High Performance Microwave Acoustic Componentsfor Mobile Radios

F. Maximilian Pitschi, Jurgen E. Kiwitt, Robert D. Koch, Bernhard Bader, Karl Wagner,and Robert Weigel∗

EPCOS AG, Anzinger Strasse 13, 81617 Munich, Germany∗University of Erlangen-Nuremberg, Cauerstrasse 9, 91058 Erlangen, Germany

Email: Maximilian.Pitschi@epcos.com

Invited Paper

Abstract— Due to their outstanding characteristics acousticcomponents, such as SAW and — later on — also BAW filters,have been key components in wireless communication systemsfrom their very beginning. Regarding mobile radio handsets theprimary field of application of acoustic components has moved tothe RF front-end sections. Here, with the increasing demand forWCDMA-FDD capable mobile phones, duplexers, which allowto separate the TX and RX paths while being simultaneouslyconnected to the antenna, have become indispensable.

Regarding the specifications of the different operating bandsthe requirements on form factors, pass band characteristics,attenuations of the other signals (TX, RX, GPS, WLAN, . . . ), andisolations make them high performance components requiring asophisticated acoustic as well as electromagnetic design. Due tothe tough requirements imposed on the duplexers in terms of, e.g.,center frequencies, spacings of RX and TX bands, relative bandwidths, and shape factors, they also require application-specificacoustic structures.

As a consequence, in contrast to standard filters, duplexers arehighly optimized components using leading-edge technologies. Inorder to achieve the best balance of the characteristic propertiesof the acoustic structures regarding, for instance, their qualityfactors, pole-zero distances, and temperature coefficients, themanufacturing processes of the acoustic chips are designed forthe specific applications. Furthermore, optimizing the electricalcharacteristics a co-design of package and acoustic chip isapplied.

In this paper we provide an overview of the different types ofduplexers that have been the result of the focused specializationand optimization efforts. We classify them according to the typeof acoustic structures, matching circuitry, and package beingused. So far, purely SAW-based and BAW-based duplexers aswell as hybrid duplexers using both SAW and BAW chips havebeen reported. Most of the matching circuitry is integrated in, forinstance, an LTCC package using a distributed approach basedon a λ/4 line or a lumped approach comprising coils and/orcapacitors. Sometimes the matching circuitry is realized exter-nally on the PCB. Furthermore, we present recent integrationsuccesses, i.e., duplexer inserts, allowing the seamless integrationof duplexers into modules without drawbacks on performance ormodule thickness.

I. INTRODUCTION

In the field of microwave acoustic components the twodominating technologies are SAW (Surface Acoustic Wave)and BAW (Bulk Acoustic Wave). Currently these technologiesroughly cover the lower decade of microwave frequencies up

to 3 GHz. Regarding the market of RF components for mobileradios, SAW components have a considerably longer historyin comparison to BAW components. While SAW componentsare available on the market since more than two decades,BAW components started their success story about one decadeago. Nowadays, SAW and BAW components are used inevery mobile terminal for the second generation (2G) or thirdgeneration (3G) mobile technology, i.e., the Global Systemfor Mobile Communications (GSM) and the Universal MobileTelecommunications System (UMTS) using wideband codedivision multiple access (WCDMA), respectively.

After a brief historical review of the development of du-plexers in Sec. II, we will highlight the methods whichare applied for the design and realization of duplexers atEPCOS in Secs. III through V before we discuss the issuesof technology selection and development in Sec. VI. Fur-thermore, we review some basics on how we characterizeour components by measurement and simulation in Sec. VIIpreparing the presentation of typical examples of EPCOS’duplexers in Sec. VIII. Finally, we start with state-of-the-art fully matched lower temperature co-fired ceramic (LTCC)duplexers for discrete usage, see Sec. VIII-A, and low-profileHTCC duplexers for module applications, see Sec. VIII-B.After discussing a recent approach regarding a matched hightemperature co-fired ceramic (HTCC) duplexer in Sec. VIII-B.2, we leave the ”stand-alone” duplexers and present duplexerinserts, see Sec. VIII-C, as well as their designated future usein highly complex modules with integrated duplexers, suchas, the front-end module with integrated duplexers (FEMiD)introduced in Sec. VIII-C.

