Characteristics of a Novel Slow-Wave Defected Ground Structure for Planar Wideband Filters

Transcription

1 2011 Internationa Conference on Information and Eectronics Engineering IPCSIT vo.6 (2011) (2011) IACSIT Press, Singapore Characteristics of a Nove So-Wave Defected Ground Structure for Panar Wideband Fiters Maryam Jadi 1 and Majid Tayarani 2 1 Eectrica Engineering Department, Iran University of Science and Technoogy, Tehran, Iran, m_jadi@eec.iust.ac.ir 2 Eectrica Engineering Department, Iran University of Science and Technoogy, Tehran, Iran, m_tayarani@iust.ac.ir Abstract. This paper presents a ne approach for designing compact band stop and band pass fiter. This technique is based on defected ground structure. Despite other fiters that use DGS just to improve the response of the fiter, in this paper Defected Ground Structure is used as the buiding bock of the fiter. The center frequency of the fiter can be easiy controed by changing the dimension of the DGS resonator. To fiters ith center frequency of 1.17GHz and 3.2GHz has been designed and simuated. The structure of the fiter has been improved for ideband and high Q appications. Current characteristics and so ave factor of the fiter have been anayzed. Keyords: Defected Ground Structure, Fiter, Microstrip, Wideband, So-ave factor. 1. Introduction Fiters ith opass, bandstop or bandpass are critica components in many microave and miimetre ave systems. They are used to suppress harmonics and spurious signas of ampifiers and osciators. An easy ay to design a microstrip fiter is stepped impedance resonators. But the main probem of such fiter is existence of harmonics and this kind of fiter has poor poer handing. So a ne technique shoud be presented ithout these probems. Defected Ground Structures as Eectromagnetic Band Gap (EBG) is a ne effective technique to design o pass and band pass fiters or to improve the response of the fiters. In recent years interest on defected ground structures as a kind of eectromagnetic band gap structures is groing. DGS is reaized by etching of a defected pattern from the backside of metaic ground and has periodic or non-periodic structure. DGS as first proposed in [1] hich has a dumbbe shaped. After that ot of DGS structures ith various shapes have been proposed. We can mention to square shaped [2], spira shaped [3] and H-shaped [4]. Defected ground structures disturbs the shieded current distribution in the ground pane and thus change the characteristics of microstrip ine such as its inductance and capacitance so they have rejection band in some frequency ranges. Aso these structures have so-ave properties. By proper use of the ground pane these structures reduce the size of microstrip component significanty. The rejection properties of defected ground microstrip ines are so usefu in many microave circuits for harmonic suppression such as poer divider [5], or improvement the response of antennas [6]. Hoever most of the researches are focused on the band rejection properties of these structures and a fe papers have been reported that use DGS structures as a buiding bock to reaize microave circuits as bandpass fiters. In this paper, e propose a ne bandpass fiter hich uses DGS as a buiding bock of the structure. The fiter consists of a DGS resonator and a microstrip ine, ith a compact size. The frequency response of the structure can be easiy controed by changing the dimension of the DGS resonator. This artice is organized as fooed: In II, based on [7], the bandgap properties of a DGS ce are demonstrated. By changing ine properties a bandpass fiter is obtained. According to that, a nove DGS fiter is proposed that is suitabe for higher frequencies. In section III, current properties and so ave effect of the structure is anaysed. Finay a concusion is reported in section IV. 135

2 2. Characteristic of the DGS Ce Fig. 1(a) shos the schematic diagram of the ground of a DGS, ith face to face couping hich as used in [7]. In Fig. 1(b) the ine of the fiter is demonstrated. The ine idth shoud be chosen for the characteristic impedance of 50 Ω, but it can be shon that by etching defected ground, effective dieectric constant ε re of the microstrip is increased thus the characteristic impedance i be greater than 50Ω. So for matching the microstrip ine to the ports, e suggest that the idth of the ine above the DGS section shoud be increased, aso e shoud connect to ine ith characteristic impedance of 50 Ω at the to ports of the structure. The substrate ith a dieectric constant of 10, oss tangent of and thickness of mm is considered. In order to investigate the frequency response of the DGS ce, it is simuated by CST EMsimuator. The dimensions of the structure are tabuated in tabe I. Dimensions are based on mm. (a) Figure 1. Laytout of a band stop fiter (a) ground pane using DGS resonators ith face to face couping, (b) microstrip ine.. TABLE I. PHYSICAL DIMENSION OF THE STRUCTURE OF DGS BANDSTOP FILTER WITH FACE TO FACE COUPLING (UNIT:MM) a b p Fig. 2 iustrates the simuated response of this structure hich is of order to and the to equa resonators are couped together via the ine. The band stop response of this structure shos to pos in the bandstop due to the to resonators that are couped together. (b) Figure 2. Simuated S-parameters characteristic of the bandstop DGS fiter ith face to face couping. Because of its tight couping, this structure shos sharper transition compared to the back to back fiter investigated in [11]. By etching a gap in the ine, the bnadstop response of this fiter is converted to a bandpass one ith the same centra frequency. The schematic vie of the ine ith interdigita gap is iustrated in Fig. 3 here e=1.8mm, d=0.25mm and g=0.4mm. Figure 3. Schematic vie of the ine ith interdigita gap 136

