74 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 15, NO. 2, FEBRUARY 2005

A Low Loss RF MEMS Ku-Band IntegratedSwitched Filter Bank

Isak C. Reines, Charles L. Goldsmith, Christopher D. Nordquist, Christopher W. Dyck, Garth M. Kraus,Thomas A. Plut, Patrick S. Finnegan, Franklin Austin IV, and Charles T. Sullivan

Abstract—A switched Ku-band filter bank has been developedusing two single-pole triple-throw (SP3T) microelectromechanicalsystems (MEMS) switching networks, and three fixed three-poleend-coupled bandpass filters. A tuning range of 17.7% from 14.9 to17.8 GHz was achieved with a fractional bandwidth of 7.7 2.9%,and mid-band insertion loss ranging from 1.7 to 2.0 dB.

Index Terms—End-coupled filters, microelectromechanicalsystems (MEMS) filters, MEMS devices, microstrip filters, RFMEMS, tunable filters.

I. INTRODUCTION

LOW loss, tunable bandpass filters are critical componentsin radio frequency [RF] front ends and transceivers [1].

RF microelectromechanical systems (MEMS) enable the con-struction of miniaturized tunable filters that exhibit low loss,low-power consumption, and excellent linearity compared totheir solid-state counterparts. Integration of RF MEMS filtersin a monolithic microwave integrated circuit (MMIC) processcan reduce overall system size, weight, and cost, making themattractive for many commercial and military applications.

A number of researchers have demonstrated tunable band-pass filters at microwave and millimeter wave frequencies byusing distributed MEMS transmission line (DMTL) designs[2], [3]. DMTL filters are tuned by loading multiple resonatorsections with MEMS capacitive switches that function tochange the phase velocity and electrical length of the resonatorsections, thus tuning the center frequency of the filter. InDMTL filter designs reported to date, the low unloaded ofthe resonator sections dominates the high- MEMS varactorsused to tune the filter. This limits the best attainable insertionloss of DMTL filters implemented in a coplanar waveguide(CPW) structure, which typically range between 3 and 4 dB at

through -band. In addition, the tuning range of DMTLfilters is limited by the available tuning of the loading varactors.The goal of this work is to develop an integrated switchedfilter bank architecture at -band that uses both high-Q fixed

Manuscript received April 24, 2004; revised July 21, 2004. This work wassupported in part by the U.S. Department of Energy’s National Nuclear SecurityAdministration under Contract DE-AC04-94AL85000. The review of this letterwas arranged by Associate Editor A. Stelzer.

I. C. Reines, C. D. Nordquist, C. W. Dyck, G. M. Kraus, T. A. Plut, and C.T. Sullivan are with the Sandia National Laboratories, Albuquerque, NM 87185USA (e-mail: icreine@sandia.gov).

P. S. Finnegan is with the L & M Technologies, Albuquerque, NM 87109-5802 USA.

C. L. Goldsmith is with the MEMtronics Corporation, Plano, TX 75075 USA.F. Austin IV is with The Plus Group, Albuquerque, NM 87112-3800 USA.Digital Object Identifier 10.1109/LMWC.2004.842823

Fig. 1. Switched filter bank based on MEMS SP3T switches.

end-coupled filters, and low loss MEMS metal contactingswitches to reduce overall insertion losses.

Unlike DMTL filters, this type of architecture is easily scal-able to broader tuning ranges, because the switched filter bankis only limited by the bandwidth of the SP3T switch.

This work presents a low loss Ku-band switched filter thatis realized with two single-pole triple-throw (SP3T) switchesand a bank of three-pole end-coupled half-wave resonator fil-ters with center frequencies of 14.9, 16.2, and 17.8 GHz (Fig. 1).Series metal contacting MEMS switches in the SP3T switchingnetworks allow for filter selection, while shunt switches sep-arated by a quarter wavelength section of microstrip transmis-sion line increase the isolation between signal paths. In total, theswitched filter bank design incorporates 18 MEMS metal con-tacting switches. In order to avoid MEMS switch yield issues,the SP3T switches were fabricated and tested separately beforebeing incorporated with the filter bank, however, these circuitsare easily implemented in an integrated fashion on a single sub-strate. The overall size of the switched filter bank is 15.4 mm by6.7 mm.

II. SP3T SWITCH DESIGN

The layout of the SP3T switching network is shown inFig. 2. The SP3T design requires high isolation ( 30 dB over14–19 GHz) between the signal paths to reduce unwantedcrosstalk that would otherwise degrade the filter response.MEMS metal contacting switches [4], which are used in botha series and shunt configuration, were modeled as a capacitor

10 fF in the up state, and in the down state as a resistorin series with a length of microstrip transmission line

with dimensions m and m. The MEMSswitch has a cantilever thickness of 5.8 m, a spring constantof 8.66, and a gap height of 3.5 m. The typical pull in voltageof the cantilever switch was 39 V. At the operational voltagewas 55 V and the switching speed was 83 s. Since only one

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REINES et al.: LOW LOSS RF MEMS Ku-BAND INTEGRATED SWITCHED FILTER BANK 75

Fig. 2. Photomicrograph of SP3T switch.

series switch is activated to the down state at any time, there arealways two series switches and one shunt switch in the up statethat have a significant effect on the impedance. These switchesyield a 30-fF up-state capacitance that was accounted for withan inductive matching section forming a “T-match” circuit forgood input and output match [5].

