1、IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 61, NO. 1, JANUARY 2013 393High- Narrowband Tunable ComblineBandpass Filters Using MEMSCapacitor Banks and PiezomotorsSiamak Fouladi, Member, IEEE, Fengxi Huang, Student Member, IEEE,Winter Dong Yan, Member, IEEE, and Raafat R. Mansour, Fell
2、ow, IEEEAbstractThispaperpresentsthedesignandimplementationofa new class of evanescent tunable combline bandpass filters basedon electronic tuning with the use of RF microelectromechanicalsystems (RF-MEMS) capacitor banks and also mechanical tuningusing piezomotors. The use of microelectromechanical
3、 systemstuning circuit results in compact implementation of the proposedfilters with high- and near to zero dc power consumption.The proposed filter structures consist of combline resonatorswith tuning disks that are either mechanically moveable usingpiezomotors or are loaded with RF-MEMS capacitor
4、banks.Two- and six-pole tunable bandpass filters are designed andmeasured based on the proposed tuning concept. The two-poletunable filter operates at 2.5 GHz with a bandwidth of 22 MHzand demonstrates a tuning range of 110 MHz, while the qualityfactor is better than 374 (1300374 over the tuning ran
5、ge). Thesix-pole tunable filter with RF-MEMS capacitor banks operatesfrom 2.634 to 2.59 GHz (44-MHz tuning range). The proposedtunable filter structures can also be implemented using alternativetechnologies such as bariumstrontiumtitanate varactors.Index TermsCombline filter, tunable filters, high ,
6、electronictuning, mechanical tuning, RF microelectromechanical systems(RF-MEMS), piezomotor.I. INTRODUCTIONAS WIRELESS devices become more and more compact,the developmentofinexpensive and miniaturized tunablefilterswithasuperiorperformanceiscrucialnotonlyformobilehandheld devices, but also to wirel
7、ess infrastructure equipment.It can benefitfromtunablefilter technologies in three differentareas; first, installing wireless infrastructure equipment, such asa remote radio unit (RRU) on top of a 15-story high commu-nication tower, is a very costly job. By using tunable filters,one installation can
8、 serve many years since if there is a needto change the frequency or bandwidth, it can be done throughManuscript received July 11, 2012; revised October 02, 2012; accepted Oc-tober 15, 2012. Date of publication December 04, 2012; date of current versionJanuary 17, 2013. This paper is an expanded pap
9、er from the IEEE MTT-S In-ternational Microwave Symposium, Montreal, QC, Canada, June 1722, 2012.S.FouladiiswiththeWirelessSemiconductorDivision,AvagoTechnologies,San Jose, CA 95131 USA (e-mail: ).F. Huang and R. R. Mansour are with the Department of Electrical and Com-puter Engineering, University
10、of Waterloo, Waterloo, ON, Canada N2L 3G1(e-mail: fxhuanguwaterloo.ca; rmansouruwaterloo.ca).W. D. Yan is with the Research and Development Center, Huawei Technolo-gies, Kanata, Canada K2K 3J1 (e-mail: ).Color versions of one or more of the figures in this paper are available onlineat http:/ieeexplo
11、re.ieee.org.Digital Object Identifier 10.1109/TMTT.2012.2226601remote electronictuning, ratherthan installing anewfilter.Sec-ondly,inurbanareas,thereisverylimitedspaceforwirelessser-vice providers to install their base stations due to expensive realestates and/or maximum weight loading constrains on
12、 certaininstallation locations such as light poles or power lines. There-fore,onceaninstallationsiteisacquired,itisnaturalforwirelessservice providers to stuff as many functions, such as multistan-dardsandmultibands,intoonesiteaspossible.Atunablefilterisa key element to enable such possibility. Fina
13、lly, the frequencyspectrum is a very limited and expensive resource. To constructa wireless network that covers large geographic locations, it isnot quite unusual for one wireless service provider to use a dif-ferent frequency spectrum and bandwidth at different locations,even within one single netw
14、ork. This complex frequency spec-trum allocation will require many different fixed filters to bebuilt. However, by using tunable filters, it is quite possible tojust use one type of filter to construct the whole network.Various tuning techniques have been developed to constructtunable filters. Mecha
15、nical tuning 1, 2 magnetic tuning 3,and electrical tuning 4, 5 are the most common. In termsof quality factor, power-handling capability, and linearity, me-chanical tuning is superior to the other two tuning techniques.Unfortunately, due to their bulky size, heavy weight, and lowtuning speed, mechan
16、ically tunable filters have limited appli-cations. Microelectromechanical systems (MEMS) technologyhas the potential to produce highly miniaturized tunable filters613. Planar tunable filters employing MEMS devices arereported in 68. These filters cover a frequency range from1to6GHzwithfilter values
17、between 85250. Since the RFmicroelectromechanical systems (RF-MEMS) devices havevery low insertion loss, of these planar filters are limited bythe resonator . In addition to a relatively low quality factor,they require complicated MEMS fabrication for monolithic in-tegration of MEMS devices with fil
