An RF-MEMS-Based Digitally Tunable SIW Filter in X-Band for Communication Satellite Applications

applied sciences

Article An RF-MEMS-Based Digitally Tunable SIW Filter in X-Band for Communication Satellite Applications

Abbas El Mostrah 1, Andrei Muller 2, Jean-François Favennec 2,*, Benjamin Potelon 2,* , Alexandre Manchec 1, Eric Rius 2 ,Cédric Quendo 2 , Yann Clavet 1, Francis Doukhan 3 and Johann Le Nezet 3

1 Elliptika, Gouesnou, Rue Gustave Zédé, 29200 Brest, France; [email protected] (A.E.M.); [email protected] (A.M.); [email protected] (Y.C.) 2 Laboratoire des Sciences et Techniques de l’Information, de la Communication et de la Connaissance (Lab-STICC), Université de Brest, 6 avenue Le Gorgeu, CS 93837, 29238 Brest, France; [email protected] (A.M.); [email protected] (E.R.); [email protected] (C.Q.) 3 Direction Générale de l’Armement (DGA), Route de Laillé, 35131 Bruz, France; [email protected] (F.D.); [email protected] (J.L.N.) * Correspondence: [email protected] (J.-F.F.); [email protected] (B.P.)

Received: 12 March 2019; Accepted: 30 April 2019; Published: 4 May 2019

Abstract: This paper presents a digitally tunable SIW (substrate integrated waveguide) filter designed for X-band, based on RF-MEMS (radio frequency micro-electrical-mechanical systems) technology. Four commercial off-the-shelf RF-MEMS single-pole single-throw (SPST) switches were directly mounted on the upper surface of the filter, with metallic tuning posts specifically located within each cavity to define the potential achievable frequency range. Fabricated on standard alumina substrate, the design of the filter and the biasing network enabled fine digital frequency control of up to four functional states by the inclusion of wire bondings between each switch and the substrate. A relative tuning range of 2.3% was achieved between the lower and upper discrete states of 2.76% and 2.89% in the 3 dB fractional bandwidths.

Keywords: bandpass tunable ﬁlter; RF-MEMS; substrate integrated waveguide SIW; X-band

1. Introduction Tunable filters have been attracting attention over recent years as they are one of the key building blocks for reconfigurable radio front ends, multiband and subsystems. Frequency tunability in planar technology has been widely investigated in the literature [1]. To bypass the Q-factor limitations of planar technology, an alternative solution consists in using substrate integrated waveguide (SIW) technology [2]. Due to a higher quality factor and good integrating abilities with planar circuits, SIW is considered a good candidate for many microwave applications. In particular, SIW seems a very good candidate to cope with constraints induced by new trends in the development of communication satellites. In that context, there is a real need for innovative tuning devices that can address several frequency plans. Recent scientific literature has described several technological options to achieve the tuning of SIW filters [3]. Amongst them, solutions based on piezoelectrical or mechanical technologies [4,5] are not suitable as they cannot match all the constraints induced by our spatial application (size, weight, speed, power consumption, etc.). Thus, a choice was made to design a solution based on electrically-controlled microelectronic devices (diodes, transistors, radio frequency micro-electrical-mechanical systems (RF-MEMS), etc.) implemented on the top of an SIW structure. In order to make an SIW filter tunable, the authors of [6] proposed a new concept using vertical metallic posts inserted into the cavity. A 5% frequency shift around 10 GHz was achieved by either connecting

