A Wireless Chipless Temperature Sensor utilizingan Orthogonal Polarized Backscatter Scheme

Bernd Kubina, Christian Mandel, Martin Schußler, Mohsen Sazegar and Rolf JakobyTechnische Universitat Darmstadt, Institute for Microwave Engineering and Photonics, Darmstadt, Germany

Email: kubina@imp.tu-darmstadt.de

Abstract—A novel passive wireless temperature sensor is pre-sented which is designed planar and chipless. The sensor taguses a backscatter scheme with two linear polarization planesto seperate transmit and receive path during wireless readout.The measured value is encoded in the spectral signature of thetag by a temperature-dependent notch filter. This filter is set upas a coupled line resonator, loaded by an interdigital Barium-Strontium-Titanate (BST) varactor. The temperature-dependencyof the permittivity of the ferroelectric BST affects the varactor’scapacity and consequently allows a temperature measurement.The sensor has been successfully operated in the temperaturerange between 20 C and 85 C.

Index Terms—Passive microwave remote sensing, temperaturesensors, sensor systems and applications, ferroelectric materials.

I. INTRODUCTION

In recent years, wireless sensors have emerged in manyfields, ranging from industrial fabrication to logistics andmedicine. In the context of this development, a particular inter-est arises in the employment of wireless sensors in specific andharsh environments. Specific areas of application can be founde. g. in extremely high temperature or in environments withhigh electric field strength, where classical semiconductor-based sensors cannot operate. Here, passive and chiplesssensors can meet these unusual requirements. These sensorsoperate without a local power supply (passive) and do notutilize integrated circuits (chipless).

To realize chipless sensors for such specific applicationsRF backscatter principles can be exploited. In correspon-dance to analog phase, amplitude and frequency modulationtechniques, chipless backscatter systems can use time-domainreflectometry [1], signal amplitude variations [2], or spectralsignatures [3], [4] to encode information.

A spectral signature approach is taken for the presentedtemperature sensor. The planar tag carries a resonator which isdetuned by the temperature. For this purpose, the temperature-dependence of the permittivity of the ferroelectric materialBarium-Strontium-Titanate (BST) is used.

For wireless readout, the sensor takes advantage of usingtwo orthogonal linear polarizations. The two polarizationplanes are employed to separate transmit and receive path.

The wireless sensor has been manufactured and successfullytested in an indoor scenario in the temperature range between20 C and 85 C. It operates in a 200 MHz transmission bandat 3 GHz.

~Ei

~Er

Fig. 1. Working principle of the wireless sensor tag. The reader signal isreceived, filtered and backscattered in an orthogonal polarization plane. Thefilter characteristic is dependent on the temperature measurement.

II. PASSIVE BACKSCATTER PRINCIPLE WITHORTHOGONAL POLARIZATIONS

The presented sensor uses a passive backscatter scheme byemployment of two orthogonal linear polarizations to separatetransmitted (Tx) and received (Rx) signals. For this purpose,two orthogonally oriented reader antennas are used, and the tagitself carries both a receiving and a transmitting antenna, eachoriented to the corresponding reader antenna (see Figure 1).Applications of this concept are found in [5] for a chiplessRFID system and in [6] for a displacement sensor.

During a wireless readout a linearly polarized wave is sentby the reader and received by the tag with the correspondingRx antenna. This signal is subsequently passing through ananalog band-stop (notch) filter on the tag. The notch fil-ter’s transmission characteristic is dependent on the measuredvalue, here the ambient temperature. The filtered signal isbackscattered to the reader by the second tag antenna in theorthogonal polarization plane. The reader is able to detect themeasurement value by evaluation of the spectrum of the Rxsignal, which carries the desired information.

This backscatter approach is taken due to two main reasons.First, it can result in a very simple reader setup. The directevaluation of the forward transmission S21 between the readerantennas can already show the measured value, since thenotch position is seen in the spectrum (this is the casewhen interferences are small). Second, one can minimize theinfluence of interfering signals, which arise from unwantedreflections of the ambient environment. Since, in theory, anincident planar wave is reflected from a metallic plane in thesame polarization-plane, the reflection does not interfere withany waves in the orthogonal plane. Thus, the orthogonalityconcept can suppress the influence of interfering reflectionsand improve the reading ability in reflective environments.

