Design of UWB Bandpass Filter with WLAN Band Rejection by DMS in Stub Loaded Microstrip Highpass Filter Pratik Mondal 1, Hiranmoy Dey *2, Arabinda Roy 3, Susanta Kumar Parui 4 Department of Electronics and Telecommunication Engineering, Indian Institute of Engineering Science and Technology, Shibpur, India 1 pratik665@gmail.com; 4 susanta_p@telecom.becs.ac.in Abstract- A novel concept of bandpass filter (BPF) using distributed stub loaded highpass filter (HPF) and Defected Microstrip Structure (DMS) is proposed in this paper, which exhibits an ultra-wideband (UWB) property. The proposed design exhibits a bandpass response with WLAN rejection by combining the proposed DMS with the highpass filter design approach, also miniaturization is proposed to minimize the structure resulting in the pass band enhancement of the proposed filter with minimum insertion loss. The design consists of shorted stub microstrip transmission line on a substrate with relative dielectric constant 4.4 and thickness 1.59 mm. Its excellent bandpass property is verified by both simulated and measured results which show a result of 90.39% Fractional Bandwidth (FBW). The proposed UWB- BPF has an advantage of compact size, low cost and easy fabrication. Keywords- Bandpass Filter; Highpass Filter; Defected Microstrip Structure; Ultra-Wideband Filter; WLAN Rejection I. INTRODUCTION Microstrip filters are used to confine the microwave signals within assigned spectral limits. In case of design and realization of filters, there is a limitation in operating frequency band, which may be due to radiation, dispersion and resonance effects within its elements. A bandpass filter is an important passive component which is able to select signals inside a specific bandwidth at a certain center frequency and reject signals of other frequency region. Optimum distributed microstrip structures are periodic structures where in the propagation of wave in certain frequency band is allowed without any attenuation [1-3]. Here, the bandpass filters are designed by integrating DMS in the signal plane with the stub loaded HPF. Improvement also been done by the compact model. The compactness of the proposed filter is obtained by folded short stubs of same electrical length and enhancement of bandwidth up to 5.19% occurs due to high coupling. Defected etched structure in the metallic ground plane of a microstrip line is an attractive solution for achieving rejection band and slow-wave characteristics due to disturbance or perturbation of the shield current distribution in the ground plane [4]. The combination of the proposed HPF and DMS represents a new methodology to construct bandpass filter. The measured result shows much higher FBW, wider bandwidth and comparatively low loss. Ultra-wideband (UWB) or broad-band microwave filters are essential components for many promising modern applications, such as UWB wireless and radar systems. Since the U.S. Federal Communications Commission (FCC) authorized the unlicensed use of the frequency band from 3.1 to 10.6 GHz for commercial purposes and their applicability may be limited by existing narrowband radio signals such as wireless local area network (WLAN) that may interfere with the UWB system in the UWB frequency band of 3.1 to 10.6 GHz [5]. So a communication system working in this UWB frequency band required a bandpass filter with a notched band to avoid being interfered by the WLAN radio signals [6]. The range of WLAN signal is 5.25 to 5.85 GHz [10]. The rejection of WLAN bandwidth has been achieved by the DMS here. II. DESIGN OF SHORT CIRCUITED STUB LOADED HIGHPASS FILTER AND IT S MINIATURIZATION A. Design of Optimum Distributed Stub Loaded Highpass Filter This type of filter consists of a cascade of shunt short-circuited stubs of electrical length (θc) at some specified frequency, fc (usually the cutoff frequency of high pass), separated by connecting lines (unit elements) of electrical length 2θc. Although the filter consists of only n stubs, it has an insertion function of degree 2n 1 in frequency so that its highpass response has 2n 1 ripples [7]. Therefore, the stub filter will have a fast rate of cutoff, and considered to be optimum in this sense. θ = θ c f fc (1) - 50 -
