Design and Simulation of Folded Arm Miniaturized Microstrip Low Pass Filter

Transcription

1 813 Design and Simulation of Folded Arm Miniaturized Microstrip Low Pass 1 Inder Pal Singh, 2 Praveen Bhatt 1 Shinas College of Technology P.O. Box 77, PC 324, Shinas, Oman 2 Samalkha Group of Institutions, Panipat, Haryana, India Abstract - In this paper we presented two simple designschebyshev 3-pole microstrip stepped-impedance lowpass filter in L-band (1 GHz) and folded arm microstrip lowpass filter which is widely being implemented in GPS systems, mobile phones and defence telemetry. Various shapes are designed, simulated and frequency is tuned by altering the stub size and its position. According to the shape of the devices these two filters can be implemented. These two filters are designed for 1 GHz cut-off frequency at -3dB with a passband ripples less than 0.1dB and it shows sharp stopband 1 GHz 4.9 GHz. Effective permittivity of the substrate is 10.8 and height The stepped impedance LPF is a traditional filter and folded arm filter is also stepped impedance LPF but its inductive arm is bent at Folded arm filter is the miniaturized form of the traditional LPF. The miniaturized LPF gives the same performance as the traditional stepped impedance LPF. The filter is miniaturized by folding its inductive arm at 90 0 and its dimensions are optimized. Area of folded arm lowpass filter is reduced by 27.9% with respect to the traditional stepped impedance lowpass filter. These LPF filters are designed and simulated on Ansoft-HFSS platform. Its gives the satisfactory results between theoretical and simulated ones. Keywords - Stepped Impedance LPF, Chebyshev, L-Band, Folded Inductive Arm, HFSS. 1. Introduction Microstrip filter is a three layered structure, ground, substrate and patch. Substrate is sandwiched between ground and the patch. Microstrip structure is made on PCB of desired specification. The advantage of microstrip filter is its compact size, not expensive, easy fabrication, light weight, easy troubleshooting. The disadvantage of microstrip filter is its poor power handling capability [1]. Power handling capability of the structure is reduced when high impedance lines are implemented in the design because thin metal lines can t tolerate the high power. At higher microwave frequencies and higher dielectric constant it is difficult to control the heat generation in the conducting structure as well in dielectric. In case of dielectric breakdown, peak power is rapidly reduced. An alternative approach to reduce power loss is, use of low constant or such type of dielectric which has very high thermal resistance e.g. alumina. Power handling capability can be easily controlled at low microwave operating frequency. Proposed LPF is operating at 1 GHz frequency eventually loss is very less. By using multilayer structure power handling capability can be optimized [2]. If surface of microstrip filter is exposed to air then there is a scope of the interference between surface waves and other unwanted radiation. This can be minimized by shielding the structure by metallic waveguide to prevent entering of any type of electromagnetic radiation inside the structure and it also suppresses a surface modes. Full wave electromagnetic analysis is performed for the modeling of shielding effect. The enclosure cancels the electric field inside the box. The proposed LPF is enclosed by metal housing. It is difficult to fabricate very high impedance line since the line is very thin. Stepped impedance LPF has its impedance high and low in steps from one end to another end [3]. Its design and mathematical formulation is very simple and accurate but its dimensions are larger so it is not always suitable for the compact devices [4]. In some applications, area of the component on the PCB is very precious and can t compromise with the size of the device. In order to design proposed miniaturized LPF, the inductive arms are folded at 90 degree and applied some compensation techniques to retain the total inductance and capacitance of the traditional stepped impedance LPF to get the same frequency response. Since in this paper we

