Modeling of Split-ring Type Defected Ground Structure and Its Filtering Applications

Transcription

1 Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 8, No. 1, June 29 6 Modeling of Split-ring Type Defected Ground Structure and Its Filtering Applications Susanta Kumar Parui 1 and Santanu Das 2 Department of Electronics and Telecommunication, Bengal Engineering and Science University, Shibpur, Howrah , INDIA 1 arkapv@yahoo.com & 2 santanumdas@yahoo.com Abstract A popular split-ring defected ground structure under a microstrip line is modeled here by an equivalent circuit. The frequency characteristics of the proposed unit cell show an attenuation zero close to its attenuation pole frequency. This is modeled by 3 rd order elliptic lowpass filter. Accordingly, equivalent circuit is proposed and equivalent LC parameters are extracted. The influences of split-gap variation of the DGS unit on pole and cutoff frequencies have been investigated. Two DGS cells with different pole frequencies cascaded underneath a microstrip line realize a sharp and deep lowpass filter. The passband insertion loss has been reduced by using a high-low impedance line in place of the standard line. A prototype filter has been fabricated with Arlon substrate. The simulated S-parameters are compared with its experimental measurement results. Index Terms microstrip, defected ground structure, elliptic filter, bandstop I. INTRODUCTION A defected structure etched in the metallic ground plane of a microstrip line is attractive solution for achieving finite pass band, rejection band and slow-wave characteristic. A dumb-bell shaped DGS was explored first time by D. Ahn and applied to design a lowpass filter [1-3]. It showed one-pole response and modeled by Butterworth lowpass filter. A filter with high selectivity would be preferable owing to the demand for currently expanding communication systems within finite spectrum resources. It is well known that a filter with its attenuation poles and zeros at finite frequencies shows high selectivity. A DGS filter with elliptical response is the solution. A few DGSs with quasi-elliptic responses were proposed recently [4-5]. An asymmetric DGS unit consisting of three circular nodeslots connected by two thin link-slots underneath a microstrip line [6] showed both attenuation zero and pole in the frequency characteristics and was modeled by elliptical filter function. Another asymmetric DGS consisting of two square slots connected by a rectangular slot under a microstrip line is proposed [7]. Its resonant elements produce two distinct poles, resulting in a sharp bandstop filtering response. In this paper, a split-ring DGS unit pattern is modeled by a LC circuit. Both attenuation zero and pole frequencies are observed in its frequency characteristics. It is modeled by 3 rd order elliptic Brazilian Microwave and Optoelectronics Society-SBMO received 2 April, 28; revised 13 April, 29; accepted 7 May, 29 Brazilian Society of Electromagnetism-SBMag 29 SBMO/SBMag ISSN

2 Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 8, No. 1, June 29 7 lowpass filter function. Moreover, owing to the increased equivalent inductance and capacitance, the required area is seen to be smaller than traditional dumbbell DGSs. By varying the split-gap, the pole and cutoff frequencies of stopband may be tuned. Here two DGS cells with different split-gaps are cascaded below a standard microstrip line for realizing a bandstop filter. The response shows high sharpness but poor insertion loss in its passband. It can be reduced by replacing the standard line with a high-low impedance line. II. CHARACTERISTICS OF DGS UNIT Fig. 1 shows the investigated split-ring DGS pattern, which is etched off on the backside ground plane underneath a microstrip line. In order to investigate its frequency characteristics, a cell is simulated by the MoM based IE3D simulator and the response is compared with measurement results as shown in Fig. 1. The different dimensions of the DGS unit are considered as: a=6 mm, b=1 mm and g=.4 mm. The substrate with dielectric constant 3.2, loss tangent.25 and thickness.79 mm is considered here. The width of the conductor strip (5Ω) is calculated to be 1.92 mm. b g a -2-4 S11(mea) S21(mea) S11(sim) S21(sim) Fig. 1. Split-ring DGS unit: schematic diagram of DGS (solid area indicates ground slots and dotted area microstrip line ) EM-Simulated and measured S-parameters The simulated attenuation zero frequency and the pole frequency are obtained at 3.9 GHz and 4.8 GHz, whereas the measured values are 3.7 GHz and 4.6 GHz, respectively. The maximum stopband attenuation is -26 db and the passband insertion loss is.32 db for both the simulated and measured results. The sharpness factor at lower transition knee is obtained as 46dB/GHz in the simulated results and 42dB/GHz in the measured results. Thus, the investigated DGS unit exhibits both attenuation zero and pole at finite frequencies and they are close to each other. As a result, a sharp transition knee is achieved with deep stopband attenuation. III. MODELING OF DGS UNIT CELL In order to apply the DGS unit to a practical circuit design, it is necessary to extract its equivalent circuit parameters. Both the simulated and experimentally measured S-parameters look like a 3 rd order Brazilian Microwave and Optoelectronics Society-SBMO received 2 April, 28; revised 13 April, 29; accepted 7 May, 29 Brazilian Society of Electromagnetism-SBMag 29 SBMO/SBMag ISSN

