Patent Publication Number: US-2020284763-A1

Title: Multiplexing surface acoustic wave sensors with delay line coding

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority of U.S. Provisional Application No. 62/529,725, including Appendix A entitled “APPARATUS AND METHOD FOR FREQUENCY MODULATION SURFACE ACOUSTIC WAVE SENSOR” and Appendix B entitled “BULK ACOUSTIC WAVE(S) AND/OR SURFACE ACOUSTIC WAVE(S),” filed therewith, filed on Jul. 7, 2017, the entire contents of which are hereby incorporated by reference. 
    
    
     FIELD 
     The present disclosure relates generally to devices and methods for simultaneously identifying, detecting, measuring or sensing multiple analytes using Surface Acoustic Wave (SAW) or Bulk Acoustic Wave (BAW) sensors. More particularly, the disclosure relates to a multiple SAW and/or BAW sensor device capable of simultaneously sensing a plurality of target materials using SAW and/or BAW sensors with differing lengths of delay lines. 
     BACKGROUND 
     A Surface Acoustic Wave (SAW) sensor and/or a Bulk Acoustic Wave (BAW) sensor is an element or a device for identifying, detecting, sensing or measuring various physical, chemical, or biological quantities or changes in quantities of various kinds of chemical or biological material, such as those in liquid media and organic or inorganic gases. There is an urgent need for point of care (time to result &lt;30 min), portable, multiplexed (can screen multiple target analytes simultaneously form a biological fluid) sensors with high sensitivity and specificity without any sample processing. 
     SUMMARY 
     The SAW sensor is a passive electronic device. An input electrical signal is applied to the pads. The transducer transforms the electrical signal into a mechanical signal, which is called a Surface Acoustic Wave (SAW). Sensor response is equivalent to the property changes (phase, amplitude and frequency or delay) of the mechanical wave. For example, a variance in at least one of amplitude, phase, frequency, or time-delay between pulses of the receiving signal (R X ) and/or the excitation signal. For example, the multiplexing SAW measurement system can include phase detection which can determine a phase corresponding to each of the plurality of pulses with respect to each other and/or the excitation signal. For example, the difference in delay line length between the SAW sensors results in a time delay between the pulses of the received signal (R X ). The shifts in time domain between the pulses of the compressed pulse train correspond to phase shifts associated with a particular SAW sensor. The phase shifts can be determined, for example, using a software program or field programmable gate array (FPGA) hardware. 
     In one aspect, the disclosure provides a surface acoustic wave (SAW) device, including: a piezoelectric substrate; and a plurality of SAW sensors attached to the piezoelectric substrate and arranged on a surface of the piezoelectric substrate, the plurality of SAW sensors including a first SAW sensor comprising a first delay line configured to propagate a first surface acoustic wave, and a second SAW sensor comprising a second delay line configured to propagate a second surface acoustic wave, wherein a length of the first delay line is greater than a length of the second delay line. 
     In an embodiment, the first SAW sensor includes: a first transducer for transmitting the first surface acoustic wave along the first delay line, and a second transducer for receiving the first surface acoustic wave upon propagation of the first surface acoustic wave along the first delay line. 
     In an embodiment, the first SAW sensor comprises a transducer positioned on the substrate and a reflector positioned on the substrate opposite the transducer, wherein the transducer transmits the first surface acoustic wave along the first delay line, and the transducer receives the first surface acoustic wave after the first surface acoustic reflects off the reflector and propagates along the first delay line twice. 
     In an embodiment, the reflector is a first reflector and wherein the first SAW sensor further comprises a second reflector positioned on the substrate proximate the first reflector relative to the transducer, wherein the transducer is configured to receive the first surface acoustic wave upon reflecting off the second reflector and propagating along the first delay line twice. 
     In an embodiment, the first reflector is configured to reflect a surface acoustic wave having a first frequency and the second reflector is configured to reflect a surface acoustic wave having a second frequency. 
     In an embodiment, the first SAW sensor comprises a first pair of electrical contacts and the second SAW sensor comprises a second pair of electrical contacts, and wherein the first and second pairs of electrical contacts are electrically connected. 
     In an embodiment, each of the saw sensors are configured to receive an excitation signal. 
     In an embodiment, the excitation signal includes at least one of a pulse voltage, a sinusoidal electrical signal, frequency modulation, linear frequency modulation, hyperbolic frequency modulation, orthogonal frequency coding, random modulation, continuous phase modulation, frequency shift key, multi-frequency shift key, phase shift key, wavelet modulation, or a wideband frequency signal. 
     In an embodiment, each of the saw sensors are configured to simultaneously receive the excitation signal. 
     In an embodiment, the device further includes: one or more processors in communication with each of the first SAW sensor and the second SAW sensor, the one or more processors configured to generate a receiving signal based at least in part on signals received from the first SAW sensor and the second SAW sensor. 
     In an embodiment, the one or more processors are further configured to determine or monitor at least one analyte based at least in part on the receiving signal. 
     In an embodiment, the one or more processors are configured to determine or monitor identify the at least one analyte by detecting a variance in amplitude, phase, frequency, or time-delay between at least two of a pulse corresponding to the excitation signal, a pulse corresponding to the first SAW sensor, or a pulse correspond to the second SAW sensor. 
     In an embodiment, the receiving signal comprises a compressed pulse train having a plurality of pulses. 
     In an embodiment, the plurality of pules of the compressed pulse train includes: a first pulse corresponding to the first SAW sensor, and a second pulse corresponding to the second SAW sensor. 
     In an embodiment, a timing of the first pulse is based at least in part on the length of the first delay line, and wherein a timing of the second pulse is based at least in part on the length of the second delay line. 
     In an embodiment, the plurality of pulses of the compressed pulse train comprises a pulse corresponding to the excitation signal. 
     In an embodiment, the piezoelectric substrate comprises at least one of 36° Y quartz, 36° YX lithium tantalite, langasite, langatate, langanite, lead zirconate titanate, cadmium sulfide, berlinite, lithium iodate, lithium tetraborate, or bismuth germanium oxide. 
     In an embodiment, the piezoelectric substrate comprises a piezoelectric crystal layer. 
     In an embodiment, the piezoelectric crystal layer comprises a thickness greater than a Love Wave penetration depth on a non-piezoelectric substrate. 
     In an embodiment, the device further includes a sensing region located at the first delay line and configured to attach to or react with an analyte. 
     In an embodiment, the device further includes a detector for measuring a phase response of surface acoustic waves as a function of an analyte added to the sensing region. 
     In an embodiment, the sensing region comprises a biologically sensitive interface for capturing analytes from a liquid media. 
     In an embodiment, the sensing region comprises a chemically sensitive interface for absorbing analytes from a liquid media. 
     In an embodiment, the device further includes a guiding layer on the first delay line. 
     In an embodiment, the guiding layer comprises at least one of a polymer, SiO2 or ZnO. 
     In an embodiment, a first surface acoustic wave corresponding to the first SAW sensors comprises a frequency greater than 100 MHz, greater than 300 MHz, greater than 500 MHz, or greater than 1000 MHz. 
     In one aspect, the disclosure provides a method including the steps of: generating an excitation signal; transmitting the excitation signal to a surface acoustic wave (SAW) device, wherein the SAW device comprises a first SAW sensor including a first delay line configured to propagate a first surface acoustic wave, and a second SAW sensor including a second delay line configured to propagate a second surface acoustic wave, wherein a length of the first delay line is greater than a length of the second delay line; receiving an output signal of the SAW device, the output signal indicative of at least one of the first delay line, the length of the second delay line, or an analyte exposed to at least one of the first SAW sensor or the second SAW sensor; and determining or monitoring the analyte based at least in part on the output signal of the SAW device. 
     In one aspect, the disclosure provides a method including the steps of: receiving an excitation signal; generating a first surface acoustic wave for propagation across a first delay line of a first SAW sensor of a SAW device; generating a second surface acoustic wave for propagation across a second delay line of a second SAW sensor of the SAW device, wherein a length of the first delay line is greater than a length of the second delay line; receiving the first surface acoustic wave after propagation across the first delay line; receiving the second surface acoustic wave after propagation across the second delay line; and generating a signal based at least in part on at least one of the received first surface acoustic wave, the received second acoustic wave, or the excitation signal. 
     In an embodiment, the first SAW sensor comprises a reflector configured to reflect the first surface acoustic wave, and wherein said receiving the first surface acoustic wave occurs after the first acoustic wave is reflected. 
     In an embodiment, the second SAW sensor comprises a reflector configured to reflect the second surface acoustic wave, and wherein said receiving the second surface acoustic wave occurs after the second acoustic wave is reflected. 
     In one aspect, the disclosure provides a method including the steps of: generating an excitation signal; transmitting the excitation signal to a surface acoustic wave (SAW) device, wherein the SAW device comprises a first SAW sensor including a first delay line configured to propagate a first surface acoustic wave, and a second SAW sensor including a second delay line configured to propagate a second surface acoustic wave, wherein a length of the first delay line is greater than a length of the second delay line; receiving the excitation signal at the SAW device; generating a first surface acoustic wave for propagation across the first delay line; generating a second surface acoustic wave for propagation across the second delay line; receiving the first surface acoustic wave after propagation across the first delay line; receiving the second surface acoustic wave after propagation across the second delay line; generating a signal based at least in part on at least one of the received first surface acoustic wave or the received second acoustic wave, wherein the signal is indicative of at least one of the first delay line, the length of the second delay line, or an analyte exposed to at least one of the first SAW sensor or the second SAW sensor; and determining or monitoring the analyte based at least in part on the generated signal. 
     In one aspect, the disclosure provides a method including the steps of: exposing at least a portion of a SAW device to a sample media comprising an analyte, wherein the SAW device comprises a first SAW sensor including a first delay line configured to propagate a first surface acoustic wave responsive to an excitation signal, and a second SAW sensor including a second delay line configured to propagate a second surface acoustic wave responsive to the excitation signal, wherein a length of the first delay line is greater than a length of the second delay line, and wherein a sensitive region of at least one of the first delay line or the second delay line reacts to the analyte such that at least one of the first surface acoustic wave or the second surface acoustic wave is altered; receiving a signal corresponding to an output of a SAW device; identifying a first pulse of the received signal, wherein the first pulse corresponds to the first SAW sensor; identifying a second pulse of the received signal, wherein the second pulse corresponds to the second SAW sensor; identifying a third pulse of the received signal, wherein the third pulse corresponds to the excitation signal; determining at least one of a phase, frequency, amplitude, or timing of at least two of the first pulse, the second pulse, or the third pulse; and based at least in part on said determining, identifying or monitoring the analyte. 
     In an embodiment, the identifying or monitoring the analyte comprises determining a variance in at least one of amplitude, phase, frequency, or time-delay between at least two of the first pulse, the second pulse or the third pulse. 
     In one aspect, the disclosure provides a method including the steps of: generating sequentially several excitations signals which are route sequentially through a multiplexer to different delay lines which generate responses which are route sequentially through the same or a different multiplexer to the receiving electronics. 
     A Surface Acoustic Wave (SAW) device including a piezoelectric substrate and a plurality of SAW sensors attached to the piezoelectric substrate and arranged on a surface of the piezoelectric substrate. The plurality of SAW sensors includes a first SAW device and a second SAW device. The first SAW sensor includes a first delay line configured to propagate a first surface acoustic wave. The second SAW sensor includes a second delay line configured to propagate a second surface acoustic wave. A length of the first delay line is greater than a length of the second delay line or the length of the second delay line is greater than the length of the first delay line. 
     The device of the preceding paragraph may also include any combination of the following features described in this paragraph, among other features described herein. In some embodiments, the first SAW sensor further includes a first transducer for transmitting the first surface acoustic wave along the first delay line and a second transducer for receiving the first surface acoustic wave upon propagation of the first surface acoustic wave along the first delay line. 
     The device of any of the preceding paragraphs may also include any combination of the following features described in this paragraph, among other features described herein. In some embodiments, the first SAW sensor can further include a transducer positioned on the substrate and a reflector positioned on the substrate opposite the transducer. The transducer is configured to transmit the first surface acoustic wave along the first delay line and the transducer is further configured to receive the first surface acoustic wave after the first surface acoustic reflects off the reflector and propagates along the first delay line twice. In some embodiments, the reflector is a first reflector and the first SAW sensor further includes a second reflector positioned on the substrate proximate the first reflector relative to the transducer. The transducer is configured to receive the first surface acoustic wave upon reflecting off the second reflector and propagating along the first delay line twice. In some embodiments, the first reflector is configured to reflect a surface acoustic wave having a first frequency and the second reflector is configured to reflect a surface acoustic wave having a second frequency. 