Appl. Sci. 2019, 9, 1838; doi:10.3390/app9091838 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 1838 2 of 8 or disconnecting the metallic posts. Depending on the purpose of the desired tuning device, digital or analog tuning is required. In this study, the need to switch between frequency plans is addressed by a focus on digital tuning. In [7], the digital tunability of cavities using packaged RF-MEMS switches from the Omron Corporation resulted in a filter with a 28% tuning range, from 1.2 to 1.6 GHz. The relative bandwidth was between 3.2% and 4.2% over the tuning range, and the use of bias and MEMS switches was demonstrated to tightly prescribe the tuning frequency range. However, the use of numerous bias currents for both coarse and fine tuning implies the degradation of the Q-factor. A different switching device was used in [8], with tunability achieved using PIN diodes implemented upon cavities. The filter presented a 25% tuning range from 1.55 to 2 GHz, with 2.3% to 3% relative bandwidth. To satisfy high-frequency applications, another tunability technique was applied on evanescent-mode cavities, as described in [9]. It was performed by using open-ended microstrip stubs, whose lengths can be adjusted by RF-MEMS switches. A frequency shift of 4% was realized. The fractional bandwidth and the minimum insertion losses were approximately 3.6% and 4.57 dB, respectively. Analysis of previous results indicates that RF-MEMS appears to be a good choice for tuning, especially when high-frequency filters are needed. In this paper, the emphasis is on experimental results from the design of a single fourth-order tunable filter with two contiguous states, able to address four contiguous frequency plans. The single filter is intended to replace a switched bank of four independent filters. However, the final aim of this study is to demonstrate the mastery of frequency shift using additional wire bondings. Tunability was achieved using a combination of metallic posts, additional wire bondings and commercial off-the-shelf RF-MEMS. The position of the metallic posts defined the potential achievable frequency range whereas the additional wire bondings determined the accessible frequency shift within this potential achievable range. Finally, the RF-MEMS played the role of a switching device which enables the selection of the desired states according to specifications. The filter was designed in X-band. The proposed filter and biasing network were built on the same alumina substrate plate.

2. Tunable Resonator Topology The design procedure of a resonant SIW cavity is similar to that of a conventional rectangular waveguide cavity. Energy is coupled to the cavity by input/output irises and tapered microstrip lines. The cavity is realized on a 635 µm thick alumina substrate (εr = 9.9, tgδ = 3 10 4). In order to achieve × − tunability, a metallic inductive post is introduced into the cavity and isolated from the top metallic face by a gap. For consistency, we use the term “tuning post” to define the metallic inductive post used as the tuning element. Depending on the position and the state of the tuning post, whether connected or not to the top metallic face of the cavity, the cavity resonance undergoes a frequency shift. Placing the tuning post in the center of the cavity, where the electric field is at its maximum, produces a large frequency shift. As the tuning post is placed further away from the center, the frequency shift decreases, and reaches a minimum close to the edges of the cavity [6]. As a first approach and in order to investigate SIW cavity behavior, the switching described here was executed using four wire bondings. The four wire bondings shown in Figure1 were manually disconnected one at a time and the electrical response was measured. For consistency, the wire bondings are labeled according to their direction (Figures1 and2)—N, W, S and E being North, West, South and East, respectively. Hence, we obtained five different results beginning from the state NWSE, where all wire bondings are connected, and finishing at the state U (standing for unconnected), where no wire bonding remains. Figure2 illustrates the measurement results. Assuming the state NWSE as the nominal starting state, the cavity resonated at a frequency of 11.68 GHz with all four wire bondings connected. Table1 summarizes measurements of the frequency shift on the transmission coefficient peak as a function of the disconnected wire bondings. The frequency shift between the two extreme states (NWSE and U) was approximately 817 MHz (Xpost = 1.9 mm). To explain this effect, the extent to which the EM field was perturbed must be Appl. Sci. 2019, 9 FOR PEER REVIEW 3

Appl. Sci. 2019, 9, 1838 3 of 8 considered. When the tuning post was connected with four wire bondings, a surface current was induced on the tuning post, causing a large perturbation inside the cavity. Appl. Sci. 2019, 9 FOR PEER REVIEW 3

Figure 1. Photograph of a substrate integrated waveguide (SIW) cavity with tuning post and four wire bondings.

Hence, we obtained five different results beginning from the state NWSE, where all wire bondings are connected, and finishing at the state U (standing for unconnected), where no wire bonding remains. FigureFigure 1. Photograph1. Photograph of aof substrate a substrate integrated integrated waveguide waveguide (SIW) (SIW cavity) cavity with with tuning tuning post post and and four four Figure 2 illustrates the measurement results. wirewire bondings. bondings.

FigureFigure 2.2. Measurement results for didifferentﬀerent connection states: (NWSE) four, (NSE) three, (SE) two, (S) oneone andand (U)(U) nono wirewire bondings.bondings.

