Computer Aided Design for Microwave Circuits

University of Perugia

Background Experience The research group at the University of Perugia has been involved since 15 years in the analysis and design of microwave and millimeter-wave circuits in various technologies, from conventional (waveguide) to integrated (both hybrid and monolithic) circuits. The activity has been focused on both theoretical and experimental aspects. From the theoretical point of view an electromagnetic simulator, based on the Finite Difference Time Domain (FDTD) method, has been developed. This simulator is suitable for the analysis of structures with arbitrary geometry and has been extended in such a way as to rigorously model the interaction between electromagnetic fields and electronic devices (diodes, BJT, etc.). From the experimental point of view a great effort has been devoted to the set-up of a Microwave Electronic Laboratory (MEL) including a clean room and a test and measurement setup. At present, such laboratory is used for manufacturing hybrid integrated circuits up to 20 GHz, and for the testing of waveguide circuits up to 60 GHz. The following presentation begins with an outline of the main features and capabilities of the FDTD simulator developed at the University of Perugia. Then, for each research topic a brief abstract of the work is given.

1 FDTD simulator The FDTD method is based on the direct solution of Maxwell's time-dependent curl equations. To this purpose, time and space are discretized according to the Yee's algorithm: in particular, the space is divided in a finite number of elementary cells used for the mapping of electric and magnetic fields. The time solution is obtained by an iterative scheme which evaluates the electric and magnetic field components at alternate half-time steps. The following features have been implemented in the code developed at the University of Perugia.

1.1 Variable mesh In order to improve the efficiency of the method, a Variable Mesh is applied so that resolution of the grid can be improved according to the variation of the electromagnetic field in the proximity of singularities. However, if a variable mesh is adopted, slight local numerical anisotropies are introduced in the grid. This is due to the variation of the numerical propagation constant along the mesh.

1.2 Boundary conditions From the theoretical point of view, the electromagnetic problem is completed once boundary conditions for Maxwell's equations are defined. In terms of numerical implementation it leads to the definition of some local constraints among the field components, according to the geometry of the original structure. In practice, it is done by defining some kinds of walls: the electric wall, the magnetic wall, the absorbing wall and the source wall. The first one is used to model perfect conductors, the second one to model symmetry planes, and the last one to avoid reflection from termination either in a waveguide problem and in an open-space one. In the FDTD simulator developed at the University of Perugia several Absorbing Boundary Condition (ABC) algorithms have been implemented. Among them it is worth mentioning: Mur's first and second order, which have the advantage of being computationally fast and easy but suffer

fairly low accuracy; Perfectly Matched Layer (PML), which is general purpose and very accurate but quite demanding in terms of computing resources; Modal Absorbing Boundary Conditions (MABC) that allow a very efficient simulation of waveguide and coaxial circuits. Source type boundary conditions are used to provide the proper field excitation of the structure. Actually sources can be seen as time-varying boundary conditions. Interfaces between materials with different dielectric permittivities and conductivities are other kind of boundary conditions that are required expecially for the analysis of planar circuits.

1.3 Antenna and scattering problems In order to study antenna and scattering problems two additional features are required, namely the near-to-far field transform and the plane wave excitation. The former is based on the time-domain equivalence theorem applied to a parallelepipedal surface enclosing the radiating structure. This method is particularly suited for the analysis of planar antennas with finite ground plane. The latter is based on the separation between incident and scattered field.

1.4 Lumped elements Combining Lumped Element (LE) models of electronic devices and FDTD electromagnetic simulation the so called LE-FDTD method is obtained. The accuracy of this method, however, relies on the careful tuning of LE model parameters against the actual fabrication technology. This is usually obtained by extracting parameters values from experimental procedures.

1.5 Global modeling Within the framework of a full-custom IC design, an “a-priori” characterization of active devices cannot be easily pursued. In these cases a more “technology-oriented” modelling technique can be adopted. In this technique, no equivalent circuit should be defined, and thus no device-parameter extraction procedure is needed. Semiconductor transport equations are instead directly solved in a distributed fashion over an arbitrary domain. This modellization strategy, conventionally referred to as “Global Modeling”, is based on the self-consistent solution of Maxwell's equations and charge transport equations. In principle, the derivation of the combined field and device simulation scheme is straightforward. It consist of solving the curl Maxwell's equation in conjunction with the Boltzmann transport equations that, in the simplest case, lead to the “drift-diffusion” approximation.