     The device of any of the preceding paragraphs may also include any combination of the following features described in this paragraph, among other features described herein. In some embodiments, the first SAW sensor includes a first pair of electrical contacts and the second SAW sensor includes a second pair of electrical contacts. The first and second pairs of electrical contacts are electrically connected. In some embodiments, each of the SAW sensors are configured to receive an excitation signal. In some embodiments, the excitation signal includes at least one of a pulse voltage, a sinusoidal electrical signal, frequency modulation, linear frequency modulation, hyperbolic frequency modulation, orthogonal frequency coding, random modulation, continuous phase modulation, frequency shift key, multi-frequency shift key, phase shift key, wavelet modulation, or a wideband frequency signal. In some embodiments, each of the SAW sensors are configured to simultaneously receive the excitation signal. 
     The device of any of the preceding paragraphs may also include any combination of the following features described in this paragraph, among other features described herein. In some embodiments, the SAW device further includes one or more processors in communication with each of the first SAW sensor and the second SAW sensor. The one or more processors can be configured to generate a receiving signal based at least in part on signals received from the first SAW sensor and the second SAW sensor. In some embodiments, the one or more processors are further configured to determine or monitor at least one analyte based at least in part on the receiving signal. In some embodiments, the one or more processors are further configured to identify the at least one analyte by detecting a variance in amplitude, phase, frequency, or time-delay between at least two of a pulse corresponding to the excitation signal, a pulse corresponding to the first SAW sensor, or a pulse correspond to the second SAW sensor. 
     The device of any of the preceding paragraphs may also include any combination of the following features described in this paragraph, among other features described herein. In some embodiments, the receiving signal includes a compressed pulse train having a plurality of pulses. In some embodiments, the plurality of pules of the compressed pulse train includes a first pulse corresponding to the first SAW sensor, and a second pulse corresponding to the second SAW sensor. In some embodiments, a timing of the first pulse is based at least in part on the length of the first delay line, and a timing of the second pulse is based at least in part on the length of the second delay line. In some embodiments, the plurality of pulses of the compressed pulse train includes a pulse corresponding to the excitation signal. 
     The device of any of the preceding paragraphs may also include any combination of the following features described in this paragraph, among other features described herein. In some embodiments, the piezoelectric substrate includes at least one of 36° Y quartz, 36° YX lithium tantalite, langasite, langatate, langanite, lead zirconate titanate, cadmium sulfide, berlinite, lithium iodate, lithium tetraborate, or bismuth germanium oxide. In some embodiments, the piezoelectric substrate includes a piezoelectric crystal layer. In some embodiments, the piezoelectric crystal layer includes a thickness greater than a Love Wave penetration depth on a non-piezoelectric substrate. 
     The device of any of the preceding paragraphs may also include any combination of the following features described in this paragraph, among other features described herein. In some embodiments, the SAW device further includes a sensing region located at the first delay line and configured to attach to or react with an analyte. In some embodiments, the sensing region includes a biologically sensitive interface for capturing analytes from a liquid media. In some embodiments, the sensing region includes a chemically sensitive interface for absorbing analytes from a liquid media. In some embodiments, the SAW device further includes a detector for measuring a phase response of surface acoustic waves as a function of an analyte added to the sensing region. In some embodiments, the SAW device further includes a guiding layer on the first delay line. In some embodiments, the guiding layer includes at least one of a polymer, SiO2 or ZnO. In some embodiments, a first surface acoustic wave corresponding to the first SAW sensors includes a frequency greater than 100 MHz, greater than 300 MHz, greater than 500 MHz, or greater than 1000 MHz. 
     As described herein, a method may include generating an excitation signal and transmitting the excitation signal to a surface acoustic wave (SAW) device. The SAW device includes a first SAW sensor having a first delay line configured to propagate a first surface acoustic wave, and a second SAW sensor having a second delay line configured to propagate a second surface acoustic wave. A length of the first delay line is greater than a length of the second delay line or the length of the second delay line is greater than the length of the first delay line. The method further includes receiving an output signal of the SAW device. The output signal is indicative of at least one of the first delay line, the length of the second delay line, or an analyte exposed to at least one of the first SAW sensor or the second SAW sensor. The method further includes determining or monitoring the analyte based at least in part on the output signal of the SAW device. 
     As described herein, a method may include receiving an excitation signal and generating a first surface acoustic wave for propagation across a first delay line of a first SAW sensor of a SAW device. The method further includes generating a second surface acoustic wave for propagation across a second delay line of a second SAW sensor of the SAW device. A length of the first delay line is greater than a length of the second delay line or the length of the second delay line is greater than the length of the first delay line. The method further includes receiving the first surface acoustic wave after propagation across the first delay line, and receiving the second surface acoustic wave after propagation across the second delay line. The method further includes generating a signal based at least in part on at least one of the received first surface acoustic wave, the received second acoustic wave, or the excitation signal. 
     The method of the preceding paragraph may also include any combination of the following steps or features described in this paragraph, among other steps or features described herein. In some embodiments, the first SAW sensor includes a reflector configured to reflect the first surface acoustic wave, and said receiving the first surface acoustic wave occurs after the first acoustic wave is reflected. In some embodiments, the second SAW sensor includes a reflector configured to reflect the second surface acoustic wave, and said receiving the second surface acoustic wave occurs after the second acoustic wave is reflected. 
     A method as described herein may also include generating an excitation signal and transmitting the excitation signal to a surface acoustic wave (SAW) device. The SAW device includes a first SAW sensor including a first delay line configured to propagate a first surface acoustic wave, and a second SAW sensor including a second delay line configured to propagate a second surface acoustic wave. A length of the first delay line is greater than a length of the second delay line or the length of the second delay line is greater than the length of the first delay line. The method further includes receiving the excitation signal at the SAW device, generating a first surface acoustic wave for propagation across the first delay line, generating a second surface acoustic wave for propagation across the second delay line, receiving the first surface acoustic wave after propagation across the first delay line, receiving the second surface acoustic wave after propagation across the second delay line; and generating a signal based at least in part on at least one of the received first surface acoustic wave or the received second acoustic wave. The signal is indicative of at least one of the first delay line, the length of the second delay line, or an analyte exposed to at least one of the first SAW sensor or the second SAW sensor. The method further includes determining or monitoring the analyte based at least in part on the generated signal. 
     A method as disclosed herein may also include generating several excitation signals and transmitting the excitation signals sequentially to one or several SAW devices. A radio-frequency multiplexer connects a first SAW device section comprising one or several delay lines to a first section containing one or several excitations signals, the multiplexer connects a second SAW device section comprising one or several delay lines to the second excitation signal section and so on. Each SAW device section can be on the same or different SAW devices and comprises one or several delay lines with the same or different length. The method further includes receiving sequentially signals corresponding to the output of the SAW sections. The multiplexer routing is used to determine which section of the SAW device is active. 
     According to the techniques herein, a method may also include exposing at least a portion of a SAW device to a sample media comprising an analyte. The SAW device includes a first SAW sensor having a first delay line configured to propagate a first surface acoustic wave responsive to an excitation signal, and a second SAW sensor having a second delay line configured to propagate a second surface acoustic wave responsive to the excitation signal. A length of the first delay line is greater than a length of the second delay line or the length of the second delay line is greater than the length of the first delay line. A sensitive region of at least one of the first delay line or the second delay line is configured to react to the analyte such that at least one of the first surface acoustic wave or the second surface acoustic wave is altered. The method further includes receiving a signal corresponding to an output of a SAW device, identifying a first pulse, second pulse, and a third pulse of the received signal. The first pulse corresponds to the first SAW sensor. The second pulse corresponds to the second SAW sensor. The third pulse corresponds to the excitation signal. The method further includes determining at least one of a phase, frequency, amplitude, or timing of at least two of the first pulse, the second pulse, or the third pulse, and based at least in part on said determining, identifying or monitoring the analyte. 
     The method of the preceding paragraph may also include any combination of the following steps or features described in this paragraph, among other steps or features described herein. In some embodiments, said identifying or monitoring the analyte comprises determining a variance in at least one of amplitude, phase, frequency, or time-delay between at least two of the first pulse, the second pulse or the third pulse. 
     Any of the features, components, or details of any of the arrangements or embodiments disclosed in this application, including without limitation any of the SAW device embodiments or method embodiments as disclosed herein, are interchangeably combinable with any other features, components, or details of any of the arrangements or embodiments disclosed herein to form new arrangements and embodiments. 
     Conditional language, such as, among others, “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements, and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment. 
     The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. Likewise the term “and/or” in reference to a list of two or more items, covers all of the following interpretations of the word: any one of the items in the list, all of the items in the list, and any combination of the items in the list. Further, the term “each,” as used herein, in addition to having its ordinary meaning, can mean any subset of a set of elements to which the term “each” is applied. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” As used herein, the terms “connected,” “coupled,” “attached” or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof. 
     Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or” in reference to a list of two or more items, covers all of the following interpretations of the word: any one of the items in the list, all of the items in the list, and any combination of the items in the list. Likewise the term “and/or” in reference to a list of two or more items, covers all of the following interpretations of the word: any one of the items in the list, all of the items in the list, and any combination of the items in the list. 
     Depending on the embodiment, certain operations, acts, events, or functions of any of the algorithms described herein can be performed in a different sequence, can be added, merged, or left out altogether (non-limiting example: not all are necessary for the practice of the algorithms). Moreover, in certain embodiments, operations, acts, functions, or events can be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors or processor cores or on other parallel architectures, rather than sequentially. 
     The various illustrative logical blocks, modules, routines, and algorithm steps described in connection with the embodiments disclosed herein can be implemented as electronic hardware, or as a combination of electronic hardware and executable software. To clearly illustrate this interchangeability, various illustrative components, blocks, modules, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware, or as software that runs on hardware, depends upon the particular application and design constraints imposed on the overall system. The described functionality can be implemented in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the disclosure. 
     Moreover, the various illustrative logical blocks and modules described in connection with the embodiments disclosed herein can be implemented or performed by a machine, such as a processor device, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A processor device can be a microprocessor, but in the alternative, the processor device can be a controller, microcontroller, or combinations of the same, or the like. A processor device can include electrical circuitry configured to process computer-executable instructions. In another embodiment, a processor device includes an FPGA or other programmable device that performs logic operations without processing computer-executable instructions. A processor device can also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Although described herein primarily with respect to digital technology, a processor device may also include primarily analog components. For example, some or all of the signal processing algorithms described herein may be implemented in analog circuitry or mixed analog and digital circuitry. A computing environment can include any type of computer system, including, but not limited to, a computer system based on a microprocessor, a mainframe computer, a digital signal processor, a portable computing device, a device controller, or a computational engine within an appliance, to name a few. 
     The elements of a method, process, routine, or algorithm described in connection with the embodiments disclosed herein can be embodied directly in hardware, in a software module executed by a processor device, or in a combination of the two. A software module can reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of a non-transitory computer-readable storage medium. An exemplary storage medium can be coupled to the processor device such that the processor device can read information from, and write information to, the storage medium. In the alternative, the storage medium can be integral to the processor device. The processor device and the storage medium can reside in an ASIC. The ASIC can reside in a user terminal. In the alternative, the processor device and the storage medium can reside as discrete components in a user terminal. 
     Further, the processing of the various components of the illustrated systems can be distributed across multiple machines, networks, and other computing resources. In addition, two or more components of a system can be combined into fewer components. Various components of the illustrated systems can be implemented in one or more virtual machines, rather than in dedicated computer hardware systems and/or computing devices. 
     Ranges provided herein are understood to be shorthand for all of the values within the range. For example, a range of 1 to 50 is understood to include any number, combination of numbers, or sub-range from the group consisting 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50 as well as all intervening decimal values between the aforementioned integers such as, for example, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, and 1.9. With respect to sub-ranges, “nested sub-ranges” that extend from either end point of the range are specifically contemplated. For example, a nested sub-range of an exemplary range of 1 to 50 may comprise 1 to 10, 1 to 20, 1 to 30, and 1 to 40 in one direction, or 50 to 40, 50 to 30, 50 to 20, and 50 to 10 in the other direction. 
     Any patents and applications and other references noted above, including any that may be listed in accompanying filing papers, are incorporated herein by reference. Aspects of the disclosure can be modified, if necessary, to employ the systems, functions, and concepts of the various references described above to provide yet further implementations of the disclosure. 
     Details of the system may vary considerably in its specific implementation, while still being encompassed by the disclosure herein. As noted above, particular terminology used when describing certain features or aspects of the disclosure should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the disclosure with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the disclosure to the specific examples disclosed in the specification, unless the above Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the disclosure encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the disclosure under the claims. 
     Disjunctive language such as the phrase “at least one of X, Y, or Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to present that an item, term, etc., may be either X, Y, or Z, or any combination thereof (non-limiting examples: X, Y, and/or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain embodiments require at least one of X, at least one of Y, or at least one of Z to each be present. 
     Unless otherwise explicitly stated, articles such as “a” or “an” should generally be interpreted to include one or more described items. Accordingly, phrases such as “a device configured to” are intended to include one or more recited devices. Such one or more recited devices can also be collectively configured to carry out the stated recitations. For example, “a processor configured to carry out recitations A, B and C” can include a first processor configured to carry out recitation A working in conjunction with a second processor configured to carry out recitations B and C. 