2 814 compared the properties of two designs at the same center frequency and we achieved to minimize the area of the proposed filter. 2. Design Equation of Stepped Impedance Lowpass 2.1 Important s in Design Consideration Consider the Fig.1 in which a traditional layout of the stepped impedance LPF is shown [5] Z0C <Z0 <Z0L, where Z0C and Z0L denote the characteristic impedances of the low and high impedance lines, respectively, and Z0 is the source impedance, which is usually 50 ohms for microstrip filters A lowerz0c results in a better approximation of a lumped-element capacitor, but the resulting line width WC must not allow any transverse resonance to occur at operation frequencies A higher value of Z0L gives a better approximation of a lumped-element inductor, but Z0L should not be too high because in that case fabrication of narrow inductive line would be difficult and it will affects its current-carryingcapability. Fig.1 (a) In Fig.1 Z 0 is the source and load impedance. L 1, L 3 L n+1 are inductive elements. C 2, C 4 C n are capacitive elements. 2.2 Design Equation of Microstrip Stepped Impedance Lowpass Design equations to realize the 3-pole Chebyshev L-C ladder type stepped impedance lowpass filter are given below. Design equations of the filter depends upon the lowpass prototype, type of response, number of elements that filter comprises. Since L-C ladder type lowpass filter consists alternate inductive and capacitive elements. = = (1) = (2) Equation (1) & (2) represent the value of inductor (nh) and value of capacitor (pf). Where, Ω c is normalized cut-off frequency. f c is the cut-off frequency based upon the guided wavelength. g 0, g 1, g 2 are the lowpass prototype element. = (3) = (4) Where l L = physical length of inductor l C = physical length of capacitor λ g = guided wavelength ω c = cut-off frequency The equations (3)& (4) doesn t include the series reactance of capacitive element and shunt susceptance of inductive element. Physical length of inductor and physical length of capacitor should be optimized by satisfying these conditions Fig.1 (b) Fig. 1 (a) General layoutof the stepped-impedance LP microstrip filter. (b) L-C ladder type LPF. = + tan (5)

3 815 = + 2 tan (6) 3. Design 3.1 Design Specification Of Stepped Impedance Open-Stub Lowpass Cut-off frequency, f c = 1 GHz Normalized frequency = 1 GHz Relative Dielectric constant, ε r = 10.2 Substrate height, h = 1.27 No. of poles =3 Function type = Chebyshev Characteristic impedance = 50 Ω Passband ripple = 0.1 db (return loss -20 db) Table 1: Of Stepped Impedance Open-Stub Lowpass Characteristic impedance = 50 Ω Passband ripple = 0.1 db (return loss -20 db) Table 2: of Folded Arm Stepped Impedance Open-Stub Lowpass g 0 1 l 1 = l g l 2 = l g l g w L 1 =L nh w C pf w l 0 g 0 1 l g l g l g l g 4 1 w L 1 =L nh w L 0.1 C pf w c 8.7 Fig. 3 Layout of Folded arm stepped impedance open-stub lowpass filter Fig. 2 Layout of stepped impedance open-stub lowpass filter. 3.2 Design Specification of Folded Arm Stepped Impedance Open-Stub Lowpass Cut-off frequency, f c = 1 GHz Normalized frequency = 1 GHz Relative Dielectric constant, ε r = 10.2 Substrate height, h = 1.27 No. of poles =3 Function type = Chebyshev In the Fig. 3 detailed structure of perpendicularly folded inductive arm stepped impedance lowpass filter is shown. Bent arm acts as a combination of inductor and capacitor like in T-network. The additional inductive and capacitive effects are compensated in the proposed filter by changing the dimension of inductive arm and dimension of central capacitive patch. It could be compensated by designing a mitered bend but in the proposed filter it not feasible or practical from the fabrication point of view, since we used a very thin inductive line i.e So dimensions of the central capacitive patch are optimized in order to maintain the center frequency 1 GHz

4 816 XY Plot 1 patch S- db(st(t1,t1)) Setup1 : Sw eep1 db(st(t2,t1)) Setup1 : Sw eep1 Fig. 4 Perpendicular bent in microstrip line. Microstrip is bent as shown in the fig. 4. When two microstrip are placed over one another at 90 degree. It is very essential to take into account the overlapped part L 2 of the microstrip. Closed-form expressions for evaluation of capacitance [6]: = (7) Fig. 5 Plot of S11 and S21 parameter vs frequency. XY Plot 5 stripline The bend in the microstrip increases the return loss since the parasitic discontinuity capacitances increases. Equation no. (7) is used to compensate this loss. This loss is more significant when the operating frequency is 3GHz. The proposed filter works on the center frequency 1GHz subsequently loss is less and moreover inductive arm width is only 0.1 so the conductive area is less. 4. Simulation and Analysis S - db(st(s1,s1)) db(st(s2,s1)) 4.1 Stepped Impedance Open-Stub Lowpass In Fig. 5 simulated S-parameter response of the 3-order stepped impedance open-stub Lowpass filter is shown. S11 and S21 are plotted w.r.t to frequency. The filter works well for the cut-off frequency 1 GHz at -3dB. This filter shows the return loss -31 db. Its shows a wide and smooth stop band from 1 GHz to 4.9 GHz. Insertion loss is -17 db. It shows a ripple of 0.1 db Fig. 6 Plot of S11 and S21 parameter vs frequency of Folded arm LPF 4.2 Folded Arm Stepped Impedance Open-Stub Lowpass In Fig.6 S-parameter (S11 & S21vs. frequency) response of proposed filter, folded arm stepped impedance LPF is

