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Magnetoresistive thin film sensor for active RF power
ELSEVIER
Sensors and Actuators
A 69 ( 1998) 21-26
Magnetoresistive thin film sensor for active RF power V. Vountesmeri *, A. Martynyuk Centro
de Irmtrumet~ios, Received
Universidad 20 August
National
Autonorna
1997; received
de Mexico,
PO Box 70-186,
in revised form 22 December
CD. Universiraria,
Mexico
1997; accepted 26 December
D.F. 04510, Mexico 1997
Abstract A new thin film sensor for active RF power based on a magnetoresistive effect in a ferromagnetic film has been developed. The sensitive element of the sensor was created by the deposition of 80Ni-20Fe alloy on a glass substrate. In a 50-a coaxial line of size 1.6 X 0.65 mm’, the sensitivity 1.2 mV/W at the frequency 13.56 MHz has been achieved. The dynamic range is 82 dB, the upper level is 1.3 W with selfabsorption less then 5%. The temperature coefficient of sensitivity (TCS) is - 4 pm “C- ’ The uncertainty in measurements of the active RF power due to complex load is no more than 1.8% for reactive circuits with power factor up to cos $=0.23 or for unmatched transmission lines with VSWR up to 8.5. 0 1998 Published by Elsevier Science S.A. All rights reserved. Keywords:
RF power;
Measurement:
Magnetoresistive
sensor; Coaxial
line
1. Introduction
RF and microwave sourceshave been used widely for communications as well as industrial and medical applications. The main parameterwhich determinesthe technological and medical processintensity is the active RF power dissipatedin the temrination. Generally, this termination is not ideally matched, hencethe presenceof a reflected wave, which changesthe matchedregime of the waveguide operation and renders the processof measuringactive power ‘in situ’ very complicated [ 11. Matching devices were usedfor the matching of generator and an arbitrary load. However, measurementof the active power in the matched part of the line doesnot permit us to judge the active power accurately in the load becauseof unknown dissipation in the matching devices. Hence, it is necessaryto usepower sensors,which have the potential to operate in unmatched
transmission
lines directly.
There are three basic methodsfor measuringactive power in unmatched lines which may be used for designing the active power sensors. The first method is a directional coupler method, where two directional couplers for forward and reflected waves are connectedin series.In addition to these,two quadratic detectors or themlocouplesareused.The active power in the transmission line is determinedby extracting the reflected power * Corresponding 8612.
author.
0924-4247/98/$19,00 Pzzs0924-4247(98)00043-0
0
Tel.:
+ 52-5-622-8619;
1998 Published
Fax:
by Elsevier
+ 52-5-622-86201
signal from the forward power one. The disadvantageof this method is in the necessityof having directional couplerswith adequatedirectivity and two identical quadratic detectors.In this method, there is also a big influence of the signal harmonics due to frequency dependentcoupling and directivity and non-quadraticbehavior of the detectors. The secondmethod consistsof amplitudemeasurementof the RF voltage ( V), the RF current (I) and the phaseshift (cp) between them in an arbitrary position of the standing wave. The active power is P=OS VI coscp
(1)
This methodis usedonly in laboratory conditions becauseof complexity, especially in the RF current and phase shift measurements. The third is the method of direct multiplication of the instantaneousvalues of the RF voltage V(t) and current Z(t) in a certain sectionof the transmissionline with the following integration: (2) where r is the integration time which is more than the period of the RF signal. This method is most suitable for measuring‘in situ’ the active power with arbitrary crosssection of the transmission line. In this case,the dimensionsof the sensorarenot depend-
Science S.A. All rights reserved.
22
V. Vountesmeri,
A. Martynyuk/Sensors
andActuators
A 69 (1998)
21-26
ent on the signal wave length in a wide frequency range. Many attempts have been made to use for this purpose the Hall and Gauss effects in semiconductors [ 2-51, but they failed because of large parasitic signals (up to 100%) due to rectification and thermoelectricity. For low frequencies of up to 500 kHz, the multiplication of electrical signals which are proportional to the instantaneous values of the voltage and current is used. For this purpose transducers of voltage and current are used [ 6,7]. We propose a new type of sensor based on galvanomagnetic phenomena (abnormal Hall effect and magnetoresistante) in ferromagnetic films [ 81. Such sensors have up to one order greater sensitivity than semiconductor sensors in weak RF magnetic fields, one order less rectification and one to two orders lower thermoelectricity. Hence, magnetoresistive sensors have properties 103-lo4 times better than semiconductor sensors. This allowed us to design magnetoresistive RF sensors with good characteristics.
2. Theory
Fig. 1.Themagnetoresistivesensitiveelement.