     While the above detailed description has shown, described, and pointed out novel features as applied to various embodiments, it can be understood that various omissions, substitutions, and changes in the form and details of the devices or algorithms illustrated can be made without departing from the spirit of the disclosure. As can be recognized, certain embodiments described herein can be embodied within a form that does not provide all of the features and benefits set forth herein, as some features can be used or practiced separately from others. The scope of certain embodiments disclosed herein is indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a diagram of a Surface Acoustic Wave (SAW) device, according to exemplary embodiments. 
         FIG. 1B  illustrates time domain excitation signals and receiving signals corresponding to the SAW device of  FIG. 1A , according to exemplary embodiments. 
         FIG. 2A  is a diagram of a Surface Acoustic Wave (SAW) device, according to exemplary embodiments. 
         FIG. 2B  illustrates time domain excitation signals and receiving signals corresponding to the SAW device of  FIG. 2A , according to exemplary embodiments. 
         FIGS. 3A-3C  illustrate block diagrams of a multiplexing SAW measurement system, according to exemplary embodiments. 
         FIG. 4  illustrates a diagram of a SAW device, according to exemplary embodiments. 
         FIG. 5  illustrates graph of an excitation signal and a receiving signal corresponding to SAW device of  FIG. 4 , according to exemplary embodiments. 
         FIG. 6  illustrates a graph of a compressed pulse train corresponding to the receiving signal of  FIG. 5 . 
         FIG. 7  illustrates real-time phase shifts of sensing and reference channels, according to exemplary embodiments. 
         FIG. 8  is a block diagram of a multiplexing SAW device, according to exemplary embodiments. 
         FIG. 9  is a flow diagram illustrative of an embodiment of a process implemented by a multiplexing SAW device, according to exemplary embodiments. 
         FIG. 10  is a flow diagram illustrative of an embodiment of a process implemented by a multiplexing SAW device, according to exemplary embodiments. 
         FIG. 11  is a flow diagram illustrative of an embodiment of a process implemented by a multiplexing SAW device, according to exemplary embodiments. 
         FIG. 12  is a flow diagram illustrative of an embodiment of a process implemented by a multiplexing SAW device, according to exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     A Surface Acoustic Wave (SAW) sensor or a Bulk Acoustic Wave (BAW) sensor is utilized to determine or monitor an analyte (sometimes referred to as a target material) present in media, such as liquid, solid, gaseous or biological media. A SAW sensor can include a receptor configured to bind to one or more analyte(s) on a surface of the SAW sensor. When a sample media containing the one or more analyte(s) is placed on the SAW sensor, a physical, chemical or electrical reaction occurs between the analyte and the receptor. The resulting change is used to determine or monitor the content of the analyte. 
     A SAW device can include a piezoelectric substrate, an input interdigitated transducer (IDT) (sometimes referred to as a transmitting IDT) on one portion of the surface of the piezoelectric substrate, and an output IDT (sometimes referred to as a receiving IDT) on another portion of the piezoelectric substrate. The transmitting IDT can be excited with an excitation signal. For example, the excitation signal can include a variety of signals including, but not limited to, a pulse voltage, a sinusoidal electrical signal, frequency modulation, linear frequency modulation, hyperbolic frequency modulation, orthogonal frequency coding, random modulation, continuous phase modulation, frequency shift key, multi-frequency shift key, phase shift key, wavelet modulation, a wideband signal, and the like. Due to the piezoelectric effect, the transmitting IDT produces a surface acoustic wave which propagates along the space between the IDTs (generally referred to as the delay line) in the direction of the receiving IDT. After propagating along the delay line, a wavelength of the surface acoustic wave may change due to a physical, chemical or electrical reaction between the analyte and the receptor. The surface acoustic wave reaches the receiving IDT, and by the piezoelectric effect, the receiving IDT converts the acoustic wave into a receiving signal, such as an electrical signal. 
     In some embodiments, the receptor (also referred to as a sensitive layer) is placed on the delay line. When the sensitive layer is exposed to an analyte such as a particular gas, chemical material, biological material, and the like, a quantifiable change occurs in the sensitive layer, such that as the surface acoustic wave propagates along the delay line, the sensitive layer modulates or changes the surface acoustic wave. For example, a phase, velocity, amplitude or frequency of the surface acoustic wave can be altered as the surface acoustic wave propagates across the sensitive layer. 
     By comparing the excitation signal and receiving signal(s), characteristics of the analyte can be quantified. For example, changes in velocity or amplitude of the surface acoustic wave can correspond to changes in amplitude, frequency, phase-shift, or time-delay in the receiving signal, as compared to the excitation signal. Accordingly, a SAW sensor advantageously provides the ability to measure nearly any physical or chemical interference which affects the propagation of SAW and would cause the change of an output electrical signal. 
     In addition, as the surface acoustic wave propagates along the delay line, there is a noticeable and measurable delay of the receiving signal, as compared to the excitation signal. This delay can be at least partially attributable to the length of the delay line. Thus, in some embodiments, multiple SAW sensors are utilized, each having a delay line of a different length. Because the length of the delay line affects the delay of the receiving signal, each of the receiving signals of the multiple SAW sensors can have different delays. Thus, in some instances, a SAW device can simultaneously utilize multiple SAW sensors (having differing delay line lengths) to measure a plurality of analytes. 
     In some embodiments, the receiving IDT is replaced by a reflector. The surface acoustic wave passes through the delay line, reflects off the reflector, and passes back through the delay line before arriving back at the transmitting IDT. 
     For a biosensor, when a biomolecule, such as a protein, antibody, antigen, deoxyribonucleic acid (“DNA”), ribonucleic acid (“RNA”), bacteria, an animal cell, a virus or tissue, and a toxin generated therefrom, binds to a surface of the biosensor, a surface mass of the sensor changes, and thereby a signal drift occurs in the sensor. As a result, the biosensor can determine or monitor the content of the target material. 
     Uni-Directional SAW Sensors 
       FIG. 1A  is a diagram of a Surface Acoustic Wave (SAW) device  100 , according to some embodiments. The SAW device  100  includes a piezoelectric substrate (not illustrated) and an array of SAW sensors  102 ,  104 ,  106 ,  108 ,  110  having delay lines  126  of different lengths  112 ,  114 ,  116 ,  118 ,  120 . In some instances herein, the SAW device  100  is described with respect to sensor  102 . However, some or all of the other SAW sensors  104 ,  106 ,  108 ,  110  can have components or features similar or different to those described with respect to SAW sensor  102 . 
     The SAW sensor  102  resides on the substrate and includes a transmitting interdigitated transducer (IDT)  122  that excites a surface acoustic wave into the piezoelectric substrate. The SAW sensor  102  also includes a receiving IDT  124  that detects the surface acoustic wave after propagation through the substrate, and two pairs of electrical contacts  132 ,  134  for electrically connecting the IDTs  122 ,  124  to electrical components. 
     The SAW device  100  can include various piezoelectric substrates, such as a combination of one or more of 36° Y quartz, 36° YX lithium tantalate, 128° YX lithium niobate, langasite, langatate, langanite, lead zirconate titanate, cadmium sulfide, berlinite, lithium iodate, lithium tetraborate, or bismuth germanium oxide. In some embodiments, a SAW device  100  utilizes a single piezoelectric substrate to which the plurality of SAW sensors  102 ,  104 ,  106 ,  108 ,  110  is attached. In some embodiments, one or more of the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  can utilize different piezoelectric substrates (e.g., a first SAW sensor  102  is attached to a first piezoelectric substrate and a second SAW sensor  104  is attached to a second piezoelectric substrate). 
     The transmitting IDT  122  (sometimes referred to as an input IDT) transduces or converts an excitation signal into a surface acoustic wave and transmits the surface acoustic wave into the piezoelectric substrate such that the surface acoustic wave propagates through the substrate, along a delay line  126 . The excitation signal can be generated by hardware, such as a waveform generator as described herein, and can include a variety of signals including, but not limited to, pulse voltages, sinusoidal electrical signals, frequency modulation, linear frequency modulation, hyperbolic frequency modulation, orthogonal frequency coding, random modulation, continuous phase modulation, frequency shift key, multi-frequency shift key, phase shift key, wavelet modulation, etc. In some embodiments, each of the transmitting IDTs  122  of the SAW device  100  are excited simultaneously with a single excitation signal. For instance, the excitation signal can be received by an RF switch, which synchronizes the transmission across some or all of the transmitting IDTs  122 . In some embodiments, at least some of the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  are not excited simultaneously. For example, two or more of the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  can be excited sequentially. 
     The transmitting IDT  122  can receive the excitation signal via the electrical contacts  132  (e.g., contact pads). For example, the SAW sensor  102  can include a first pair of electrical contacts  132  for receiving the excitation signal. The pair of electrical contacts  132  includes a positive and negative component that can be used for electrically connecting the transmitting IDT  122  with internal or external electrical components, such as a voltage source. For example, to generate a surface acoustic wave, a voltage source is connected to the transmitting IDT  122  through the electrical contacts  132 , which includes a positive contact for connecting to a positive voltage of an excitation source and a negative contact for connecting to a negative voltage (e.g., an electrical ground of the system). 
     Once excited (e.g., when voltage or an excitation signal is applied), the array of transmitting IDTs generates a plurality of surface acoustic waves propagating through the substrate, along the delay lines  126  of each SAW sensor  102 ,  104 ,  106 ,  108 ,  110 . As a non-limiting example, a first surface acoustic wave is generated and transmitted along a first delay line corresponding to SAW sensor  102 , a second surface acoustic wave is generated and transmitted along a second delay line corresponding to SAW sensor  104 , a third surface acoustic wave is generated and transmitted along a third delay line corresponding to SAW sensor  106 , a fourth surface acoustic wave is generated and transmitted along a fourth delay line corresponding to SAW sensor  108 , and a fifth surface acoustic wave is generated and transmitted along a fifth delay line corresponding to SAW sensor  110 . The surface acoustic waves can have various frequencies. For instance, the frequency of a surface acoustic wave can be approximately 50, 100, 150, 200, 250, 300, 350, 400, 500, 600, 700, 800, 900, or 1000 MHz (+/− approximately 25 MHz). Similarly, the frequency of the surface acoustic waves can be less than 100 MHz, greater than 100 MHz, greater than 300 MHz, greater than 500 MHz, or greater than 1000 MHz. 
     In some instances, the frequency of the surface acoustic wave can at least partially depend on a type or composition of the piezoelectric substrate. For example, the frequency of the surface acoustic wave can be greater than 100 MHz for a SAW sensor having a piezoelectric substrate that excites pure or leaky shear-horizontal mode generation (non-limiting examples: 36° Y quartz, 36° YX lithium tantalate, langasite, langatate, langanite, lead zirconate titanate, cadmium sulfide, berlinite, lithium iodate, lithium tetraborate, and bismuth germanium oxide). 
     In some embodiments, the SAW device  100  includes a thin guiding layer (not shown) that confines the surface acoustic wave as it propagates through the substrate. 
     The receiving IDT  124  (sometimes referred to as an output IDT) receives a surface acoustic wave after a delay of some finite time (e.g., after the surface acoustic wave propagates from the transmitting IDT  122 , through the delay line, to the receiving IDT  124 ). The receiving IDT  124  transduces the propagated surface acoustic wave (e.g., the surface acoustic wave after it propagates along the delay line  126 ) into a receiving signal (e.g., an electrical signal). As described herein, in some embodiments, all of the surface acoustic waves of the SAW device  100  can arrive at or reach a corresponding receiving IDT  124  at separate times due to the differing delay line lengths  112 ,  114 ,  116 ,  118 ,  120 . For example, each of the transmitting IDTs  122  can simultaneously transmit a surface acoustic wave along a delay line  126 . Because the delay line lengths  112 ,  114 ,  116 ,  118 ,  120  can be different for each SAW sensor  102 ,  104 ,  106 ,  108 ,  110 , the surface acoustic waves require different periods of time to propagate across a corresponding delay line  126  before reaching a corresponding receiving IDT  124 . Thus, the various delays of the receiving signals (or pulses of the receiving signal) can be based at least in part on a length variation between the delay lines  126  of the different SAW sensors  102 ,  104 ,  106 ,  108 ,  110 . 
     The SAW device  100  can include an array of electrical contacts  132 ,  134  (e.g., contact pads) on each side of the delay lines  126 . For example, a SAW sensor  102  can include two pairs of electrical contacts  132 ,  134 , each pair having a positive and negative component. The positive and negative components can be used for IDT electrical connections with internal or external electrical components such as a voltage source or phase detection integrated circuit, to name a few. For example, to generate a surface acoustic wave, a voltage is connected to the transmitting IDT  122  through the electrical contacts  132 , which includes a positive contact for connecting to a positive voltage of an excitation source and a negative contact for connecting to a negative voltage (e.g., an electrical ground of the system). Similarly, to receive the surface acoustic wave after it propagates through the substrate, the receiving IDT  124  includes or is connected to two contacts (positive and negative) for connecting with positive and negative electrodes of an external measurement system (such as an RF switch or an RF amplifier). 