978-2-87487-027-9 © 2012 EuMA 29 Oct -1 Nov 2012, Amsterdam, The Netherlands

Proceedings of the 42nd European Microwave Conference

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Cvar

ϑ

towardsantenna 1

towardsantenna 2

Zw

Fig. 2. Schematic drawing of the used notch filter. A temperature-dependentvaractor is placed on a coupled line resonator and detunes the resonancefrequency.

III. BST-BASED TEMPERATURE-DEPENDENT NOTCHFILTER

Barium-Strontium-Titanate (BST) is a ferroelectric with ahigh permittivity, that can be changed by applying an elec-trostatic field across the material. The ferroelectric behaviourhas been investigated for several years and has been appliedin many tunable RF circuits, such as phase shifters designedwith BST-based Interdigital Capacitors (IDCs) [7], [8].

Beside the electrostatic tunability, BST’s permittivity showsa significant temperature dependence. Figure 9 depicts thetemperature dependency of the permittivity of a BST thickfilm between −50 C and 100 C. Above the so-called Curie-point the permittivity drops continuously. This behaviour isexploited in the presented sensor. Furthermore, the temperaturedependency of the permittivity can be adjusted by the materialcomposition [7], which allows the presented sensor to beadapted to various temperature ranges.

The tag carries a transmission line resonator, which is cou-pled to the transmission line connecting the two antennas. ABST varactor is placed on the resonator, so that the varactor’scapacity affects the resonant frequency (see Figure 2). Thus,the forward transmission characteristic S21 between the tagantennas shows a notch-filter characteristic. The temperaturecan be determined by measuring the notch frequency.

IV. DESIGN AND FABRICATION OF THE TAG

The sensor tag includes two patch antennas for Rx andTx in orthogonal orientation. In order to optimize the tag’swireless performance, the radiation properties of the antennasare examined during the design process. Here, high axialratio, high directivity and low mutual coupling between thetwo antennas is of importance. To achieve these goals, patchantennas have been chosen. For broadbanded transmission, aRogers RT/duroid 5880 substrate with 3.157 mm thickness hasbeen used. The two patches have been matched to 100 Ω.

Simulation results of the tag are depicted in Figure 4. Thetwo patches are placed at a distance of 7 cm from each otherin orthogonal orientation on one substrate. A coupling below−30 dB is achieved, while the axial ratio of the radiated fieldsat 3 GHz is larger than 15 dB for observation angles betweenΘ = ±50 in all ϕ-planes. The directivity of each patch inthe main radiating direction is 7.4 dBi.

Θ ϕ

Port 1

Port 2

Fig. 3. Simulation setup of two orthogonally oriented patch antennas on thetag substrate.

2.8 3 3.2

−40

−20

0

f/GHzS11,S

21/d

B

S11

S21

(a) Simulated S-Parameters

−50 0 500

10

20

30

40

Θ/

Dir./d

Bi,

AR/d

B AR 0

AR 90

Dir. 0

Dir. 90

(b) Simulated radiation characteristics of the patch connected to port 1

Fig. 4. Simulation results of the patch antennas of the tag (compare Figure3). The matching (S11) and coupling (S21) of ports 1 respective 2 is shown aswell as the axial ratio (AR) and directivity (Dir.) in the ϕ = 0 and ϕ = 90

planes.

The transmission line notch filter has been realized withmicrostrip lines (see Figure 2). A λ/2 resonator is partiallycoupled to the transmission line, which connects the antennas.The used BST varactor has a capacity of 0.5 pF at roomtemperature. It is placed in the middle of the resonator tomaximize the tuning range of the resonant frequency.

Both, the resonator and the patch antennas have been de-signed to operate in a 200 MHz band with the center frequencyat 3 GHz.

