[ieee 2012 ieee asia-pacific conference on applied electromagnetics (apace) - melaka, malaysia...

An Aperture-Backed Sextuple-Mode Ultra-Wideband Bandpass Filter

Albin Sui Hian Kuek, Hieng Tiong Su, and Manas Kumar Haldar School of Engineering, Computing and Science

Swinburne University of Technology (Sarawak Campus) Kuching, Sarawak, Malaysia [email protected]

Abstract—In this paper, a sextuple-mode ring resonator ultra-wideband bandpass filter is presented. The objective is to enhance the selectivity of a quintuple-mode filter by introducing an additional transmission pole within the passband. The extra pole comes from two open stubs inserted symmetrically inside the ring resonator. Analysis of the filter is done to show the relationship between the six resonant frequencies, and characteristic impedance and electrical length of the stubs. The filter is designed, fabricated and measured. The measured responses are in good agreement with simulations. The filter exhibits 108.02% 3dB fractional bandwidth at 6.85GHz centre frequency.

Keywords - Ultra-wideband; bandpass filter; aperture-backed; sextuple mode; dual symmetry.

I. INTRODUCTION Ultra-wideband (UWB) radio frequency (RF) technology,

which operates on low energy and high bandwidth, has gained tremendous research interest in recent decade. Applications of UWB bandpass filter (BPF) is needed to tap the unlicensed use of the frequency range covering from 3.1GHz to 10.6GHz, which is the UWB spectrum released by US Federal Communications Commission (FCC) [1]. Existing UWB BPF can be broadly categorized into two main categories based on the resonators used, namely Stepped Impedance Resonator (SIR) and Ring Resonator.

Ring resonator, which can be circular or square, is essentially a one wavelength loop resonating at the frequency corresponding to the wavelength. One of the benefits of ring resonator is its compact size. Ring is naturally a single-mode resonator of which is undesired as more modes translate to higher selectivity. To make a ring resonator dual mode, one of the methods is to attach a quarter-wavelength open stub perturbation outside the ring [2] or two one-eight folded open stubs inside the ring[3]. Instead of open stubs, perturbation patch may be added at the corner of a square ring to make it dual mode [4]. Another technique is to design a orthogonally-loaded dual-mode double-ring resonator [5]. To design a triple mode ring, two quarter-wavelength stubs can be added, internally or externally [6] of the ring. An alternative way to add two extra transmission poles to ring resonator is by implementing stepped-impedance feed lines [7-9].

Defected ground structure (DSG) is defects on a ground plane which alters effective inductance and capacitance. One

of the benefits of DSG is disturbed shielding field of the ground plane helps in providing tighter coupling. To achieve the advantage of tighter coupling, a wide aperture can be formed on the ground plane directly under the gaps of upper-plane microstrip coupling structures [10]. By effectively weakening the coupling between the coupling structures and the ground plane, tight coupling is achieved. Narrow line width and gap width of the conventional SIR filters can then be increased or be more relaxed while maintaining the coupling coefficient [11-14].

The objective of this paper is to improve on our previous work of a quintuple H-ring resonator UWB BPF [15]. The previous work is constructed with a triple-mode H-shape ring resonator with two open stubs. To make the filter quintuple mode, it is coupled with interdigital lines at its two ports to stepped-impedance feed lines. In order to improve on this filter, two additional open stubs are introduced inside the ring resonator to add one transmission pole within the UWB passband making the filter sextuple-mode. The increase of mode in the filter results in better selectivity and hence higher fractional bandwidth. The stubs are introduced symmetrically to preserve the dual-symmetry planes of the original configuration resulting in simplified even- and odd-mode analysis of the filter. The analysis yields a design graph which shows the effects of characteristic impedance and electrical length of the two open stubs on the six resonant frequencies. Based on the graph, a sextuple-mode UWB BPF is designed and implemented on RT/Duroid 6010.2LM substrate. Due to the limitation of in-house fabrication, apertures are etched on the ground plane to increase the gap width of the interdigital line coupler. The filter is fabricated and measured to be in good agreement with simulated results.

II. ANALYSIS OF THE SEXTUPLE-MODE BANDPASS FILTER

Figure 1. Layout of sextuple-mode filter with two symmetry planes

This research work is funded by the Ministry of Higher Education (MOHE), Malaysia under a Fundamental Research Grant Scheme No. FRGS/2/2010/TK/SWIN/03/01.

