5 Gi-Re Kim : DESIG OF COUPLED RESOATORS BADPASS FILTER WITH DEFECTED GROUD STRUCTURE Design o Coupled Resontors Bndpss Filter with Deected Ground Structure Gi-Re Kim, ember, KIICS Abstrct In this pper our-pole elliptic unction bndpss ilter is designed with two ground slots. A reserch o microstrip bndpss ilters (BPF) using deected ground structures (DGS) is presented. DGS technique llows designs o tight couplings without the necessity o using very nrrow coupling gps. The simultor Sonnet is used to design the resontor nd to clculte the coupling coeicient o the bsic coupling structure. Compred to similr microstrip ilters without deected ground structure, the simulted perormnces o these novel structures indicte some technologicl dvntges. Index Terms compct coupled resontors, bndpss iler (BPF), deected ground structure (DGS). I. ITRODUCTIO I wireless communiction systems, smll size nd high perormnce ilters re needed to reduce the cost nd improve the system perormnce. They cn be designed in mny dierent wys. However, urther minituriztion becomes more diicult or this ilter. Plnr ilters provide good minituriztion bility [ 4]. Thereore, there hs been much reserch conducted on plnr ilters nd their components. Since microstrip resontors re the bsic components o plnr ilter design, it is necessry to select proper resontor types used in ilter design. A conventionl ht-wvelength open-line microstrip resontor is too lrge to be used in the modern communiction system such s 9 Hz, 8 Hz or personl communiction systems (PCS). The hirpin ilters [, 5 7] were olded rom the open line wvelength microstrip resontor to become U-shped resontors nd mke progress in circuit size reduction rom the prllel-coupled line structure. Ground slots hve mny pplictions in microwve techniques. Slot ntenns nd slot coupled ntenns [8] hve been continuously developed nd re widely used in communictions. The slot coupling is convenient wy to couple microstrip lines in multilyer circuits [9]. In this pper re presented investigtions on the eects o ground slots on the couplings between hirpin resontors. Ground slots in the ground plne cn enhnce the electric coupling, or the electric prt o mixed nuscript received rch, ; revised rch 6, ; ccepted April 9,. Gi-Re Kim is with the Deprtment o Electronics Engineering, Sill University, Busn, 67-736, Kore, (Emil: grkim@sill.c.kr) coupling between two djcent resontors. The results o these investigtions were used in the design o some ourpole cross coupled plnr microwve bndpss Filters with pir o ttenution poles t imposed inite requencies. The Filters were designed with two ground slots. II. FILTER CHARACTERISTICS The ilter hving only one pir o trnsmission zeros (or ttenution poles) t inite requencies gives much improved skirt selectivity, mking it vible intermedite between the Chebyshev nd elliptic-unction ilters, yet with little prcticl diiculty o physicl reliztion. The trnser unction o this type o ilter is F n S Ω + ( Ω ) () ( ) ε Fn ε r LR ΩΩ ΩΩ+ Ω Ω Ω +Ω ( Ω ) cosh ( n ) cosh ( Ω ) + cosh + cosh where Ω is the requency vrible tht is normlized to the pssbnd cut-o requency o the lowpss prototype ilter, ε r is ripple constnt relted to given return loss L R log S in db, nd n is the degree o the ilter. It is obvious tht Ω ±Ω (Ω >) re the requency loctions o pir o ttenution poles. ote tht i Ω the iltering unction F n (Ω) degenertes to the milir Chebyshev unction. The trnsmission requency response o the bndpss ilter my be determined using requency mpping, i.e., ω ω Ω FBW ω ω in which ω is the requency vrible o bndpss ilter, ω is the mid-bnd requency nd FBW is the rctionl bndwidth. The loctions o two inite requency ttenution poles o the bndpss ilter re given by () (3) (4)
ITERATIOAL JOURAL OF KIICS, VOL. 9, O., April 5 ( FBW ) FBW 4 ω ω Ω + Ω + (5) w. ormlly, smller gp nd nrrower line result in stronger I/O coupling or smller externl qulity ctor o the resontor. ( FBW ) FBW 4 ω ω Ω + Ω + (6) III. COUPLIG COFIGURATIOS The microstrip circuit ws designed on Telon dielectric substrte, with thickness o.7874 mm, dielectric constnt ε r o.7. In order to develop pplictions or the.8 GHz requency bnd, the sizes o microstrip hirpin resontors were conirmed. The hirpin resontors size is: W5.5mm, L3.7mm, S8mm. The ground slots re ll rectngulr, with dierent lengths l slot nd widths w slot. The geometries o the min coupling conigurtions re shown in Figs.. Fig. (). Electric coupling conigurtion. Fig.. The coupling o resontor with its eed line. The design prmeters o bndpss ilters, i.e., the coupling coeicients nd externl qulity ctor in Fig. 3, cn be determined in terms o circuit elements o lowpss prototype ilter o Fig. 4, which consists o lumped cpcitors nd idel dmit The design prmeters o bndpss ilters, i.e., the coupling coeicients nd externl qulity ctor in Fig. 3, cn be determined in terms o circuit elements o low-pss prototype ilter o Fig. 4, which consists o lumped cpcitors nd idel dmittnce inverters. The reltionships between the bndpss design prmeters nd the low-pss elements re Q C ei Qeo (7) FBW n, n n, n+ FBW, or nto CC n n+ (8) Fig. (b). gnetic coupling conigurtion. FBW Jm mm, +, or m (9) C m FBW J, or m m m, m+ () Cm Fig.