IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 2, FEBRUARY 2007 425

Insertion Loss in Reflection-Type Microwave PhaseShifter Based on Ferroelectric Tunable Capacitor

Orest G. Vendik, Member, IEEE

Abstract—A tunable capacitor based on a ferroelectric thinfilm grown on a dielectric substrate is a promising componentfor reconfigurable and tunable devices for numerous RF andmicrowave applications such as phase shifters, tunable filters,and tunable matching networks. The goal of this paper is char-acterization of the insertion loss in a reflection-type microwavephase shifter based on the ferroelectric tunable capacitor. Thereflection-type phase shifter based on ferroelectric planar tunablecapacitor was designed, manufactured, and tested. Its simulatedand measured insertaion loss and phase shift are compared. Twoloss mechanisms in the phase shifter were distinguished: the lossin the tunable capacitor and the loss in the associated circuitry(metallization and substrate). The properly organized investiga-tions of the dissipation of energy in metallization and dielectricsubstrate can result in a sufficient improvement of the quality ofthe devices. As a final result, one may anticipate that the figure-of-merit of a ferroelectric tunable or switchable phase shifter canreach 200 deg/dB.

Index Terms—Ferroelectric tunable capacitor, figure-of-merit ofphase shifter, reflection-type phase shifter.

I. INTRODUCTION

ATUNABLE capacitor based on a thin film barium–strontium–titanate (BST) on a dielectric substrate is a

promising component for reconfigurable and tunable devicesfor numerous RF and microwave applications such as phaseshifters, tunable filters, and tunable matching networks. TheBST thin-film technology for the microwave application wasdeveloped during a period of over 30 years [1]. The physicalproperties of ferroelectrics at microwave frequencies andprinciple of operation were formulated in [2] and [3]. It wasproclaimed that the application of ferroelectrics at microwaveswas in a crucial period of developing production technologyand delivering the technology to industrial enterprises [2]. Thereflection-type microwave phase shifter itself or in combinationwith a hybrid junction is used as a promising part of a steerableantenna array and other microwave systems [4], [5]. Now thelow-cost production technology of BST microwave compo-nents in combination with high performance is considered asa main reason for designing the BST microwave devices formass production [6].

Manuscript received May 30, 2006; revised October 11, 2006. This work wassupported by the Network of Excellence “Metamorphose” under Project 500252(the 6th Framework Programme of the European Commission).

The author is with the Department of Electron-Ion Technology, St. PetersburgElectrotechnical University, St. Petersburg 197376, Russia.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMTT.2006.889348

Fig. 1. Equivalent diagram of the phase shifter. Two states of the phase shifterare considered. The states are determined by the two values of capacitance ofthe tunable capacitor.

The insertion loss is an important characteristic of any mi-crowave device [4]. The goal of this paper is rating of the inser-tion loss in the reflection-type microwave phase shifter based onthe ferroelectric tunable capacitor.

II. DESIGN OF THE PHASE SHIFTER

An equivalent circuit diagram of the phase shifter is shownin Fig. 1. Two states of the tunable capacitor are considered.The states are characterized by the following values of thecapacitance:

(1)

where is the middle value of the capacitance andis the tunability of the capacitor.

The impedance of the tunable capacitor for two states is pre-sented as follows:

(2)

(3)

where is the resistance responsible for the loss in the tun-able capacitor

(4)

(5)

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426 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 2, FEBRUARY 2007

Fig. 2. Dielectric permittivity and loss factor as a function of the biasingvoltage for BSTO film used in the planar capacitor with the spacing betweenplanar electrodes 10 �m.

where and are loss factors of the tunable capacitorin the two states.

