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Short Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011

Compact Planar Filter for Ultra Wide Band Applications Bindu C J1, S Mridula1 and P.Mohanan2 1

Division of Electronics Engineering, School of Engineering, Cochin University of Science and Technology, Kochi -22 email:bindu@ipath.net.in 2 Department of Electronics, Cochin University of Science and Technology, Kochi – 22 Abstract—The paper presents a maximally flat compact planar filter for ultra wide band applications. In order to get strong coupling between the input and output line an interdigitated structure is employed. On the other hand, transmission zeros are produced by half wavelength open stubs. As a consequence, the filter has low insertion loss in its pass band and sharp attenuation in its lower and upper stop bands. The measured frequency property of the fabricated filter shows good agreement with the simulated response.

III. F ILTER

The proposed filter uses conventional coupled lines of quarter wavelength, but in the inter-digital structure to achieve maximum coupling. Half wavelength open stubs which act as LC resonant circuits are used to introduce transmission zeros [2]. Instead of using an open stub of uniform thickness, if stepped impedance stubs are used, the position of the transmission zeros can be adjusted by adjusting the width ratio W1/W2, according to the requirements. A bottom up design strategy is used whereby the design starts with a simple coupled line of quarter wavelength long. The structures are simulated using FEM based simulator HFSS from Ansoft Corporation. The filter structure is fabricated on double sided FR4 substrate with relative permittivity of 4.4, dielectric loss tangent of 0.02 and a thickness of 1.6mm.The response of a simple parallel coupled structure in Fig 1(a) is shown in Fig.1.(b). It can be seen that the roll off is not sharp. Also the fractional bandwidth seems to be small (around 70%). The response can be improved by adding one more parallel coupled line. For improved coupling, inter-digital structure is used. Fig.2(a) shows the structure modified using three coupled lines. There are slight changes in the dimensions of the structure due to the change in effective dielectric constant.The S21 plot is shown in Fig.2(b).

I. INTRODUCTION The relevance of UWB is unquestionable in this era of information, on account of its high data rate and low power. As FCC has deregulated the use of the band of 7.5GHz, the UWB communication has gained great attention among industry and academia [1]. Several works have been reported to meet the design challenges of UWB filters. While using the conventional method of coupled lines for bandpass filters, most of the designs pose fabrication problems, as they reduce the coupling gap to the order of 0.1mm in order to achieve this wide bandwidth. In the recent past different planar filter configurations have been reported for ultra-wideband applications. But the issues such as reducing size and simultaneously enhancing the performance have not necessarily been prioritized in ultrawideband filter designs. This work concentrates on simple coupled line structure with adjustable transmission zeros to realize the desired bandwidth. The structure exhibits sharp roll off at both lower and upper cut off frequencies and low insertion loss in the pass band.

II. LITERATURE

STRUCTURE

REVIEW

A detailed review of various filter design methods has been conducted.[2][3][4][8]. Conventional configurations of the stepped-impedance resonator (SIR) and traditional microstrip filter designs require very small gaps between the coupled segments. This results in high manufacturing accuracy, and thus, higher expenses Works using short circuited stubs were reported, [4] but necessitates via holes to be fabricated. Also the structure is not compact. Aperture backed techniques were seen developed to compensate for the reduced coupling of parallel coupled lines with larger gaps. But these offer only around 70 % fractional band widths.

Figure 1a. Simple coupled line Structure

Figure 1b. S21 plot for the simple coupled line Structure

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Short Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011

Figure 2a. Structure using three Interdigital Coupled Lines

Figure. 3b S21 Plot of Single Open Stub

The effect of addition of a single open stub on the interdigital coupled line structure is shown in Fig 4(a) with its S21 plot in Fig 4(b). The effective permittivity of the whole structure will be altered by the introduction of the open stubs. Hence the dimensions of the sections have to be changed for proper impedance matching. But the attenuation characteristics at the upper stop band are not so promising.

Figure.2b S21 Plot of Three Coupled Lines

It can be seen from the plot that the fractional bandwidth is improved to around 150%. Still this structure does not give any control over the lower and upper cut off frequencies. Also it exhibits the periodicity in its response which is inherent of the quarter wavelength lines. The lower and upper cut off frequencies are controlled by introducing open stubs of varying impedance. Each of the impedance sections have been made one quarter wavelength. The impedance ratio is adjusted by varying the widths of the low and high impedance sections. Fig.3(a) shows the layout and Fig 3(b) shows the S parameter plot for a stepped impedance open stub that is used as an LC resonator. Each section makes an effective half wavelength resonator.[6]

Figure 4a. Layout with Single OpenStub

Figure 3a. Layout of Stepped Impedance Open Stub

It is seen introducing transmission zeros at 2.2GHz and 9.73GHz. The frequencies at which the transmission zeros are introduced can be varied by varying the impedance ratio of the stub, that is, by varying the ratio W1/W2 The number of filter sections is determined by the requirement of the attenuation level at the stop bands. Pramod K. Singh, et al.[5] proposed similar structure using RO4003C substrate with thickness 0.81 mm, dielectric constant 3.38, and loss tangent 0.004

ÂŠ 2011 ACEEE DOI: 02.CEMC.2011.01. 524

Figure. 4b S21 Plot for Strcture with Single Open Stub

To improve the performance, one more open stub can be included. In this paper, one more stub is shown introduced to get better performance in the upper stop band. However addition of further units in this will make the structure complicated and will cause more floor area. 115

Short Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011 The final structure coupled to a 50 &! transmission line section shown in Fig.5(a) is very simple to fabricate and has an area of 17.52mm x 21mm2.The simulated response for the structure is shown in Fig.5(b). All the dimensions are shown in Table 1.It can be seen that the minimum dsigned dimension is 0.3mm, which can easily fabricated. It seen that the stop band atenuation is improved by the introduction of one more stub.