The prototype has been built using standard printed circuitboard lithography and etching process. The BST varactor hasbeen connected to the metal layer with conductive glue. Aphotograph of the manufactured tag is shown in Figure 5.

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Fig. 5. Photograph of the 12 cm× 12 cm manufactured passive chipless tagand a microscopic top view of the varactor.

reader antennastag

heat gun

Fig. 6. Photograph of the wireless setup. The tag has been placed 1m infront of the reader antennas. The heat gun has been used to change the tag’sambient temperature.

V. MEASUREMENTS

The manufactured tag has been tested in a wireless indoorscenario. In order to detect the spectral signature of the taga Vector Network Analyzer (VNA) has been used, whosetwo ports have been connected to two orthogonally orientedhorn antennas. These horn antennas as well as the tag havebeen mounted on tripods and placed in a hall, as shown inFigure 6. While the reader antennas were placed 25 cm apart,the distance between these and the tag has been 1 m. In orderto adjust the tag’s ambient temperature a heat gun has beenplaced about 30 cm next to it. For the reference temperaturemeasurements an infrared camera has been used.

In Figure 7(a) the direct measurement of the forward trans-mission S21 between the two reader antennas is depicted. Here,the measurements at 27 C and 49 C as well as a measurementof the room without tag is depicted. These measurements showthat undesired depolarized reflections from ambient objects inthe hall interfere with the desired backscatter of the tag, so thatthe sensor’s resonant frequency cannot be clearly recognized.

If the received signals are examined in Time Domain (TD)as obtained from the Inverse Fast Fourier Transform (IFFT)the multiple backscatters can be analyzed. Figure 7(b) showsthe S21-measurements in the equivalent baseband.

One way to improve the direct measurements is to calculatethe difference between the “no tag”-measurement and each

2.7 2.8 2.9 3 3.1 3.2 3.3−70

−60

−50

−40

f/GHz

S21/d

B

No Tag27 C49 C

(a) Directly measured S21

0 10 20 30 40

0

1

2

t/ns

ampl

itude

/a.u

. No Tag27 C49 C

desired signal

(b) TD signals

2.7 2.8 2.9 3 3.1 3.2 3.3−70

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27 C49 C77 C

(c) Differential measurement

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−55

−50

−45

f/GHz

S21/d

B

27 C49 C77 C

(d) TD-filtered measurement

Fig. 7. Measurement results of the wireless setup (compare Figure 6). Thedirectly measured S21 is shown in (a); the results of the differential methodin (b); the equivalent time-domain base-band signals of (a) are shown in (c)and the results of the time-gated measurement in (d).

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20 40 60 802.9

2.95

3

3.05

3.1

ϑ/C

f res/G

Hz

MeasurementRegression

Fig. 8. Measured course of the resonance frequency fres in dependence ofthe temperature ϑ.

measurement at different temperatures, which leads to theresults shown in Figure 7(c). They clearly show a transmissionband between 2.9 GHz and 3.1 GHz as well as a temperature-dependent transmission notch. Only slight interferences arevisible.

Alternatively to the differential method, a time domain filtermethod has been applied. A time gate has been set duringthe slot of the arrival of the desired signal (as indicated inFigure 7(b)) and the obtained signal has been transformedback into frequency domain. Now, the transmission charac-teristic of the tag and the notch frequencies are extracted withnearly no disturbing influence of room reflections as shownin Figure 7(d). One recognizes, that the TD- filter methodshows better results than the differential method. The TD-filtermethod has obviously a higher capability to handle multiplereflections between the tag and objects in the room.

The resonant frequencies of the sensor have been measuredunder assistance of the TD-filter method for ambient tempera-tures between 20 C and 85 C leading to the characteristiccurve shown in Figure 8. An overall shift of the resonantfrequency fres of 198 MHz has been achieved. This results inan approximated linear sensitivity of ∆fres/∆ϑ = 3.1 MHz/K.