The operation of the sensor consists of in the multiplication of the RF voltage V= V, expCjwt) and current Z=1, X exptjwz) with the following integration for sharing out of the DC component which corresponds to active component of the complex power fall through the cross section of the coaxial transmission line: P=O.5 Re (V,Z,*)
(3)
Here V,,, and Z, are the complex amplitude of the RF voltage and current in an arbitrary cross section of the transmission line, respectively. This multiplication is made by using the magnetoresistance effect in ferromagnetic films [ 91. For a_ftmagneticfilm strip (Fig. 1) magnsized up to saturation MO by the effective magnetic fiel+d& - created in the film by the external magnetic field Ho - and taking into account the demagnetizing form factors of the film and uniaxial magnetic anisotropy field, for the p. direction, the change of resistance due to a change of the magnetized angle of Acp is R-=-AR
sin2tpoAp
(4)
where AR is the change in magnetoresistance of the magnetic films magnetized in the direction of the film’s axis OX, and the normal to this axis is the OX, direction. In magnetic film, the RF current If is created by the transmission line voltage K I,=YV
(5)
where Y is the admittance of the magnetic film circuit. The change of the film resistance R, is caused by the excitat$n of the magnetization vectordby the RF magnetic field h caused by the RF current I flowing in the coaxial central conductor and it is possible to write it as
R,=R,I
(6)
where R, is the resistive-current sensitivity of the magnetoresistive sensor which depends on the magnetoresistive parameter of the magnetic film AR,,,. It is a function of the sensor position relative to the central conductoftthrough h” and intensity of the external displacement field Ho. So the RF current If created by the line voltage V flows in the magnetic film, the resistance of which changes in time in accordance with the RF current I flowing in the coaxial central conductor. As a result of the parametric detection of the RF current If, a DC voltage V, appears at the film’s ends. Taking into account Eqs. (5) and (6) and combining the variables, we can write: Vo=0.5 Re(Z,R,*)=0.5
Re(YRI*VZ*)
(7)
Comparing Eqs. (7) and (3), we see that in order to have the output power proportional to the active power rhe product YRI* must be real value or Im( YR[*) = 0. Only in this case will the DC output voltage be proportional to the active power: V,=K,0.5
Re(VZ*)=K,P
(8)
where Kp = Re( YR,*) is the active power sensitivity of the sensor. The dependence of the sensor’s sensitivity with respect to frequency variations is connected to the frequency dependence of magnetic film circuit admittance Y (Eq. (5) ) and the resistive-current sensitivity of the magneto-resistive sensor R, (Eq. (6) ) . These dependences led to varying active power sensitivity with frequency and increasing uncertainty of the active power measurement in the presence of reactive power, because Im( YR,*) # 0.
V. Vountesmeri,
A. Marpnyuk
/Sensors
and Acwztors
A 69 (1998)
23
21-26
Hence, for a particular frequency it is necessary to satisfy the equality Im( YR;+) =O. The maximum of the detection effect will be when the direction of the magnetization
R=368 Ohm R&54 Ohm c,=c=o.01uF
3. Sensor A sensitive element based on a magnetoresistive 80Ni20Fe alloy magnetic film deposited on a polished glass substrate by the electronic beam method. The sensitive element consists of two loop-like branches which are connected in shunt for the RF signal and in series for the output signal, Fig. 2. For elimination of secondary parasitic detection effects, each branch consists of two magnetoresistors (R, = 554 R, AR/R, = 1.53%, 1.4 mm length, 30-km width, separated by 0.3 mm spacing) connected in series by copper strips 30 pm wide. Contacts are prepared on one side of the substrate. The size of the sensitive element is 7.5 X 5.0 mm’. The thickness is 0.5 mm. The sensitive element is set up under the central wire of the microcoaxial line ( 1.6 X 0.65 mm2) so the magnetoresistors are situated symmetrically with respect to the coaxial axis normal, Fig. 3. The distance from the coaxial axis to the magnetoresistors is 0.85 mm. This construction is placed in a non-magnetic box so that it is possible to magnetize the magnetic films with an external magnetic field. Thz magnetoresistors operate in the external magnetic field Ho created by a permanent magnet. The magnetic field is parallel to the substrate’s surface and its intensity and direction are changed by changing the distance between the substrate and perrnanent magnet and its (magnet) rotation. The field was varied from 400 to 10’ A/m and was measured by a Hall sensor. In the experiment the amplitude of the field was larger than the demagnetizing fields of the magnetic film in order tozbtain a>mall difference in direction between the vectors M0 and Ho and a rather large amplitude of the magnetic field to avoid ferromagnetic resonance in the film at the operating frequencies. The electrical scheme is shown in Fig. 3. Here, four magnetoresistors (MR) are connected in series relative to rnagntt ,- - -------<
-,-\
,’
4. Experimental
results and discussion
The test set for investigation of the power sensor is shown in Fig. 4. The sensor (3) at one side is connected to the RF generator ( 1) (Advanced Energy, P,,, = 600 W, f = 13.56 MHz) through attenuator (2) (20 dB, P,, = 100 W) which serves to exclude the influence of the unmatched load on the RF generator. At the other side the sensor is connected to the
$7
R,
d
j ,,’ ,/’ ” //
.l-\\) . Fig. 2. Geometric film.
detected signal V, and in parallel relative to the RF voltage V which exists in the coaxial line in the region where the magnetoresistive sensor is situated. The capacitors C serve to ground the RF currents and, together with the resistance of the four magnetoresistors R,, they also serve to integrate the multiplying signals and share out the detecting signal V, which is proportional to the active RF power in the coaxial line. The capacitor C, serves in the separation of the DC voltage (which is possible in the coaxial line) from the small detecting signal V, to avert the influence of the large DC voltage on the exact measurement of the small signal V,,. Resistor R limits the RF power dissipated in the magnetoresistive sensitive elements where large RF power in the coaxial line is measured. V is the RF voltage at the point of the connection of the electrical input of the magnetoresistive sensitive element to the coaxial line, and I is the current in the inner coaxial conductor.
