Surface Acoustic Wave (SAW) Wireless Passive RF Sensor System Tutorial Donald C. Malocha Department of Electrical Engineering & Computer Science University of Central Florida Orlando, Fl. 32816-2450 donald.malocha@ucf.edu

Don Malocha University of Central Florida Don Malocha, Professor, University of Central Florida BS, MS and PhD, Univ. of Illinois, UIUC Texas Instruments, Corporate Research Laboratory, Dallas, MTS Sawtek, Orlando, Mgr. of Advanced Product Development Motorola, Visiting/Member of the Technical Staff, Phoenix and Ft. Lauderdale Visiting Faculty, ETH, Switzerland, and Univ. of Linz, Austria Past President, IEEE Ultrasonics, Ferroelectrics and Frequency Control Society WEB site: http://caat.engr.ucf.edu/ UCF – nations 2nd largest university

Mnemonics Inc. (MNI), Melbourne, Fl. Acknowledgment The author wishes to thank continuing support from everyone who has aided us at NASA, and especially Dr. Robert Youngquist, NASA-KSC. The foundation of this work was funded through NASA Graduate Student Research Program Fellowships, the University of Central Florida – Florida Space Grant Consortium, and NASA STTR and SBIR contracts. Continuing research is funded through NASA STTR/SBIR contracts and industrial collaboration with our industrial partner Mnemonics Inc. (MNI), Melbourne, Fl.

General Background

Multi-Sensor TAG Approaches Silicon RFID – integrated or external sensors Requires battery, energy scavenging, or transmit power Radiation sensitive Limited operating temperature & environments SAW RFID Tags - integrated or external sensors Passive – powered by interrogation signal Radiation hard Operational temperatures ~ 0 - 500+ K Resonator – coding in frequency CDMA- time coding, 40-60 dB loss, wideband OFC - time & frequency coding, 6-20 dB loss, ultra wide band

Why Use SAW Sensors and Tags? Frequency/time are measured with greatest accuracy compared to any other physical measurement (10-10 - 10-14). External stimuli affects device parameters (frequency, phase, amplitude, delay) Operate from cryogenic to >1000oC Ability to both measure a stimuli and to wirelessly, passively transmit information Frequency range (practical) ~100 MHz – 3 GHz Monolithic structure fabricated with current IC photolithography techniques, small, rugged

What is a typical SAW Device? A solid state device Converts electrical energy into a mechanical wave on a single crystal substrate Provides very complex signal processing in a very small volume Approximately 4-5 billion SAW devices are produced each year Applications: Cellular phones and TV (largest market) Military (Radar, filters, advanced systems Currently emerging – sensors, RFID

SAW Principle - Piezoelectricity Squeezing a piezo-crystal creates a voltage. A voltage can compress or dilate a piezo-crystal.

SAW Basics Transduction & Reflection fro SAW Sensors SAW - mechanical wave trapped to the surface Transduction via piezoelectric effect Velocity ~ 3000 - 4000 m/sec Wavelength @ 1 GHz ~ 3 um Line resolution at 1 GHz ~ .75 um Reflection via Bragg reflector structure DC Effect RF to SAW Bragg reflector

SAW Materials to Meet Sensor Needs Crystal cut Coupling coefficient Temperature coefficient SAW Velocity Max Temp LiNbO3 Y,Z 4.6% 94 ppm/ºC 3488 m/s ~500 ºC 128ºY,X 5.6% 72 ppm/ºC 3992 m/s LiTaO3 0.74% 35 ppm/ºC 3230 m/s Quartz ST 0.16% 0 ppm/ºC 3157 m/s 550 ºC Langasite Y,X 0.37% 38 ppm/ºC 2330 m/s >1000 ºC 138ºY,26ºX 0.34% ~0 ppm/ºC 2743 m/s SNGS 0.63% 99 ppm/ºC 2836 m/s SAW travels ~ 105 slower than EM wave SAW wavelength @ 1 GHz ~ 3 um

SAW/IC Fabrication Techniques Lines are ~ .8 um SAW Transducer SAW reflector gratings The dark line in each micrograph is a 23 um gold wire SAW reflector gratings SAW devices @ 1 GHz require submicron lithography. Standard IC thin films, photolithography and processing are used.

