INVESTIGATIONS ON SPECIFIC MICROSTRIP FILTERS WITH DEFECTED GROUND STRUCTURE

Abstract

\ licrowave filters are very esseiitial comf)onents of iiiodern wireless comiminica-ti()11 SVSt('11)5 [1]. A filter is a. two l)ort network that provides perfect transmission of signals with frequencies in certain passl)ancl and infinite all enual.ion in the stop I )a)1(l regions [2]. 1\ Ii(TOWaVo' systelil olteii requires a meaiis for suppressing 1111\Vn1ited signals and/or separating signals having (hiferent frequencies. These fiuictions are performed by filters. Filters arc' geiierallv categorized in terms of their frequency characteristics, such as, low-pass. lngh-pass. handpass. and ])l1i(lst()1). A llli('rostril) filter consists >f a coiuliio'tiiig 5t1'i1) lili(' on ()I1(' si( Ic' of a di-electric sul)stral e and a ground plane OH the oilier side. The performance of liiicro\vnve filters can be analyzed in terms of sharl)ness or roll-off factor. inser-lion loss, ret urn loss. s11l)l)1'essioii of higher liarnioiin's. l)an(lwi(ll ii and selectivit, Geiiciallv. a iincrosi 111) fillet call he designed by 11sing, two iiiellaals. One is in- - sen ion loss iiiet 110(1 and af1011I(1' one is image l)a1'allletel' iiiei ho(l. 13oth the image parammieter and insertion loss methods of filter design provide hunped el-enieni circuits. These litnipeci element circiuts are convert e(l into (list nibuted circuit -'leiiient s using Richard's transfhrmations [2]. However, these techniciues for designing microwave filters miiakc the circuits bulky. Since the late 198Os. 1 echniques eniploviiig the electromagnetic bandgap (EBG) or photonic bandgap (PBG) struct imnes and defected ground structures (DGS) attracte(l the atten-tion of the researchers, to achieve mimaturizat ion. reducing surface wave prob- lenis and reinovuig an arl)it 101'V SI ()1)I)uii(l iii nhjerowave coni1xoieh1t s. Re(ei it lv. - various other te(-hm(jlles. such as. met aniaterials or complement ary split ring 1(Soi1lt 015 (CSRR ) low I ehlll)('ial liFe (:0-hl'Ni ceranmics ( LT((_' ) low I cmnpera- - ture co-fired ferril ('S ( LT('F) defected mnicrostrip (D\IS) . ground plane al)ertlmre (CPA.) and substrate integrated waveguides (SIW) are also eXploit e( I for size reflilci 1011 and improving the 1)erlormimamlce of nnowave circuits. But DCS 'is a simple, popular and ('lh(ieihl mud 110(1 ahuiolig all Illes(' I edlliii(Ii1es of mmiiiiiatiim'iza-tioii of microwave coml)oneflt 5. part icularlv. hiters [3-10]. The (oii(el)t of DGS is originated from I lie electromuiagnetic/plioi onic bamalgap structures. Basically. the DGS is a I runcated version of an EBG/PBC struict iim'e vit ii one or two Ilimit - cells. The EBC/PBCs are pem'i( idic structures. These teclumu(!ues have an attrac- tive l)rol)eltr to stop a l)alti(111a1 l)mual of frequency and size reduction in filter - design [ii 14]. It is c()iumpleX to use EBC/PBC structures for the design of the microwave Or niillimiwter wave (oilll)oilents mine to the (lifhdillties of modeling. The other (lilli(llltV in using the L13C/P13C dilduit is (alise(l lmv the rmahiatioui lromii the l)erio(li(aIlv (I (lied (lelects but in DGS radiation loss is less due to the eniplov-weimt of limit em! intiumi mer of I 1w unit cells of EBC /PBC (on(, or two only). DGS is ml simuul)le slouiw. \Vhii(]i is ('I cued in the ground l)lhlue of lfli(T05t1ij) line. The (lilileimsions of defect list miii) the dm1-remit (list i-il on ion iii the ground plane of iiui-11(1St nj) line \vlmi(hi comitiols the dX(ittlliofl and pro)agaI ion of electmomuuagmuet ii waves t liromigli simi )st mat Liver [15 21)]. The shape of the defect may vary from a simuiple to a complex geolnel ry [11]. The defect iii I lie ground plane of phi-iiar I rmummsmissioii hues also changes the clmaract eristims of the transmission line, sudi as, inductance and capacitance [ii. The l)CSs have been used or can he used for the design of microwave (omponemll s such as imower amplifiers [21. 22]. microwave oscillators [23]. dividers [24]. couplers [25] (ollmi)iiiers [26]. anton-nas [27-31] and filters 95-211 [32-361 in ma(lar systems, o]tical and wireless coiiiiiiiiiii<i-il ion systellis [37 59]. A lot of literature has been reported on mi- - (rostrip filters mith (lefecte(l groiui(l stl11(tlll(' to unprove the f)elforllian(e of filters [5-- 29] [:32 36.60 64]. H()w('Ver, there is still a scope for in1)rovn1g the l)('1foi'lllain(' of iiiicrost.rip filters with DGS Various aspects of the DGS ap-I)livat ion in planar iiiiciost lii) filters have iiol. Vet hilly lx'eii exp1ore I, all lioiigli it has been deveh)ped to a great extent for oilier iiiicrowave CO11II)ofleilts. The I asic topologies of DGS were not eXplore(l to achieve sharpness in iilicrosl.rip filters. Fractal geoiiietries are also not investigated and discussed iii detail with (lilTerent positiolls of fractals in (lulnbbell-shaped (conventional) DGS [65]. The nnnuuhmml ('SB R-lXSs (square C'SRR a.11(l