1. University of Central Florida School of Electrical Engineering and Computer Science 1 Passive, Wireless Surface Acoustic Wave Technology for Identification and Multi- Sensor Systems D. C. Malocha School of Electrical Engineering & Computer Science, University of Central Florida, Orlando, Fl., 32816-2450 dcm@ece.engr.ucf.edu

2. University of Central Florida School of Electrical Engineering and Computer Science 2 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 It is estimated that approximately 4 billion SAW devices are produced each year Applications: Cellular phones and TV (largest market) Military (Radar, filters, advanced systems Currently emerging – sensors, RFID

3. University of Central Florida School of Electrical Engineering and Computer Science 3 SAW Introduction Operates from 10MHz to 3 GHz Fabricated using IC technology Manufactured on piezoelectric substrates Operate from cryogenic to 1000 o C Small, cheap, rugged, high performance 2mm 10mm Quartz Filter SAW packaged filter showing 2 transducers, bus bars, bonding, etc.

4. University of Central Florida School of Electrical Engineering and Computer Science 4 General SAW Background 2.5mm 10mm LiNbO 3 Filter From: Siemens

5. University of Central Florida School of Electrical Engineering and Computer Science 5 SAW Advantage

6. University of Central Florida School of Electrical Engineering and Computer Science 6 Basic Operation of a SAW Electromechanical Transduction Velocity*time=distance Velocity=distance/time= A SAW transducer is a mapping of time into spatial distance on the substrate

7. University of Central Florida School of Electrical Engineering and Computer Science 7 SAW Reflector Array With ¼ wavelength electrodes, all reflections add in phase (synchronous) which makes a distributed reflector. This is an acoustic mirror. Perturbation at each electrode is small which minimizes losses and mode conversion (BAW generation) SAW Input SAW Output

8. University of Central Florida School of Electrical Engineering and Computer Science 8 SAW Tag & Sensor Advantages Extremely robust Operating temperature range: cryogenic to ~1000 o C Radiation hard, solid state Wireless and passive Coding and spread spectrum embodiments Security in coding Multi-sensors or tags can be interrogated Wide range of sensors in a single platform Temperature, pressure, liquid, gas, etc.

9. University of Central Florida School of Electrical Engineering and Computer Science 9 Why Use SAW Sensors and Tags? External stimuli affects device parameters (frequency, phase, amplitude, delay) Operate from cryogenic to >1000 o C Ability to both measure a stimuli and to wirelessly, passively transmit information Frequency range ~10 MHz – 4 GHz Monolithic structure fabricated with current IC photolithography techniques, small, rugged

10. University of Central Florida School of Electrical Engineering and Computer Science 10 Response of SAW Reflector Test Structure Measurement of S 21 using a swept frequency provides the required data. Transducer response Reflector response is a time echo which produces a frequency ripple Time Response Frequency Response

11. University of Central Florida School of Electrical Engineering and Computer Science 11 Current SAW RF ID Tag Schematic Good for ID tags in close proximity All reflectors are at the same frequency Typical insertion loss is from 40 to 60 dB

12. University of Central Florida School of Electrical Engineering and Computer Science 12 Orthogonal Frequency Coded (OFC) SAW Sensors – a New Embodiment Simultaneous sensing and tagging possible using multiple frequencies Interrogation using RF chirp is possible Reduced time ambiguity of compressed pulses Improved security using spread spectrum Ultra-wide band (UWB) possible

13. University of Central Florida School of Electrical Engineering and Computer Science 13 Orthogonal Frequency Coded (OFC) SAW Device Concept Orthogonal Frequency Coded (OFC) SAW Device Concept Wideband input transducer- coded or uncoded, connected to antenna. A chip in time is represented by a reflector at a given Bragg frequency. Each reflector approximates an ideal Rect (t/T) * cos (w o t) time response with a specified carrier frequency. Multiple chips (reflectors) constitutes a bit ( entire reflector bank). Coding is contained in chip frequency, phase and delay. Example OFC Tag

14. University of Central Florida School of Electrical Engineering and Computer Science 14 UCF OFC Sensor Successful Demonstrations Temperature sensing –Cryogenic: liquid nitrogen –Room temperature to 250 o C –Currently working on sensor for operation to 750 o C Cryogenic liquid level sensor: liquid nitrogen Pressure sensor Hydrogen gas sensor

15. University of Central Florida School of Electrical Engineering and Computer Science 15 Schematic of OFC SAW ID Tag

16. University of Central Florida School of Electrical Engineering and Computer Science 16 Chip length Bit Length τ B = N·τ C The peak of one chip is at the null of all others

17. University of Central Florida School of Electrical Engineering and Computer Science 17 Bit, PN, OFC Signal Comparison Matched Filter Correlated Response OFC format: 7 chips & 7 frequencies, PG=49 Bit Frequency Response Processing Gain ~ time- bandwidth product

18. University of Central Florida School of Electrical Engineering and Computer Science 18 OFC Sensor Platform for Many Sensor Applications OFC reflectors repeated on both sides of transducer Transceiver yields two compressed pulses Pulse separation proportional to sensed information Different free space delays (τ 1 ≠ τ 2 ) yield temperature For gas, chemo or bio sensing a sensitive film, such as palladium for hydrogen gas, is placed in one delay path and a change in differential delay senses the gas (τ 1 = τ 2 )