1.6 Modal Analysis The Compact-2D-FDTD method is a specialized version of the 3D-FDTD simulator suited to perform the modal analysis of planar guiding structures, operating in the ranges of microwave, millimeter-wave and optical frequencies. Well known examples of such structures are microstrips, coplanar waveguides and rib-waveguides. Since all of them are typically uniform along the propagation direction, their modal character is influenced by the transverse geometry and physical parameters only. This feature is exploited to obtain an accurate and computationally efficient mode-solver known as Compact-2D-FDTD method.

2 Packaged microwave integrated circuits Because of the increasing operating frequency and complexity of the circuits, the prediction of the package behavior and of the interaction between the package and the enclosed circuit itself is becoming more and more significant. In most of the commercial microwave simulators, the presence of the package is taken into account by simply considering the reactance introduced by the electric walls close to the circuit; unfortunately, when the package supports resonant modes and when electromagnetic couplings between different parts of the circuit are present, this approach is not valid anymore. To overcome this limitation the full-wave FDTD simulator has successfully been used to investigate packaged MMIC circuits. In particular, the behaviour of packaged single and coupled MMIC via-hole grounds have been investigated; the theoretical analysis has been compared with experimental results showing excellent agreement. Moreover, since the package introduces resonances, we have investigated several different possibilities to choke off these resonances. It is shown that the common practice of inserting a damping layer just below the lid is often not effective. In particular, the importance of placing damping layers also on the side walls is demonstrated.

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3 MMIC commercial package The behaviour of a commercial MMIC package operating in the range 0-40 GHz has been investigated. It consists of a mechanical support of kovar with a fused quartz substrate above it. The area where the MMIC must be placed is enclosed between glass side walls and a top metallic lid. The latter is connected to a ground plane by via-holes through the glass walls. Bias lines and metal backed CPW terminations are also present. It is interesting to note that this package is substantially an open structure. Within the operating frequency range, the electromagnetic shielding is realized by metallic via-holes connecting the lid with the ground plane. In order to compare theoretical and experimental results, a simple microstrip transmission line, inserted between the input and output ports has been simulated.

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4 Coplanar waveguide to slotline transitions The uniplanar configuration has having a growing popularity in the area of microwave integrated circuits. Fundamentals uniplanar configurations are the coplanar waveguide (CPW) and the slotline(SL). Well known advantages with respect to the microstrip line configuration are: easy shunt and series mounting, elimination of via holes, reduced radiation loss (for CPW), use of thicker substrates overcompensate the necessity for air bridges to suppress the spurious slotline mode in the CPW. The lack of accurate design tools however has prevented an extensive application of uniplanar technology, of which the transition between CPW and SL is a key element. The FDTD method has been employed to analyze several types of CPW-SL transitions including the effects of the coaxial connectors, air bridges, shielding effects as well as interactions between discontinuities. Theoretical simulations show excellent agreement with experimental data appeared in the literature.

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5 Hybrid integrated circuit bonding The bonding wire interconnection is a key element for the fabrication of hybrid integrated circuits. It is employed to connect solid state devices to passive circuit elements as well as multichip modules. In spite of its small physical length, when millimeter-wave operations are required, the discontinuity introduced by the bonding wire can significantly affect the performance of the whole circuit. At the University of Perugia, the bonding wire interconnection has been studied from the point of view of its modeling and electrical characterization. In particular, two electrical models of the bonding wire have been developed. First, the Finite Difference Time Domain (FDTD) method has been adopted to rigorously analyze several bonding wire configurations (including multi-chip, single- and double-wire structures) and to produce reference results. Then, a quasi-static model of the bonding wire has been derived. This model is based on the representation of the structure with four uniform transmission line sections. Such an approach is suitable for commercial microwave CAD tools since the model parameters can be evaluated analytically from the dimensions of the structure.

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Figure 4: Realized bonding wire interconnection and comparison of the developed models with the measured data.

