Published in IET Microwaves, Antennas & PropagationReceived on 20th September 2011Revised on 22nd February 2012doi: 10.1049/iet-map.2012.0077

ISSN 1751-8725

Precise frequency and bandwidth control of switchablemicrostrip bandpass filters using diode andmicroelectro-mechanical system technologiesZ. Brito-Brito1 I. Llamas-Garro2 G. Navarro-Muñoz3 J. Perruisseau-Carrier4 L. Pradell3

F. Giacomozzi 5 S. Colpo5

1ITESO, Jesuit University of Guadalajara, 44604 Jalisco, México2Centre Tecnologic de Telecomunicacions de Catalunya (CTTC), 08860 Barcelona, Spain3Technical University of Catalonia (UPC), 08034 Barcelona, Spain4Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland5Fondazione Bruno Kessler (FBK), 38123 Povo, Trento, ItalyE-mail: llamasi@theiet.org

Abstract: In this study, two reconfigurable bandpass filters are presented. The first filter is able to switch between WiFi anduniversal mobile telecommunications (UMTS) transmit band standards. Centre frequency, bandwidth and power specificationsare precisely met by a switchable filter topology that includes two folded resonator extensions switched by only two PINdiodes. Design specifications require two filter states, one at 2.440 GHz with a 80 MHz bandwidth and a second state at1.955 GHz with a 140 MHz bandwidth for the WiFi and UMTS transmit bands, respectively. The filter should handle amaximum power of 16 and 21 dBm for the WiFi and UMTS states, respectively. Measured results show very good agreementwith the simulations. The filter topology meets power requirements of both standards. The second filter was designed usingohmic-contact cantilever-type MEMS able to switch between two different states with a centre frequency tunable range of24% in C band. This design includes two additional switches to provide precise input and output couplings for each state.The filter was designed to have centre frequencies of 5 and 6.2 GHz, with a fractional bandwidth of 7 and 3%, respectively.Filter specifications were successfully met with the proposed topologies.

1 Introduction

The goal of the work is to design a precise frequency andbandwidth controllable filter topology using diodes andmicroelectro-mechanical systems (MEMS) switchingelements. This paper not only presents power handling andthird-order inter-modulation results and a more detaileddesign procedure compared to previous work [1], but also adesign using ohmic-contact cantilever MEMS switches ableto commute between two different states with a centrefrequency tunable range of 24% in C band. Other works ontunable filters include switchable bandpass filter usingMEMS switches [2, 3], Barium Strontium Titanate (BST)varactors [4, 5] and PIN diodes [6–8]. Most of the designsreconfigure centre frequency and/or bandwidth. Filters witha reconfigurable bandwidth do not consider specific designvalues for each filter state. The bandpass filter discussed in[3] uses MEMS switches to achieve a steppedreconfigurable centre frequency range from 1.65 to2.34 GHz, but bandwidth control was not included in thefilter design. In [4], a filter was designed using BSTvaractors as tuning elements with a continuous variablecentre frequency range from 1.8 to 2.04 GHz. Bandwidth

increased with frequency and its control was not included inthe design. In [6], two centre frequency states at 1.91 and2.07 GHz were obtained by using PIN diodes. Here againbandwidth control was not considered in the designprocedure. The reconfigurable filter for WiFi and UMTSreceive standards in [8] was implemented to adjust theresponse at the receive standards and uses significantlymore switching elements compared to the filter discussed inthis paper.

The microstrip filters presented in this paper consists of twoswitchable resonators. Centre frequency is controlled byadjusting the length of the resonators, and bandwidth iscontrolled by adjusting the coupling between resonators bymeans of a folded resonator extension. The external qualityfactor Qe related to the input and output coupling to thefilter was selected in such a way that it maintains a verygood input and output match for both filter states. The filtertopology is able to produce reconfigurable centrefrequencies and bandwidths. Two reconfigurable bandpassfilters are presented. The first filter is able to switchbetween the transmit standards for WiFi and UMTS usingPIN diodes, including maximum power handlingcapabilities with low inter-modulation distortion. The

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second filter was designed using ohmic-contact cantilever-type MEMS switches able to switch between two differentstates with a centre frequency tunable range of 24% in Cband, with a fractional bandwidth of 7 and 3%, at each state.