II. HISTORY

Already starting with first generation (1G) mobile phonesduplexers were required. Then, microwave ceramic (MWC)duplexers were used in mobile phones supporting, e.g., theAdvanced Mobile Phone Service (AMPS), the German B-net,or the Nordic Mobile Telefone (NMT). In the very beginningthe stringent requirements of low insertion loss in combinationwith high close-in attenuation as well as isolation could onlybe achieved by microwave ceramic duplexers [1]. Despite

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their outstanding properties regarding frequency stability overtemperature and power handling capability SAW duplexerssuperseeded them from the market of mobile phones. Theirobvious drawback was that they were bulky regarding boththeir form factors and thicknesses. Furthermore, there was nosignificant potential for miniaturization. Nevertheless, MWCduplexers are still relevant in the market of base stations [2].It was the development of high power durable metalizationsfor SAW devices that allowed to replace the dielectric filtersby low-loss SAW filters in the 1980s, which were used for thefirst SAW duplexers [1]. With SAW technology having a greatpotential of miniaturization, it was tipping point regarding theapplication of SAW duplexers in mobile phones and — lateron — terminals. Nowadays, this technology is used for nearlyevery WCDMA band. Later on, BAW duplexers followed inthe late 1980s and the early 1990s [3], [4], [5]. They are mainlyused for the WCDMA duplexers for band II.

III. DESIGN TECHNIQUES

Regarding the electrical performance the major design goalsof duplexers are low insertion loss (IL), high close-in at-tenuation as well as high isolation. Furthermore, the devicesshall be compact regarding both their form factors and theirthicknesses. Finally, their design shall be simple and robust toallow batch processing with high yield in the fabrication.

Both technologies allow the realization of different kindsof resonating building blocks which are essential to achievelow insertion loss. Up to now, the most important resonatingbuilding blocks have one or two resonant modes. Thus, bothSAW and BAW one-port resonators have one resonant moderesulting in a series resonance of high quality and — in com-bination with their inevitable static capacitance — a parallelresonance. Most often the one-port resonators operate as seriesand shunt resonators of ladder-type filters, but other topologiesare feasible as well.

In SAW components, building blocks with two or more res-onant modes are well established. The dual mode SAW (DMS)or sometimes called multi-mode SAW (MMS) structures arerealized by cascading several interdigital transducers (IDT)that are typically enclosed between two short-circuited reflec-tors [6], [7], [8], [9], [10]. Besides the filtering function DMSstructures can additionally provide a balun function and animpedance transformation. BAW components using buildingblocks with two resonant modes are rare. Coupled resonatorfilters (CRF) consist of two stacked BAW resonators whichare tightly placed one above the other. The thin coupling layerbetween the two resonators and both piezoelectric layers canbe used to tune the performance of the CRF, i.e., especially theamount of acoustic coupling of both resonators. CRF structurescan be used beneficially in filters with advanced isolationand stop band suppression requirements. Additionally, CRFscan easily be used to include a balun function in the filter.Challenging manufacturing demands in processing multiplestacked high precision thin films, however, make it hard tobring batch fabrication to good volume yield [11].

Using acoustic waves the wave lengths of which are byorders of magnitude smaller than the wave lengths of electro-magnetic waves at the same operating frequency, the buildingblocks of both SAW and BAW components are rather small,allowing the realization of very compact devices.

The standard impedance in RF front-ends is 50Ω in caseof single-ended operation. In case of balanced operation thedifferential mode is naturally referenced to 100Ω; likewise,the common mode is referenced to 25Ω.

A. Topology of RX and TX filtersStarting with single-ended duplexers, i.e., both the TX and

RX filters have single-ended inputs and outputs, the filters aretypically formed as ladder-type filters, which are composedof one-port SAW or BAW resonators. It should be noted thatbesides resonators other reactances can be used in addition toor instead of resonators. Nowadays, more and more duplexerswith balanced RX output are demanded. Therefore, in case ofSAW duplexers, MMS/DMS filters are introduced at the RXoutputs providing the required single-ended/balanced transfor-mation and, by the way, replacing some of the resonators.

B. Diplexing at antenna portIn the frequency range of the TX pass band (FTX) the

reflection coefficient of the RX filter itself is nearly -1, i.e.,a kind of short-circuit to ground. This would not allow theTX signal to be transmitted to the antenna port. Therefore,matching networks are needed, see Fig. 1.

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Fig. 1. Block diagram of a duplexer.

