Vol.29 No.1/2 JOURNAL OF ELECTRONICS (CHINA) March 2012

COMPACT BANDPASS FILTER WITH HIGH SELECTIVITY AND WIDE STOPBAND USING SLOTTED STEPPED-IMPEDANCE RESONATOR1

Xu Jin Ji Yuxue Miao Chen Wu Wen (Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, Nanjing 210094, China)

Abstract In this paper, a Slotted Stepped-Impedance Resonator (SSIR) is proposed. Due to the slots in the low-impedance section of the conventional SIR, the new resonator has a lower fundamental resonance f0 and can provide a potential finite transmission zero fz close to f0. Based on the proposed SSIR, a fourth-order Chebychev BandPass Filter (BPF) is designed at f0=1 GHz. The measured results show that a better than –65 dB rejection is achieved on both the lower and the upper stopband. Moreover, the new filter has a wide –30 dB rejection upper stopband from 1.13f0 to 6.52f0. The fab-ricated filter exhibits a size of 0.078 0.062 .g gλ λ× The new filter has a planar topology and is easily integrated with modern portable communication systems.

Key words BandPass Filter (BPF); Stepped-Impedance Resonator (SIR); High selectivity; Wide stopband

CLC index TN407

DOI 10.1007/s11767-012-0780-4

I. Introduction With the rapid growth of the miniaturized

portable communication systems, bandpass filter with compact size and high performance has been great in demand. For this purpose, many structures, such as spiral-like resonator[1], patch-via-spiral resonator[2], net-type /8λ resonator[3], hybrid reso-nator[4], have been studied. Stepped-Impedance Resonator (SIR), as one of the most popular structure in filter design, has been received a great attention[5–8]. In Ref. [5], double split-end SIR is introduced to reduce the resonator size while pro-viding additional transmission zeros. In Ref. [6], an additional open-stub is inserted between the low- and high-impedance lines of the resonator to fa-cilitate the generation of cross-coupled path and lower its resonant frequency. In Ref. [7], a filter with the 8th harmonic suppression is designed by embedding the bandstop resonator into the quar-ter-wavelength SIR. By inserting several ground strips in the low-impedance section of SIR, the size of conventional SIR can be miniaturized and the characteristic of fabrication tolerance is generated[8].

1 Manuscript received date: September 5, 2011; revised date:

February 23, 2012. Communication author: Xu Jin, born in 1987, male, Ph.D.. Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, Nanjing 210094, China. Email: xujin2njust@126.com.

However, these filters suffer from complicated forms and design procedure[1,3–6], the etched gr- ound[2], embedded bandstop resonators[7], and mass of shorted vias[8], which limit their application in communication systems.

In this paper, a Slotted SIR (SSIR) is proposed by etching the slots in the low-impedance section of the conventional SIR. The new SSIR has a lower fundamental resonance and a potential finite transmission zero compared to the conventional SIR. In addition, the slots can provide an extra freedom to tune the first harmonic resonance of the resonator. As an example, a fourth-order Cheby-chev BPF is designed at f0=1 GHz. The filter is fabricated on the substrate Rogers 4003C (h= 0.508 mm, re 3.55).ε = Due to the high performance of the proposed SSIR, the new filter shows some great advantage of compact size, high selectivity, wide stopband and simple topology. This paper is divided into four sections and is organized as fol-lows. Section II investigates the characteristics of the proposed SSIR. Section III gives a BPF design procedure based on SSIR. Finally, Section IV draws the brief conclusions.

II. SSIR Analysis The basic topology of the proposed SSIR under

folded configuration is shown in Fig. 1(a). It com-prises a shorted SIR with four slots in its low-

24 JOURNAL OF ELECTRONICS (CHINA), Vol.29 No.1/2, March 2012

impedance section. For the purpose of investigating its resonant property, a 50 Ω microstrip tapped line is directly connected to the resonator as shown in Fig. 1(b). After de-embedding the phase shift introduced by the microstrip tapped line, Fig. 2 gives the calculated input impedance (related to f0) and input admittance (related to fz) under the physical dimensions chosen as l1=10 mm, l2= 0.5 mm, w1= 3 mm, w2 = 0.2 mm, s= 0.3 mm, g = 0.3 mm, d= 0.4 mm, re 2.2,ε = h=0.508 mm[4]. In Fig. 2, the fundamental resonance of the conventional SIR without slots is also given. It can be seen that the fundamental resonance is apparently lower from 1.34 GHz to 1.05 GHz due to the effect of the slots. Furthermore, the proposed resonator has a poten-tial transmission zero fz at 1.22 GHz.

Fig. 1 Proposed SSIR and its tapped line configuration

Fig. 2 Input impedance and admittance of the proposed SSIR and the conventional SIR.