The short-circuited transmission line section with the char-acteristic impedance and electric length at the middlefrequency has the following reactance:

(6)

Let us assume that the circuit presented as a series connec-tion of the tunable capacitor and the transmission line section

is in resonance at the middle value of the tunable ca-pacitor and the middle frequency. Thus, one has the followingequality:

(7)

The impedance of the circuit considered in two states is (seeFig. 1)

(8)

(9)

The simple optimization procedure makes it possible to find, , , and providing the following equalities:

(10)

In Fig. 2, the dielectric permittivity and loss factor of theBSTO film used in the planar capacitor are shown as a function

of applied voltage at room temperature [7]. For a rough estima-tion, let us take into consideration the following values, whichcharacterize the properties of the planar capacitor at the fre-quency GHz: pF, , , and

. Our experience shows that use of pFis unacceptable because of the influence of the parasitic capaci-tance. The quarter-wavelength transformer is used. The parame-ters of the transformer at are ,(Fig. 1). The simple calculation gives the imaginary and realparts of the impedance (Fig. 1) as follows:

In the plane with the impedance (Fig. 1), the reflec-tion coefficient is

(11)

where . The phase shift of the reflected wave is

(12)

The insertion loss in two states is

(13)

For the middle frequency , one has

and dB

The phase shifter is formed as a thin film integrated circuitbased on the BSTO layer on an alumina substrate. Layout ofthe phase shifter is shown in Fig. 3. Inset 1 shows the structureof the planar capacitor shaped as a section of a coplanar line,which is an inherent part of the integrated circuit. The spacingbetween the planar capacitor electrodes is 10 m. Inset 2 showsthe coplanar line section with parameters and , as shownin Fig. 1. Furthermore, in Fig. 3, 3 is the quarter-wavelengthtransformer, 4 is the input transmission line with characteristicimpedance , 5 is the quarter-wavelength section,which is used as a ground plane for the tunable capacitor, and6’s are the quarter-wavelength stubs for dc/RF isolation of thebiasing voltage feeders, which are connected with the contactpads, i.e., 7’s. The size of the alumina substrate is 0.5 1015 mm.

Fig. 4 presents a photograph of two versions of the phaseshifter placed on the same substrate in the brass housing.

III. SIMULATED AND MEASURED DATA

The electromagnetic full-wave analysis was used for a simu-lation of the reflection coefficient of the phase shifter with thelayout presented in Fig. 3. The tunable capacitor was introducedin the simulation as a lumped component with the impedance

given for the two states by (2) and (3). The first cycleof the simulation was performed for the case of a perfect metal

VENDIK: INSERTION LOSS IN REFLECTION-TYPE MICROWAVE PHASE SHIFTER 427

Fig. 3. Layout of the phase shifter. Inset 1 shows the structure of the planarcapacitor shaped as a section of a coplanar line. Inset 2 shows the coplanar linesection (Z and � , as shown in Fig. 1). 3 is the quarter-wavelength transformer,4 is the input transmission line, 5 is the quarter-wavelength section, which isused as a ground plane for the tunable capacitor, 6’s are the quarter-wavelengthstubs for dc/RF isolation of the biasing voltage feeders, 7’s are contact pads.

and a lossless substrate. The losses in the structure were pre-sented only by the energy dissipation in the ferroelectric tunablecapacitor. The second cycle of the simulation was performed forthe case of copper film ( m) with a thicknessof m and a loss factor of the substrate .

The insertion loss and phase shift obtained as results of thesimulation are presented in Fig. 5(a) and (b). Curve 1 corre-sponds to the case of the perfect metal. Curve 2 correspondsto the case of the real copper films. The most remarkable resultof the simulation is the difference in the insertion loss for a per-fect conductor and the real copper films. The contribution of theconductor loss contribution due to the circuit is approximatelyequal to the contribution of the tunable ferroelectric component.

Fig. 4. Two versions of the phase shifter placed on the same substrate in thebrass housing.

The frequency band in which the digital phase shift is kept inthe limit 180 10 is determined as 8.6–9.4 GHz. It turned outthat the frequency band is sufficiently influenced by the quarter-wavelength section included in series with the tunable capacitor(5 in Fig. 3) and the dc/RF isolators (6 and 7 in Fig. 3).

