Compact UWB Band-pass Filter with Ring Slot Resonator Structure

for Improved Upper Stop-band Suppression Characteristic

Kun Song, Yingzeng Yin, Shoutao Fan, and Yuqing Wei

National Key Laboratory of Science and Technology on Antenna and Microwave, Xidian University, Xi’an, Shaanxi, 710071, China

In this paper, a novel ultra-wideband band-pass filter (UWB-BPF) is designed with the ring slot resonator (RSR), which is a hybrid micro-strip and ground slot resonator structure to yield a broad pass-band filtering characteristic. Using the modulated micro-strip stubs and a pair of slits etched on the RSR cell, the poor selectivity and almost no suppression at the upper spurious pass-band can be improved effectively. S imulated results are confirmed via experiment, showing a good UWB filtering performance with a fractional bandwidth of about 112% from 2.9 to 10.3GHz, a widened upper stop-band up to 20GHz with a rejection level better than 17.6dB, and a sharp skirt at 11.9GHz, simultaneously. In addition, a small area is only about 42.5% of the previous ones.

Index Terms—additional slits, ring slot resonator (RSR), stop-band suppression, and UWB-BPF

I. INTRODUCTION

Recently, variety of UWB-BPFs have been invented and

reported in Refs. [1-8]. These published articles provide some new structures such as double-/multiple-/quintuple-mode resonators[1-2, 7-8], hybrid micro-strip/CPW transition structure[3], ring resonator[4], fork-form resonator[5], and U-slot quarter wavelength resonator[6] for UWB-BPFs designs. Among these designs in the literatures [1-5], the poor or unmentioned suppression at the upper sideband out of frequency range over 3.1~10.6GHz is still unsatisfied in practice. For a good rejection characteristic on the harmonics in the upper stop-band, some latest methods including an embedded band-stop structure [9], additional coupling slots [10-11], aperture-backed inter-digital coupled lines and a stub loaded folded stepped-impedance resonator (SLFSIR) [12], and ring open stub [13] in LPFs and UWB-BPFs applications are introduced for high selectivity and wide stop-band suppression. These larger UWB-BPFs have periodic uniform DGS array [11] and complex structures [12-13].

In this paper, we propose a novel UWB-BPF with the ring slot resonator (RSR), which has a simple hybrid micro-strip and ground slot resonator structure different from the above micro-strip/slot transition s tructures[3, 6] to form a broad pass-band filtering characteristic. Using the modulated micro-strip open micro-strips and a pair of additional slits etched on the RSR cell, widened/deepened upper stop-band suppression and sharp rejection skirt at upper side of the UWB pass -band can be obtained. Simulated and measured results show that the proposed UWB-BPF has a fractional bandwidth of 112% from 2.9 to 10.3GHz, an improved upper stop-band up to 20GHz with a rejection level better than 17.6dB and a sharpened skirt at about 11.9GHz, simultaneously. In addition, a small size of

sLs additional slots

R

g

RSR

W1

W0

Top layer Bottom layer

Fig.1 The geometry of the proposed UWB-BPF

about 7.5×7.5mm2 makes it a good candidate for the portable UWB systems and applications.

II. MODELING DESIGN OF UWB-BPF The geometry of the proposed UWB-BPF is shown in Fig.1. The proposed UWB-BPF is simulated and fabricated on a 0.508mm thick Rogers RO4003 substrate with a relative permittivity 3.35 and dielectric loss tangent 0.0027.

In our previous design, we use the simple RSR cell structure to generate the UWB filtering characteristic because it can realize a wide microstrip-CPW-microstrip transition like Refs. [2-3], which is superior to the traditional micro-strip band-pass filters with relatively narrow pass-band. However, the poor selectivity is unavailable in practice. From Fig.2 (a), it is obvious that the simple RSR cell exhibits a wide pass-band characteristic with a poor selectivity at the upper stop-band. When the radius R of ring slot is varied from 5 to 6mm, the bandwidth can be adjusted effectively. When R is equal to 5.0mm, the filter shows clearly three transmission poles with the low return loss across the pass-band in Fig.2 (b). This indicates the proposed filter is a three-pole band-pass filtering

___________________________________ 978-1-4244-8559-8/11/$26.00 ©2011 IEEE

(a)

(b)

Fig.2 Simulated S-parameters of the simple RSR cell performance. (a) S21, and (b) S11.