It is known that f0 and f1 of the SIR can be tuned by its impedance ratio[9,10]. To further ob-serve the impact of the slots g and numbers N on f0, f1/f0, and fz/f0, a parameter study is done as shown in Fig. 3. It can be seen that as g increases, the ratio of f1/f0 increases firstly and then decreases. As N increases, f0 moves towards the lower frequency

while f1/f0 increases. In addition, it is interesting that the potential transmission zero fz is close to f0 (around fz/f0=1.15) and do not change dramatically with the physical parameter.

Fig. 3 Impact of the slots g and numbers N on 1 0/f f and 0/zf f

If the fundamental resonance f0 and the first harmonic resonance f1 are specified as 1 GHz and 4.3 GHz, respectively, then, the physical parame-ters can be adjusted. f0 can be mainly tuned by l1 while f1/f0 can be tuned by w1, w2, and g. The op-timized physical parameters of the resonator are given as wf = 1.13 mm, w1 = 2.7 mm, w2 = 0.2 mm, l1 = 10.39 mm, l3 = 0.65 mm, g = 0.3 mm, and d = 0.4 mm. Under these physical sizes, SSIR has a potential transmission zero at 1.15 GHz.

III. Fourth-order Chebychev BPF De-sign

Fig. 4 depicts the layout of the designed fourth- order Chebychev BPF based on proposed SSIRs. Tapped lines are directly connected to the first and fourth resonators used as the input/output ports. t indicates the position of the tapped line, g12 indi-cates the gap size between the first and second

XU et al. Compact Bandpass Filter with High Selectivity and Wide Stopband Using Slotted Stepped-impedance 25

resonators while g23 indicates the gap size between the second and third resonators. The fourth-order Chebychev low-pass filter prototype with a ripple of 0.04321 dB and Fractional BandWidth (FBW) of 12.5% is applied to the filter design. The lumped circuit element values of the lowpass prototype filter are found to be g0=1, g1=0.9314, g2=1.292, g3=1.5775, g4=0.7628, and g5=1.221[11]. The inter- stage coupling coefficients kij and the external quality factor Qe can be calculated using the fol-lowing equations.

Fig. 4 Layout of the proposed fourth-order Chebychev BPF

FBW/ij i jk g g= (1a)

ei 0 1 /FBWQ g g= (1b)

eo 1 /FBWN NQ g g += (1c)

The calculated inter-stage coupling coefficients and the external quality factor are k12=k34=0.115, k23=0.088, Qei=Qeo=7.39. Fig. 5 shows the coupling coefficients and the external quality with respect to the corresponding physical parameters which are extracted by the full-wave EM simulator HFSS.

Fig. 5 Extracted coupling coefficient and the external quality factor Qe

IV. Simulation and Measurement Re-sults

The simulated and measured results of the fabricated filter are plotted in Fig. 6. Good agree-ment can be observed between simulation and measurement and there are some discrepancies due to the fabrication error. The measured results show that the fabricated filter has a center frequency of 1 GHz with FBW of 12.6%. The minimum meas-ured insertion loss is 3.1 dB while the measured return loss is better than 13 dB over the whole passband. The deterioration of the insertion loss may due to the dielectric loss and fine transmission lines used. The proposed filter has a deep rejection (<–55 dB) upper stopband from 1.26f0 to 4.24f0. The first harmonic frequency is measured around 4.3 GHz and without any extra measurements, it can be suppressed below –30 dB due to the im-pedance mismatch at the harmonic frequency. Thus, a wide –30 dB rejection stopband is achieved from 1.13f0 to 6.52f0. The size of the fabricated filter is 0.078 0.062 ,g gλ λ× where gλ is the guided wave-length of 50Ω line on the substrate at f0. Tab. 1 shows the size and performance of the proposed filter compared to some other compact and high performance BPFs. Obviously, the proposed BPF has several advantages of simpler topology, com-pacter size and a wider –30 dB rejection stopband. In addition, less vias are used. Photograph of the fabricated filter is shown in Fig. 7.

Tab. 1 Filter performance comparison

Design Size Stopband (–30 dB rejection)

Ref. [2] 0.188 gλ × 0.043 gλ 1.14f0 × 5.71f0

Ref. [4] 0.217 gλ × 0.100 gλ 1.14f0 × 2.78f0

Ref. [7] 0.09 gλ × 0.074 gλ 1.14f0 × 5.28f0

Ref. [8] 0.065 gλ × 0.083 gλ 1.69f0 × 4.57f0

This work 0.078 gλ × 0.062 gλ 1.13f0 × 6.52f0

V. Conclusion In this paper, a compact fourth-order microstip

bandpass filter is presented by using the proposed SSIRs. Due to the high performance of the pro-posed resonator, the new filter exhibits the ad-vantages of compact size (0.078 0.062 ),g gλ λ× high selectivity, deep rejection (better than –65 dB) and

26 JOURNAL OF ELECTRONICS (CHINA), Vol.29 No.1/2, March 2012

a wide –30 dB rejection stopband from 1.13f0 to 6.52f0. Therefore, the proposed filter will be very attractive in the design of high isolation and compact diplexer.

Fig. 6 Simulated and measured S-parameters

Fig. 7 Photograph of the fabricated filter

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