To allow for minimum insertion loss in both the TX andRX paths conditions (1) and (2) for reflection coefficients Γ1

and Γ2 at the frequency ranges of the RX and TX pass bands,FRX and FTX, respectively, have to be fulfilled. Reflectioncoefficients Γ1 and Γ2 are seen at the TX output and the RXinput looking from the junction point of the two signal paths,i.e., the antenna port.

Γ1(FRX) ≈ 1 (1)Γ2(FTX) ≈ 1 (2)

The most common matching configurations shall be brieflydescribed with reference to Fig. 1 with the two genericmatching networks M1 and M2 at the antenna port:

• Matching network M1 is absent, which is possible, sincenormally a SAW filter can be designed to fulfill (1). M2

consists of a line with a length of approximately λ/4for TX frequencies. Especially some years ago, whenthe form factors were bigger, this approach was verycommon. A variant of this matching topology, whichallows further miniaturization, is to replace the λ/4-lineby a Π-network of two shunt capacitors and a seriesinductor in between.

• M1 is a shunt inductor, and M2 consists of a shuntinductor and a series capacitor. The two coils are mergedtogether and the capacitor is normally integrated in theRX chip. This is a very compact approach.

Regarding the performance, both matching topologies arecomparably good. Nevertheless, there may be an advantagefor one of the approaches for a specific design goal.

C. Impedance matching at RX port and TX port

An optional impedance matching network at the RX outputport or at the TX input port might be advantageous asacoustic filters tend to have capacitive input impedances.Accepting additional matching components during the designand optimization phase of the filters may offer new degreesof freedom that outweigh the extra effort. Typically series orparallel inductors are required to match the impedance of thefilter in the frequency range of its pass band to the demandedimpedance of 50Ω for single-ended applications or 100Ω forbalanced applications.

D. Transfer function shaping

Unlike SAW filters for IF applications, RF filters prefer res-onating acoustic structures in order to allow for low IL. Theirfilter characteristics are obtained by appropriately dimension-ing the acoustic building blocks, such as, e.g., resonators, aswell as choosing the topologies in which they are connected.Most often the acoustic structures are connected in a ladder-type configuration. See, e.g., the popular zick-zack connectionof one-port resonators.

As a matter of fact the immittances of acoustic structuresand, as a consequence, the complete filter characteristics arerather sensitive with respect to extra reactances. Thus, bymeans of extra capacitors or inductors the transfer functionscan be largely modified. For instance, using inductors inshunt branches of ladder-type filters allows to shape thetransfer function ranging from (a) some sort of fine tuningby slightly modifying the pole-zero-distance of at least oneshunt resonator to (b) completely reviewing by introducingnew electromagnetic poles.

IV. MICRO-ACOUSTIC TECHNOLOGIES

Apart from these similarities there is one significant differ-ence in their intended main propagations of the acoustic wavesthat along with some processing details being rather specificfor EPCOS are discussed in the next two sections.

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A. SAW technologyWith the intended main propagation of surface acoustic

waves being in the x-y-plane a horizontal design of theelectrodes that are deposited on the surface of a piezoelectriccrystal is required. Many degrees of freedom arise allowingto control, for instance, the resonance frequencies as well asthe spacing of resonance frequencies. Typical piezoelectriccrystals are, e.g., LiTaO3, LiNbO3, and quartz. We use100 mm wafers, the main patterning of which is performedby lift-off and wet etching.

B. BAW technologyIn contrast to SAW techology, the intended main prop-

agation of bulk acoustic waves is in z-direction. Thus, avertical design of the layer stack forming the BAW structuresis required. There exist at least two essentially different ap-proaches: solidly mounted resonators (SMRs) and membrane-type resonators (FBARs). Both feature a piezoelectric layer,such as, e.g., AlN, which is enclosed between two electrodesfollowed by acoustic mirrors. Both types of resonators realizethe acoustic top mirror by the interface to the air. While themembrane-type resonators also achieve the reflection of theacoustic waves at the interfaces to the air serving as an acousticmirror, SMRs use a sandwich of layers with alternating highand low acoustic impedances as a distributed acoustic mirror.At EPCOS, we use 200 mm wafers featuring AlN on Si aspiezoelectric material. Due to their moderate requirement onthe shapes of the BAW structures, the main patterning can beperformed by dry etching.

