bi-monthly technical progress report # 1

SBIR PHASE I FINAL REPORT

Contract Number NNX09CE49P

SAW Passive Wireless Sensor-RFID Tags with Enhanced Range

ASRD R1012 F

Jacqueline H Hines, PhD - PI July 22, 2009

Jackie Hines

Text Box

Copyright © 2009 Applied Sensor Research & Development Corporation. All rights reserved.

Table of Contents

.............................................................................................................ii Table of Contents

............................................................................................................. 3 Project Summary

................................................................................................... 4 1. Executive Summary

.............................................................................................. 4 Program Objectives

.......................................................................... 4 Summary of Technical Approach

......................................................... 5 Summary of Technical Merit and Feasibility

............................ 6 2. Statement of Work and Description of Program Accomplishments

............................ 6 Task 4.1.1 Program Technical & Administrative Management

............................................................................... 6 Task 4.1.2 Literature Search

..................................... 7 Task 4.1.3 External Sensor Selection and Procurement

....................................................... 11 Task 4.1.4 SAW Sensor-RFID Tag Design

........................................................... 12 Task 4.1.5 Device Fabrication & Testnig

............................................... 43 Task 4.1.6 Evaluation of Commercial Feasibility

........................................................................ 44 Task 4.1.7 Phase I Final Report

.......................................................................................... 44 3. Patents and Publications

.................................................................................... 44 4. Program Financial Summary

................................................................ 45 Report Documentation Page (NASA SF298)

ASRDR1012F Page ii of 44



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Project Summary The originally proposed Phase I project had one main objective: To demonstrate the technical feasibility of producing SAW sensor-RFID tag devices capable of providing a wireless interface to external sensors, with enhanced range relative to conventional SAW RFID tags. When developed, a wireless multisensor system using these passive sensor-RFID tags would allow elimination of the wiring harness to numerous types of sensors on airframes and in test facilities and would enable remote monitoring of large groups of conventional sensors. Conventional SAW RFID systems suffer from serious problems with code collision avoidance, and limited range capabilities. While current SAW RFID tags are capable of generating billions of unique codes, the readers run into problems when trying to read more than a couple of tags in the same field of view, particularly if the tags are located in close proximity to one another. In addition, currently available systems are not capable of interfacing with and reading external sensors. Thus, we evaluated methods to address all three of these issues (code collision avoidance, range, and interfacing with load sensors) by analyzing and modifying both tag device structure and interrogation system architecture. This Phase 1 project evaluated several SAW sensor-RFID tag device structures theoretically and experimentally, and established their performance characteristics with particular emphasis on applying these devices as remote sensor interfaces for various types of external impedances and sensors. The results of this effort demonstrated the technical feasibility of producing SAW sensor-RFID tag devices capable of providing a wireless interface to external sensors, including switches, thermistors, and strain gages. A simple passive impedance transformation technique was developed that allows SAW devices to interface directly with external load sensors with impedances of up to 5 kΩ and above. The feasibility of using SAW sensor-tag interface devices with external sensors that generate voltages such as AE sensors was also established. Voltage producing load sensors require the use of a zero-bias FET to transform voltage variations into changes in impedance that can be measured with the SAW device. Feasibility of measuring AE sensors was evaluated by analyzing SAW sensor-tag devices loaded with FETs with applied DC voltages. Interrogation system sampling time considerations may restrict the use of these interface devices with AE sensors to measuring the low frequency content of the AE sensor signals.

Enhanced tag read range was shown to be possible using several approaches. Pulse expansion and compression in the interrogation system and in the sensor-tags can introduce processing gain and increase range, although it does nothing to avoid the code collision problems that occur in current SAW RFID readers. ASR&D devised two new interrogation system architectures, both of which are approaches that should provide enhanced read range while enabling code collision prevention, even with multiple tags operating simultaneously in the field of view of one reader. One of these ASR&D proprietary system architectures capable of interrogating up to 100 sensors at a range of about 100 feet is described in the Phase II proposal submitted concurrently with this report.



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1. Executive Summary

1.1. Program Objectives The originally proposed Phase I project had one main objective: To demonstrate the technical feasibility of producing SAW sensor-RFID tag devices capable of providing a wireless interface to external sensors, with enhanced range relative to conventional SAW RFID tags. When developed, a wireless multisensor system using these passive sensor-RFID tags would allow elimination of the wiring harness to numerous types of sensors on airframes and in test facilities and would enable remote monitoring of large groups of conventional sensors. Discussions with NASA representatives early in the project indicated that it would also be highly desirable for this work to produce a summary of the potential performance characteristics of the various possible SAW sensor-tag device topologies and associated interrogation system architectures in order to provide NASA with an understanding of the tradeoffs involved in applying these devices as remote sensor interfaces for various types of sensors. This effort evaluated several SAW sensor-RFID tag device structures theoretically and experimentally, and established their performance characteristics with external impedances and sensors to the extent needed to accomplish this objective. The technical feasibility of using specific different types of SAW sensor-RFID tags as interfaces for different types of external sensors was demonstrated. Interrogation system architectures were evaluated for use with the sensor-tags.

1.2. Summary of Technical Approach Our review of current conventional SAW RFID systems revealed serious problems with code collision avoidance, and limited range capabilities. While current SAW RFID tags are capable of generating billions of unique codes, the readers run into problems when trying to read more than a couple of tags in the same field of view, particularly if the tags are located in close proximity to one another. In addition, currently available systems are not capable of interfacing with and reading external sensors. Thus, we evaluated methods to address all three of these issues (code collision avoidance, range, and interfacing with load sensors) by analyzing and modifying both tag device structure and interrogation system architecture. Various techniques to enhancing range were considered, including modifying device characteristics and developing improved interrogation system architecture approaches. The first approach utilizes pulse expansion and compression in the interrogation system and in the sensor-tags to introduce processing gain and increase range. While this approach can increase range for SAW RFID tags, it does nothing to avoid code collision problems. ASR&D devised two new interrogation system architectures, both of which are approaches that should provide enhanced read range while enabling code collision prevention, even with multiple tags operating simultaneously in the field of view of one reader. Several SAW sensor interface device structures were designed, fabricated, and tested as described in detail below, and potential impedance transformation techniques were



