Design and Improvement of a Compact Bandpass Filter using DGS Technique for WLAN and WiMAX Applications

In this article, A compact size band pass filter based on octagonal resonators is presented to give sharp response at desired frequency bands along with very low insertion loss. The proposed filter structure is composed of octagonal microstrip resonators, backed by Quasi-Yagi slots ‘Quasi-Yagi Defected Ground Structure’ (Quasi-Yagi-DGS). By controlling the electrical coupling between the octagonal–strip and the Quasi-Yagi-DGS, the bandpass filter’s stopband is optimized for better rejection. The proposed BPF has low insertion loss and compact size because of the slow-wave effect. Meanwhile, sharp rejection bands induced by the presence of two transmission zeros. The simulated center frequency and passband insertion loss are 2.4 GHz and 0.6 dB, respectively.

One of the very successful solutions to achieve significant size reduction is to use DGS components which also have the capability of suppressing undesired periodicity effect [25][26][27], easy design, high compactness, high quality factor and is suitable for integration into various microwave subsystems. Thus, the DGS adds an extra degree of freedom in microwave circuit design and opens the door to a wide range of application. The DGS can be designed in different geometries, depending on the application and on the desired frequency characteristics. DGS is realized by etching off a defected pattern from the ground plane [1].
In the context of this work, a bandpass filter structure is designed in a first part. This bandpass filter structure is designed, and simulated. In order to achieve the desired characteristics, some DGS resonators are simulated and optimized to obtain the desired rejection bands.

Characteristics of Octagonal Resonator
The octagonal resonator has a compact geometry. The use of this structure as it can be seen in Figure 1 and Figure 2 will be shown to give sharp cutoff frequency response as well as  [28,29]. A circuit model has been developed to characterize the proposed bandpass filter. Figure. 1 The Three-dimensional view of the octagonal resonator Figure. 2 Simulated S-parameters of octagonal resonator The equivalent-circuit parameters can be calculated from the S-parameters based on an electromagnetic (EM) simulation ( Figure 3). Once the and parameters have been computed at the resonant frequency by the CST simulator, the required circuit parameters can be defined by using the relationship between the S-ABCD parameters and the Y parameters as described by the following equations: where: = the series admittance of the π equivalent circuit, = the parallel admittance of the π equivalent circuit, and = the characteristic impedance of the transmission line,   Figure 3 shows the equivalent circuit model of the octagonal resonator, which composes of a parallel capacitance , and is the parallel inductance to . The parallel capacitance, , between the resonator and the metallic plane is formed by the influence of the fringing field between the resonator and the ground plane.

Characteristics of The Quasi-Yagi-DGS
Typically, DGS units are used to design and improve filters [2], patch antennas [3][4], branch line couplers, power dividers, and power amplifiers [5]. One technique for realizing DGSs is to etch a few defect patterns in the backside metallic ground plane under a microstrip transmission line. The defect patterns disturb the shield current distribution in the ground plane, modifying the transmission line characteristics of the microstrip (an increase in the effective inductance and capacitance) and achieving a slow-wave effect with bandstop properties. In conventional microstrip lines, the value of the line impedance increases when the width of the microstrip line becomes narrower. However, for the case of a microstrip line with DGS, because the additional inductance results in a highly increased characteristic impedance, the line width is broader than that of a standard microstrip line for the same characteristic impedance [6].
A quasi-Yagi DGS (Figure 4) was used to control the coupling between the cascaded resonators to improve the rejection band response, reduce passband loss, increase the sharpness of the transition domain, and thus minimize the size of the filter structure [7], [30][31].  Figure 5 shows the equivalent-circuit model of the Quasi-Yagi-DGS unit, the circuit parameters are extracted from an electromagnetic simulation by matching to a one pole Butterworth band stop filter response, as such in reference [27,32], using a simple relation chips and the optimization's method (14) and (15), where represents the sum of the capacitances in the ground plane and is the parallel inductance to . The parallel capacitance, , between the microstrip feed and the metallic plane is formed by the influence of the fringing field around the DGS unit.
Values for the resonator's inductor and capacitor, and , can be computed by means of Equation (14) and (15) [8]. The simulation results of the investigated EM structure and its corresponding circuit are illustrated in Figure 6, which shows identical values of 3 dB cutoff frequency ( ) and pole frequency ( ) at 5.25 and 7 GHz, respectively. The transmit band show a very low insertion loss (<0.4 dB). The proposed DGS-resonator with its dimensions is shown in Figure 6.

Design Procedure of Planar Band Pass Filter Using Identical Octagonal Resonators
In order to obtain WLAN, and WiMAX, two-pole microstrip band pass filter with two transmission zeros, low insertion loss in the pass band, and high selectivity, a structure with tow octagonal resonators bandpass filter were proposed. The included configurations use slowwave open stub tapped resonator filters. One of the very successful topologies to achieve significant size reduction is to use coupled Quasi-Yagi-DGS components as shown in Figure 9. The proposed filter is designed on substrate RO4003 with relative permittivity , a dielectric loss tangent and thickness . From the simulation response of two poles BPF (Figure 7 and Figure 8) the simulated performance has 3 dB bandwidth from 2.33-2.46 GHz and shows second passband around 7 GHz. The undesired second passband must be minimized. The deployment of a bandstop filter as cascading section with the bandpass filter is a classical technique to reduce unwanted passband. By taking the advantages of an attenuation pole of the designed DGS having resonant frequency at the second passband (Figure 6), the unneeded passband is drastically reduced. The structure of the BPF which significantly suppresses unwanted passband is shown in Figure 9.  Figure 10. The Simulated |S 21 | achieve below -20 dB in the stopband and less than 0.6 dB in the pass band. Comparing with |S 11 | of the previous BPF, |S 11 | of the DGS BPF is better. The unwanted passband is replaced by a transmission zero around the 7 GHz frequency.

Field Distribution Along of the BPF
In order to investigate the filter characteristics, current distribution is examined. Figure  11(a) and Figure 11(b) show the current distributions for the BPF in its passband (f = 2.4 GHz) and in its rejection band (f = 7 GHz). From Figure 11(a), it can be clearly noticed that at a frequency of 2.4 GHz, the energy is transmitted from input to output ports.

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On the other hand, the current shows minimum distribution (-40 dB) in one port (the input feed port) with no energy flow around port 2, thus displaying strong stopband behavior, at the frequency of 7 GHz which operates in the stopband as shown in Figure 11

Conclusion
A novel 2 nd order WLAN (and WiMAX) bandpass filter with two transmission zeros for sharp transition band using DGS has been presented in this paper, the undesired harmonic has been suppressed using DGS. In order to realize a compact, symmetrical structure and to simplify the implementation, DGS-method and electric coupling method have been used. The proposed BPF consists of two octagonal-strip resonators located on the top surface and two Quasi-Yagi-DGS resonators in the ground plane. The filter has been designed and simulated for a center frequency of 2.4 GHz. The simulated insertion loss and return loss in the passband are better than 0.6 and 15 dB, respectively.