Fig.3 Improved upper stop-band characteristic comparison among the different structures evolved from the RSR cell.

structure and two coupling micro-strips can contribute to the degree of filter as well.

To design a UWB-BPF with a good stop-band performance, other different structures evolved from the ring slot resonator(RSR) cell are presented and their responses are simulated in Fig.3, respectively, from which we can see that the improved RSR cell with the modulated coupling micro-strips generates a

Table I. The optimum parametric values for proposed UWB-BPF

Parameters R g s Ls W1 W0

Values (mm) 3.4 0.35 0.65 3.15 1.0 1.15

(a)

(b)

(c)

Fig.4 Simulated and measured results of the proposed UWB-BPF. (a) Photograph, (b) S-parameter responses, and (c) Group delay.

transmission pole at about 12.2GHz improving the poor selectivity. By adjusting the length of modulated coupling micro-strips to enhance the input/output source coupling, the selectivity at upper stop-band is improved greatly.

Unfortunately, almost no suppression at the upper stop-band like in Refs. [2-4], where the spurious signals can pass, is still unaccepted. For the upper stop-band suppression

improvement, the proposed UWB-BPF with a pair of additional slits etched on the ring slot resonator (RSR) is shown in Fig.1. These slits can change the coupling between the modulated micro-strips. Adjusting the dimensions (Ls, s) of the additional slits, a good band-notched function can be brought at the upper spurious pass-band to produce a wider upper stop-band up to 20GHz. Furthermore, the previous resonant value of R can be decreased from 5 to 3.4mm. So, this novel and compact UWB-BPF structure with a smaller size and wider stop-band suppression better than 20dB can be obtained from the simulated results in Fig.3. In addition, a good skirt response can be also achieved by controlling the dimensions of the modulated coupling micro-strips and a pair of additional slits.

III. FABRICATION AND RESULTS DISCUSSIONS

According to the above design process, the proposed UWB-BPF with the optimum geometrical parameters in Table I is fabricated and measured in Fig.4 (a) and (b), respectively.

In Fig.3, both the simple RSR cell(R=5.50mm, g=0.25mm, s=0.25mm) and the improved RSR cell(R=5.50mm, g=0.35mm, s=0.35mm) with the modulated coupling micro-strips (3.0m � 1.0mm) can't achieve an excellent UWB-BPF performance because of their own shortcomings such as the former's poor selectivity and the latter's spurious pass -band. In fact, it is observed that the miniaturized RSR cell and the modulated coupling micro-strips have a strongly significant mutual effect on improving the upper stop-band characteristic. Owning to the additional slits coupled with the modulated coupling micro-strips, a widen stop-band rejection with a sharp skirt response is obviously brought up to 20GHz. Moreover, the previous resonant value of R is greatly decreased from 5.5 to 3.4mm, which leads to a smaller area only about 42.5% of the previous designs, simultaneously.

From Fig.4 (b), the proposed UWB-BPF shows a small insertion loss about 0.92dB within the measured 3dB pass-band from 2.9 to 10.3GHz, and the widened stop-band is well extended to 20GHz with an actual rejection better than 17.6dB. The sharp skirt is generated at about 11.9GHz, simultaneously. In addition, the simulated and measured group delay varies between 0.075 and 0.325ns over the most central pass-band inFig.4(c). Although there are some differences with the simulations at higher frequency range mainly because of deterioration of material properties with the increasing frequency, the measured responses still agree well with simulated ones.

IV. CONCLUSION

A compact UWB-BPF with ring slot resonator (RSR) is proposed and designed for improved upper stop-band suppression. The proposed UWB-BPF shows an about 112% fractional bandwidth from 2.9 to 10.3GHz and a widen stop-band suppression up to 20GHz with a rejection level better than 17.6dB. The improved RSR cell and the additional slits also show a significant effect on improving the stop-band

characteristics. In addition, the proposed UWB-BPF with asmall size about 7.5×7.5mm2 is a good candidate for UWB systems and portable applications.

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