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evaluated. In order to develop an understanding of the potential performance characteristics of the various possible SAW sensor-tag device topologies, with particular emphasis on how these devices operate when used as a wireless link to external impedance varying sensors, ASR&D evaluated several device topologies. These sensor-tags need to be able to perform two functions: (i) device coding (for identification) and (ii) load sensor measurement. As there are various methods to accomplish both of these goals, ASR&D is considering each goal separately, with the final objective being a device topology that satisfies both goals. The device topologies under consideration are: (a) Reflective differential delay line (RDDL) [essentially the approach taken by Bob Brocato in the Sandia system, no coding]; (b) Reflective differential delay line with multistrip coupler (MSC); (c) Reflective tapped delay line RFID tags (RTDL); (d) Dispersive reflective tapped delay line RFID tags (DRTDL); (e) Orthogonal frequency coded (OFC) tags; (f) Discrete Frequency Coded (DFC) tags; (g) Polarity modulated frequency coded (PMFC) tags, and (h) Power spectral density (PSD) reflective differential delay line tags. For each device topology, it is necessary to evaluate the ability to identify individual coded sensor-tags correctly, and also to evaluate the change in sensor-tag performance with change in external load impedance. These two factors will be discussed separately below for the sensor-tag device approaches evaluated. 1.3. Summary of Technical Merit and Feasibility The Phase I effort successfully demonstrated the technical feasibility of producing SAW sensor-RFID tag devices capable of providing a wireless interface to external sensors, including switches, thermistors, and strain gages. A simple passive impedance transformation technique was developed that allows SAW devices to interface directly with external load sensors with impedances of up to 5 kΩ and above. The feasibility of using SAW sensor-tag interface devices with external sensors that generate voltages such as AE sensors was also established. Voltage producing load sensors require the use of a zero-bias FET to transform voltage variations into changes in impedance that can be measured with the SAW device. Feasibility of measuring AE sensors was evaluated by analyzing SAW sensor-tag devices loaded with FETs with applied DC voltages. Interrogation system sampling time considerations may restrict the use of these interface devices with AE sensors to measuring the low frequency content of the AE sensor signals. Enhanced tag read range was shown to be possible using several approaches. Pulse expansion and compression in the interrogation system and in the sensor-tags can introduce processing gain and increase range, although it does nothing to avoid the code collision problems that occur in current SAW RFID readers. ASR&D devised two new interrogation system architectures, both of which are approaches that should ameliorate the anti-collision problem and allow multiple tags to operate simultaneously in the field of view of one reader, while allowing enhanced reading range. One of these ASR&D proprietary system architectures capable of interrogating up to 100 sensors at a range of about 100 feet is described in the Phase II proposal submitted concurrently with this report.



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2. Statement of Work and Description of Program Accomplishments The following provides the project tasks as originally proposed, along with a summary of accomplishments for each task. Task 4.1.1 Program Technical & Administrative Management

ASR&D will maintain technical, administrative, and financial management of the program, and will deliver a mid-point progress report (as required). The Company will also deliver a comprehensive final report on the program, as well as recommendations for a Phase II program, to NASA upon completion of the Phase I effort. Accomplishments: ASR&D signed this contract with NASA on January 22, 2009. A formal program kick-off telecom meeting was held on Friday March 27, and input was received from NASA program representatives from JSC, LaRC, MSFC, GRC, and others. Data on destructive and non-destructive test methods used for composite overwrapped pressure vessels (COPVs) from NASA WSTF was also received and reviewed. Bi-monthly program status reports were prepared and submitted to NASA. ASR&D also filed a New Technology Report on a novel SAW sensor-tag approach for which a provisional patent has been filed, through NASA’s New Technology reporting web site. A New Technology Summary Report (NTSR) was also filed. This final report represents the conclusion of ASR&D’s program management responsibilities regarding this Phase I STTR program.

Task 4.1.2 Literature Search ASR&D will expand its already fairly detailed literature search to ensure that the team is aware of the most recent and relevant work related to SAW sensor-tags. Additionally, evaluations of additional mechanism for impedance transformation will be investigated. Based on the results of this research, any promising impedance matching approaches will be considered for inclusion in the experiments, and SAW sensor simulation and fabrication work will be planned. Accomplishments: ASR&D completed its literature search on SAW sensor-tags. This included a thorough review of the SAW RFID tag work of RF-SAW (the Global SAW Tag), and CTR (Corinthian Technologies), among others. Based on an understanding of these approaches, ASR&D’s sensor-tags will fill in a technological niche for longer-range RFID tags capable of operating with a small quantity (up to 100 or more) sensors interrogated in the same reader field of view (FOV). They would not compete with the very high data capacity RFID tags used for inventory control, which can produce billions of codes (although with the drawback of limited read range and inability to sort out more than a few tags simultaneously in the reader FOV). We are also focused on tying the SAW devices to external sensors and providing a feedback with both the tag ID and the measured response of the external sensor. This is unlike the RF-SAW and CTR approaches, wherein the SAW device is itself used as a temperature sensor and an RFID tag. While our approach could be used in this way (i.e. with the SAW being the RFID and the temperature sensor at the same time), the use of the SAW as a wireless link to other sensors opens up the possibility for many



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more sensor types and applications than could be implemented with the SAW as the sensing element.

Task 4.1.3 – External Sensor Selection and Procurement 4.1.3.1 Sensor Technology Survey

ASR&D personnel will survey commercially available passive sensors to identify a selection of devices that are potentially useful for technology demonstration and/or application purposes. Specifically, a mechanical switch, an acoustic emission sensor, and other sensors as appropriate will be identified. Accomplishments: ASR&D personnel discussed sensor selection with personnel from NASA Langley and NASA JSC. Sensors of interest include acoustic emission (AE) sensors, strain gages (for static strain measurement), and thermistors or similar temperature sensors. Based on this input, ASR&D selected a Digital Wave B225.5 broadband AE sensor (50 kHz – 400 kHz) for testing with aluminum plates. A mechanical (pushbutton) switch (Judco Manufacturing P/N 50-0015-0) was identified and purchased for use as an open circuit/short circuit PCB load for baseline testing. Strain gages and thermistors were also evaluated. Based on information provided by Quality Thermistor, Inc., industry standard practice is to use NTC (negative temperature coefficient) thermistors in applications requiring direct measurement of temperature, while positive temperature coefficient (PTC) thermistors are used for applications where temperature compensation is being performed within circuits. Quality Thermistor Inc. produces NASA-grade thermistors along with commercial grade thermistors, and the latter were available as samples, so these were selected for Phase I test purposes. Various strain gage technologies were reviewed. Film strain gages such as those produced by Strain Measurement Devices were selected as good candidates for use, along with gages from Micro Measurement.

4.1.3.2 Evaluation of impedance matching techniques and requirements ASR&D personnel will evaluate the impedance matching requirements of the sensors identified above in light of the SAW device characteristics. Impedance matching techniques will be evaluated, and an appropriate technique will be selected for each sensor requiring matching. Accomplishments: ASR&D utilizes proprietary in-house SAW device simulation tools that are based on a combination of coupling-of-modes (COM) theory and P-matrix formulations. This software allows for rapid design of SAW devices with high confidence that the desired performance will be obtained. In addition, by using these simulation tools ASR&D was able to evaluate the impact of varying impedance loads on SAW tag reflective elements, and optimize reflective transducer design and passive matching circuits to provide enhanced sensitivity to variations in external loads. Traditionally, it has been thought that SAW devices would only be useful as interface devices for load sensors with impedance low enough to match the SAW device, which is typically designed to be approximately 50Ω. Figure 1 below exemplifies this



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effect. This figure shows part of the simulated time domain S11 response of a typical RDDL device designed at 250 MHz, with two reflecting transducers located on either side of a central transducer that is used for input and output. The device response consists of reflections from the two outer transducers. The reference transducer response is at about 0.5 μsec, while the reflection from the transducer attached to the external test load impedance occurs at about 0.7 μsec. As both of these transducers are fairly reflective, multiple reflections occur, as can be seen in Figure 1 by the second set of reflections from each transducer. Reflections further out in time become more complicated, as reflections caused by waves transiting between the two outer transducers overlap in time. For evaluation of the effects of load impedance, we consider the first two reflections only.