6 SPDT switch A millimeter-wave “Single Pole, Double Throw” (SPDT) switch, realized with a Si-MMIC microstrip technology has been studied. The analysis has been performed with both commercial, equivalent circuit based simulator and mixed electromagnetic-device simulator. The sinusoidal RF signal (ampl. 0.2 V, freq. 76 GHz) is injected at one end of the structure, and then forwarded along either output branches, depending on the bias supplied by the square-wave generator (ampl. 2.5 V, freq. 214 MHz). Microstrip stubs and matched loads complete the structure. To isolate the inactive branch, {p-i-n} diodes have been used, due to their high impedance in the off-state, and to the compatibility with the high-resistivity silicon, planar process. Feasibility of such Si-MMICs, suitable for operating frequencies up to 100 GHz, has been demonstrated.

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Figure 5: SPDT switch structure and performances: comparison between FDTD and hp-MDS simulations.

7 Quasi-optical frequency multipliers The basic quasi-optical frequency doubler is shown in Fig.6a. In this device the frequency multiplication is achieved by a diode bridge connected to the center of two �/2-dipoles. These dipoles are placed in a cross configuration (referred to as crossed dipole): the longest one receives the incoming power at the fundamental frequency (3.5 GHz in our prototype), while the shortest one transmits the generated power at the doubled frequency (7.0 GHz in our prototype). The advantages of such structure are the following. First, a good isolation between input and output signals is achieved since incoming and outgoing waves are orthogonally polarized. Second, the symmetry of the structure has been exploited to obtain a balanced topology which is characterized by a good conversion efficiency (only even harmonics are generated within the multiplier). However, the conversion efficiency of the above structure is intrinsically limited by the omnidirectional nature of both receiving and transmitting antennas. To overcome the above mentioned problem, the front-to-back ratio (and thus the directivity) of both receiving and transmitting dipoles has been increased by additional layers with parasitic elements. The resulting structure is essentially constituted by two Yagi-Uda antennas working at orthogonal polarizations. The new, multi-layer structure can be better understood with the example of Fig. 6b. In this case only two parasitic elements are adopted, one for each antenna of the frequency doubler. The longest element (left of the photograph) is used as a reflector for the fundamental frequency dipole; the shortest element (right of the photograph) is used as reflector for the doubled frequency dipole.

(a) basic crossed-dipole multiplier structure (b) improved Yagi-Uda multiplier structure

(c) conversion gain: experiments (d) radiation patterns at f1=2f0: experiments

Figure 6: Crossed-dipole quasi-optical frequency doubler.

8 GaAs MMIC prototypes Simple GaAs MMIC prototypes have been designed and experimented with the purpose to set-up and refine both the design tools and the test and measurement facilities. The chips have been manufactured by the Gec-Marconi foundry using the F20 process.

(a) low-pass filter with bonding pads (b) MeSFET with coplanar test ports

(c) low-pass filter measurement (d) MeSFET measurement

Figure 7: Manufactured GaAs MMICs and measured performances.

9 Radio-over-Fibre modulating devices Radio-over-Fibre (RoF) networks integrate optical fibre backbones and wireless cellular networks to provide broadband services to mobile, nomadic and fixed users. To generate and distribute RF-modulated optical signals, two schemes are used: direct modulation schemes, where a semiconductor laser is directly intensity-modulated by the RF-signal, and external modulation schemes, where the optical carrier generated by the laser is subsequently modulated by the RF-information through an electro-optical modulator. The main modulating devices of such schemes have been analysed and modelled: • the optical behaviour of an electro-absorption modulator has been analysed by using the

Compact-2D-FDTD method; • a circuit model has been developed and used to study the non-linear behaviour of directly

modulated lasers.

(a) structure. (b) 1st optical mode: dispersion curve and

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Figure 8: Electro-absorption modulator analysis for external modulation schemes: structure (a) and results (b).

(a) equivalent circuit model. (b) non-linear behaviour: comparison between

measurements (solid) and predictions (dashed) in two different bias conditions (38 mA circles; 50 mA triangles).

Figure 9: Semiconductor laser model for direct modulation schemes: schematic (a) and performance (b).