This paper is divided into five sections. Section 2 containsa discussion on the proposed filter topology, describing howthe filter design parameters frequency and bandwidth werecontrolled. Section 3 discusses simulated and measuredfilter responses, bias circuitry, power handling and third-order inter-modulation results of the filter that has beendesign using PIN diodes. Section 4 presents the simulatedand measured responses of the filter designed using ohmic-contact cantilever MEMS switches. Finally, Section 5 givesan overall conclusion of this work.

2 Filter design

Fig. 1 shows the compact two-pole switchable bandpass filtertopology using two PIN diodes. In this filter topology all PINdiodes are reverse biased to produce the WiFi transmit state,whereas the UMTS transmit state is produced when all PINdiodes are forward biased. Table 1 shows the required filterdesign specifications. The filter should be able to switchbetween the transmit bands for WiFi and UMTS standardsand satisfy the maximum power handling specification withlow third-order inter-modulation distortion. Centrefrequency, bandwidth and power handling designparameters are precisely achieved using the filter topologydescribed in this paper.

The design of a narrow bandpass filter can be based on thefollowing design parameters: external quality factor Qerelated to the input and output coupling to the filter, and thecoupling coefficient K between resonators [9]. The relationbetween bandwidth (BW) and Qe or K is given in (1) and(2), respectively; where, f0 is the filter centre frequency fora given state.

BW = 0.4489f0Qe

(1)

BW = 0.4278f0K (2)

Table 2 shows the theoretical Qe and K for the twospecified bands of operation. To find optimum filter layoutusing the ADS/MOMENTUM simulator, the couplingbetween feed lines of the filter and the first or last resonatorare calculated and matched to the theoretical values of Qe inTable 2. Similarly, by simulating the coupling betweenresonators, the values of K are found using the well-knownmethods in [9].

The relation between the external quality factor and filterbandwidth for the two standards is shown in Fig. 2. Toobtain Qe using the ADS/MOMENTUM simulator, a singleresonator weakly coupled on one side must be simulated asdiscussed in [9], and then an appropriate value of Qe can bedetermined by finding the appropriate spacing between theresonator and the feed line using the following expression

Qe =f0

Df−3 dB(3)

In (3), f0 is the resonant frequency and Df23 dB is the 3 dBbandwidth [9].

The coupling between the feed line and the WiFi resonator(Qe) can be determined by the coupling gap S1 for a fixed feedline width step w1, as shown in Fig. 3. A smaller gap ornarrower feed line step section will result in strongercouplings [9]. Similarly, Qe is set for the UMTS bandconsidering the coupling between the feed line and theUMTS resonator that includes the folded extension inFig. 1. The coupling between the feed line and the UMTSresonator (Qe) can be determined by the UMTS resonatorextension length L for a fixed feed line width step w1,coupling gap S1 and UMTS resonator width step w2, asshown in Fig. 4.

The coupling between both feed lines, marked as theoverlapping distance Y in Fig. 1, fixes the transmission zeroposition for the WiFi state as shown in Fig. 5. The positionof the transmission zero is chosen in the design to provide ahigh roll-off on the upper side of the stopband when thefilter is operating in the WiFi state.

Fig. 1 Switchable bandpass filter topology of the WiFi and UMTStransmit standards

Table 1 Filter specifications of the WiFi and UMTS transmitstandards

Centre frequency,

GHz

Bandwidth,

MHz

Maximum power

handling, dBm

WiFi 2.440 80 16

UMTS 1.955 14 21

Table 2 External quality factor and coupling coefficient of theWiFi–UMTS reconfigurable filter

Qe K

WiFi 13.6915 0.0766

UMTS 6.2686 0.1674

Fig. 2 External quality factor Qe for different bandwidths

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The relation between the coupling coefficient and bandwidthof the filter for the two standards is shown in Fig. 6. Thecoupling coefficient K for the WiFi band was fixed bycoupling gap S2; a smaller S2 results in a stronger coupling.The WiFi resonator is essentially a half-wavelengthresonator. To obtain K using a simulator, both resonators areweakly coupled to a pair of feed lines as discussed in [9],and then the design value of K can be determined for theWiFi state by finding the appropriate spacing between theresonators using the following expression

K = f2

2 − f 21f 22 + f 21

(4)

In (4), f1 and f2 are the resonant frequencies of the coupledresonator circuit [9].