RDDL with OC reference and varying loads

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-25

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0.25 0.35 0.45 0.55 0.65 0.75 0.85 0.95 1.05 1.15 1.25

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(dB

)

OC 1 kOhm 500 Ohm 100 Ohm 50 Ohm10 Ohm 5 Ohm 1 Ohm

Figure 1. Reflective Differential Delay Line (RDDL) with open circuit reference load and test load impedance varying from open circuit to 1 Ω. First peak is reference reflection, second peak shows change in reflected response from varying load. This device is only responsive to load impedances below 500 Ω.



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The first peak in the time response of Figure 1 is the reflection from the reference transducer, which in this case has a constant open circuit load impedance. The second peak is the reflection from the transducer attached to the external load impedance, which is varied from open circuit to 1 Ω. As can be seen from the top three curves in Figure 1, there is almost no change in reflection characteristics for open circuit, 1 kΩ, and 500 Ω loads. As the load is changed from 500 Ω to 100 Ω, 50 Ω, and 10 Ω, large changes in reflected response are observed. As the load impedance changes from 10 Ω to 5 Ω, and to 1Ω, smaller changes occur. Hence this transducer would be directly useful for sensor load impedances between 10 Ω and slightly less than 500 Ω. Impedance transformation will be needed for this transducer to interface to higher impedance sensor loads. Robert Brocato at Sandia National Labs introduced an impedance transformation approach using a zero-bias FET to allow the low impedance SAW device to be matched to sensors with much higher impedance [1]. After reviewing this approach and discussing it with Bob Brocato, ASR&D personnel selected two zero bias MOSFET devices with nominal impedances of 1 kΩ and 500 Ω (ALD114935SAL and LND250K1-G respectively) for use. The FETs used are in a normally on state, meaning that no gate voltage is required for there to be a drain-source (D-S) current. Changes in gate voltage modulate the D-S current, effectively varying the D-S resistance. This change in effective D-S resistance is what affects the SAW device performance when voltage generating sensors tied to FETs are used as loads. Thus the FET provides a means of utilizing SAW devices to read sensors that respond with changes in voltage rather than directly with changes in impedance. This is quite useful, as the SAW device cannot respond directly to load voltages (unless they are time varying with frequency content within the passband of the SAW device), and the use of a FET to transform voltage to impedance allows acoustic emission (AE) and similar sensors to be used with SAW devices as wireless interfaces. The results of testing with FETs is shown below in task 4.1.5.5. ASR&D evaluated other possible impedance transformation techniques that will work with passive sensors that exhibit varying impedance directly (i.e. that do not produce voltages in response to sensed parameters), and identified a new, simple method that allows properly designed SAW devices to interface directly with load sensors that have impedances of up to about 5kΩ. The approach ASR&D implemented utilizes an appropriately sized inductor placed across the SAW transducer to which the load sensor is connected (the “load transducer”). By tuning out the capacitance of the transducer, one sets up a tank circuit, establishing a high impedance node. Any resistance applied by the external sensor then loads down that node and the impact of the load sensor can be measured by the SAW device. Figure 2 shows part of the simulated time domain S11 response of a simple RDDL device like that shown in Figure 1. In this simulation, an inductor was connected across the load transducer. Notice that in both figures, the reference transducer reflection remains at about -19dB. In Figure 1, without an inductor across the load transducer, the open circuit reflection from the load transducer is identical to that of



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the reference transducer, at -19 dB. From Figure 2, we can see that addition of the inductor causes the open circuit reflection from the load transducer to increase by over 8 dB, to –10.8 dB. Application of various load resistances demonstrates that this same device, which previously could only respond to impedances well below 500Ω can, with the addition of an inductor, response to load impedances up to about 5kΩ. This simple matching step, combined with proper transducer design, will allow ASR&D to develop SAW interface devices optimized for use with a wide range of external impedance element sensors. As is well known in circuit theory, a network of inductors can also be used to transform impedance. Thus, if interfacing with higher impedance sensors is required, additional impedance transformation can be accomplished with appropriate passive circuits.

RDDL with OC reference and varying loads with inductor load matching

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0.25 0.35 0.45 0.55 0.65 0.75 0.85 0.95 1.05 1.15 1.25

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OC 5 kOhm 1 kOhm 500 Ohm 250 Ohm 100 Ohm

50 Ohm 10 Ohm 5 Ohm 1 Ohm

Figure 2. Reflective Differential Delay Line (RDDL) with open circuit reference load and test load impedance varying from open circuit to 1 Ω. An inductor has been placed across the load to allow the device to interface with higher impedance sensors. Comparing this to Figure 1, we see that this device is now responsive to load impedances below 5 kΩ.

Simulation indicates that the design of the SAW transducer can substantially impact the sensitivity of the reflected response to load impedance. To evaluate this effect experimentally, devices were built with transducers with a range of impedances. As expected, transducers with higher impedance provided the best match to external high impedance sensors.



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4.1.3.3 Sensor procurement Once suitable sensors have been identified and an approach to impedance matching these devices has been established, ASR&D will purchase each of these sensors for use in program testing. Accomplishments: A Digital Wave B225.5 broadband AE sensor (50 kHz – 400 kHz) was purchased, and NTC Thermistors with 25°C nominal resistances of 1 kΩ (P/N QTI1206X0192J) , 10 kΩ, and 100 kΩ were procured as samples from Quality Thermistor Inc. Strain gages from Micro-Measurements with several nominal impedances were also procured (P/N CEA-13-125UR-350, EA-00-250F-350, EA-06-250AF-120, and CEA-13-125UT-350).

Task 4.1.4 – SAW Sensor-RFID Tag Design and Mask Layout Discussions with NASA representatives indicated that one primary result desired from this Phase I program is an understanding of the potential performance characteristics of the various possible SAW sensor-tag device topologies, with particular emphasis on how these devices operate when used as a wireless link to external impedance varying sensors. In order to address this need, ASR&D is evaluating several device topologies. These sensor-tags need to be able to perform two functions: (i) device coding (for identification) and (ii) load sensor measurement. As there are various methods to accomplish both of these goals, ASR&D is considering each goal separately, with the final objective being a device topology that satisfies both goals. The device topologies under consideration are: (a) Reflective differential delay line (RDDL) [essentially the approach taken by Bob Brocato in the Sandia system, no coding]; (b) Reflective differential delay line with multistrip coupler (MSC); (c) Reflective tapped delay line RFID tags (RTDL); (d) Dispersive reflective tapped delay line RFID tags (DRTDL); (e) Orthogonal frequency coded (OFC) tags; (f) Discrete Frequency Coded (DFC) tags; (g) Polarity modulated frequency coded (PMFC) tags, and (h) Power spectral density (PSD) reflective differential delay line tags. For each device topology, it is necessary to evaluate the ability to identify individual coded sensor-tags correctly in a multi-tag environment, and also to evaluate the change in sensor-tag performance with change in external load impedance. These two factors will be discussed separately below for the approaches being considered, and impact of device topology on tag interrogation system architecture will be discussed. 4.1.4.1 SAW device simulation

ASR&D personnel will utilize proprietary in-house SAW CAD tools to simulate relevant aspects of the devices being developed. Modification of the CAD tools will be undertaken if necessary. Accomplishments: ASR&D completed simulation of several of the device approaches being considered using proprietary in-house SAW device simulation tools that are based on a combination of coupling-of-modes (COM) theory and P-matrix formulations. Some results of this simulation (showing the effects of varying impedance loading on the devices) are shown above in task 4.1.3.2. Simulation of remaining device topologies



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was sufficient to allow device design and mask layout, and to evaluate experimental results. Given the number of devices designed and built, additional simulation results will not be included here.