%, z.,jwt >
1 ,“,// I
Fig. 3. The elecrnc scheme of the sensor
1
N-S relarion
for electromagnetic
detection
in a ferromagnetic Fig. 4. A block diagram
of the test set.
a
24
V. Vountesmeri,
A. Murtyyuk
/Sensors
matching load (4)) (Hewlett-Packard 438 A Power meter). It is possible to connect different values of reactive elements (L,C) in parallel to the matched load creating an unmatched complex load. The magnitude and direction of the magnetic field is changed by changing the distance and rotation of the permanent magnet (N * S). The output signal is measured by the Hewlett Packard 3458 A Multimeter (5). 4.1. Dynamic
range
The dynamic range of the sensoris determined in the matching regime as the ratio between the maximal power where the amountof the non-linearity exceedsacertain level, in this case 3%, and the minimal power?which createsan output signal equal to the noise signal of the sensorat room temperature.In Fig. 5, the volt-watt characteristicsfor different values of the external displacementmagnetic field are shown. Deviation 3% from linearity occurs at a power level of 1.3 W. The insertion lossdueto self-absorptionis no more then 5% of the incident power. The sensor sensitivity is inversely proportional to the displacement magnetic field intensity. This is clear from geometry, Fig. 2, wherethe angle of the magnetized deviation (in the linear approach) is inversely propozional to the internal displacementmagnetic field intensity HOi. Hence, the change of the film resistance (4) and, consequently, sensitivity (8) have the same dependences. The misalignmentof the sensitiveelementwith respectto the power line leads to the reduction of sensitivity and an increasein the parasitic detection effects. The lower power level isdeterminedby both the sensitivity and the level of the Johnson’snoisevoltage V,,=d4kTR,Af
(9)
where R, = 4R, is the internal resistanceof the sensorat the output clamps, T is the temperatureof the sensor,k is Boltzmann’s constant, and Af is the frequency band of the measurement amplifier. For the given parametersof the sensor,the lower value of the dynamic range in the unit frequency band is 7.9X
0
ml
Fig. 5. The voltage-power ment magnetic fields.
403
600 603 Powr,mW
characteristics
IW
1200
of the sensor for different
and Actuators
A 69 (1998)
21-26
lo-’ W. Hence, this sensorhas82 dB dynamic range in the frequency band equal to 1 Hz. 4.2. Angular
dependence
The angular dependenceof the output signal was investigated for matching, short and open operation regimes of the transmissionline. The angle dependencefor the matching regime is shownin Fig. 6. The active power dissipatedin the load is equal to zero for the short andopenoperation regimes, Hence, for an ideal active power sensorthe output signalmust be zero. In reality, however, there is an output signal due to secondaryparasitic effects in the sensorbecauseof thermoelectricity and a self-detectioneffect. This effect takesplace when there is a detection signal at the output of the sensorin the presenceof only one signal component, which can be either RF current in the film or RF magnetic field. It is found that the maximum value of the useful signalfor the matching load is two orders greater than the parasitic signalsof the two other operation regimes-short and open. This result provides more evidence about the high quality of this kind of power sensor.There is an angularposition of the displacementmagneticfield wherethesesignalsare separated (8=0, T). 4.3. Temperature
dependence
The temperaturedependenceof the conversion coefficient has also been investigatedIt was found that the temperature coefficient of sensitivity (TKS) hasa value of - 4 pm “C- ’ in the temperaturerange ( - 50- + 5O)“C. 4.4. Complex
load dependence
It is very important to check the operation of the sensorin the regime of the complex load. This complex load is createdby the connection of reactive elements(capacitors or inductors) to the coaxial line in paralle1to the coaxial termination which is an absorbingelement of the powermeter (HP 438 A). The distance between the point of the connectionof the reactive elementsand the center
1400
displaceFig. 6. The normalized
sensor output signal vs. magnetized
angle.
V. Vountesmeri,
A. Martynyuk/Sensors
of the sensitive element is 3 mm. So, the phases andresistance of the load transformed to the sensor’s center region are approximately the same like the load for the operating frequency 13.56 MHz. The sensitivity of the sensor is the relation between output signal and active power in the section of the coaxial line where the sensor is situated (8). However, it is very difficult for us to measure this power directly due to losses in the transmission line and in the reactive elements. The only possibility we have is to measure active power using the terminal powermeter and to determine the sensor sensitivity for the regime of the powermeter termination and the regimes of the different complex loads, assuming that the transmission line and the reactive elements are ideal, without losses, as the relation between the output sensor’s signal V, and the powermeter display PM
and Acruarors
A 69 (1998)
25
21-26
Table 2 The corrected value of the active power sensitivity and its deviation from the termination regime of operation for different normalized load admittances taking into account the losses (measured Q) in the reactive elements Normalized (50 a) load admittance
Reactive element’s
(6)
(QLc)
1.00-j 1.00-j 1.00-j 1.00-j 1.00-j 1.oo l.OO+j I .OO + 1.OO + l.OO+j
4.24 2.60 1.35 1.07 0.70
86 95 14.5 155 196 -
0.73 j 1.02 j 1.67 3.42
535 269 209 76
Q
Corrected sensitivity (K, mV/W)
Deviation from termination sensitivity (6% 5%)
1.233 1.227 1.224 1.221 1.219 1.212 1.209 1.212 1.220 1.221
I .I3 1.24 0.99 0.74 0.58 0.00 -0.25 0.00 0.66 0.74
K,+ M
