A review of centrally loaded multimode microstrip resonators for bandpass filter design

G Model

ARTICLE IN PRESS

AEUE 51431 1–14

Int. J. Electron. Commun. (AEÜ) xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue

REVIEW

1

A review of centrally loaded multimode microstrip resonators for bandpass filter design

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Q1

4

Ker Chia Lee ∗ , Hieng Tiong Su, Manas Kumar Haldar Faculty of Engineering, Computing and Science, Swinburne University of Technology (Sarawak Campus), Jalan Simpang Tiga, Kuching, Sarawak, 93350, Malaysia

5 6 7

a r t i c l e

8 18

a b s t r a c t

i n f o

9

Article history: Received 5 September 2014 Accepted 16 June 2015

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Keywords: Microstrip filter Bandpass filter Multimode resonator

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1. Introduction

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The increasing demand for good performance filters in the field of radio frequency and wireless communication leads to the advancement in the design and development of compact microstrip resonator filters. This paper reviews the development of multimode microstrip resonators in recent decade. Halfwavelength open loop resonator is a well-known design approach for a single-mode resonator design, the half-wavelength line can be centrally loaded with an open-circuited stub, a short-circuited stub or a grounded via to form a dual-mode resonator. For symmetrical resonator structure, the resonant frequencies can be analysed using odd- and even-mode analysis. Triple-mode, quadruple-mode, quintuple-mode resonators are considered for wide bandwidth filters. The multimode resonators, in general, can be designed into filter using three different design methods. The first method uses cross-coupling resonators topology. The second method considers the distribution of the fundamental resonant frequencies of the resonator according to the Chebyshev’s insertion loss function. The third approach is to design all fundamental resonant frequencies of the resonator to be located within the 3-dB frequencies. All these three design approaches will be reviewed and discussed in this paper. In addition, a design example using the last approach for a fourth-order bandpass filter using a quadruple-mode resonator is given. © 2015 Published by Elsevier GmbH.

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Many applications in telecommunication and wireless technology required compact radio frequency and microwave bandpass filters. Compact microstrip resonators with superior performance are continuously developed to meet the demands. These microstrip resonators can be classified into single-mode and multimode resonators. The single-mode resonator can be categorised into six sub-classes. They are the half-wavelength resonators, quarter-wavelength resonators, stepped impedance resonators, quarter-wavelength stepped impedance resonators, patch resonators and ring resonators. These single-mode resonators are the basic structure for the theory and design method of the lowpass, highpass, bandpass and bandstop filters [1]. N number of singlemode resonators is required for an N-th order filter. One way of reducing the filter size is to use multimode resonators. If each multimode resonator has N modes, the number of resonators in the filter design is reduced by a factor of N. Thus if the dual-mode resonators (N = 2) are used, the filter size is halved and so on.

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

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∗ Corresponding author. Tel.: +60 0168855102. E-mail address: [email protected] (K.C. Lee).

Multimode resonators can also be categorised according to the number of resonant modes. They include the dual-mode resonator, triple-mode resonator, quadruple-mode resonators, quintuplemode resonators and so on. The following sections summarise the design and development of these multimode resonators, from dualmode (Section 2) to quintuple-mode (Section 5), with emphasis on centrally loaded open loop resonators in recent decade. Section 6 summarises the different design approaches for these compact multimode resonators. Section 7 gives a design example using a compact quadruple-mode resonator, the simulated and measured results are in good agreement. 2. Dual-mode resonators As reported by Matsuo et al. [2], conservative dual-mode resonators have the following common design approaches: 1. There must be at least a 90◦ separation between the input and output ports, 2. The resonator should have a discontinuity/perturbation to generate a reflected wave against an incident wave in the resonator, and 3. The resonator must be symmetric.

http://dx.doi.org/10.1016/j.aeue.2015.06.006 1434-8411/© 2015 Published by Elsevier GmbH.

Please cite this article in press as: Lee KC, et al. A review of centrally loaded multimode microstrip resonators for bandpass filter design. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.06.006

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Fig. 1. Open-loop resonators centrally loaded with (a) stepped impedance open-circuited stub [12], (b) T-shaped open-circuited stub [13], (c) Y-shaped open-circuited stub [14], (d) short-circuited stub [16], (e) short-circuited stub [17], and (f) grounding via [19].

