university of central florida school of electrical engineering and computer science 1 passive,...

University of Central FloridaSchool of Electrical Engineering and Computer Science

1

Passive, Wireless Surface Passive, Wireless Surface Acoustic Wave Technology Acoustic Wave Technology for Identification and Multi-for Identification and Multi-

Sensor SystemsSensor SystemsD. C. Malocha

School of Electrical Engineering & Computer Science, University of Central Florida,

Orlando, Fl., [email protected]

Piezoelectric Substrate

f1 f4 f6 f0f2 f5 f3

0 1 2 3 4 5 6 71

0.5

0

0.5

1

Normalized Time (Chip Lengths)

mailto:[email protected]


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What is a typical SAW Device?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 systemsCurrently emerging – sensors, RFID


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SAW IntroductionSAW 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

2mm

10mm

Quartz Filter SAW packaged filter showing 2 transducers, bus bars, bonding, etc.


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General SAW BackgroundGeneral SAW Background

2.5mm

10mm

LiNbO3 Filter

From: Siemens


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SAW AdvantageSAW Advantage


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Basic Operation of a SAW Basic Operation of a SAW Electromechanical TransductionElectromechanical Transduction

Velocity*time=distance

Velocity=distance/time= T

A SAW transducer is a mapping of time into spatial distance on the substrate


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SAW Reflector ArraySAW 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


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SAW Tag & Sensor AdvantagesSAW 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?Why Use SAW Sensors and Tags?

• 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 ~10 MHz – 4 GHz

• Monolithic structure fabricated with current IC photolithography techniques, small, rugged


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Response of SAW Reflector Test StructureResponse of SAW Reflector Test Structure

Measurement of S21 using a swept frequency provides the required data.

62 64 66 68 70 72 74 76 78 80-90

-80

-70

-60

-50

-40

-30

-20

Frequency (MHz)

dB

(S21

)

Transducer response

Reflector response is a time echo which produces a frequency ripple

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-80

-70

-60

-50

-40

-30

-20

-10

Time (s)

dB

(s

21)

Direct SAW response

Reflector response

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-80

-70

-60

-50

-40

-30

-20

-10

Time (s)

dB

(s

21)

Direct SAW response

Reflector response

Time ResponseFrequency Response


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Current SAW RF ID Tag SchematicCurrent 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 Orthogonal Frequency Coded (OFC) SAW Sensors – (OFC) SAW Sensors –

a New Embodimenta 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


f1 f4 f6 f0f2 f5 f3f1f4f6f0 f2f5f3


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Orthogonal Frequency Coded Orthogonal Frequency Coded (OFC) SAW Device Concept(OFC) SAW Device Concept


f1 f4 f6 f0f2 f5 f3

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.

Example OFC Tag


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UCF OFC Sensor Successful UCF OFC Sensor Successful DemonstrationsDemonstrations

• 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 TagSchematic of OFC SAW ID Tag

0 1 2 3 4 5 6 71

0.5

0

0.5

1



f1 f4 f6 f0f2 f5 f3


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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.80

0.2

0.4

0.6

0.8

Normalized Frequency

Mag

nit

ud

e (L

inea

r)

0 1 2 3 4 5 6 71

0.5

0

0.5

1


Chip length

Bit Length

c/11 cc ff

τB = N·τC

constantlength, Chip c

cc N* cf

The peak of one chip is at the null of all others


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Bit, PN, OFC Signal ComparisonBit, PN, OFC Signal Comparison

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-40

-35

-30

-25

-20

-15

-10

-5

0

Normalized Frequency

No

rma

lize

d to

Pe

ak

of S

ing

le C

arr

ier

(dB

)

7 chips/bit PN-OFC7 chips/bit PN Single CarrierBPSK

0 1 2 3 4 5 6 70

0.2

0.4

0.6

0.8

1

Time Normalized to a Chip Length

No

rma

lize

d A

mp

litu

de

7 chips/bit PN-OFC7 chips/bit PN Single CarrierBPSK

Matched Filter Correlated Response

OFC format: 7 chips & 7 frequencies, PG=49

Bit Frequency Response

Processing Gain ~ time-bandwidth product


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OFC Sensor Platform for Many OFC Sensor Platform for Many Sensor ApplicationsSensor Applications

• OFC reflectors repeated on both sides of transducer• Transceiver yields two compressed pulses• Pulse separation proportional to sensed information

• Different free space delays (τ1 ≠ τ2) 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 (τ1 = τ2)


f1 f0f2 f3f1f0 f2f3


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Schematic and Actual OFC Gas SensorSchematic and Actual OFC Gas Sensor


f1 f0f2 f3f1f0 f2f3

•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


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Orthogonal Frequency Coded Orthogonal Frequency Coded (OFC) SAWs(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 predictions

Experimental Measurement



Dual delay 250 MHz, BW~28%, 7 chips/bank, YZ LiNbO3

•Ng*r = .72 @fo

Chip reflector loss~4dB

COM Simulation versus COM Simulation versus Experimental Results – Example 1 Experimental Results – Example 1

Ng*r ~ .72 Ng*r ~ .72


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COM Simulation versus COM Simulation versus Experimental Results Experimental Results

Example 2 - Example 2 - Ng*r~2.38Ng*r~2.38



2 3 4 5 6 7 8-70

-60

-50

-40

-30

-20

-10

Time (s)

Imp

uls

e R

esp

on

se (

dB

)

SimulationExperiment

First reflectorbank

response

Second reflectorbank response

250 MHz, YZ LiNbO3,

8 chips %BW=11.5

Al shorted-electrode reflectivity was ~3.4% Ng=70 @f0

Ng*r~2.38

Chip reflector loss<.5dB


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Chirp Interrogation Transceiver Chirp Interrogation Transceiver SchematicSchematic

SAWsensor

RF Oscillator

Digital control and analysis circuitry

SAW up-chirp filter

SAW down-chirp filter

IF Oscillator

A / D

IF Filter

Picture of RF Section Transceiver, A/D and Post Processing is Accomplished in Computer

Transceiver Block Diagram


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Compressed Pulse ResponsesCompressed Pulse Responses



Temperature Sensor Example


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Example of Current Hardware Example of Current Hardware Simulator ResultsSimulator 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 ResultsTemperature 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

0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

120

140

160

180

200Temperature Sensor Results

Time (min)

Te

mp

era

ture

( C)

LiNbO3 SAW Sensor

Thermocouple


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OFC Cryogenic Sensor ResultsOFC Cryogenic Sensor Results

0 5 10 15 20 25-200

-150

-100

-50

0

50

Time (min)

Tem

pera

ture

( C

)

ThermocoupleLiNbO

3 SAW Sensor

ScaleVertical: +50 to -200 oC

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 oC and -200 oC and horizontal scale is relative time in minutes.


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OFC Bench Marks - CodingOFC Bench Marks - Coding• Number of possible codes: >2N*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, >2M*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

OFC Bench Marks – OFC Bench Marks – Time & FrequencyTime & 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


29

ccc

c

OFC Bench Marks – OtherOFC 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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DiscussionDiscussion

• 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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ApplicationsApplications

• 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 Graduate Research Student ContributorsContributors

• Daniel Gallagher• Brian Fisher

• Nick Kozlovski• Matt Pavlina

• Bianca Santos• Mike Roller• Rick Puccio

• Nancy Saldanha


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AcknowledgmentAcknowledgment

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.

university of central florida school of electrical engineering and computer science 1 passive,...

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