V. PACKAGING TECHNOLOGY

Fig. 2 shows the package road map of duplexers. Meanwhilethe market for duplexers has been split into fully integratedduplexers for discrete usage in case of single or dual bandapplications and low-profile duplexers for module duplexersin case of triple band and more sophisticated applications.

Due to difficulties in achieving small package sizes (withdimensions smaller than 3 × 3 × 1mm3) and performancelimitations caused by the inductive behavior of the bonding

wires, the major manufacturers of SAW components switchedfrom ceramic packages with wire bonding to flip-chip bondingin the first years of this decade.

At EPCOS a fundamentally new approach, i.e., the so calledChip Sized SAW Package (CSSP), was successfully imple-mented. Meanwhile three generations of CSSP technologies— CSSP1, CSSP2 (also known as CSSPlus), and CSSP3 —have been introduced in the market. All of them feature aflat cofired ceramic interposer and solder balls for electricallyconnecting the pads on the SAW chip with the traces of theinterposer. They differ primarily in the way in which theexistence of the crucial cavity between chip and substrate isensured, while reducing the form factor and improving theratio of area offered for the acoustic structures to total arearequired. For more details refer to Feiertag et al. [12].

VI. TECHNOLOGY SELECTION AND DEVELOPMENT

Table I shows the frequency ranges of the TX and RX passbands, FTX and FRX, respectively, of the WCDMA bands Ithrough XV in order to provide an impression of the pluralityof the characteristics of the various WCDMA bands. Thefrequency ranges are given in terms of the correspondinglower and upper cut-off frequencies, f1,TX and f1,RX aswell as f2,TX and f2,RX, of the TX and RX pass bands,respectively. Obviously, there are significant differences interms of center frequencies as well as absolute bandwidthsand relative bandwidths. Summarizing the center frequencies,absolute (relative) bandwidths, and absolute (relative) duplexdistances roughly vary in the ranges of 707 MHz–2655 MHz,10 MHz–75 MHz (1.2 %–4.3 %), and 10 MHz–355 MHz (1 %–18.4 %), respectively.

Regarding Table I the relative duplex distances, i.e., thespacings of RX and TX pass bands, listed in the fourth columnof the table turned out to be the primary parameter for theselection of the micro-acoustic technology. Of course, thecenter frequencies and relative band widths are important, butin comparison with the significance of the duplex distance theyend up as secondary parameters.

According to their relative duplex distances, d, the WCDMAduplexers are assigned to three groups. Groups 1, 2, and 3,which are listed in the fifth column of the table, denote small(d < 1.75%), intermediate (1.75% < d < 5%), and large(d > 5%) relative duplex distances.

In case of small duplex distances, see Group 1, the duplexdistance is shared by fabrication margins and frequency shiftsdue to the range of operating temperatures as well as resonatorand design parameters such as coupling coefficients, qualityfactors, additional passive components that affect the roll-offand slope steepness of both the right hand side TX pass bandedge as well as the left hand side RX pass band edge. In caseof large duplex distances, see Group 3, achieving sufficientclose-in rejection as well as IL becomes difficult with too littlecoupling coefficients.

TABLE IKEY CHARACTERISTICS OF THE VARIOUS WCDMA BANDS

REGARDING ACOUSTIC COMPONENTS [13].

Band FTX [MHz] FRX [MHz] d [%] Group

I 1920–1980 2110–2170 6.4 3

II 1850–1910 1930–1990 1.0 1

III 1710–1785 1805–1880 1.1 1

IV 1710–1755 2110–2155 18.4 3

V 824–849 869–894 2.3 2

VI 830–840 875–895 4.1 2

VII 2500–2570 2620–2690 1.9 2

VIII 880–915 925–960 1.1 1

IX 1749.9–1784.9 1844.9–1879.9 3.3 2

X 1710–1770 2110–2170 17.5 3

XI 1427.9–1452.9 1475.9–1500.9 1.6 1

XII 698–716 728–746 1.7 1

XIII 777–787 746–756 2.7 2

XIV 788–798 758–768 2.6 2

Each WCDMA duplexer application exhibits a unique com-bination of filter requirements like filter bandwidth, duplexdistance, center frequency, and frequency stability. So far, thereseems to be no universal micro-acoustic technology availableto fulfill the requirements of all 3G/4G frequency bands.At EPCOS both micro-acoustic technologies are available.We consider the two micro-acoustic technologies, i.e., SAWand BAW, as complementing, rather than as competing. Thecritical question about which technology in combination withwhich amendments matches best the needs of a specificWCDMA application, however, remains.

