International Journal of Innovative Research in Engineering & Science ISSN 2319-5665 (April 2014, issue 3 volume 4) Improve the Frequency Characteristic of Stepped Impedance Microstrip Low Pass Filter Using Dumbbell Shape Dhruv Singh Thakur#1 , Krishna Kant Nayak#2 , Manorma Kushwah#3 #1 Guide, #2HOD, #3Research Scholar Department of Electronics Engg. Bansal Institute of Science and Technology Bhopal (M.P.) India ABSTRACT Demand for newer microwave and millimeter wave system with respect to size, cost and performance. This paper describes a compact dumbbell-shaped defected ground structure (DGS) unit and also Comparison the frequency characteristic between conventional step impedance filters and dumbbell shaped DGS. Using micro strip layout which works at 2 GHz for permittivity 4.4 values with a substrate thickness 1.6 mm the design and simulation are performed using 3D full wave simulator IE3D. Key words: Low Pass Filter, IE3D, Micro Strip Filter, Defected ground structure (DGS). INTRODUCTION Applications of defected ground structure (DGS) in radio frequency/microwave (RF/MW) circuits find numerous advantages like circuitry size, sharp cut off and superior response suppression. DGS has simple structure, equivalent L-C circuit model, and extensive applicability to design filters, couplers, dividers, and amplifiers [9-12]. Recently, the defected ground structure (DGS) for microstrip lines [14-15][9] has become one of the most interesting areas of research owing to their extensive applicability in microwave circuits. 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. Dumb-bell shaped DGS is explored first time by D. Ahn and applied to design a low pass filter [6-8]. It shows one-pole response and modeled by Butterworth low pass filter. These DGS units are used for designing all pole type filters. In [1] New microstrip low pass filters with ultra-wide stopband using periodical non-uniform DGS are designed, and a comparison between low pass filters with uniform 50-ohm and steppedimpedance microstrip line are evaluated, shows with stepped-impedance microstrip line, the filter has better performances of enhanced return loss, steeper out band suppression and better harmonics suppression. In [2] a novel dumb-bell defected ground structure with lengthened etched gap (LDGS),which improved dumb-bell DGS by adding additional etched gap in etched square area. Simulation and results show the resonance frequency of the LDGS is 2.48 GHz, while general dumb-bell DGS is 5.49 GHz with the same area. In [3], A dumbbell shaped defected ground structure (DGS) RF choke is proposed for UWB broadband RF choke design to cover the 3.1 to 10.6 GHz UWB band with 15 dB rejection throughout the band. In this paper we design dumbbell shape on ground plane for improving the frequency characteristic of the filter. 38 International Journal of Innovative Research in Engineering & Science ISSN 2319-5665 (April 2014, issue 3 volume 4) LOW-PASS FILTER DESIGN In this section a microstrip low pass filter has been designed. Subsequently the same design has been implemented using the defected ground structure technique and the performance of both the filters has been compared. Both filters have been designed to have a maximally flat response and a cut-off frequency of 2 GHz. The impedance of both input and output ports is assumed to be 50Ω. Low-Pass Filter without DGS: In this design the conventional metallic ground plane is used without any defects. The lumped element low pass filter circuit to meet the above specifications is obtained by using the prototype filter available for a maximally flat response. The element values are obtained by proper impedance and frequency scaling. To implement this design using microstrip technology, alternating sections of very high and very low characteristic impedance lines are used. These kinds of filters are known as stepped impedance filters or hi-Z; low-Z filters [4]. Such filters are easier to fabricate and take up less space. The characteristic impedance of the low- impedance lines is chosen as 11Ω and that for the high-impedance lines is chosen to be 150Ω the filter design steps are as follows: Filter Design Specifications: Cut-off frequency=2 GHz, Relative Dielectric Constant ε r = 4.4, Height of substrate, h=1.6mm. The loss tangent tanδ=0.02. The filter impedance Z0 =50Ω., The highest line impedance ZH=ZOL=150Ω. The lowest line impedance Zl=Zoc=20Ω. Ωc=1. We have taken