The normalized admittance of the complex load Y,, power factor cos cpof the load, sensitivity KM and its deviation 8, from the termination regime are shown in Table I. It is shown (see Table 1) that the active power sensitivity detemrined in this way changes with a change of the complex load and the sign of its deviation from the termination regime is positive and does not depend on the sign of the load susceptance (capacitive or inductive). This means that there are additional dissipation in the reactive elements which is not measured by the powermeter but are taken into account by the sensor. The inclusion of the dissipations in the reactive elements leads to the formula for the corrected sensitivity which differs from ( 10) and is better approach to the real active power sensor sensitivity
KM
K,=
(11)
l+QJQ,C
Table 1 The measured active power sensitivity and its deviation from the termination regime of operation for different normalized load admittances without taking into account the Iosses in the reactive elements Normalized ( 50 R ) load admittance (Y,)
1.00-j 4.24 1.00-j 2.60 1.00-j 1.35 1.00-j 1.07 1.00-j 0.70 1.00 1.00 + j 0.73 1.00-l-j 1.02 l.OO+j 1.67 I.OO+j 3.42
Load power factor (co5 q)
0.23 0.36 0.59 0.68 0.82 1.00 0.808 0.701 0.514 0.280
Measured sensitivity (K,,, mVIW
1.294 1.261 1.235 1.229 1.219 1.212 I.211 1.217 1.230 1.276
Deviation from termination sensitivity ( &,, %I 6.8 4.0 1.9 1.4 0.9 0.0 0.1 0.4 1s 5.3
where Q, is the quality factor of the load without accounting for the dissipation in the reactive element, and Q,, is the quality factors of the reactive L and C elements accounting for the connecting wires, The quality factor of the reactive elements was measured by Q-meter (HP 4342 A) at the operation frequency. The corrected values of the sensitivity and their deviations are shown in Table 2. It is shown that the deviation of the sensitivity is less than ( 1.7-2)% for a quality factor of the load up to 5. 4.5. Unmatched txwsmission
line
The maximum phase shift between the RF current and the voltage in an unmatched transmission line is connected to the reflection coefficient Tof the load as cp,,,=*2arctan/rl
(12)
Hence, the uncertainty of the active power sensor does not exceed ( 1S-2) % in the load reflection coefficient / Tl = 0.79 (VSWR = 8.5) for arbitrary position of the sensor in the transmission line.
5. Conclusions A magnetoresistive sensor has been proposed for measuring active RF power in unmatched transmission lines. This type of sensor element is very suitable for this purpose because it has two independent inputs-electrical and magnetic, which are connected to two components of the electromagnetic field which determine the Poynting’s vector and therefore, the electromagnetic power. The electrical input of the sensor is connected to the RF electrical field (voltage) and the magnetic input-with RF magnetic field (current) _ Hence, there is no need to use a current transformer which strongly limits the operation frequency band. Magnetoresis-
26
V. Vountesmeri,
A. Martynyuk
/Sensors
tive sensors operate, in principle, in a very wide frequency band. The elementary theory of the magnetoresistive sensor for RF active power to explains the principle of its operation has been presented. The sensor has been investigated at the frequency 13.56 MHz and power levels up to 1.3 W. It showed good performance for measurement of active RF power in a wide dynamic range up to 82 dB for transmission lines with VSWR up to 8 or for reactive circuits with quality factor up to 5. This type of sensor has high sensitivity and stability, small size and it is useful for different applications in a wide frequency range.
References
Ill
V.A. Godyak, R.B. Piejak, In situ simultaneous radio frequency discharge power measurements, J. Vat. Sci. Technol. A 8 (1990) 38333837. [21 H.E.M. Barlow, The application of the Hall effect in semiconductor to the measurement of power in an electromagnetic field, Proc. IEE 102B (1955) 179-185. of the magnetoresistance effect in semicon[31 S. Kataoka, Application ductors to microwave power measurements, Proc. IEE 113 (1966) 948. Electron. Ind. 21 (6) ( 1962) 141 A. Rugari, A Hall effect powermeter, 6-16. L. Katz, V. Tereshova, D. Shahter, Microwave pulse [51 U. Arhipov, passing powermeter, Prib. Tekh. Eksp. 1 (1975) 156-1.58 (in Russian). [61 K.K. Clarke, D.T. Hess, A 1000 A/25 kV/25 kHz-500 kHz VoltAmpere-Wattmeter for loads with power factor from 0.001 to 1.00, IEEE Trans. Instrumen. Meas. 45 ( 1996) 142-145.
and Actuators
A 69 (1998)
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[71 W.Z. Fam, A novel transducer to replace current and voltage transformers in high-voltage-measurement, IEEE Trans. Instrumen. Meas. 45 (1996) 190-194. transform[81 VS. Vuntesmery, General theory of galvanogyromagnetic ers based on ferromagnetic films, Proc. of the 7th Colloquium on Microwave Communication, Budapest, 1982, pp. 775-779. films, Fiz. Tverd. Tela 26 [91 W.D. Ku, Planar effect in ferromagnetic ( 1968) 565-569 (in Russian).
Biographies Valery Vountesmeri (M’96) was born in 1941 in the Sumy region of Ukraine. He received his MSc and PhD degrees in radio engineering from the Kiev Polytechnic Institute, Kiev, Ukraine, in 1969 and 1973, respectively. He received a DSc degree in solid state electronics and micro-electronics, antennas and microwave techniques from the Kiev Institute of Cybernetics in 1988. In 1974, he joined the Radio Engineering Faculty, Kiev Polytechnic Institute, as Lecturer. In 1990, he was promoted to Full Professor. In 1979, he served as a visiting scientist at the Institute of Physics, University of Oslo, Norway. Since 1995, he has been a Guest Researcher and Head of Microwave Laboratory at the Instrument Center of the National University of Mexico. His current research interests are RF and microwave measurement devices and systems, RF magnetoresistive sensors, millimeter-wave circuits and devices. He has authored more than 85 scientific papers and 25 patents. AEexander Martynyuk. Biography not available at the time of
publication.