Assuming the state NWSE as the nominal starting state, the cavity resonated at a frequency of Table 1. Frequency shift as a function of switched states. 11.68 GHz with all four wire bondings connected. Table 1 summarizes measurements of the frequencyXpost (mm) shift on the transmission States coefficient U–S peak as a function S–SE of the disconnected SE–NSE NSE–NWSEwire bondings. Figure 2. Measurement results for different connection states: (NWSE) four, (NSE) three, (SE) two, (S) The frequency1.9 shift Freq.between Shift (MHz)the two extreme 508 states (NWSE 225 and U) was 67approximately 17 817 MHz one and (U) no wire bondings. (Xpost = 01.9 mm). To Freq. explain Shift (MHz)this effect, the 1670 extent to which 640 the EM field 390 was perturbed 210 must be consideredAssuming. When the the state tuning NWSE post a wass the connected nominal starting with four state, wire the bond cavityings, resonate a surfaced at currenta frequency was of induced11.68Consequently, on GHz the with tuning each all post, four time causing wirea wire bondings a bonding large perturbation wasconnected disconnected. inside Table the (transitioning1 summarizescavity. from measurements states NWSE of to the NSEfrequency toConsequently SE to Sshift toU), on, each the inducedtransmissiontime a wire surface bonding coefficient current was peak decreased, disconnected as a function causing (transitioning of a the smaller disconnected perturbationfrom states wire NWSE than bonding the to s. to previousNSEThe to frequencySE stateSuntil to U),shift no the currentbetween induced was the surface induced two extremecurrent on the decreasestates tuning (NWSE post,d, causing i.e., and state aU) smaller (U).was approximately perturbation than 817 theMHz previous(XpostFigure state= 31.9 shows until mm). ano To simulation current explain wa ofthiss induced induced effect, onthe currents the extent tuning on to a whichpost, tuning i.e. the post, state EM inside (U).field a was cavity, perturbed as a function must be of theconsideredFigure number 3 .shows ofWhen connected a the simulation tuning wire bondings.postof induced was connected The currents induced withon currenta tuningfour wire on post the bond inside tuningings, a post cavity,a surface increased as acurrent function as the was numberof inducedthe number of connectedon theof connected tuning wire post, bondings wire causing bonding increased. a larges. The perturbation Anotherinduced current cavity inside prototype on the the cavity. tuning was post also realized,increasedwith as the a tuningnumber postConsequently of connected at the center, wireeach of bondingstime the cavity a wire increased. (Xpost bonding= 0 Anotherwas mm). disconnected The cavity same prototype strategy (transitioning was and measurementsalso from realized, states NWSEwith were a to appliedtuningNSE postto as SE previously at to the S to center U), described, the of induced the followingcavity surface (Xpost the current procedure= 0 mm). decrease ofThe disconnecting dsame, causing strategy a wiresmaller and bondings measurements perturbation one at athan were time. the Theappliprevious results,ed as previously state summarized until nodescribed, incurrent Table 1wafollowing, shows induced that the theon procedure the frequency tuning of shiftpost, disconnecting withi.e., state the tuning (U). wire bondings post at the one center at a wastime higher. TheFigure results, than 3 shows when summarized thata simulation at the in edgeTable of induced(2910 1, show MHz currents that for the Xpost onfrequency a =tuning0 mm). shift post In with inside both the cases, a tuningcavity, the post maximumas a functionat the of the number of connected wire bondings. The induced current on the tuning post increased as the number of connected wire bondings increased. Another cavity prototype was also realized, with a tuning post at the center of the cavity (Xpost = 0 mm). The same strategy and measurements were applied as previously described, following the procedure of disconnecting wire bondings one at a time. The results, summarized in Table 1, show that the frequency shift with the tuning post at the Appl. Sci. 2019, 9 FOR PEER REVIEW 4 Appl. Sci. 2019, 9, 1838 4 of 8 center was higher than when that at the edge (2910 MHz for Xpost = 0 mm). In both cases, the maximum frequency shift was obtained between configurations U and S, whereas the lowest one was frequency shift was obtained between conﬁgurations U and S, whereas the lowest one was between between states NWSE and NSE (see Table 1). The unloaded quality factor of this resonator varied states NWSE and NSE (see Table1). The unloaded quality factor of this resonator varied between 440 between 440 (state NWSE) and 500 (state U) (Xpost = 1.9 mm), and was significantly better than that (state NWSE) and 500 (state U) (Xpost = 1.9 mm), and was signiﬁcantly better than that of standard of standard planar resonators, e.g., stub resonators. planar resonators, e.g., stub resonators.

(a) (b) (c) (d)

FigureFigure 3. 3. ElectricElectric current density alongalong thethe tuningtuning post post for for four four cases: cases: (a ()a four,) four, (b ()b three,) three, (c )( twoc) two and and (d) (oned) one wire wire bonding bonding connected connected to theto the tuning tuning post. post.