(c). ixed coupling conigurtion. Typicl input/output (I/O) coupling structures or coupled microstrip resontor ilters, nmely the coupled line structures, re shown with the microstrip open-loop resontor. The coupling o the coupled line stricture in Fig. cn be ound rom the coupling gp g nd the line width where denotes the rctionl bndwidth o the bndpss ilter, is the cpcitnce o the lumped cpcitor nd is the chrcteristic dmittnce o the inverter, nd is the degree o the ilter. To ind the element vlues o low-pss prototype, one my use n pproximte synthesis method described in []. This method is simple, but it suers rom inccurcy nd cn even il or highly selective ilters tht require moving the ttenution poles closer to the cuto requencies o the pssbnd. However, the exct synthesis process is rther complicted nd time consuming becuse there does not exist ny closed-orm ormuls or the element vlues. The vlues o the ttenution pole requency were chosen such tht they
5 Gi-Re Kim : DESIG OF COUPLED RESOATORS BADPASS FILTER WITH DEFECTED GROUD STRUCTURE cover wide rnge o prcticl designs or selective microstrip bndpss ilter responses. Reerring to Fig., the side lobe t the stopbnd would be too high i is smller thn the given vlues. obtined by using E simultion tool Sonnet. By using the design curves o the coupling coeicient nd the externl Q, this ilter cn be relized. Fig. 3. Generl coupling structure o the bndpss ilter with single pir o inite-requency zeros. Fig. 5(). Electric coupling coeicient. Fig. 4. Lowpss prototype ilter or the ilter synthesis IV. COUPLIG COEFFICIETS The requency responses o the coupling structures were obtined by using method o moments (o) commercil simultion sotwre []. The coupling coeicient ws clculted rom the two split-resonnce requencies []. The cross-coupled structure provides electric, mgnetic, nd mixed coupling. Using prmeter-extrction technique, we crry out Esimultions to extrct the externl Q nd coupling coeicient ginst the physicl dimensions. The coupling coeicient ij o ech pir o coupled resontors cn be clculted by using the eqution s ollows: Fig. 5(b). gnetic coupling coeicient. ij + () where, nd re the lower nd higher split resonnt requencies. Thereore, the design curves or these coupling structures cn be presented nd shown in Fig.. The externl Q o such eed structure cn be chrcterized by the ollowing: Q () Δ 3dB where, is the resonnt requency o the resontor nd Δ 3dB is the 3 db bndwidth. The design curve o the eed loction on the resontor or given externl Q cn be Fig. 5(c). ixed coupling coeicient. V. FILTERS DESIG AD SIULATIO A our-pole ellipticl bndpss ilter response cn be implemented using the cross coupling between nondjcent resontors. The cross-couplings give the input signl two pths rom the input port to the output. The mgnitude nd phse o the signl re chnged dierently though dierent pths. As mentioned bove,
ITERATIOAL JOURAL OF KIICS, VOL. 9, O., April 53 the multipth eect my cuse ttenution poles t inite requencies i the couplings mong the resontors re properly designed. Fig. 6 shows the our-pole elliptic unction bndpss ilter with using miniturized hirpin resontors. In the conigurtion, signiicnt couplings exist between ny two neighboring miniturized hirpin resontors. The structure cn be extended to orm crosscoupled ilters o higher orders. Cross-coupled bndpss ilters with using compct hirpin resontors re designed to hve rctionl bndwidth o Hz t mid-bnd requency o.8 GHz. The ilter ws bricted on Telon substrte. The reltionships between the bndpss design prmeters nd coupling coeicient re s ollows [4]: K.3, K 3.393, K 34.373 nd K 4.3. The spcing designted in the cross-coupled ilter is s ollows: w. mm, w.4 mm, w 3 5.8 mm, w 4.9 mm, w 5. mm nd w 6.6mm. The eects on the center requency nd bndwidth re negligible s long s the gp tuning is smll. The mesured pssbnd insertion loss is -3 db, good greement with the simultion. Fig. 7 shows the simulted nd mesured chrcteristics o the ilters. Both ilters exhibit good rejection t second hrmonic s predicted. Fig. 8 shows the prototype o bndpss ilter. Fig. 8. Fbricted prototype. VI. BADPASS FILTERS WITH DGS DESIG AD SIULATIO Bsed on the bove results, some 4-pole cross-coupled plnr microwve BPFs with single or with two ground slots were designed. The slots re chosen in the center o ground plne (9mm 3mm). Fig. 9 shows the lyout o the microstrip our-pole elliptic unction compct iler with DGS. Fig. shows the simulted nd mesured chrcteristics o the ilters. Fig. shows the top plne o proposed structure, nd Fig. shows the ground plne o proposed structure. Both ilters exhibit good rejection t second hrmonic s predicted. But the spcing is chnged. The spcing is s ollows: W. mm, W.4 mm, W 3 5.8 mm, W 4.9 mm, W 5.mm nd W 6. mm. Fig. shows the prototype o bndpss ilter with DGS. Fig. 6. Lyout o the microstrip our-pole elliptic unction compct iler. Fig. 9. Lyout o the microstrip our-pole elliptic unction compct iler with DGS. Fig. 7. Simultion nd mesured results o BPF.
