A Review of SAW Sensor Technology
DEC. 2015
Arthur R. Weeks
Electrical and Computer Engineering
University of Central Florida
Orlando, Fl. 32816-2450
[email protected]
Donald C. Malocha
Center for Applied Acoustics Technology
University of Central Florida
Orlando, Fl. 32816-2450
[email protected]
Wireless SAW Sensor Applications
• Temperature –Braking, Motors, High Voltage
Arrestors
• Relative Humidity
• Gas H2, Co2, Isopropyl Alcohol
• Stress
• ID-Tag for Toll System (Norway)
• ID system for Munich Subway (Siemens)
• Accelerometers
• Pressure - tire pressure (Siemens)
• Fluids (biomedical applications)
• Magnetic (wireless hall sensor - Reed switch)
Why SAW
Passive Sensors?
• A game-changing approach
• Wireless, “infinite-life”, and
multi-coded
• Single communication
platform for diverse sensor
embodiments
• Broad frequency range of
operation and range
(.25-2.5 GHz)
• Many different
embodiments
• Multiple sensor operations
on a single chip
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Physical
Gas
Liquid
other
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Highlights
Solid state
Piezoelectric
Freq: 0.2 – 2.4 GHz
Temp: 0.1 – >1000K
Rad Hard
SS-RFID
Current Sensors: temp,
gas, strain, liquid,
magnetic
Single IDT SAW Sensor
Drawing of a single IDT and a seven chip OFC SAW device grating frequencies
match chip center frequencies (RED – Interrogation signal, BLUE- sensor
signal)
SAW DEVICE MODEL
I
Input
from Ant
A1
B2
B1
A2
Zload V
I
A = Forward waves
B =Reverse waves
V =Voltage across the Load
I = Current through the Load
1 = port 1, 2 = Port 2
Yload = Admittance of the load
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•
•
•
•
P1,3, P2,3 - Voltage to SAW Transfer Function
P1,3 , P2,3 - SAW to Voltage Transfer Function
P3,2 = -2P2,3 P3,1 = -2P1,3
P1,2 , P2,1 - Acoustic Port Transmission Coefficient
For a Symmetrical Transducer P1,2 = P2,1
P1,1 , P2,2 - Acoustic Port reflection Coefficient
P3,3 - Transducer Admittance
𝑃 𝑃 𝑃
P = 𝑃12,11 𝑃12,22 𝑃12,33
,
,
,
𝑃3,1 𝑃3,2 𝑃3,3
𝑏1
𝑏2
𝐼
=
𝑃1,1 𝑃1,2 𝑃1,3
𝑃2,1 𝑃2,2 𝑃2,3
𝑃3,1 𝑃3,2 𝑃3,3
𝑎1
𝑎2
The Reflection Coefficient
of the Transducer
2𝑃1,32
𝑆1,1 ≈
𝑃3,3 + 𝑌𝑙𝑜𝑎𝑑
𝑉
Common SAW Device Materials
Vel(m/s)
Dv/v (%)
Lithium Niobate
LiNbO3 ,Y-Z
3488
2.4
94
LiNbO3 , 128˚Y-X
3979
2.7
75
SiO2Quartz
ST-X (42.75˚Y-X)
3159
0.06
0(32)
Lithium Tantalate
LiTaO3, X-112˚Y
3300
0.35
18
Lithium Tetraborate
Li2B4O7, 45˚X-Z
3350
0.45
0(270)
LiNbO3 ,64˚Y-X
4742
5.5
80
LiNbO3 ,41˚Y-X
4792
8.5
80
LiNbO3 ,36˚Y-X
4212
2.4
32
Quartz, 36˚Y-X +90˚
5100
-
0(6)
Material
Coupling Constant K2 = 2 ·Dv/v
TCD (ppm/˚C)
SAW DEVICE PROCESS STEPS
(Liftoff Method)
1. SAW Sensor Design
2. Mask Generation
3. Wafer Cleaning
4. Photo Resist (PR) Coating
5. Exposure of Mask to PR (exposed areas PR is removed during development)
6. Development (exposed areas PR removed for metallization)
7. Deposition for metallization
8. Lift-off (removes remaining PR)
9. Dice Wafer
10. SAW Sensor Cleaning
11. Install SAW Sensor in a package or mount to an Antenna
Example of Seven Chip Orthogonal Frequencies
Grating Electrode Count
Example 915 MHz SAW OFC Sensor
Light Micrograph
f4 f3 f1 f5 f2
5 Chip
SAW OFC Reflector Chip Code
Wideband SAW Sensor Antenna Design
The Saw Sensor is located inside the Gold Chip Carrier
Center Frequency 915 MHz
Wideband SAW Sensor Antenna Design
The reflection coefficient at the plane
of the antenna balun 𝛤p, 𝑍𝐴𝑁𝑇 , and
SAW microstrip matching network,
𝑍𝑆𝐴𝑊, is:
𝑍𝑆𝐴𝑊 − 𝑍𝐴𝑁𝑇
𝑆1,1 = 𝛤𝑝 =
𝑍𝑆𝐴𝑊 +𝑍𝐴𝑁𝑇
The best match occurs when
𝑍SAW =𝑍𝐴𝑁𝑇
Calculated Reflection Coefficient
Including SAW Reflectors
Wideband SAW Sensor Antenna Design
S11 for Wideband Antenna - Center Frequency 915 MHz
Wireless SAW Sensors and Wideband Antenna
Without back reflector (smaller size, broader bandwidth)
Center Frequency 915 MHz
Wireless SAW Multi-Sensor System
Shown are several different types of sensors,
an interrogator and a computer system for
post processing of the received sensor data.
