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Ultrawide stopband low-pass filterusing triangular resonators defectedgroundHany Taheraba Electrical Engineering Department, Faculty of Engineering andIslamic Architecture, Um Alqura University, Alabdia, PO (5555),21955 Makkah, KSAb Microstrip Department, Electronic Research Institute, Giza,EgyptPublished online: 09 Jan 2014.

To cite this article: Hany Taher (2014) Ultrawide stopband low-pass filter using triangularresonators defected ground, Journal of Electromagnetic Waves and Applications, 28:5, 542-550,DOI: 10.1080/09205071.2013.879048

To link to this article: http://dx.doi.org/10.1080/09205071.2013.879048

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Ultrawide stopband low-pass lter using triangular resonatorsdefected ground

Hany Tahera,b*

aElectrical Engineering Department, Faculty of Engineering and Islamic Architecture,Um Alqura University, Alabdia, PO (5555), 21955 Makkah, KSA; bMicrostrip Department,

Electronic Research Institute, Giza, Egypt

(Received 10 July 2013; accepted 22 December 2013)

In this article, a novel complementary triangular split ring resonator (CTSRR)using defected ground structure (DGS) is introduced. The unit is implemented inmicrostrip technology. The CTSRR has unique lter characteristics, including sixdimension-dependent nite attenuation poles with wide bandgap, very highattenuation rate, and very low level of passband ripples. CTSRR units with differentdimensions CTSRR units are cascaded together to design a high-performancelow-pass lter (LPF) with a cutoff frequency ( fc) of 1.80 GHz. The designed LPFexhibits an attenuation rate of 250 dB/GHz and passband ripples of less than 0.30dB. Moreover, it has a wide 20-dB stopband at up to 16.5 times fc. To ourknowledge, this is the best obtained stopband value among the published resultsuntil now. The designed lter has been fabricated and validated using small signalparameter measurements. Excellent agreement is noticed between transmissioncoefcient measurements and its simulated counterpart using electromagneticsimulator.

Keywords: low pass lters; microstrip; defected ground structure; split ring resonators

1. Introduction

Modern microwave communication systems require LPFs to suppress unwantedharmonics generated by nonlinear devices. A conventional LPF uses a steppedimpedance structure. Although its design procedure is simple, it suffers from seriousdisadvantages such as a low attenuation rate and narrow stopband.[1] LPFs can berealized using a periodic transmission line structure. Defected ground structure (DGS)has recently been used to implement this periodic form. DGS periodic structures haveattractive properties including their cutoff and bandgap characteristics. These propertiescan be controlled by selecting the physical dimensions of the DGS units.

In the past, periodic DGS LPFs were realized with DGS units of uniformdimensions.[2] Later, DGS LPFs of nonuniform dimensions were investigated andreported. They were implemented using different shapes, such as square,[3] cross-shaped,[4] double equilateral U-shaped,[5] bow tie resonators,[6] and complementarysquare split ring resonators (CSSRR).[7] Moreover, LPF is implemented with steppedimpedance combined with shunt open stubs.[8] DGS is utilized with defectedmicrostrip lines to improve frequency response of LPF.[9] All these structures offer

*Email: [email protected], [email protected]

2014 Taylor & Francis

Journal of Electromagnetic Waves and Applications, 2014Vol. 28, No. 5, 542550, http://dx.doi.org/10.1080/09205071.2013.879048

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lower passband ripples and a wider stopband than uniform-dimension DGS structures.It is noticed that all these different structures can provide up to 5fc 20-dB stopband.However, there is still room to improve the performance, especially increasing thestopband, through investigation of new shapes of DGS units.

The performance of the used DGS unit determines the global behavior ofthe designed LPF. Consequently, the solution is to nd a DGS unit that fullls allhigh-performance criteria of LPF simultaneously. It is very clear that sharp attenuationrate is the main disadvantage of the nonuniform DGS LPFs. To obtain very sharp atten-uation rate, a DGS with elliptic response has to be used. Square Split Ring Resonator(SRR) DGS unit has been modeled with elliptic function from third order.[10] SSRRDGS exhibits high attenuation rate with one attenuation pole and very small passbandripples. In a different context, a double equilateral U-shaped DGS [5] unit has beenutilized to produce two attenuation poles.

To exploit the advantages of the two concepts, they have been merged. In otherwords, double SSR DGS units with elliptic function have been combined to form asingle unit. This unit is considered the Complementary (negative image) of SSRResonator (CSSRR).[7] Two advantages have been obtained from using this approach.Firstly, every DGS unit has the ability to nullify the transmission characteristics twiceon predetermined locations instead of only one. Secondly, the bandgap transmissioncharacteristics of double units are wider than the one that can be provided by the singleunit. It is arguable that DGS unit with more attenuation poles results in improvedattenuation characteristics of LPF.

