198 PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011

Microstrip Ultra-Wide-Band Filter

Abdel-Fattah Sheta and Ibrahim ElshafieyElectrical Engineering Department, College of Engineering

King Saud University, P. O. Box 800, Riyadh 11421, Saudi Arabia

Abstract— A new UWB filter is proposed. The filter structure is based on identical linesand short circuited stubs. The merit of the proposed structure lies in the ability to design asimple structure with arbitrary bandwidth. The proposed filter can be designed for a fractionalbandwidth from 40% to more than 100%. The concept is validated through the design of abandpass filter to cover the entire band of the UWB standard and another filter is designed tocover few groups of the UWB standard (from 6 to 10GHz). Simulations have been performedusing IE3D software package.

1. INTRODUCTION

Since the Federal Communications Committee (FCC) authorized the unlicensed use of the ultra-wideband (UWB) frequency spectrum for short-range and high-speed wireless communication in2002 [1], remarkable research interests in the analysis and design of various microwave deviceshave been investigated [2]. UWB bandpass filters (BPFs) with low insertion loss, high out-of-bandrejection, and a flat group delay performance within that band, are highly needed to meet therequired UWB frequency mask (3.1 to 10.6GHz). Several prototype UWB filters based on differenttechniques have been studied. Coupled microstrip resonators are proposed for 128% fractionalbandwidth by cascading two stages fork resonator [3]. Another microstrip coupled resonators isdesigned by cascading two interdigital hairpin resonators [4]. The filter in this case designed tosuppress wireless local area network in the band 5.7 GHz to 5.8 GHz. Another method based oncircuit model for an optimum distributed high pass filter (HPF) [5] has been implemented in varioustechniques [6, 7]. The filter structure in its simplest form consists of cascaded short circuit stubsseparated by identical connecting lines. The electrical length of the connecting lines is twice that ofthe length of the short circuited stubs at the lower cutoff frequency. The characteristic impedancesof the short-circuited stubs and the connecting lines are calculated based on the HPF model [5].

In this paper, a filter consists of short circuited stubs separated by identical connecting lines isinvestigated. Based on this arrangement, UWB filter with fractional bandwidth from about 40%to 115% can be designed with one or more short circuited stubs. The stubs electrical length is halfof the connecting lines electrical length. The structure is sensitive to the relative electrical lengths.

2. FILTER STRUCTURE

As shown in Fig. 1, the filter structure under considerations consists of four stubs of electricallength θs and characteristic impedances Zo1, Zo2, Zo3, and Zo4 separated by connecting lines ofelectrical length θc and characteristic impedance Zoc.

θc θc θc

θs θs θsθs

Zo1 Zo2 Z4Zo3

ZocZoc Zoc

Z Zo o

Figure 1: Circuit diagram of the proposed UWB filter. Zo1 = Zo4 and Zo2 = Zo3.

Progress In Electromagnetics Research Symposium Proceedings, Marrakesh,Morocco, Mar. 20–23, 2011 199

The stubs electrical lengths are half of the connecting lines electrical length, such that θc = 2θs.The stubs length is one quarter wavelength at the filter center frequency. In this design the middlestubs characteristic impedances are equal. The end stubs characteristic impedances are also equal,such that Zo1 = Zo4 = Z1 and Zo2 = Zo3 = Z2. It is observed also that the characteristic impedanceof the middle lines are half of the end lines, i.e., Z1 = 2Z2. The filter bandwidth depends on thelines impedances Zoc and Z1. Design curves that relate the filter fractional bandwidth to linescharacteristic impedances are shown in Fig. 2. These parameters are obtained based on return lossbetter than 16 dB.

It is thus clear that, the filter bandwidth can be controlled by the selection of the characteristicsimpedances of the stubs as well as the connecting lines. The filter in this case can be designed tocover either the entire ultra-wide band (3.1 to 10.3 GHz) or some groups of the sub-bands of theentire this band. UWB-filter with fractional bandwidth from 40% to 120% can be achieved.