4.1.4.2 SAW device design The results of simulation will be used to design SAW sensor-RFID tags with properties desirable for operation as wireless links to the selected sensors. Modification of transducer impedance within limited ranges will be performed if appropriate, using the flexibility allowed through tapered transducer device layouts. Up to two design and photomask generation passes will be conducted as part of the phase I effort if required. Accomplishments: Sensor-tag devices were designed using reflective delay line (regular and MSC), reflective tapped delay line RFID (dispersive and non-dispersive), OFC, DFC, PMFC, and PSD device configurations. The general device layouts are shown below with test results for the most promising device types. Where appropriate, multiple different implementations of each of these device topologies were built to evaluate tradeoffs in device design. These various device structures were consolidated in layout for mask generation. In total, 16 different device structures were designed, with between 2 and 16 variations of each structure to evaluate tag devices with different codes, plus test structures.

4.1.4.3 Photomask generation The SAW device design will be translated into a .gdsII file format, and cross checked with the original design for accuracy. Accomplishments: Photomask files were generated for 4 masks, each of which included multiple sensor-tag device structures.

4.1.4.4 Photomask procurement The mask file will be forwarded to a commercial photomask supplier for mask generation. Accomplishments: Four photomasks were procured from CompUGraphics USA, and were used for device fabrication.

Task 4.1.5 – Device Fabrication and Testing 4.1.5.1 SAW device fabrication & assembly

ASR&D personnel will utilize the nanofabrication facilities at the University of Maryland College Park to fabricate the sensor-tag devices designed above. Photolithography, metal deposition, liftoff, and wafer dicing will all be conducted at UMd. Wafer probing, and device assembly and bonding will be completed in ASR&D’s in-house clean room facility. Accomplishments: ASR&D fabricated RDDL, RFID, OFC, DFC, PMFC, and PSD sensor-tag devices



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orm

previously designed. The devices (more than 100) were assembled onto 14-pin DIP headers and bonded with gold wirebonds for testing.

4.1.5.2 Test fixture design and assembly A test fixture capable of making the necessary electrical connections to the SAW device and to impedance transforming, matching, and external sensor components will be designed. It is anticipated that this will be a PCB. ASR&D will use it’s in-house PCB prototyping facilities to manufacture and assemble test fixtures for use with the SAW devices and sensors being used. Accomplishments: Test fixtures were designed, fabricated, and assembled for the non-FET impedance loading tests of PSD devices and for the code correlation testing of OFC, DFC, and PFMC devices. Additional test fixtures have were designed to accommodate the SAW device and various combinations of FET, impedance matching, and sensor load devices. Wireless test fixtures incorporating fractal antennas were also produced and used for testing. These fixtures were fabricated, assembled, and used for testing. Figure 3 shows a few of the many test fixtures constructed for use. The fixture on

the lower left incorporates a fractal antenna from fractus®. The fractal antenna used (P/N FR01-B3-V-0-054) was designed for mobile TV use, in the 180-220 MHz range. As can be seen in Figure 4, this omnidirectional antenna has an extremely small ffactor for an antenna operating in such a low frequency range. Since we are utilizing the antenna outside of its designed frequency range, we estimate that antenna loss

Figure 3. Typical fixtures for wired and wireless testing is about 40-45 dB per antenna. However, the antenna does have a wide frequency band of operation suitable for use with our SAW sensors. For future use, a fractal antenna optimized for operation over the frequency range of interest should allow us to reduce antenna losses to no

Figure 4. Fractal antenna used in wireless tests. more than about 20 dB per



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antenna, possibly less. Alternatively, increasing SAW operating frequency may allow for even more efficient antennas with small footprints. From Figure 3, one can see that the footprint of this antenna is comparable to that of the SAW device itself, making it substantially smaller than other antennas in the 250 MHz range.

4.1.5.3 Baseline electrical testing The SAW devices will next be tested for baseline electrical performance (wired) using an RF network analyzer. Proper one-port SAW RFID tag responses should be observed. If necessary, a revised SAW design pass will be completed and additional devices fabricated to achieve proper performance. Accomplishments: All of the devices fabricated were tested for baseline electrical performance (wired) using an Agilent E5070B RF network analyzer. The generic device topology, details of the test setup for each device, and selected baseline test results are shown below, grouped by device type. The results obtained using a RDDL with MSC device structure were not favorable and this structure will not be discussed further. Reflective Differential Delay Line (RDDL): The first device evaluated is a simple reflective differential delay line, shown in Figure 5 below. This device was built in order to provide a direct comparison to the RDDL device built and tested by Bob Brocato at 70 MHz. Depending on the spacings τ and τ1 2, this device configuration will produce a RDDL or a PSD response.

Figure 5. Reflective Differential Delay Line (RDDL) device structure. If τ1 and τ2 are nearly equal, this device produces a PSD response (discussed below), otherwise a RDDL response is produced.

τ1 τ2



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In Figure 5, the center transducer is the input/output transducer, and the two outer transducers reflect responses. The load sensor can be connected across either reflecting transducer, and a reference load can be connected across the other. If the time delays τ and τ1 2 are different, a RDDL response as shown in Figures 1, 2, and 15 is produced. If the time delays τ and τ1 2 are nearly equal, the reflections combine to produce a power spectral density (PSD) response, shown below in Figure 16. RFID (non-dispersive): Reflective tapped delay line RFID SAW tags were designed using single, double, and triple-track devices. Each device included one start tap and one stop tap, a sensor load tap, and ten coding taps. Codes were effected by on-off keying (OOK). Using OOK with 10 taps allows production of 210 (1,024) unique codes.

Z

Figure 6. Non-dispersive RFID device structure with all taps. Coded devices had taps selectively removed to effect code.

RFID (dispersive, full): As previously mentioned, in order to enhance device interrogation range, techniques commonly used in pulse compression radar systems were utilized. Two such SAW RFID device embodiments were built and tested, each of which will require a specific system implementation. Figure 7 below illustrates the first dispersive device structure, which we will refer to as “fully” dispersive. Figure 8 shows a representation of the functionality of the corresponding interrogation system required for operation with this device. Figure 8 shows a transmitter generating an “up chirp” signal to send an expanded signal to the SAW tag. The tag has a corresponding “down chirp”, which compresses the incoming signal into a narrow pulse, which propagates down the tag and reflects off the reflectors, producing a chain of narrow pulses. This chain of narrow pulses is then expanded by the same transducer, producing a chain of expanded (“down chirp”) pulses. This pulse chain returns to the receiver, where another matched “up chirp” filter compresses the expanded pulses into narrow pulses in time. Thus the receiver sees a response, after compression, similar to that of a conventional SAW RFID tag.



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Figure 8. RFID tag system using full pulse compression and expansion in the device.