Fig. 1 Optimum distributed short circuited stub high pass filter Optimum distributed microstrip structures are periodic structures as shown in Fig. 1, where in the propagation of wave in certain frequency band is allowed without any attenuation. A highpass filter consisting of five short-circuited stubs distributed along the transmission line is designed using the conventional technique, for which the design parameters are: 3dB Cut-off frequency (fc) = 3.1 GHz, Substrate used is FR4 of Dielectric Constant (ε_r) = 4.4 & Height (h) =1.59 mm, Passband ripplee = 0.1dB, Characteristic impedance (Z) = 50Ω, Corresponding width= 3mm, Number of stub elements (n) = 5 and Electrical length (θ_c) = 30. The Layout and simulated S-parameters for the proposed filter is shown in Fig. 2 and Fig. 3 respectively. Table 1 shows the normalized values of the elements under consideration and Table 2 shows computed design parameters of proposed filter. Let us consider the design of an optimum distributed highpass filter having a cutoff frequency fc = 3.1 GHz and 0.1 db ripple passband up to 15.5 GHz. Fig. 2 Layout of proposed HPF. (a=32 mm, b=5 mm, c=5.5 mm, d=5.4 mm, e=5.4 mm, f=5.11 mm, g=5.12 mm, W=3 mm, W1=0.2 mm, W2=0.48 mm, W3=0.63 mm) Fig. 3 Simulated S-Parameters of the proposed HPF TABLE 1 NORMALIZED VALUES OF THE ELEMENTS y (mhos) y 1=y 5= 0.34252 y 2=y 4= 0.43985 y 3= 0.48284 y 1,2=y 4,5= 1.67119 y 2,3=y 3,4= 1.05095 Z (ohms) Z 1=Z 3= 146 Z 2=Z 4= 113.67 Z 3= 103.55 Z 1,2=Z 4,3= 46.672 Z 2,3=Z 3,4= 47.57 TABLE 2 COMPUTED DESIGN PARAMETERS 51
W (mm) W 1=W 5= 0.2 W 2=W 4= 0.48 W 3= 0.63 W 1,2=W 4,5= 3.39 W 2,3=W 3,4= 3.2 λ g (mm) λ g1=λ g5= 21.98 λ g2=λ g4= 21.70 λ g3= 21.59 λ g1,2=λ g4,5= 20.44 λ g2,3=λ g3,4= 20.49 l (mm) l 1=l 5=5.5 l 2=l 4=5.4 l 3=5.4 l 1,2=l 4,5=5.11 l 2,3=l 3,4=5.12 The simulated result shown in Fig. 3 shows that the highpass response is up to 11.8 GHz instead of 15 GHz which is the desired one. So, in order to get high coupling and wider bandwidth, other approaches should be made. B. Miniaturization of HPF Using Folded L-Shaped Shorted Stubs The miniaturization is done in the proposed HPF by folding the shorted stubs with L-Pattern as shown in Fig. 4, maintaining the same electrical length (θc). The stubs are arranged in parallel, close to each other so that they are coupled. The folded stubs with L-Pattern provides tight coupling and enhance the bandwidth up to 5.19%. Fig. 4 Layout of folded L-Shaped HPF (a=32 mm, b=5 mm, c=2.7 mm, d=5.11 mm, e=5.12 mm, W=3 mm) As shown in Table 3 and Fig. 5, the performance of the miniaturized HPF is compared with the proposed methodology, as shown in Table 3 showing the overall size reduction up to 33%. TABLE 3 PERFORMANCES COMPARISON Type Passband (GHz) Occupied Area (mm 2 ) Normal stub loaded HPF 3.1-10.8 32*8.5 Folded stub loaded HPF 3.2-11.3 32*5.7 Fig. 5 Comparison of simulated results of S- Parameters III. BAND STOP PROPERTY OF DEFECTED MICROSTRIP STRUCTURES Defected etched structure on the metallic plane of a microstrip line is an attractive solution for achieving certain rejection band and slow-wave characteristics due to disturbance or perturbation of the field current distribution. DMS has the characteristics of band rejection as mentioned previously and also shown in Fig. 6. 52
0 Amplitude(dB) -10-20 -30 S 11-40 S 21 4 5 6 7 8 Frequency(GHz) Fig. 6 Simulated S parameters of the DMS(a=1mm,b=7mm,w=0.2mm) as mentioned in figure The schematic diagram of the proposed L shaped DGS structure is shown in Fig. 6 and its corresponding S parameters are also shown. It shows a narrow stop band with a pole frequency at 5.8 GHz. Now, as the length of the DMS is increased, i.e. inductance increased, the operating frequency is shifted towards lower value as shown in Fig. 7(a). The variation of slot width (w) with pole frequency of notch is also shown in Fig. 7(b). Here, as the width of the DMS is