5 817 shown. In designing of LPF with a wide stop-band techniques are proposed in [7], but the obtained response and transition band equal to 0.4 GHz is not sharp enough. In Fig. 6the cut-off frequency is 1 GHz at -3dB. Smooth and wide stop-band from 1 GHz to 4.95 GHz. The return loss of the filter is -21 db. The insertion loss of the filter is- 24 db. 4.3 Combined Analysis of Stepped Impedance and Folded Arm Lowpass In the Fig. 7 comparative results of S-parameter are made. In this graph, response of stepped impedance lowpass filter and proposed folded arm stepped impedance lowpass filter are compared. Both the filters show a cut-off frequency 1 GHz at -3 db. Return loss of folded arm LPF is 10 db higher than the stepped impedance LPF. Insertion loss of stepped impedance LPF is more than folded arm LPF. Both filter show a good stop-band response. Both filter show same stop-band range 1 GHz to 4.9 GHz. S - P a r a m e t e r -3 S11 ( LPF Folded arm ) S11 ( LPF Straight ) db(st(s1,s1)) db(st(s2,s1)) db(st(t1,t1)) Imported airbox_h='20' airb db(st(t2,t1)) Imported airbox_h='20' airb XY Plot 5 S21 (LPF Straight ) S21 ( LPF Folded Arm ) stripline Fig. 7 Comparison of simulated S-parameter response of stepped impedance open-stub lowpass filter and folded arm lowpass filter. 5. Conclusion Traditional 3-pole stepped impedance microstrip lowpass filter and the proposed 3-pole folded arm microstrip stepped impedance lowpass filter show very good response for the center frequency 1 GHz at -3 db with minor variations. In this paper we achieved to design for the compactness of the filter without changing the specification of the stepped impedance lowpass filter. Proposed filter shows better and wide stop-band characteristic and sharp rejection at stop-band. Total dimensions occupied on the PCB board by stepped impedance are x Area of stepped impedance lowpass filter is Whereas total dimensions occupied on the PCB board by folded arm stepped impedance lowpass filter are x Area of folded arm stepped impedance lowpass filter is We achieved to design miniaturized folded arm stepped impedance lowpass filter by 27.9% in the area occupied on the board. Insertion loss of folded arm filter is 7 db less than the stepped impedance lowpass filter. References [1] K.Rajasekaran, J.Jayalakshmi,T.Jayasanka, International Journal of Scientific and Research Publications, Volume 3, Issue 8, August [2] J. Inder Bhal, Average power handling capability of multilayer microstrip lines, International Journal of RF and Computer-Aided Engineering, Volume 11, Issue 6, 2001, pp [3] G.L.Mathaei, L.Young & E.M.T. Jones, Microwave impedance matching networks and coupling structures, Artech House, Dedham, Mass, [4] S. Das, Dr. S.K.Chowdhury, Design Simulation and Fabrication of Stepped Impedance Microstrip line Low Pass for S-band Application using IE3D and MATLAB IJECT Vol. 3, Issue 1, Jan. - March 2012 [5] J. S. Hong, M. J. Lancaster, Microstrip s for RF/Microwave Applications, John Wiley & Sons, Inc., 2001, pp [6] K. C. Gupta, R. Garg, I. Bahl, and P. Bhartis, Microstrip Lines and Slotlines, Second Edition, Artech House, Boston, [7] J.-L. Li, S.-W. Qu, and Q. Xue, Compact Microstriplowpass with Sharp Roll-Off and Wide Stop-Band, Electron. Lett., vol. 45, no. 2, Jan. 2009, pp