With the simulation model of the resonator, the temperaturedependency of the varactor’s capacity can be extracted fromthe measurements. In the mentioned temperature range, thecapacity changes about 32.6 % as shown in Figure 9. Thecapacitance matches quite well with the measured curve ofthe permittivity of the BST thick film.

VI. CONCLUSIONS

A passive chipless temperature sensor tag has been intro-duced that uses an orthogonal backscatter scheme at 3 GHz.A temperature-dependent notch filter was implemented onthe tag, that uses a BST varactor to encode the measuredvalue in the spectral signature of the backscattered signal. Theconcept of the sensor has been proven in a wireless indoormeasurement series between room temperature and 85 C,where the sensor showed a sensitivity of 3.1 MHz/K.

In order to gain a clear detection of the sensor’s measuredvalue, two different processing methods have been presented.Both, a differential measurement and a time-domain filtering

0 50 100

0.3

0.4

0.5

ϑ/C

Cvar/p

F

Cvar

200

300

400

ε r

BST εr

Fig. 9. Course of the permittivity of a BST thick-film at 3GHz derived froma coplanar waveguide measurement in a closed chamber and derived courseof the used varactor’s capacity Cvar in dependence of the temperature.

method show good results. The time-domain filtering methodhas been in particular able to deliver a clear measurement ofthe tag’s transmission characteristic.

This work is a starting point to adapt the sensor to spe-cific applications in high temperature environments. For thispurpose, the course of the BST permittivity can be adjustedto different temperature ranges by changing the materialcomposition. In this scenario, the presented tag allows fora realization on ceramic substrates, to achieve operability inranges above 150 C.

ACKNOWLEDGMENT

The authors would like to thank the CST company forproviding their Microwave Studio as well as the Institute forMaterials Research at the Karlsruhe Institute of Technologyfor their support on BST.

REFERENCES

[1] C. Mandel, H. Maune, M. Maasch, M. Sazegar, M. Schußler, andR. Jakoby, “Passive Wireless Temperature Sensing with BST-BasedChiplesss Transponder,” Proceedings of the 6th German MicrowaveConference, Mar. 2011.

[2] S. Preradovic and N. Karmakar, “Chipless RFID Tag with IntegratedSensor,” 2010 IEEE Sensors, pp. 1277–1281, Nov. 2010.

[3] T. Thai, J. Mehdi, H. Aubert, P. Pons, G. DeJean, M. Tentzeris, andR. Plana, “A Novel Passive Wireless Ultrasensitive RF TemperatureTransducer for Remote Sensing,” IEEE MTT-S International MicrowaveSymposium Digest, pp. 473–476, May 2010.

[4] A. Traille, M. Tentzeris, S. Bouaziz, P. Pons, and H. Aubert, “A NovelWireless Passive Temperature Sensor Utilizing Microfluidic Principles inMillimeter-Wave Frequencies,” 2011 IEEE Sensors, pp. 524–525, Oct.2011.

[5] S. Preradovic and N. Karmakar, “Design of Fully Printable PlanarChipless RFID Transponder with 35-bit Data Capacity,” Proceedings ofthe 39th European Microwave Conference, Sep. 2009.

[6] C. Mandel, B. Kubina, M. Schußler, and R. Jakoby, “Passive ChiplesssWireless Sensor for Two-Dimensional Displacement Measurement,” Pro-ceedings of the 41st European Microwave Conference, Oct. 2011.

[7] A. K. Tangantsev, V. O. Sherman, K. F. Astafiev, J. Venkatesh, andN. Setter, “Ferroelectric Materials for Microwave Tunable Applications,”Journal of Electroceramics, Vol. 11, pp. 5–66, Nov. 2003.

[8] H. Maune, M. Sazegar, Y. Zheng, X. Zhou, A. Giere, P. Scheele,F. Paul, J. R. Binder, and R. Jakoby, “Nonlinear Ceramics for TunableMicrowave Devices, Part II: RF-characterization and component design,”Microsystem Technologies, Vol. 17, No. 2, Feb. 2011.

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