A Compact UWB filter for Wireless communication

Transcription

1 A Compact UWB filter for Wireless communication Manidipa Nath AICTR, Department of ECE, New Delhi ABSTRACT Ultra-Wide Band (UWB) is a promising technology for many wireless applications due to its large bandwidth, good ratio of transmission data and low power cost. the main goal of this work is to design an UWB filter suitable for that purpose. In order to achieve that goal, one UWB filter configuration have been investigated, designed and characterized after analyzing the typical filter parameters, such as the return loss, insertion loss and attenuation characteristics over the full frequency band. Setting the dimensions of the proposed filter in a miniaturized size is a requirement as per user specification. The size of this filter has also been studied because of its important aspect on the frequency behavior. Keywords- UWB filter, resonator, miniaturization, matching, stub, shortin I. INTRODUCTION As defined by the Federal Communications Commission (FCC), UWB technology[1] is to transmit and receive information over a large bandwidth These are the two conditions of UWB technology: BW > 500MHz or BW /fc >0:2 where fc is the central frequency and BW is the bandwidth. UWB can operate between 3.1 and 10.6 GHz at limited transmission powers for indoor communications, as defined in FCC. In 2002, the Federal Communications Commission (FCC) of the United States released the frequency band GHz for ultra-wideband (UWB) commercial communications. So recently, more attention has been paid to applications of ultrawideband (UWB) technology on wireless communication system. UWB technology is promising and attractive for local area networks, position location and tracking, and radar systems, because UWB has thecharacteristics of low cost, high data transmission rate and very low power consumption. Many UWB devices and circuits have been proposed and investigated widely. It is important to reducetheir size and weight in order to integrate them with other components as a compact system. Compact and broadband bandpass filter (BPF) is a key passive component and highly demanded in a UWB system. A planar BPF, based on a microstrip structure, can provide the advantages of easy design, low cost, compact size, and is widely used in a variety of RF/microwave and millimeterwave systems to transmit energy in passband and to attenuate energy in one or morestopbands. So, compact UWB microstrip BPF can be used in a UWB communication system. UWB filters must have a fractional bandwidth of more than 70%, and it is very difficult to achieve such a wide passband with a traditional parallel-coupled transmission line structure. Therefore, there is a requirement for UWB BPF with a strong coupling structure that can be easily fabricated. In this design, a dual-line coupling

