Sextuple-Mode Ultra-Wideband Bandpass Filter with Wide Upper Stopband

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

Swinburne University of Technology (Sarawak Campus) Kuching, Sarawak, Malaysia akuek@swinburne.edu.my

Abstract—A sextuple-mode ultra-wideband bandpass filter with wide upper stopband is presented in this paper. The wide upper stopband is achieved by attaching two open-circuited stubs 90° from the feed lines inside a ring resonator. These stubs introduce two transmission zeroes above the UWB passband to enhance the upper stopband. Analytical design of the six transmission poles and two transmission zeroes is described. The filter is then fabricated on RT/Duroid 6010.2LM substrate and measured. The simulated and measured responses are in good agreement showing upper stopband greater than 20dB up to 28GHz.

Keywords-ultra-wideband; sextuple order; bandpass; filter; wide upper stopband; 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. Design of a triple-mode ring resonator can be achieved by modifying the conventional resonator with a pair of open-ended coupled lines and a shunt open stub for a transmission-line path [2]. Another method is to attach two quarter-wavelength open-circuited stubs internally or externally of the ring [3, 4]. Meanwhile, a quintuple-mode UWB BPF filter [5] can be designed with stepped-impedance feed lines which introduce two transmission poles in the passband. The authors in [6] propose a quintuple-mode ring resonator with four short-circuited stubs. The novelty of this paper lies in the direct-connected feed lines for strong coupling and ease of fabrication. Alternatively, two stepped-

impedance stubs can be introduced to a ring resonator to possess five resonances [7].

One of the desired characteristics of a bandpass filter is to have wide upper stopband. Abbosh [8] realises this by having three transmission zeroes in the upper stopband of his triple-mode filter. It is done by connecting three subsections of different lengths and coupling factors to form a stepped-impedance parallel-coupled microstrip structure. On the other hand, defected ground structures (DGS) which alters effective inductance and capacitance are implemented in [9-12] to improve upper stopband performance. Authors in [13-16] design a UWB BPF with improved out-of-band performance using quasielectromagnetic bandgap (EBG) structure. One can also incorporate a novel inverted-F impedance transformer at the input and output to generate out-of-band transmission zeroes [17].

In this paper, we propose an improvement to our previous sextuple-mode ring resonator UWB BPF [18] to widen the 20dB upper stopband. This is accomplished by introducing two transmission zeroes in the upper stopband. The method is to attach two open-circuited stubs inside the ring resonator. The two stubs are at the lower and upper sides of the ring, 90° from the feed lines. They are designed to minimally affect the analysis of the filter rendering high reusability of previous analytical equations. This improved filter still preserves the nature of dual symmetry. Thus, even- and odd-mode analysis is carried out on only one quarter of the structure. Analytical equations and design graphs are then given to determine the six resonant frequencies in the passband as well as the two transmission zeroes in the upper stopband. The filter is then simulated to optimise its dimensions for good responses before being fabricated on RT/Duroid 6010.2LM substrate. Measured results experimentally verify the simulation responses, showing wide 20dB upper stopband until 28GHz and slightly improved passband selectivity.

II. DESIGN AND ANALYSIS OF THE UWB BANDPASS FILTER

Fig. 1 shows the layout of the improved sextuple-mode filter with two symmetry planes. The filter fundamentally maintains the structure of previous filter and its property of dual symmetry. However, two extra open-circuited stubs are attached at the lower and upper sides of the ring, 90° from the feed lines. The objective is to introduce two transmission zeroes above the UWB passband effectively widen the upper

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stopband rejection. The stubs are designed to be half of the characteristic impedance of the ring resonator, . The reason is to simplify the analysis of the structure as even-mode characteristic impedance of the stubs, which is twice as large, will be the same as . ½ and denote the characteristic impedance and electrical length of the two new stubs, respectively. As a result, the analytical equations of previous filter to determine the resonant frequencies can be reused with minor modification in (1).

Even- and odd-mode analysis is performed on a quarter of the structure as it possesses two symmetry planes. Nevertheless, only magnetic wall can be applied for the horizontal plane as electric wall will short the input and output. Fig. 2 shows the vertical-plane even mode with perfect magnetic walls placed along both vertical and horizontal symmetry planes. Fig. 3 shows vertical-plane odd mode with perfect magnetic wall and electric wall placed along the horizontal and vertical symmetry planes, respectively. By applying the boundary condition, = 0, resonant conditions of the equivalent circuits can be represented by the following two 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 + + + − = / = /

Fig. 1. Layout of the improved sextuple-mode filter with two symmetry planes

in dashed lines.