2012 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2012), December 11 - 13, 2012, Melaka, Malaysia

9978-1-4673-3115-9/12/$31.00 ©2012 IEEE 221

The layout of the sextuple-mode filter is shown in Fig. 1. Two open stubs are inserted inside the ring resonator, which is similar to our previous quintuple-mode design, to gain one extra transmission pole within the passband. In spite of the additional stubs, the configuration remains symmetrical both vertically and horizontally as represented by the dashed line in Fig. 1. The benefit of retaining these two symmetry planes is simplified analysis of the filter. Magnetic or electric wall can then be applied along both the planes to perform odd- and even-mode analysis. However, electric wall is not placed along the horizontal plane as the input and output ports are shorted. This left us with two configurations when magnetic wall is placed along the horizontal plane as shown in Fig. 2. One is to apply electric wall along the vertical symmetry line to get vertical-plane odd mode, and another is applying magnetic wall to get vertical-plane even mode. This simplified layout can be represented by the equivalent circuit shown in Fig. 3. The characteristic impedance and electrical length of the newly-introduced open stubs are denoted by �� and �� respectively. Solving for �� of the equivalent circuit in Fig. 3 yields the following algebraic equations:

4�� + 4��

�� + 4�� +

2�� +

2�� +

2�� −

2�� + � = 0�� (1)�

for vertical-plane even mode, and 4��

�� + 4�� − 4��

� �� +

2�� +

2�� +

2�� −

2�� − � = 0 (2)

for vertical-plane odd mode.

Where, � = 2��

�� + 2�� + 2��

�� +

2�� + �� +

�� + �� −

��

�� = �/�

� = ��/�

Figure 2. Magnetic wall placed along horizontal symmetry plane. (a) Vertical-

plane odd mode. (b) Vertical-plane even mode.

The roots of (1) and (2) are solved by Wolfram Mathematica computational software [16] to find six resonant frequencies of the filter; three even-mode and three odd-mode. Fig. 4 plots the resonant frequencies normalised by UWB centre frequency, �� = 6.85GHz , against electrical length ratio, ��/� with two different impedance ratio, �. It can be observed that the first even-mode frequency, �� is independent of both � and ��/�. It is also evident from the plot that �� and �� are merging with increasing ��/� . Hence, smaller ��/� has to be chosen in order to maintain sextuple mode of the filter. The bandwidth of the filter can be estimated from �� − �� , and it decreases with increasing electrical length ratio for both �. � = 0.5 is selected as it exhibits larger bandwidth compared to � = 0.2. To design as close to equispaced resonant frequencies and to achieve the desired UWB passband, ��/� is chosen to be 0.2. The equations and methodologies of our previous work used to design the other parameters, namely ��, ��, ��, ��, � and �� are not repeated in this paper.

Figure 3. Equivalent circuit of the simplified layout.

O.C.

2�, �

(a)

��, ��

(b)

��, ��

��, ��

��

2��, ��

O.C. O.C.

O.C.

�, �


222

Figure 4. Design graph of resonant frequencies normalized by centre

frequency against electrical length ratio, ��/�.

III. IMPLEMENTATION AND FABRICATION OF THE SEXTUPLE-MODE BANDPASS FILTER

Based on the design graph in Fig. 4, a sextuple-mode bandpass filter is implemented on RT/Duroid 6010.2LM substrate of thickness 6.35mm and dielectric constant 10.2 to prove the analysis. The dimensions of the filter shown in Fig. 5 are optimised using Sonnet EM Simulator [17].

Figure 5. Optimised layout of the sextuple-mode filter. (Dimensions in mm)

In-house fabrication capability is constrained to minimum line gap and line width of 0.2mm. In order to achieve strong coupling between the interdigital line [10], aperture is etched along the ground directly below it as shown in Fig. 6. The stronger coupling results in smaller ripple size within the passband. Fig. 7 shows the Sonnet-simulated �� response of decreasing ripple size when aperture width, W increases. However, the collateral effect of increasing the aperture width is increase of passband bandwidth. Fig. 8 shows the almost linear relation between bandwidth and aperture width. Thus, we have to first design the filter for slightly smaller bandwidth before implementing the aperture to conform to UWB masks. For our design, W = 1.8mm is chosen which leads to an approximate 4% increase in 3dB bandwidth.

Figure 6. Apertures etched on the ground below the interdigital lines.

Figure 7. Simulated response showing the decrease in ripple size with

increasing aperture width.

Figure 8. Relation between 3dB bandwidth and aperture width.

The optimised filter in Fig. 5 with the prescribed ground aperture is fabricated on RT/Duroid 6010.2LM substrate using photoresist-masking technique. Fig. 9 is photograph of the fabricated filter showing top and bottom views of the filter.

-60

-50

-40

-30

-20

-10

00 2 4 6 8 10 12

|S21

| in

dB

Frequency (GHz)

Arrows indicating direction of increasing aperture width.