3 Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 8, No. 1, June 29 8 elliptic lowpass filter response. So an equivalent circuit consisting of a parallel network composed of L and C and two parallel capacitances C p as shown in Fig. 2 is proposed to model the DGS unit. No resistance element is included here. For the given dimensions of DGS unit, L-C parameters are extracted as L =.955 nh, C= pf and C p =.64 pf using NuHertz make Filtersim software. The S-parameters obtained from the circuit-simulation as shown in Fig. 2 complies with both the simulation and experimentally measurement results. L V Zo Cp C Cp Zo -2 S21(cir) -4 S11(cir) Fig. 2 Proposed equivalent circuit of the DGS S-parameters from circuit model IV. INFLUENCE OF SPLIT GAP WIDTH In order to investigate the influence of the split gap (g) of the ring, the DGS unit is simulated with different values of g. The g-values are varied to.2 mm,.4 mm, 2 mm and 4 mm. The pole frequency of stopband is affected appreciably as observed in Fig. 3. Transmission coefficient db -2-4 g=.2mm g=2.mm g=4.mm g=.4mm Frequency,GHz Split gap, mm Fig. 3. S-parameters for different split-gap pole frequency (f p ) /zero frequency (f z ) versus split gap plots fp fz As the gap width increases, the effective length of the resonator (perimeter of the split ring) decreases and therefore, the pole location moves up to a higher frequency keeping sharpness factor, stopband attenuation and passband insertion loss almost constant. The pole and zero frequencies are plotted against g in Fig. 3 and it is observed that both of them vary with g in considerable Brazilian Microwave and Optoelectronics Society-SBMO received 2 April, 28; revised 13 April, 29; accepted 7 May, 29 Brazilian Society of Electromagnetism-SBMag 29 SBMO/SBMag ISSN

4 Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 8, No. 1, June 29 9 amount. Thus, the dimensions of the DGS may be determined from its pole frequency using plot in Fig. 3. The extracted LC parameters for different g are given in Table-I. TABLE-I: EQUIVALENT LC PARAMETERS OF DGS UNIT FOR DIFFERENT SPLIT GAP g (mm) f z (GHz) f p (GHz) L (nh) C (pf) C p (pf) V. REALIZATION OF LOWPASS FILTER A lowpass filter has been designed with a pair of DGS cells having different pole frequencies below a 5Ω microstrip line as shown in Fig. 4. The pole frequencies are varied by split-gap width (g) using the plot of Fig. 3. The g-values are taken as: g 1 =.4 mm and g 2 = 1. mm. The separation between DGS cells is taken as 3 mm. The other dimensions of the DGS units are: a= 6 mm, b= 1 mm as before. The simulated results in Fig. 4 show lowpass filter characteristics. The cut-off frequency at 3.9 GHz, center frequency at 4.9 GHz and 2-dB rejection bandwidth of 1.1 GHz are obtained. The insertion loss in the passband is observed as.31 db, which is quite high for any practical filter. W d g 1 g 2 Magnetude, db -2-4 S11(sim) S21(sim) Fig. 4. schematic diagram of two-cell DGS filter with g =.4 mm and 1. mm EM-simulated S-parameters The insertion loss is reduced by a high-low impedance line (HI-LO line) in place of the standard line as shown in Fig. 5. In Fig. 5 the DGS filter has been simulated with different width of the low impedance line (w c ) = 3 mm and 4 mm and the insertion loss is observed to be reduced from.31 db to.16 db and.1 db respectively. It is also found that both the pole and cutoff frequencies are reduced with w c. Thus the insertion loss can be manipulated by changing the impedance of the low impedance line and hence an optimum w c may be obtained. Brazilian Microwave and Optoelectronics Society-SBMO received 2 April, 28; revised 13 April, 29; accepted 7 May, 29 Brazilian Society of Electromagnetism-SBMag 29 SBMO/SBMag ISSN

5 Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 8, No. 1, June 29 1 d W Wc g 1 g 2 Transmission coefficient, db wc=4mm wc=w=1.92 mm for standard line wc=3mm Fig. 5: layout of 2-cell bandstop filter simulated transmission coefficient with different value of w c For different combination of g-values (g 1 and g 2 ) of the pair of DGS cells, some stopband filters may be realized with different center frequency and bandwidth. It is noted that when the two gapwidths are very close, the stopband filter has narrow bandwidth but deep stopband attenuation. But if the gap-widths differ much, the stopband filter gives wider bandwidth with low stopband attenuation. For g =.4 mm and 4 mm, the bandstop filter gives bandwidth of 1.8 GHz, whereas it is 1. GHz for g =.4 mm and 1 mm, as shown in Fig. 6. Transmission coeff., db g=.4mm-1mm g=.4mm-2mm g=.4mm-4mm Transmission coeff., db g=.4mm-1mm g= 1mm-2mm g=2mm-4mm Fig. 6 Simulated S-parameters of 2-cell bandstop filter with different pair of g Brazilian Microwave and Optoelectronics Society-SBMO received 2 April, 28; revised 13 April, 29; accepted 7 May, 29 Brazilian Society of Electromagnetism-SBMag 29 SBMO/SBMag ISSN