Consequently, thereTable are two1. Frequency ways to s controlhift as a frequencyfunction of s shift,witched depending states. on the position of the tuning post and the number of connected wire bondings. The first parameter (post location on the Xpost (mm) States U–S S–SE SE–NSE NSE–NWSE cavity) enabled the definition of the frequency range of the tuning, whereas the second (number of 1.9 Freq. Shift (MHz) 508 225 67 17 wire bondings) allowed the control of the magnitude of frequency shift. However, the variation in 0 Freq. Shift (MHz) 1670 640 390 210 magnitude was limited by the tuning range as defined by the position of the post.

3. TunableConsequently Filter Design, there are two ways to control frequency shift, depending on the position of the tuning post and the number of connected wire bondings. The first parameter (post location on the cavity)3.1. Specifications enabled the and definition SIW Filter of Design the frequency range of the tuning, whereas the second (number of wire bondings) allowed the control of the magnitude of frequency shift. However, the variation in In the context of communication satellite application, one possible targeted digitally tunable magnitude was limited by the tuning range as defined by the position of the post. two-state filter should be able to discretely control a bandwidth of 250 MHz, with a central frequency ranging from 10.825 GHz to 11.075 GHz, through 250 MHz steps. 3. Tunable Filter Design The design here of a two-state tunable filter electrically controlled using RF-MEMS fits the above specifications. 3.1. Specifications and SIW Filter Design First, this required the design of a simple fourth-order Chebyshev SIW bandpass filter. SIW band-passIn the filters context have of communication been broadly studied satellite in application, the literature one [10 ] possible and the targeted design procedure digitally tunable is quite twosimilar-state to filter that should of a conventional be able to discretely rectangular control waveguide a bandwidth filter. of The 250 circuit MHz, was with realized a central on frequency a 635 µm 4 rangingthick alumina from 10.825 substrate GHz (ε tor = 11.0759.9, tg GHzδ = 3, through10− ) and 250 wasMHz fed steps by. a tapered microstrip line. The design here of a two-state tunable× filter electrically controlled using RF-MEMS fits the above specifications.3.2. Implementation of Tuning Elements First,The challenge this require thend becamethe design how of to a make simple the fourth filter tunable.-order Chebyshev So far, we have SIW detailed bandpass our filter. tunability SIW bandsolution-pass using filters wire have bondings. been broadly This technique studied wasin the then literature adapted [10] to our and specifications the design procedure with a combination is quite similarof electrically to that controlledof a conventional devices, rectangular using a commercial waveguide RF-MEMS filter. The switch circuit from was Radant. realized In on comparison a 635 µm −4 thickwith alumina a semi-conductor substrate junction,(εr = 9.9, tgδ RF-MEMS = 3 × 10 technology) and was fed off ersby a low tapered insertion microstrip losses, line. near zero power consumption, large power handling and very high linearity (>60 dBm) [11–15]. In [11], the authors 3.2. Implementation of Tuning Elements claimed that RF-MEMS switches have a figure of merit (FOM) of ~10–20 THz, and that a comparable FOMThe for challenge semiconductor then became technologies how to ismake 250–500 the filter GHz, tunable. which So is 20–40far, we times have detailed less effective our tunability than that solutioof RF-MEMS.n usingA wire 90 V bondings. actuation appliedThis technique to the gate was provides then adapted an electrostatic to our force specifications on the RF-MEMS with a combinationcantilever, allowing of electrically the RF controlled signal to pass devices through, using the a source-drain commercial RF path.-MEMS For high-frequency switch from Radant. structure In comparison with a semi-conductor junction, RF-MEMS technology offers low insertion losses, near Appl.Appl. Sci. Sci. 2019 2019, 9 FOR, 9 FOR PEER PEER REVIEW REVIEW 5 5

