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Performance of Microstrip Low-Pass Filter on Electromagnetic Band Gap Ground Plane Anjini Kumar Tiwary and Nisha Gupta Department of Electronics and Communication Engineering, Birla Institute of Technology, Mesra, Ranchi (Jharkhand) 835 215, India
ABSTRACT In this work, the performance characteristics of a printed low-pass filter (LPF) were analyzed on an electromagnetic band gap (EBG) ground plane. A ninth order Chebyshev LPF was designed at 2.44 GHz cut-off frequency using Stepped Impedance Resonator (SIR) method. The simulation results show that the EBG pattern of rectangular slots etched on the ground plane and located offset from the center line not only offers an improved stopband characteristic but also reduces the overall size of the filter. Finally, a prototype model was developed based on the results obtained from the simulation and tested. The simulated and experimental results show good agreement. Keywords: Chebyshev filter, Electromagnetic band gap, EMI/EMC, Low-pass filter.
1. INTRODUCTION RF/microwave filter design techniques have been subjects of active interest for several decades as RF/ microwave filters are important components in most RF/microwave applications. Low-pass filters (LPFs) of high quality, compact size and flexible reconfiguration are always desirable in modern RF/microwave communication systems to remove undesired harmonics or spurious mixing products. By carefully defining the signal band, filters limit the system noise and reduce the potential effect of out-of-band interference essential for many Electromagnetic Interference/Electromagnetic Compatibility (EMI/EMC) applications [1-4]. In the practical application, one of the requisite qualities is sharp passband to stopband transition. But all the filters mentioned above have gradual cut-off response. The rejection characteristic can be improved by increasing the number of cascade sections, which, however, would unfortunately deteriorate passband insertion loss (IL) and lead to larger physical size of the filter. Despite several techniques proposed to extend out-ofband rejection bandwidth, their designs require large area, large step in conductor width or multiple layers. However, use of electromagnetic band gap (EBG) structures in the filter design can overcome the above shortcomings to a greater extent because of their ability to suppress unwanted electromagnetic mode transmission and radiation in microwave and millimeter waves [3,4], which makes them important in EMI/EMC applications [5-9]. The EBG structures are periodic structures in which the propagation of electromagnetic waves is forbidden in certain frequency bands. In these EBG structures, the constructive and destructive interference of electromagnetic waves results in transmission and 230
reflection bands. The EBG structure has also been called a photonic bandgap (PBG) structure or a frequency selective surface (FSS). A common feature of periodic structures is the existence of frequency bands where electromagnetic waves are highly attenuated and do not propagate. Thus, the EBG structures suppress the propagation of surface waves over a specific frequency band that directly depends on the dimensions and type of the constitutive elements within the EBG structures. The planar EBG structures are highly compatible with microstrip line circuits and make contributions to a number of high-performance compact microstrip filter designs. This EBG structure, when etched on the ground plane of the filter configuration [10-14], maximizes destructive wave interference in the stopband frequency range, which produces excellent isolation level in the bandgap frequency range which is desirable in most EMI/EMC applications. The modern RF/microwave filter prototypes are categorized according to their transfer functions. The important filter types are Butterworth (maximally flat), Chebyshev, elliptic function, Gaussian filters and allpass filters. Among them, the Chebyshev class of filters is very popular because of their characteristics such as equiripple in the passband, together with sharp cutoff and high selectivity, which gives a compromise between lowest signal degradation and highest noise/interference rejection. Its usefulness is further enhanced by its ability to build in prescribed transmission zeros for improving the close-to-band rejection slopes. In [15], an LPF design is discussed using hybrid EBG structure for its application in wideband rejection filters. The dimensions of the LPF are obtained from the Kaiser IETE JOURNAL OF RESEARCH | VOL 56 | ISSUE 5 | SEP-OCT 2010
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distribution. However, in the proposed work, a compact multisection sharp-rejection ninth order Chebyshev microstrip LPF etched on an EBG ground plane is designed using the conventional stepped impedance method without using the Kaiser distribution. It is shown that similar improvement in the stopband characteristics is obtained using the current approach.
as depicted in Figure 4(a) and (b). The improvement in the stopband characteristics is further depicted by the S-parameter characteristics. Figure 3 shows the proposed LPF structure with rectangular offset EBG pattern on the ground plane. The offset parameter x = 7.02 mm.