Basic Passive Wireless SAW System Sensor #1 Gas Sensor #3 Temperature Sensor #2 Pressure Goals: Interrogation distance: 1 – 50 meters # of devices: 10’s – 100’s - coded and distinguishable at TxRx Aerospace applications – rad hard, wide temp., solid state, etc. Single platform and TxRx for differing sensor combinations

RFID and SAW Introduction

RFID Sensor RFID Acquisition Measurand Extraction Two primary system functions: RFID and extraction of the measurand. The RFID must first be acquired and then the measurand extracted. The presentation will address these issues for a temperature sensor system. RFID Acquisition Priority for system Coding approach Demodulation approach System Parameters Measurand Extraction RFID is acquired S/N ratio Accuracy Acquisition rate

Diversity for Identification Frequency Spectrum Diversity per Device Coding Divide into frequency bands Time Delay per Device Different offset delays per device Pulse position modulation Time allocations minimize code collisions Spatial Diversity – device placement Sensor & Tx-Rx Antenna Polarization Use combinations of all to optimize system

Brief Introduction to Wireless SAW Sensors One port devices return the altered interrogation signal Range depends on embodiment Range increased using coherent integration of multiple responses Interrogator used to excite devices Several embodiments are shown next

Reflective Delay Line Sensor “Wireless Interrogator System for SAW-Identification-Marks and SAW-Sensor Components”, F. Schmidt, et al, 1996 IEEE International Frequency Control Symposium First two reflectors define operating temperature range of the sensor Time difference between first and last echoes used to increase resolution of sensor No coding as shown

SAW Chirp Sensor “Spread Spectrum Techniques for Wirelessly Interrogable Passive SAW Sensors”, A. Pohl, et al, 1996 IEEE Symposium on Spread Spectrum Techniques and Applications Increased sensitivity when compared with simple reflective delay line sensor Multi-sensor operation not possible due to lack of coding

Impedance SAW Sensors “State of the Art in Wireless Sensing with Surface Acoustic Waves”, W. Bulst, et al, IEEE UFFC Transactions, April 2001 External classical sensor or switch connected to second IDT which operates as variable reflector Load impedance causes SAW reflection variations in magnitude and phase No discrimination between multiple sensors as shown

SAW RFID Practical Approaches Resonator Fabry-Perot Cavity Frequency selective, SAW device Q~10,000 Code Division Multiple Access (CDMA) Delay line – single frequency Bragg reflectors Pulse position encoding Orthogonal Frequency Coding (OFC) Delay line, multi-frequency Bragg reflectors Frequency coupled with time diversity

SAW Resonator Q~10,000 Resonant cavity Frequency with maximum returned power yields sensor temperature High Q, long time response Coding via frequency domain by separating into bands “Remote Sensor System Using Passive SAW Sensors”, W. Buff, et al, 1994 IEEE International Ultrasonics Symposium

SAW CDMA Delay Line CDMA Tag Concept CDMA Tag Single frequency Bragg reflectors Coding via pulse position modulation Large number of possible codes Short chips, low reflectivity - (typically 40-60 dB IL) Early development by Univ. of Vienna, Siemens, and others

SAW OFC Delay Line OFC Tag OFC Tag Multi-frequency (7 chip example) DUT - RF probe connected to transducer Bragg reflector gratings at differing frequencies Micrograph of device under test (DUT) OFC Tag Multi-frequency (7 chip example) Long chips, high reflectivity Orthogonal frequency reflectors –low loss (6-10 dB) Example time response (non-uniformity due to transducer)

Discussion Resonator, CDMA, and OFC embodiments have all been successfully demonstrated and applied to various applications. Devices and systems have been built in the 400 MHz, 900 MHz and 2.4 GHz bands by differing groups. Resonator Minimal delay Narrowband PG~1 Fading Frequency domain coding High Q – long impulse response Low loss sensor CDMA Delay as reqd. ~ 1usec Spread Spectrum Fading immunity Wideband PG >1 Time domain coding Large number of codes using PPM OFC Delay as reqd. ~ 1usec Spread Spectrum Fading immunity Ultra Wide Band PG >>1 Time & frequency domain coding Large number of codes using PPM and diverse chip frequencies

OFC Sensor Embodiment

SAW OFC Sensor Introduction Conventional wisdom at the time: “ Orthogonality in frequency is not feasible with coded reflective passive SAW sensors.”, “Spread Spectrum Techniques for Wireless Interrogable Passive SAW Sensors”, A. Pohl, et. al., IEEE 4th International Symposium on Spread Spectrum Techniques and Applications, 1996, pp. 730. “D. Malocha and coworkers recently developed Orthogonal Frequency Coding for SAW tags [25]. …….This approach can be applied to sensors and for identification of a limited number of sensors, but it can hardly be used for ID tags with large numbers of codes.” Review on SAW RFID Tags, V. P. Plessky, and L.M. Reindl, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 57, no. 3, March 2010, pp.654 First OFC publication by UCF group in 2004 and working system in 2009. The use of spread spectrum frequency and time coding had been overlooked as either not possible or too complicated. For RFID sensors, the approach is both feasible, advantageous, and demonstrated.

