Hindawi Publishing Corporation Active and Passive Electronic Components Volume 2011, Article ID 919240, pages doi:10.1155/2011/919240 Research Article Comb-Line Filter with Coupling Capacitor in Ground Plane Toshiaki Kitamura Faculty of Engineering Science, Kansai University, Suita, Osaka 564-8680, Japan Correspondence should be addressed to Toshiaki Kitamura, kita@kansai-u.ac.jp Received 18 January 2011; Accepted March 2011 Academic Editor: Tzyy-Sheng Horng Copyright © 2011 Toshiaki Kitamura This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited A comb-line filter with a coupling capacitor in the ground plane is proposed The filter consists of two quarter-wavelength microstrip resonators A coupling capacitor is inserted into the ground plane in order to build strong coupling locally along the resonators The filtering characteristics are investigated through numerical simulations as well as experiments Filtering characteristics that have attenuation poles at both sides of the passband are obtained The input susceptances of even and odd modes and coupling coefficients are discussed The filters using stepped impedance resonators (SIRs) are also discussed, and the effects of the coupling capacitor for an SIR structure are shown Introduction Filter Structure Miniaturization of microwave filters is highly demanded For mobile telephones especially, ceramic laminated filters [1–4] have been widely used, and in particular, comb-line filters have extensively made practical use In this study, comb-line filters in which both sides of the substrate are utilized are considered Comb-line filters consist of two quarter-wavelength resonators, and attenuation poles can be created in the frequency characteristics of the transmission parameter by changing the coupling locally along the resonators [5] The stopband characteristics can be improved by arranging the attenuation poles around the passband In [1, 2], strong coupling between two resonators is obtained by installing a patch conductor on the dielectric substrate above the resonators The patch conductor is referred to as a coupling capacitor (Cc ) The method of installing a coupling capacitor by inserting slots into the ground plane is discussed By this method, a coupling capacitor can be achieved without using a multilayered structure As a method of inserting slots into a ground plane, a defected ground structure (DGS) has been attracting much attention [6–8] In a broad sense, the proposed structure is a kind of DGS The filtering characteristics are investigated through numerical simulations as well as experiments The filters using stepped impedance resonators (SIRs) are also discussed, and the effects of the coupling capacitor for an SIR structure are shown Figure shows an overview of the proposed comb-line filter Two microstrip resonators are arranged on the substrate, and each resonator is terminated through the ground plane at one end using a through hole As an I/O port, a microstrip line with a characteristic impedance of 50 ohm is directly connected to each resonator A square-shaped coupling capacitor is fabricated by inserting slots into the ground plane The coupling capacitor is a patch conductor that is not terminated through the ground plane and produces strong coupling locally along the resonators The thickness and relative permittivity of the substrate are assumed to be 1.27 mm and 10.2, respectively, and the diameter of the through hole is 0.3 mm The metallization patterns and dimensions of the filter are shown in Figure The dimensions and position of the coupling capacitor are also shown The coupling capacitor is a × 5.2 mm2 and is located d mm from the open ends of the microstrip resonators The filtering characteristics are investigated with a and d as parameters The other structural parameters are also shown in Figure Results and Discussion The filtering characteristics are investigated through numerical simulations by the full-wave EM simulator Ansoft HFSS Active and Passive Electronic Components −10 I/O port −20 I/O port |S11 | (dB) −30 1.27 mm |S21 | (dB) −10 Shading: conductor Symmetric plane −20 −30 −40 −50 −60 Figure 1: Whole structure of filter −70 0.4 5.2 Frequency (GHz) 0.4 With Cc Without Cc Figure 3: Frequency characteristics of scattering parameters 10.2 d 0.4 a j 0.4 j o e Bin + Bin j e o Bin − Bin o e Bin − Bin j e o Bin − Bin j o e Bin + Bin 1.2 0.4 Figure 4: Equivalent circuit 2.4 0.4 Unit: mm Shading: conductor Symmetric plane Figure 2: Metallization patterns Ver 11 The frequency characteristics of the scattering parameters when a = 5.2 mm and d = 1.0 mm are shown in Figure For comparison, the results when there is no coupling capacitor (Cc ) are also shown It can be seen that an attenuation pole was created at each side of the passband by installing the coupling capacitor The center frequency of the passband was also increased slightly by inserting slots into the ground plane The slots also cause radiation loss, and it is understood from Figure that − |S11 |2 − |S21 |2 = 0.090 at 2.1 GHz when a coupling capacitor is used Figure illustrates the equivalent circuit of the comb-line filter Attenuation poles appear at the frequencies where the input susceptances of the even and odd modes are equal to each