2.
In the first attempt, a ninth order conventional Chebyshev LPF, as shown in Figure 1, was analyzed without any EBG pattern. The filter was designed for a cut-off frequency of 2.44 GHz. The current distribution pattern for the conventional LPF in the passband and the stopband are depicted in Figure 4 (a) and (b). As seen in the passband, the current flow is un-attenuated while in the stopband it is seen to be attenuated. The S-parameter characteristics of the LPF are shown in Figure 5.
LOW-PASS FILTER GEOMETRY AND MATERIAL PARAMETERS
In the proposed work, the characteristics and properties of LPF are determined for a ninth order Chebyshev LPF configuration designed for a cut-off frequency of 2.44 GHz with 0.1 dB ripple response in the passband. In order to attain steep filter transition, ninth order filter is selected. The filter is designed using a Stepped Impedance Resonator (SIR) method where high impedance and low impedance are assumed as 120 and 20 Ω, respectively. The 3D commercial simulator IE3D from Zeland Software Inc., USA, based on Method of Moments (MoM) solution, is used for simulating the proposed LPF structure. The different dimensions of the conventional ninth order Chebyshev LPF unit, as shown in Figure 1, are considered as: Ws = 9.49 mm, L1 = L5 = 4.12 mm, L2 = L4 = 7.36 mm, L3= 7.61 mm, s = 6.56 mm, t = 5.85 mm and g = 0.39 mm. The proposed structure is printed over a low-cost FR4 material which is readily available. The substrate with a dielectric constant of 4.4, loss tangent of 0.016 and thickness of 1.6 mm is considered. The width (w) of the conductor strip on the top plane is 2.62 mm, corresponding to 50-Ω characteristic impedance. Figure 2 shows the proposed LPF structure with rectangular EBG pattern on the ground plane. The structure on the signal line and the ground plane overlaps for zero offset position. Hence, the EBG slot parameters are same as that of the signal line and are given as follows. Dimension of first slot: length L1 = 4.12, width Ws = 9.49; dimension of second slot: length L2 = 7.36, width Ws = 9.49; dimension of third slot: length L3 = 7.61, width Ws = 9.49; dimension of fourth slot: length L4 = 7.36, width Ws = 9.49; dimension of fifth slot: length L5 = 4.12, width, Ws = 9.49. All the parameters are expressed in millimeters. Figure 3 shows the proposed LPF structure with rectangular offset EBG pattern on the ground plane. The offset parameter is x = 7.02 mm. The optimized EBG dimensions are obtained after rigorous simulation steps for several offset positions of the EBG structure and examining the current distribution for each of the offset parameters. The offset is varied in steps of 0.25 times the EBG period. It is found that the best results in terms of stopband width are obtained for the case when the offset is one quarter of the EBG period. This is first confirmed from the current distribution plot, which shows that the current passes freely in the passband and is obstructed to the maximum in the stopband region IETE JOURNAL OF RESEARCH | VOL 56 | ISSUE 5 | SEP-OCT 2010
3. RESULTS AND DISCUSSION
Figure 1: The conventional ninth order Chebyshev LPF.
Figure 2: The proposed ninth order Chebyshev LPF with rectangular EBG pattern.
Figure 3: The proposed ninth order Chebyshev LPF with rectangular offset EBG pattern.
(a)
(b)
Figure 4: (a) Current distribution for conventional LPF in passband 0.285 GHz; (b) current distribution for conventional LPF in stopband 4 GHz. 231
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Next, the analysis of the filter was carried out with rectangular EBG pattern present on the ground plane. The analysis of the filter was carried out with respect to the location of the EBG structure on the ground plane. The best response in terms of wider rejection bandwidth and better rejection ratio was obtained when the EBG pattern was offset by one quarter of an EBG period from the center line. The current distribution pattern plotted for the two cases as shown in Figures 6(a), (b), and 7(a), (b) with the rectangular EBG without and with offset clearly indicates the advantage of the offset in improving the stopband characteristics. It is also observed that the EBG pattern on the ground plane shifts the cut-off frequency of the filter from 2.44 to 2.02 GHz. Therefore, in order the design the filter with EBG pattern for the same cut-off frequency, i.e. 2.44 GHz, the size of the filter is to be reduced and it is found that a size reduction of approximately 12.25% offers the same cut-off frequency. The S-parameter characteristics of the proposed LPF are shown in Figure 8. The prototype models developed on the inexpensive FR4 substrate with a dielectric constant of 4.4 and thickness of 1.6 mm are shown in Figures 9(a), (b), and 10(a), (b).