     In some embodiments, the number of contacts  132 ,  134  increases proportionally as the number of SAW sensors  102 ,  104 ,  106 ,  108 ,  110  increases. For example, although the SAW device  100  is illustrated at including five SAW sensors  102 ,  104 ,  106 ,  108 ,  110 , any number of SAW sensors can be utilized (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more). Thus, because the number of contacts can increase proportionally as the number of SAW sensors increases, the size or number of the contacts sometimes constitutes a limiting factor on the SAW device  100  size. 
     Accordingly, although not illustrated in  FIG. 1A , in some embodiments, the contacts  132 ,  134  of some or each of the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  of the multiplexing SAW device  100  are joined or connected together. This advantageously can result in a reduction in size of the multiplexing SAW device  100 , a reduction in cost (e.g., since costs increase proportionally with chip size), or an increase in the number of possible SAW sensors  102 ,  104 ,  106 ,  108 ,  110  (thereby increasing the number of analytes which can be detected). For example, the positive contact pads of each of the transmitting IDTs can be joined together, and the negative contact pads of each of the transmitting IDTs can also be joined together. Similarly, the corresponding positive or negative contact pads of each of the receiving IDTs can be joined together. This connection can occur on the SAW device  100  itself (such as at the piezoelectric substrate with a multi-layer metallization process and common contact pads), or can occur off the SAW device  100  (such as with an external printed circuit board (PCB)). The connection of common contact pads (e.g., positive with positive, negative with negative) contributes to a reduction in the size of the SAW device  100  chip. For example, with reference to  FIG. 1A , the total number of contacts of SAW device  100  for external connection can be reduced to four types (e.g., positive and negative contacts for transmitting IDTs  122 , and positive and negative contacts for receiving IDTs  124 ). The total number of contacts can be reduced to four, irrespective of the number of SAW sensors in the SAW device  100 . 
     Although the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  are arranged in a sequencing format where the delay-line lengths  112 ,  114 ,  116 ,  118 ,  120  are gradually increased in size from a first sensor  102  to a last sensor  110 , it should be noted that the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  can be arranged in any sequence (e.g., no order corresponding to delay line length  112 ,  114 ,  116 ,  118 ,  120 ). In addition, although the delay-line length  112 ,  114 ,  116 ,  118 ,  120  of each SAW sensor  102 ,  104 ,  106 ,  108 ,  110  is different in the illustrated example, in some embodiments, one or more of the delay-line-lengths  112 ,  114 ,  116 ,  118 ,  120  can be the same. 
     In some embodiments, a delay line  126  includes an aluminum or gold layer, or a quidded layer with a polymer, SiO2, or ZnO. The delay lines are rendered biologically active by conjugating a layer of receptors such as antibodies, proteins, aptamers, or ligands that bind analytes from a fluid. Similarly, the sensor can detect chemicals in fluids through binding to a chemically sensitive interface. 
     In some embodiments, the delay line  126  (or a guiding layer, sensitive layer, or sensing area positioned on or near the delay line) provides a mechanism for attachment of an analyte (such as a biological or chemical analyte) from a medium (such as a liquid). For example,  FIG. 1A  illustrates a fluidic cell  128  which covers at least a portion of the array of SAW sensors  106  (e.g., a portion of the delay lines  126 ) and is configured to provide the delivery of analytes to the delay lines  126  or sensing area. 
     In some embodiments, a sensitive layer is attached to the surface of each of the SAW sensors (e.g., on the delay line  126 ), between the transmitting IDT  122  and receiving IDT  124 . When the sensitive layer is exposed to an element (non-limiting examples: a gas, a chemical material, a biological material), the sensitive layer is altered such that it causes a quantifiable change in the propagating wave (e.g., in the amplitude, velocity, etc.). The change can be measured by, for example, detecting the variance of the excitation signal and the receiving signals in terms of amplitude, phase, frequency, or time-delay. 
     It some instances, it can be desirable to detect, monitor or measure multiple analytes simultaneously using a single SAW device. For example, detecting multiple analytes can be beneficial for biological material such as infectious disease diagnostics, or volatile organic compounds detection, to name a few. In some embodiments as described herein, multiple analytes can simultaneously be detected or measured by the SAW device  100 . For example, the different delay-line lengths  112 ,  114 ,  116 ,  118 ,  120  of each SAW sensor  102 ,  104 ,  106 ,  108 ,  110  advantageously result in a time delay between receiving signals associated with the SAW sensors  102 ,  104 ,  106 ,  108 ,  110 . By delaying the receiving signals such that they are each separated by a time delay, the SAW device  100  advantageously allows the testing of one or more analytes, for instance, in a sample media. For example, the receiving signals can be combined into a compressed pulse train. The pulses of the compressed pulse train each have a specific time delay corresponding to the length difference of delay lines. In some embodiments, phase or other information of the compressed pulse train can be extracted. 
       FIG. 1B  illustrates time domain excitation signals (T X )  156 ,  158  and receiving signals (R X )  140 ,  150  corresponding to the SAW device  100  of  FIG. 1A , according to some embodiments. As described herein, the SAW device  100  of  FIG. 1A  includes an array of five SAW sensors  102 ,  104 ,  106 ,  108 ,  110 , each having a delay line  126  of different lengths  112 ,  114 ,  116 ,  118 ,  120 . For example, the length of each delay line can be determined from Equation 1, below: 
       Delay line length= L   1 +( n− 1)*Δ L  
 
     where L 1  is the length of the shortest delay line (e.g., length  112 ), n is a number corresponding to an order number of a SAW sensor when all of the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  are sorted from shortest delay line to longest delay line (e.g., n=1 for SAW sensor  102  having the shortest delay line  112 , n=2 for the SAW sensor having the next shortest delay line  114 , n=5 for SAW sensor  110  having the longest delay line  120 ), and ΔL  130  is the difference in delay line length between subsequent SAW sensors when the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  are ordered by delay line length. It should be noted, however, that although each of the delay line lengths  112 ,  114 ,  116 ,  118 ,  120  are different by a factor of ΔL  130  in the illustrated example, the delay lines  126  can have any length and are therefore not required to increase in a lock-step or other patterned manner. Thus, it follows that Equation 1 for determining length of a delay line may change depending on the specific delay line lengths. Alternatively, it might be the case that no equation for determining each of the delay line lengths  112 ,  114 ,  116 ,  118 ,  120  is available. In some embodiments, the delay line lengths are stored in memory. In some embodiments, the delay line lengths are predetermined. 
     With continued reference to  FIGS. 1A and 1B , an excitation signal (T X )  156  is received by the SAW device  100  and each of the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  are simultaneously excited. In this example, the excitation signal (T X )  156  is a pulse voltage. However, as described herein, the excitation signal (T X )  156  can be one or more of various signals. For example, the excitation signal (T X )  156  can be a frequency-modulated signal that covers a spectrum of frequency. In some embodiments, the frequency-modulated signal can advantageously provide a higher power gain than can an excitation signal at a fixed frequency. 
     The excitation signal (T X )  156  excites the arrays of transmitting IDTs  122  and generates an array of surface acoustic waves, which propagate along the delay line  126  of each SAW sensor  102 ,  104 ,  106 ,  108 , and  110 . The receiving IDTs  124  receive the propagated surface acoustic waves and convert the surface acoustic waves into pulses  141 ,  142 ,  143 ,  144 ,  145  of receiving signals (R X )  140 ,  150 . As described herein, the delay-line lengths  112 ,  114 ,  116 ,  118 ,  120  of each SAW sensor  102 ,  104 ,  106 ,  108 ,  110  are different. Thus, the surface acoustic waves will reach the various receiving IDTs  124  at various times. Accordingly, the individual pulses  141 ,  142 ,  143 ,  144 ,  145  of the receiving signal (R X )  140  are each delayed by a different period of time, which corresponds to the different lengths  112 ,  114 ,  116 ,  118 ,  120  of the delay lines  126 . This delay in time between the individual pulses  141 ,  142 ,  143 ,  144 ,  145  of the receiving signal (R X )  140  occurs even though the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  are excited simultaneously. 
     Stated another way, the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  generate a pulse train of electrical signals  141 ,  142 ,  143 ,  144 ,  145  due to the propagation delay of different lengths  112 ,  114 ,  116 ,  118 ,  120  of the delay lines  126 . With respect to the example of  FIG. 1B , the time delay (T d )  136  between the excitation pulse (T X )  156  and the first pulse  141  of the receiving signal (R X )  140  (e.g., the pulse  141  corresponding to the SAW sensor  102  having the delay line  126  with the shortest length  112 ) is given by Equation 2, below: 
     
       
      
       T 
       d 
       =L 
       1 
       /V  
      
     
     where L 1  is the delay line length of the shortest delay line (e.g., length  112 ), and v is the surface acoustic wave velocity, wherein the surface acoustic wave velocity (v) of a wave is the rate at which the surface acoustic wave propagates in a particular space (e.g., through a substrate). 
     The time delay (ΔT d )  138  between each of the subsequent pulses  141 ,  142 ,  143 ,  144 ,  145  of the receiving signal (R X )  140  is given by Equation 3, below: 
       Δ T   d   =ΔL/v  
 
     where ΔL  130  is the difference in delay line length between subsequent SAW sensors when the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  are ordered by delay line length, and v is the surface acoustic wave velocity. 
     The time delay (T d )  136  between the first pulse  141  and the excitation signal  156  can be measured in variety of ways. For example, the time delay (T d )  136  can correspond to the time difference between the center, beginning, end or each pulse. In some instances, the time delay (T d )  136  can correspond to the time difference between a corresponding region of each of the pulses  141 ,  156 . The time delay (ΔT d )  138  can be determined using similar techniques. 
     As a non-limiting example, a sensor has a shortest delay line length (L) (e.g., delay length  112  of SAW sensor  102 ) of 4 mm and is attached to a 36° LiTaO3 piezoelectric substrate. A surface acoustic wave traveling through a 36° LiTaO3 piezoelectric substrate has a velocity (v) of 4212 m/sec. Thus, the delay line of the shortest length (L 1 )  112  will delay the first pulse  141  approximately 0.95 μs for unidirectional SAW sensors. In addition, with a difference of delay line length (ΔL)  130  of approximately 0.3 mm, the time delay (ΔT d ) between each receiving pulse  141 ,  142 ,  143 ,  144 ,  145  of the receiving signal (R X )  140  is approximately 71 ns. 
     In some embodiments, the SAW sensors  102 ,  104 ,  106 ,  108 ,  110  are continuously excited at a constant period (T p )  152 ,  154  (e.g., at intervals of 10, 20, 30, 40, 50, 100, 200, 400, or 500 μs) to generate multiple receiving signals (R X )  140 ,  150 . In examples such as these, the receiving signals (R X )  140 ,  150  (e.g., the pulses  141 ,  142 ,  143 ,  144 ,  145  in the receiving signals  140 ,  150 ) can be averaged to, for example, determine a receiving signal (R X ) having reduced receiving noise. In some embodiments, the time delays (e.g., T d , ΔT d ) can be on the order of ns to μs. 
     Bi-Directional SAW Sensors 
       FIG. 2A  is a diagram of a surface acoustic wave (SAW) device  200 , according to some embodiments. The multiplexing SAW device  200  includes a piezoelectric substrate (not illustrated) and an array of SAW sensors  202 ,  204 ,  206 ,  208 ,  210 , wherein the delay-line lengths  212 ,  214 ,  216 ,  218 ,  220  of each SAW sensor  202 ,  204 ,  206 ,  208 ,  210  are different. In some instances herein, the SAW device  200  will be described with respect to sensor  202 . Each of the other SAW sensors  204 ,  206 ,  208 ,  210  can have components or features similar or different to those described with respect to SAW sensor  202 . 
     SAW sensor  202  can include a transmitting/receiving IDT  222  that transmits a surface acoustic wave into the piezoelectric substrate and detects a reflected SAW. The SAW sensor  202  can also include a reflector  224 , which reflects the SAW back towards the transmitting/receiving IDT  222 , and a pair of electrical contacts  232  for IDT electrical connections. 
     The SAW device  200  can include various piezoelectric substrates, such as a combination of one or more of 36° Y quartz, 36° YX lithium tantalate, 128° YX lithium niobate, langasite, langatate, langanite, lead zirconate titanate, cadmium sulfide, berlinite, lithium iodate, lithium tetraborate, or bismuth germanium oxide. In some embodiments, a multiplexing SAW device  200  utilizes a single piezoelectric substrate on which the plurality of SAW sensors  202 ,  204 ,  206 ,  208 ,  210  rest. In some embodiments, one or more of the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  can utilize different piezoelectric substrates (e.g., a first SAW sensor  202  resides on a first piezoelectric substrate and a second SAW sensor  204  resides on a second piezoelectric substrate). 