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Figure 7. Fully dispersive SAW RFID tag

Δt

Δt

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RFID (dispersive, half): In addition to the “fully dispersive” SAW RFID tag implementation shown above, a

“half-dispersive” SAW RFID tag was built and tested. In the system as shown in Figure 9, an up chirp filter identical to that in the previous system is used as a pulseexpansion filter in the transmitter. The input/output transducer of the SAW RFID tag is a down chirp filter, designed to have the same bandwidth but half the time extent of the pulse expansion filter. Thus, upon being received by the RFID tag, the up chirp signal is compressed half way. This half-compressed pulse propagates down the SAW surface and reflects off the taps, producing a chain of partially compressedpulses. Each of these partially compressed pulses becomes fully compressed after passing through the input/output transducer for the second time. Thus the signal sent back to the receiver is a chain of compressed pulses similar to a conventional SAW RFID signal.



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P

Figure 9. SAW RFID tag system using half pulse compression in the tag device.

SD (simple RDDL):

The simplest PSD device is a RDDL as shown in Figure 5 above, where the time arly equal. The difference in delays will determine the delays τ1 and τ2 are ne

frequency at which the null occurs.

PSD (tapered RDDL): Tapered transducers are known to provide flat passband responses over wide

t lower loss than can be achieved by comparable non-y ance

d

r

frequency bandwidths atapered transducers. The design flexibility provided by tapered transducers maprove advantageous in achieving lower loss and in optimizing transducer impedto match external loads. ASR&D designed, built, and tested three different tapereRDDL structures intended for use as PSD devices. Figure 10 below shows a schematic of the generic device structure. The three variations built utilized transducers that were 12λ, 36 λ, and 96 λ long, with electrodes distributed in 1, 4, and 4 subtransducers, respectively. Tests of these devices with open circuit reference loads and open and short circuit sensor loads (using a switch as a sensoload) showed that the 12λ long transducers were the most sensitive to variations inload impedance. Results of this testing are shown below under task 4.1.5.4. Frequency coded tags: Three types of frequency coded devices were tested. These were Orthogonal frequency coded (OFC) tags, Discrete Frequency Coded (DFC) tags, and Polarity

Δt

modulated frequency coded (PMFC) tags. The basic structure of the frequencycoded tag devices is shown in Figure 10 below.

Rx

Tx

Δt/2

Z



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Figure 9. SAW RFID tag system using half pulse compression in the tag device.

Figure 10. Generic frequency coded SAW tag device.

ZL

ZR



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As shown in Figure 10, all of the frequency coded tags had a centrally located input/output transducer, with reflective sections distributed on the left and right in a mirror image fashion, at different delays from the central transducer. Coding was defined by the order in which the reflective sections at different frequencies were arranged on the die surface. For OFC, strict mathematical orthogonality relationships defined the frequency, bandwidth, and time extent of each reflective section. PMFC was OFC with sign changes allowed for the reflecting frequency chips, to allow for greater flexibility in code generation. For the DFC devices, we deliberately relaxed the orthogonality conditions required by OFC and PMFC, in order to reduce overlapping frequency content between the chips. This enables construction of codesets that are functionally “more orthogonal” than OFC codes. For these devices, a die with multiple parallel acoustic tracks was used, so that individual frequency chips could be longer in time extent and overlap with one another in time (conditions not allowed by OFC or PMFC). Code Correlation Baseline Testing: For any tag ID system that utilizes correlation properties for tag acquisition and identification, the auto- and cross-correlation properties of different codesets becomes relevant. For each of the coded OFC, DFC, and PMFC devices, two devices were cascaded in order to produce the correlation response that would be observed in a multisensor system using a set of codes. Auto and cross-correlation was evaluated for each set of devices, and typical results are shown in Figures 11 through 14 below. Figures 11 and 12 show typical autocorrelation (Figure 11) and cross correlation (Figure 12) performance for sensor tags utilizing orthogonal frequency coding (OFC) for sensor identification. OFC devices utilize reflectors with a set of different and mathematically orthogonal center frequencies, organized in specific orders on the

lity conditions dictate that the null of ne reflector frequency response occurs at the peak of the adjacent reflector

frequency resp in the which

10 easured auto-correlation peaks for OFC devices

-

y the own,

SAW chip to create a code. Since orthogonao

onse, there is a large degree of overlap of the signalsfrequency domain, as can be seen from the yellow S21 response in Figure 11 (corresponds to two identical devices cascaded). Correlation of any of the OFC codes with itself (autocorrelation) produces a single correlation peak in time, provided the responses are synchronous. This peak is shown in Figure 11 as the green curve, which has a peak value of -74.479 dB, and sidelobes approximately dB or more below this peak. Mranged from -73.4 dB to -74.5dB, with sidelobe suppression of at least 10 dB. Inorder to properly identify individual sensor-tags, the interaction of two tags with different codes should produce a cross-correlation response that does not exhibit peaks, and has a level much lower than the correlation peak response. The crosscorrelation performance observed for the OFC codes tested did not exhibit this desired characteristic. A typical OFC cross-correlation response is shown bgreen curve in Figure 12. As can be seen, while no clear correlation peak is shthe chip responses that make up the code overlap enough to produce a small peak,



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Figure 12. Crosscorrelation response typical of OFC sensor-tag (green), and corresponding frequency domain response (yellow).

Figure 11. Autocorrelation response typical of OFC sensor-tag (green), and corresponding frequency domain response (yellow).



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which in this example has a peak at -79.4dB (only 5 dB lower than the autocorrelation peak used for device recognition). Given the fact that these responses were taken under ideal, synchronous conditions in a wired test setup, it is clear that the auto- and cross-correlation responses are not separated by as much as would be desired to give correct sensor identification in practical applications. Under another program, ASR&D has been working with the University of Central Florida (UCF) on modified OFC-like coding techniques with a goal of enhanced sensor identification. PMFC device results did not show significant improvement over OFC, and hence are not included. Based on the observed OFC correlation properties, ASR&D developed another coding approach, called discrete frequency coding (or DFC). This approach utilizes reflector (or transducer) sections with different center frequencies located in different orders on the device to create the codes, as in orthogonal frequency coding. The difference in this approach is that we no longer require the mathematical conditions of orthogonality to be met, and instead we deliberately separate the frequency ranges of the chips so there is less inter-chip interference in the frequency domain. Figure 13 shows a typical autocorrelation response produced by cascading of two identical DFC devices. From the frequency response (the yellow curve in Figure 13), the eight chips in the device code are clearly visible as frequency channels. The green curve in Figure 13 shows the time domain response, which exhibits an aut ch side. Note that for DFC devices, the sidelobes also fall off much more rapidly farther from the main peak than for OFC devices. The cross-correlation response for two DFC devices with different codes is shown in Figure 14. As can be seen, the cross-correlation in time (green curve) does not exhibit any clear peaks, but rather appears to correspond to the sidelobe response level of the autocorrelation response, with a minimum response level of -77.4 dB. This auto- to cross-correlation comparison was typical of all of those tested, with clear single peaks seen for autocorrelation, and with cross-correlation responses that were at least 8-10 dB lower than the correlation peak. While these results are an improvement over OFC devices, additional refinement of the coding technique may improve sensor identification capability.

ocorrelation peak (at -68.8dB), with sidelobes more than 12 dB down on ea



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Figure 14. Cross-correlation response typical of DFC sensor-tag (green), and corresponding frequency domain response (yellow) showing eight channels.