Sensors and Actuators
A 69 ( 1998) 21-26
Magnetoresistive thin film sensor for active RF power V. Vountesmeri *, A. Martynyuk Centro
de Irmtrumet~ios, Received
Universidad 20 August
National
Autonorna
1997; received
de Mexico,
PO Box 70-186,
in revised form 22 December
CD. Universiraria,
Mexico
1997; accepted 26 December
D.F. 04510, Mexico 1997
Abstract A new thin film sensor for active RF power based on a magnetoresistive effect in a ferromagnetic film has been developed. The sensitive element of the sensor was created by the deposition of 80Ni-20Fe alloy on a glass substrate. In a 50-a coaxial line of size 1.6 X 0.65 mm’, the sensitivity 1.2 mV/W at the frequency 13.56 MHz has been achieved. The dynamic range is 82 dB, the upper level is 1.3 W with selfabsorption less then 5%. The temperature coefficient of sensitivity (TCS) is - 4 pm “C- ’ The uncertainty in measurements of the active RF power due to complex load is no more than 1.8% for reactive circuits with power factor up to cos $=0.23 or for unmatched transmission lines with VSWR up to 8.5. 0 1998 Published by Elsevier Science S.A. All rights reserved. Keywords:
RF power;
Measurement:
Magnetoresistive
sensor; Coaxial
line
1. Introduction
RF and microwave sourceshave been used widely for communications as well as industrial and medical applications. The main parameterwhich determinesthe technological and medical processintensity is the active RF power dissipatedin the temrination. Generally, this termination is not ideally matched, hencethe presenceof a reflected wave, which changesthe matchedregime of the waveguide operation and renders the processof measuringactive power ‘in situ’ very complicated [ 11. Matching devices were usedfor the matching of generator and an arbitrary load. However, measurementof the active power in the matched part of the line doesnot permit us to judge the active power accurately in the load becauseof unknown dissipation in the matching devices. Hence, it is necessaryto usepower sensors,which have the potential to operate in unmatched
transmission
lines directly.
There are three basic methodsfor measuringactive power in unmatched lines which may be used for designing the active power sensors. The first method is a directional coupler method, where two directional couplers for forward and reflected waves are connectedin series.In addition to these,two quadratic detectors or themlocouplesareused.The active power in the transmission line is determinedby extracting the reflected power * Corresponding 8612.
author.
0924-4247/98/$19,00 Pzzs0924-4247(98)00043-0
0
Tel.:
+ 52-5-622-8619;
1998 Published
Fax:
by Elsevier
+ 52-5-622-86201
signal from the forward power one. The disadvantageof this method is in the necessityof having directional couplerswith adequatedirectivity and two identical quadratic detectors.In this method, there is also a big influence of the signal harmonics due to frequency dependentcoupling and directivity and non-quadraticbehavior of the detectors. The secondmethod consistsof amplitudemeasurementof the RF voltage ( V), the RF current (I) and the phaseshift (cp) between them in an arbitrary position of the standing wave. The active power is P=OS VI coscp
(1)
This methodis usedonly in laboratory conditions becauseof complexity, especially in the RF current and phase shift measurements. The third is the method of direct multiplication of the instantaneousvalues of the RF voltage V(t) and current Z(t) in a certain sectionof the transmissionline with the following integration: (2) where r is the integration time which is more than the period of the RF signal. This method is most suitable for measuring‘in situ’ the active power with arbitrary crosssection of the transmission line. In this case,the dimensionsof the sensorarenot depend-
Science S.A. All rights reserved.
22
V. Vountesmeri,
A. Martynyuk/Sensors
andActuators
A 69 (1998)
21-26
ent on the signal wave length in a wide frequency range. Many attempts have been made to use for this purpose the Hall and Gauss effects in semiconductors [ 2-51, but they failed because of large parasitic signals (up to 100%) due to rectification and thermoelectricity. For low frequencies of up to 500 kHz, the multiplication of electrical signals which are proportional to the instantaneous values of the voltage and current is used. For this purpose transducers of voltage and current are used [ 6,7]. We propose a new type of sensor based on galvanomagnetic phenomena (abnormal Hall effect and magnetoresistante) in ferromagnetic films [ 81. Such sensors have up to one order greater sensitivity than semiconductor sensors in weak RF magnetic fields, one order less rectification and one to two orders lower thermoelectricity. Hence, magnetoresistive sensors have properties 103-lo4 times better than semiconductor sensors. This allowed us to design magnetoresistive RF sensors with good characteristics.
2. Theory
Fig. 1.Themagnetoresistivesensitiveelement.
