bandstop filter

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A Novel Compact Microstrip Bandstop Filter Based on Spiral Resonators Ho Lim, Jong-Hyuk Lee*, Sang-Ho Lim, Dong-Hoon Shin, and Noh-Hoon Myung School of Electrical Engineering and Computer Science Korea Advanced Institute of Science and Technology 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea [email protected] *Network Quality Engineering Team, LG TeleCom, Ltd. 4 th Fl., ING Tower, 679-4 Yoksam1-dong, Gangnam-gu, Seoul, 135-977, Korea Abstract— A novel compact microstrip bandstop filter using spiral resonators (SRs) is proposed. An array of SRs topology which is etched on the center line has stopband characteristics. And this microstrip bandstop filter can be implemented with very small size because of the sub-wavelength effect of SRs. The 3-cell SR filter is fabricated and verified by measurements. We have obtained low insertion loss in the passband and high rejection level in the stopband with steep cutoff. This novel structure is easy to fabricate and is a promising structure that is compatible with MMIC or PCB process. Keywords-Spiral resonators, bandstop filter, microstrip filter. I. INTRODUCTION The miniaturization of microwave filters founded on planar technology is an essential issue for integrated and miniaturized transceiver front-ends. In this viewpoint, a novel design method using spiral resonators (SRs) showed the feasibility of a compact-size filter. In the last decade, it can be realized by virtue of electronic band gap (EBG) structure, split-ring resonators (SRRs), and SRs. All of them exhibit unusual characteristics which can pass specific frequency bands or suppress parasitics, unwanted spurious bands, and harmonics. Traditionally, these filtering characteristics are implemented by using lumped elements and half wavelength short-stubs. These methods have, however, shortcomings of large size, narrow bandwidth, and poor performance owing to noise. In the last several years, numerous researches on EBG structure have been done for reducing these drawbacks [1]-[2]. The EBG structure is originated from [3]-[4]. Yablonivitch introduced band gap which can control radiation of light randomly induced, and John presented band gap which can concentrate light waves into focus. Whereafter experiments about 3D EBG were carried out [5]-[6]. In the early EBG structure, only dielectric was used. As the EBG structure used both a dielectric and a conductor, more various experiments were made. The EBG structure is generally designed to have a periodicity so that it inhibits wave propagation at the specific frequency band [7]. The periodicity is assured through at least six repetitive stages. So the length of the EBG structure is at least 3 times of wavelength in that the electrical length of a period determining stopband is a half of wavelength at resonant frequency. The merit of those structures is that they can be integrated in a device without additional space. This is a promising method compared to conventional ways in point of size reduction view. However, there are still some problems in miniaturizing because those structures require several stages for satisfactory bandstop characteristics. This drawback has been improved in [8]-[13]. Recently, several structures having effectively slow wave effect have exhibited for more compactness than early EBG structures. One of them is the SRs which is depicted in Fig. 1. r ε W L L1 W1 W2 r ε Fig. 1. Spiral resonator topology. In this paper, we present a novel compact microstrip bandstop filter based on the SR approach. First, we investigate the equivalent LC circuit model of the SR unit cell and design filters using the commercial simulation tool. Then, we perform simulations and measurements for the fabricated filter in order to verify the performance. II. DESIGN METHODOLOGY OF BANDSTOP FILTER BASED ON SPIRAL RESONATORS The electromagnetic analysis of a SR was studied in [14]- [16]. In summary, filtering characteristics of the SR can be Proceedings of Asia-Pacific Microwave Conference 2007 1-4244-0749-4/07/$20.00 @2007 IEEE. 2221

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Page 1: Bandstop Filter

A Novel Compact Microstrip Bandstop Filter Based on Spiral Resonators

Ho Lim, Jong-Hyuk Lee*, Sang-Ho Lim, Dong-Hoon Shin, and Noh-Hoon Myung

School of Electrical Engineering and Computer Science Korea Advanced Institute of Science and Technology

373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea [email protected]

*Network Quality Engineering Team, LG TeleCom, Ltd. 4th Fl., ING Tower, 679-4 Yoksam1-dong, Gangnam-gu, Seoul, 135-977, Korea

Abstract— A novel compact microstrip bandstop filter using spiral resonators (SRs) is proposed. An array of SRs topology which is etched on the center line has stopband characteristics. And this microstrip bandstop filter can be implemented with very small size because of the sub-wavelength effect of SRs. The 3-cell SR filter is fabricated and verified by measurements. We have obtained low insertion loss in the passband and high rejection level in the stopband with steep cutoff. This novel structure is easy to fabricate and is a promising structure that is compatible with MMIC or PCB process.