18、ters. A variety of nonplanartunable filters using RF-MEMS technology have been reportedin 911. The evanescent-mode cavity filter in 9 utilizescapacitive RF-MEMS switch networks and operates from 4.07to 5.58 GHzwitha from 300 to 500. Tunable dielectricresonator filters using RF-MEMS devices with exce
19、ptionalvalues above 500, over the entire tuning range, are reportedin 10. Although these filters have higher values comparedto the planar tunable filters, they still require complicatedRF-MEMS fabrication and assembly of the tuning elementsinside the cavities. Tunable filters using substrate integra
20、tedwaveguide (SIW) technology and off-the-shelf RF-MEMS0018-9480/$31.00 2012 IEEE394 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 61, NO. 1, JANUARY 2013Fig. 1. Combline tunable resonator with tuning disk.switches are presented in 12 and 13. The two-pole filter in13 operates from 1.2 t
21、o 1.6 GHz with valuesfrom93to 132over the tuning range. A novel tuning approach of comblineevanescent-mode cavity filters using commercially availableRF-MEMS switches is reported by the authors in 14. Atwo-pole tunable bandpass filter is demonstrated that achievesa high- above 374 over the entire tu
22、ning range from 2.503to 2.393 GHz (110-MHz tuning). In this paper, the concept isthoroughly analyzed and we also expand the proposed tuningconcept to higher order combline filters with more stringentrequirements for multiband wireless applications. For the firsttime, we present six-pole tunable filt
23、ers using both RF-MEMStuning circuits and piezomotors with superior performance interms of values.II. PROPOSED TUNING CONCEPTFig. 1 shows the 3-D view and cross-sectional view of a con-ventionalcomblineresonator.Theresonatorcanbetunedbyad-justing the gap between the metallic post and tuning disk. Th
24、iscanbeachievedbymanualtuningusingascreworbyautomatictuning using a driving mechanism such as motors. The use ofconventionalmechanical tuning using stepper motors can resultinfinetuningsteps,high-powerhandling,andhigh ;however,these motors are usually very expensive and bulky, increasingthe total we
25、ight and size of the tunable filter. In this paper, weare proposing the use of inexpensive tuning mechanisms basedon RF-MEMS switched capacitor banks and piezomotors.Fig. 2 demonstrates the structure of the proposed tunable res-onatorbasedonRF-MEMSswitchedcapacitorbanks.Asshowninthisfigure,thetuning
26、diskisisolatedfromthecavitywallwithaTeflon spacer. An RF-MEMS capacitor bank is implementedon a printed circuit board (PCB) and is assembled on top of thecavity. The tuning disk is connected to the PCB board througha threaded insert on the PCB board. The variable loading ef-fect of the capacitor ban
27、k on the tuning disk is used to tunethe resonator. A simplified schematic view and the equivalentcircuit diagram of the RF-MEMS tuning circuit is presented inFig. 3. The switched capacitor bank consists of high- capaci-tors in series with RF-MEMS contact type switches. By turningthe MEMS switches on
28、 and off, using a dc actuation voltage,it is possible to change the value of the capacitor bank and ad-just the resonant frequency of the cavity. The use of RF-MEMSFig. 2. Schematic drawing of the proposed tunable resonator.Fig. 3. (a) Schematic diagram of the RF-MEMS tuning circuit and (b) the cir-
29、cuit model.switched capacitor bank results in improved performance interms of , insertion loss, power handling, and linearity for theproposed tunable filter.III. TUNABLE RESONATORA tunable resonator is designed using the proposed struc-ture and the tuning circuit shown in Fig. 3. The capacitor bankc
30、onsists of four high- capacitors in series with fourRF-MEMS switches. Each one of the MEMS switches has aseparate bias voltage and can be actuated separately. Using thiscircuit,theresonatorcanbetunedto16differentstatesbasedonthevalueofthecapacitorbank.Fig.5showsthesimulatedreso-nance frequency of a
31、single resonator for different values of thecapacitor bank when pF, pF, pF,and pF. As seen in this figure, a tuning range of168 MHz from 2.5 to 2.332 GHz is achieved.A photograph of the assembled tunable resonator is shownin Fig. 4(a). The resonator was machined from copper and in-side the cavity wa
32、s sputtered with silver for a higher value.The tuning circuit was mounted on top of the lid and glued withsilver epoxy. Fig. 4(b) shows the RF-MEMS capacitor bank.High- ( at 2.5 GHz) multilayer ceramic capaci-tors from Johanson Technology (S-series) and Radant single-polesingle-throw(SPST)RF-MEMSswi
33、tches(RMSW101)aremounted on a Rogers RO4350 PCB board. Each MEMS swicthisactuatedseparatelywithanactuationvoltageof90Vandzerodc current (near to zero dc power consumption). The total sizeFOULADI et al.:HIGH- NARROWBAND TUNABLE COMBLINE BANDPASS FILTERS USING MEMS CAPACITOR BANKS AND PIEZOMOTORS 395F