3 The simuated response of the fiter is shon in Fig. 4 here the centra frequency of the fiter is 1.17GHz and fractiona bandidth of it, is equa to 35%. The fractiona bandidth of the fiter can be cacuated from (1), here f 0, f 1 and f 2 are centra frequency, first and second 3dB frequencies, respectivey. FBW = f /( f ) 1 (1) 0 2 f Figure 4. Simuated S parameters characteristic of DGS bandpass fiter using ine ith interdigita gap. By changing the dimensions of DGS ces, centra frequency of the bandpass fiter can be changed easiy. But the structure mentioned above, interfaces a great probem due to the need to shorten the dimensions of the ce for the second band design and the spacing ( 0 ) is bounded to a oer imit due to technoogy restriction. So, e propose a ne DGS ce, ith meander section. The meander ines, ead to an increase in the current path, compensating the effects caused by the increase in spacing. The proposed ce and its response are iustrated in Fig. 5 and Fig. 6, respectivey. Centra frequency of this bandpass fiter is 3.2 GHZ. The dimension of the structure is tabuated in tabe II. So if e have difficuty ith the dimension if Fig. 1(a), structure of Fig. 5 can be used aternativey. TABLE II. 1 ' PHYSICAL DIMENSIONS OF THE PROPOSED DGS BANDPASS FILTER. 2 ' a ' b ' ' m m 2 ' Figure 5. Schematic vie of the ground pane of the proposed DGS bandpass fiter. Figure 6. Simuated S parameter characteristic of the proposed DGS band pass fiter. 137

4 3. Improvement of the Response In the simuated response of Fig. 4, it can be seen that the stopband rejection is ess than 30 db aso e ant to improve the sharpness fiter. For that, e propose a ne structure, hich is composed of cascade connection of three ces, ith separation of 9.8 mm. The schematic diagram of the structure is iustrated in Fig. 7 and the simuation resuts are shon in Fig. 8. The stopband rejection is no more than 40 db and the sharpness of the passband have been improved. The bandidth of the passband is 51%. Figure 7. Schematic vie of ground pane of the proposed utra ide band DGS band pass fiter ith cascaded resonators. Figure 8. Simuated S parameter characteristic of proposed bandpass fiter ith cascaded resonators. 4. Surviving So-Wave Effect As mentioned above, defected ground structure resonators disturb the shieded current in the ground pane. Fig.7 shos the current path on the ground pane of fig. 1 ith bandpass response. As can be seen, the current initias from the input port, turn around the DGS resonator and exits from the output port. Figure 9. Current path on the ground paane of band pass fiter of fig.1 at 1.17GHz. Aso the simuated S parameter shos a resonant frequency in the response of the structure. From that, e can concude that the DGS structure has so-ave characteristics and the propagation constant differs from a singe microstrip ine. To prove this statement, e have computed the so-ave factor of this structure and compare it ith a microstrip ine ithout DGS resonator. The so-ave factor of a DGS can be computed from (2) and (3), here L is the physica ength of the microstrip ine, λ0 is the aveength in free space, Δθ is the phase difference of microstrip ine beteen ith and ithout DGS and ε eff is the effective microstrip permittivity. SWF ( f ) eff, D = λ0 = ε Δθ ( f ) / 360L + p ε eff (2) 138

5 = ( ε + 1) / 2 + ( ε 1) / 2(1 + 12h / )^( 0.5) ε (3) eff r r Fig. 8 shos a comparison of the so-ave factor ith and ithout DGS resonators. It is cear that by use of DGS, the so-ave factor of DGS is improved. 5. Concusion Figure 10. SLWF of DGS In this paper, a nove approach to design bandpass and bandstop fiter has been proposed. This technique is based on impementation of defected ground structures as a buiding bock of the fiter. By changing the dimension of the fiter, frequency response of the fiter, can be easiy controed. By cascading three simiar resonators, the response of the fiter, has been improved greaty. The consequent structure has a ideband response. Surveying of the so-ave factor of this resonator shos that this factor is greater than a singe microstrip ine. 6. References [1] C. S. Kim, J. S. Park, D. Ahn and J. B. Lom, A Nove 1-D Periodic Defected Ground Structure for Panar Circuitss, IEEE Micro. Wireess. Compon. Lett. 2000, Vo. 10, No. 4, pp [2] D. Ahn, J. Park, C. Kim, J. Kim, Y. Qian, and T. Itoh.A. A Design of the Lopass fiter Using the Nove Microstrip Defected Ground Structure. IEEE Transaction Microave Theory and Technoogy, 2001, Vo. 49, No. 1, pp [3] C. Kim, J. Lim, S. Nam, K. Kang, J. Park, G. Kim, and D. Ahn. The Equivaent Circuit Modeing of Defected Ground Structure ith Spira Shape. IEEE MTT-S Int. Micro. Symp. Dig. 2002, vo. 3, pp [4] M. K. Manda and S. S.. A nove defected ground structure for panar Circuits. IEEE Micro. Wireess Compon. Lett. 2006, vo. 16, no. 2, pp [5] H. T. Kang, J. S. Yun, C. S. Kim, J. S. Park, D. Ahn, and G.Y. Kim, Radisic. A Study on immpemenetation of so-ave structures using photonic bandgap configuration. IEEE MIT/AP/EMC Korea Chapter Microave and Wave propagation., 1999, Vo. 22, No. 1, pp [6] I. S. Lim and B. S.Lee. Design of Defected Ground Structures for Harmonics Contro for Active Microstrip Antennas. IEEE AP-S Int. Symp. 2002, Vo. 2, pp, [7] X.-H. Wang and B.-Z. Wang. Compact Broadband Dua-band Bandpass Fiters Using Sotted Ground Structures, Progress In Eectromagnetics Research, 2008, PIER 82,