A simple transceiver block diagram
• The FPGA controls the timing of the transmit signal, the actuation of
the receiver, the sampling of the transmitter signal (for reference)
and the start of the A/D to acquire sensor data.
• The Bandpass filters limit the bandwidth to 15MHz to meet the
required maximum bandwidth (26 MHz) for the 915 MHz ISM band.
Sub-Sampling
Sampling in of the time signal makes the frequency
domain periodic.
Given 𝑓 𝑡 has a Fourier Transform of 𝐹 f , then 𝑓𝑠(𝑛𝑇𝑠)
has Fourier Transform as:
∞
F(f) =
𝐹 f − 𝑘·F𝑠
,
𝑘=−∞
whereTs is the sampling period and
F𝑠 = 1/Ts is the sampling frequency.
Sub-Sampling
• The periodic nature of a sampled signal can be used to alias any part of the
un-sampled spectrum down to baseband.
• The sampling frequency Fs must be twice of the Bandwidth.
• Sampling a 915 MHz Sensor signal at 100 MHz moves 900
MHz to DC and the maximum frequency of 950 MHz to 50
MHz .
• The A/D analog input must have a bandwidth of at least 950 MHz.
• The A/D sample and hold (S/A) settling time tset must be fast enough.
• The S/A bandwidth ≈ 1/tset
SNR Analysis of The Wireless SAW Sensor System
[Po ·GSensor·GTx−ant·GRx−ant·Nsum·B·t]
SNR =
[PL·kT·B·NF∗]
Typically the minimum detectable
signal is set to input noise power
NF∗= NF + γ
PMDS = NF∗ · kT/τ
γ = [Next + (NADC/GRx)/[kT · B]
Where,Next is any external noise sources
SNR =
[Po·GSensor·GTx−ant·GRx−ant·Nsum]
[PMDS · PL]
[V2r ·GSensor·GTx−ant·GRx−ant·Nsum]
SNR =
[V2MDS · PL]
where Vr is the transmit voltage level and VMDS is
the voltage level detectable at the ADC
GSensor = Sensor Gain (incl. GSAW *G2Sensor−Ant)
GRx−Ant = Gain of Receive Antenna
G Tx−Ant = Gain of Transmit Antenna
GRx = Receiver Gain from Antenna Input to ADC
τ = Output signal time length
B = Bandwidth of signal at ADC
P0 = Power to transmit antenna
ADC = Ideal Analog-to-Digital Converter Gain
MDS = Minimum Detectable Signal @ ADC
S = Signal Power Measured at ADC
N = Noise Power Measured at ADC
kT = Thermal Noise Energy
PG = Signal Processing Gain of the System (τB)
PL = Path Loss
NF = Receiver Noise Figure
Next = External Noise Source at Antenna
Output
NADC = ADC Equivalent Noise
Nsum = Number of Synchronous Integrations at
the ADC
Random Noise Source
One Sample of a White Noise Source
Normalized time
Average Power Spectrum
(20 averages)
Normalized Frequency to Fs/2
Example Chirp Source
Chirp signal as a Function of Time
Its corresponding Spectrum
• Tx(t) = A cos[wotK +wc)·t] , where wc is the starting
frequency time t= 0.
• And K = wf /(wo · t1), where at t1 the final frequency
is wf
SNR Versus Range for Isotropic Radiation
PL = [ Vem / (4·p·R·fo) ]-4 , Friis equation,
where fo is the frequency of interest,
Vem is the free space velocity and
R is the Range
The Propagation Loss (PL)
increases at the rate of 40 dB
per decade increase in path
length (R).
915 MHz Coherent Receiver
fo = 915 MHz,
BW = 15 MHz,
τsignal = 1μs,
τwindow = 10μs
PG = 15,
P0 = 8.5dBm,
PMDS = −72dBm,
Nsum = 1,
NF = 7dB
GRx and GTx= 9dB.