In the experimental investigations, the capacitors based on thefilm of Ba Sr TiO by magnetron spattering on aluminasubstrate were used. The film thickness was 0.5 m. Thicknessof the copper film was 1.5–2 m. Phase-shifter layout was pat-terned with a photolithography process. The phase shifter wasmanufactured as a unit integrated circuit.

The tunable capacitors and the assembled phase shifter wereinvestigated in a two-stage procedure [8].

The capacitors were first manufactured separately. The ca-pacitance and tunability were measured at MHz. Theloss factor was measured with the microstrip resonator at

GHz. The measured loss factor was .The biasing voltage for was 200 V.

The complex reflection coefficient of the phase shifter wasthen measured in the frequency range of 8–10 GHz. The resultof the measurement is presented in Fig. 5(a) and (b) denotedby the filled circles. The phase shift and losses were measuredand simulated, taking into account that the microwave powerdissipation in the ferroelectric capacitor, conducting films, andsubstrate are in reasonable agreement.

IV. DISCUSSION

The common characteristic of a microwave phase shifter isits figure-of-merit defined as the ratio of the phase shift in de-grees and the insertion loss in decibels. The simulation for aperfect metallization and a perfect substrate givesand dB. That gives deg/dB.

The result of the measurements is followed byand dB, which gives deg/dB.

428 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 2, FEBRUARY 2007

Fig. 5. (a) Simulation and measurements of the insertion loss of the phaseshifter. (b) Simulation and measurements of the phase shift. Curve 1 correspondsto the simulation with the perfect metal. Curve 2 corresponds to the simulationof the copper films. Curve 3 corresponds to the result of measurement of thephase shifter manufactured.

The widely used method for estimation of the figure-of-meritis the commutation quality factor of the electrically controlleddevice or materials [2], [3], [9], [11]. The commutation qualityfactor ( ) generalizes the tunability and loss factors of a deviceor materials as follows:

(14)

The commutation quality factor is strongly connected withthe figure-of-merit. For the digital phase shifter – , onehas [9]

(15)

For the tunable capacitors used in the phase shifter, ,, and . Thus, . In accor-

dance with (15), one obtains deg/dB. That is in a goodagreement with the simulated deg/dB.

The experimental measurements exhibiteddeg/dB.

Two main parts of the losses in the phase shifter can be dis-tinguished: the loss in the tunable capacitor and the loss in therest of the phase shifter circuitry (metallization and substrate).One may write

(16)

Our simulations and measurements show thatdB. Thus, for , one has

deg/dB.Analysis of the literature [3], [10] shows that the best fer-

roelectric tunable capacitors at the frequency – GHzare characterized by . In accordance with (15)for a digital phase shifter with and ,we have deg/dB, but in combination with

deg/dB, we obtain only deg/dB.Thus, the figure-of-merit of the tunable (switchable) phaseshifter is mainly limited by the circuit losses.

V. CONCLUSION

It has been shown that two main contributing losses in the fer-roelectric phase shifter should be distinguished: the loss in thetunable capacitor and the loss in the associated circuitry (met-allization and substrate). At the level of modern technology, thevalues of these two kinds of losses are quite comparable. Thereis a lack of information about investigation of the loss in cir-cuitry of a microwave integrated circuit specified for the ferro-electric tunable or switchable phase shifters. One should believethat the properly organized investigations of the dissipation ofenergy in metallization and in the dielectric substrate aimed todecrease the loss in the tunable or switchable devices can resultin a sufficient improvement of the quality of the devices. As afinal result, one may anticipate that the real figure-of-merit ofa ferroelectric tunable or switchable phase shifter can reach thevalue – deg/dB.

ACKNOWLEDGMENT

The author is grateful to S. Karmanenko, St. Petersburg Elec-trotechnical University, St. Petersburg, Russia, for supply withthe substrates covered by the BSTO layer, which were used inthe experiment described.