2 structure has been used to implement a strong coupling between the input/output port and the resonator, which is more compact than the interdigital coupling structure. A compact UWB microstrip BPF and Notched BPF with low insertion loss have been presented and analyzed. In addition, the UWB filters have extremely compact size of 25 mm X 10 mm for BPF and 25 mm X 28 mm for Notched BPF when the length of the feed lines is ignored. After the release of UWB, bandpass filters with a passband of the same frequency range (3.1 GHz GHz, a fractional bandwidth of 110%) were challenges for conventional filter designs. Before mid 2003, the bandwidth of the passband for a bandpass filters was extended from 40% to 70% [2]. These filters are named broad bandpass filters. They were not covering the whole UWB frequency range yet. In [3], a bandpass filter covering the whole UWB frequency range with a fractional bandwidth of 110% was realized by fabrication signal lines on a lossy composite substrate. A successful transmission of the UWB pulse signal was demonstrated using the proposed bandpass filter. This is one of the early reported filters that possess an ultra-wide passband. However, it has a high insertion loss in the passband due to the lossy substrate. Not much research work was reported in 2003 and In 2004, a ring resonator with a stub was proposed which shows a bandwidth of 86.6% [4]. A bandpass filter covering the whole UWB frequency band was a challenge for microwave filter designers and researchers in that period of time. In 2005, there are 11 conference papers in total published in International Microwave Symposium, International Conference on Ultra-Wideband, Asia- Pacific Microwave Conference, or European Microwave Conference. In the same year, there are four journal publications. There are mainly four types of structures that are able to realize an ultra-wide passband. One is a microstrip structure shown in Fig. 1 [5]. It consists of a microstrip multi-mode resonator (MMR) and a parallel-coupled line at each end of the network. The MMR has two identical highimpedance sections with a length of quarter guided wavelength at two sides and a low-impedance section with a length of half guided wavelength in the middle. The MMR in the filter generates first and third resonant mode at the edges of the UWB passband. The parallel-coupled lines are modified to obtain the ultra-wide passband. This could be done by adjusting the coupling length, Lc [5], for example. The second type is a hybrid coplanar waveguide (CPW) and microstrip structure. This type of structure consists of a CPW MMR on one side and a microstrip input and output on the other side [6]. The CPW MMR is responsible for generating the first and third resonant mode for the UWB passband, which is similar to a microstrip MMR in [5]. Its geometry can be varied. Fig. 2 shows the CPW MMR in [6]. The third type of filter which is also able to have a fractional bandwidth of 110% is the broadsidecoupled microstrip-cpw structure [7] shown in Fig. 3. There is a broadside-coupled microstrip line on one side of the substrate [see Fig. 3 (a)] and an open-end CPW on the other side of the substrate [see Fig. 3 (b)]. The length of the coupled line equals to λg/2 in order to obtain a 110% bandwidth. The last type of filters that has a bandwidth as high as around 100% is the combination of a highpass filter and a lowpass filter [8]. In [8], a stepped-impedance lowpass filter is embedded into a highpass filter with quarter-wavelength short-circuited stubs, achieving a passband from 3 GHz to 10 GHz. New fabrication technique, such as Low Temperature Co-fire Ceramic (LTCC), is applied in UWB bandpass filter designs [9]. In [9], a LTCC bandpass filter shows a bandwidth of 48.75%. This filter has a small physical size due to the multi-layer configuration. However, the bandwidth of the passband is relatively small compared to other UWB bandpass filters and the insertion loss is as high as around 2.2 db. In 2006, microstrip MMR based UWB bandpass filters are further optimized with improvement in the rejection of the upper stopband. It can be done by introducing interdigital microstrip coupled lines at the two sides of the MMR in [10]. A highpass filter consisting of a transmission line with two embedded U-shaped slots is cascaded with a lowpass filter which is a dumbbell-shaped defected ground structure array in the ground plane, to obtain a passband from 3 GHz to 10.9 GHz [11]. With novel highpass and lowpass structures, the bandpass filter obtains a wider bandwidth than the filter taking a similar approach in 2005 [8]. With regards to the UWB bandpass filter designs by cascading a highpass