In order to switch to the UMTS band, diodes D1 and D2(see Fig. 1) are forward biased to increase the electricallength of the resonator which fixes the required UMTScentre frequency. As shown in Table 2 the couplingcoefficient K for the UMTS band is larger than that for theWiFi band. To obtain a high inter-resonator coupling withrespect to the one required for the WiFi state, the UMTSresonators were designed to include a folded section thatproduces a strong electric-type coupling between resonatorswhen the UMTS band is selected, and hence produces awide passband, which has been optimised to achieve designspecifications.

The proper selection of UMTS resonator extension widthw2 to produce the required inter-resonator coupling toproduce the UMTS bandwidth was optimised as shownin Fig. 7. In addition, the proximity between resonatorextensions and the WiFi resonator sections that fixed theoverlapping distance X (see Fig. 1) was optimised as shownin Fig. 8. To define filter layout, each state of the filter wasoptimised using ADS/MOMENTUM to produce therequired theoretical design parameters Qe and K inTable for each filter state.

3 Results using PIN diodes

Using the techniques described above, a bandpass filter wasdesigned to switch between two states; one having a centrefrequency of 1.955 GHz with a passband bandwidth of

Fig. 6 Coupling coefficient K for different bandwidth or spacingS2 between WiFi resonators

Fig. 7 Coupling coefficient K for different UMTS resonatorextension width w2

Fig. 3 External quality factor Qe for different resonator spacing S1from the feed line

Fig. 4 External quality factor Qe for different UMTS resonatorextension length

Fig. 5 Transmission zero position for the WiFi state

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140 MHz, and a second state having a centre frequency of2.440 GHz, with a passband bandwidth of 80 MHz. Thesestates correspond to the UMTS and WiFi transmit standardsrespectively, the filter must also be able to handlemaximum power levels of 16 and 21 dBm for the WiFi andUMTS states, respectively, without generation of third-orderinter-modulation distortion.

The filter was designed using a Rogers 1.524 mm thicksubstrate (1r ¼ 3.55, d ¼ 0.0021) and HPND-4028 AvagoTechnologies PIN diodes. The fabricated device is shownin Fig. 9. The layout including the DC bias lines waspatterned on the substrate using standard photolithographictechniques. The Bias network consisted of a choke inductor

to isolate DC bias lines and circuitry from the microwavecircuit [10]. The current on each diode was limited to 10 mAby placing a 1 kV series resistor in the forward bias state; avoltage of 210 V was supplied in the reverse bias state.

Lumped element models for the PIN diodes and chokeinductors were calculated for both forward and reverse biasstates. The lumped element models were obtained fromregressions after measuring a single PIN diode or chokeinductor and fitting RLC models to experimental data,Table 3 shows the model values for both elements. Full-wave simulations of the filter topology were made includinglumped element models for the PIN diodes and chokeinductors. Table 4 contains the reconfigurable bandpassfilter dimensions.

The filter was optimised using ADS/MOMENTUM toprecisely produce the two discrete states. The measurementswere taken using an N5242A PNA-X Agilent networkanalyser. Table 5 contains a summary of results. A goodagreement in terms of centre frequency, bandwidth andpower handling was obtained for both filter states.

A comparison between simulated and measured responsesfor the WiFi state (with the two diodes in reversepolarisation), is shown in Fig. 10; a centre frequencydeviation of only 12 MHz is obtained between simulationsand measurements. The return loss at the passband ofthe filter is about 20 dB in simulations, and 24 dB inmeasurements. The difference between the simulated andmeasured bandwidth is only 9 MHz.

A comparison between simulated and measured widebandresponses of the switchable bandpass filter in the WiFi state isshown in Fig. 11, where it is possible to see the centrefrequency but also the resonances at 2 and 4 fc, which havelevels about 10 dB below.