OFC Historical Development Chose 1st devices at 250 MHz for feasibility Several different OFC sensors demonstrated Demonstrated harmonic operated devices at 456, 915 MHz and 1.6 GHz Fundamental device operation at 915 MHz Devices in the +1 GHz range in 2010 First OFC system at 250 MHz Current OFC system at 915 MHz First 4 device wireless operation in 2009 Mnemonics demonstrates first chirp OFC correlator receiver in 2010

Why OFC SAW Sensors? A game-changing approach All advatageous of SAW technology Wireless, passive and multi-coded sensors Frequency & time offer greatest coding diversity Single communication platform for diverse sensor embodiments Radiation hard Wide operational temperature range

Schematic of OFC SAW ID Tag Sensor bandwidth is dependent on number of chips and sum of chip bandwidths. Frequency domain of Bragg reflectors: contiguous in frequency but shuffled in time Time domain chips realized in Bragg reflectors having differing carrier frequencies and frequencies are non-sequential which provides coding

Example 915 MHz SAW OFC Sensor US Quarter SAW Sensor SAW OFC Reflector Chip Code f4 f3 f1 f5 f2 FFT Show picture of device and explain microwave operation Point out the parts of the tag and how we extract a time delay Trace out the transducer response Explain the time response and the decreasing transducer response

SAW OFC RFID signal – Target reflection as seen by antenna absorber S11 w/ absorber and w/o reflectors S11 w/o absorber and w/ reflectors Coded SAW chips are bound in frequency and received sequentially in time

OFC: # codes=N!*2N where N= #chips OFC vs CDMA Number of possible codes versus number of chips for same chip configuration CDMA: # codes=2N OFC: # codes=N!*2N where N= #chips

Effect of Code Collisions from Multiple SAW RFID Tags -Simulation Due to asynchronous nature of passive tags, the random summation of multiple correlated tags can produce false correlation peaks and erroneous information

OFC Coding Time division diversity (TDD): Extend the possible number of chips and allow delay and phase modulation # of codes increases dramatically, M>N chips, >2M*N! Reduced code collisions in multi-device environment f1 f4 f3 f2 Sensor #1

456 MHZ SAW OFC TDD Coding A 456 MHz, dual sided, 5 chip, tag COM-predicted and measured time responses illustrating OFC-PN-TDD coding. Chip amplitude variations are primarily due to polarity weighted transducer effect and fabrication variation.

OFC FDM Coding f1 f4 f3 f2 f6 f7 f5 f8 Sensor #1 Sensor #2 Frequency division multiplexing: System uses N-frequencies but any device uses M < N frequencies System bandwidth is N*Bwchip OFC Device is M*BWchip Narrower fractional bandwidth Lower transducer loss Smaller antenna bandwidth f1 f4 f3 f2 f6 f7 f5 f8 Sensor #1 Sensor #2 36

Antenna and SAW Sensor Design Considerations

SAW Electrically Small Antenna Gain and Bandwidth The plots show that there is a minimum size at a given frequency to attain a desired fractional bandwidth. As the frequency increases, a larger fractional bandwidth is achievable for a smaller antenna size. As the effective size of the antenna increases, the gain and bandwidth both increase.

SAW Target – SAW + Antenna UCF Initial Design 250 MHz Disk Monopole Antennas Large dinner plate design met fractional bandwidth, but hardly miniature compared to SAW sensor size

Target Gain vs. Frequency Analysis points to ~1 GHz   where f is in GHz Good fo region %BW SAW, antenna and net gain in dB, and fractional bandwidth, versus frequency for a 3cm radius ESA. Assumes a SAW propagation length of 5 usec.