Appl. Sci. 2019, 9 FOR PEER REVIEW 5 zerozero power power consumption, consumption, large large power power handling handling and and very very high high linearity linearity (>60 (>60 dBm) dBm) [11 [11–15–].1 5In]. In[11], [11], the authors claimed that RF-MEMS switches have a figure of merit (FOM) of ~10–20 THz, and that a thezero authors power claimconsumption,ed that RF large-MEMS power switches handling have and a figure very ofhigh merit linearity (FOM )(>60 of ~10 dBm)–20 THz,[11–1 and5]. In that [11], a comparable FOM for semiconductor technologies is 250–500 GHz, which is 20–40 times less effective comparablethe authors FOMclaim edfor that semiconductor RF-MEMS switches technologies have is a 250figure–500 of GHz, merit which (FOM is) of20 ~10–40– times20 THz, less and effective that a than that of RF-MEMS. A 90 V actuation applied to the gate provides an electrostatic force on the RF- thancomparable that of RF FOM-MEMS. for semiconductor A 90 V actuation technologies applied to is the 250 gate–500 provides GHz, which an electrostatic is 20–40 times force less on effective the RF- MEMS cantilever, allowing the RF signal to pass through the source-drain path. For high-frequency MEMSthan that cantilever, of RF-MEMS. allowing A 90 the V actuationRF signal applied to pass tothrough the gate the provides source- drainan electrostatic path. For forcehigh- frequencyon the RF- structureAppl. Sci. simulator2019, 9, 1838 (HFSS) purposes, RF-MEMS were modeled by a similar 3D package with an5 of 8 structureMEMS cantilever, simulator allowing (HFSS) the purposes, RF signal RF -toMEMS pass through were modeled the source by - adrain similar pat h. 3D For package high-frequency with an incorporated RLC Model (shown in Figure 4) based on S-parameter data supplied by the incorporatedstructure simulator RLC Model (HFSS) ( pshownurposes, in RFFigure-MEMS 4) were based modeled on S-parameter by a similar data 3D supplied package with by the an manufacturer.manufacturer. simulatorincorporated (HFSS) RLC purposes, Model RF-MEMS (shown in were Figure modeled 4) based by a similar on S-parameter 3D package data with supplied an incorporated by the RLCmanufacturer. Model (shown in Figure4) based on S-parameter data supplied by the manufacturer.

Figure 4. RLC circuit model of an RMSW201 Radant micro-electrical-mechanical system (MEMS) Figure 4. RLC circuit model of an RMSW201 Radant micro-electrical -mechanical system (MEMS) switch.Figureswitch. 4. RLC circuit model of an RMSW201 Radant micro-electrical-mechanical system (MEMS) switch. Figure 4. RLC circuit model of an RMSW201 Radant micro-electrical-mechanical system (MEMS) switch. In In Table Table 1,1 1,the, thethe results resultsresults show show show that thatthat there therethere are are are two twotwo configurations configurations configurations that that that can can can be be be used used used to to to obtain obtain obtain a a a frequencyfrequency shift shift close close to tothe the specifications specifications (250 (250 MHz): MHz): states states S to S SE to SEwith with the the tuning tuning post post at the at the edge edge frequencyIn Table shift 1 close, the toresults the specifications show that there (250 areMHz): two states configurations S to SE with that the can tuning be usedpost at to the obtain edge a (Xpost(Xpost = 1.9= 1.9 mm) mm, and), and states states NSE NSE to toNWSE NWSE with with the the tuning tuning post post at atthe the center center (Xpost (Xpost = 0= mm).0 mm). It is It is (Xpostfrequency = 1.9 shift mm) close, and to states the specifications NSE to NWSE (250 with MHz): the tuningstates S post to SE at withthe center the tuning (Xpost post = 0 at mm). the eItdge is worthworth noting noting that that the the quality quality factor factor is less is less impacted impacted when when the the tuning tuning post post is placed is placed at atthe the edge. edge. worth(Xpost noting = 1.9 mm) that ,the and quality states factorNSE to is NWSEless impacted with the when tuning the posttuning at thepost center is placed (Xpost at the = 0 edge. mm). It is WeWe chose chose to to use use the the first first configuration configuration (Xpost (Xpost= 1.9 = mm). 1.9 mm). In order In order to command to command the filter the electrically, filter worthWe noting chose that to the use quality the first factor configuration is less impacted (Xpost when = 1.9 the mm). tuning In post order is placed to command at the edge. the filter electrically,it was necessary it was necessary to substitute to substitute one of theone twoof the wire two bondings wire bondings of the of SE the state SE state with with the RF-MEMSthe RF- electrically,We chose it was to necessary use the first to substitute configuration one of (Xpost the two = wire 1.9 mm). bondings In order of the to SE command state with the the filterRF- MEMSswitch switch for each for each cavity. cavity. Figure Figure5 shows 5 shows the desiredthe desired configuration configuration with with the the RF-MEMS RF-MEMS switch switch and MEMSelectrically, switch it wasfor each necessary cavity. to Figure substitute 5 shows one ofthe the desired two wire configuration bondings ofwith the the SE RFstate-MEMS with theswitch RF- anda a wire wire bonding. bonding. AccordingAccording toto Radant,Radant, the the equivalent equivalent serial serial inductance inductance of of the the two two wire wire bondings bondings andMEMS a wire switch bondin for g.each According cavity. Figureto Radant 5 shows, the equivalent the desired serial configuration inductance with of the the two RF -wireMEMS bondings switch usedused to toconnect connect the the packaged packaged MEMS MEMS Switch Switch is 0 is.16 0.16 nH nH,, while while the the parallel parallel capacitance capacitance of the of the pad pad is is usedand a to wire connect bondin theg. packaged According MEMS to Radant Switch, the is equivalent 0.16 nH, while serial the inductance parallel capacitanceof the two wire of the bondings pad is 0.010.01 pF. pF. The The filter filter was was designed designed to toresonate resonate at at10.825 10.825 GHz GHz when when the the four four RF RF-MEMS-MEMS were were not not biased biased 0used.01 pF. to Theconnect filter the was packaged designed MEMS to resonate Switch at is10.825 0.16 nGHzH, while when the the parallel four RF capacitance-MEMS were of notthe biasedpad is (OFF(OFF state state = S= state).S state Figure). Figure 6 and6 and Table Table 2 present2 present the the top top view view of ofthe the tunable tunable filter filter and and its its dimensional dimensional (OFF0.01 pF. state The = S filter state). was Figure designed 6 and toTable resonate 2 present at 10.825 the top GHz view when of the the tunable four RF filter-MEMS and wereits dimensional not biased parameters,parameters, respectively. respectively. parameters,(OFF state = respectively.S state). Figure 6 and Table 2 present the top view of the tunable filter and its dimensional parameters, respectively.