IV. FABRICATION AND RESULTS Figure 6a shows the photograph of the fabricated filter structure along with the SMA connectors. The structure is compact as can be seen in figure.

Figure 6a. Photo of the Fabricated Prototype Filter Structure

Figure.5a

Standard photolithography is used for the fabrication process and the S parameter measurements were taken using HP8510C Vector Network Analyzer. The experimental results [Fig.6(b)] agree well with the simulated results [Fig 5b]. The small discrepancies in the result can be attributed to the inaccuracies in the fabrication process and/or numerical errors in simulation.

Dimensions and Layout of Final Structure with two Open Stubs TABLE I. DIMENSIONAL DETAILS

Figure 6b. Measured S 21 Plot of the fabricated Prototype Filter

The insertion loss is less than -2dB including those introduced by the SMA connectors which may contribute around 0.3 dB. The attenuation in the upper stop band is less than -12dB, except for the spurious 2 GHz frequency in the lower stop band. This may be attributed to the errors in fabrication process. The roll-off rate is sharp at both band edges and the fractional bandwidth attained is approximately 109%. The measured reflection characteristics of the filter are shown in Fig.7(a)

Figure.5b Simulated S21 Plot for the Final Structure

The insertion loss for the filter is well within 0.5dB. Also there is sharp rejection at lower and upper cut off frequencies. The stop band attenuation is around 12 -15dB, which can be further reduced by cascading another section, if required at the cost of still higher insertion loss. The insertion loss can be reduced further by reducing the gap between the coupled lines and by using a less lossy dielectric.

ÂŠ 2011 ACEEE DOI: 02.CEMC.2011.01. 524

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Short Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011 The roll off achieved is steep and the attenuation in the stop bands is reasonable. It can be further brought down by cascading more sections as desired. However, there is a limitation that the structure may become complicated as more sections are to be incorporated. ACKNOWLEDGEMENTS The authors owe a great deal to the friendly help of all research scholars at CREMA, Department of Electronics, CUSAT REFERENCES [1] Figure 7a Measured reflection characteristics of the fabricated Prototype filter

[2]

The group delay plot reveals that the filter designed has good linear phase characteristics making it suitable for all sorts of communication applications. The group delay is almost flat in the pass band and has a value less than 5ns.

[3]

[4]

[5]

[6]

[7]

[8]

Figure. 7b Experimental Group Delay Plot

V. CONCLUSION The filter designed is a stepping stone for future implementations. The design is simple and dimensions are easy to fabricate. Further, it can be tuned for any bandwidth of interest, by varying the lengths of open stubs and coupled lines. The fundamental restriction, namely, the gap between the coupled lines is made practically feasible by allowing a minimum of 0.3mm gap.

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FCC, “Federal Communications Commission Revision of Part 15 of the Commission’s Rules Regarding UltraWideband Transmission Systems,” First Report and Order FCC, 02.V48, Feb. 2002. David M. Pozar, Microwave Engineering , Third Edition, John Wiley & Sons Inc. ISBN 978-81-265-1049-8., 1989. Jia-Sheng Hong, M. J. Lancaster, Microstrip Filters for RF/ Microwave Applications, John Wiley and Sons Inc, ISBN 0-471-22161-9, 2010 Jia-Sheng Hong and Hussein Shaman, “ an optimum ultrawideband microstrip filter”, microwave and optical technology letters / vol. 47, no. 3, november 5 2005 Pramod K. Singh, Sarbani Basu, and Yeong-Her Wang, Member, IEEE, “Planar Ultra-Wideband Bandpass Filter Using Edge Coupled Microstrip Lines and Stepped Impedance Open Stub “ IEEE Microwave and Wireless Components Letters, Vol. 17, No. 9, September 2007 Lei Zhu, Senior Member, IEEE, and Wolfgang Menzel, Fellow, IEEE, “Compact Microstrip Bandpass Filter with Two Transmission Zeros Using a Stub-Tapped Half Wavelength Line Resonator”, IEEE Microwave and Wireless Components Letters, Vol. 13, No. 1, January 2003 Y.-C Chiou and P S Yang, J, -T Kuo” Transmission zero design graph for Dual mode Dual Band filter with periodic Stepped Impedance ring Resonator”.Progress In Electromagnetics Research, Vol. 108, 23{36, 2010 Marjan Mokhtaari, Jens Bornemann, Smain Amari,” A Modified Design Approach for Compact Ultra-Wideband Microstrip Filters”, International Journal of RF and Microwave Computer-Aided Engineering/Vol. 20, No. 1, January 2010