REFERENCES

[1] O. G. Vendik, Ferroelectrics at Microwaves. Moscow, Russia: Soyet-skoye Radio, 1979.

[2] ——, “Microwave tunable components and subsystems based on fer-roelectrics: Physics and principles of design,” Integr. Ferroelect., vol.49, pp. 181–190, 2002.

[3] A. K. Tagantsev, V. O. Sherman, K. F. Astafiev, J. Venkatesh, and N.Setter, “Ferroelectric materials for microwave tunable applications,” J.Electroceram., vol. 11, pp. 5–66, 2003.

[4] S. K. Koul and B. Bhat, Microwave and Millimeter Wave PhaseShifters. Boston, MA: Artech House, 1991, vol. 2, Semiconduct. andDelay Line Phase Shifters.

[5] V. Sherman, K. Astafiev, N. Setter, A. Tagantsev, O. Vendik, I.Vendik, S. Hoffmann-Eifert, and R. Waser, “Digital reflection-typephase shifter based on a ferroelectric planar capacitor,” IEEE Microw.Wireless Compon. Lett., vol. 11, no. 10, pp. 407–409, Oct. 2001.

VENDIK: INSERTION LOSS IN REFLECTION-TYPE MICROWAVE PHASE SHIFTER 429

[6] nGimat, “A low cost analog phase shifter product family for military,commercial and public safety applications,” Microw. J., vol. 49, no. 3,pp. 152–160, Mar. 2006.

[7] O. G. Vendik, S. P. Zubko, and M. A. Nikol’ski, “Microwaveloss-factor of Ba Sr1 TiO as a function of temperature, biasing field,barium concentration, and frequency,” J. Appl. Phys., vol. 92, no. 12,pp. 7448–7452, Dec. 2002.

[8] M. A. Nikol’skiï, “Investigation of nonlinear microwave componentsbased on ferroelectrics,” (in Russian) Ph.D. dissertation, Dept. Elec-tron-Ionic Vacuum Technology, St. Petersburg Electrotech. Univ., St.Petersburg, Russia, 2003.

[9] I. B. Vendik, O. G. Vendik, and E. L. Kollberg, “Commutation qualityfactor of two-state switching devices,” IEEE Trans. Microw. TheoryTech., vol. 48, no. 5, pp. 802–808, May 2000.

[10] O. G. Vendik, I. B. Vendik, and V. O. Sherman, “Commutation qualityfactor as a working tool for optimization of microwave ferroelectricdevices,” Integr. Ferroelect., vol. 43, pp. 81–89, 2002.

[11] V. V. Pleskachev and I. B. Vendik, “The commutation quality factorof electrically controlled microwave device components,” Tech. Phys.Lett., vol. 29, no. 12, pp. 1018–1020, 2003.

Orest G. Vendik (M’92) was born in Leningrad,Russia, in 1932. He received the Diploma of radioengineer, Ph.D., and Doctor of Sc. degrees fromthe Leningrad Electrical Engineering Institute (nowSt. Petersburg Electrotechnical University), St.Petersburg, Russia, in 1954, 1957, 1966 respectively.

From 1967 to 1968, he was a Researcher on leavewith Surrey University, London, U.K. From 1969 to1989, he was with the St. Petersburg ElectrotechnicalUniversity, as the Head of the Department of Elec-tron-Ion Technology. He is currently a Professor with

the Department of Electron-Ion Technology, St. Petersburg ElectrotechnicalUniversity. From 1992 to 2002, he was regularly with Chalmers University,Göteborg, Sweden. His general research interests have been in solid-stateelectronics, theory of antennas, and microwave physics. He has authored fivebooks and has authored or contributed to over 250 papers in Russian andinternational journals. His citation index is over 1500.

Prof. Vendik is a member of the St. Petersburg Association of Scientists since1989. He was a recipient of the U.S.S.R. State Prize for Science and Technology(1988) and a Soros Professor Grant presented by the Soros Foundation (1994).He was named a Man of Science of the Russian Federation (1999).