Figure 5. 3D model of an RMSW201 Radant MEMS switch. Figure 5. 3D m modelodel of a ann RMSW201 Radant MEMS switch. Figure 5. 3D model of an RMSW201 Radant MEMS switch.

FigureFigure 6. Layout 6. Layout of the of thetunable tunable SIW SIW four four-pole-pole filter. ﬁlter. Figure 6. Layout of the tunable SIW four-pole filter. TableFigure 2. Dimensional6. Layout of the parameters tunable SIW of the four ﬁlter-pole (in filter. mm). TableTable 2. Dimensional2. Dimensional parameters parameters of theof the filter filter (in (inmm) mm). . W W W W l l g Table1 2. Dimensional2 parameters of3 the filter (in mm)1 . 2 6.3 3.5 2.42 2.17 5.8 6.43

lt d b Xpost m n 4 0.3 0.45 1.9 0.6 0.1 Appl. Sci. 2019, 9 FOR PEER REVIEW 6

Wg W1 W2 W3 l1 l2 6.3 3.5 2.42 2.17 5.8 6.43 Appl. Sci. 2019, 9, 1838 lt d b Xpost m n 6 of 8 4 0.3 0.45 1.9 0.6 0.1

4. Fabricating,4. Fabricating, Mounting Mounting and and Measurements Measurements TheThe photograph photograph of the of therealized realized filter ﬁlter is shown is shown in Figure in Figure 7. 7It. wa It wass built built on onstandard standard alumina alumina substratesubstrate.. Measurements Measurements were were performed performed on a on Keysight a Keysight E8364 E8364 A 45 A MHz 45 MHz-50-50 GHz GHz—VNA—VNA and and a a CascadeCascade Microtech Microtech summit summit 9000 9000 probe probe station. station. It is It worth is worth noting noting the the simplicity simplicity of the of the RF- RF-MEMSMEMS biasingbiasing network. network. Indeed, Indeed, a bias a bias line linewas was etched etched on the on theside side of the of thefilter ﬁlter,, allowing allowing the thedistribution distribution of of actuationactuation voltage voltage for forall RF all- RF-MEMSMEMS gates gates through through long long DC DC wire wire bondings. bondings.

FigureFigure 7. Photograph 7. Photograph of the of RF the-MEMS RF-MEMS-based-based tunable tunable bandpass bandpass filter and ﬁlter zoom and on zoom the mounted on the mounted RF- MEMSRF-MEMS switch with switch the with additional the additional wire bonding. wire bonding.