circular ('SBR) are available for - filter design [5.6.66-69], hail these ('SRR-DGSs are not extensively explored and investigated for filter design. \'Tery less has been reported on the design of THz filters with DGS. This has l)rovi(led the basic iIlil)et US to (arry out the design and realizat.ioii of microwave filters with DGS. Most. of the above mentioned prol)lemns are discussed in this work which fhriiis the basic embo(lilnent of the thesis. l'his thesis l)1esents a (letaile(l analysis of microstrip filters with various DGS stnictures with t.11('il eiilianccd )iopeit iCS. lli(' first (liapt er of I his thesis l)r(seii1.s an iii! roduct 1011. mnol iviit.ion and scope of the work and orgailizat ion of the thesis. Iii chapter 1 wo of time thesis. a (onlprehlensive oVerview 01 ii he releva.iit lit eral iire survey is gjy [16]. (iiaptei I lI1('(' I >ieseiul.s the design and realization of nhicrostril) filters with a new (lelected ground structure ( l)CS) . \'ariolis conventional DGSs (dinubbell-shaped) are available in literature but these DGSs have sonic liiiiii ations. The imiaiii limitation of conventional DGS filters is the DGS itself which is responsi-ble for losses duc' to its large area. The sliaipiiess of filter depends oii effective (apacit ance and this effective capacitance depends on the slot, gap which can be - a(iJust.e(l 1i1)f() certain extent to increase the sharpness of the filter. So. in this chapter a new DGS is prol)(.)sed and cornl)ared with (.()iiVent.ioflai DGS. This new ill 1)CS l)rovides an (Xl ia ( legiee ol lree(li)In to iiiiirove I he ixrliaiiiaiiev of filters. ( )n the I)aSi5 of (01111)aral ive StlI(lies. l)?Ui(lstOl). l)an(Il)ass and low pass filters - are designed with the j)1OI)OSC(I IICW DGS tO)Ol0gV [it)]. The l)I'Ol)Os('(I filters are designed and fabricated using 50ft ,\/4 illicrostrip line vliicli is very coin-pact when (olllparc( I wit 11 (oulv('lit ioiial nilurost lii) filters. A I We Step Vkuli(lal 1011 P10(e(11lr(' is ?ldal)ted. Design afl(i co-design are (allied out using HFSS (1cc-ti'oiiiagiie6c(EM) siiiinlator and ADS (ircilit simulator respectively. and these results are (01n1)are(i vit Ii the exl)eiimelil al results. For this p111])05('. all the de-signed hit ers were fahrical ('(1 and 521/S1 respoiises were nieasured using Rolide Schvarz \'eclor Network Analyzer 1127.8500. (1lIal)t 01' louii' l)1'('s('lits the (letaile(l (lesigli si 11(Ii('s 01 ( iele('t e(i grouw(l Stifle-htues wit Ii lra(tal geounet ries [ii]. The ('oliVeiili(ilDli DGSs ( dulul)b Il-sli 1x'd) have a flat rejeclion l)1'01)01'tY. To design and u'ealizc' lOW l' filters \vit Ii StOOl) reject i011 Pi'Ol)('FtV. iiiauiV of these couV('iil ional DGSs are neede(l ho be cisca(le(l. which iiieaiis higher insert ion loss and larger size. Fractal DGS has two 11111)01'-taut properties: one is sell-smiilaril v for 1111111 il)iuld application and other one is s)a('c-hil1i1ig For (( )111j)a('l hess [72]. Ihe design si uidies of uiiici'owave filters \vil 11 Iraclal DGSs are thus presented in t his ('1011)! ('F. Tlles(' Iriut al geoweh lies have l)01'fl eluul)loVec I at (liffereuut 1)ositjOflS of dulnl)1)ehl-shlape(i DGS and i'ornpared. \liikowski nuol Peauio frutals are eluuplove(l Ihi' the design of low pass filters at the citt -oil 11e(luiencies 5.1 Cllz and 1.9 Gllz respectively [3-79I All the de-signed fillers ai'e fabricated and I este(l in VNA and the un('asure(i results are in good agreeuuleiul with the siiuiulah cC! results. Chapter five fhcuses (il riucrostril) filters with couuiplementary split ring res-onahors (CSH ii) as defect e(l grouuuud structure [$0 81. In I his chapter various ('SRR-DGSs (including conventional DGS) are compared. On the basis of these comparison a new CSRflDGS is pps'l with enhanced filter properties. This - DOW CSRR-DGS topolgv is used to design filter (bandpass. lowpass. and dual- lv l)alld bail(lI)ass). All t.11(' (lesiglie(l filter.,: with DC1S, discussed aov' are fabri- - cated and tested and the measured results are aniiable with those Ol)taifle(l using the full wave simulators. Chapter six deals with design studies of niicrostrip bancistop filters with the defected ground structure at terahertz frequency. in this chapter, a planar mi-crostrip terahertz (THz) bancistop filter has been proposed with defected ground structure with high insertion loss (S21) in stopband of -25.8 dB at 1.436 THz. A dielectric substrate of Benzocyclolmten (BCB) is used to realize a compact band-stop filter using modified hexagonal dumbbell shaped defected ground structure (DB-DGS) in a. A/41 50f1 microstrip line at THz frequency range for compact-ness [86-89]. The proposed filter can be used for sensing and (letectiorl in biorned-ical instruments in DNA testing [90 93]. Finally, in chapter seven, the continutious made in the thesis are given and scope for the future work is outlined. In summary, the thesis contributes towards the improvement of filter performance.