Non-contact Radiative Temperature Measurement ofan Object in a Closed Cavity

!Institute of Measurement Engineering and Sensor Technology, University of Applied Sciences Ruhr WestPO box 100755, D-45473 Mulheim an der Ruhr, Germany

Email: 1dawei.xu@hs-ruhrwest.de,2dirk.rueter@hs-ruhrwest.de, 3joerg.himmel@hs-ruhrwest.de,5klaus.thelen@hs-ruhrwest.de"General and Theoretical Electrical Engineering (ATE), Faculty of Engineering, University of Duisburg-Essen

and CENIDE – Center for Nanointegration Duisburg Essen, D-47048 Duisburg, GermanyEmail: 4daniel.erni@uni-due.de

Abstract—Referring to the wide application of non-contactsurface temperature measurements, a method is presented hereusing the thermal noise in the microwave band of the measuredobject in a closed cavity. The thermal noise coupled from anantenna in this frequency range allows the measurement of thetemperature within the object and even the core temperature.The measurement of the scattering parameters of the antenna inthe cavity is needed to correct the coupled thermal noise powerfrom the object. Using a mathematical algorithm, an online real-time temperature measurement can be achieved.

Index Terms—Non-contact temperature measurement in mi-crowave, power of thermal noise, microwave radio-thermometry

I. INTRODUCTION

Non-contact temperature measurement of an object in themicrowave frequency range is usually applied in astronomicalobservation. For a relatively small object in daily life, there areseveral applications in the medical field based on the close-proximity measurement method [1]. However these applica-tions are limited by the requirement of strict self-calibration.

In this paper, a method of non-contact temperature mea-surement is presented for the object enclosed in a cavity.In the described method the emitted thermal noise power ofthe object and the scattering parameter of the antenna aremeasured alternately. This method has no strictly defined limitin terms of position of the object, and does not need to referto the noise source in a real-time measurement. One possibleapplication is the temperature control of organic objects beingwarmed up in a microwave heating process.

Electromagnetic shielding is used to form a cavity resonator.In the cavity, the electromagnetic waves are resonant at certainfrequencies. In this paper the resonant modes are used forthe coupling between the antenna and the measured object.The stored energy of these modes is distributed inhomoge-neously within the cavity [2]. This inhomogeneity can greatlyinfluence the coupling between the antenna and the measuredobject. Additionally, the changing of the electrical propertiesof the measured object as conductivity, permeability andpermittivity as well as the object shape are influencing themode characteristics. As these quantities might change with

thermometer

spiral hosepipe pump

valve

aluminium cavity

LNAvector network analyzer

spectrum analyzer

Fig. 1. Experiment setup

the object temperature during the measurement, the couplingbetween the antenna and object is barely manageable withoutfurther information and not sufficiently stable for an accuratemeasurement of the thermal noise of the object. Measuringthe antenna’s scattering parameter helps to obtain the inputresistance of the antenna, which is needed for calculatingthe thermal noise in the cavity. It corrects simultaneously thecoupling between the antenna and the measured object, whichis needed for an online real-time evaluation of the temperatureof the measured object [3].

networkanalyzer

spectrumanalyzer

aluminiumcavity pump

watercontainer

LNA

thermometerantennaconnector

Fig. 2. Photograph of the experimental setup

Dawei Xu *, ,1, Dirk Rüter *,2, Jörg Himmel *,3, Daniel Erni ,4, and Klaus Thelen *,5

978-2-87487-047-7 © 2017 EuMA 10–12 Oct 2017, Nuremberg, Germany

Proceedings of the 47th European Microwave Conference

872

II. EXPERIMENTAL SETUP

A. Cavity descriptionThe 300 mm ! 230 mm ! 250 mm test cavity shown in

Fig. 3(a) is made of 6 aluminum sheets with a thickness of 2mm each. The top cover can be moved in order to allow theloading of the measured object. The antenna is mounted onone side of the top cover as shown in Fig. 3, and is connectedto the 50 ! SMA connector, which is fixed on the cover. Aspiral hosepipe with a volume of around 800 mL serves asthe measured object, which can be placed at arbitrary positionwithin the cavity.