Right now, there seems to be no general answer in sight,since, firstly, there are many trade-offs that have to be consid-ered. Although the basic filtering function is being determinedby the acoustic die, the trade-offs extend to the packagetechnology and the design techniques. The development ofduplexers is typically based on a co-design of the acousticdies and the packages.

Secondly, the portfolio of processes is continuously im-proved and extended to meet upcoming requirements. Eachmicro-acoustic technology, i.e., SAW or BAW, offers a specificcombination of resonator characteristics. The most importantresonator characteristics are the quality factor, the piezoelectriccoupling coefficient or pole-zero-distance (PZD), the temper-ature coefficient of frequency, the frequency accuracy, and theresonance frequency.

The following examples are typical. They are organizedin the same way: If a modification of a device feature is

demanded in a certain direction, e.g., increased or decreased,an appropriate techology development is triggered in orderto implement the change regarding the physical context. (a)Filters with larger bandwidth trigger the technology develop-ment of new piezoelectric materials in order to obtain a higherpiezoelectric coupling. (b) In case of a demand for lowerinsertion loss, an improved electrode system, for instance,contributes to reduce losses in the signal path. Of course,there are many other loss mechanisms. (c) For duplexers withsmaller duplex distances a bunch of technology developments,such as, the overall loss minimization within the acousticstructures in order to increase the quality factor of the res-onators, the reduction of fabrication tolerances or trimming,or the introduction of functional layers and multi-layer thinfilm technology for the reduction of TCF, help.

Having analyzed the physical contexts of a device featureappropriate measures can be taken to optimize the set oftechnologies in order to adapt the resonator characteristics tothe specific needs of a given WCDMA standard. Thus, anoptimum solution can be obtained by considering the differentduplexer requirements and carefully selecting the technologies.As a matter of fact, sometimes the optimum results from thecombination of the two micro-acoustic technologies.

VII. MEASUREMENT AND SIMULATION

In our era of ever fast change the time-to-market of new orenhanced products is absolutely crucial. One key to successhas been the consequent use of virtual prototyping.

At first glance, it demands considerable extra effort forsetting up and maintaining a sophisticated simulation envi-ronment. Regarding the highly specialized market for acousticcomponents major tasks associated with the simulation envi-ronment comprise the selection of general purpose, commer-cial simulation tools, the development of specialized, propri-etary simulation tools and, most important, the overall model-ing of the device under test (DUT). The latter includes a rig-orous characterization of the DUT by measurement. Further-more, it requires the characterization of many test structuresby measurement and simulation for, e.g., the determination ofmodel/material parameters and the physical/chemical analyticsof geometric parameters as well as material properties.

It is a common misconception that the sole use of ex-pensive, general purpose, commercial simulation tools is theguarantee for decent simulation accuracy. From the pointof view of a user of such a tool the simulation accuracyis neither just the accuracy of the numerical solution oftypically large mathematical systems of equations nor thedifference between measurement and simulation for usuallyrather simple, extremely fine discretized configurations forwhich analytical solutions exist. The simulation accuracy boilsdown to the agreement of measurement and simulation withinthe engineer’s problem domain having rather limited resourcesin terms of computation time or main memory available.

Regarding the early 1990s, expensive workstations couldhardly perform numerical field simulations of simple, smallHTCC packages of GSM RX filters due to their limited main

memory. Since then the size of main memory increased byabout three orders of magnitude. Also the efficiency of thesimulation tools evolved dramatically. Nevertheless, limitedmain memory and computation time still exist due to theincreased complexity of the components. The complexity ofthe problems to be simulated stepped with the transition fromsingle filters to duplexers. To stay with that picture, it actuallyjumped with modules with integrated duplexers.

We stress the classical engineering approach of modular-ization in order to cope with the complexity of the overallproblem. For this purpose the search for interfaces and thestrict compliance with them are crucial to break down theoverall system into subsystems of manageable complexity.