other The frequency characteristics of the input susceptances e o and Bin are shown in Figures of the even and odd modes Bin 5(a) and 5(b), respectively The structural parameters were the same as those in Figure The input susceptances of the even and odd modes were calculated by setting the magnetic and electric wall, respectively, on the symmetric plane shown in Figure As shown in these figures, the odd-mode susceptances hardly changed, whether or not there was a coupling capacitor The coupling capacitor does not have much effect on the electromagnetic fields of the odd mode On the other hand, the even-mode susceptances were decreased by inserting the coupling capacitor, and they intersected with the odd-mode ones at 1.62 and 2.81 GHz, as shown in Figures 5(a) and 5(b), respectively The frequencies of the intersections correspond with the attenuation-pole frequencies in Figure The frequency characteristics of the scattering parameters with d as a parameter when a = 5.2 mm are shown in Figures 6(a) and 6(b) Here, the parameter d corresponds to the position of the coupling capacitor As shown in Figure 6(a), when d is small (from to 1.0 mm), two attenuation poles appear at both sides of the passband, and the parameter d mainly affects the attenuation-pole frequency above the passband On the other hand, as shown in Figure 6(b), no attenuation pole appears when d is large (from 4.0 to 6.0 mm) The resonant frequency of the odd Active and Passive Electronic Components 0.002 −0.002 Bin (S) Bin (S) 0.001 −0.004 −0.006 1.55 1.6 Frequency (GHz) Even mode (with Cc ) Odd mode (with Cc ) −0.001 1.65 2.75 2.8 Frequency (GHz) Even mode (with Cc ) Odd mode (with Cc ) Even mode (without Cc ) Odd mode (without Cc ) (a) 2.85 Even mode (without Cc ) Odd mode (without Cc ) (b) Figure 5: Frequency characteristics of input susceptances around attenuation-pole frequencies (a) below and (b) above passband −20 |S11 | (dB) −10 −10 −20 −30 −30 −10 −10 |S21 | (dB) −20 −20 −30 −40 −40 −50 −50 −60 −60 |S21 | (dB) −30 −70 |S11 | (dB) 0 −70 Frequency (GHz) Frequency (GHz) d = mm d = mm d = mm d = mm d = 0.5 mm d = mm (a) (b) Figure 6: Frequency characteristics of scattering parameters when (a) d = ∼ 1.0 mm and (b) d = 4.0 ∼ 6.0 mm (a = 5.2 mm) mode is about 2.0 GHz and changes very little when changing d However, the resonant frequency of the even mode decreases as d increases It was confirmed that the even-mode resonant frequency becomes lower than the odd-mode one when d exceeds about 2.5 mm The frequency characteristics of the input susceptances of an even mode are shown in Figure Here, the frequency range is chosen so as to be close to the attenuation-pole frequency above the passband The structural parameters were the same as in Figure 6(a) As shown in this figure, Active and Passive Electronic Components 0.01 Coupling coefficients 0.1 Bin (S) 0.005 0.08 0.06 0.04 0.02 −0.005 2.8 Frequency (GHz) 3.2 3.4 0.5 d (mm) 1.5 Figure 8: Coupling coefficients as a function of d (a = 5.2 mm) d = mm d = 0.5 mm d = mm Figure 7: Frequency characteristics of input susceptances of even mode (a = 5.2 mm) −10 −20 f − fo2 k = e2 fe + fo2 (1) Here, fe and fo are the resonant frequencies of the even and odd modes, respectively, and they are determined from the zero-crossing points of the input susceptance curves of the even and odd modes As can be seen, the coupling coefficients decrease steadily as d increases Figure shows the frequency characteristics of the scattering parameters with a as a parameter when d = mm Here, the parameter a is the length of the patch conductor that makes up the coupling capacitor As shown in this figure, the attenuation-pole frequencies decrease, and, in contrast, the passband frequency shifts slightly higher as a increases The coupling coefficients as a function of a are shown in Figure 10 It can be seen that the coupling coefficients increase almost linearly as a increases Next, an SIR filter shown in Figure 11 is studied Filters can be miniaturized by using an SIR structure The frequency characteristics of scattering parameters are shown in Figure 12 The solid line shows the results of a normal SIR −30 −10 |S21 | (dB) the input susceptances of an even mode change almost in parallel to each other with changing d According to this, the attenuation-pole frequencies above the passband change as shown in Figure 6(a) However, from Figure 6, the parameter d also has a large effect on the bandwidth of the passband The coupling coefficients as a function of d are shown in Figure The coupling coefficient k is calculated using the following equation |S11 | (dB) 2.6 −20 −30 −40 −50 −60 −70 Frequency (GHz) a = 5.2 mm a = 6.2 mm a = 7.2 mm Figure 9: Frequency characteristics of scattering parameters (d = mm) filter (without Cc ) when s1 = 0.1 mm and s2 = 0.2 mm It is shown that the passband frequency becomes lower compared with that in Figure 3, meaning that it is possible to miniaturize the filter In addition, two attenuation poles can be created at both of the passbands by choosing appropriate values for parameters s1 and s2 However, the values of s1 and s2 are quite small The dotted line shows the results of an SIR filter with Cc when s1 = 0.4 mm and s2 = 0.9 mm It is understood that attenuation poles