CHAPTER-4 WIDE BAND PASS FILTER DESIGN 4.1 INTRODUCTION

Transcription

1 CHAPTER-4 WIDE BAND PASS FILTER DESIGN 4. INTRODUCTION The bad pass flters suested last chapter are hav the FBW less tha the 2%. I cotrast of that ths chapter deals wth the des of wde bad pass flter whch are applcable for UWB applcatos. I 22 the FCC (Federal Commucatos Commsso) approved the use of ultra wde bad for commercal purpose wth the frequecy from 3.GHZ to.6ghz [9]. For the UWB purpose, the fractoal badwdth of BPFs usually exceeds %. To acheve such wde bad respose a very stro coupl s requred f the parallel le resoators are used. The practcal mplemetato of such flter s hhly dffcult because to et the tht coupl the spac betwee the ed resoators are very small [-3]. Varous des techque have bee reported the lterature to des the BPF for UWB applcato [93], [94], [43]-[47]. A lare coupl may be acheved by the dess of coplaar waveude BPF [43]; however lare FBW have ot bee realzed. The BPF wth ultra-wde badwdth ad flat roup delay was realzed us lossy materal but wth a hh serto loss of 6.7 db [45]. Aother flter preseted [45] s developed by us shelded mult-layer structure. A electromaetc bad ap (EBG) embedded multple-mode resoator s proposed to costtute a upper stop bad-mproved ad sze-maturzed ultra-wdebad bad pass flter [46]. Three ter dtal ede coupled mcrostrp les are used for coupl ehacemet to des a UWB flter [95]. I ths flter a stepped mpedace ope stub s used for realz trasmsso zero smultaeously upper ad lower stop bads as well as mpedace match. I [94] the flter uses a sle CPW resoator whch s oly a quarter-waveleth lo ad coupled to two mcrostrp ope-crcuted 97

2 stubs parallel o the other sde of a commo substrate. Ultra wde bad flter based o quarter waveleth short crcuted stubs has bee demostrated [47], ths flter s based o quarter-wave leth short-crcuted stubs model. A UWB flter s desed based o a crcut model for a optmum short crcuted stub flter whose ut elemets or coect les are o redudat [93]. A wdebad BPF s costructed by a drect cascade of a LPF ad a HPF. Both upper ad lower trasto bads ca be determed dvdually as lo as the put ad output mpedaces of both flters are matched. I [9] a hh pass flter s combed wth a stepped mpedace low pass flter to realze UWB BPF. I ths chapter two dfferet structures are proposed for wde bad pass flter for UWB applcato. These two structures are the based o the combatos of hh pass ad low pass flter. The hh pass flter des for both the structures s same where as the LPF used are dfferet. I the ext secto the des techque of HPF s dscussed based o the des techque [], [2]. 4.2 WIDE PASS FILTER DESIGN USING TRIANGULAR PATCH Ths hh pass flter may also be rearded as pseudo hh pass flter [] because the basc des techque s based o the bad pass flter des for wde bad applcato us short crcuted stubs. The hh pass flter structure desed here s based o the fure show (4.). Ths cossts of quarter wave leth lo short crcuted stubs coected wth half waveleth lo le. For a partcular flter order the hh pass flter respose depeds upo the characterstcs admttace of the coect les ad characterstc admttace of stub les. The des equatos for determ the characterstc admttace are descrbed [2] are ve as follows: 98

3 h FBW (4.) F. 4. basc crcut of hh pass flter, l,2 to l -, :leth of coect les, l to l : leth of short crcuted stubs, to : admttace of short crcuted stubs,,2 to -, characterstc admttace of coect les, characterstc admttace of source ad load.,, 2,2, h h h (4.2) For = 2 to ,, 2 ta h N (4.3),2,2 ta 2 N h (4.4)