10 RoF-Predistortion circuit Radio-over-Fibre (RoF) networks performances are typically influenced by the non-linearities due to the modulating devices: lasers in direct modulation schemes, electro-optical modulators in external modulation schemes. A completely analog predistortion circuit has been developed to compensate the non-linearities generated by directly modulated semiconductor lasers. This low-cost circuit, of industrial interest, has a multi-decade correction capability, from 300 MHz up to 2 GHz; thus, it can be used effectively to reduce second and third order harmonic (HD2 and HD3) and intermodulation (IM2 and IM3) distortions generated inside the CATV, GSM/DCS, GPRS and UMTS bands. An average broadband compensation of at least 10 dB has been recorded during measurements; the compensation magnitude can be further enhanced by specifically tuning the circuit for CATV or cellular bands, alternatively.

(a) predistorter prototype. (b) predistorter schematic.

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Figure 10: Predistortion circuit: structure (a)-(b) and performance (c)-(d).

11 Analyis of the EM fields interaction with biological tissues

The Em field inside a tissue, rapresented with spherical cells has been computed. The Hodgkin-Huxley model has been implemented on the cell’s membrane by using the LE-FDTD method. Long simulation time has been avoided using a quasistatic FDTD technique and reducing the computational domain with the Periodic Boundary Conditions (PBC) based on the Floquet theorem. The computation has been carried out at mobile communication frequencies (GSM900, GSM1800). The basic structure is shown in Fig. 11. In Fig. 12 the electric field is showed on a plane parallel to the direction of propagation. . The modulus of the electric field along the centre of the structure is showed in Fig. 13.

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References [1] P. Mezzanotte, M. Mongiardo, L. Roselli, R. Sorrentino, and W. Heinrich,”Analysis of Packaged Microwave Integrated Circuits by FDTD,” IEEE Trans. Microwave Theory Tech., vol. MTT-42, Sept. 1994, pp.1796-1801. [2] P. Mezzanotte, M. Mongiardo, L. Roselli, R. Sorrentino, “FDTD Analysis of High Performance MMIC Package,” 1994 IEEE MTT-S International Symposium Digest, San Diego, May 1994, pp. 337-340. [3] P. Ciampolini, P. Mezzanotte, L. Roselli and R. Sorrentino, “Accurate and efficient circuit simulation with lumped-element FDTD technique,” IEEE Trans. Microwave Theory Tech., vol.MTT-44, no. 12, 1996. [4] P. Ciampolini, L. Roselli, G. Stopponi, “Integrated FDTD and Solid-State Device Simulation,”IEEE Microwave and Guided Wave Letters, Vol. 6, n. 11, Nov. 1996, pp. 419-421. [5] F. Alessandri, M.Dionigi, R. Sorrentino, and L.Tarricone, “Rigorous and Efficient Fabrication-Oriented CAD and Optimization of Complex Waveguide Networks,” IEEE Trans. Microwave Theory and Tech., vol. MTT-45, No. 12 , Dec.1997, pp. 2366-2374. [6] F. Alessandri,M. Dionigi, M. Mongiardo, and R. Sorrentino, “Efficient Full-wave Automated Design and Yield Analysis of Waveguide Components,” Int J. of RF and Microwave Computer –Aided Engineering, Vol.8 no 3, May 1998, pp. 200-207. [7] P. Ciampolini, L. Roselli, G. Stopponi, R. Sorrentino, “Global Modeling Strategies for the Analysis of High Frequency Integrated Circuits,” IEEE Trans. Microwave Theory Tech., vol. 47, n.6, June 1999. [8] S. Helbing, M. Cryan, F. Alimenti, P. Mezzanotte, L. Roselli and R. Sorrentino, “A Novel Crossed Dipole Structure for Quasi-Optical Frequency Doubler Applications,” in Proc. 29th Eu. Microw. Conf., München, Oct. 1999, pp. 193-196. [9] M. Cryan, S. Helbing, F. Alimenti, P. Mezzanotte , L. Roselli and R. Sorrentino, “Simulation, Measurements and Structure Improvement of Quasi-Optical Multipliers,” in Proc. 29th Eu. Microw. Conf., München, Oct.. 1999, pp. 37-40. [10] F. Alimenti, P. Mezzanotte, L. Roselli, R. Sorrentino, “Modeling and Characterization of the Bonding Wire Interconnection,” IEEE Trans. Microwave Theory Tech., vol. 49, n.1, Jan 2001, pp. 142-150. [11] F. Zepparelli, F. Alimenti, P. Bassi, P. Mezzanotte, L. Roselli, G. Tartarini and R. Sorrentino, “Rigorous Analysis of 3D Optical and Optoelectronic Devices by the Compact-2D-FDTD Method”, Optical and Quantum Electronics, vol.31, n.9/10, pp. 827-841, Oct. 1999 [12] A. Schiavoni, G. Emili, M. Francavilla, P. Bertotto, L. Roselli, R. Sorrentino “Electromagnetic field in a cluster of cells” BEMS, St. Paul, Minnesota, Giugno 2001