DrainDrain and and source source pads pads were were connected connected to the to thetuning tuning post post and andSIW SIW top topsurface surface,, respectively respectively.. Two Two wirewire bondings bondings were were used used for each for each connection connection in order in order to minimize to minimize the inductive the inductive effect. e ffMoreover,ect. Moreover, a 1 MΩa 1 resistor MΩ resistor was mounted was mounted directly directly on the on other the other side sideof the of filter the filter in order in order to limit to limit the thecurrent. current. The The firstfirst (OFF) (OFF) and and the the second second (ON) (ON) states states were were obtained obtained for actuationfor actuation voltages voltages of 0 and of 93 0 V,and respectively. 93 V, respectively.The simulated The simulated/measured/measured responses responses are shown are in Figureshown8 .in Figure 8. ThanksThanks to the to thefour four additional additional wire wire bonding bondingss (one (one for each for each RF-MEMS), RF-MEMS), the theresultant resultant tuning tuning range range betweenbetween the thetwo two states states was was found found to match to match the thedesired desired specifications. specifications. The The center center frequencies frequencies of the of the twotwo sta statestes were were 10.825 10.825 and and 11.075 GHz, GHz, respectively. respectively. Thus, Thus, according according to the to specifications, the specifications, we designed we designeda tuning a tuning range range of 2.3% of 2.3% (250 (250 MHz). MHz). However, However, it seemsit seems that that the the model model ofof thethe RF RF-MEMS-MEMS needs needs to to bebe further further improved improved in interm term of losses. of losses. The Themeasured measured insertion insertion losses losses of the oftwo the states two were states 4.4 were and 4.4 2.6 anddB, respectively. 2.6 dB, respectively. The measured The measured 3 dB relative 3 dB bandwidths relative bandwidths of the two of states the two were states 2.76% were and 2.76%2.89%, and respectively.2.89%, respectively. Appl. Sci. 2019, 9, 1838 7 of 8 Appl. Sci. 2019, 9 FOR PEER REVIEW 7

Figure 8. High-frequency structure simulation (HFSS) and measured S-parameters (S21 top and S11 down)Figure of 8. the High tunable-frequency filter. structure simulation (HFSS) and measured S-parameters (S21 top and S11 down) of the tunable filter. 5. Conclusions 5. ConclusionsIn this paper, we highlighted the possibility of achieving four-state digital tuning of SIW filters with theIn possibilitythis paper,to we finely highlighted tune the the frequencies. possibility of achieving four-state digital tuning of SIW filters withFeasibility the possibility was demonstratedto finely tune the through frequencies. the design and realization of a two-state tunable filter using commercialFeasibility was RF-MEMS demonstrated switches. through One of the their design remarkable and realization advantages of a istwo the-state simplicity tunable of thefilter biasingusing network.commercial RF-MEMS switches. One of their remarkable advantages is the simplicity of the biasingFurthermore, network.one major difficulty when designing tunable filter arises when attempting to master the tuningFurthermore, range. one major difficulty when designing tunable filter arises when attempting to masterIn this the work, tuning the range. specified tuning range of the filter was set to about 2.3%, but it is possible to widenIn it tothis 24.3% work, by the locating specified the tuning tuning range posts of in the the filter center was of set the to cavities about 2.3%, andby but correctly it is possible using to RF-MEMSwiden it to switches 24.3% by and locating wire bondings. the tuning posts in the center of the cavities and by correctly using RF- MEMSIndeed, switches this paper and wire also bondings. highlighted that the position of the tuning posts defined the tuning range. In theIndeed, tuning this range, paper frequency also highlighted shifts between that the states position were of the controlled tuning posts by the define numberd the andtuning type range. of switchingIn the tuning elements. range, frequency shifts between states were controlled by the number and type of switchingFinally, eleme it is possiblents. to extend the number of states electronically addressed. For example, one solutionFinally, would it be is topossible use a four-pole to extend RF-MEMS the number (SP4T of states RF-MEMS) electronically for each cavity,addressed which. For would example, control one thesolution electrical would connectivity be to use in a each four direction-pole RF- (N,MEMS E, S, (SP4T W) of RF one-MEMS) properly for located each cavity tuning, which post. Thus, would acontrol specification the electrical of four targetedconnectivity sub-bands in each could direction be properly (N, E, S, addressed. W) of one properly located tuning post. Thus, a specification of four targeted sub-bands could be properly addressed. Author Contributions: A.E.M., A.M. (Andrei Muller), J.-F.F., B.P., A.M. (Alexandre Manchec), E.R., C.Q., Y.C., F.D. andAuthor J.L.N. Contributions: contributed to theA.E design,.M., A.M the., J implementation,.-F.F., B.P., A.M., theE.R realization,., C.Q., Y.C. the, F.D measurements. and J.L.N. contributed of the circuits to the and design, the analysisthe implementation, of the results. the realization, the measurements of the circuits and the analysis of the results.