54 Gi-Re Kim : DESIG OF COUPLED RESOATORS BADPASS FILTER WITH DEFECTED GROUD STRUCTURE The min dvntge o the BPF with ground slots stys in the possibility o using lrger gps between resontors. This solution is especilly convenient when tight couplings re needed. The ilters lyouts were designed ter study o the coupling coeicients versus gps, bsed on E-ield simultion. The designed ilters with ground slots under the mixed coupling were bricted nd tested. The mesured requency responses o these proposed ilter structures demonstrted the vlidity o the design nd o the Eield simultions. The DGS design cn be lso pplied to mny other types o bndpss ilters, llowing relxtion o the briction tolernces. I better perormnces re required, lower loss substrtes should be used. REFERECES Fig.. Compre o simultion results o BPF. Fig.. Fbricted prototype o bndpss iler with DGStop lyer. [] S.B. Cohn, Prllel-coupled trnsmission line resontor ilters, IEEE Trns. icrowve Theory Tech., vol. 6, pp. 3 3, 958. [] S. Cspi nd J. Adelmm, Design o combline nd interdigitl ilter with tpped-line input, IEEE Trns. icrowve Theory Tech., vol. 36, pp.759 763, 988. [3] G.L. tthei, Comb-line bndpss ilter o nrrow or moderte bndwidth, IEEE Trns. icrowve Theory Tech., vol. 6, pp. 8 9, 963. [4] U.H. Gysel, ew theory nd design or hirpin-line ilters, IEEE Trns. icrowve Theory Tech., vol., pp. 53 53, 974. [5] E.G. Cristl nd S. Frnkel, Hirpin-line nd hybrid hirpinline/hl-wve prllel-coupled-line ilters, IEEE Trns. icrowve Theory Tech., vol., pp. 79 78, 97. [6] J.S. Hong nd.j. Lncster, Cross-coupled microstrip hirpinresontor ilters, IEEE Trns. icrowve Theory Tech., vol. 46, pp. 8. 998 [7] G.L. tthei,.o. Fenzi, R.J. Forse, nd S.. Rohling, Hirpin combilters or HTS nd other nrrow-bnd pplictions, IEEE Trns. icrowve Theory Tech. vol. 45 (997), 6 3. [8] RAO Q., JOHSTO R. H., odiied Aperture Coupled icrostrip Antenn, IEEE Trns. Antenns Propgt., vol. 5, no., pp. 3397-34, 4. [9] KI J. P., PARK W. S., An Improved etwork odelling o Slot-Coupled icrostrip Lines, IEEE Trns. icrowve Theory nd Tech., vol. 46, no., pp. 484-49, 998 [] User's nul, Sonnet Sotwre, Inc., ew York - Sonnet Proessionl, version.5, 5. [] HOG J.S., LACASTER.J., Couplings o icrostrip Squre Open-Loop Resontors or Cross-Coupled Plnr icrowve Filters, IEEE Trns. icrowve Theory nd Tech., vol. 44, pp. 99-9, 996. [] CAERO R., Advnced Coupling trix Synthesis Techniques or icrowve Filters, IEEE Trns. icrowve Theory nd Tech., vol. 5, 3. Fig.. Fbricted prototype o bndpss iler with DGS bottom lyer VII. COCLUSIOS Gi-Re Kim received the B.S.,.S. in Electronic Engineering rom the Sogng University, Seoul, Kore, in 986, 988, respectively. He lso received Ph.D. rom Kyungnm University, Kore in 999. From 988 to 993 he ws resercher in Communiction Reserch Center o Smsung Electronics Co. Ltd. Since 999, he hs been on the culty o Electronics Engineering Deprtment t the Sill University, Busn, Kore.