the element value for low pass filters from [12] for n=6. The values of various inductance Li and capacitance Ci are obtained from the equations, as Li = (Zo/go) (Ωc/2πfc) gi Ci = (go/Zo)(Ωc/2πfc) gi Physical length (in mm) of the high and low impedance lines (inductance and capacitance respectively) is found out by the formulae given below, lL = λgl /2π Sin-1 (ωc Li / ZOL) lC = λgc /2π Sin-1 (ωc Ci Zoc) L and C are the normalized element values of the low pass prototypes To calculate the width of capacitor and inductor we use the following formula [9]. W/h = 8 exp (A)/exp(2A)-2 Here A= [(ZC / 60){(εr+1)/2}^0.5+ (εr+1/ εr-1)(0.23+0.11/ εr )] Zc = η / 2 π √ εre [ln (8h/w+ 0.25 w/h)] The effective dielectric constant can be found by the following formula εre = (εr+1)/2+(εr-1)/2 [(1+12h/W)-0.5] The effective wavelength is also found as λge = 300/(1.5√ εre) The length of widths and lengths of the alternating low and high impedance lines have been calculated and summarized in table 1. (Some value of length and width has modified according to demand of the geometry) Table 1: Summary of stepped impedance low pass filter without DGS 39 International Journal of Innovative Research in Engineering & Science Section Zi= Zl or ZH (Ω) Wi(mm) li(mm) 1 2 3 4 5 6 7 20 150 20 150 20 150 50 11.39 0.1789 11.39 0.1789 11.39 0.1789 3.04 2.1 5.42 7.79 7.4 6.1 1.98 6.85 ISSN 2319-5665 (April 2014, issue 3 volume 4) Fig. 1: shows the physical layout of the top layer of the stepped impedance low pass filter. Fig 2: Frequency response of the stepped impedance low pass filter without DGS 40 International Journal of Innovative Research in Engineering & Science ISSN 2319-5665 (April 2014, issue 3 volume 4) Figure 2 shows the Frequency response of the stepped impedance low pass filter without DGS. The graph clearly indicates a maximally flat response at 2GHz cut off frequency with a rejection of approximately -43 dB at 5.7GHz and a return loss performance of mostly better than -28dB (36dB in the best case) is observed. Low-Pass Filter with DGS: In this design dumbbell shape defects are incorporated in the ground plane. A DGS unit can be represented by an LC equivalent circuit. The etched lattice increases the series inductance to the microstrip line and the etched gap area which is placed under the inductance line provides the parallel capacitance with effective line inductance. The dimensions of the dumbbell are a=6mm, b=3mm, l=6mm and w=0.2mm. The superimposed view of the filter shown below Fig 3: Superimposed view of top layer and ground plane of the low pass filter with DGS Fig 4: Frequency response of the stepped impedance low pass filter with DGS 41 International Journal of Innovative Research in Engineering & Science ISSN 2319-5665 (April 2014, issue 3 volume 4) The response obtained is maximally flat at 2GHz cut off frequency with a rejection of approximately -55 dB at 4GHz which is much better than the design requirement. Furthermore, it can be said that the power handling capability of the stepped impedance low pass filter with DGS will be better than that for the stepped impedance low pass filter without DGS since DGS helps in implementing high impedance inductance line with broader conductor width when compared to conventional micro strip CONCLUSION The papers present an efficient approach to improve rejection of unwanted frequency with compare to conventional stepped impedance low pass filter. From the simulated results at 2GHz cut off frequency the low pass filter without DGS reject frequency approximately 43 dB at 5.7GHz using DGS offers significant improvement approximately -55 dB at 4GHz with same cut-off frequency and size reduction when compared with the conventional low pass filter. ACKNOWLEDGMENT I would like to express my special appreciation and thanks to my guide Asst Prof Dhruv Singh Thakur Bansal Institute of Science and Technology Bhopal (M.P.) REFERENCES . [1] Jian-Kang Xiaol, Yu-Feng Zhu Jeffrey S. Fu, “ Non uniform DGS Low Pass Filter with ultra wide stopband” 978-1-4244-6908-6/10/$26.00 ©2010 IEEE [2] Bin You Junqing Zhang, “ A novel defected ground structure with lenghtened etched gap and microwave filter design” 978-1-4244-3693-4/09/$25.00 ©2009 IEEE [3] Chin-I Yeh, Dong-Hua Yang, Yung-Nan Chen, Tsung-Han Liu, Jeffrey S. Fu* “Dumbbell DGS Based Broadband RF Choke for UWB LNA” 978-1-4244-64180/10/$26.00 ©2010 IEEE [4] D. M. 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