The operation of the sensor consists of in the multiplication of the RF voltage V= V, expCjwt) and current Z=1, X exptjwz) with the following integration for sharing out of the DC component which corresponds to active component of the complex power fall through the cross section of the coaxial transmission line: P=O.5 Re (V,Z,*)
(3)
Here V,,, and Z, are the complex amplitude of the RF voltage and current in an arbitrary cross section of the transmission line, respectively. This multiplication is made by using the magnetoresistance effect in ferromagnetic films [ 91. For a_ftmagneticfilm strip (Fig. 1) magnsized up to saturation MO by the effective magnetic fiel+d& - created in the film by the external magnetic field Ho - and taking into account the demagnetizing form factors of the film and uniaxial magnetic anisotropy field, for the p. direction, the change of resistance due to a change of the magnetized angle of Acp is R-=-AR
sin2tpoAp
(4)
where AR is the change in magnetoresistance of the magnetic films magnetized in the direction of the film’s axis OX, and the normal to this axis is the OX, direction. In magnetic film, the RF current If is created by the transmission line voltage K I,=YV
(5)
where Y is the admittance of the magnetic film circuit. The change of the film resistance R, is caused by the excitat$n of the magnetization vectordby the RF magnetic field h caused by the RF current I flowing in the coaxial central conductor and it is possible to write it as
R,=R,I
(6)
where R, is the resistive-current sensitivity of the magnetoresistive sensor which depends on the magnetoresistive parameter of the magnetic film AR,,,. It is a function of the sensor position relative to the central conductoftthrough h” and intensity of the external displacement field Ho. So the RF current If created by the line voltage V flows in the magnetic film, the resistance of which changes in time in accordance with the RF current I flowing in the coaxial central conductor. As a result of the parametric detection of the RF current If, a DC voltage V, appears at the film’s ends. Taking into account Eqs. (5) and (6) and combining the variables, we can write: Vo=0.5 Re(Z,R,*)=0.5
Re(YRI*VZ*)
(7)
Comparing Eqs. (7) and (3), we see that in order to have the output power proportional to the active power rhe product YRI* must be real value or Im( YR[*) = 0. Only in this case will the DC output voltage be proportional to the active power: V,=K,0.5
Re(VZ*)=K,P
(8)
where Kp = Re( YR,*) is the active power sensitivity of the sensor. The dependence of the sensor’s sensitivity with respect to frequency variations is connected to the frequency dependence of magnetic film circuit admittance Y (Eq. (5) ) and the resistive-current sensitivity of the magneto-resistive sensor R, (Eq. (6) ) . These dependences led to varying active power sensitivity with frequency and increasing uncertainty of the active power measurement in the presence of reactive power, because Im( YR,*) # 0.
V. Vountesmeri,
A. Marpnyuk
/Sensors
and Acwztors
A 69 (1998)
23
21-26
Hence, for a particular frequency it is necessary to satisfy the equality Im( YR;+) =O. The maximum of the detection effect will be when the direction of the magnetization
R=368 Ohm R&54 Ohm c,=c=o.01uF
3. Sensor A sensitive element based on a magnetoresistive 80Ni20Fe alloy magnetic film deposited on a polished glass substrate by the electronic beam method. The sensitive element consists of two loop-like branches which are connected in shunt for the RF signal and in series for the output signal, Fig. 2. For elimination of secondary parasitic detection effects, each branch consists of two magnetoresistors (R, = 554 R, AR/R, = 1.53%, 1.4 mm length, 30-km width, separated by 0.3 mm spacing) connected in series by copper strips 30 pm wide. Contacts are prepared on one side of the substrate. The size of the sensitive element is 7.5 X 5.0 mm’. The thickness is 0.5 mm. The sensitive element is set up under the central wire of the microcoaxial line ( 1.6 X 0.65 mm2) so the magnetoresistors are situated symmetrically with respect to the coaxial axis normal, Fig. 3. The distance from the coaxial axis to the magnetoresistors is 0.85 mm. This construction is placed in a non-magnetic box so that it is possible to magnetize the magnetic films with an external magnetic field. Thz magnetoresistors operate in the external magnetic field Ho created by a permanent magnet. The magnetic field is parallel to the substrate’s surface and its intensity and direction are changed by changing the distance between the substrate and perrnanent magnet and its (magnet) rotation. The field was varied from 400 to 10’ A/m and was measured by a Hall sensor. In the experiment the amplitude of the field was larger than the demagnetizing fields of the magnetic film in order tozbtain a>mall difference in direction between the vectors M0 and Ho and a rather large amplitude of the magnetic field to avoid ferromagnetic resonance in the film at the operating frequencies. The electrical scheme is shown in Fig. 3. Here, four magnetoresistors (MR) are connected in series relative to rnagntt ,- - -------<
-,-\
,’
4. Experimental
results and discussion
The test set for investigation of the power sensor is shown in Fig. 4. The sensor (3) at one side is connected to the RF generator ( 1) (Advanced Energy, P,,, = 600 W, f = 13.56 MHz) through attenuator (2) (20 dB, P,, = 100 W) which serves to exclude the influence of the unmatched load on the RF generator. At the other side the sensor is connected to the
$7
R,
d
j ,,’ ,/’ ” //
.l-\\) . Fig. 2. Geometric film.
detected signal V, and in parallel relative to the RF voltage V which exists in the coaxial line in the region where the magnetoresistive sensor is situated. The capacitors C serve to ground the RF currents and, together with the resistance of the four magnetoresistors R,, they also serve to integrate the multiplying signals and share out the detecting signal V, which is proportional to the active RF power in the coaxial line. The capacitor C, serves in the separation of the DC voltage (which is possible in the coaxial line) from the small detecting signal V, to avert the influence of the large DC voltage on the exact measurement of the small signal V,,. Resistor R limits the RF power dissipated in the magnetoresistive sensitive elements where large RF power in the coaxial line is measured. V is the RF voltage at the point of the connection of the electrical input of the magnetoresistive sensitive element to the coaxial line, and I is the current in the inner coaxial conductor.