Keywords-Spiral resonators, bandstop filter, microstrip filter.

I. INTRODUCTION The miniaturization of microwave filters founded on planar

technology is an essential issue for integrated and miniaturized transceiver front-ends. In this viewpoint, a novel design method using spiral resonators (SRs) showed the feasibility of a compact-size filter. In the last decade, it can be realized by virtue of electronic band gap (EBG) structure, split-ring resonators (SRRs), and SRs. All of them exhibit unusual characteristics which can pass specific frequency bands or suppress parasitics, unwanted spurious bands, and harmonics. Traditionally, these filtering characteristics are implemented by using lumped elements and half wavelength short-stubs. These methods have, however, shortcomings of large size, narrow bandwidth, and poor performance owing to noise. In the last several years, numerous researches on EBG structure have been done for reducing these drawbacks [1]-[2]. The EBG structure is originated from [3]-[4]. Yablonivitch introduced band gap which can control radiation of light randomly induced, and John presented band gap which can concentrate light waves into focus. Whereafter experiments about 3D EBG were carried out [5]-[6]. In the early EBG structure, only dielectric was used. As the EBG structure used both a dielectric and a conductor, more various experiments were made. The EBG structure is generally designed to have a periodicity so that it inhibits wave propagation at the specific frequency band [7]. The periodicity is assured through at least six repetitive stages. So the length of the EBG structure is at least 3 times of wavelength in that the electrical length of a period determining

stopband is a half of wavelength at resonant frequency. The merit of those structures is that they can be integrated in a device without additional space. This is a promising method compared to conventional ways in point of size reduction view. However, there are still some problems in miniaturizing because those structures require several stages for satisfactory bandstop characteristics. This drawback has been improved in [8]-[13].

Recently, several structures having effectively slow wave effect have exhibited for more compactness than early EBG structures. One of them is the SRs which is depicted in Fig. 1.

W

L

L1 W1

W2

W

L

L1 W1

W2

Fig. 1. Spiral resonator topology.

In this paper, we present a novel compact microstrip bandstop filter based on the SR approach. First, we investigate the equivalent LC circuit model of the SR unit cell and design filters using the commercial simulation tool. Then, we perform simulations and measurements for the fabricated filter in order to verify the performance.

II. DESIGN METHODOLOGY OF BANDSTOP FILTER BASED ON SPIRAL RESONATORS

The electromagnetic analysis of a SR was studied in [14]-[16]. In summary, filtering characteristics of the SR can be

Proceedings of Asia-Pacific Microwave Conference 2007

1-4244-0749-4/07/$20.00 @2007 IEEE. 2221

Page 2: Bandstop Filter

analyzed using the LC equivalent circuit model. When time-varying magnetic field penetrates the SR specifically, a current can be induced along the SR. Then a distributed inductance is generated in proportion to the length of the SR, and a mutual inductance is also generated between the lines of SR. The charge distribution in the SR is depicted in Fig. 2.

- - - - -

+ +

+ +

+

- - - - -

+ +

+ +

+

Fig. 2. Charge distribution of a spiral resonator. The gray

region denotes metal.

Distributed capacitance between inside and outside lines and fringing capacitances at the ends of lines are generated. These fringing capacitances are equivalently connected in series. Consequently, the equivalent circuit model of the SR can be represented as Fig. 3 [14].

Fig. 3. Equivalent circuit model of a spiral resonator

The circuit parameters of Cd, R, L, and Cf are a distributed capacitance, a resistance of spiral line, a distributed inductance (mutual inductance is neglected), and a fringing capacitance, respectively. As can be seen in Fig. 3, the equivalent circuit of the SR corresponds to the LC equivalent circuit model of a bandstop filter. The resonance frequency can be estimated as

01 , (1)

T TL Cω =

where CT is sum of the distributed capacitance and the fringing capacitance, and LT is sum of the distributed inductance and the mutual inductance.