Figure 13. Autocorrelation response typical of DFC sensor-tag (green), and corresponding frequency domain response (yellow) showing eight channels.



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4.1.5.4 Testing with external sensor # 1 Once proper baseline performance has been observed, the SAW devices will be connected to the simplest of the external sensor devices – a mechanical switch. The network analyzer will again be used to measure the device response with the switch in two positions – open and closed. This represents the widest possible swing in impedances and will allow the team to bound device performance capabilities (for a given impedance transformation and matching network and SAW device). Wireless testing utilizing the network analyzer as a transceiver (with appropriate external signal amplification and antennas) will be performed to demonstrate wireless operation. Accomplishments: Once baseline performance of all of the SAW sensor-tags fabricated had been measured, wired network analyzer testing with load impedances was started by connecting the devices to an external mechanical switch as an initial load impedance. Initial tests were performed using a pushbutton switch from Judco. Evaluation of the data from initial tests clearly indicated that this switch taken together with stray reactive impedances on the test fixture did not represent a true short circuit or a true open circuit at the frequency of operation, as will be shown below. A modified test utilizing a header with jumpers was developed and used for the remaining tests, with improved results. Test results indicate that these factors mu n with the parasitics, clear changes in device performance were observed for changes in load impedance for all SAW devices tested.

RDDL Device switch testing

st be carefully taken into account in order to optimize tag performance. Eve

Fig ads. Note the change in the reflected response from the load transducer is about 8 dB.

ure 15. RDDL device response (unmatched) to short circuit and open circuit lo

RDDL Time Domain Response

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PSD (non-tapered) switch testing

Figure 16. RDDL PSD device response (unmatched) to short circuit and open circloads. Note the change in the reflected response from the load transducer is 22 dB.

uit

PSD (tapered) switch testing The sensor-tag device structure that exhibited the most promising response to the external switch load was the power spectral density (PSD) tag device. This structure utilizes two responses that are equal in amplitude and closely spaced in time to generate a frequency domain response signal with a PSD that has a null located at a desired frequency. Variation in the delay difference between the two responses can cause the null to move in frequency in a very sensitive manner. Changes in the relative amplitude of the reflections can also make the null change in depth – with matching responses producing a deep null and responses that are very different in amplitude producing no null. The location and depth of the null can be measured using a time integrating correlator system approach that compares the integrated energy in the PSD of two portions of the passband where the null is expected to occur. Changes in the ratio of the energy in these two passbands will correspond to changes in null characteristics. Until such a system is fully developed, the null characteristics can be measured using a network analyzer. Baseline performance of a PSD tag device comprised of two responses reflected from transducers connected to external loads (one a reference load and one a test load) is shown in Figure 17. For these baseline tests, short circuit loading was effected by bonding across transducer bus bars on the die to minimize parasitics. Again to minimize parasitics, open circuit loads were achieved by leaving the transducers unbonded (i.e. the transducer was not bonded out to the DIP package

PSD RDDL SC/OC tests unmatched

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ote that the difference in the response between open and short circuit on one h of the SAW response, a very large

PSD t

Figure 17. Response of PSD sensor-tag with: both transducers open circuit (brown curve) resulting in deep null; one transducer open circuit and the other transducer short circuit (green curve) resulting in no null; and both transducers short circuit (pink curve) resulting in low reflectivity from both transducers. Note that short circuits were effected by bonding transducer bus bars together on the die, and open circuits were not bonded out to the package.

pins). Note that on the high coupling SAW substrates being used (lithium niobate in this case), a large component of the reflection is electrical. The largest possible reflection should occur when the transducers are open circuit. From Figure 6, we see (brown curve) that with both transducers open circuit, we obtain a deep (over 40dB) symmetric null due to the interaction of the two equal amplitude reflected responses. The null has been designed to occur at the frequency seen, which places it in the upper half of the overall device response passband. When one of the transducers is left open circuit and the other is short circuited, the two responses no longer match in amplitude and (as shown by the green curve) no null is produced. Ntransducer is over 35 dB in the null dept

ag baseline performance

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Figure 18. Response of PSD sensor-tag with external switch. The reference transducer has an open circuit termination on the PCB fixture. The device is tested with the mechanical switch in open (red) and closed (blue) positions. Note that while the response is significantly influenced by parasitics on the PCB, we still see a large change from open to short circuit loads on the test transducer.

response change compared to prior SAW sensor-tag loading results. Short circuiting both transducers results in low reflectivity from both transducers (pink curve). Next, this PSD tag was connected to a PCB test fixture equipped with an external mechanical switch, for testing with the switch in open (open circuit) and closed (short circuit) positions. Figure 18 shows the results of these tests, where the reference load is open circuit on the PCB (i.e. the die has been bonded to the pins of the DIP header, and these pins terminate in short traces on the PCB that are not connected to each other or to other loads). The device is tested with the mechanical switch in open and closed positions (red and blue curves in Figure 18, respectively). Note

PSD tag with external switch

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ss (d

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that due to fixture parasitics, the open circuit null is not as deep as the reference case shown in Figure 17. Also due to parasitic inductance on the PCB, the closed switch does not present an ideal short circuit, but rather matches the SAW transducer better than a short circuit, resulting in a response level of about -43 dB, more than 11 dB lower loss than the loss exhibited by the reference open circuit/short circuit case in Figure 17. This indicates that matching may be used to advantage to enhance the desired response. It should be noted that the difference in response between open circuit and short circuit cases for this sensor-tag is quite large. By comparison, consider the SAW tag responses measured by Bob Brocato for 70 MHz reflective delay lines shown in Figure 19 [1]. Figure 19(a) shows the tag response with the switch in a closed position, and Figure 19(b) shows the response with the switch in an open position. Sensor measurement points are indicated by the white

(a) (b)

Figure 19. (a) Tag measurement, switch response in closed position. (b)Tag measurement, switch response in open position.

arrows, and it can be seen that the response with the switch in an open position is almost twice as large as that with the switch in a closed position (6dB change). By

t er

oses,

o

comparison, the current PSD tag shows an comparatively very large change of 25-30 dB with comparable load variation. Although the baseline PSD testing shown above was wired and the Sandia tests were wireless, we expected that the inherenhigh sensitivity of the PSD devices will produce wireless test results with much largresponses as well, and these expectations were met (see wireless tests below). ASR&D obtained the Sandia device and test setup for comparative testing purphowever the system was received too late in the program to perform tests during the Phase 1 project. Additional tests will be performed in the near future. In addition to testing with a switch, the PSD device was preliminarily tested with loadimpedances of 5 kΩ (which based on simulations should be roughly equivalent t