     The transmitting/receiving IDT  222  transduces or converts the excitation signal into a surface acoustic wave and transmits the surface acoustic wave into the piezoelectric substrate such that the surface acoustic wave propagates through the substrate, along a delay line  226 . The excitation signal can include a variety of signals including, but not limited to, a pulse voltage, a frequency modulated signal, a sinusoidal electrical signal, etc. In some embodiments, each of the transmitting IDTs of the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  within the SAW device  200  are excited simultaneously with an excitation signal, for instance, using an RF switch to synchronize the transmission. In some embodiments, at least some of the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  are not excited simultaneously (for example, the SAW sensors  202  and  204  can be excited sequentially). 
     The transmitting/receiving IDT  222  can receive the excitation signal via an array of electrical contacts  232  (e.g., contact pads). For example, the SAW sensors  202  can include a pair of electrical contacts  232  for receiving the excitation signal. The pair of electrical contacts  232  includes a positive and negative component that can be used for electrically connecting the transmitting/receiving IDT  222  with internal or external electrical components, such as a voltage source. For example, to generate a surface acoustic wave, a voltage is connected to the transmitting/receiving IDT  222  through the electrical contacts  232 , which includes a positive contact for connecting to a positive voltage of an excitation source and a negative contact for connecting to a negative voltage (e.g., an electrical ground of the system). The contacts  232  can also be utilized for connecting with positive and negative electrodes of an external measurement system (such as an RF switch or an RF amplifier). 
     Once excited, the array of transmitting/receiving IDTs generates an array of surface acoustic waves propagating through the substrate, along the delay lines  226  of each SAW sensor  202 ,  204 ,  206 ,  208 ,  210 . The surface acoustic waves can have various frequencies. For instance, the frequency of a surface acoustic wave can be approximately 50, 100, 150, 200, 250, 300, 350, 400, 500, 600, 700, 800, 900, or 1000 MHz (+/− approximately 25 MHz). Similarly, the frequency of a surface acoustic wave can be less than 100 MHz, greater than 100 MHz, greater than 300 MHz, greater than 500 MHz, or greater than 1000 MHz. 
     In some instances, the frequency of the surface acoustic wave can at least partially depend on a type or composition of the piezoelectric substrate. For example, the frequency of the surface acoustic wave can be greater than 100 MHz for SAW sensor  206  having a piezoelectric substrate that excites pure or leaky shear-horizontal mode generation (non-limiting examples: 36° Y quartz, 36° YX lithium tantalate, langasite, langatate, langanite, lead zirconate titanate, cadmium sulfide, berlinite, lithium iodate, lithium tetraborate, and bismuth germanium oxide). 
     In some embodiments, multiplexing SAW device  200  includes a thin quidding layer (not shown) that confines the surface acoustic wave as it propagates through the substrate. 
     The transmitting/receiving IDT  222  also receives the surface acoustic after the surface acoustic reflects off reflector  224  and the surface acoustic wave propagates back through the substrate. The transmitting/receiving IDT  222  transduces the propagated acoustic wave into a receiving signal. In some embodiments, each of the plurality of surface acoustic waves arrives at or reaches the plurality of transmitting/receiving IDTs at separate times. For example, as described above, each of the transmitting/receiving IDTs can simultaneously transmit a surface acoustic wave into the substrate. Because the lengths  212 ,  214 ,  216 ,  218 ,  220  of the delay lines of the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  are different for each SAW sensor  202 ,  204 ,  206 ,  208 ,  210 , the surface acoustic waves propagate across different delay line lengths and reach each of the transmitting/receiving IDTs at various times, based on the length variation of delay line. A compressed pulse train can be generated from the receiving signals. Pulses of the compressed pulse train can have a specific time delay corresponding to the length difference of delay lines. In some embodiments, phase or other information of the compressed pulse train can be extracted. 
     The SAW device  200  includes an array of electrical contacts  232  (e.g., contact pads) on each side of the delay lines  226 . For example, each of the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  can include a pair of electrical contacts  232  each pair having a positive and negative component. The positive and negative components can be used for IDT electrical connections with internal or external electrical components such as a voltage source or phase detection integrated circuit, to name a few. For example, to generate a surface acoustic wave, a voltage is connected to the transmitting/receiving IDT  222  through the electrical contacts  232 , which includes a positive contact for connecting to a positive voltage of an excitation source and a negative contact for connecting to a negative voltage (e.g., an electrical ground of the system). Similarly, the positive and negative contacts of the transmitting/receiving IDT  222  can connect with positive and negative electrodes of an external measurement system (such as an RF switch or an RF amplifier). 
     In some embodiments, the number of contacts  232  increases proportionally as the number of SAW sensors  202 ,  204 ,  206 ,  208 ,  210  increases. For example, although the SAW device  200  is illustrated at including five SAW sensors  202 ,  204 ,  206 ,  208 ,  210 , any number of SAW sensors can be utilized (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more). Thus, because the required number of contact can increase proportionally as the number of SAW sensors increases, the size or number of the contact sometimes constitutes a limiting factor on the device size. 
     Accordingly, although not illustrated in  FIG. 2A , in some embodiments, the contact pads of some or each of the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  of the multiplexing SAW device  200  are joined or connected together. This advantageously can result in a reduction in size of the SAW device  200 , a reduction in cost (e.g., since costs increase proportionally with chip size), or an increase in the number of possible SAW sensors (thereby increasing the number of analytes which can be detected). For example, the positive contact pads of each of the transmitting/receiving IDTs can be joined together, and the negative contact pads of each of the transmitting/receiving IDTs can also be joined together. This connection can occur on the SAW device  200  itself (such as at the piezoelectric substrate with a multi-layer metallization process and common contact pads), or can occur off the SAW device  200  (such as with an external printed circuit board (PCB)). The connection of common contacts (e.g., positive with positive, negative with negative) contributes to a reduction in the size of the sensor chip. For example, the number of contacts for external connection can be reduced to two types (e.g., positive and negative contacts for the transmitting/receiving IDTs), irrespective of the number of SAW sensors  202 ,  204 ,  206 ,  208 ,  210  in the multiplexing SAW device  200 . 
     The different delay-line lengths  212 ,  214 ,  216 ,  218 ,  220  of each SAW sensor  202 ,  204 ,  206 ,  208 ,  210  cause the surface acoustic waves to reach the transmitting/receiving IDTs  222  at separate times. Thus, a receiving signal from each SAW sensor is delayed at various times, based on the length variation of delay line. A compressed pulse train can be generated with a specific time delay according to the length difference of delay lines. Phase or other information of the compressed pulse can be extracted. 
     Although the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  are arranged in a sequencing format where the delay-line lengths  212 ,  214 ,  216 ,  218 ,  220  are gradually increased from a first sensor to a last sensor, it should be noted that the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  can be arranged in any sequence (e.g., no order corresponding to delay line length  212 ,  214 ,  216 ,  218 ,  220 ). In addition, although the delay-line length  212 ,  214 ,  216 ,  218 ,  220  of each SAW sensor  202 ,  204 ,  206 ,  208 ,  210  is different in the illustrated example, in some embodiments, one or more of the delay-line-lengths can be the same. 
     In some embodiments, a delay line  226  includes an aluminum or gold layer, or a quidded layer with a polymer, SiO2, or ZnO. The delay lines are rendered biologically active by conjugating a layer of receptors such as antibodies, proteins, aptamers, or ligands that bind analytes from a fluid. Similarly, the sensor can detect chemicals in fluids through binding to a chemically sensitive interface. 
     In some embodiments, the delay line  226  (or a guiding layer, sensitive layer, or sensing area positioned on the delay line) provides a mechanism for attachment of an analyte (such as a biological or chemical analyte) from a medium (such as a liquid). For example,  FIG. 2A  illustrates a fluidic cell  228  which covers at least a portion of the array of SAW sensors  202 ,  204 ,  206 ,  208 ,  210  (e.g., a portion of the delay lines  226 ) and is configured to provide the delivery of analytes to the delay lines  226  or sensing area. 
     In some embodiments, a sensitive layer resides on the surface of each of the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  (e.g., on the delay line  226 ), between the transmitting/receiving IDT  222  and the reflector  224 . When the sensitive layer is exposed to an element (non-limiting examples: a gas, a chemical material, a biological material), the sensitive layer is altered such that it causes a quantifiable change in the propagating wave (e.g., in the amplitude, velocity, etc.). The change can be measured by detecting the variance of input and output electrical signals, for instance, in terms of amplitude, phase, frequency, or time-delay. 
     It some instances, it can be desirable to detect, monitor or measure multiple analytes simultaneously using a single SAW device. For example, a single SAW device may be more time-efficient. In addition, detecting multiple analytes can be beneficial for biological material such as infectious disease diagnostics, or volatile organic compounds detection, to name a few. In some embodiments as described herein, multiple analytes can simultaneously be detected or measured by the SAW device  200 . For example, the different delay-line lengths  212 ,  214 ,  216 ,  218 ,  220  of each SAW sensor  202 ,  204 ,  206 ,  208 ,  210  advantageously result in a time delay between receiving signals associated with the SAW sensors  202 ,  204 ,  206 ,  208 ,  210 . By delaying the receiving signals such that they are each separated by a time delay, the SAW device  200  advantageously allows the testing of multiple analytes. For example, the receiving signals can be combined into a compressed pulse train. The pulses of the compressed pulse train each have a specific time delay corresponding to the length difference of delay lines. In some embodiments, phase or other information of the compressed pulse train can be extracted. 
       FIG. 2B  illustrates time domain excitation signals (T X )  256 ,  258  and receiving signals (R X )  240 ,  250  corresponding to the SAW device  200  of  FIG. 2A , according to some embodiments. As described herein, the SAW device  200  of  FIG. 2A  includes an array of five SAW sensors  202 ,  204 ,  206 ,  208 ,  210 , each having a delay line  226  of a different length  212 ,  214 ,  216 ,  218 ,  220 . For example, the length of each delay line can be determined from Equation 4, below: 
         L   2 +( n− 1)*Δ L   2  
 
     where L 2  is the length of the shortest delay line (e.g., length  212 ), n is a number corresponding to an order number of a SAW sensor when all of the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  are sorted from shortest delay line to longest delay line (e.g., n=1 for SAW sensor  202  having the shortest delay line  212 , n=2 for the SAW sensor having the next shortest delay line  214 , n=5 for SAW sensor  210  having the longest delay line  220 ), and ΔL is the difference in delay line length between subsequent SAW sensors when the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  are ordered by delay line length. It should be noted, however, that although each of the delay line lengths  212 ,  214 ,  216 ,  218 ,  220  are different by a factor of ΔL 2    230  in the illustrated example, the delay lines  226  can have any length and are therefore not required to increase in a lock-step or other patterned manner. Thus, it follows that Equation 4 for determining length of a delay line may change depending on the specific delay line lengths. Alternatively, it might be the case that no equation for determining each of the delay line lengths  212 ,  214 ,  216 ,  218 ,  220  is available. 
     With continued reference to  FIGS. 2A and 2B , an excitation signal (T X )  256  is received by the SAW device  200  and each of the SAW sensors  102 ,  104 ,  106 ,  108 ,  210  are simultaneously excited. In this example, the excitation signal (T X )  256  is a pulse voltage. However, as described herein, the excitation signal (T X )  256  can be one or more of various signals. For example, the excitation signal (T X )  256  can be a frequency-modulated signal that covers a spectrum of frequency. In some embodiments, the frequency-modulated signal can advantageously provide a higher power gain than can an excitation signal at a fixed frequency. 
     The excitation signal (T X )  256  excites the array of transmitting/receiving IDTs  222 , which generate an array of surface acoustic waves. The surface acoustic waves propagate along the delay line  226  of each SAW sensor  202 ,  204 ,  206 ,  208 , and  210  before reaching a reflector  224 . The reflectors  224  echo or reflect the surface acoustic waves back along the delay line. The surface acoustic waves again propagate through the delay line  226 , before being received by the transmitting/receiving IDTs  222 , which generate receiving signals (e.g., by transducing a surface acoustic wave into an electrical signal). The transmitting/receiving IDTs  222 . It should be noted that, in some instances, a reflective delay line SAW sensor (e.g., a SAW sensor having reflectors), can include multiple IDTs (e.g., a transmitting IDT and a receiving IDT) or a single IDT that transmits and receives. 
     The transmitting/receiving IDTs  222  receive the propagated surface acoustic waves and convert the surface acoustic waves into pulses  241 ,  242 ,  243 ,  244 ,  245  of receiving signals (R X )  240 ,  250 . As described herein, the delay-line lengths  212 ,  114 ,  216 ,  218 ,  220  of each SAW sensor  202 ,  204 ,  206 ,  208 ,  210  are different. Thus, the surface acoustic waves will reach the various receiving IDTs  222  at various times. Accordingly, the individual pulses  241 ,  242 ,  243 ,  244 ,  245  of the receiving signal (R X )  240  are each delayed by a different period of time, which corresponds to the different lengths  212 ,  214 ,  216 ,  218 ,  220  of the delay lines  226 . This delay in time between the individual pulses  241 ,  242 ,  243 ,  244 ,  245  of the receiving signal (R X )  240  occurs even though the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  are excited simultaneously. 