Wideband Open-Sleeve Dipole Antenna Designed on 32mil FR4 (εr=4.7 and tan(δ)=0.015) Entire structure optimized in IE3D between 800MHz and 1GHz Explain antenna design 41

Fully integrated on-wafer SAW OFC sensor and antenna SAWtenna @ 915 MHz Fully integrated on-wafer SAW OFC sensor and antenna Wireless OFC SAWtenna time domain response Test wafer-level SAW & antenna integration

Miniature 915MHz Integrated OFC SAW-Patch Antenna

Synchronous Correlator Transceiver

Synchronous Transceiver - Software Radio Pulse Interrogation: Chirp or RF burst Correlator Receiver Synchronous Software Radio Based 915 MHz Pulsed RF Transceiver Block Diagram

Temperature Extraction Using Adaptive Correlator Comparison of ideal and measured matched filter of two different SAW sensors : 5-chip frequency(below) Normalized amplitude (dB) versus time NS401 NS403 Stationary plots represent idealized received SAW sensor RFID signal at ADC. Adaptive filter matches sensor RFID temperature at the point when maximum correlation occurs.

Synchronous Correlator Receiver Block diagram of a correlator receiver using ADC OFC Single Sensor Signal Correlation Output Temperature Extraction

250 MHz Wireless Pulsed RF OFC SAW System - 2nd Pass An OFC SAW temperature sensor data run on a free running hotplate from an improved 250 MHz transceiver system. The system used 5 chips and a fractional bandwidth of approximately 19%. The dashed curve is a thermocouple reading and the solid curve is the SAW temperature extracted data. The SAW sensor is tracking the thermocouple very well; the slight offset is probably due to the position and conductivity of the thermocouple.

SAW 915 MHz Correlator Transceiver

MNI Transceiver Design Pulsed RF Chirp Correlator Receiver Synchronous operation Integration of multiple “pings” OFC processing gain Adaptive filter temperature extraction Software radio based approach for versatility

Current Sensor System Results 915 MHz transceiver developed by Mnemonics, Inc. (MNI), Melbourne, Fl RF Chirp 700nsec, 28dBm peak power Synchronous receiver OFC SAW temperature sensors developed by UCF YZ LiNbO3, 5 chip OFC delay line sensor 915 MHz fundamental, 0.8 um electrodes Correlator software developed at UCF

Critical Transceiver Operational Parameters EM Path Loss Considerations Electrically Small Antennas (ESA) SAW Device Propagation Loss Target Gain versus Center Frequency Integrated SAW and Antenna

EM Path Loss versus Range EM isotropic two-way path loss for 3 differing operational frequencies: 0.25, 0.5 and 1 GHz - solid lines. The dotted traces are the thermal noise levels at 3 differing bandwidths, 25, 72, and 200 MHz. Path loss increases @ 40dB/decade w/ increasing range or frequency

RF Transceiver: Sensor Overview OFC with single wideband transducer Center Frequency: 915 MHz Bandwidth: Chirp - ~78 MHz Number of Chips: 5 Chip length 54ns/each, total reflector length 270ns Substrate: YZ LiNbO3

SAW 915 MHz OFC Sensor SAW sensor acts as RFID and sensor All antenna & transducer effects are doubled Antenna gain and bandwidth are dependent on size scaled to frequency SAW propagation loss is frequency dependent

Parameter Definitions (extensive list of variables) ADC= ideal analog-to-digital converter MDS= minimum detectable signal at ADC S= signal power measured at ADC N= noise power measured at ADC kT= thermal noise energy EIRP= equivalent radiated power GRFIDS= RFIDS gain (less than unity for passive device) GRx-ant= gain of the receiver antenna GRx= receiver gain from antenna output to ADC PG= signal processing gain of the system (= τ·B) PL= path loss NF= receiver noise figure Next= external noise source referenced to antenna output NADC= ADC equivalent noise Nsum= number of synchronous integrations in ADC PGC = pulse compression gain from chirp interrgogation

Range Prediction For passive RFIDS, the range is given from Friis equation as Range =r = PL.25·[vEM/(4·π·fo)] ; where vEM=free space velocity A minimum S/N is determined for detection, and the maximum range, in meters, achievable, given in dB, is obtained as rmax-dB=.25·{GPDL+Gsys+Nsum-[S/Nmin]} -10·log[(4·π·f)/vEM], where GPDL=[EIRP/(NF*·kT/τau)] = power-detection level gain and Gsys = [(GRFIDS·GRx-ant)]

RF Chirp Transceiver Parameters Power to antenna = 30dBm Pulse-length = 700ns, 20Vpp Antenna Gain = 9dB Bandwidth = 74MHz Receiver Gain = 45dB NF = 15dB PGC= 49 = 17 dB

Range Prediction for MNI Receiver for RFID Detection (not sensor) Range is a function of the complete system loop gain, shown in solid line (red). Loop gain is dependent on the transmit power, noise and gain in the system. Typical loop gains are realistically achievable between 100 to 180 dB. The box shows the predicted loop gain for the MNI/UCF system, which is very close to measurements obtained.

