducers. The devices with continuously tapered transducers are generally functionally equivalent to the step tapered devices but the use of continuously tapered transducers and/or reflector can provide greater levels of selectivity in the filtering operations of the devices.
The continuously tapered transducers have a more continuous frequency response than the stepped tapered transducer, both in terms of SAW generation and SAW sensing. Instead of four discrete acoustic tracks being used, a continuous acoustic track is formed between the elements and the various frequencies are carried at different positions on the track relative to the transverse of the propagation direction. In other words, instead of a stepped frequency signal, the acoustic wave is a continuous frequency acoustic wave when viewed across the transverse to the propagation direction. Input and output devices are connected to the transducers/reflectors as described above.
The illustrated SAW devices have one transducer or reflector with the subsections out of alignment with respect to one another at the transverse to the propagation direction and the other transducer or reflector with the subsections in alignment with respect to one another along the transverse to the propagation direction. It is possible to exchange these for one another or to have both SAW elements of an acoustic track with the subsections out of alignment. In embodiments with three transducer and/or reflector elements, it is foreseeable that all three would have the subsections out of alignment relative to on another along the transverse.
Other embodiments that utilize discrete frequency coding on SAW devices for communication and sensor applications are within the scope of this invention, including but not limited to the use of selective coatings to realize chemical sensors and biological sensors, both for vapor and liquid phase, and different device configurations and mounting techniques to effect physical sensors for temperature, pressure, strain, torque, and liquid measurements (viscosity, flow rate, etc.). It would be understood by one skilled in the art that the advantages of the present invention can be realized in devices made using various acoustic wave modes, including but not limited to Rayleigh waves, flexural plate waves, acoustic plate modes, transverse waves, and guided waves such as Love waves, Lamb waves, and layer guided waves. Similarly, any of a wide range of interrogation system architectures can be used to interrogate such devices. Additional aspects of a practical system utilizing the invention include the ability to store data and calculation results, and devices for transmitting the data and/or results to entities interested in the results. Such transmission of information may include but is not limited to communicating to external computers, web sites, cell phones, and other devices.
It is within the scope of the present invention that the input element(s) and/or output element(s) may include interrogation devices, control circuits, sensor circuits, display device, or any number of other well known devices or systems such as can be connected to a surface acoustic wave device. One application of the present device is as a sensor wherein the device is configured or constructed in a know way so that a parameter to be measured affects the acoustic wave propagation within the device and/or affects an electrical impedance of an external device connected to the surface acoustic wave device.
Although other modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.
Patent Application
20070296305