4 ,, ta 2 N h (4.5),,,, N N for =2 to - (4.6),, for = to - (4.7) where are the elemet values of a ladder-type lowpass prototype flter such as a Chebyshev, ve for a ormalzed cutoff frequecy =..rad/sec ad h s a dmesoless costat whch may be assed to aother value so as to ve a coveet admttace level the teror of the flter [2]. By us the above set of equatos of the wde bad pass flter a pseudo hh pass flter has bee desed. I the hh pass flter desed here, oly two quarter wave leth (of the ceter frequecy) lo short crcuted stubs of characterstc admttace /35 mho are used. These are coected wth les wth characterstcs admttace /5mho. Us the smulatos the leth are optmzed. The lay out of the HPF s ve fure (4.2) F. 4.2 Hh pass flter structure us optmum method. The hh pass flter s smulated ad the results for scatter parameters are show fure (4.3). The 3dB cut off frequecy of the hh pass flter s

5 2GHz. The lowpass flter of fure (2.37) s cascaded wth the above HPF to des a wde bad flter. The cascaded structure s show fure (4.4) S parameters S2 parameters S parameters db Frequecy GHz F 4.3 S-parameters of Hh pass flter. F 4.4 Cascaded structure of HPF ad LPF structures. The smulated scatter parameters of ths cascaded structure are show fure (4.5). The lower ad upper 3dB cut off frequeces of the cascaded flter are 2.84GHz ad.448 GHz respectvely. The maxmum retur loss pass bad s.33db. The FBW of ths flter s 3% ad the total sze of ths cascaded flter s 26.4mm.

6 S parameters S2 parameters S-parameters db Frequecy Ghz F.4.5 Smulated S-parameters of cascaded structure. The sze of the cascaded flter ca be further reduced by modfy the structure as show fure (4.6). Throuh ths modfcato the sze of the cascaded structure s reduced from 26.4mm to 8.25 mm. Ths structure s smulated us IE3D ad the smulated results are show fures (4.7). F. 4.6 The modfed structure wth traular patch placed o the HPF. 2

7 S parameters S2 parameters S-parameters db frequecy GHz F.4.7 S-parameters of cascaded wde bad flter. From the smulated results t s observed that the lower ad upper 3dB cut off frequeces of the modfed structure are.5ghz ad.5ghz ad the FBW s about 5%. The maxmum retur loss pass bad s observed as about 3.7dB whch eeds some mprovemet. The structure s further modfed as show fure (4.8). Ths ew structure s smulated ad the S-parameters S ad S 2 are show fure (4.9). F. 4.8 Wdebad flter embedded wth HPF ad LPF. 3

8 S parameters S2 parameters S-parameters db Frequecy GHz F. 4.9 S-parameters of the embedded structure. The lower ad upper cut off frequeces of the embedded structure s 2.GHz ad.8ghz ad the FBW s 3%. The maxmum retur loss pass bad s observed as less tha 5dB ad the mmum serto loss the stop bad s 5dB up to more tha 2GHz. The total leth of the flter show s 2mm. The smulated parameters of the structure show fure (4.8) are compared wth the scatter parameters of the structure show fure (4.4). These comparsos of S ad S2 parameters are show fures (4.) ad (4.). 4

9 Cascaded Structure Embeded Structure S db Frequecy GHz F 4. Comparso of S parameters of cascaded ad embedded structure. Cascaded Structure Embeded Structure S parameters db Frequecy GHz F. 4. Comparso of S 2 parameters of cascaded ad embedded structure. From the comparsos of S-parameters t s evdet that the wdebad flter show fure (4.8) s much superor to the flter show (4.4). The UWB flter show fure (4.8) s fabrcated o the substrate wth delectrc costat 3.2 ad the heht.762mm ad s show fure (4.2). The measured scatter parameters S 5

10 ad S 2 compared wth the smulated values of the embedded structure. These comparsos are show fures (4.3) ad (4.4). F. 4.2 Fabrcated wde bad flter us traular patch resoators. S db Measured S Smulated S Frequecy GHz F. 4.3 S parameters of smulated ad fabrcated structures. 6