The proposed LPF and conventional LPF both show 20-dB rejection bandwidth from 2.85 to 6 GHz, i.e. 3.15 GHz. However, the proposed LPF has 40-dB rejection bandwidth from 3.69 to 5.19 GHz, i.e. 1.5 GHz, whereas the conventional LPF shows zero bandwidth at 40 dB. Also, maximum attenuation is around 67.17 dB at 4.04 GHz in proposed LPF, whereas in conventional LPF it is 39.67 dB at 3.76 GHz. Hence, the proposed LPF offers better attenuation characteristics in the stopband along with 40-dB rejection bandwidth. The experimental results are also depicted in Figures 5 and 8 for the conventional and proposed LPF structures. The results are to be found in good agreement with respect to the simulation. Some discrepancies in the experimental results may be attributed to the manufacturing tolerances and the variation in material characteristics of the sample supplied.
4. CONCLUSION A ninth order conventional LPF and the proposed LPF with EBG structure on the ground plane were designed
(a)
(b)
Figure 6: (a) Current distribution for slot without offset LPF in passband 0.24 GHz; (b) current distribution for slot without offset LPF in stopband 4.08 GHz.
Figure 5: Simulated and experimental S-parameter characteristics of conventional LPF.
(a)
(b)
Figure 7: (a) Current distribution for proposed LPF in passband 0.42 GHz; (b) current distribution for proposed LPF in stopband 4.06 GHz. 232
Figure 8: Simulated and experimental S-parameter characteristics of proposed LPF. IETE JOURNAL OF RESEARCH | VOL 56 | ISSUE 5 | SEP-OCT 2010
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(a)
(a)
(b)
Figure 9: (a) and (b) Top and bottom views, respectively, of conventional LPF (fabricated). and prototype models were developed. The filters were designed to operate at a cut-off frequency of 2.44 GHz suitable for WLAN application. It is observed that the proposed LPF not only offers better attenuation characteristics in the stopband and higher 20-dB rejection bandwidth compared to the conventional LPF, but also reduces the size of the LPF by approximately 12% in comparison to the conventional LPF structure.
(b)
Figure 10: (a) and (b) Top and bottom views, respectively, of proposed LPF (fabricated). 7.
8.
9.
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AUTHORS Anjini Kumar Tiwary was born in Jamshedpur, India in 1972. He received M.E. degree in Wireless Communications from Birla Institute of Technology, Ranchi, India in 2009. Currently, he is an Assistant Professor for Department of Electronics and Communication Engineering, Birla Institute of Technology, Ranchi, India and working towards Ph.D. degree. His research interests are Design and Development of Printed Filter Configurations for EMI/EMC applications. E-mail:
[email protected] Nisha Gupta received Bachelor’s and Master’s degrees in Electronics Telecommunication and Electrical & Electronics engineering both from Birla Institute of Technology, Mesra, Ranchi, India and also Ph.D. degree from the Indian Institute of Technology, Kharagpur, India. She was a Maintenance Engineer at Shreeram Bearings Ltd., Ranchi during 1982-83 and a Programmer
at Ranchi University, Ranchi from 1983 to 1986. She was a Junior Scientific Officer in a DRDO sponsored project, at Department of Electronics and Electrical Communication Engineering, Indian Institute of Technology, Kharagpur, from 1986-1989 and a Institute Research Scholar and Research Associate (CSIR) in the same department from 1990-1996. She was a post doctoral fellow at University of Manitoba, Canada from 1997-1998 before joining the Department of Electronics and Communication Engineering, Birla Institute of Technology in 1999 as a Reader. Currently, she is a Professor and Head in the same department. She has authored and coauthored more than 35 technical journal articles. Her research interests are Computational Electromagnetics, RF circuits and Antennas for Wireless Communication and AI techniques in Wireless and Mobile Communication. E-mail:
[email protected]
DOI: 10.4103/0377-2063.72771; Paper No JR 666_09; Copyright © 2010 by the IETE
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