increased, i.e. capacitance is decreased, the pole frequency is shifted towards higher value as shown in the graph also. Fig. 7 (a) Variation of frequency with length of the DMS; (b) Variation of frequency with slot width (w) IV. UWB BANDPASS FILTER USING DMS WITH WLAN BAND NOTCH The bandstop filter is combined with the proposed miniaturized L-shaped stub with the highpass filter to obtain the desired bandpass response. Also a WLAN rejection has been introduced by the DMS unit. The ultra wideband of 3.1GHz to 10.6GHz is interfered with the WLAN rejection at 5.8GHz is introduced here as shown in Fig. 8. The behaviors of the DMSs are integrated with highpass response to get the proposed BPF. The Simulated S-parameters shows a 3dB passbond of 3.4 GHz to 10.7 GHz with the WLAN rejection at 5.46 GHz is achieved by the desired filter. The centre frequency of the passband is 7.05 GHz with insertion loss (IL) of 0.91 db with a FBW of a 103.54% is obtained. The Simulated S-parameters are shown in Fig. 10 which is simulated by Method of Moment (MOM) based IE3D software. The pole frequency of notch band is 5.46 GHz at - 22.1 db and 3dB BW ranging from 5.1 GHz to 6 GHz has been obtained having a FBW of 16.48%. Also a prototype of the proposed filter is fabricated on FR4 substrate as shown in Fig. 9. 53
Fig. 8 Schematic Diagram of compact UWB Bandpass filter with WLAN notch.(a=50mm,b=4mm,c=5mm) 0 Fig. 9 Photographic view of the fabricated structure Magnitude(dB) -10-20 -30-40 -50 Simlated Result(S21) Measured Result(S21) 0 5 10 Frequency(GHz) Fig. 10 Comparison between the simulated and measured S21-Parameter The fabricated unit has been measured using Agilent make vector network analyzer (model N5230A). The measured results in Fig. 10 shows center frequency at 6.934 GHz, 3dB bandwidth of 6.268 GHz and fractional bandwidth of 90.39% with respect to the center frequency at 7.05 GHz, 3dB bandwidth of 7.3 GHz and fractional bandwidth of 103.54 % of the EMsimulated result. The band notch performance of the measured value is 5.28 GHz at 38.31 db at pole frequency and 3dB BW ranging from 4.8 to 6.3 GHz has been obtained with a FBW of 28.40% with respect to the pole frequency of 5.46 GHz at -22.1 db and 3dB BW from 5.1 GHz to 6 GHz with a FBW of 16.48% for the simulated one. V. CONCLUSION UWB bandpass filter with notched band is proposed and fabricated. The proposed filter has been made with the combination of Highpass and Bandstop filter. The highpass filter here is made up with optimum distributed loaded stub and the stopband is achieved by DMS structure. The measured result shows a very good agreement with the simulated ones. The notched band is also introduced by embedding DMS on the signal plane. For the compactness of the filter, folded stub or L- shaped stub is used. This type of BPF is very easy to integrate in application of Ultra Wide-band wireless communication system. REFERENCES [1] D. M. Pozar, Microwave Engineering, 2nd ed., John Wiley, 2000. [2] G. L. Mathaei, L. Young and E. M. T. Jones, Microwave Filter, impedance matching networks and coupling structures, Artech House, Dedham, Mass, 1980. [3] Jia-Shen G. Hong and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, 1st ed., John Wiley & Sons Inc., 2001. [4] S. Roy Choudhury, S. K. Parui and S. Das, Improvement of stopband using a spiral defected microstrip and defected ground structures Applied Electromagnetics Conference (AEMC), pp. 1-3, 2011. [5] Revision of Part 15 of the Commission s Rules Regarding Ultra-Wide band Transmission Systems, Federal Communications Commission, ET-Docket 98-153, 2002. [6] Guo-Chun Wu, Jie Peng, Bian Wu and Chang-Hong Liang, Compact UWB filter with notched band and improved out-of-band performance, Proceedings of International Symposium on Signals, Systems and Electronics, 2010. [7] Subodh Singhal, Deepak Sharma and Ram Mehar Singh Dhariwal, Design of 1.3 GHz Microstrip Highpass Filter using Optimum Distributed Short Circuited Stubs, International Conference on Computational Intelligence, Communication Systems, Networks, 2009. 54