6 Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 8, No. 1, June Thus, an optimization between bandwidth and stopband attenuation should be made for designing a practical filter. By cascading more number of such DGS cells with different pole frequencies, a stopband filter with wider bandwidth may be realized. VI. EXPERIMENTAL MEASUREMENT RESULTS A prototype bandstop filter has been fabricated on Arlon make PTFE substrate by photo etching technique using two DGS units with g 1 =.4 mm and g 2 = 1 mm underneath a HI-LO line. The other dimensions are: a = 6 mm, b = 1 mm and d = 3 mm. The width (w c ) of the low impedance line is obtained as 4 mm for optimum insertion loss of.1 db and the high impedance line (w) is 1.92 mm, corresponding to 5Ω. It is measured by a vector network analyzer of model N523A. The photographic views of top plane and ground plane of the prototype filter are shown in Fig. 7. Fig. 7. Photographic views of two-cell DGS based bandstop filter ground plane, signal plane The cutoff frequency, pole frequency and rejection bandwidth (2dB) are obtained as 3.8 GHz, 4.75 GHz and.78 GHz, respectively in experimental measurement as shown in Fig. 8, whereas, these are 3.9 GHz, 4.7 GHz and.8 GHz, respectively in IE3D-simulation as shown in Fig S11(sim) S21(sim) S11(mea) S21(mea) Fig. 8. S-parameters of bandstop filter simulated, measured Brazilian Microwave and Optoelectronics Society-SBMO received 2 April, 28; revised 13 April, 29; accepted 7 May, 29 Brazilian Society of Electromagnetism-SBMag 29 SBMO/SBMag ISSN

7 Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 8, No. 1, June A sharpness factor of 46 db/ghz at transition knee and passband insertion loss of.12 db are observed in the measured results whereas their simulated values are 23 db/ghz and.49 db respectively. The total dimension of this filter is 2 mm 6 mm including the feed line, which is equivalent to.425λ.127λ. Here, λ is the guided wavelength at the center frequency of stopband. Thus this filter becomes compact in size. VII. CONCLUSION The unit cell of well-known split-ring DGS is modeled here by 3 rd order elliptical lowpass filter. The equivalent circuit is represented by a -type LC network. A scheme for designing a lowpass filter by using two DGS units having different pole frequencies underneath a high-low impedance line has been demonstrated. As the filters give better sharpness, narrow bandwidth and compact size, they may be suitable for designing wireless communication systems. Acknowledgement The work is funded by All India Council of Technical Education (AICTE), Govt. of India. REFERENCES [1] C.S.Kim, J.S.Park, D.Ahn and J.B. Lim, A novel one dimensional periodc defected ground structure for planar circuits, IEEE Microwave and Guided wave Letters, vol. 1, No. 4, pp , 2 [2] D. Ahn, J.S.Park, C.S.Kim, J.Kim, Y Qian and T. Itoh, A design of the lowpass filter using the novel microstrip defected ground structure, IEEE Trans. on Microwave Theory and Techniques, vol. 49, no. 1, pp , 21 [3] Lim J., Kim C., Lee Y., Ahn D., and Nam S., Design of lowpass filters using defected ground structure and compensated microstrip line, Electronics Letter, vol. 38, no. 25, pp , 22 [4] Chen J.-X., Li J.-L., Wan K.-C. and Xue Q., Compact quasi-elliptic function filter based on defected ground structure, IEE Proc.-Microwave Antennas propagation, vol. 153, no. 4, pp , Aug. 26 [5] Susanta Kumar Parui and Santanu Das, A novel asymmetric defected ground structure for implementation of Elliptic filters, Proc. of SBMO/ IEEE-MTTS International Microwave and Opto-electronics Conference (IMOC-27), Brazil, pp , 27 [6] Susanta Kumar Parui, and Santanu Das, A simple defected ground structure with elliptical lowpass filtering response, Proc. of Asia Pacific microwave conference (APMC), 27 [7] Susanta Kumar Parui, and Santanu Das, A new defected ground structure with elliptical band-reject and band-accept filtering characteristics, Proc. of Asia Pacific microwave conference (APMC), 27 Brazilian Microwave and Optoelectronics Society-SBMO received 2 April, 28; revised 13 April, 29; accepted 7 May, 29 Brazilian Society of Electromagnetism-SBMag 29 SBMO/SBMag ISSN