Fig. 9 Photograph of the switchable bandpass filter for WiFi andUMTS transmit standards

Table 3 Lumped element models of surface mount components

Model

diode (off state)

diode (on state)

inductor

Table 4 Filter dimensions of the WiFi and UMTS transmit standards

W1, mm S1, mm W2, mm S2, mm S3, mm UMTS resonator

extension length L, mm

Overlapping

distance X, mm

WiFi resonator

Length, mm Width, mm

0.5 0.97 1.2 5.16 1.01 14.5 5.7 35.1 2.8

Table 5 Simulated and measured results of the WiFi and UMTS transmit standards

Centre frequency Bandwidth Power saturation point IIP3

WiFi, GHz UMTS, GHz WiFi, MHz UMTS, MHz WiFi, dBm UMTS, dBm WiFi, dBm UMTS, dBm

simulated 2.440 1.955 80 140 — — — —

measured 2.428 1.939 71 144 16 19 32.5 32

Fig. 8 Coupling coefficient K for different bandwidth oroverlapping distance X

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A comparison between simulated and measured responsesfor the UMTS state (with the two diodes in forwardpolarization) is shown in Fig. 12. Deviation betweensimulations and measurements is of only 16 MHz for thecentre frequency and 4 MHz for the bandwidth. The returnloss at the passband of the filter is found to be at about30 dB in simulations, and 36 dB for the measurements.Table 6 shows the comparison between simulated andmeasured filter insertion loss, where a very good agreementis obtained for each filter state.

Simulated and measured wideband responses for theUMTS state are shown in Fig. 13, where it is possible tosee the centre frequency but also the resonances at 3 and5 fc, which have levels about 8 dB below.

The filter should handle maximum powers of 16 dBm forthe WiFi state and 21 dBm for the UMTS state. Powerhandling of the filter was tested up to 25 dBm, saturationlevels were found at 16 dBm for the WiFi state and 19 dBmfor the UMTS state, thus the filter presents a linear powerresponse up to the maximum power handling specifications

as shown in Fig. 14. The measured third-order inter-modulation intercept point for each state was 32.5 dBm forthe WiFi state and 32 dBm for the UMTS state, thesemeasurements were done considering a separation betweentones of 25 MHz for the WiFi state, and 5 MHz for theUMTS state, according to channel spacing for eachstandard. Good agreement has been obtained between initialdesign specifications and final experimental results.

4 MEMS reconfigurable bandpass filter

A similar filter was also implemented in MEMS technology,since it offers the advantage of electronic reconfigurabilitywith negligible power consumption and high linearitycompared to conventional solid-state technology-baseddevices. The fabrication technology for the MEMSreconfigurable bandpass filter is an integrated eight-masksurface micromachining process from FBK [11]. In Fig. 15,a cross section that summarises the structural elementsprovided by the process is shown. In this technology,movable bridges/cantilevers are manufactured using a2 mm-thick electrodeposited gold layer. Another 3 mm-thick

Fig. 11 Simulated and measured wideband response for the WiFistate

Fig. 14 Output power versus input power of the filter for bothstates

Fig. 13 Simulated and measured wideband response for theUMTS state

Fig. 12 Simulated and measured results for the UMTS state

Fig. 10 Simulated and measured results for the WiFi state

Table 6 Filter insertion loss of the WiFi and UMTS transmitstandards

WiFi, dB UMTS, dB

simulated 3.339 3.796

measured 3.345 3.837

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gold film is selectively superimposed to increase the rigidityof the central part of the beam and for the patterning of themicrostrip lines. A third 150 nm gold layer is evaporatedover the underpass metal line for manufacturing thelow-resistance metal-to-metal electromechanic contacts. A

high-performance ohmic-contact series cantilever MEMSswitch, with structure and dimensions similar to the onereported in [12] (fabricated using the same technologicalprocess), has been used to realise the integrated filter. Thecantilever is suspended above an interrupted microstripsignal line and anchored at one end. The membranedimensions are 180 mm long and 110 mm wide. In order togenerate a good ohmic contact between the beam and theline, some dimples have been placed in the contact area ofthe microstrip line, by using small poly-silicon bumpsdeposited underneath. The membrane embeds10 mm × 10 mm holes for the easier removal of thesacrificial layer, increased flexibility and reduced damping.The switch is used here to vary the total length of a microstripline. The filter comprises four MEMS switches, two of whichare used to modify the resonator lengths (similar to the filterpresented in Section 3), whereas the others two (located onthe feed lines) provide a reconfigurable input and outputcoupling to the filter. The cross coupling has been designed toplace the transmission zero on the lower side of the passbandat the low-frequency state.