11 Measured S Smulated S S2 db Frequecy GHz F. 4.4 S 2 parameters of smulated ad fabrcated structures. From the scatter parameters show fures (4.3) ad (4.4) t s observed that the fabrcated results are well areemet wth smulated values. 4.3 WIDE BAND FILTER DESIG WITH FRACTAL DGS A ovel structure of ultra wde bad flter has bee proposed ad mplemeted ths secto. The des techque cossts of three parts. I frst part a ultra wde bad low pass flter has bee desed us the applcato of fractal structure as defectve roud plae whch ter works as a electromaetc bad ap (EBG) structure based o Serpsky trale o the roud plae of a 5 ohm le as show fure (4.6). The 3dB cut off frequecy of the LPF s.6ghz. The Serpksky trale based DGS structure s show fure (4.5). The values of the dfferet dmesoal parameters lke a, b, d ad are obtaed throuh lare umber of smulatos. The dmesos of the slots are obtaed as a=.2 mm, b=.4mm, =3.9mm ad d=5.48mm. 7

12 F. 4.5 The proposed DGS based o Serpsk trale. F. 4.6 The proposed lowpass flter structure. The smulated S ad S 2 parameters of ths LPF are show the fures (4.7). 8

13 S parameters S2 parameters S-parameters db Frequecy GHz F. 4.7 Respose of the low pass structure wth 3-dB cutoff frequecy.6 GHz. The HPF s the desed us the techque descrbed prevous secto. The dmesos of the HPF are show fure (4.8). F. 4.8 Structure of the Hh pass flter, L=5.9mm, c=.2mm, l=6.65. The LPF of fure (4.6) cascaded wth the pseudo HPF of fure (4.8) to acheve the wde bad flter show fure (4.9). The leth of ths cascaded structure s 25.5mm. The structure s smulated wth IE3D ad the smulated S-parameters are show fure (4.2). The lower ad upper cut off frequeces of the cascaded flter s 2.3 GHz ad.55 GHz respectvely. The FBW s acheved as 35%. The maxmum retur loss the pass bad s 9

14 .5dB ad the mmum serto loss stop bad up to 6 Ghz s about 2dB. F. 4.9 Cascade structure of HPF ad LPF. F. 4.2 The smulated S ad S 2 parameters of the cascaded structure. The sze of the above wde bad pass flter ca be made more compact by embedd both the flter LPF ad HPF oe cofurato as show fure (4.2). Ths flter s also smulated wth IE3D ad the smulated S- parameters are show fure (4.22).

15 F. 4.2 Structure of BPF for Ultra wde bad. S parameters S2 parameters S-parameters db Frequecy GHz F Respose of Bad pass flter whch s the combato of resposes for lowpass ad hh pass flters. From the smulated results of the structure show fure (4.27) t s observed that the lower ad upper cut off frequeces are 2GHz ad.35ghz ad the fractoal bad wdth s acheved as 35%. The maxmum serto loss up to 6GHz the stop bad s more tha 25dB. The results of the structure show fure (4.2) are compared wth the results of the structure show fure (4.9). These comparsos of S ad S 2 parameters are show fures (4.23) ad (4.24) respectvely.

16 Cascaded strucure Embedded Structure S-parameters GHz Frequecy GHz F.4.23 Comparso of the S parameters of cascaded ad embedded structures. Cascaded structure Embedded structure - S2 db Frequecy GHz F Comparso of the S 2 parameters of cascaded ad embedded structures. From these results t ca be observed that the structure show fure (4.2) s more compact where the sze reduces from 25.5mm to 8.5mm ad ves better pass bad characterstcs. 2

17 4.4 CONCLUSIONS The two dfferet structures of ultra wde bad flters are desed. The des methodoloy s based o the combatos of hh pass flter ad low pass flter. I oe structure a low pass flter respose s acheved us traular patch resoators whch have bee embedded wth two short crcuted stubs. The resultat crcut s very compact as show fure (4.2). The measured ad smulated results ve a very ood match whch s depcted fures (4.3) ad (4.4). Aother structure of LPF for UWB BPF s desed us the fractal patter o the roud plae of a 5 ohm mcrostrp le as show fure (4.6). The hh pass respose s acheved from the shorted stubs. The embedded structure whch combes hh pass ad low pass flters toether ca be vewed from the fure (4.2) as a UWB flter. I both the cases the FBWs are more tha %., ad crcut sze s very small. 3