     Stated another way, the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  generate a pulse train of electrical signals  241 ,  242 ,  243 ,  244 ,  245  due to the propagation delay of different lengths,  212 ,  214 ,  216 ,  218 ,  220  of the delay lines  226 . With respect to the example of  FIG. 2B , the time delay (T d )  236  between the excitation pulse (T X )  256  and the first pulse  241  of the receiving signal (R X )  240  (e.g., the pulse  241  corresponding to the SAW sensor  202  having the delay line  226  with the shortest length  212 ) is given by Equation 5, below: 
         T   d =2* L   2   /v    
     where L 2  is the delay line length of the shortest delay line (e.g., length  212 ), and v is the surface acoustic wave velocity. Because the surface acoustic wave propagates the delay line twice, the delay time (T d )  236  is double the time delay (T d )  136  of the SAW device  100  (having unidirectional SAW sensors), even though the delay lines lengths are the same (see e.g.,  FIGS. 1B and 2B ). 
     The different in time delay (ΔT d )  238  between the each of the subsequent pulses  241 ,  242 ,  243 ,  244 ,  245  of the receiving signal (R X )  240  is given by Equation 6, below: 
       Δ T   d =2*Δ L   2   /v  
 
     where ΔL 2  is the difference in delay line length between subsequent SAW sensors when the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  are ordered by delay line length, and v is the surface acoustic wave velocity. Because the surface acoustic wave propagates the delay line twice, the difference in delay time (ΔT d )  238  is double the difference in time delay (ΔT d )  138  of the SAW device  100  (having unidirectional SAW sensors), even though the delay lines lengths are the same. 
     The time delay (T d )  236  between the first pulse  241  and the excitation signal  256  can be measured in variety of ways. For example, the time delay (T d )  236  can correspond to the time difference between the center, beginning, end or each pulse. In some instances, the time delay (T d )  236  can correspond to the time difference between a corresponding region of each of the pulses  241 ,  256 . The time delay (ΔT d )  238  can be determined using similar techniques. 
     As a non-limiting example, a sensor has a shortest delay line length (L) (e.g., delay length  212  of SAW sensor  202 ) of 4 mm and is attached to a 36° LiTaO3 piezoelectric substrate. A surface acoustic wave traveling through a 36° LiTaO3 piezoelectric substrate has a surface acoustic wave velocity (v) of 4212 m/sec. Thus, the delay line of the shortest length (L 2 )  212  will delay the first pulse  241  approximately 1.9 μs for unidirectional SAW sensors. In addition, as shown in  FIG. 2A , with a difference of delay line length (ΔL 2 )  230  of approximately 0.3 mm, the time delay (ΔT d )  238  between each receiving pulse  241 ,  242 ,  243 ,  244 ,  245  of the receiving signal (R X )  240  is approximately 142 ns. 
     In some embodiments, the SAW sensors  202 ,  204 ,  206 ,  208 ,  210  are continuously excited at a constant period (T p )  252 ,  254  (e.g., at intervals of 10, 20, 30, 40, 50, 100, 200, 400, or 500 μs) to generate multiple receiving signals (R X )  240 ,  250 . In examples such as these, the receiving signals (R X )  240 ,  250  (e.g., the pulses  241 ,  242 ,  243 ,  244 ,  245  in the receiving signals  240 ,  250 ) can be averaged to, for example, determine a receiving signal (R X ) having reduced receiving noise. In some embodiments, the time delays (e.g., T d , ΔT d ) can be on the order of ns to μs. 
       FIGS. 3A-3B  illustrate block diagrams of a multiplexing SAW measurement system, according to some embodiments. The multiplexing SAW measurement system  300 A of  FIG. 3A  includes a waveform generator  360 , a first amplifier  362 , an array of SAW sensors  364 , a second amplifier  366 , a deconvolution module  368 , a filter module  370 , and a phase detection module  372 . The multiplexing SAW measurement system  300 B of  FIG. 3B  further includes an RF switch  376 . 
     The waveform generator  360  generates an excitation signal. For example, the excitation signal can include a pulse voltage (e.g., as illustrated in  FIGS. 1B and 2B ), a frequency modulated signal (e.g., linear frequency modulation, hyperbolic frequency modulation, etc.), a chirp signal, etc. In some embodiments, the waveform generator can be controlled by a controller which may include one or more hardware processors (non-limiting example, a start button). The waveform generator  360  can generate signals at any one of a number of frequencies. For example, the waveform generator  360  can generate signals at a frequency of approximately 50, 100, 150, 200, 250, 300, 350, 400, 500, 600, 700, 800, 900, or 1000 MHz (+/− approximately 25 MHz). In addition or alternatively, the waveform generator can generate a frequency modulated signal coving a spectrum of frequency. In some instances, a spectrum of frequency has a higher power gain than a fixed frequency. 
     The first amplifier  362  amplifies the excitation signal generated by the waveform generator  360 . In some embodiments, the amplifier  362  is a radio frequency amplifier (RF amplifier). 
     The amplified excitation signal is applied to the array of SAW sensors  364 . The array of SAW sensors  364  can be similar to any of the SAW sensors as described herein. For example, the array of SAW sensors  364  can include SAW sensors with transmission delay lines (e.g., as described with respect to  FIG. 1A ), such that the SAW sensors include a transmission IDT opposite a receiving IDT. The SAW sensors can additional or alternatively the array of SAW sensors  364  can include SAW sensors with reflective delay lines (e.g., as described with respect to  FIG. 2A ), such that the SAW sensors include a transmission/receiving IDT opposite a reflector. As described herein, each of the SAW sensors of the array  364  can have a delay line of a different length. The excitation signal is received by the array of SAW sensors  364  and the array of SAW sensors  364  generate a receiving signal (R x ), such as the receiving signal (R x )  140 ,  240  described with respect to  FIGS. 1B and 2B . 
     In some embodiments, as illustrated in  FIG. 3B , the system  300 B can include an RF switch  376  to simultaneously apply the excitation signal to each of the SAW sensors. For example, the RF switch  376  can synchronize the excitation signals to the sensor array  364 . By synchronizing the excitation signals to the sensor array  364 , the resulting phase detection (as described below) can provide a more accurate measurement, as compared to non-synchronized excitation of the sensor array  364 . 
     A second amplifier  366  amplifies the received signal (R x ) generated by the array of SAW sensors  364 . In some embodiments, the amplifier  366  is an RF amplifier. The received signal (R X ) is further processed with signal deconvolution  368  and then match filtered  370  to produce a compressed pulse train. 
     The signal deconvolution  368  can, for example, be utilized to reverse the effects of physical or chemical interferences that affected the surface acoustic wave during propagation. For example, physical or chemical interferences (e.g., associated with environmental temperature, viscosity, stress, pressure, velocity, etc.) might affect the propagation of the surface acoustic wave, thereby altering the receiving signals. In some instances (e.g., as described with respect to  FIG. 4 ) a reference channel can be utilized to measure physical or chemical interferences that are not associated with the analyte to be measured. By measuring or determining the physical chemical interferences, the receiving signals can be adjusted during signal deconvolution  368  to remove alternations that may have been caused by the interferences. 
     As described herein, the received signal (R x ) includes a plurality of pulses or signals which correspond to each of the SAW sensors of the array of SAW sensors  364 . A pulse can be used to determine variance in at least one of amplitude, phase, frequency, or time-delay between another pulse and/or the excitation signal. For example, the multiplexing SAW measurement system  300 A,  300 B includes phase detection  372  which can determine a phase  374  corresponding to each of the plurality of pulses with respect to each other and/or the excitation signal. For example, the difference in delay line length between the SAW sensors results in a time delay between the pulses of the received signal (R x ). The shifts in time domain between the pulses of the compressed pulse train correspond to phase shifts associated with a particular SAW sensor. The phase shifts can be determined, for example, using a software program, field programmable gate array (FPGA) hardware, a hardware processor, and the like. 
     The resulting system  300 A,  300 B offers the advantage of simultaneous excitation and sensing. Measurement of the received sensor signal (R x ) in a matched filter system allows for sensing of multiple targets or bio-agents simultaneously. An individual sensor can thus indicate the presence of an individual substance to which it reacts specifically. An indication regarding the amount of the substance present can be provided by the phase change of the measurement signal. As a whole, the sensor chip (e.g., the SAW device  100 , SAW device  200 , array of SAW Sensors  364 ) thus forms a detector with its multiple individual sensor elements, the detector being able to simultaneously identify a large number of various substances during a single test procedure. 
       FIG. 4  illustrates a diagram of a SAW device  400 , according to some embodiments. The SAW device  400  includes a plurality of SAW sensors  402 ,  404  with reflective delay lines  420 ,  427 ,  410 ,  417 . The SAW device  400  includes two delay lines serving as sensing channel  480 , where the delay line surface was immobilized with a biomaterial (such as antibody) for capturing specific analytes. The SAW device  400  also includes two delay lines serving as reference channels  482  for measuring any environmental effects such as temperature, stress, etc. In some instances herein, the SAW device  400  will be described with respect to sensor  402 . Each of the other SAW sensors  404  can have components or features similar or different to those described with respect to SAW sensor  402 . 
     The SAW sensor  402  or sensing channel  480  includes two IDTs  483 ,  484 , two delay lines  420 ,  427 , and four reflectors  424 ,  425 ,  421 ,  423 , and a sensing area  428 . As illustrated, in some embodiments (non-limiting example: when the excitation signal is a wideband), a delay line can have multiple reflectors. For example, delay line  420  includes a first reflector  424  and a second reflector  425 . The first reflector  424  can be configured to reflect surface acoustic waves at a first frequency and the second reflector can be configured to reflect surface acoustic waves at a second frequency (e.g., different than the first frequency). In some instances, the inclusion of multiple reflectors on a single delay line advantageously enhancing the performance of the matched filter. The SAW sensor  404  or reference channel  482  includes two IDTs  485 ,  486 , two delay lines  410 ,  417 , four reflectors  414 ,  415 ,  411 ,  413 , and a reference area  429 . As illustrated, the delay lines of all of the reference channels and sensing channels of the SAW device  400  have a different length  412 ,  419 ,  416 ,  418 . 
     In some instances, the sensing channel  480  can have the same or similar features as delay line  226  of  FIG. 2A . For example, the sensing channel  480  can be utilized to detect or measure analytes which are attached to sensing area  428 . A surface acoustic wave is transmitted in the sensing channel  480 , along the delay lines  420 ,  427 . An analyte, such as a biomaterial, can be placed in the sensing area  428 . As the surface acoustic wave propagates along the delay line, the analyte modulates the wave (e.g., phase, frequency, amplitude modulation, etc.). The SAW sensor  402  can generate a receiving signal which corresponds to the modulated surface acoustic wave. The receiving signal can then be compared to the excitation signal to determine in what ways the receiving signal was modulated by the analyte. Characteristics of the analyte can then be determined based at least in part on the modulation of the receiving signal. 
     In some instances, the reference channel  482  can be utilized to measure physical or chemical interferences that are not associated with the analyte to be measured. For example, physical or chemical interferences might affect the propagation of the surface acoustic wave, thereby altering the receiving signals. By measuring or determining the physical chemical interferences (e.g., associated with environmental temperature, viscosity, stress, pressure, velocity, etc.), the receiving signals can be adjusted to remove alternations that may have been caused by the interferences. 
     For example, SAW sensors can be sensitive to the affections of, among other things, environmental temperature fluctuations, stress or strain applied on the piezoelectric substrate, viscosity of a biological liquid (such as whole blood, serum, and urine), etc. Accordingly, a reference channel  482  can be used in conjunction with a sensing channel  480 . The surface acoustic wave of the sensing channel  480  will be modulated or modified by the analyte as well as the environmental, chemical, or physical interferences, as described above. In some instances, the reference channel  482  can be utilized such that a surface acoustic wave is not modulated by an analyte but is modulated by the same interferences as the sensing channel. The receiving signal resulting from the sensing channel can then be altered to compensate for the inferences determined with the reference channel  482 . 
       FIG. 5  illustrates a graph  500  of an excitation signal (T x )  556  and a receiving signal (R x )  540  corresponding to SAW device  400  of  FIG. 4 , according to some embodiments. As described above, the SAW device  400  includes two SAW sensors  402 ,  404  having reflective delay lines. Each SAW device  400  has two channels (a sensing channel  480  and a reference channel  482 ). The sensing channels  420 ,  427  correspond to where the delay line surface was immobilized with a biomaterial (such as antibody) for capturing specific analytes. The reference channels  410 ,  417  are for measuring any environmental effects such as temperature, stress, etc. Each channel has a corresponding delay line  420 ,  427 ,  410 ,  417 , where each of the delay lines is different in length  412 ,  419 ,  416 ,  418 . 
     In this example, the SAW device  400  was excited with a chirp signal (T X )  556  having a center frequency of 520 MHz and a bandwidth of 56 MHz. The response signal (R X )  540  generated by the SAW device  400  contained encoded information from the eight reflectors (e.g., 2 reflectors on each of the four delay lines). The length (L) of the shortest delay line (e.g., length  419  of  FIG. 4 ) is approximately 5.4 mm, and the length difference (ΔL) is approximately 0.4 mm between the reflectors. Accordingly, the first peak or pulse of the response signal (R X )  540  was received approximately 2.63 μs (T d )  536  after the excitation signal (T X )  556  was transmitted. In addition, an approximate 0.2 μs delay (ΔT d ) exists between each subsequent peak. 
