Design of New DGS Hairpin Microstrip Bandpass Filter Using Coupling Matrix Method

Transcription

1 Progress In Electromagnetics Research Symposium 27, Prague, Czech Republic, August Design of New DGS Hairpin Microstrip Bandpass Filter Using Coupling Matrix Method A. Boutejdar 1, A. Elsherbini 2, A. Balalem 1, J. Machac 3, and A. Omar 1 1 Microwave and Communication Engineering, University of Magdeburg, 3916 Magdeburg, Germany 2 Department of Electrical and Electronic Engineering, Ain Shams University, Cairo, Egypt 3 Faculty of Electrical Engineering, Czech Technical University, Technick 2, Prague 6, Czech Republic Abstract In this paper we present a novel compact Hairpin bandpass (BPF) microstrip filter employing two U-slots etched in the ground plane (DGS) and two 5 Ω feeds on the top. A new type of microstrip BPF based on coupled DGS resonators is designed using coupling matrix method. The new BPF is very compact, in addition, the filter has a very wide stopband with two transmission zeroes. A good agreement between the measured and simulated results is achieved. 1. INTRODUCTION THE coupled microstrip bandpass filter (BPF) has been extensively investigated and widely used in many microwave and millimeter-wave systems in order to achieve high performance, small size, and low cost and to comply with strictly required transmission specifications. There are many types of bandpass filter design techniques to meet the above requirements, such as the use of highpermittivity materials, variation of resonator structures, and use of multiple resonant modes. In the conventional microstrip and strip line BPFs having parallel coupled lines, the size is quite large because of use of λ/2 resonators, while the realization is simple. On the other hand, the BPFs having Hairpin resonator structures, which is a modification of parallel coupled lines, have relatively smaller size than the BPF having parallel coupled lines. However, these filters are of limited utility due to their typically high insertion loss and the practical problem to achieve less than 5% bandwidth. In order to solve these problems, the DGS structures will be used. The stringent requirements of modern microwave communication systems are often met only by high performance and compact filtering structures. Several of such filters have been reported using generic structures called the defected-ground structures (DGS). Since DGS cells have inherently resonant properties, they have been used in filtering circuits to achieve narrow bandwidth, and to suppress these spurious passbands The DGS-resonators have several advantages such as compact size, low radiation loss. Therefore, the DGS-Filters are widely used in the design of filters, oscillators, and antennas. In this paper, we introduce a new DGS-Hairpin structure in order to suppress higher harmonics and to realize sharp edges by introducing two transmission zeros to filter response [1], and locate them at either sides of the passband. This DGSs operate as two magnetic coupled resonators and also as a stop resonator to suppress harmonics, simultaneously. 2. HAIRPIN-DGS RESONATOR A defect for the microstrip line, which has been etched in the backside metallic ground plane, disturbs the current distribution in the ground, and increases the effective inductance and capacitance of the microstrip line. Therefore, the DGS is usually modeled as a parallel LC resonance circuit by using a circuit-analysis method. The proposed DGS shape with its dimensions is illustrated in Fig. 1, wile, Fig. 2 shows its equivalent circuit, where L p and C p denote the inductance and capacitance, which are the results of the electromagnetic field disturbances in the ground plane. For more accurately modeling the DGS section, capacitance C 1 and inductance L 1 should be considered as a part of the equivalent circuit models, which are result from the fringing field around the discontinuity area. In order to extract the values of the equivalent circuit elements, the S-parameters of a DGS unit at the metallic ground plane should be calculated using an EM-simulator, in addition the relationship between the S-parameter and ABCD-matrix will be used. To confirm the validity of the proposed equivalent circuit model of a DGS unit, shown in Fig. 1, the DGS-Hairpin slot has been simulated using EM simulators Microwave Office. The dimensions of the slot shown in Fig. 3, are as follows: = 6 mm, l 2 = 1 mm, d = 5 mm, and w = 1.9 mm.

2 262 PIERS Proceedings, August 27, Prague, Czech Republic, 27 5Ω-Microstrip-Line Substrate Metallic Ground L p Z= 5 Ohm Z= 5 Ohm DGS C p Figure 1: Three-dimensional view of the Hairpin- Figure 2: Equivalent circuit of the DGS-Hairpin slot. 3. INFLUENCE OF HAIRPIN DGS DIMENSIONS ON THE ATTENNUATION POLE FREQUENCY The proposed slot shown in Fig. 1, can provide attenuation pole at certain frequency without any periodic array of In order to investigate the frequency characteristics of the etched slot, we simulated the DGS unit section using Microwave Office. The placement of the DGS under the microstrip line involves the appearance of a resonance frequency. This effect is due to the decreased the effective permittivity which results with increasing the effective inductance of the microstrip. The variation of the dimensions of the DGS-length shifts the attenuation pole location in the frequency domain. It is well known, a resonant frequency can be generated by a combination of inductive and capacitive elements. Thus, in order to explain the simulated frequency response of the proposed DGS section, we introduced a capacitance in the equivalent circuit. The etched gap area, which is placed under the microstrip line, corresponds to capacitance and the Metallic bridge between the DGS-arms is equivalent to a series inductance. So, the DGS [2] unit is equivalent to a resonant circuit, which is shown in Fig. 2. The parameters of this DGS equivalent circuit have been found using curve-fitting. They are: C p =.33 pf and L p = 2.33 nh. In order to investigate the effect of the DGS-arm dimensions, the length (d) of intern etched gaps were kept constant at 5mm and the length ( ) of extern etched rectangular area was varied. The simulated results are illustrated in Fig. 4. As the arm-length ( ) are increased, both the characteristic impedance and the series inductance of the microstrip line increased, while the cut-off and resonance frequency decrease. w d l 2 & S 12-2 =2.5mm -4 =3.6mm =3.9mm =4.6mm =5mm S 12 Figure 3: Three-dimensional view of the Hairpin- Figure 4: Simulated S-parameters for different values of of the Hairpin-DGS cell.