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open circuit loading for this device), 1 kΩ, 536 Ω, 100Ω, and 10 Ω. These load resistances were tested with both open circuit and 50 Ω reference loads. The results of these tests are shown in Figures 20 and 21. Figure 20 shows the response with the reference transducer open circuit. As expected from simulations (without load impedance matching), the 5 kΩ and 1 kΩ responses are barely distinguishable from the reference case with both transducers open circuit (on the PCB fixture). At 536 Ω, the null is seen to translate in frequency slightly, and to become slightly deeper (gold curve). At 100 Ω (light blue curve), the null has become more than 10 dB deeper, and has also shifted in frequency by about 6 MHz. Further reduction of the load impedance to 10 Ω results in a reduction in null depth of about 6 dB, and an additional shift in frequency of 14 MHz. Figure 21 shows the same device tested with the same test loads, but with the reference transducer connected to a 50 Ω load. Once again, little change in device performance is observed for the high load impedances (as expected), and substantial changes are seen for the 100 Ω and the 10 Ω loads. The light blue curve in Figure 21 represents the device response with a 100 Ω load impedance. The red curve shows how the response changes with a 10 Ω load. Use of a 50Ω reference load seems to stabilize the frequency at which the null occurs, but the change in null depth is still pronounced with changes in load impedance. The 100 Ω load (light blue curve) shows a null that is fully 20 dB less deep than the reference open circuit case. Changing the load impedance to 10 Ω causes the null to completely disappear, and there is instead an increase in the energy in the portion of the passband where the null was previously located. The change in response level is more than 25 dB in this case. It is important to understand the magnitude of the changes that are being observed in these tests. Conventional SAW sensors utilized in oscillator (and other)

cy shifts -

e observed for conventional SAW sensor-tags devices. While these devices have not yet been

r

configurations routinely measure parameter changes by interpreting frequenof 10-100ppm (or less), which for a 250 MHz device correspond to shifts of 2.5 kHz25 kHz. By comparison, the frequency shifts observed for the PSD devices are approximately 3 orders of magnitude larger, in the MHz range. Substantially larger changes in attenuation are also observed for PSD devices than ar

tested with high impedance sensors connected using FET impedance transformation, we anticipate that we will see correspondingly larger changes fothese sensor loads as well. The PSD sensor-tags being developed by ASR&D utilize a correlative code recognition process similar to that of the OFC and DFC devices discussed previously. In addition, our interrogation system approach utilizes time diversity to allow re-use of identical codes at different delays, effectively expanding the numberof useful codesets. ASR&D has filed for and obtained patent protection on a temperature sensor and system that utilizes this approach, and additional patent filings on this proprietary approach are pending.



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Figure 20. PSD tag with open circuit reference load and varying test loads.

PSD tag with OC reference and varying loads

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q (

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Figure 21. PSD tag with 50 Ω reference load and varying test loads.

PSD tag with 50 Ohm reference and varying loads

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on-dispersive RFID tag switch testingN Switch testing was also performed with the RFID tag devices fabricated. A typical switch response for a non-dispersive RFID tag is shown in Figure 22 below. This device had multiple acoustic tracks, with the load tap being the only tap in one track. This configuration resulted in a device where only the load tap response changed when the load sensor impedance changes. In Figure 22, as in each of the RFID devices designed and tested in the Phase 1 project, the first tap is a “start” tap, and the second tap is the load tap. Note that a change in load tap response of about 10 dB is observed for the change in switch position.

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0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00

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(dB

)

Switch Open Switch Closed

Figure 22. Non-dispersive RFID tag with open/closed switch test loads.

Fully dispersive RFID Tag switch testing Switch testing was also performed on the fully dispersive RFID tag devices. For these tests, an upchirp filter was used to generate the transmit chirp signal using

ent to the SAW tag, where the full downchirp transducer compressed the pulse, ausing a pulse to propagate towards the reflectors. Reflected pulses generated

a train of downchirps that propagated back, where they were compressed by another upchirp filter and receivsplitte

igure 23 shows the measured open circuit response (shown as the light emory trace) and the short circuit response (the darker trace). A change in sponse level of approximately 10 dB was observed.

the network analyzer sweep out of port 1 as a source signal. This signal was sc

ed by port 2 of the network analyzer. A signal r was used to facilitate testing of the 1-port tag in a 2-port test setup.

Fmre



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Figure 23. Fully-dispersive RFID tag open and short circuit test loads.

Figure 24. Half-dispersive RFID tag open and short circuit test loads.

Open Circuit

Short Circuit

Open Circuit

Short Circuit

Open Circuit

Short Circuit



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Half-dispersive RFID Tag switch testing Switch testing was also performed on the half-dispersive RFID tags, utilizingexternal upchirp SAW filter and the network analyzer as a signal source, andreflected response from the SAW tag as input to port 2 on the analyzer. The results of this testing is shown in Figure 24, where the difference between open circuit and short circuit can be seen as about 12-13 dB.

one the

Wireless Sensor-tag Switch testing Once baseline wired testing had been completed, wireless tests with switch loading were performed for most of the sensor-tags. The wireless tests were conducted using an Agilent E5070B Network Analyzer as both a signal source and a response “reader”. The test setup is shown schematically in Figure 25, and selected wireless sensor-tag responses are shown below. The antennas used for wireless testing were fractal antennas, each of which contributed approximately -40 dB gain (I,e, 40 dB of loss) to the system on transmit and on receive. This 160 dB of loss, combined with the limited power output capability of the network analyzer and external amplifiers on hand at ASR&D seriously limited the interrogation range that could be demonstrated. For all of the wireless test results shown below, the distance D1 was between 3 feet and 5 feet, while the distance D2 was maintained at 1-3 inches. While this range is extremely limited, calculations can show that these results will enable much greater sensor-tag read range with appropriate reader system design.

Figure 25. Test setup used for wireless testing of SAW sensor-tags.

Agilent E5070B Network Analyzer

DUT

LNA

PA

D2

D1



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Non-dispersive tag Figures 26 and 27 below show the wireless response of one of the nondistag devices with open circuit (Figure 26) and short circuit (Figure 27)

persive

Figure 27. Wireless test of nondispersive RFID tag with short circuit load.

Figure 26. Wireless test of nondispersive RFID tag with open circuit load.



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less and wired results showed the For this non-dispersive RFID device, the wiresame change in response level between open and short circuit, of about 8 dB. Half-dispersive RFID Tag Wireless testing of the half-dispersive RFID tags also showed results that

imilar in magnitude to the wired tests. As s observed from open circuit (memory) to

Figure 28. Half-dispersive RFID tag with open and short circuit load. Several features of the response shown in Figure 28 are worth considering. The large signal occupying the first third of the time domain response is the expanded chirp signal from the source chirp filter. This does not constitute interference for the desired RFID tag signal, which necessarily will occur later in time. Thus this signal does not constitute interference or noise in the system, nor is it multipath. It should be noted that the substantial noise level in the second third of the response is due to the source chirp filter being poorly matched and thus producing multiple reflections. Proper matching of the source chirp filter can

exhibited tap response level changes sshown in Figure 28, a13 dB change washort circuit (data trace) load.

reduce the noise level in this region to that of the later response, or about -85 dB.



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4 external sensor # 2

reak test) will be con ing

.1.5.5 Testing withAfter the switch data has been analyzed and acceptable performance has been observed, testing will begin with the acoustic emission sensor. Various tests that are characteristically done with AE sensors (such as the pencil lead b

sidered, and if feasible will be performed. Wireless testing utilizthe network analyzer as a transceiver (with appropriate external signal amplification and antennas) will be performed to demonstrate wireless operation. Accomplishments: Wired testing with varying impedance loads and with a switch was described above. Wired testing was also performed with thermistor and/or strain gage loads, and tests were done with an acoustic emission sensor. AE Sensor Testing Direct testing of the AE sensors with the SAW sensor-tag devices was not feasible due to test equipment sample time limitations. AE sensor responses were measured using an Agilent 1 GHz mixed signal oscilloscope. The sensor substrate was a ¼ in thick aluminum plate, with the sensor centrally mounted. Lead break tests and tests with a center punch were performed, and the resulting voltage signals evaluated. A typical lead break test response is shown in Fig29 below. As shown in Figure 30, the energy content of the center punch responses is much higher.

ure

Figure 29. Typical lead break test AE sensor response.