Structure of Organization

Organization of the Perugia research group

Ing. R. RussoTechnician

Responsable for the Clean room

Dr. M. StovaliTechnician

Public relationsSoftware communications

Dr. M. DionigiAssistant Researcher

Frequency-Domain Modelling Responsable

Dr. P. MezzanotteAssistant Researcher

Time-Domain Modelling Responsable

Dr. F. AlimentiContract Researcher

Microwave Circuits Responsable

Ing. F. ZepparelliPhD. Student

Microwave photonics

Ing. G. EmiliPhD. Student

Global Modelling strategies

Ing. L. RoselliAssistant ProfessorResearch Manager

Prof. R. SorrentinoMicrowave Electronic Laboratory Head

Facilities

CAD tools for MMICs Design 1. 3D-LE-FDTD + Compact-2D-FDTD elecromagnetic simulator developed at the University

of Perugia. 2. Agilent-EEsof ADS 1.5 3. hp-MDS 4. hp-Momentum 5. hp-EEsof Series IV 6. Microwave Office (AWR) 7. CST Microwave Studio 3.0 8. Eagleware 9. Cadence 2001

Test and measurement equipment 1. Clean room (30 square meters; class 10000, 1000 and 100 under laminar air flow)

equipped with photo-etching process for the manufacturing of microwave hybrid planar circuits.

2. Vector network analyzed Hewlett/Packard model hp-8720C; 45 MHz – 20 GHz. 3. Scalar network analyzer Hewlett/Packard model hp-8757A; up to 60 GHz. 4. Spectrum Analyzer Tektronik model 492P; up to 20 GHz. 5. Equipment for the manufacturing of bonding wire interconnections model West Bond

7400C (wedge-wedge wire bonder). 6. Power meter up to 20 GHz 7. Optical microscope 8. Probing station (Alessi) for on-wafer microwave measurements up to 40 GHz .

Figure 8: Clean room.

Key Personnel

Roberto Sorrentino Roberto Sorrentino received the Doctor degree in Electronic Engineering from the University of Rome "La Sapienza", Rome, Italy, in 1971, where he became an Assistant Professor of Microwaves in 1974. He was adjunct professor at the University of Catania, at the University of Ancona and at the University of Rome "La Sapienza" (1977-1982), where he then was an Associate Professor from 1982 to 1986. In 1983 and 1986 he was appointed as a Research Fellow at the University of Texas at Austin, Austin, USA. From 1986 to 1990 he was a Professor at the University of Rome "Tor Vergata". Since November 1990 he has been a Professor at University of Perugia, Perugia, Italy, where he was the Chairman of the Electronic Department and Director of the Computer Center (1990-1995). He is presently the Dean of the Faculty of Engineering. His research activities have been concerned with various technical subjects, such as the electromagnetic wave propagation in anisotropic media, the interaction of electromagnetic fields with biological tissues, but mainly with numerical methods for passive microwave structures and the analysis and design of microwave and millimeter-wave circuits. He is the author or co-author of about 70 technical papers in international journals and 100 refereed conference papers. He has edited a book for IEEE Press, and has recently co-authored a book on Advanced Modal Analysis. In 1990 he became a Fellow of the IEEE "for contribution to the modeling of planar and quasi-planar microwave and millimeter-wave circuits". From 1984 through 1987 he was the Chairman of the IEEE Section of Central and South Italy and was the founder of the local MTT/AP Chapter, that he chaired from 1984 to 1987. In 1993 he was the recipient of the MTT-S Meritorious Service Award. From Jan. 1995 through April 1998 he was the Editor-in-Chief of the IEEE Microwave and Guided Wave Letters. He presently serves on the Administrative Committee of the IEEE Microwave Theory and Techniques Society. He served the International Union of Radio Science as Vice Chair (1993-1996) then Chair (1996-1999) of the Commission D (Electronics and Photonics). Since 1996 he has been the Chairman of the Management Committee of the European Microwave Conference. He has been the Chairman of the European Microwave Association since its constitution in 1998. He is a member of the High Technical Council of the Italian Ministery of Communications.