Chirp Transceiver: SAW OFC Sensor Range Experiment Single sensor only; no signal integration Multiple distances from 1.2m to 20m 0 to 20dB additional attenuation at each step 128 readings taken per distance per attenuation Longest distance of successful interrogation 7m Reading error .07 corresponds to 60% of all data points within 5°C (3.5%)

Practical Extension NF = 18dB → 8dB (∆G = 10dB) GSAW = -23dB → -10dB (∆G = 13dB) GPSI = 12dB → 22dB (∆G = 10dB) Total improvement: 33dB Approximately extended range: 80m 1.21 jigga watts

915 MHz OFC Temperature Sensor System Measured Device Data in a Hallway Data is measured in a hallway approximately 2.1 meters wide. Antennas: transmit is a wideband 1 dB dipole; receive is a 9 dB Yagi. The system loop gain is calculated at ~40 dB (+/-3 dB). Transmit signal is a single, 700 nsec, 915 MHz chirp pulse. The OFC SAW device uses 5 chips, each with an approximate 15 MHz bandwidth. SAW device processing gain is 25. Slope of the fit measured data is -38.7 dB/decade; close to the 40 dB/decade expected for isotropic radiation path loss. The hallway is probably producing a waveguiding effect and external noise was low during testing. Test shows that some environments can produce long ranges.

OFC SAW Correlator Receiver Tag Ranging Distance from interrogator to the sensor can be extracted based on EM delay (8m per chip length – 54ns) X-axis indicates various distances at which sensor was placed away from interrogator Cross-marks indicate distance from interrogator on y-axis 128 Measurements were made for each step Blue box indicates spread of a half of all data Black boundaries indicate spread of 99.3% of all data Red pluses indicate outliers

UCF Sensor Development There is an extensive body of knowledge on sensing Wired SAW sensing has quite an extensive body of knowledge and continues Wireless SAW sensing has been most successfully demonstrated for single, or very few devices and in limited environments The following are a few of the successful UCF sensor projects The aim is to enable wireless acquisition of the sensors data The further goal is to develop a multi-sensor system for aerospace applications Successful wireless sensing has been demonstrated for temperature, liquid, closure, and range

UCF OFC Sensor Successful Demonstrations Temperature sensing Cryogenic: liquid nitrogen Room temperature to 250oC Currently working on sensor for operation to 750oC Cryogenic liquid level sensor: liquid nitrogen Pressure/Strain sensor Hydrogen gas sensor Closure sensor with temperature

Four-sensor operation Four OFC SAW sensors are co-located in close range to each other at a distance of 0.8m to 1.2m Sensors NS402 and NS404 remained at room temperature Sensor NS401 heated to 140°C Sensor NS403 cooled to -130°C Data was taken simultaneously from all four sensors and then temperature extracted in the correlator receiver software Error is within ±5°C (±3.5% for given dynamic range) Explain the set-up better, this is very important stuff, speak to the slide, read the plot

Differential SAW OFC Thin Film Gas Sensor Embodiment

Temperature Sensor using Differential Delay Correlator Embodiment Temperature Sensor Example 250 MHz LiNbO3, 7 chip reflector, OFC SAW sensor tested using temperature controlled RF probe station

Temperature Sensor Results 250 MHz LiNbO3, 7 chip reflector, OFC SAW sensor tested using temperature controlled RF probe station Temp range: 25-200oC Results applied to simulated transceiver and compared with thermocouple measurements

OFC Cryogenic Sensor Results Scale Vertical: +50 to -200 oC Horizontal: Relative time (min) OFC SAW temperature sensor results and comparison with thermocouple measurements at cryogenic temperatures. Temperature scale is between +50 to -200 oC and horizontal scale is relative time in minutes. Measurement system with liquid nitrogen Dewar and vacuum chamber for DUT

Schematic and Actual OFC Gas Sensor Differential mode OFC Sensor Schematic Actual device with RF probe For palladium hydrogen gas sensor, Pd film is in only in one delay path, a change in differential delay senses the gas (τ1 = τ2) (in progress)