3 and a lowpass filter, a systematic consistent and analytical method is proposed [12]. There are a good number of new structures proposed that exhibit an ultra-wide passband [13] [16]. In [13], 3λg/4 parallel-coupled line resonators shown in Fig. 4 are used to realize a passband from 3 GHz to 10 GHz. With the introduction of lumped components to a microstrip line, a miniaturized UWB BPF with a length of 0.18λg is realized at a fractional bandwidth of 127% at a center frequency of 6.5 GHz [14]. The small physical size is attributed to the lumped components used. A broadside coupled line in suspended substrate stripline [15] can also be used to realize an UWB bandpass filter. A filter with shortcircuited stubs could giverise to a UWB bandpass filters with a bandwidth of 110% [16]. In 2007, there are 26 papers reporting new UWB bandpass filters which is much more than the previous two years (15 papers in 2005, 18 papers in 2006). UWB bandpass filters with a notch stopband from 5 GHz to 6 GHz for filtering the wireless localarea network (WLAN) is a new topic branched out in this area [17] - [19]. Additional components are introduced providing the notch stopband at the desired frequency. In [17], an embedded open-circuit stub is proposed providing a sharp notch stopband. It is integrated into a UWB bandpass filter providing the stopband from 5 GHz to 6 GHz. A stub is introduced in the broadside-coupled microstrip-cpw structure [18] to generate a notch stopband at WLAN frequency range. Other than adding stubs to the structure, in [19], a notch stopband is generated in the UWB passband by an asymmetric parallel-coupled line at two sides of a microstrip MMR. There are three main existing approaches to realize a UWB bandpass filter. One is a microstrip or CPW MMR with the assistance of coupling mechanisms, such as microstrip coupled line or coupling at the transit between a microstrip line and a CPW. Broadside-coupled microstrip line with a CPW at the back is another important configuration. The third one is a direct or indirect combination of a lowpass and a highpass filter. In terms of miniaturization, the employment of LTCC or lump components is an effective means to significantly reduce the size of the structure. For future development and research in this area, miniaturization of UWB filters is important for the application in hand-held devices. A system integrating both filters and antennas in UWB frequency range is very attractive to wireless communications using signals in this frequency band. UWB was originally developed for military communications and radar. In the field of UWB technology different methods and structures [20-24]has pushed development of new UWB filters. Lumped-element filter design is generally unpopular due to the difficulty of its use at microwave frequencies along with the limitations of lumpedelement values [25] II. THEORY The size is another important factor in this work, because the final application requires a small filter with a diameter around 3.1 cm. This restriction is the hardest specification due to the relationship between the size and the frequency.for lower frequencies as required in the specifications, the size should be bigger.thus, the design of a small filter becomes a challenging issue. UWB was originally developed for military communications and radar. In the field of UWB technology different methods and structures [2-6]has pushed development of new UWB filters. Lumped-element filter design is generally unpopular due to the difficulty of its use at microwave frequencies along with the limitations of lumpedelement values [25]. Hence, conventional microstrip filters are often used. The new proposed filter design is based on quarter wavelength short-circuited stub. In order to reduce filter size, bending connecting line and let five short-circuited stubs via the same hole is designed.consequently,half hexagon UWB filter was simulated and optimized for its best achievable performance. The paper focuses the on systematic design and realization of a ultra wideband filter in printed circuit configuration.the UWB filter has as one input and one output port.the filter is of hexagonal shaped microstrip configuration.the length and width of fingers of the connecting lines are taken as design parameter for optimization. The MOM simulation tools used to investigate the performance of the this filter and the combined responses for ultrawideband.it is designed as per FCC recommended band from GHz. It was

4 observed that the design dimensions are critical in deciding the filter responses. The line dimension and coupling gaps are optimized to meet the specification and final pcb design is generated. The UWB filter is designed to provide an Insertion Loss 1 db and average roll off of 30 db/decade. Simulated results predicts performance of the filter as per FCC Standard. The filter hardware based on the optimized design has been fabricated and tested.the measurement results are quite encouraging. The structure under consideration consist of a TEM mode or quasi-tem mode transmission line resonator elements arranged in a half hexagonal pattern. Each resonator element has an electrical length of 90 degree at the midband frequency and is short circuited at one end. The resulting filter is compact and the tolerance required in their manufacture are relatively relaxed. The second pass band of this filter is centered at about 3 times the midband frequency of the desired first pass band while there is no spurious response in between. Another advantage is that the filter can be fabricated in structural forms which are self supporting so that dielectric material is not required to be used. stubs and the characteristic impedances of the connecting lines are choosen at 3.1 GHz. III. DESIGN To meet the design criterion particularly the bandwidth and size a half hexagonal microstrip structure with shorting pin configuration has been choosen. The filter design has been implemented on a high frequency circuit board.here the substrate used is 25 mil(dielectric constant 10, loss tangent 0.009).A detail analysis has been done to find the response of the structure under consideration(fig 2) using moment method and FDTD solver. It has been seen that the bandwidth criterion ( GHz.) is fulfilled with this structure and the size is perfect to put in a predefined package for testing and measurement.the shorting pin is used to suppress the unwanted mode that may lead to additional loss for this filter configuration.final design has been optimized several times for getting best filter response over the frequency band of operation. An exact analysis of the structure is very tedious. Hence a synthesis procedure is followed which involves a number of simplifying approximations that permit straightforward, easy to-use design calculations. However these approximate design equations are found to be sufficiently accurate for most practical applications. The new proposed filter design is based on quarter wavelength short-circuited stubs. Here five shortcircuited stubs was designed for an optimum distributed band pass filter performance whose connecting lines are non-redundant [26]. Thus, the filter can exhibit a frequency selectivity equivalent to that of a conventional 9-pole Chebyshev filter. But an optimum distributed high-pass filter requires a greater area. In order to reduce the filter size, the connecting line had been reduced.thus, let 9-pole decrease to 5-pole, even though frequency selectivity of 5-pole is not better than 9-pole. However, circuit dimension can be reduced very much. The characteristic impedances of these short-circuited Figure 1.Simulation result of UWB filter(before optimization)