A. Neutral measurement setupRegarding the measurement of our components we favor

a neutral test environment, which allows the precise electricalcharacterization and operation of the filter in different applica-tion situations. There are neither intentionally nor unintention-ally artifacts included in the test setup (e.g., the sample printedcircuit board (PCB)) or the application setup (e.g., PCB ofwireless terminal). The measurements throughout the wholepaper have been obtained using neutral PCBs, the electricalproperties of which have been optimized in order to yield (a)minimal reflections, (b) minimal feed-through or cross-talk,and (c) minimal losses over the complete signal paths fromthe SMA connectors to the landing areas of the footprint [14],[15], [16].

B. Sophisticated simulationUsing our highly modularized and optimized simulation

environment the components to simulate, e.g., duplexers ormodules, are pieced together of structures whose dimensionsspan about five orders of magnitude from fractions of mi-crometers to centimeters. Again the key to success is theidentification of interfaces and the compliance with them. Weuse the standard 50Ω interface as in the case of the neutralPCBs. The components typically provide the standard 50Ωinterface as in the case of fully matched LTCC duplexers ornon-standard interfaces as in the case of low-profile HTCCduplexers that need external matching components.

The interface between the acoustic structures and the sur-rounding connecting structures is very fine grained and tightlycoupled. Nevertheless, the chosen interface allows to sepa-rately determine the electrical characteristics of the acousticstructures, such as SAW and/or BAW resonators, as well asthe electrical characteristics of the connecting structures. Thelatter cover everything from the bus bars of SAW structures ortop/bottom electrodes of BAW structures to the traces of thePCB. Doing so is of crucial importance, since the immittancesof the acoustic structures, which make up the filter character-istic, are considerably masked by parasitic reactances [17].

Using consequent modularization we can obtain simulationsin an efficient way with different levels of accuracy. In orderto avoid overloading of Figs. 3 and 4, we only added thecorresponding simulations to Figs. 7 through 9, which show

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Fig. 3. Measured scattering parameters: transfer functions of TX (top) and RX (middle) filters as well as the isolations between TX inputand RX output (bottom) of duplexers for WCDMA bands I (magenta), II (red), IV (black), V (blue), and VIII (green).

typical comparisons of measurements and simulations. Byinspection, we find good agreement of measurement andsimulation.

VIII. TYPICAL EXAMPLES

In the following sections, we will discuss a variety ofcomponents. We start in Secs. VIII-A and VIII-B with thetwo classical stand-alone duplexers for discrete usage, i.e.,the fully matched LTCC duplexers and the unmatched low-profile HTCC duplexers. We will not consider earlier solutionswith separate RX and TX filters. Although HTCC duplexersare usually unmatched, we present a result of a feasibilitystudy of a matched HTCC duplexer. In Sec. VIII-C, whichbridges the gap between individual duplexers and duplexerbanks, we look at the recent approaches of duplexer inserts aswell as their designated future usage in sophisticated front-endmodules (FEMs).

A. Fully matched duplexersIn this section a selection of the classical, fully matched

LTCC duplexers being in volume production is presented.These duplexers are housed in CSSPlus packages. Further-more, they feature single-ended/single-ended TX filters and

single-ended/balanced RX filters. As usual, the single-endedports are matched to 50Ω, the balanced ports to 100Ω.Stressing the trend from individual duplexers to ”banks” ofduplexers as required in modern wireless multi-band terminals,the scattering parameters of all duplexers are shown in Figs. 3and 4. By the way, even showing only five duplexers theplots also nicely visualize the fragmentation of the frequencyspectrum as well as the differences between the variousWCDMA bands regarding center frequencies, band widths,and duplex distances.1) Duplexers for 1GHz applications: Our components

B7683 [18] and B7681 [19] are WCDMA band V andband VIII duplexers, respectively. The SAW filter structuresfor both the corresponding TX and RX filters are realizedon a single LiTaO3 chip. They are mounted flip-chipped on3025 LTCC packages, which provide the required matchingelements. They are fully matched without the need for addi-tional components.2) Duplexers for 2GHz applications: In the 2 GHz fre-

quency range duplexers B7696 [20], B7680 [21], andB7686 [22] handle WCDMA bands I, IV, and II, respectively.

WCDMA band I duplexer B7696 [20] is composed of

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Fig. 4. Measured scattering parameters: reflection coefficients TX input (top), antenna (middle), RX output (bottom) of duplexers for WCDMAbands I (magenta), II (red), IV (black), V (blue), and VIII (green).

a single LiTaO3 chip, which holds the TX and RX SAWfilters. It is flip-chip mounted on a small 2520 LTCC packageproviding most matching elements required. Only one externalparallel coil of about 10 nH is needed at the balanced RX portto fully match the duplexer.