       FIG. 6  illustrates a graph  600  of a compressed pulse train (R X )  640  corresponding to the receiving signal (R X )  540  of  FIG. 5 . In this example, a deconvolution and match filtered process, such as those described with respect to  FIGS. 3A-3B , were performed on the signal of  FIG. 5  to generate the compressed pulse train (R X )  640 . As illustrated, the compressed pulse train (R X )  640  has eight peaks or pulses  641 ,  642 ,  643 ,  644 ,  645 ,  646 ,  647 ,  648 , each corresponding to a respective reflector  421 ,  423 ,  424 ,  425 ,  411 ,  413 ,  414 ,  415  of  FIG. 4 . 
     Non-Limiting Bi-Directional Example 
       FIG. 7  illustrates a graph  100  of real-time phase shifts of sensing channel  780  and reference channel  782 , according to exemplary embodiments. As a non-limiting example, and with reference to  FIG. 4 , a SAW array was fabricated using standard photolithographic techniques on a 36° y-cut, x-propagating lithium tantalate (LiTaO3) wafer of 500 μm thick and 100 mm in diameter. The SAW device  400  was excited with an excitation signal having a frequency of 525 MHz. The wafer was first cleaned in a barrel asher and dipped in 1 volume percent hydrofluoric acid (HF). A photoresist was then applied onto the wafer, and patterned with photolithographic process, followed by a titanium (10 nm)/Aluminum (70 nm) metallization and liftoff process to create the IDT, aluminum waveguide and reflectors. The wafer was then diced into individual dies. 
       FIG. 7  shows the real-time response when the device was introduced with phosphate-buffered saline (PBS) buffer, 10 pg human chorionic gonadotropin (HCG), and 100 pg HCG at time of 0 sec, 1 min and 5 min. The sensors  402 ,  404  (see e.g.,  FIG. 4 ) were measured using an RF reader as a waveform generator and an RF switch, such as those described with respect to  FIGS. 3A and 3B . The SAW device  400  was connected to the RF reader through the RF switch which synchronizes the transmission and receiving signals between the RF reader and the SAW sensors  402 ,  404 . The RF reader provided a linear frequency modulation chirp signal with a center frequency of 520 MHz, and 56 MHz bandwidth. 
     A data acquisition system measured all-four channels simultaneously. Data was recorded in real time using a 12-bit A/D converter at a rate of 56 MHz, using a desktop computer, and the deconvolution and matched filter were conducted using a software program to extract the phase shift. 
     The SAW device  400  was treated with oxygen plasma to activate the surface and the whole device was coated with silane PEG-600 biotin (Nanocs). The central area (e.g., approximately 1.5 mm×1 mm) of the delay line of the sensing channels were immobilized with neutravidin followed by biotinylate anti-HCG (human chorionic gonadotropin) antibody. The excess anti-HCG was washed with HPLC water. Then, a liquid cell made of polydimethylsiloxane (PDMS) (approximately 1.5 mm wide, 4 mm long, and 0.25 mm thick) was put on top for liquid introduction. The phase shifts of both sensing  780  and reference  782  channels and the differential  784  were recorded. 
     Block Diagram 
       FIG. 8  is a block diagram  800  of a multiplexing SAW device, according to exemplary embodiments. As illustrated, the SAW device  802  can include a plurality of SAW sensors  810 ,  812 ,  814 . The SAW device  802  can receive an excitation signal  806 . For example, the excitation signal can be directly or indirectly transmitted from a waveform generator, as described herein. As illustrated in  FIG. 8 , in some embodiments, the excitation signal is transmitted to one or more contacts of the SAW device  802 . In some embodiments, the excitation signal is transmitted to an RF switch (not shown). The RF switch can synchronize the transmission of the excitation signal across one or more SAW sensors  810 ,  812 ,  814  of the SAW device  802 . In some embodiments, the SAW device  802  has an onboard RF switch (not shown). 
     As described herein, the plurality of SAW sensors  810 ,  812 ,  814  receive the excitation signal  806  and generate an electrical signal  820 ,  822 ,  824 . A hardware processor  804  receives the electrical signals  820 ,  822 ,  824  and generates a receiving signal  808 , as described herein. For example, the receiving signal  808  can include a compressed pulse train, wherein each of the pulses corresponds to at least one of the SAW sensors  810 ,  812 ,  814 . In some embodiments, the SAW device  802  saw device includes an onboard hardware processor. 
     Flow Diagrams 
       FIG. 9  is a flow diagram illustrative of an embodiment of a process  900  implemented by a multiplexing SAW device for determining or monitoring one or more analytes using a plurality of SAW sensors having delay lines of different lengths. One skilled in the relevant art will appreciate that the elements outlined for process  900  may be implemented by one or more computing devices or components of the multiplexing SAW device (such as a processor), another computing device, in software, etc. Accordingly, process  900  has been logically associated as being generally performed by a processor, and thus the following illustrative embodiments should not be construed as limiting. 
     At block  902 , the process  900  generates an excitation signal. In some embodiments, the excitation signal is generated by hardware, such as by a waveform generator as described herein. In some embodiments, the excitation signal is generated by software. As described herein, in some embodiments, the excitation signal includes at least one of a variety of signals including, but not limited to, a pulse voltage, a sinusoidal electrical signal, frequency modulation, linear frequency modulation, hyperbolic frequency modulation, orthogonal frequency coding, random modulation, continuous phase modulation, frequency shift key, multi-frequency shift key, phase shift key, or wavelet modulation. 
     At block  904 , the process  900  transmits the excitation signal to a SAW device. The excitation signal can be directly or indirectly transmitted (e.g., through another element) to the SAW device. For example, in some embodiments, the excitation signal is transmitted to one or more contacts of the SAW device such as a positive and negative contact of the SAW device. In some embodiments, the excitation signal is transmitted to an RF switch. The RF switch can synchronize the transmission of the excitation signal across one or more SAW sensors of the SAW device. 
     At block  906 , the process  900  receives an output signal from the SAW device. In some embodiments, the signal is indicative of one or more differences in length of delay lines of the SAW sensors of the SAW device. For example, as described herein, the SAW device can include a plurality of SAW sensors. Each SAW sensor includes a delay line of a different length. As the excitation signal propagates across the delay lines, the signals received after propagation across the delay line are each time delayed relative to each other. For example, the time delay corresponds to the difference in delay line lengths. In some embodiments, the signal received from the SAW device includes a plurality of portions, wherein each portion corresponds to a time-delayed signal associated with each SAW sensor of the SAW device. For example, the signal can include a compressed pulse signal, where each of the pulses correspond to a different SAW sensor and a difference in time between the pulses corresponds to the different in delay line length of the SAW sensors. 
     In some embodiments, the signal received from the SAW device is indicative of one or more analytes in a sample that was added to the SAW device. For example, each SAW sensor of the SAW device can include a receptor (also referred to as a sensitive layer) configured to bind to one or more analyte(s) on a surface of the SAW sensor. When a sample media containing the one or more analytes is placed on the SAW sensor, a physical, chemical or electrical reaction occurs between the analyte and the receptor. This physical, chemical or electrical reaction can alter (e.g., in phase, frequency, or amplitude) a surface acoustic wave as the surface acoustic wave propagates along a delay line corresponding to the physical, chemical or electrical reaction. Thus, the signal received from the SAW device can be indicative of the one or more analytes by being responsive to the physical, chemical or electrical reaction between the analyte and the receptor. 
     At block  908 , the process  900  can determine or monitor the one or more analytes introduced to the SAW sensor based at least in part on the output signal of the SAW device. As described herein, the output signal can include portions of the signal having one or more differences in phase, frequency, amplitude, etc. The differences in phase, frequency, amplitude, etc. between the portions of the output signal and/or the excitation signal can analyze to determine or monitor the one or more analytes. 
     It will be understood that the various blocks described herein can be implemented in a variety of orders, and that the process  900  can implement one or more of the blocks concurrently and/or change the order, as desired. Furthermore, it will be understood that fewer, more, or different blocks can be used as part of the process  900 . For example, the process  900  can include blocks similar to those of process  1000 ,  1100 ,  1200  (see e.g,  FIG. 10 ). 
       FIG. 10  is a flow diagram illustrative of an embodiment of a process  1000  implemented by a multiplexing SAW device for generating a receiving signal using at least a SAW device have a plurality of SAW sensors having different delay lines of different lengths. One skilled in the relevant art will appreciate that the elements outlined for process  1000  may be implemented in hardware, such as by a SAW device having a plurality of SAW sensors or a hardware processors, by one or more computing devices or components of the multiplexing SAW device (such as a hardware or other processor), another computing device, in software, etc. Accordingly, process  1000  has been logically associated as being generally performed by a processor, and thus the following illustrative embodiments should not be construed as limiting. 
     At block  1002 , the process  1000  receives an excitation signal. In some embodiments, process  1000  receives the excitation signal directly or indirectly from hardware, such as from a waveform generator as described herein. The excitation signal includes at least one of a variety of signals including, but not limited to, a pulse voltage (e.g., T x    156 , T x    256 ), a chirp signal (e.g., T x    556 ), a sinusoidal electrical signal, frequency modulation, linear frequency modulation, hyperbolic frequency modulation, orthogonal frequency coding, random modulation, a signal covering a spectrum of frequency, continuous phase modulation, frequency shift key, multi-frequency shift key, phase shift key, or wavelet modulation. In some embodiments, the process  1000  receives the excitation signal at a SAW device, such as at one or more IDTs of one or more SAW sensors. 
     In some embodiments, the excitation signal is received at one or more contacts of the SAW device such as a positive and negative contact of the SAW device. In some embodiments, the excitation signal is received at to an RF switch. The RF switch can be included or not included in the SAW device. The RF switch can synchronize the transmission of the excitation signal across one or more SAW sensors of the SAW device. 
     At block  1004 , the process  1000  generates a plurality of surface acoustic waves. For example, each of one or more IDTs can convert or transduce the excitation signal into a surface acoustic wave. The process  1000  (e.g., the one or more IDTs) transmits the surface acoustic waves across delay lines of a plurality of SAW sensors. In some embodiments, each of the SAW sensors includes a delay line of a different length. In some embodiments, one or more of the delay lines are the same or substantially the same length. 
     At block  1006 , the SAW device includes one or more reflectors configured to reflect the surface acoustic waves. For example, each of the plurality of SAW sensors can include a bidirectional sensor array such that each includes a reflector. A reflector can reside on the substrate, opposite the IDT. The surface acoustic wave is transmitted from the IDT and propagates though the substrate, before reaching the reflector. The reflector generates an echo of the surface acoustic wave (or reflects the wave), which causes the surface acoustic wave to propagate from the reflector to an IDT (e.g., the transmitting IDT, a receiving IDT, etc.). Thus, in some embodiments, the surface acoustic wave propagates through a substrate, or across a delay line multiple times. It should be noted that, in some embodiments, the SAW device does not include reflectors. Instead, the SAW sensors can include a unidirectional sensor array such that the surface acoustic wave propagates through a substrate once, and is then received, for instance, with an IDT. 
     At block  1008 , the process  1000  receives the plurality of surface acoustic waves. In some embodiments, such as with a SAW sensor including a bidirectional sensor array, the surface acoustic waves are received by an IDT after propagating across the delay line twice. In some embodiments, such as with a SAW sensor including a unidirectional sensor array, the surface acoustic waves are received by an IDT after propagating across the delay line once. As described herein, due to the different lengths of the delay lines, the propagation time of each surface acoustic wave is different. 
     At block  1010 , the process  1000  generates a receiving signal based at least in part on the received surface acoustic waves. For instance, the process  1000  can convert each of the propagated waves into an electrical signal. For example, the propagated waves can be received by a plurality of IDTs. The plurality of IDTs can convert or transduce the propagated surface acoustic waves into electrical or other signals. In some embodiments, each of the electrical or other signals is combined into a single signal (termed receiving signal). In some embodiments, the process  1000  generates a compressed pulse train, wherein one or more of the pulses correspond to the different SAW sensors of the SAW device. For example, each pulse can correspond to a different SAW sensor. In addition or alternatively, the pulses can each include a different time delay based on the length of a corresponding delay line. In some embodiments, an electrical signal corresponding to each SAW sensor is combined in a compressed pulse train. In some embodiments, signal processing software separates the signals or pulses of the compressed pulse train. 
     It will be understood that the various blocks described herein can be implemented in a variety of orders, and that the process  1000  can implement one or more of the blocks concurrently and/or change the order, as desired. Furthermore, it will be understood that fewer, more, or different blocks can be used as part of the process  1000 . For example, process  1000  may not include block  1006  because, for example, the SAW device includes a unidirectional sensor array. In addition or alternatively, the process  1000  can include blocks similar to those of processes  900 ,  1100 ,  1200 . 