%, z.,jwt >
1 ,“,// I
Fig. 3. The elecrnc scheme of the sensor
1
N-S relarion
for electromagnetic
detection
in a ferromagnetic Fig. 4. A block diagram
of the test set.
a
24
V. Vountesmeri,
A. Murtyyuk
/Sensors
matching load (4)) (Hewlett-Packard 438 A Power meter). It is possible to connect different values of reactive elements (L,C) in parallel to the matched load creating an unmatched complex load. The magnitude and direction of the magnetic field is changed by changing the distance and rotation of the permanent magnet (N * S). The output signal is measured by the Hewlett Packard 3458 A Multimeter (5). 4.1. Dynamic
range
The dynamic range of the sensoris determined in the matching regime as the ratio between the maximal power where the amountof the non-linearity exceedsacertain level, in this case 3%, and the minimal power?which createsan output signal equal to the noise signal of the sensorat room temperature.In Fig. 5, the volt-watt characteristicsfor different values of the external displacementmagnetic field are shown. Deviation 3% from linearity occurs at a power level of 1.3 W. The insertion lossdueto self-absorptionis no more then 5% of the incident power. The sensor sensitivity is inversely proportional to the displacement magnetic field intensity. This is clear from geometry, Fig. 2, wherethe angle of the magnetized deviation (in the linear approach) is inversely propozional to the internal displacementmagnetic field intensity HOi. Hence, the change of the film resistance (4) and, consequently, sensitivity (8) have the same dependences. The misalignmentof the sensitiveelementwith respectto the power line leads to the reduction of sensitivity and an increasein the parasitic detection effects. The lower power level isdeterminedby both the sensitivity and the level of the Johnson’snoisevoltage V,,=d4kTR,Af
(9)
where R, = 4R, is the internal resistanceof the sensorat the output clamps, T is the temperatureof the sensor,k is Boltzmann’s constant, and Af is the frequency band of the measurement amplifier. For the given parametersof the sensor,the lower value of the dynamic range in the unit frequency band is 7.9X
0
ml
Fig. 5. The voltage-power ment magnetic fields.
403
600 603 Powr,mW
characteristics
IW
1200
of the sensor for different
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A 69 (1998)
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lo-’ W. Hence, this sensorhas82 dB dynamic range in the frequency band equal to 1 Hz. 4.2. Angular
dependence
The angular dependenceof the output signal was investigated for matching, short and open operation regimes of the transmissionline. The angle dependencefor the matching regime is shownin Fig. 6. The active power dissipatedin the load is equal to zero for the short andopenoperation regimes, Hence, for an ideal active power sensorthe output signalmust be zero. In reality, however, there is an output signal due to secondaryparasitic effects in the sensorbecauseof thermoelectricity and a self-detectioneffect. This effect takesplace when there is a detection signal at the output of the sensorin the presenceof only one signal component, which can be either RF current in the film or RF magnetic field. It is found that the maximum value of the useful signalfor the matching load is two orders greater than the parasitic signalsof the two other operation regimes-short and open. This result provides more evidence about the high quality of this kind of power sensor.There is an angularposition of the displacementmagneticfield wherethesesignalsare separated (8=0, T). 4.3. Temperature
dependence
The temperaturedependenceof the conversion coefficient has also been investigatedIt was found that the temperature coefficient of sensitivity (TKS) hasa value of - 4 pm “C- ’ in the temperaturerange ( - 50- + 5O)“C. 4.4. Complex
load dependence
It is very important to check the operation of the sensorin the regime of the complex load. This complex load is createdby the connection of reactive elements(capacitors or inductors) to the coaxial line in paralle1to the coaxial termination which is an absorbingelement of the powermeter (HP 438 A). The distance between the point of the connectionof the reactive elementsand the center
1400
displaceFig. 6. The normalized
sensor output signal vs. magnetized
angle.
V. Vountesmeri,
A. Martynyuk/Sensors
of the sensitive element is 3 mm. So, the phases andresistance of the load transformed to the sensor’s center region are approximately the same like the load for the operating frequency 13.56 MHz. The sensitivity of the sensor is the relation between output signal and active power in the section of the coaxial line where the sensor is situated (8). However, it is very difficult for us to measure this power directly due to losses in the transmission line and in the reactive elements. The only possibility we have is to measure active power using the terminal powermeter and to determine the sensor sensitivity for the regime of the powermeter termination and the regimes of the different complex loads, assuming that the transmission line and the reactive elements are ideal, without losses, as the relation between the output sensor’s signal V, and the powermeter display PM
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Table 2 The corrected value of the active power sensitivity and its deviation from the termination regime of operation for different normalized load admittances taking into account the losses (measured Q) in the reactive elements Normalized (50 a) load admittance
Reactive element’s
(6)
(QLc)
1.00-j 1.00-j 1.00-j 1.00-j 1.00-j 1.oo l.OO+j I .OO + 1.OO + l.OO+j
4.24 2.60 1.35 1.07 0.70
86 95 14.5 155 196 -
0.73 j 1.02 j 1.67 3.42
535 269 209 76
Q
Corrected sensitivity (K, mV/W)
Deviation from termination sensitivity (6% 5%)
1.233 1.227 1.224 1.221 1.219 1.212 1.209 1.212 1.220 1.221
I .I3 1.24 0.99 0.74 0.58 0.00 -0.25 0.00 0.66 0.74
K,+ M