34、ig.4. (a)Proposedtunableresonator.(b)RF-MEMSswitchedcapacitorbank.Fig.5. Simulatedandmeasuredresonancefrequencyand versuscapacitancevalue for single resonator.of the PCB for each capacitor bank is 2.5 cm 2.5 cm. Themeasured tuning response and ofthetunableresonatorispre-sentedinFig.5.Theresonatorist
35、unedfrom2.503to2.393GHz(110 MHz), while the measured is from 1301 to 374 for allthe tuning range.IV. TWO-POLE TUNABLE BANDPASS FILTEREmploying the constructed tunable resonator, a two-poletunable bandpass filter is designed and simulated using HFSS.Fig. 6 shows the full-wave simulation model of the
36、filter. Forelectromagnetic (EM) simulations, the MEMS tuning circuitsare included in the model. A factor of 150 at 2.5 GHz isassumed for the fixed ceramic capacitors. The MEMS switch ismodeled with a small series capacitor and resistor for the “off”and “on” states, respectively ( fF and ).Fig. 7 sho
37、ws the simulated -parameters of the designed tun-able bandpass filter for three (three out of 16) different tuningstates. Simulation results demonstrate that the filter operates at2.5 GHz with a bandwidth of 22 MHz (0.9%) and a return lossbetter than 20 dB. It exhibits an insertion loss of 0.44 dB f
38、orthe first state, when all the switches are “off” and the capacitiveloading on the tuning disk is almost zero. The center frequencycan be tuned to different states from 2.5 to 2.345 GHz, whilethe bandwidth varies from 20 to 22 MHz and the return loss isbetter than 12 dB.Fig. 6. 3-D EM model of the
39、two-pole tunable filter.Fig. 7. Simulated -parameters of the designed filter.A two-pole tunable combline filter was fabricated and mea-sured. The filter housing was made of aluminum and platedwithcopperforimprovedperformance.ThetuningcircuitswithRF-MEMS capacitor banks were assembled on top of the l
40、id, asshown in Fig.8. There are screwsconnected to thetuning disks.Thesescrewsareusedonlyfortheinitialtuningofthefilterafterassembly. There are biasing wires for each one of the capacitorbanks and each set of MEMS switches. The MEMS switchesare turned on and off in a way that the two capacitor banks
41、 havethe same capacitance values and the two resonators are tunedsynchronously. Measurement results for the two-pole tunablebandpass filterarepresentedinFig.9.Asshowninthisfigure,for the first state, the filter operates at 2.5 GHz with a 23-MHzbandwidthand1.32-dBinsertionloss.Amaximumtuningrangeof 1
42、10 MHz from 2.503 to 2.393 GHz is achieved using theRF-MEMS tuning circuits. The return loss is better than 14 dBand the insertion loss is less than 2.4 dB for all the tuning states.The measured return loss when pF( are “off”) shows that the two resonators are not com-pletely tunedtothesamefrequency
43、.Thisisdueto thesmalldif-ference in the capacitance values of the two ceramic capacitors396 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 61, NO. 1, JANUARY 2013Fig. 8. Assembled two-pole tunable filter with MEMS capacitor bank.Fig. 9. Measured -parameters of the two-pole tunable filter
44、., in each capacitor bank, which was not accounted for inthe simulations. For this type of discrete tuning, it is necessaryfor the two capacitor banks to have exactly the same capaci-tance values. The measurement results validate the simulationsand the feasibility of using the proposed tuning approa
45、ch forcombline bandpass filters.TABLE ISUMMARY OF SPECIFICATIONS FOR WiMAX TUNABLE BANDPASS FILTERFig. 10. Coupling scheme and matrix of the six-pole tunable filter.V. WiMAX TUNABLE BANDPASS FILTERTight channel spacing requirements for the current wirelessstandards call for filters with a very sharp
46、 frequency roll-off. Aschannels are packed more closely together, higher order cou-pled resonator filters are required and for this reason filter specsbecome more demanding. In this section, we present the de-sign and implementation of a tunable bandpass WiMAX filteroperating at 2.5 GHz. Some of the
47、 filter requirements are pre-sented in Table I. The sharp frequency roll-off at the band edge( 25 dB, 5-MHz offset from passband) requires a comblinefilter with a higher number of cavities. Based on filter synthesisin order to meet the filter requirements, an Elliptic filter with atleastsixcoupledca
48、vitiesisrequired.Fig.10showsthecouplingscheme and the coupling matrix from the filter synthesis usingcoupling matrix method 15. Two different implementations ofthis tunable filter are presented in this paper. The two filtershave the same coupled cavity combline structure with two dif-ferent tuning m
49、echanisms. The first filter is using piezomotorsas tuning elements, while the second filter employs RF-MEMSswitched capacitor banks.A. Tunable WiMAX Filter With PiezomotorsThe same resonator structure with the dimensions shown inFig. 1 is used to design the six-polefilter. EM optimization withHFSS is used to find the dimensions of the coupling irises inorder to achieve the coupling matrix elements listed in Fig. 10.For positive coupling values, as shown in Fig. 11(a), a rect-angular iris with