       FIG. 11  is a flow diagram illustrative of an embodiment of a process  1100  implemented by a multiplexing SAW device for determining or monitoring one or more analytes using a plurality of SAW sensors having delay lines of different lengths. One skilled in the relevant art will appreciate that the elements outlined for process  1100  may be implemented by one or more computing devices or components of the multiplexing SAW device (such as a hardware processor or other processor), another computing device, in hardware, software, etc. Accordingly, process  1100  has been logically associated as being generally performed by a processor, and thus the following illustrative embodiments should not be construed as limiting. 
     At block  1102 , similar to block  902  of process  900 , process  1100  generates an excitation signal. In some embodiments, the excitation signal is generated by hardware, such as by a waveform generator as described herein. In some embodiments, the excitation signal is generated by software. As described herein, in some embodiments, the excitation signal includes at least one of a variety of signals including, but not limited to, a pulse voltage, a sinusoidal electrical signal, frequency modulation, linear frequency modulation, hyperbolic frequency modulation, orthogonal frequency coding, random modulation, continuous phase modulation, frequency shift key, multi-frequency shift key, phase shift key, or wavelet modulation. 
     At block  1104 , similar to block  904  of process  900 , process  1100  transmits the excitation signal to a SAW device. The excitation signal can be directly or indirectly transmitted (e.g., through another element) to the SAW device. For example, in some embodiments, the excitation signal is transmitted to one or more contacts of the SAW device such as a positive and negative contact of the SAW device. In some embodiments, the excitation signal is transmitted to an RF switch. The RF switch can synchronize the transmission of the excitation signal across one or more SAW sensors of the SAW device. 
     At block  1106 , similar to block  1002  of process  1000 , process  1100  receives an excitation signal. In some embodiments, process  1100  receives the excitation signal directly or indirectly from hardware, such as from a waveform generator as described herein. The excitation signal includes at least one of a variety of signals including, but not limited to, a pulse voltage, a sinusoidal electrical signal, frequency modulation, linear frequency modulation, hyperbolic frequency modulation, orthogonal frequency coding, random modulation, continuous phase modulation, frequency shift key, multi-frequency shift key, phase shift key, or wavelet modulation. In some embodiments, the process  1100  receives the excitation signal at a SAW device, such as at one or more IDTs of one or more SAW sensors. 
     In some embodiments, the process  1100  receives the excitation signal at a SAW device, such as at one or more IDTs of one or more SAW sensors. In some embodiments, the excitation signal is received at one or more contacts of the SAW device such as a positive and negative contact of the SAW device. In some embodiments, the excitation signal is received at to an RF switch. The RF switch can be included or not included in the SAW device. The RF switch can synchronize the transmission of the excitation signal across one or more SAW sensors of the SAW device. 
     At block  1108 , similar to block  1004  of process  1000 , process  1100  generates a plurality of surface acoustic waves. For example, each of one or more IDTs can convert or transduce the excitation signal into a surface acoustic wave. The process  1100  (e.g., the one or more IDTs) transmits the surface acoustic waves across delay lines of a plurality of SAW sensors. In some embodiments, each of the SAW sensors includes a delay line of a different length. In some embodiments, one or more of the delay lines are the same or substantially the same length. 
     At block  1110 , similar to block  1006  of process  1000 , the SAW device includes one or more reflectors configured to reflect the surface acoustic waves. For example, each of the plurality of SAW sensors can include a bidirectional sensor array such that each includes a reflector. A reflector can reside on the substrate, opposite the IDT. The surface acoustic wave is transmitted from the IDT and propagates though the substrate, before reaching the reflector. The reflector generates an echo of the surface acoustic wave (or reflects the wave), which causes the surface acoustic wave to propagate from the reflector to an IDT (e.g., the transmitting IDT, a receiving IDT, etc.). Thus, in some embodiments, the surface acoustic wave propagates through a substrate or across a delay line multiple times. It should be noted that, in some embodiments, the SAW device does not include reflectors. Instead, the SAW sensors can include a unidirectional sensor array such that the surface acoustic wave propagates through a substrate once, and is then received, for instance, with an IDT. 
     At block  1112 , similar to block  1008  of process  1000 , the process  1100  receives the plurality of surface acoustic waves. In some embodiments, such as with a SAW sensor including a bidirectional sensor array, the surface acoustic waves are received by an IDT after propagating across the delay line twice. In some embodiments, such as with a SAW sensor including a unidirectional sensor array, the surface acoustic waves are received by an IDT after propagating across the delay line once. As described herein, due to the different lengths of the delay lines, the propagation time of each surface acoustic wave is different. 
     At block  1114 , similar to block  1010  of process  1000 , process  1100  generates a receiving signal based at least in part on the received surface acoustic waves. For instance, the process  1100  can convert each of the propagated waves into an electrical signal. For example, the propagated waves can be received by a plurality of IDTs. The plurality of IDTs can convert or transduce the propagated surface acoustic waves into electrical or other signals. In some embodiments, each of the electrical or other signals is combined into a single signal (termed receiving signal). In some embodiments, the process  1100  generates a compressed pulse train, wherein one or more of the pulses correspond to the different SAW sensors of the SAW device. For example, each pulse can correspond to a different SAW sensor. In addition or alternatively, the pulses can each include a different time delay based on the length of a corresponding delay line. 
     At block  1116 , similar to block  906  of process  900 , process  1100  receives a signal from the SAW device. In some embodiments, the signal is indicative of one or more differences in length of delay lines of the SAW sensors of the SAW device. For example, as described herein, the SAW device can include a plurality of SAW sensors. Each SAW sensor includes a delay line of a different length. As the excitation signal propagates across the delay lines, the signals received after propagation across the delay line are each time delayed relative to each other. For example, the time delay corresponds to the difference in delay line lengths. In some embodiments, the signal received from the SAW device includes a plurality of portions, wherein each portion corresponds to a time-delayed signal associated with each SAW sensor of the SAW device. For example, the signal can include a compressed pulse signal, where each of the pulses correspond to a different SAW sensor and a difference in time between the pulses corresponds to the different in delay line length of the SAW sensors. 
     In some embodiments, the signal received from the SAW device is indicative of one or more analytes in a sample that was added to the SAW device. For example, each SAW sensor of the SAW device can include a receptor (also referred to as a sensitive layer) configured to bind to one or more analyte(s) on a surface of the SAW sensor. When a sample media containing the one or more analytes is placed on the SAW sensor, a physical, chemical or electrical reaction occurs between the analyte and the receptor. This physical, chemical or electrical reaction can alter (e.g., in phase, frequency, or amplitude) a surface acoustic wave as the surface acoustic wave propagates along a delay line corresponding to the physical, chemical or electrical reaction. Thus, the signal received from the SAW device can be indicative of the one or more analytes by being responsive to the physical, chemical or electrical reaction between the analyte and the receptor. 
     At block  1118 , similar to block  908  of process  900 , process  1100  determines or monitors the one or more analytes introduced to the SAW sensor based at least in part on the output signal of the SAW device. As described herein, the output signal can include portions of the signal having one or more differences in phase, frequency, amplitude, etc. The differences in phase, frequency, amplitude, etc. between the portions of the output signal and/or the excitation signal can analyze to determine or monitor the one or more analytes. 
     It will be understood that the various blocks described herein can be implemented in a variety of orders, and that the process  1100  can implement one or more of the blocks concurrently and/or change the order, as desired. Furthermore, it will be understood that fewer, more, or different blocks can be used as part of the process  1100 . For example, the process  1100  can include blocks similar or different to those of process  1000 ,  1100 ,  1200 . 
       FIG. 12  is a flow diagram illustrative of an embodiment of a process  1200  implemented by a multiplexing SAW device for determining or monitoring one or more analytes using a plurality of SAW sensors having delay lines of different lengths. One skilled in the relevant art will appreciate that the elements outlined for process  1200  may be implemented by one or more computing devices or components of the multiplexing SAW device (such as a processor), another computing device, in software, etc. Accordingly, process  1200  has been logically associated as being generally performed by a processor, and thus the following illustrative embodiments should not be construed as limiting. 
     At block  1202 , similar to block  1106  of process  1100  and block  906  of process  900 , process  1200  receives a signal from the SAW device. In some embodiments, the signal is indicative of one or more differences in length of delay lines of the SAW sensors of the SAW device. For example, as described herein, the SAW device can include a plurality of SAW sensors. Each SAW sensor includes a delay line of a different length. As the excitation signal propagates across the delay lines, the signals received after propagation across the delay line are each time delayed relative to each other. For example, the time delay corresponds to the difference in delay line lengths. In some embodiments, the signal received from the SAW device includes a plurality of portions, wherein each portion corresponds to a time-delayed signal associated with each SAW sensor of the SAW device. 
     At block  1204 , the process  1200  identifies or determines one or more pulses of the receiving signal. For example, the receiving signal can include a compressed pulse train such as described herein. Each of the peaks or pulses can corresponds to a different surface acoustic wave which propagated across a delay line of a different length. Accordingly, because the delay lines are of a different length, each of the pulses occurs in the compressed pulse train at a different time. 
     At block  1206 , the process  1200  compares each of the identified pulses of the receiving signal to another one of the one or more pulses or the excitation signal. As a non-limiting example, a SAW device includes a plurality of SAW sensors, each having a delay line of a different length. At least some of the SAW sensors also include a receptor configured to bind to one or more analytes. A sample media (potentially including one or more analytes) is introduced to the SAW sensors such that it contacts the one or more receptors of the SAW sensors. When a sample media containing the one or more analytes is placed on the SAW sensor, a physical, chemical or electrical reaction occurs between the analyte and the receptor. An excitation single is introduced into the SAW device using a waveform generator. When the excitation signal is introduced to the SAW device, each of the SAW sensors are simultaneously excited such that each generate a surface acoustic wave from the excitation signal. The surface acoustic waves propagate along the delay lines and depending on whether the sample media contains the one or more analytes, some of the surface acoustic waves can be altered in phase, frequency, amplitude, etc. Each of the identified pulses correspond at least one of the surface acoustic waves which may have been altered. Thus, at block  1206 , the process  1200  can compare the phase, frequency, amplitude, etc. of each of the pulses or the excitation signal. 
     At block  1208 , based at least in part on the comparison at step  1206 , the process  1200  determines one or more differences in phase, frequency, amplitude, etc. between the one or more pules or the excitation signal. For example, a SAW device or system can include phase detection which can determine a phase corresponding to each of the plurality of pulses with respect to each other and/or the excitation signal. For example, the difference in delay line length between the SAW sensors results in a time delay between the pulses of the received signal (R X ). The shifts in time domain between the pulses of the compressed pulse train correspond to phase shifts associated with a particular SAW sensor. The phase shifts can be determined, for example, using a software program or FPGA (field programmable gate array) hardware. 
     At block  1210 , the process  1200  determines or monitors the content of the sample media introduced to the SAW device. For example, as described herein, if an analyte exists in a sample, it will create a physical, chemical or electrical reaction with the receptor, which will ultimately alter at least a pulse of the receiving signal. For example, changes in velocity or amplitude of the surface acoustic wave can correspond to changes in amplitude, frequency, phase-shift, or time-delay in the receiving signal, as compared to the excitation signal. By comparing the pulses to other pulses or to the excitation signal, the process  1200  can determine in what way (if at all) the surface acoustic wave was altered as it propagated across the delay. Using this information, the process can identify analytes present in the sample or can monitor the analytes in the sample. For example, the process  1200  may utilize a local or remote database including information on how a surface acoustic wave may be altered by a specific physical, chemical or electrical reaction, as described herein. Once the process  1200  determines how the surface acoustic wave (or the pulse) was altered, it matches or compares the alterations to identified alterations in the database. In some instance, the process can include a learning feature which can update the database based on determined results. 
     The resulting system offers the advantage of simultaneous excitation and allows for sensing of multiple analytes, targets or bio-agents simultaneously. A SAW sensor can thus indicate the presence of an individual substance to which it reacts specifically. An indication regarding the amount of the substance present can be provided by the phase change of the measurement signal. As a whole, the sensor chip (e.g., the SAW device  100 , SAW device  200 , array of SAW Sensors  364 ) thus forms a detector with its multiple individual sensor elements, the detector being able to simultaneously identify a large number of various substances during a single test procedure. 
     It will be understood that the various blocks described herein can be implemented in a variety of orders, and that the process  1200  can implement one or more of the blocks concurrently and/or change the order, as desired. Furthermore, it will be understood that fewer, more, or different blocks can be used as part of the process  1200 . For example, the process  1200  can include blocks similar or different to those of process  900 ,  1000 ,  1100 . 
     It will be understood that although the various embodiments described herein reference surface acoustic waves, SAW sensors, and/or SAW devices, any of the embodiments described herein are compatible with bulk acoustic waves, BAW sensors, and/or BAW devices, or a combination of BAW and SAW sensors or devices. Accordingly, the embodiments described herein should not be limited to surface acoustic waves.