can be created near the Active and Passive Electronic Components −10 −20 −30 −10 −20 |S21 | (dB) Coupling coefficients 0.12 |S11 | (dB) 0.16 0.08 −30 −40 −50 −60 0.04 −70 a (mm) 5.2 Frequency (GHz) SIR (without Cc ) SIR (with Cc ) Straight (with Cc ) Figure 10: Coupling coefficients as a function of a (d = mm) 0.4 Figure 12: Frequency characteristics of scattering parameters (solid line: s1 = 0.1 mm and s2 = 0.2 mm (without Cc ), dotted line: s1 = 0.4 mm and s2 = 0.9 mm (with Cc ), and dashed line: a = 5.2 mm and d = 1.0 mm (Figure 2)) 0.4 3.6 −10 s1 −20 0.4 6.2 −10 |S21 | (dB) 0.4 1.2 0.4 −30 5.2 |S11 | (dB) −20 −30 −40 −50 −60 −70 2.4 s2 Symmetric plane Unit: mm Shading: conductor Figure 11: Metallization patterns of SIR filter Frequency (GHz) Experiment Simulation Figure 13: Frequency characteristics of scattering parameters (a = 5.2 mm and d = 1.0 mm (Figure 2)) passband by choosing s1 and s2 that are easy to fabricate, and therefore, the structural limitation can be relaxed by using a coupling capacitor [2] However, a drawback is that the passband frequency becomes higher compared with a normal SIR filter This is due to the decrease of the effective relative permittivity by the installation of slots in the ground plane For comparison, the results of the filter shown in Figure (a = 5.2 mm, and d = 1.0 mm) are also shown (dashed line) Finally, the filtering characteristics of our developed filter are investigated through experiments The proposed filter shown in Figure is manufactured on an RT/duroid 6010LM substrate of 1.27-mm thickness and 10.2 relative permittivity The frequency characteristics of the scattering parameters when a = 5.2 mm and d = 1.0 mm are shown in Figure 13 Here, the solid and dashed lines indicate the experimental and numerical results, respectively As can be seen, bandpass characteristics with an attenuation pole both below and above the passband were achieved The experimental results were also in good agreement with the numerical ones From this figure, it is estimated that the influence of process variation may cause the degradation of impedance matching Conclusion A comb-line filter with a coupling capacitor in the ground plane was proposed The insertion of the coupling capacitor builds strong coupling locally along the resonators in the filter The filtering characteristics were investigated through numerical simulations as well as experiments, and the filtering characteristics having attenuation poles at both sides of the passband were obtained The input susceptances of even and odd modes and coupling coefficients were discussed The filters using SIRs were also discussed, and the effects of the coupling capacitor for an SIR structure were shown References [1] T Ishizaki, M Fujita, H Kagata, T Uwano, and H Miyake, “Very small dielectric planar filter for portable telephones,” IEEE Transactions on Microwave Theory and Techniques, vol 42, no 11, pp 2017–2022, 1994 [2] T Ishizakl, T Uwano, and H Miyake, “An extended configuration of a stepped impedance comb-line filter,” IEICE Transactions on Electronics, vol 79, no 5, pp 671–677, 1996 [3] T Ishizaki, T Kitamura, M Geshiro, and S Sawa, “Study of the Influence of Grounding for Microstrip Resonators,” IEEE Transactions on Microwave Theory and Techniques, vol 45, no 12, pp 2089–2093, 1997 [4] T Kitamura, M Geshiro, T Ishizaki, T Maekawa, and S Sawa, “Characterization of triplate strip resonators with a loading capacitor,” IEICE Transactions on Electronics, vol 81, no 12, pp 1793–1798, 1998 [5] H Egami, T Kitamura, and M Geshiro, “Study on meandershaped microstrip comb-line filter,” The Institute of Electrical Engineers of Japan, vol 125, no 10, pp 1596–1601, 2005 [6] A M E Safwat, F Podevin, P Ferrari, and A Vilcot, “Tunable bandstop defected ground structure resonator using reconfigurable dumbbell-shaped coplanar waveguide,” IEEE Transactions on Microwave Theory and Techniques, vol 54, no 9, Article ID 1684152, pp 3559–3564, 2006 [7] M Wang, Y Chang, H Wu, C Huang, and Y Su, “An inverse s-shaped slotted ground structure applied to miniature wide stopband lowpass filters,” IEICE Transactions on Electronics, vol 90, no 12, pp 2285–2288, 2007 [8] J Yang, C Gu, and W Wu, “Design of novel compact coupled microstrip power divider with harmonic suppression,” IEEE Microwave and Wireless Components Letters, vol 18, no 9, pp 572–574, 2008 Active and Passive Electronic Components Copyright of Active & Passive Electronic Components is the property of Hindawi Publishing Corporation and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission However, users may print, download, or email articles for individual use ... the influence of process variation may cause the degradation of impedance matching Conclusion A comb- line filter with a coupling capacitor in the ground plane was proposed The insertion of the coupling. .. 0.4 a j 0.4 j o e Bin + Bin j e o Bin − Bin o e Bin − Bin j e o Bin − Bin j o e Bin + Bin 1.2 0.4 Figure 4: Equivalent circuit 2.4 0.4 Unit: mm Shading: conductor Symmetric plane Figure 2: Metallization... each side of the passband by installing the coupling capacitor The center frequency of the passband was also increased slightly by inserting slots into the ground plane The slots also cause radiation