Funding: This study was supported by “Direction Générale de l’Armement: DGA” as part of the FENDER RAPID project. Appl. Sci. 2019, 9, 1838 8 of 8

Funding: This study was supported by “Direction Générale de l’Armement: DGA” as part of the FENDER RAPID project. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Hong, J.-S. Reconfigurable planar filters. IEEE Microw. Mag. 2009, 10, 73–83. [CrossRef] 2. Deslandes, D.; Wu, K. Single-substrate integration technique of planar circuits and waveguide filters. IEEE Trans. Microw. Theory Tech. 2003, 51, 593–596. [CrossRef] 3. Entesari, K.; Saghati, A.P.; Sekar, V.; Armendariz, M. Tunable SIW Structures: Antennas, VCOs, and Filters. IEEE Microw. Mag. 2015, 16, 34–54. [CrossRef] 4. Lee, B.; Nam, S.; Lee, T.-H.; Lee, J. Third-Order Frequency-Agile Substrate-Integrated Waveguide Filter With a Pair of Transmission Zeros. IEEE Microw. Wirel. Compon. Lett. 2017, 27, 566–568. [CrossRef] 5. Zhang, H.; Kang, W.; Wu, W. Balanced bandpass filter with tunable centre frequency based on substrate integrated waveguide technology. IET Electron. Lett. 2018, 54, 886–888. [CrossRef] 6. Bohorquez, J.C.; Potelon, B.; Person, C.; Rius, E.; Quendo, C.; Tanne, G.; Fourn, E. Reconfigurable Planar SIW Cavity Resonator and Filter. In Proceedings of the 2006 IEEE MTT-S International Microwave Symposium Digest, San Francisco, CA, USA, 11–16 June 2006; pp. 947–950. 7. Sekar, V.; Armendariz, M.; Entesari, K. A 1.2–1.6-GHz Substrate-Integrated-Waveguide RF RF-MEMS Tunable Filter. IEEE Trans. Microw. Theory Tech. 2011, 59, 866–876. [CrossRef] 8. Armendariz, M.; Sekar, V.; Entesari, K. Tunable SIW bandpass filters with PIN diodes. In Proceedings of the European Microwave Conference (EuMC), Paris, France, 28–30 September 2010; pp. 830–833. 9. Schulte, B.; Ziegler, V.; Schoenlinner, B.; Prechtel, U.; Schumacher, H. RF-MEMS tunable evanescent mode cavity filter in LTCC technology at Ku-band. In Proceedings of the European Microwave Integrated Circuits Conference (EuMIC), Manchester, UK, 10–11 October 2011; pp. 514–517. 10. Diedhiou, D.L.; Rius, E.; Favennec, J.-F.; El Mostrah, A. Ku-Band Cross-Coupled Ceramic SIW Filter Using a Novel Electric Cross-Coupling. IEEE Microw. Wirel. Compon. Lett. 2015, 25, 109–111. [CrossRef] 11. Rebeiz, G.M.; Patel, C.D.; Han, S.K.; Ko, C.-H.; Ho, K.M.J. The Search for a Reliable MEMS Switch. IEEE Microw. Mag. 2013, 14, 57–67. [CrossRef] 12. Van Caekenberghe, K. Modeling RF MEMS Devices. IEEE Microw. Mag. 2012, 13, 83–110. [CrossRef] 13. Rebeiz, G.M.; Entesari, K.; Reines, I.C.; Park, S.; El-tanani, M.A.; Grichener, A.; Brown, A.R. Tuning in to RF MEMS. IEEE Microw. Mag. 2009, 10, 55–72. [CrossRef] 14. Blondy, P.; Peroulis, D. Handling RF Power: The Latest Advances in RF-MEMS Tunable Filters. IEEE Microw. Mag. 2013, 14, 24–38. [CrossRef] 15. Gevorgian, S. Agile microwave devices. IEEE Microw. Mag. 2009, 10, 93–98. [CrossRef]

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