In this study, spiral-shaped slots on the center line makes filtering characteristic. This slot is the dual counterpart of a spiral resonator made of metal patterns. Therefore dual

electromagnetic behaviors between them are expected due to the duality theorem. This means that a time-varying electric field parallel to the ring axis rather than a magnetic field is required. In general, the complementary of a metallic planar structure is accomplished by replacing the metal parts of the original structure with apertures and vice versa [17]. In this way, spiral-shaped slots are etched on the center line of microstrip technology. This makes sure that slots are adequately exited by electric field applied parallel to the ring axis.

III. NUMERICAL AND EXPERIMENTAL RESULTS The proposed bandstop filter is designed and verified with

both simulations and measurements. The filter has 3 unit cells and each cell consists of a microstrip and a complementary SR on the center line. The SR used in our design is shown in Fig. 4. To increase flexibility of size, we have changed the shape from circle type to rectangular type.

W

L

L1 W1W2

W

L

L1 W1W2

Fig. 4. Rectangular type spiral resonator etched on the

center line of the microstrip. The gray region denotes metal.

W3

L1

W4 W5L2 L3 L2

d dW3

L1

W4 W5L2 L3 L2

d d

Fig. 5. Dimension of the designed filter. The gray region

denotes metal.

The target resonance frequency is 7.2 GHz. To achieve this, we have determined the dimension of the SR using Ansoft HFSS (W=1.4 mm, W1=0.2 mm, W2=0.2 mm, L=3.0 mm, and L1=1.6 mm). The spacing (d) between two adjacent SRs (See Fig. 5.) is 6.2 mm, which is about a quarter of a guided wavelength. Steep skirt and bisymmetry characteristics of S21 are achieved. The width of the center line (W4) is 1.67 mm

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Page 3: Bandstop Filter

(corresponding to characteristic impedance of 50 ohms for the region in which there is no complementary SRs) and W5=2.4 mm (in the region with complementary SRs). And we have used a tapered line to match discontinuity between the wide line and the 50-ohm line. The size of a substrate (L1 × W3) is 30 × 25 mm2.

Fig. 6. Photograph of a fabricated filter.

The filter was fabricated on a Taconic RF-35 substrate ( rε =3.5, thickness h=0.76 mm) and shown in Fig. 6. Total length of the fabricated filter is 15.4 mm, which does not include the length of the tapered line for matching. This is electrically smaller than the guided wavelength of 24.8 mm. Thus we can implement a compact bandstop filter utilizing proposed structures.

1 2 3 4 5 6 7 8 9 10 11

-60

-50

-40

-30

-20

-10

0

|S| [

dB]

Frequency [GHz]

S11 S21

(a) Simulation result

1 2 3 4 5 6 7 8 9 10 11

-60

-50

-40

-30

-20

-10

0

|S| [

dB]

Frequency [GHz]

S11 S21

(b) Measurement result

Fig. 7. (a) Simulated S21 and S11 (b) Measured S21 and S11

The simulation was performed with Ansoft HFSS and compared with the measured result with a HP 8722ES vector network analyzer. The results are shown in Fig. 6.

The measured result agreed well with the simulation result. The resonant frequency is 7.3 GHz and the bandwidth of stopband is 0.5 GHz (7.1 ~ 7.6 GHz) with the reference level of |S21| = -10 dB. A deep rejection band (S21 = -53 dB) is obtained at resonant frequency with steep cutoff. And a flat passband (S21 < 1.2 dB) is presented, which implies that the proposed filter has low insertion losses.

IV. CONCLUSION In this paper, a novel compact microstrip bandstop filter

using SR structures was proposed. The designed filter is very compact. The fabricated filter shows good performance, high bandstop level, steep skirt, and low insertion loss. The proposed structure with bandstop characteristics based on SRs can be useful for various applications of MMIC or PCB process.

ACKNOWLEDGMENT This research was supported by KAIST BK 21 (Brain

Korea 21) and ADD (Agency for Defense Development) through the RDRC (Radiowave Detection Research Center) at KAIST.

REFERENCES

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[7] Y. Yablonovitch, “Photonic bandgap structure for microstip lines,” IEEE Microw. Guided Wave Lett., vol. 8, pp. 69-71, Feb. 1993.

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