Fig. 2. Vertical-plane even mode and its equivalent circuit.

Fig. 3. Vertical-plane odd mode and its equivalent circuit.

To investigate the two transmission zeroes, mutual admittance between the input and output is to be zero as expressed by:

[2 + ] [2 + ] − = 0Solving the roots of (1) and (2) using Wolfram

Mathematica computational software [19] yields six resonant frequencies of the filter; three even-mode and three odd-mode Similarly, roots of (3) solve for the two transmission zeroes;

and . Fig. 4 plots the resonant frequencies and transmission zeroes normalised by UWB centre frequency, = 6.85GHz, with respect to electrical length ratio, / . It can be observed that the first even-mode frequency, is independent of / .The passband bandwidth of the filter is estimated by the two outermost resonances, − . The estimated bandwidth is seen to get smaller with increasing / . Meanwhile, the two transmission zeroes are always above in the upper stopband as desired. These transmission zeroes will suppress the spurious harmonics enhancing the out-of-band performance. To design as close to equispaced resonant frequencies and to achieve the desired UWB passband, / is chosen to be 0.2. The procedures used to design the other parameters, namely , , , , ,

, and , can be referred to in our previous work.

2 ,

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Fig. 4. Design graph of resonant frequencies and transmission zeroes normalised by centre frequency against electrical length ratio, / .

III. IMPLEMENTATION OF THE UWB BANDPASS FILTER A wide upper stopband sextuple-mode bandpass filter,

based on the design graph in Fig. 4, is implemented on a substrate of thickness 6.35mm and dielectric constant 10.2 to validate the design. Optimisation of the dimensions of the filter shown in Fig. 5 is executed using Sonnet EM Simulator [20] and CST Microwave Studio [21].

Due to in-house fabrication’s constraint that limits the minimum line gap and line width to 0.2mm, the filter suffers from high insertion loss within the passband. Coupling between the interdigital lines has to be increased to overcome the limitation. This is solved by etching a rectangular aperture at the ground layer directly below each interdigital lines [22] as shown in Fig. 6. Following the methodology described in our previous work, the optimal aperture width is found to be = 2.0 .

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

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

The filter is first simulated under weak couplings between the feed lines and resonator to validate that the six transmission poles as well as the two transmission zeroes are as prescribed by the design graph in Fig. 4. The plot in Fig. 7 shows the simulated S21 response under weak couplings.

Fig. 7. Transmission poles and zeroes simulated under weak couplings.

The optimised filter shown in Fig. 5 along with the prescribed ground aperture is then fabricated on RT/Duroid 6010.2LM substrate using photoresist-masking technique. Fig. 8 shows the photographs of the top and bottom sides of the fabricated filter.

Fig. 8. Photograph of the fabricated filter. (a) Bottom view. (b) Top view.

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The fabricated filter is then clamped to Anritsu 3680 Universal Test Fixture and measured using Agilent Technologies N5230A network analyser. The measured and simulated responses of the proposed filter up till 30GHz compared with UWB indoor mask are shown in Fig. 9. The two responses are in good agreement and they fit under the UWB indoor mask except for the lower stopband. The insertion loss within the passband is less than 0.65dB. Fig. 10 shows the comparison of responses between the current proposed filter and the previous work reported in [18]. The improved filter exhibits wider 20dB upper stopband up till 28GHz, approximately 4 . It is also evident from the graph that the roll-off at the higher passband is steeper attributable to the close proximity of the transmission zero to the higher passband. Consequently, there is a slight increase of 3dB fractional bandwidth to 108.72% from 108.02%.

Fig. 9. Simulated and measured responses of the proposed filter.

Fig. 10. Comparison of responses between the current improved filter with the

previous design.

IV. CONCLUSION This paper presents a method to enhance the out-of-band

performance of a sextuple-mode UWB BPF by introducing two transmission zeroes within the upper passband. This is achieved by adding two open-circuited stubs inside the ring resonator 90° from the feed lines. The procedure of designing the stubs is described in the article. As a result, the 20dB upper stopband is extended until 28GHz. On top of that, the 3dB fractional bandwidth has slightly increased as the result of one transmission zeroes nearby the passband. The measured results of the fabricated filter experimentally validate the simulation as well as the analysis.

ACKNOWLEDGMENT Albin Kuek thanks Swinburne University of Technology

(Sarawak Campus) for his Ph.D. studentship, and Rogers Corporation for free substrate samples.

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