WCDMA band IV duplexer B7696 [21] has a 3025 LTCCpackage, which holds all necessary matching elements. Asalready outlined in Table I, duplexers for bands I and IVmainly differ in the pass bands of the TX filters. The passband of the RX filter of band IV duplexer is a subband of theband I RX filter. Seeking for minimal IL we didn’t reuse theRX filter of the band I duplexer, but implemented an optimizedRX filter. Therefore, the frequency characteristics of the RXfilters differ in both their center frequencies and their shapesas can been seen in the plot in the middle of Fig. 3.

At EPCOS, WCDMA band II duplexer B7686 [22] is anexcellent example regarding the selection and optimization ofdesign techniques, micro-acoustic technology, and packagingtechnology. Being a 3025 BAW/SAW hybrid duplexer, it com-bines a single-ended TX BAW die and a single-ended/balancedRX SAW die. The duplexer needs only one parallel coil of15 nH at the balanced RX port to achieve full matching. This

Fig. 5. Photograph showing top view and footprint of WCDMAband II hybrid BAW/SAW duplexer B7686.

combination of a BAW and a SAW die is optimal, since theBAW technology offers low insertion loss (IL) in the single-ended TX path, while the SAW technology is appreciatedfor providing the balun functionality required to obtain thebalanced RX output. In Fig. 5 the two dies can be clearlyidentified on the package. The electrical performance of theduplexer can be observed in Figs. 3 and 4.

Fig. 6. Planar inductors on an HTCC substrate. An extensionof EPCOS’ CSSP3 package technology enables the integration ofminiaturized inductors on top of the ceramic substrates.

B. Low-profile duplexers1) Unmatched HTCC duplexers: Due to the high ohmic

losses of tungsten and molybdenum which are used as con-ductor materials in the HTCC technology, the integration ofreactive elements with high quality factors is not possible. Butdue to the higher mechanical stiffness of HTCC in comparisonto LTCC, it is a good choice as a thin multilayer substrate forthe realization of low-profile duplexer packages.2) Matched HTCC duplexers: To overcome the limitation

of classical HTCC packages, which normally do not provideintegrated reactive elements of sufficient quality factor, anextension of EPCOS’ CSSP3 technology was developed. Itallows to pattern miniaturized spiral coils on the top surfaceof an HTCC substrate, as depicted in Fig. 6. The use of copperas conducting material offers the fabrication of inductors witha high Q factor. But, being on the top side of the HTCCsubstrate, the planar coils are in close vicinity of the filterstructures after flip-chip mounting of the die. The resultingstrong electromagnetic coupling between the coils and the chipmetalization as well as among the coils have the potentialto degrade the duplexer performance. Therefore, the designof such duplexers is very challenging. It can be done byprecise electromagnetic modeling of the component [23]. Weused the extended CSSP3 technology to realize a prototypicalband I SAW duplexer with an 2520 HTCC package. Due tothe new integration approach, we have been able to integratefour planar coils which are used for enabling the duplexingand forming the transfer function in a package with an overallheight of only 450 µm. Only one external coil at the balancedRX port is needed to obtain fully matched operation. Themeasured and simulated transfer functions of our low-profileband I SAW duplexer are shown in Fig 7.

C. Modules with integrated duplexersFor the integration of duplexers into modules stringent

thickness requirements of the complete module need to beconsidered. The overall thickness is limited to 1.2 mm [24].Therefore, low-profile matched duplexers or duplexer in-serts [25], [26], [27] are used. Here, duplexer inserts are acombination of a low-profile, unmatched HTCC duplexer anda building block inside the host module, which holds therequired passive components needed for operation. Therefore,the HTCC interposers only rewire from the fine footprint of