In order to acquire consistent results, the geometrical con-ditions inside the cavity should be kept stable during themeasurement process. The setup of the water circulationallows to change the water temperature without opening thecavity, and thus no change of geometric parameters occurs. Asshown in Fig. 1 a water container placed outside the cavitycan be easily filled with cold or hot water. A pump forces thewater from the outside container through the spiral hosepipeinto the cavity. The ejected water from the spiral returns backto the container. After a few cycles, the temperature in thespiral and the container matches. The water temperature of thecontainer outside is measured with an ordinary thermometer.The liquid in the water circulation is normal tap water witha quantified amount of salt, in order to increase the electricalconductivity. Thus a more reproducible electrical characteristicwill be ensured in comparison to the use of normal water [4].Additionally salt water is a more representative replacementfor organic materials used in the real application. In this way,a stable mode distribution in the cavity is achieved.

B. Antenna descriptionThe antenna used in this experiment is a wideband design

as shown in Fig. 3. The side view in Fig. 3 (b) shows thatthe part indicated in red was designed as a 50 ! strip line,which is used as transmission line between the coupler ofthe antenna and the SMA connector. The other part of theantenna indicated in blue, forms the coupling loop for themagnetic field of the electromagnetic mode. With the gradualwidth increase of the conductor, the antenna couples to theEM wave in a wide frequency range. This is confirmed by theS11 parameter as shown in Fig 4 (red line). The bandwidthamounts to 620 MHz for a S11 parameter less than -10 dB.With a loading of water (22.6#C) in the spiral hosepipe, severalresonant peaks emerge, shown by the blue curve, such as theresonance at 1.122 GHz with |S11| = -8.653 dB. This peakshows a lower Q-factor than the other resonant peaks couplingwith the antenna.

C. Low noise amplifierThe low noise amplifier (LNA) is assembled in three stages

of RF-amplifiers. They are connected in cascade using theamplifiers: AN11035, ZX60-P103LN+ and BB 0515. The|S21| parameter of the cascade is shown in Fig. 5. In thefrequency range of 0.2-1.4 GHz, the LNA has a total gain> 50 dB, with an input signal power of -60 dBm.

300mm

250m

m

230m

m

SMA

side view

antenna

GND

air stripline coupler

(a) (b)

Fig. 3. Antenna setup and its placement in the cavity

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Frequency [GHz]

-40

-35

-30

-25

-20

-15

-10

-5

0

|S11

| [dB

]

|S11| Antenna

Antenna in free spaceAntenna in cavity with object

1.122 GHz-8.653 dB

620 MHz

Fig. 4. Measured spectral response of the antenna’s input reflection

The spectrum analyzer Agilent MXA N9020a is used formeasuring the thermal noise power of the signal amplifiedby the LNA. The Resolution bandwidth (RBW) is set to 3MHz. The network analyzer Agilent PNA E8257A is used tomeasure the scattering parameter of the LNA as correctiondata. It is also used to measure the real-time values of thecomplex S11 parameter of the cavity using the SMA connectorof the antenna as reference plane.

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Frequency [GHz]

0

10

20

30

40

50

60

70

|S21

| [dB

]

|S21| LNA

Fig. 5. Measured S21 spectrum of the LNA

873

III. THEORY OF THE MEASUREMENT

Vobj

Tlna

Z0 V VmLNA

Z0 V Vm

Tlna

LNA

Slna22

Slna21

Slna11Sant

11

Zant

Z lnaVobj

Sant11

Zant

cavity withantennaandobject

spectrumanalyzer

Fig. 6. Equivalent circuit of the thermal noise measurement

Fig. 6 shows the equivalent circuit of the whole measure-ment setup. The cavity including the object is electricallyrepresented at the reference plane by the impedance Zant,while the voltage Vobj is representing the thermal noise source.The walls of the cavity are nearly perfect conductors. Thus theEM-wave absorption is to neglect in comparison to the objectabsorption. Therefore, the real part of the cavity impedanceis determined only by the object resistivity. The thermalnoise resistance in Eq. (1) yields the open-circuit root meansquare noise voltage Vobj depending on the temperature of themeasured object [5]:

Vobj =!4kBTobj Re{Zant}"f (1)

Where:• kB : is the Boltzmann’s constant 1.3806! 10$23J/K• "f : is the resolution bandwidth (in experiment 3 MHz)• Tobj : is the temperature within the object, which is

assumed to be uniformly distributed• Zant: is the impedance of the system, consisting of cavity,

antenna and object at the antenna SMA connectorThe voltage over the input impedance of the LNA V lna

input canbe calculated using Eq. (2):

V LNAinput =

""""Zlna

Zlna + Zant

"""" · Vobj (2)

The transmission coefficient S21 represents the ratio oftransmitted and incident wave amplitudes. If impedancematching at the output of the LNA is given, the output voltagecan be calculated using the S-parameters of the LNA (3):

V lnaoutput =

"""""Slna21

1 + Slna11

""""" · Vlnainput (3)

The port impedance can be calculated from the correspond-ing S11 parameter by:

Zant =1 + Sant

11

1" Sant11

· Z0 (4)

and

Zlna =1 + Slna

11

1" Slna11

· Z0 (5)

Where:• Z0: is the input impedance of measurement instrument

(50 !)• Zlna: is the input impedance of LNA

After rearranging Eq. (1) - (5), with additional intrinsic thermalnoise temperature of LNA, we obtain Eq. (6):

Tobj =Pm · Z0

4kB"f Re{Zant} ·""" Slna

21

1+Slna11

"""2·""" ZlnaZlna+Zant

"""2

" Tlna

(6)

Where:• Pm: is the noise power of the output voltage Vm of the

LNA, Pm = V 2m

Z0

• Tlna: is the equivalent intrinsic thermal noise temperatureof the LNA cascade depending on its inherent electricalnoise after the self amplification

Because the operating temperature of the LNA is notchanged during the experiment, we consider the intrinsic noiseof the LNA as independent of the measured temperature. Thusthe equivalent intrinsic temperature Tlna of the LNA is alsokept constant (see Fig. 6).

IV. EXPERIMENTAL PROCEDURE

For the first step, the calibration of the LNA should becarried out once. The S-Parameters of the LNA are consideredto be constant throughout the experiment, as it operatesin the linear range, and the supply voltage as well as theenvironmental conditions are kept stable during the procedure.

After calibration, two measurements are to be carried outalternately. First, the SMA connector of the antenna is con-nected to the network analyzer for the measurement of Sant

11 .Second, the connector of the antenna is hooked up to the LNA.In the experimental setup, the switch action between these twomeasurement steps is done manually. For practical purposes,it could be switched with an RF dicke switch or a comparableRF switch with neglectable insertion loss, as shown in Fig. 1.

In Eq. (6), there is still one unknown parameter Tlna, whichis considered to be constant. This is experimentally confirmedby several real-time measurements at known temperatures ofthe measured object. This step is necessary in this experimentdue to the unknown LNA noise temperature. When the LNAis fully characterized, this step is not needed. However, it isimportant throughout the experiment to drive the LNA witha controlled voltage source to maintain a constant interiortemperature. The experiment is taking place in an RF-shieldedenclosure cabin, mainly to avoid the intrusion of externalnoise.

874

V. EXPERIMENTAL RESULTS

There are four measurement samples with different objecttemperatures (22.6#C, 34.8#C, 44.6#C, 54.0#C and 65.5#C),measured in the same frequency range of 0.8-1.4 GHz.