In this work, CTSRR unit is investigated. It is found that the unit can provide sixattenuation poles, i.e. with four additional poles more than double equilateral U-shaped[45] and (CSSRR) [7] supply. Therefore, CTSRR unit enhances the rejectionperformance compared with other shapes. A spurious response in the stopband of aLPF is effectively suppressed utilizing a proper dimension of cascaded CTSRR units.A microstrip LPF prototype with fc of 1.8 GHz and 20-dB stopband up-to 7.5 times fcis designed, fabricated, and experimentally measured. The stopband width value isobtained using the available limited measurement range vector network analyzer(VNA). However, simulations results indicate that the 20-dB stopband extends up to16.5 times fc.

This article is organized as follows: Section 2 investigates the suitability of theproposed CTSRR unit for designing a high-performance LPF. Moreover, parametricdesign study is shown in this study. The design procedure of the LPF is demonstratedin Section 3. Enhancement of design methodology is presented in Section 4. Thevalidation of the design with fabricated circuit measurements is presented in Section 5.Conclusions are drawn in Section 6.

2. Performance of CTSRR unit

The CTSRR is the complementary form of the TSRR. The CTSRR is excited by anelectric eld which is normally incident unlike the TSRR which is excited by aperpendicular magnetic eld. Since the CTSRRs require electrical coupling, they areloaded in the ground plane of the microstrip line underneath the signal line. Thestructure inhibits the signal propagation at a resonance frequency and within a narrowband. This phenomenon reects the negative effective permittivity of the medium.Therefore, this unit is a perfect candidate to build frequency-selective structures such aslters and antennas.

Journal of Electromagnetic Waves and Applications 543

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The proposed CTSRR unit is depicted in Figure 1 with its geometric dimensions. Ithas a 50- microstrip line of width (d) on top and an etched CTSRR section in theground plane. The triangular unit is equilateral and has an outer side length (L). Todecrease the number of design parameters, thicknesses of the two slots and widths ofthe splits, as well as the distance between the two slots, have the same dimension (g).The frequency response of the proposed unit is investigated with the method ofmoments (MOM) using the Zeland IE3D software package.[11]

The dimensions of the simulated CTSRR unit are L = 12 mm and g = 0.5 mm. Thesubstrate used in all the simulations is Teon Duroid with a dielectric constant (r) of10.20 and thickness of 0.635 mm. The width d of the 50- microstrip line is 0.58 mm.Figure 2 clearly shows the superiority of the ltering performance of the CTSRR unitin rejection behavior. The insertion loss behavior of the CTSRR unit has six attenuationpoles at different frequencies, in which three of them reach more than 20-dB level.Therefore, CTSRR unit has six bandgaps which are four more than those provided byCSSRR and U-shaped DGS units.

The effect of L on the frequency response of the CTSRR unit is studied by keepingg constant at 0.50 mm and varying L (12, 11, and 10 mm). The simulation results, asdepicted in Figure 3, show that the fc and attenuation poles locations fon are inversely

Figure 2. Insertion loss of CTSRR.

Figure 1. Geometric dimensions of the CTSRR unit.

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proportional to L. The reason is that as L decreases, the effective series inductance ofthe line decreases, and consequently values of fc and attenuation pole locations areincreased.

3. Design and optimization of CTSRR LPF

To demonstrate the effectiveness of the proposed CTSRR unit, an LPF is built. Thelter is designed for GSM applications, with an fc of 1.8 GHz. A 50- microstrip lineis placed on top of a Teon Duroid substrate having the specications listed in theprevious section. A nonuniform CTSRR DGS periodic structure with N units is etchedat the bottom of the substrate as shown in Figure 4. Constant g equals to 0.5 mm forall CTSRR units is assumed to decrease the number of design variables.

The overall performance of the lter is considered a linear resultant of individualbehavior of all units. fc and the attenuation rate of the LPF are determined mainly fromthe performance of the unit that has the lowest fc. As discussed earlier, the unit withlargest L has lowest fc. The concerned unit is the rst one from the left side of theperiodic structure, as depicted in Figure 4. The rst step in the design procedure is tond the proper dimensions of this unit. As shown in Figure 2, the CTSRR unit with

Figure 3. Insertion loss of CTSRR, with different L.

Figure 4. CTSRR LPF with same g and different L.


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L = 12 mm has fc equals 1.8 GHz. Therefore, this unit is chosen to be the rst unit inthe designed LPF.