The normalized admittance of the complex load Y,, power factor cos cpof the load, sensitivity KM and its deviation 8, from the termination regime are shown in Table I. It is shown (see Table 1) that the active power sensitivity detemrined in this way changes with a change of the complex load and the sign of its deviation from the termination regime is positive and does not depend on the sign of the load susceptance (capacitive or inductive). This means that there are additional dissipation in the reactive elements which is not measured by the powermeter but are taken into account by the sensor. The inclusion of the dissipations in the reactive elements leads to the formula for the corrected sensitivity which differs from ( 10) and is better approach to the real active power sensor sensitivity
KM
K,=
(11)
l+QJQ,C
Table 1 The measured active power sensitivity and its deviation from the termination regime of operation for different normalized load admittances without taking into account the Iosses in the reactive elements Normalized ( 50 R ) load admittance (Y,)
1.00-j 4.24 1.00-j 2.60 1.00-j 1.35 1.00-j 1.07 1.00-j 0.70 1.00 1.00 + j 0.73 1.00-l-j 1.02 l.OO+j 1.67 I.OO+j 3.42
Load power factor (co5 q)
0.23 0.36 0.59 0.68 0.82 1.00 0.808 0.701 0.514 0.280
Measured sensitivity (K,,, mVIW
1.294 1.261 1.235 1.229 1.219 1.212 I.211 1.217 1.230 1.276
Deviation from termination sensitivity ( &,, %I 6.8 4.0 1.9 1.4 0.9 0.0 0.1 0.4 1s 5.3
where Q, is the quality factor of the load without accounting for the dissipation in the reactive element, and Q,, is the quality factors of the reactive L and C elements accounting for the connecting wires, The quality factor of the reactive elements was measured by Q-meter (HP 4342 A) at the operation frequency. The corrected values of the sensitivity and their deviations are shown in Table 2. It is shown that the deviation of the sensitivity is less than ( 1.7-2)% for a quality factor of the load up to 5. 4.5. Unmatched txwsmission
line
The maximum phase shift between the RF current and the voltage in an unmatched transmission line is connected to the reflection coefficient Tof the load as cp,,,=*2arctan/rl
(12)
Hence, the uncertainty of the active power sensor does not exceed ( 1S-2) % in the load reflection coefficient / Tl = 0.79 (VSWR = 8.5) for arbitrary position of the sensor in the transmission line.
5. Conclusions A magnetoresistive sensor has been proposed for measuring active RF power in unmatched transmission lines. This type of sensor element is very suitable for this purpose because it has two independent inputs-electrical and magnetic, which are connected to two components of the electromagnetic field which determine the Poynting’s vector and therefore, the electromagnetic power. The electrical input of the sensor is connected to the RF electrical field (voltage) and the magnetic input-with RF magnetic field (current) _ Hence, there is no need to use a current transformer which strongly limits the operation frequency band. Magnetoresis-
26
V. Vountesmeri,
A. Martynyuk
/Sensors
tive sensors operate, in principle, in a very wide frequency band. The elementary theory of the magnetoresistive sensor for RF active power to explains the principle of its operation has been presented. The sensor has been investigated at the frequency 13.56 MHz and power levels up to 1.3 W. It showed good performance for measurement of active RF power in a wide dynamic range up to 82 dB for transmission lines with VSWR up to 8 or for reactive circuits with quality factor up to 5. This type of sensor has high sensitivity and stability, small size and it is useful for different applications in a wide frequency range.
References
Ill
V.A. Godyak, R.B. Piejak, In situ simultaneous radio frequency discharge power measurements, J. Vat. Sci. Technol. A 8 (1990) 38333837. [21 H.E.M. Barlow, The application of the Hall effect in semiconductor to the measurement of power in an electromagnetic field, Proc. IEE 102B (1955) 179-185. of the magnetoresistance effect in semicon[31 S. Kataoka, Application ductors to microwave power measurements, Proc. IEE 113 (1966) 948. Electron. Ind. 21 (6) ( 1962) 141 A. Rugari, A Hall effect powermeter, 6-16. L. Katz, V. Tereshova, D. Shahter, Microwave pulse [51 U. Arhipov, passing powermeter, Prib. Tekh. Eksp. 1 (1975) 156-1.58 (in Russian). [61 K.K. Clarke, D.T. Hess, A 1000 A/25 kV/25 kHz-500 kHz VoltAmpere-Wattmeter for loads with power factor from 0.001 to 1.00, IEEE Trans. Instrumen. Meas. 45 ( 1996) 142-145.
and Actuators
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[71 W.Z. Fam, A novel transducer to replace current and voltage transformers in high-voltage-measurement, IEEE Trans. Instrumen. Meas. 45 (1996) 190-194. transform[81 VS. Vuntesmery, General theory of galvanogyromagnetic ers based on ferromagnetic films, Proc. of the 7th Colloquium on Microwave Communication, Budapest, 1982, pp. 775-779. films, Fiz. Tverd. Tela 26 [91 W.D. Ku, Planar effect in ferromagnetic ( 1968) 565-569 (in Russian).
Biographies Valery Vountesmeri (M’96) was born in 1941 in the Sumy region of Ukraine. He received his MSc and PhD degrees in radio engineering from the Kiev Polytechnic Institute, Kiev, Ukraine, in 1969 and 1973, respectively. He received a DSc degree in solid state electronics and micro-electronics, antennas and microwave techniques from the Kiev Institute of Cybernetics in 1988. In 1974, he joined the Radio Engineering Faculty, Kiev Polytechnic Institute, as Lecturer. In 1990, he was promoted to Full Professor. In 1979, he served as a visiting scientist at the Institute of Physics, University of Oslo, Norway. Since 1995, he has been a Guest Researcher and Head of Microwave Laboratory at the Instrument Center of the National University of Mexico. His current research interests are RF and microwave measurement devices and systems, RF magnetoresistive sensors, millimeter-wave circuits and devices. He has authored more than 85 scientific papers and 25 patents. AEexander Martynyuk. Biography not available at the time of
publication.