Using the techniques described in Section 2, the bandpassfilter is designed to switch between the two states reported inTable 7.

The fabricated device is shown in Fig. 16 and Table 8contains the filter dimensions. It was realised on a 500 mm-thick quartz substrate (1r ¼ 3.78, d ¼ 0.0001). The biasnetwork is made of poly-silicon high-resistivity lines toreduce losses. Full-wave simulations were made using ADS/MOMENTUM to precisely define the two discrete states. Inthe on-state, the MEMS switch introduces a low seriesresistance of 1 V associated with the metal contact betweenthe beam and the line [12], and was included in thesimulations. The measurements were taken using a N5242APNA-X Agilent network analyser and a wafer-probe station.The measured actuation voltage of the MEMS switches isabout 50 V. Tables 9 and 10 contains a summary of results.A good agreement in terms of centre frequency andbandwidth was obtained for both filter states. The increase inmeasured insertion loss for both filter states (compared tosimulations) is associated with higher-than-expected lossesin the CPW-to-microstrip input/output transitions.

A comparison between simulated and measured responsesof the MEMS switchable bandpass filter is shown in Fig. 17.For the low-frequency state the centre frequency deviationbetween simulations and measurements is 66 MHz. The

Table 7 MEMS reconfigurable bandpass filter specifications

Centre frequency, GHz Fractional

bandwidth, %

low-frequency state 5 7

high-frequency state 6.2 3

Table 8 MEMS reconfigurable bandpass filter dimensions

W1, mm S1, mm W2, mm S2, mm S3, mm High resistivity

lines

Low-frequency

resonator

extension

length L, mm

Overlapping

distance

X, mm

Overlapping

distance

Y, mm

High-

frequency

resonator

Mean

length, mm

Width,

mm

Length,

mm

Width,

mm

0.11 0.1 0.11 2.13 0.16 8.5 0.01 4.6 0.95 4.6 13.8 0.34

Table 9 Simulated and measured results of the MEMS reconfigurable bandpass filter

Centre frequency Fractional bandwidth

Low-frequency state, GHz High-frequency state, GHz Low-frequency state, % High-frequency state, %

simulated 5 6.2 7 3

measured 4.934 6.131 8.4 4.8

Fig. 15 Diagram of the cantilever MEMS switch on quartzsubstrate

Fig. 16 Photograph of the MEMS reconfigurable bandpass filter

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return loss at the passband of the filter is about 12 dB insimulations, and 13 dB in measurements. The differencebetween the simulated and measured bandwidth is 1.4%.Concerning the high-frequency state, the centre frequencydeviation between simulations and measurements is of69 MHz. The difference between the simulated andmeasured bandwidth is 1.8%. The return loss at thepassband of the filter is found to be at around 22 dB insimulations, and 19 dB for the measurements.

5 Conclusions

Two bandpass filters switchable between two discretefrequency bands have been demonstrated. One filter hasbeen designed for a gateway able to switch between WiFiand UMTS transmit bands used in vehicle-to-vehiclecommunications. The proposed filter is compact and able toswitch between the two centre frequencies with thespecified bandwidth by using only two PIN diodes, therebyminimising cost and power consumption. A very goodagreement between simulations and measurements has beenobtained for centre frequency, bandwidth and powerhandling requirements. The second filter has been designedusing ohmic-contact cantilever MEMS switches to commutebetween two different states with a centre frequency tunablerange of 24% in C band. A very good agreement betweensimulations and measurements has been obtained for centrefrequency and bandwidth.