5 Figure 4. UWB filter configuration with package. Fig2. Simulated S-Parameters of UWB Filter Figure 3.Simulation Result of UWB Filter(after optimization) V. MEASUREMENT The fabricated filter was measured for transmission and reflection performance with the help of Network Analyzer (E8363B).The measured attenuation and VSWR plot of the filter is shown in figure(3-4).the fractional bandwidth is 0.76 instead of the design value & simulation value 0.78, which indicates a shrinkage of bandwidth of 12.6 % as a result of various approximation involved in the design equations and due to fabrication inaccuracies.the insertion loss is found to be 3.5dB (average) over the band. The fabrication process is required to be better to improve this loss figure. The other performance is seen to be satisfactory. IV. CHARACTERIZATION The filter layout has been fabricated using CER-10 with best precession available.the Final circuit after integration and packaging undergone for testing.inital measurement results shows good filter characteristic over the whole UWB band.the measured return loss over the band is 1.0 db(average).this loss can be further reduced using low loss substrate and SMA connectors. Figure 5 Attenuation Measurement of UWB Filter

6 passband are equal to 3.9 GHz and 11.2 GHz against their counterpart frequencies of GHz and GHz in the simulation. Furthermore, the UWB filter has achieved the return loss of less than -10 db from 4 to 10.5 GHz. gure 6. VSWR Plot of UWB Filter Fi VII. CONCLUSION Compact UWB microstrip BPF have been designed using the structure having half hexagon resonator with the five stubs. The compact UWB filters have beenfabricated, and tested. The fabricated UWB BPF has a 7.3 GHz passband, -10 db return lossbandwidth of 6.5 GHz. These results have indicated a very goodagreement between simulation and measurements. VIII. REFERCES Figure 7&8. S21 & S11 Plot-Comparison of Simulated and Measured results VI. OBSERVATION From the simulation and measurement results it is observed that the filter passband bandwidth is 7.3 GHz as compared to GHz in simulation. The lower and higher cutoff frequencies of the UWB 1. Federal Communications Commission, "Revision of Part 15 of the Commission's Rules Regarding Ultra-wideband Transmission Systems", Tech. Rep.,ET-Docket , FCC02-48, April J.-T. Kuo and E. Shih, "Wideband bandpass filter design with three-line microstrip structures", IEE Proc.-Microw. Antennas Propag., Vol. 149, No. 5/6, October/December 2002, pp A. Saito, H. Harada, and A. Nishikata, Development of Band Pass Filter for Ultra Wideband (UWB) Communication Systems, International Microwave Symposium, Philadelphia, Pennsylvania, USA, June H. Ishida and K. Araki, A Design of tunable UWB Filters, International Microwave Symposium, Fort Worth, Texas, USA, June L. Zhu, S. Sun, and W. Menzel "Ultra-wideband (UWB) Bandpass Filters Using Multiple-Mode Resonator", IEEE Microwave and Wireless Components Letters, Vol. 15, No.11, November 2005, pp H. Wang and L. Zhu, "Ultra-wideband Bandpass Filter Using Back-to-back Microstrip-to-CPW Transition Structure", Electronic Letters, Vol. 41, No.24, November K. Li, D. Kurita, and T. Matsui, An Ultra- Wideband Bandpass Filter Using Broadside-Coupled Microstrip-Coplanar Waveguide Structure, International Microwave Symposium, Long Beach, CA, USA, June 2005.

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