3. DEIGN CASES

The design concept has been verified through the design of two UWB filters. The first is designedto cover the entire band from 3.1 to 10.3 GHz. The characteristic impedance of the connectinglines is 60 Ω. The characteristic impedance of the two stubs located at the edge is 80 Ω, while thecharacteristic impedance of stubs in the middle is 40 Ω. The filter is designed at 6.85 GHz filtercenter frequency with approximately 110% fractional bandwidth. From Fig. 1, the connecting linesimpedance is Zc = 58 Ω and the stubs impedances are Z1 = 94 Ω, and thus Z2 = 47 Ω. The filter hasbeen simulated using the IE3D simulator and the simulation results are shown in Fig. 3. Dielectricmaterial with 4.5 dielectric constant and 0.78mm thickness is used in this analysis. The filter layoutis shown in Fig. 4. Very good performance is observed with return loss better than 12 dB in theentire frequency band. The mid band insertion loss is less than 0.5 dB. Another narrow band filterhas been designed with the aid of the design curves presented in Fig. 2. The filter is designed for afractional bandwidth of 50% centered at 8 GHz. The connecting line characteristics impedance is102Ω, and the stubs impedance is Z1 = 20Ω and so, Z2 = 10 Ω. Since the middle lone stubs havelow impedances, two parallel lines with 20 Ω characteristic impedance are used instead. The filteris analyzed using a dielectric substrate with 6.15 dielectric constant and 0.635 mm thickness. Thesimulation results are shown in Fig. 5, and the layout is shown in Fig. 6. The midband insertionloss is about 1 dB, and the return loss is better than 10 dB in the entire band.

Zc

Z

Fractional Bandwidth

1 = Zo1=Zo4

Z1

Z2 = Zo2 = Zo3

Z1 = 2Z2

Figure 2: Stubs and connecting lines characteristicimpedances against filter fractional bandwidth.

S21

dB

S11

Figure 3: Simulation results of the proposed UWBfilter.

13.7 mm 14.26 mm14.26 mm

1.9 mm

8.4 mm

8.57 mm

Figure 4: Layout of the UWB filter designed on FR4 material of 4.5 dielectric constant and 0.78 mm thickness.

200 PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011

dB

S21 S11

Figure 5: Simulation results of the proposed UWB filter designed to cover the band from 6 GHz to 10 GHz.

9 mm 9 mm 9 mm

3.76 mm 0.3 mm

3.16 mm

Figure 6: Layout of the UWB filter designed on Duroid material of 6.15 dielectric constant and 0.635 mmthickness.

4. CONCLUSIONS

A new UWB filter structure is introduced. The bandwidth can be controlled using only two linesimpedances. Fractional bandwidth from 40% to 120% can be achieved using the proposed structure.Two filters have been designed to verify the design concept. Design curves prove to be helpful forarbitrary fractional bandwidth requirements.

ACKNOWLEDGMENT

This research is funded by The National Plan for Science & Technology, Kingdom of Saudi Arabia,under project No. 08-ELE262-2.

REFERENCES

1. “Revision of Part 15 of the Commission’s Rules Regarding Ultra-Wideband TransmissionSystems,” ET-Docket 98-153, First note and Order, Federal Communications Commission,Feb. 14, 2002.

2. Aiello, G. R. and G. D. Rogerson, “Ultra-wideband wireless systems,” IEEE Microw. Mag.,Vol. 4, No. 2, 36–47, Jun. 2003.

3. Chen, H. and Y. Zhang, “A novel and compact UWB bandpass filter using microstrip fork-formresonators,” Progress In Electromagnetics Research, Vol. 77, 273–280, 2007.

4. Wei, F., L. Chen, X.-W. Shi, X. H. Wang, and Q. Huang, “Compact UWB bandpass filterwith notched band,” Progress In Electromagnetics Research C, Vol. 4, 121–128, 2008.

5. Hong, J. S. and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, Wiley,New York, 2001.

6. Razalli, M. S., A. Ismail, M. A. Mahdi, and M. N. Hamidon, “Ultra-wide band microwavefilter utilizing quarter-wavelength short-circuited stubs,” Microwave and Optical TechnologyLetters, Vol. 50, No. 11, 2981–2983, Nov. 2008.

7. Shaman, H. and J. S. Hong, “A novel Ultra-Wideband (UWB) Bandpass Filter (BPF) withpairs of transmission zeroes,” IEEE Microw. Wireless Compon. Lett., Vol. 17, No. 2, 121–123,Feb. 2007