Attenuation region width is mainly dependent on the CTSRR unit that has thehighest f0. The unit with the smallest L has the highest f0. The concerned unit is the lastunit from the right side as shown in Figure 4. The target attenuation width is at leasteight times fc, i.e. f0 must be >14.5 GHz. It is found that CTSRR DGS with L equals7.8 mm has an attenuation pole at 14.80 GHz, as depicted in Figure 5. Consequently,L = 7.8 mm is chosen as the last CTSRR DGS unit.

The values of N, L, and the space between the units (W) are the design parametersthat have to be determined. The initial values of WS have been xed at 0.50 mm; thesevalues are optimized during the design procedure to improve the nal behavior of theLPF. Consequently, N, Ls, and Ws are tuned until the target attenuation level, >20 dB,is achieved. It is found that N equals 12, with Ls and Ws that are listed in Table 1,yields to a LPF with performance depicted in Figure 6.

The designed LPF exhibits an attenuation rate of 200 dB/GHz and passband ripplesof less than 0.30 dB. Moreover, it has a wide 20-dB stopband at up to 8.5 times fc. It isnoticed that the insertion loss exceed 20-dB level at 4.2 and 11.4 GHz. In the next

Figure 5. Insertion loss of CTSRR unit with L = 7.6 mm.

Table 1. Dimensions of the designed LPF.

N L (mm) W (mm)

1 12.1 11.92 11.78 11.73 11.3 8.54 11 7.55 10.63 7.66 10.26 7.17 9.87 6.78 9.5 6.69 9.12 6.710 8.36 6.511 8 5.712 7.6

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section, it is explained how the performance of the same design, without changing theLPF dimensions, is enhanced. Substrate with lower r and wider microstrip line is usedto achieve this target.

4. Enhancing the performance of the designed LPF

Performance of individual used DGS unit determines the global behavior of thedesigned LPF. It is noticed that using lower r substrate enhances the bandgap charac-teristics of the CTSRR unit. The CTSRR unit with L = 9.12 mm which lies under thesubstrate with a r value of 6.15 is investigated. As depicted in Figure 7, the rst two5-dB bandgaps have 0.45 and 0.63 GHz width, respectively. Consequently, incrementsof 32 and 110%, respectively, are obtained compared to the performance of the sameCTSRR unit that utilizes substrates with r equals 10.2. It is noticed from Figure 7 thatfc of the CTSRR unit, with substrate has r equals 6.15, is shifted upward with 0.4GHz. To enhance the performance of the current design of LPF, a substrate with lowerr has to be used. However, in this case, the fc will degrade and move upward. Theproblem is how to utilize the beforehand-designed LPF and substrate with lower rsimultaneously without changing the value of fc.

Since the effect of decreasing r is lowering slow wave factor (SWF) of thestructure, it is then proper to cancel this effect using a different mechanism. Utilizingwider microstrip line, characteristic impedance less than 50 increases SWF due to theincrease in the capacitance of the line. In other words, SWF of the initial design, onsubstrates with high r, will not change noticeably if a suitable wide microstrip line isused in conjunction with substrates with lower r. Therefore, fc is still xed, and more-over, the wide bandgap width privilege still appears in the behavior of the structure.For example, the CTSRR unit with L = 9.12 mm is tested in two different setups. Therst, on substrate that has r equals 10.2 and a microstrip line with 0.58 mm width and50. The second case, the same CTSRR unit lies on substrate that has r equals 6.15and utilizes 2.2 mm microstrip line. In this case, the characteristic impedance equals29. As shown in Figure 8, the same fc is obtained in both cases. Furthermore, theenhancement of the bandgap width is still preserved.

From the previous discussion, it is concluded that the physical dimensions of thebeforehand-designed LPF can be used without changing the substrate which has lower

Figure 6. Insertion loss of LPF composed of 12 CTSRR units.


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r and a wide microstrip line to enhance its performance. Therefore, the only valueneeded to be modied is d of the microstrip line. The lter is simulated when d equals2.2 mm.

Comparisons between the performance of the designed LPF and some of theavailable previously published works in [12][15] are listed in Table 2. CTSRR LPFexhibits the highest performance among these works on different aspects of the compar-ison, especially for obtained stopband value and rejection rate. Since the CTSRR LPFhas the lowest fc, it has the largest area among others. This design is adopted as a naldesign, fabricated and measured, as it is shown in the next section.

5. Validation of the enhanced design of CTSRR LPF

The enhanced design of CTSRR DGS LPF is fabricated, as shown in Figure 9, on theTeon Duroid substrate with r equals to 6.15 and a thickness of 0.635 mm. The

Figure 7. Insertion loss of CTSRR unit with L = 9.12 mm, and (a) r = 10.2 ( ) and (b)r = 6.15 ( ), the microstrip line characteristic impedance is 50.