Luca Roselli Luca Roselli was born in Florence, Italy, in 1962. He received the Laurea degree in electronic engineering from the University of Florence, Florence, Italy, in 1988. From 1988 to 1991 he worked at the University of Florence on SAW devices. In November 1991, he joined the Department of Electronic and Information Engineering at the University of Perugia, Perugia, Italy, as a Research Assistant. Since 1992 he has been an IEEE member. Since 1994 he has tought the ``Electronic Devices'' course at the same University. Since 1996 he has been in the reviewer list of Microwave and Guided Wave Letters (MGWL). Since 1998 he has been in the reviewer list of the transactions on Microwave Theory and Techniques (MTT). In the same year he has been included in the technical committee of the Electrosoft conference, of the European Microwave Conference and of the Microwave Theory and Techniques Symposium. His research interests include the design and development of microwave and millimeter-wave active and passive circuits by numerical techniques.

Paolo Mezzanotte Paolo Mezzanotte was born in Perugia in 1965. He received the Laurea degree in Electronic Engineering from the University of Ancona, Italy in 1991 with a thesis on FDTD analysis of the GTEM cell and the Ph.D. degree from the University of Perugia, Italy in 1997. Since 1992 he has been working on FDTD analysis of microwave structures in co-operation with the Department of

Electronic and Information Engineering of the University of Perugia, Italy. In November 1999, he joined the same Department as a Research Assistant. Paolo Mezzanotte is member of IEEE and reviewer for Microwave and Guided Wave Letters (MGWL) and Microwave Theory and techniques (MTT) transactions. His main field of interest is the application of numerical methods to the study of components and structures for microwave and millimetre-wave circuits.

Marco Dionigi He received the Laurea degree (cum laude) in 1992 and Ph.D. degree in Electronic Engineering in 1996 from the University of Perugia, Italy. In 1995 he was visiting scholar at the McMaster University (Hamilton, Ontario, Canada) working on fullwave optimization of waveguide structures. In 1997 he became researcher in electromagnetic fields at University of Perugia. His current research interests are in the field of microwave and millimeter-wave waveguide components modeling and optimization, antenna modeling, nondestructive testing and characterization of materials.

Federico Alimenti Federico Alimenti was born in Foligno, Italy, in 1968. He received the Laurea degree (cum laude) and the Ph.D. degree from the University of Perugia, Perugia, Italy, in 1993 and 1997 respectively, both in Electronic Engineering. In 1993 he held a scholarship from Daimler Benz Aerospace, Ulm, Germany, where he worked at the microwave and millimeter-wave department. In 1996 he was awarded as young scientist from URSI and he was guest scientist at the “Lehrstuhl fur Hochfrequenztechnik” of the Technical University of Munchen, Germany. In 1997 he visited the Technical University of Munchen, Germany in the frame of the Vigoni programme. Since 1993 Federico Alimenti has been with the Department of Electronic and Information Engineering of the University of Perugia. His research interests concern the FDTD simulation of microwave and millimeter-wave devices, the study of MMIC interconnection and packaging technologies and the design, realization and measurement of high frequency circuits. Since 1997 he has been in the reviewer list of Microwave and Guided Wave Letters (MGWL).

Fabio Zepparelli Fabio Zepparelli received the electronic engineering diploma degree from the University of Perugia, Italy, in 1998. Since the same year he has been at the Department of Electronic and Information Engineering (DIEI) of the University of Perugia, working towards the Ph.D. degree. Since 2000 he has been a tutor at the course of Microwaves for the telecommunication engineering diploma, held at the same University. His research interests are connected with the modelling of optical waveguides, lasers and microwave/photonic devices, mainly oriented to the development of Radio-over-Fibre systems. Recent publications cover the development of RF predistortion techniques for laser transmitters.