7

7.1

7.2

7.3

7.4

7.5

7.6

7.7

7.8

0 0.5 1 1.5 2

3dB

Ban

dwid

th (G

Hz)

Aperture Width (mm)

��/��

1.0

�

��/��

Line gap = 0.2 Line width = 0.2

��/��

1.1

��/��

0.8

��/��

0.2

��/��

4.5

��/�

2.0

� = 0.2 � = 0.5

5.6


223

Figure 9. Photograph of the fabricated filter. (a) Bottom view. (b) Top view.

The fabricated filter is then measured using Agilent Technologies N5230A network analyser while clamped to Anritsu 3680 Universal Test Fixture. Fig. 10 shows the measured and simulated responses of the proposed filter plotted with UWB indoor mask. The proposed filter exhibits higher 3dB fractional bandwidth at 6.85GHz centre frequency of 108.02% compared to 104.2% from our previous quintuple-mode filter reported in [15]. The responses of the filter fit nicely under the UWB indoor mask except for the lower stopband. Fig. 11 shows the comparison between simulated responses of the proposed sextuple-mode filter and the original quintuple-mode filter [15]. From the graph, it can be seen the proposed filter has steeper roll-off at both sides. Hence, wider passband bandwidth is achieved.

Figure 10. Simulated and measured responses of the proposed filter.

Figure 11. Comparison of simulated responses of the improved sextuple-mode BPF and the original quintuple-mode BPF [15].

IV. CONCLUSION This paper presents a method to enhance the selectivity of a

quintuple-mode UWB BPF by introducing extra transmission pole within the passband to make it sextuple mode. This is achieved by adding two open stubs inside the ring resonator. The design procedure of adding the stubs is described. As a result, there is an improvement of 3.82% in its 3dB fractional bandwidth at 6.85GHz centre frequency. In future work, response in lower stopband of the filter can be improved by introducing a transmission zero in the stopband.

REFERENCES [1] FCC, "Revision of Part 15 of the Commission’s Rules

Regarding UltraWideband Transmission System First Report and Order," F. C. Commission, Ed., ed, February 2002, pp. 98-153.

[2] H. Ishida and K. Araki, "Design and analysis of UWB BPF with ring resonator," Electronics and Communications in Japan (Part II: Electronics), vol. 88, pp. 1-8, 2005.

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[4] P. Cai, Z. Ma, X. Guan, Y. Zhang, B. Chen, and B. Xu, "Dual-mode square ring resonators for design of millimeter-wave ultra-wideband bandpass filter," Microwave and Optical Technology Letters, vol. 50, pp. 879-883, 2008.

[5] J.-C. Liu, C.-Y. Wu, M.-H. Chiang, and D. Soong, "Improved dual-mode double-ring resonator with radial stub for UWB-filter design," Microwave and Optical Technology Letters, vol. 44, pp. 219-222, 2005. -90

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0

|S21

| and

|S11

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dB

Frequency (GHz)

UWB Indoor MaskSimulated ResponseMeasured Response

0 2 4 6 8 10 12 14 16

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-70

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-50

-40

-30

-20

-10

0

|S21

| an

d |S

11|

in d

B

Frequency (GHz)

UWB Indoor Mask

Sextuple Mode

Quintuple Mode

2 4 6 8 10 12

(b)

(a)


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[6] S. Sun and L. Zhu, "Wideband Microstrip Ring Resonator Bandpass Filters Under Multiple Resonances," Microwave Theory and Techniques, IEEE Transactions on, vol. 55, pp. 2176-2182, 2007.

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[9] C. H. Kim and K. Chang, "Ultra-Wideband (UWB) Ring Resonator Bandpass Filter With a Notched Band," Microwave and Wireless Components Letters, IEEE, vol. 21, pp. 206-208, 2011.

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[12] L. Zhu and H. Wang, "Ultra-wideband bandpass filter on aperture-backed microstrip line," Electronics Letters, vol. 41, pp. 1015-1016, 2005.

[13] Q. X. Chu and S. T. Li, "Compact UWB bandpass filter with improved upper-stopband performance," Electronics Letters, vol. 44, pp. 742-743, 2008.

[14] Q.-X. Chu and X.-K. Tian, "Design of UWB Bandpass Filter Using Stepped-Impedance Stub-Loaded Resonator," Microwave and Wireless Components Letters, IEEE, vol. 20, pp. 501-503, 2010.

[15] A. S. H. Kuek, H. T. Su, and M. K. Haldar, "An aperture-backed H-ring ultra-wideband bandpass filter with floating conductor for spurious suppression," submitted for publication.

[16] Mathematica, Version 8.0. Champaign, IL: Wolfram Research, Inc., 2010.

[17] Sonnet User's Guide, Release 12. North Syracuse, NY: Sonnet Software, Inc., 2009.


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