Figure 8. Insertion loss of CTSRR unit with L = 9.12 mm and (a) r = 10.2 and d = 0.58 mm( ), and (b) r = 6.15 and d = 2.2 mm ( ).

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fabricated circuit has same dimensions listed in Table 1 with d equals 2.2 mm. Thesimulation results show that the 20-dB bandwidth extends to 30 GHz, 16.5 times fc. Asit is mentioned in the introduction, due to the measurement capabilities of VNA, thefabricated LPF is measured in the range of 0.0513.5 GHz. The results of S parametermeasurements agree well with the simulation results, as shown in Figure 9.

6. Conclusion

In this article, a new DGS shape, CTSRR unit, is investigated and used to design aperiodic LPF. The CTSRR unit exhibits interesting ltering properties such as six niteattenuation poles in its stopband in addition to a very sharp attenuation rate. Thesecharacteristics can be controlled by proper selection of the geometric dimensions of theunit. The designed lter demonstrates superiority in various features compared withother previously published works, particularly in offering very wide rejection band.Very good agreement is noticed between simulations of the designed LPF andexperimental measurements of the fabricated circuit.

Table 2. Comparisons between the performance of the designed CSSRR LPF and some of thepreviously published works.

Work

Performance

Rejection rate(dB/GHz)

fc(GHz)

20-dB isolationbandwidth/fc

Passbandripples level

(dB)

Physicaldimensions(mm mm) r

This work 250 1.8 16.5 0.30 89 11 6.15[12] 120 2.8 2.5 0.30 20 19 3.38[13] 106 3.68 4.4 0.39 48 8.5 2.2[14] 40 2.4 0.8 0.41 55 11.3 4.4[15] 43.21 3.5 4.6 0.44 50 30 6.15

Figure 9. Insertion loss of LPF, r = 6.15 and d = 2.2 mm, simulated ( ) up to 30 GHz and(b) measured up to 13.5 GHz ( ).


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microstrip line. IEEE Microw. Wireless Compon. Lett. 2004;14:180182.[4] Chen HJ, Huang TH, Chang CS, Chen LS, Wang NF, Wang YH, Houng MP. A novel

cross-shape DGS applied to design ultra-wide stopband low-pass lters. IEEE Microw.Wireless Compon. Lett. 2006;16:252254.

[5] Ting SW, Tam KW, Martins RP. Miniaturized microstrip lowpass lter with wide stopbandusing double equilateral U-shaped defected ground structure. IEEE Microw. WirelessCompon. Lett. 2006;16:240242.

[6] Xie H-H, Jiao Y-C, Huang J-X, and Zhang L. Ultra-wide stopband low-pass lter withBowtie defected ground structure. Proc. of Microwave and Millimeter Wave Technology(ICMMT); 2010 May 811:16551657.

[7] Taher H. High performance low pass lter using complementary square split ring resonatorsdefected ground structure. IET, Microw. Anten. Propag. 2011;5: 771775.

[8] Yang R-Y, Lin Y-L, Hung C-Y, Lin C-C. Design of a compact and sharp-rejection low-passlter with a wide stopband. J. Electromagn. Waves Appl. 2012;26:22842290.

[9] Tirado-Mendez A, Jardon-Aguilar H, Flores-Leal R. Improving frequency response ofmicrostrip lters using defected ground structures and defected microstrip structures. Prog.Electromagnet. Res. Lett. 2010;13:7790.

[10] Parui SK, Das S. Modeling of split-ring type defected ground structure and its lteringapplications. J. Microw. Opt. Electrom. Appl. 2009;8:612.

[11] IE3D Users Manual. Release 11. Freemont, CA: Zeland Software Inc; 2004.[12] Abdulhadi AAA, Ashoka A, Jeberson W. Combined shaped microstrip line and DGS

techniques for compact low pass lter design. Int. J. Eng. Technol. 2013;13:7477.[13] Wu GC, Wang GM, Wang YW, Hu LZ. A compact microstrip low-pass lter using

D-CRLH transmission line with ultra-wide stopband and high selectivity. Radio Eng.2013;22:734738.

[14] Tiwary AK, Gupta N. Performance of microstrip low-pass lter on electromagnetic bandgap ground plane. IETE J. Res. 2010;56:230234.

[15] Martin K, Zbynk R. Comparison of planar fractal lters on defected ground substrate.Radio Eng. 2012;21:10191024.

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Abstract1. Introduction2. Performance of CTSRR unit3. Design and optimization of CTSRR LPF4. Enhancing the performance of the designed LPF5. Validation of the enhanced design of CTSRR LPF6. ConclusionReferences

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