Hydrogen Gas sensor Palladium Background Information The bulk of PD research has been performed for Pd in the 100-10000 Angstrom thickness Morphology of ultra-thin films of Pd are dependent on substrate conditions, deposition and many other parameters Pd absorbs H2 gas which causes lattice expansion of the Pd film – called Hydrogen Induced Lattice Expansion (HILE) – Resistivity reduces Pd absorbs H2 gas which causes palladium hydride formation – Resistivity increases Examine these effects for ultra-thin films (<5nm) on SAW devices HILE - Each small circle represents a nano-sized cluster of Pd atoms

Pd Films on SAW Devices Schematic of Test Conditions Control: SAW delay line on YZ LiNbO3 wafers w/ 2 transducers and reflector w/o Pd film Center frequency 123 MHz (A) SAW delay line w/ Pd in propagation path between transducer and reflector (B) SAW delay line w/ Pd on reflector only 1.27 mm

Hydrogen Gas Sensor Results: 2% H2 gas Nano-Pd Film – 25 Ang. Hydrogen Gas Sensor Results: 2% H2 gas Theory (lines) versus measurement data The change in IL indicates a <20 dB sensitivity range and further tests were < 50 dB! Sensitive hydrogen sensor is possible.

Cantilever Sensor Results Initial cantilever sensor results Apply results to strain and pressure sensors

OFC Cantilever Strain Sensor Measure Delay versus Strain

OFC Cantilever Strain Sensor Plot generated by ANSYS demonstrating the strain distribution along the z-axis of the crystal. Test fixture, this shows the surface mount package, which contains the cantilever device, securely clamped down onto a PC board which is connected to a Network Analyzer.

Applications Current efforts include OFC SAW liquid level, hydrogen gas, pressure and temperature sensors Multi-sensor spread spectrum systems Cryogenic sensing High temperature sensing Space applications Turbine generators Harsh environments Ultra Wide band (UWB) Communication UWB OFC transducers Potentially many others

Current to Future

Vision for Future Multiple access, SAW RFID sensors SAW RFID sensor loss approaching 6 dB Unidirectional transducers Low loss reflectors New and novel coding New and novel sensors New materials for high temperature (1000oC) and harsh environments SAW sensors in test space flight and support operations in 1 to 5 years

Ultra Wide Band BW defined by chirp, not by individual sensors Could use a frequency hopped chirp system Frequency diversity is increased Code coliision reduction Multi-bands for multi-sensors Subsets of sensors activated at any given time Narrower band antennas, lower loss devices

SAW Research at UCF UCF Center for Acoustoelectronic Technology (CAAT) has been actively doing SAW and BAW research for over 25 years Research includes communication devices and systems, new piezoelectric materials, & sensors Capabilities include SAW/BAW analysis, design, mask generation, device fabrication, RF testing, and RF system development Current group has 6 PhDs & 1 Post-doc

Capabilities Proprietary software: COM analysis & design, parameter extraction, data acquisition and test UCF device fabrication to < .8um resolution In-house mask fab & thin film capabilities Complete RF SAW characterization facility Extensive RF laboratory for system development

UCF SAW OFC Contracts & Intellectual Property A. 6 – Phase I and 4 –Phase II STTR/SBIRs on SAW OFC Sensors B. NASA KSC, Langley, and JSC contracts C. Fellowships from NASA, NSF, Motorola, NSDEG, UCF, McKnight, and Florida Space Grant D. Patents on SAW OFC: #7,642,898 D.C. Malocha and Puccio, Orthogonal Frequency Coding for Surface Acoustic Wave Communications, Tag, and Sensors, Jan. 5, 2010. #7,623,037 D.C. Malocha, Multi-transducer/antenna surface acoustic wave device sensor and tag, November 24, 2009. #7,825,805, D.C. Malocha and D. Puccio, Delayed Offset Multi-Track OFC Sensors and Tags, Nov. 2, 2010. #7,777,625, D.C. Malocha and D. Puccio, Weighted Reflectors for OFC Coding, Aug. 17, 2010. #7,791,249, D.C. Malocha and N.Y. Kozlovski, SAW Coding for OFC Devices, Appl # 12,618,034, D.C. Malocha and N. Kozlovski, Coding for Surface Acoustic Wave Devices, Filed Nov. 13, 2009. Several in process

Conclusion 915 MHz OFC SAW temperature sensor system has been demonstrated Current tests show 10 meter open range 4 sensors have been simultaneously interrogated and measured Range predictions and measured data have been shown Wireless passive SAW sensors are a “game-changing” technology