0.8 0.9 1 1.1 1.2 1.3 1.4Frequency [GHz]

0

0.5

1

1.5

2

P m[W

]

10-9 measured power Pm after LNA

0.8 0.9 1 1.1 1.2 1.3 1.4Frequency [GHz]

250

300

350

400

T obj[K

]

calculated temperature Tobj from Pm

(a)

(b)

Fig. 7. Calculation of temperature Tobj from noise power

Fig. 7 (a) shows the measured noise power at LNAs output.As shown in Fig. 7 (b), the calculated result of Tobj usingEq. (6) shows a noise level between ±5 K and ±30 K .Theoretically it should be a straight line, running in parallelto the frequency axis with a temperature of 317.6 K (44.6#C).However, on average, this demonstrates that the calculatedtemperature was in a correct physical range close to themeasured temperature identified by the reference thermometer.In order to obtain a reliable temperature value, the Tobj isaveraged in the frequency range of 1-1.2 GHz, and S11 of theantenna shows one of the coupling peaks as shown in Fig. 4.

Fig. 8 depicts the evaluated temperature compared to thevalues measured by the thermometer. The evaluated tempera-tures indicated by asterisks fit well to the experimental linearrelation. However, there is a rising angle offset between theline of fitted temperature and ideal temperature. The possiblereason for this mismatch is that the intrinsic noise of theLNA is considered as a stationary intrinsic noise source in thisexperiment and independent of the antenna impedance Zant.In order to obtain a higher accuracy, the intrinsic noise sourceof the LNA should however be considered as a function of

270 290 310 330 350Measured temperature by thermometer [K]

270

290

310

330

350

Eval

uate

d te

mpe

ratu

re [K

]

evaluated temperature vs. real temperature

evaluated termperaturefitted temperatureideal temperature

calibrationpoint

Fig. 8. Temperature evaluation in the frequency range 1-1.2 GHz

the input impedance and calibrated by help of a noise figuremeasurement.

VI. CONCLUSION

The real-time measurement of the reflection parameter andnoise power is adapted for the online evaluation of the objecttemperature in a closed cavity. This approach greatly simplifiesthe power meter method which is often used in the process ofnon-contact temperature measurement in microwave bands.

Compared to the temperature measurement using the poweraverage of many cavity modes in the high frequency range, themethod presented in this paper is more accurate. It could bewidely used in setups without the need of a reference sourceand in objects at random positions, and can be extended tomulti-frequency ranges in order to measure the temperature indeeper layers, and even in the core of the object. In medicaltechnology this method can be used for an accurate non-contact on-line control of the blood plasma temperature duringa microwave heating process. It could avoid some damagefrom local overheating, and may improve the quality of thewarmed plasma. [6]

REFERENCES

[1] Q. Bonds, A microwave radiometer for close proximity core body temper-ature monitoring: Design, development, and experimentation. ScholarCommons, 2010.

[2] D. Kajfez and P. Guillon, “Dielectric resonators,” Norwood, MA, ArtechHouse, Inc., 1986, 547 p. No individual items are abstracted in thisvolume., vol. 1, pp. 26,38, 1986.

[3] R. C. Willson, “Active cavity radiometer,” Applied Optics, vol. 12, no. 4,pp. 810–817, 1973.

[4] T. S. Light, “Temperature dependence and measurement of resistivity ofpure water,” Analytical Chemistry, vol. 56, no. 7, pp. 1138–1142, 1984.

[5] W. M. Leach Jr, “Fundamentals of low-noise analog circuit design,”Proceedings of the IEEE, vol. 82, no. 10, pp. 1515–1538, 1994.

[6] D. Xu, D. Ruter, and D. Erni, “ non-contact radiative temperature monitor-ing of blood plasma and packed red blood cells in a powerful microwaveenvironment,” in 1 st YRA MedTech Symposium (YRA-MedTech 2016),YRA–Young Researchers Academy MedTech in NRW, April, vol. 8, pp.66–67.

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