University of Central Florida

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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_at_ece.engr.ucf.edu

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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
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SAW Introduction

• Operates from 10MHz to 3 GHz
• Fabricated using IC technology
• Manufactured on piezoelectric substrates
• Operate from cryogenic to 1000 oC
• Small, cheap, rugged, high performance

SAW packaged filter showing 2 transducers, bus
bars, bonding, etc.
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General SAW Background
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SAW Advantage
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Basic Operation of a SAW Electromechanical
Transduction
A SAW transducer is a mapping of time into
spatial distance on the substrate
Velocitytimedistance Velocitydistance/time
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SAW Reflector Array
SAW Input
SAW Output
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)
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SAW Tag Sensor Advantages

• Extremely robust
• Operating temperature range cryogenic to 1000
  oC
• 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.

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Why Use SAW Sensors and Tags?

• External stimuli affects device parameters
  (frequency, phase, amplitude, delay)
• Operate from cryogenic to gt1000oC
• 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

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Response of SAW Reflector Test Structure
Frequency Response
Time Response
Reflector response is a time echo which produces
a frequency ripple
Transducer response
Measurement of S21 using a swept frequency
provides the required data.
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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

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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

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Orthogonal Frequency Coded (OFC) SAW Device
Concept
Example OFC Tag
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 (wot) 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.
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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 sensor
• Hydrogen gas sensor

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Schematic of OFC SAW ID Tag
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The peak of one chip is at the null of all others
tB NtC
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Bit, PN, OFC Signal Comparison
Bit Frequency Response
Processing Gain time-bandwidth product
Matched Filter Correlated Response
OFC format 7 chips 7 frequencies, PG49
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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 (t1 ? t2) 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 (t1 t2)

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Schematic and Actual OFC Gas Sensor
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 (t1 t2)

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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

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COM Simulation versus Experimental Results
Example 1 Ngr .72
Dual delay 250 MHz, BW28, 7 chips/bank,
YZ LiNbO3
COM predictions

• Ngr .72 _at_fo

Chip reflector loss4dB
Experimental Measurement
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COM Simulation versus Experimental Results
Example 2 - Ngr2.38
250 MHz, YZ LiNbO3, 8 chips BW11.5 Al
shorted-electrode reflectivity was 3.4 Ng70
_at_f0 Ngr2.38 Chip reflector losslt.5dB
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Chirp Interrogation Transceiver Schematic
Picture of RF Section Transceiver, A/D and Post
Processing is Accomplished in Computer
Transceiver Block Diagram
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Compressed Pulse Responses
Temperature Sensor Example
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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
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Temperature Sensor Results

• 250 MHz LiNbO3 OFC SAW sensor tested using
  temperature controlled RF probe station
• Temp range 25-200oC
• Results applied to simulated transceiver and
  compared with thermocouple measurements

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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 oC and -200 oC and
horizontal scale is relative time in minutes.
Measurement system with liquid nitrogen Dewar and
vacuum chamber fro DUT
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OFC Bench Marks - Coding

• Number of possible codes gt2NN! For N chips
• For N7 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, MgtN chips,
  gt2MN!
• Reduced code collisions in multi-device
  environment
• Frequency division multiplexing System uses
  N-frequencies but any device uses M lt N
  frequencies
• of codes decreases
• Reduced code collisions in multi-device
  environment

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OFC Bench Marks Time Frequency

• System Bandwidth N -1
• For single frequency -1
• Processing gain BW N2
• 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

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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

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Discussion

• 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

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Applications

• 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

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Graduate Research Student Contributors

• Daniel Gallagher
• Brian Fisher
• Nick Kozlovski
• Matt Pavlina
• Bianca Santos
• Mike Roller
• Rick Puccio
• Nancy Saldanha

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Acknowledgment

• 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.

Thank you for your attention!