6 Acknowledgments

The authors would like to thank Pablo Pardo-Carrera atEADS-CASSIDIAN, Getafe, Spain, for assisting the inter-modulation measurements. The authors would like to thankMoises Espinosa at CTTC and Adrian Contreras at UPC,for assisting the measurements and the staff of FBK MT-Lab for the fabrication of the RF-MEMS device. This workhas been financed by research projects TEC2010-20318-C02-01 and PIB2010BZ-00585 from the Spanish Ministry ofScience and Innovation and research grant Torres QuevedoPTQ-11-04792 from the Spanish Government. Z. Brito-Britowishes to thank CONACYT, Mexico for scholarship no.207926/302540.

7 References

1 Brito-Brito, Z., Llamas-Garro, I., Navarro-Muñoz, G., Perruisseau-Carrier, J., Pradell, L.: ‘UMTS-WiFi switchable bandpass filter’. Proc.39th European Microwave Conf., Rome, Italy, 29 September–1October 2009, pp. 125–128

2 Palego, C., Pothier, A., Crunteanu, A., et al.: ‘A two-polelumped-element programmable filter with MEMS pseudodigitalcapacitor banks’, IEEE Trans. Microw. Theory Tech., 2008, 56, (3),pp. 729–735

3 Reines, I., Brown, A., El-Tanani, M., Grichener, A., Rebeiz, G.:‘1.6–2.4 GHz RF MEMS tunable 3-pole suspended combine filter’.IEEE MTT-S Int. Microwave Symp. Digest, 15–20 June 2008,pp. 133–136

4 Kim, K.-B., Park, C.-S.: ‘Application of RF varactor usingBaxSr1-xTiO3/TiO2/HR-Si substrate for reconfigurable radio’, IEEETrans. Ultrason. Ferroelectr. Freq. Control, 2007, 54, (11),pp. 2227–2232

5 Nath, J., Ghosh, D., Maria, J.-P., et al.: ‘An electronically tunablemicrostrip bandpass filter using thin-film barium–strontium–titanate(BST) varactors’, IEEE Trans. Microw. Theory Tech., 2005, 53, (9),pp. 2707–2712

6 Mahe, F., Tanne, G., Rius, E., et al.: ‘Electronically switchable dual-band microstrip interdigital bandpass filter for multistandardcommunication applications’. Thirtieth European Microwave Conf.,October 2000, p. 4

7 Chen, C.-C., Wang, S.-M.: ‘Design of an LTCC switchable filter fordual- band RF front-end applications’. IEEE TENCON Conf., 30October–2 November 2007, p. 3

8 Brito-Brito, Z., Llamas-Garro, I., Navarro-Muñoz, G., Perruisseau-Carrier, J., Pradell, L.: ‘Switchable bandpass filter for WiFi – UMTSreception standards’, Electron. Lett., 2010, 46, (13), pp. 930–931

9 Hong, J.-S., Lancaster, M.J.: ‘Microstrip filters for RF/microwaveapplications’ (John Wiley & Sons Inc., New York, USA, 2001)

10 Xue, H., Kenington, P.B., Beach, M.A.: ‘A high performance ultra-broadband RF choke for microwave applications’. IEE Colloquium onEvolving Technologies for Small Earth Station Hardware, February1995, p. 4

11 Giacomozzi, F., Mulloni, V., Colpo, S., Iannacci, J., Margesin, B.,Faes, A.: ‘A flexible technology platform for the fabrication ofRF-MEMS devices’. Proc. Int. Semiconductor Conf. (CAS), Sinaia,Romania, 2011, pp. 155–158

12 Ocera, A., Farinelli, P., Cherubini, F., et al.: ‘A MEMS-reconfigurablepower divider on high resistivity silicon substrate’. IEEE/MTT-S Int.Microwave Symp., 2007, pp. 501–504

Fig. 17 Simulated and measured results of the MEMSreconfigurable bandpass filter

Table 10 MEMS reconfigurable bandpass filter insertion loss

Low frequency, dB High frequency, dB

simulated 1.14 1.13

measured 4.602 4.565

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1 Introduction2 Filter design3 Results using PIN diodes4 MEMS reconfigurable bandpass filter5 Conclusions6 Acknowledgments7 References