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hich is clearly not affected by the change in the load FET voltage. The load tap ges

Figure 30. Typical center punch test AE sensor response.

Both responses show that the voltage output of the AE sensors varies fairly rapidly in time. Because we were using a network analyzer to measure the SAW sensor-tag responses, and the sweep time of the analyzer is limited, we were unable to directly observe load voltages varying at these rates. Instead, the zero-bias FETs were connected across the SAW tags, and DC voltages were applied to induce effective drain to source resistance changes that were then measured by observing changes in the SAW response. Figure 31 below shows a typical response of a non-dispersive RFID tag with a FET connected across the load transducer, and with DC voltages from 0V to 10V applied to the gate of the FET. The left peak in the response is a reference tap, wexhibits a change in response level of about 1.5 dB over the range of voltaapplied. The magnitude of these changes could be increased by optimizing the choice of FET to provide a wide change in drain to source impedance for anticipated changes in voltage. Figure 32 shows a typical response of a tapered PSD tag with a FET and load voltages of 0V and 10V. For this device, a change in null response level of about 10 dB is observed, substantially larger than that see for the RFID tag.



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Non-dispersive RFID Tag FET Testing

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Figure 31. Example of a non-dispersive RFID tag sensor response to FET load.

th 0V (upper curve) and 10V (lower curve). Figure 32. Example of a tapered PSD tag sensor response to FET load wi



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Thermistor Testing Non-dispersive RFID tags loaded with Thermistors were tested over temperature (wired) in an oven. Figure 33 shows the tag responses at - 30C and at 120C.

Figure 33. Temperature response of a non-dispersive RFID tag at -30C and 120C.

Note that both amplitude changes of the load tap and delay changes of the SRFID taps occur, providing two methods for measuring temperature. Figure 34below shows the temperature as measured using the load tap amplitude as a measurand. This test involved heating, cooling, and then

AW

heating again, and the

36) Since our environmental chamber is a

results show good agreement with no hysteresis effects. Figure 35 shows the temperature as measured by load tap delay for a fully dispersive RFID tag. While precise temperature measurement reolutions have not yet been determined, it is clear that these methods are capable of producing good, repeatable measurement results. Wireless testing was also performed with thermistors. Figures 36 and 37 show reponses typical of a non-dispersive tag with thermistor load at hot (Figureand cold (Figure 37) temperatures.Faraday cage, wireless testing was performed using headspace gas from liquidnitrogen for cooling and a soldering iron for heat. For the specific tests shown in Figures 36 and 37 precise temperatures were not determined, but cold was about 6°C and hot was about 50°C.



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Non-dispersive RFID Thermistor Testing

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Figure 34. Temperature response of non-dispersive RFID tag based on amplitude.

Figure 35. Temperature response of fully-dispersive RFID tag based on delay.

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Figure 37. Wireless test of non-dispersive RFID tag loaded with thermistor, “cold”.

Figure 36. Wireless test of non-dispersive RFID tag loaded with thermistor, “hot”.



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Strain Gage Tests Preliminary (wired) testing was also conducted with several of the SAW sensor-tags loaded with two strain gages. Figure 38 below shows the effects of a 120Ω strain gage loading a non-dispersive RFID tag, with applied strain producing changes in gage resistance of about 1Ω.

120 Ohm Strain Gage Tests

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Figure 38. Non-dispersive RFID tag loaded with 120Ω strain gage.

The results of a tapered PSD tag being loaded with a 350Ω strain gage are shown in Figure 39. The overall change in load resistance for the strain conditions shown in Figure 39 corresponds to about 3Ω. Clearly the observed changes for the strain gage measurements are quite smalyet they are significant when we consider that the change in resistance causthe change in SAW performance are also quite small (1Ω - 3Ω). Use ofwith higher gage factors and proper impedance transformation for the highimpedance gages could increase m

l, ing

gages er

easured SAW responses. Measurement of static strain is the only type of strain evaluated thus far, although piezoelectric measurements of dynamic strain might be possible using a FET transformation similar to that used with AE sensors.



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Taperes PSD Strain Gage Tests

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‐43

‐42

1.8E+08 2.0E+08 2.2E+08 2.4E+08 2.6E+08 2.8E+08 3.0E+08

Frequency (Hz)

S11 (dB)

No deflection +12 in. deflection ‐12 in. deflection

Figure 39. Tapered PSD tag loaded with 350Ω strain gage.

4.1.5.6 Data analysis Data will be analyzed as collected, and the experimental procedures will be modified as necessary to obtain useful information about the devices under test. Additional testing with other external sensors will be conducted if time allows. Accomplishments: Due to the large number of devices designed, built, and tested during this Phase I program, a substantial amount of data was generated. The significant results of tests conducted have been summarized in this report. Original data and data analysis is being retained at ASR&D for further evaluation and development.

Task 4.1.6 – Evaluation of Commercial Feasibility

Accomplishments: ASR&D has continued preliminary discussions of SAW sensor-tag technology with representatives of the aerospace industry from both government and industrial sectors. These discussions were initiated through CANEUS, and

Montreal this past June. Discussions with representatives of major aerospace industry corporations indicate that the development of SAW wireless sensor

ASR&D has presented portions of this work at the Fly-by-wireless workshop in



ASRDR1012F of 44

3.

4. ThePhase 1 effort resulted in a total project cost of $121,349, which represents a cost overrun of 31.5 % that was absorbed by ASR&D. Selected References

. Brocato, Robert W. “Passive Wireless Sensor Tags”. Sandia Report SAND2006-1288, March 2006

interface devices is of great interest, and talks are ongoing between ASRD and specific companies regarding the potential of collaborative development and/or R&D funding availability. ASR&D has briefly discussed experimental findings with NASA program representatives and has determined that the Phase I program results are relevant to specific NASA programs. A summary of the market study and commercial feasibility evaluation is provided in the Phase II proposal.

Task 4.1.7 Phase I Final Report Accomplishments: This final report summarizes the activities and key findings of the Phase I program.

Patents and Publications One provisional patent (US application No. 61/086,715, entitled “SAW Sensor Tags with Enhanced Performance) was filed by ASR&D covering some of the technologies investigated in this program, and a NTR and NTSR submitted through the eNTRe site. ASR&D is currently evaluating whether or not to file for U.S. Utility patent coverage on these innovations.

Program Financial Summary budgeted total cost for the Phase 1 effort was $92,528.50. Completion of the

1

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4. TITLE AND SUBTITLE SAW Passive Wireless Sensor-RFID Tags with Enhanced Range

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14. ABSTRACT This report describes ASR&D’s Phase 1 SBIR program on the development of passive wireless surface acoustic wave (SAW) sensor-RFID tags. These tags will enable wireless measurement of external impedance varying sensors attached to the SAW tags.

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