Microstrip transmission lines in the SP3T switching networkwere designed for a 50- impedance on a 250- m thick alu-mina substrate with 9.8 and 0.003. Fig. 3 showsthe inductive matching network and all three series switches.A complete model was created for the SP3T switch using Ag-ilent’s advanced design system (ADS). Critical microstrip ele-ments such as “tee” and “cross” junctions were simulated usingSonnet 2.5D EM simulator and imported into ADS.

The SP3T switch was tested in lab ambient with a HP 8510Cnetwork analyzer and cascade RF probe station. Fig. 4 showsthe measured isolation of the SP3T switch with two of the threeoutput ports unterminated. A reduction in the isolation is ob-served since the SP3T switch is designed to exhibit maximumisolation when all three ports are terminated. Fig. 4 also in-cludes ADS simulations with all ports properly terminated andwith two ports unterminated, which matches well with measureddata. Although no data was collected with all ports terminated,the ADS simulation shows that the SP3T switch should exhibitgreater than 29 dB of isolation up to 19 GHz. Measured inser-tion losses of the SP3T switch range between 0.41 dB at 15 GHzto 0.47 dB at 18 GHz as shown in Fig. 4. The insertion loss isdominated by radiation losses at the coplanar waveguide to mi-crostrip discontinuity at the input and output, which accountsfor 0.25 dB at 15 GHz and 0.28 dB at 18 GHz.

Fig. 3. Close up of the SP3T switch.

Fig. 4. Measured and simulated isolation and insertion loss of the SP3Tswitch.

III. END-COUPLED FIXED FILTER DESIGN

The three-pole fixed end-coupled filters have center frequen-cies of 14.9, 16.2, and 17.8 GHz, with passband bandwidthsof %. Each filter consists of three resonator sectionsthat are approximately long at , that are separated byfour coupling capacitors. The length of the resonator sectionsand the values of the coupling capacitors were calculated from[5]. The smaller coupling capacitors in each filter were realizedusing transmission line gaps, while the larger coupling capaci-tors were implemented with interdigital structures. To reduce in-sertion losses, each filter was fabricated in microstrip with 2- mthick gold lines on a 250- m thick alumina substrate. The inser-tion loss of the 14.9-GHz filter was 1 dB, with more than 15 dBof return loss. Insertion loss of the 16.2-GHz filter was 1.1 dB,with greater than 15 dB of return loss. The 17.9-GHz filter ex-hibited 1.5 dB if insertion loss and greater than 20 dB of returnloss. The unloaded in each case was greater than 228.

IV. SWITCHED FILTER BANK RESULTS

The switched filter bank components were fabricated sepa-rately and assembled on an indium sheet to connect the back-side gold ground planes of the SP3T switches and end-cou-pled filters. The individual components were wire bonded to-gether to connect the signal paths. Fig. 5 shows the layout ofthe switched filter bank. Measured insertion loss and return lossfor the switched filter bank are shown in Fig. 6. The mid-band

76 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 15, NO. 2, FEBRUARY 2005

Fig. 5. Layout of the switched filter bank.

Fig. 6. Insertion loss and return loss for the switched filter bank.

insertion loss ranges from 1.7 dB to 2 dB, with a tuning rangeof 17.7% centered at 16.3 GHz. A fractional bandwidth of 7.7

2.9% was measured, with better than 10-dB return loss for allthree filter states. The return loss of the individual filters was de-graded by 5 dB when connected to the SP3T switches, becausethe MEMS cantilever switches exhibited an up-state capacitanceof 18 fF instead of 10 fF that was used in the initial design. The8-fF capacitance difference was due to an inaccurate up-stateswitch model. The rejection of the end-coupled filters is reducedat the higher frequencies because the capacitors couple more en-ergy and higher order transmission line resonances come intoplay.

V. CONCLUSION

This work presents a Ku-band switched filter bank with lowmid-band insertion loss ranging from 1.7 dB to 2 dB, and re-turn loss greater than 10 dB for all three states. A tuning rangeof 17.7% was achieved from 14.9 to 17.8 GHz with a fractionalbandwidth of 7.7 2.9%. The switched bank architecture facil-itates the integration of high- end-coupled filters and low lossMEMS metal contacting switches to provide lower losses com-pared to previously reported DMTL filters. This architecture isscalable to broader tuning ranges, because the range is only lim-ited by the bandwidth of the SP3T switch. The MEMS switchcount could be reduced in future switched filter bank designsby using a higher isolation series switch and a shorter electricallength shunt switch. To the best of our knowledge, this is thefirst MEMS switched filter bank reported to date.

REFERENCES

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[2] Y. Liu, A. Borgioli, A. Nagra, and R. York, “Distributed MEMS trans-mission lines for tunable filter applications,” Int. J. RF Microwave CAE,vol. 11, no. 5, pp. 254–260, Jun. 2001.

[3] A. Abbaspour-Tamijani, L. Dussopt, and G. Rebeiz, “Miniature and tun-able filters using MEMS capacitors,” IEEE Trans. Microw. Theory Tech.,vol. 51, no. 7, pp. 1878–1884, Jul. 2003.

[4] C. W. Dyck, T. A. Plut, C. D. Nordquist, P. S. Finnegan, F. Austin, I.Reines, and C. Goldsmith, “Fabrication and characterization of ohmiccontacting RF MEMS switches,” in Proc. SPIE., vol. 5344, Jan. 2004,pp. 79–88.

[5] G. L. Matthaei, L. Young, and E. M. T. Jones, Microwave Filters,Impedance-Matching Networks, and Coupling Structures. Dedham,MA: Artech, 1980.