19. University of Central Florida School of Electrical Engineering and Computer Science 19 Schematic and Actual OFC Gas Sensor For palladium hydrogen gas sensor, Pd film is in only in one delay path, a change in differential delay senses the gas (τ1 = τ2) OFC Sensor Schematic Actual device with RF probe

20. University of Central Florida School of Electrical Engineering and Computer Science 20 Orthogonal Frequency Coded (OFC) SAWs Multiple access operation using Spread Spectrum Coding Improved range due to enhanced processing gain and low loss (due to OFC reflectors) One platform for diverse sensors Inherent security using spread spectrum Fractional bandwidth can meet ultra- wideband (UWB) specifications

21. University of Central Florida School of Electrical Engineering and Computer Science 21 COM predictions Experimental Measurement Dual delay 250 MHz, BW~28%, 7 chips/bank, YZ LiNbO 3 Ng*r =.72 @f o Chip reflector loss~4dB COM Simulation versus Experimental Results – Example 1 Ng*r ~.72

22. University of Central Florida School of Electrical Engineering and Computer Science 22 COM Simulation versus Experimental Results Example 2 - Ng*r~2.38 COM Simulation versus Experimental Results Example 2 - Ng*r~2.38 250 MHz, YZ LiNbO 3, 8 chips %BW=11.5 Al shorted-electrode reflectivity was ~3.4% N g =70 @f0 N g *r~2.38 Chip reflector loss<.5dB

23. University of Central Florida School of Electrical Engineering and Computer Science 23 Chirp Interrogation Transceiver Schematic Picture of RF Section Transceiver, A/D and Post Processing is Accomplished in Computer Transceiver Block Diagram

24. University of Central Florida School of Electrical Engineering and Computer Science 24 Compressed Pulse Responses Temperature Sensor Example

25. University of Central Florida School of Electrical Engineering and Computer Science 25 Example of Current Hardware Simulator Results A simple RF front end and wired SAW device with digital oscilloscope captures trace and simulates A/D and processor Picture scale: –Vertical: 5mV / div –Horizontal: 50ns / div Auto-Correlation

26. University of Central Florida School of Electrical Engineering and Computer Science 26 Temperature Sensor Results 250 MHz LiNbO 3 OFC SAW sensor tested using temperature controlled RF probe station Temp range: 25-200 o C Results applied to simulated transceiver and compared with thermocouple measurements

27. University of Central Florida School of Electrical Engineering and Computer Science 27 OFC Cryogenic Sensor Results Scale Vertical: +50 to -200 o C Horizontal: Relative time (min) Measurement system with liquid nitrogen Dewar and vacuum chamber fro DUT OFC SAW temperature sensor results and comparison with thermocouple measurements at cryogenic temperatures. Temperature scale is between +50 o C and -200 o C and horizontal scale is relative time in minutes.

28. University of Central Florida School of Electrical Engineering and Computer Science 28 OFC Bench Marks - Coding Number of possible codes: >2 N *N! For N chips –For N=7 chips & 7 frequencies, #Codes = 645,000 –For equivalent single frequency tag: #Codes = 128 PN coding: +/- phase of chip Time division multiplexing: Extend the possible number of chips and allow +1, 0, -1 amplitude –# of codes increases dramatically, M>N chips, >2 M *N! –Reduced code collisions in multi-device environment Frequency division multiplexing: System uses N- frequencies but any device uses M < N frequencies –# of codes decreases – Reduced code collisions in multi-device environment

29. OFC Bench Marks – Time & Frequency System Bandwidth: ~N * -1 –For single frequency: ~ -1 Processing gain: BW * ~ N 2 –For single frequency: ~N –Synchronous time integration using multiple “pings” can yield increased PG OFC and single frequency devices use approximately the same time lengths University of Central Florida School of Electrical Engineering and Computer Science 29

30. OFC Bench Marks – Other Device insertion loss: OFC reflector losses can be dramatically reduced yielding 30-60 dB less insertion loss –Ideal OFC devices can have near zero loss Size Number of sensors/codes: Using the OFC diversity, 25 - 100 devices per interrogator Interrogation Distance University of Central Florida School of Electrical Engineering and Computer Science 30

31. University of Central Florida School of Electrical Engineering and Computer Science 31Discussion Current efforts include OFC SAW liquid level, hydrogen gas, pressure and temperature sensors Transceiver is under development for complete wireless, passive SAW OFC sensor system –A/D sampled –Near zero IF –Software radio demodulation –Adaptive filter integration Small, efficient antenna design OFC Code development for multi-sensors New OFC device embodiments

32. University of Central Florida School of Electrical Engineering and Computer Science 32Applications 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

33. University of Central Florida School of Electrical Engineering and Computer Science 33 Graduate Research Student Contributors Daniel Gallagher Brian Fisher Nick Kozlovski Matt Pavlina Bianca Santos Mike Roller Rick Puccio Nancy Saldanha

34. University of Central Florida School of Electrical Engineering and Computer Science 34 Acknowledgment Thank you for your attention! The authors wish to thank continuing support from NASA, and especially Dr. Robert Youngquist, NASA-KSC. The foundation of this work was funded through a NASA Graduate Student Research Program Fellowship, the University of Central Florida - Florida Solar Energy Center (FSEC), and a NASA STTR Phase I contract NNK04OA28C. Continuing research is funded through NASA contracts and industrial collaboration with Applied Sensor Research and Development Corporation, contracts NNK05OB31C, NNK06OM24C, and NNK06OM24C and Mnemonics Corp. Under a new NASA 2007 Phase I STTR.