3 Progress In Electromagnetics Research Symposium 27, Prague, Czech Republic, August THE BASIC IDEA With the modification of the DGS-size it will be possible and easy to shift the resonant frequency band, thus it will be systematically the Filters answer to controlled. The new idea is: how the resonance position will be controlled, while keeping constant the DGS-size? In order to realize that, the length of intern DGS-arm [3] will be simply changed. While d will be increased, the position will be shifted in lower frequency. Thus the compactness will be improved with the length of the intern-dgs-arms. As the Fig. 6 shows. d w & d=1mm d= 2mm d=3mm d=4mm d=5mm d=6mm -35 Figure 5: BPF. Two-dimensional view of the proposed Figure 6: Simulated S-parameters for different values of d of the BPF. 5. THE THEORY OF COUPLING MATRIX METHOD In order to realize a coupling matrix which conforms to a chosen topology, it is necessary to give first the specifications of the filter. The desired parameters will be then extracted by using an optimization-based scheme [1]. The coupling coefficient and quality factor curves [1] are then used to realize the obtained coupling coefficients In our case the second order filter is with a bandwidth BW = 5 MHz, return loss RL = 2 db, and centre frequency f = 2.1 GHz. The obtained coupling matrix from the optimization scheme is [ ] M =, and the external quality factors are q 1 = q 2 = To realize the normalized coupling matrix and quality factors, we use the required fractional bandwidth F BW = BW/f, the actual (denormalized) coupling matrix becomes. m = [ and Q 1 = Q 2 = 7 where m = F BW M, and Q = q/f BW. The m-coupling coefficients will be inserted in the experimental curve [1] in order to get the optimal distance between the DGS resonators. The unknown distance s is 2 mm. See the Fig DESIGN AND MEASUREMENT OF THE IMPROVED DGS-BANDPASS FILTER The optimized DGS has been used to design a BPF, which was fabricated on a (2 15 mm 2 ) substrate with a relative dielectric constant ε r of 3.38 and a thickness h of.813 mm. Photographs of the filter are shown in Fig. 9. Measurements were carried out on an HP8719D network analyser. One can see from Fig. 1 that the measured results show good consistency with both simulations. The fabricated BPF has a center frequency at 2.1 GHz and a suppression level of 2 db from 2.85 to 8.5 GHz; the insertion loss in the passband is about.15 db. Thus we have demonstrated that the proposed coupled DGS bandpass filter is very favourable than the designed bandpass filters in [1]. The experimental results show excellent agreement with simulated result. Fig. 1 and Fig. 11 show the simulated and measured data of the two layers of the proposed Hairpin-DGS bandpass filter [4]. ],

4 264 PIERS Proceedings, August 27, Prague, Czech Republic, 27 Fig. 8 shows the field distribution resonant frequency (at the transmission pole), it can be clearly seen that no power it transmitted to port 2. Gap Substrate s Metallic Ground DGS (b) Figure 7: 3D-view of the DGS BPF. Figure 8: The EM-field distribution at resonance frequency. (b) at center frequency of the DGS-BPF. (b) Figure 9: Fabricated Hairpin-DGS-BPF. Bottom view. (b) Top view Measurement EM-simulation Theoretical & -4-5 & Measurement EM-simulation Figure 1: Schematics of the designed DGSbandpass filter. Figure 11: Three-dimensional view of the Hairpin-

5 Progress In Electromagnetics Research Symposium 27, Prague, Czech Republic, August CONCLUSION In this paper we have introduced a new DGS Hairpin resonators and investigated different geometrical modification. Controlling the center frequency and improving the characteristics of the proposed BPF have been addressed. The use of DGS cells shows a good effect on the stopband. The second order filters with quasielliptic response were presented. Controlling the center frequency and archiving a good matching at the passband can be simply realized by changing the length or the width of the investigated structure without changing the area occupied by the filter. The filters were designed, fabricated and measured. Good agreement between simulated and measured results was achieved. REFERENCES 1. Hong, J.-S. and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, Wiley, New York, Abdel-Rahman, A., A. R. Ali, S. Amari, and A. S. Omar, Compact bandpass filters using Defected Ground Structure (DGS) coupled resonators, IEEE MTT-S International Microwave Symposium, Honolulu, Hawaii, June Boutejdar, A., A. Elsherbini, and A. S. Omar, Improvement of passband and sharpness factor of parallel coupled microstrip bandpass filter using discontinuities correction and DGS cells, MMS 27 Mediterranean Microwave Symposium Budapest, Hungary, May Awida, M., A. Boutejdar, A. Safwat, H. El-Hannawy, and A. S. Omar, Multi-bandpass filters using multi-armed split ring resontors with direct feed, IEEE MTT-S International Microwave Symposium, Honolulu, Hawaii, June 27.