Volume ≈ 350 cu. In.
Hsystem(w) = Htrans(w)·Hsensor(w) ·Hsensor(w)*·Hrec(w)
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Hsensor(w) = HIDT(w) · Gbit(w)
915 MHz Transceiver (interrogator) using off the self components.
USB interface to a standard computer for processing of the sensor data.
The integrated FPGA controls the timing for a receiver and transmitter functions.
Uses a wideband noise source for the transmit signal (band limited to 15 MHz).
Experimental Temperature Results
OFC SAW temperature sensor (7 Chip OFC) 250 MHz
compared to thermocouple measurements (15°C -45°C).
Experimental Temperature Results
OFC SAW temperature sensor (7 Chip OFC) at 250 MHz
compared to thermocouple measurements (10°C -180°C).
Experimental Temperature Results
Wireless SAW temperature Sensor at 250 MHz using OFC
compared to a thermocouple.
Experimental Temperature Results
Wireless SAW temperature Sensor at 915 MHz using OFC
compared to a thermocouple.
H2 Gas Sensor
• < 50 ppm
• RT reversible
• Response <1s
Software Design Radio (SDR) Transceiver 915 MHz
Current Research
• Wireless gas sensing
• Wireless strain sensor
• Miniature low-cost
hand-held TxRx
• High data rate
acquisition
• Wired handheld POC
diagnostics for
biological liquid
sensing
Future Research
• Higher frequencies
• NASA space
qualification
• Hydrogen and gas
sensing
• Biological POC
handheld system
• Networking of multinode multi-sensor
TxRxs
REFERENCES
1. Derek Puccio, Donald C. Malocha, Nancy Saldanha, Daniel R. Gallagher,
and Jacqueline H. Hines, ”Orthogonal Frequency Coding for SAW Tagging
and Sensors” , IEEE transactions on ultrasonics, ferroelectrics, and
frequency control, vol. 53, no. 2, february 2006.
2. D. Puccio, D. C. Malocha, D. Gallagher, and J. Hines, “SAW sensors
using orthogonal frequency coding,” in Proc. IEEE International Frequency
Control Symposium, Aug. 2004, pp. 307–310.
3. D. R. Gallagher, M. W. Gallagher, N. Saldanha, J. M. Pavlina, and
D. C. Malocha, “Spread spectrum orthogonal frequency coded SAW
tags and sensors using harmonic operation,” in Proc. IEEE International
Microwave Symposium, Boston, MA, Jun. 2009, pp. 105–108.
4. D. C. Malocha, Nikolai Kozlovski, “Saw passive wireless multi sensor
system,” in Proc. IEEE International Ultrasonics Symposium, 2009.
5. A. R. Lopez, “Fundamental limitations of small antennas: Validation of
wheeler’s formulas,” IEEE Antennas Propag. Mag., vol. 48, no. 4, pp.
28–36, August 2006.
6. D. C. Malocha, J. Pavlina, D. Gallagher, N. Kozlovski, B. Fisher,
N. Saldanha, and D. Puccio, “Orthogonal frequency coded SAW sensors
and RFID design principles,” in Proc. IEEE International Frequency
Control Symposium, Honolulu, HI, 2008, pp. 278–283.
REFERENCES
7. D. Morgan, Surface Acoustic Wave Filters: With Application to Electronic
Communications and Signal Processing. Academic Press, 2007.
8. Nikolai Y. Kozlovski, Donald C. Malocha, and Arthur R. Weeks, ”A 915 MHz
SAW Sensor Correlator System,” IEEE Sensors Journal, VOL. 11, NO. 12, Dec.
2011.
9. L. Reindl, G. Scholl, T. Ostertag, C.C.W. Ruppel, W.-E. Bulst, and F. Seifert,
“SAW Devices as Wireless Passive Sensors,” 1996 IEEE Ultrasonic Symposium.
10. Alfred Pohl’, G. Ostermayer’, L. Reindl’, and F. Seifert’, “Monitoring the Tire
Pressure at Cars Using Passive SAW Sensors,” 1997 IEEE Ultrasonic Symposium.
11. Andrea E. Hoyt, Antonio J. Ricco, John W. Bartholomew, and Gordon C.
Osbourn “SAW Sensors for the Room-Temperature Measurement of CO2 and
Relative Humidity,” Anal. Chem. 1998, 70, 2137-2145.
12. LeonhardM. Reindl, “Wireless Passive SAW Identification Marks and
Sensors,” 2002 IEEEInt`l Frequency Control Symposium & PDA Exhibition, New
Orleans, LA, USA, 29-31 May 2002.
13. Bianca Maria Cabalfin Santos, SAW Reflective Transducers and Antennas for
Orthogo-nal Frequency Coded SAW Sensors, Master Thesis University of Central
Florida, Spring 2007.