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Fig. 7. Comparison of simulated and measured scattering parametersof a matched HTCC duplexer for band I: pass bands (top) andisolation (bottom). i

the under bump metalization (UBM) on the acoustic dies tothe rather coarse footprint of the soldering pads on the LTCC.1) Duplexer banks: Being an integrated part of a module,

the isolated design and characterization by measurement ofduplexer inserts demand for some additional effort. In orderto provide a realistic module environment, test modules areused [25], [26]. These modules contain one or more duplexerinserts. Instead of the additional module components, such asswitches, LC filters and so on, electrically neutral connectionsare inserted. They connect the internal interfaces of the du-plexer inserts with the soldering pads of the module. Beingthe extension of the feed lines on the test PCB, the layout ofsuch internal feed lines has to be done with the same care.Especially, the PCB/module transition has to be optimized forlow reflection. The impedances of the lines have to be closeto the values of the PCB lines. Additionally, the crosstalkbetween the lines has to be minimized. The measured andsimulated results of a duplexer bank with three duplexer insertsare shown in Figs. 8–9 for WCDMA bands I and V and in [25](see Fig. 9) for WCDMA band II.2) FEMiDs: The front-end module with integrated du-

plexers (FEMiD), the block diagram of which is shown inFig. 10, supports four GSM and five WCDMA bands. Withsupporting the various bands being the duty of the filteringsection, the filtering section can be considered as the coreof the module. By co-banding the number of filters requiredis minimized. Where applicable due to compatible frequencybands, co-banding requires RX paths to handle both GSM andWCDMA signals. Thus, the RX filters of the duplexers forWCDMA bands I, II, IV, V, and VIII need to meet the GSMspecifications as well. With duplexers allowing co-banding,i.e., the common use of one band in two different mobiletelephone standards by one component, they also provide the

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RX filtering of the three GSM bands: GSM 850, GSM 900,and GSM 1900. Just one external RX filter is required forGSM 1800. The TX filtering of the four GSM bands is realizedby integrated LC filters.

Looking at the module shown in Fig. 10 five (strictlyspeaking four and a half) integrated duplexers for WCDMAbands I, II, IV, V, and VIII can be seen. With the RXpass bands of WCDMA bands I and IV being identical, one”triplexer” is integrated instead of two duplexers. It consists ofone RX filter for bands I and IV and two TX filters for bands Iand IV. Thus, one RX filter can be avoided. The triplexing

E

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GPIO Decoder

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Fig. 10. Block diagram of quad GSM and quint WCDMA FEMiD.

function between these bands is achieved by an appropriatematching circuit.

The filtering section is encapsulated by two switchingsections that — on the left hand side — connect the TX inputsof the module to the TX inputs of the filters or duplexers and— on the right hand side — connect the antenna terminalsof the module to the antenna terminals of the duplexers. Thebalanced RX outputs of the filters and duplexers are routeddirectly to the soldering pads of the module.

By inspection, the module can be split into two signalprocessing blocks for low and high frequency bands. Thus,there are two TX inputs for two multi-mode power amplifiersand two antenna terminals. Splitting the antenna terminalsbased on the bands (e.g., 1 GHz and 2 GHz) shall ease therealization of the antennas by the manufacturer of the mobilephones.

Due to the two modes and the many bands supported,the complexity of the module is tremendous. Regarding onlythe requirements on the linear electrical performance, themodule covers more than 200 specification items. In additionto the linear electrical performance, a variety of non-linearelectrical characteristics is considered. Some of the electricalspecification items depend on certain relations of the phase orphases of signals. For these specification items the integrationinto a module providing well-defined electrical lengths of the

Band IDPX RX

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Fig. 11. Schematic top view of quad GSM and quint WCDMAFEMiD: drawn to scale (left) and magnified (right).

connecting traces as well as well-defined component values isparticularly advantageous.

Regarding the number of components that are integratedin the module, the simplification achieved for the logistics ofthe original equipment manufacturers by the reduction of thebill of material is significant. Furthermore, the effort that theoriginal equipment manufacturers have to spend on hardwaredevelopment is considerably reduced, because besides attenu-ation and isolation requirements, many interoperability issuesare already considered on component level within the module.Finally, with this sophisticated filtering section covering fiveWCDMA bands and four GSM bands, one single, compactmodule measuring 8.7×6.2×1.2mm3 is able to cover multiplemarkets, such as North America and Europe.

IX. CONLUSION

We provided an overview of the evolution of acousticcomponents focusing on duplexers. Over the last years, weactively contributed to the evolution from single duplexers tomulti-band multi-mode FEMs with integrated duplexers. Theresult has been a plurality of duplexers for discrete usageand module applications. Giving an outlook, we presented asophisticated FEMiD supporting five WCDMA and four GSMbands. Looking backwards there has been a lot of challenges.With the ever increasing complexity of the RF front-ends, thereseem to be many interesting tasks on which to work in future.

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