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96

In a later development, the dual-mode resonators can be further classified into dual-mode patch or ring resonators with perturbation and dual-mode centrally loaded open loop resonators (Fig. 1). Ring or patch resonators can be perturbed by adding stubs or cuts onto the symmetrical plane of the resonators. The most common design is to add open circuited stub(s) on ring resonators with the input and output ports are 90◦ in separation [3]. This will split the fundamental resonant frequency into two. The ring resonators can be meandered [4] to decrease the size of the filter. Dual-mode ring resonators are designed with defected grounding structure [5] and T-shaped coupling feed structure to improve the stopband response [6,7]. Dual-band filters can be designed by stacking two ring resonators on top of each other using multilayer of substrates [8]. Complement split-ring resonator defected ground structures are implemented on the grounding surface of the dual-mode ring resonator microstrip filter to design a dual-band filter to improve passband selectivity [9]. Dual-mode ring resonator designed with composite-right/left hand transmission lines was proposed by [10]. The basic structure of the dual-mode centrally loaded open-loop resonator is a half-wavelength open loop resonator. The halfwavelength open loop resonator is then centrally loaded with an open-circuited stub, a short-circuited stub or a grounded via (Fig. 1). The centrally loaded element creates a grounding plane along the symmetrical plane of the resonator in odd mode condition, hence splitting the resonant frequency into two. The open-circuited stub has various shapes. They include uniform impedance stubs [11], stepped impedance stubs [12], T-shaped stubs [13], Y-shaped stubs [14], and Q-shaped stubs [15]. The short-circuited elements consist of a uniform impedance stub with grounding via [16,17], a grounding via only [18,19], a resistor with a grounding via at the short-circuited side [20] and others. The advantages of centrally loaded dual-mode resonators are simple structures and the resonant frequencies can be controlled easily or can be made into tuneable filters [13,16]. The dual-mode resonators also halved the filter footprint. Besides, two of these resonators can be coupled for the design of fourth order bandpass filter [13]. If the filter design requires three resonant frequencies for the formation of the passband, either a dual-mode resonator coupled with a single-mode resonator [11] or a triple-mode resonator can be used for the filter design.

3. Triple-mode resonators Triple-mode microstrip resonators are categorised into patch or ring resonators with perturbation(s) and centrally loaded open loop resonators (Fig. 2). The patch or ring resonators designs follow the design guidelines as stated by Matsuo et al. [2], similar to the dual-mode resonators. Disc patch resonator with two pairs of etched slots [21] degenerates and perturbs the fundamental mode frequency of the resonator to generate three mode resonant frequencies. Two branch-lines are added to a ring resonator [22] to realise a triple-mode resonator. Three branch-lines are added into a hexagonal-shaped ring resonator. It is then centrally loaded with capacitive stubs to become a triple-mode resonator [23]. A branch-line is added into the square ring resonator [24] to form another triple-mode resonator. This branch-line can be placed in the diagonal of the square ring resonator and two perturbations are introduced to the symmetrical plane of the resonator to form another triple-mode resonator [25]. Two concentric open-loop resonators [26] will produce three mode resonant frequencies. A straight line resonator can be centrally loaded with a patch to become a three-section stepped impedance triple-mode resonator as proposed by [27]. U-shaped three sections stepped impedance resonator can be centrally loaded with two open-circuited T-shaped stubs to form a triplemode resonator (Fig. 2(a)) [28]. Three sections stepped impedance resonator can be bent into an open-loop resonator. It is then centrally loaded with an open-circuited T-shaped stub and a shortcircuited stub (Fig. 2(b)) [29]. L-shaped resonator can be symmetrically loaded with two capacitive radial-line stubs to realise a triple-mode resonator (Fig. 2(c)) [30]. A triple-mode resonator (Fig. 2(d)) using a U-shaped uniform impedance resonator that is centrally loaded with an opencircuited stub and a short-circuited stub is designed by [31]. The open circuited stub in the resonator [31] can be replaced with a radial-line stub to become the triple-mode resonator (Fig. 2(e)) proposed by [32]. This design is proposed for a tri-band filter since three of the resonant frequencies can be controlled separately. The open-loop resonator can be stretched into a straight line and two open-circuited stubs are located at the symmetry plane of the straight line (Fig. 2(f)) as proposed by [33]. The straight line can be

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Fig. 2. Centrally loaded open-loop triple-mode resonators. (a) [28], (b) [29], (c) [30], (d) [31], (e) [32], (f) [33], (g) [36] (h) [37], (i) [38], and (j) [39].

136 137 138 139 140 141 142 143 144 145

146

147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169

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171 172 173 174

replaced with a U-shaped three sections stepped impedance line [34]. Open loop resonator can be centrally loaded with an opencircuited stub and a grounding via to become a triple-mode resonator as proposed by [35]. The open-circuited stub of this triple-mode resonator [35] can be replaced with an open-circuited stepped impedance stub (Fig. 2(g)) as illustrated in [36]. Deng et al. [37–39] researched a series of work on a triple-mode resonator designed using U-shaped resonator centrally loaded with opencircuited T-shaped stub and two short-circuited stubs. 4. Quadruple-mode resonators Quadruple-mode resonators (Fig. 3) generally can be classified into four categories according to their structures. (i) Coupled-ring resonators [40,41]. The authors in [40] have designed a concentric ring structure which has an inner ring with two open-circuited stubs (Fig. 3(a)). The two periodic stepped impedance concentric rings with two open loop resonators coupled to the outer ring (Fig. 3(b)) are proposed by [41]. (ii) Five-section stepped impedance resonators (Fig. 3(c)) [42]. (iii) Tri-section stepped impedance resonators with embedded stubs (Fig. 3(d)) [43]. (iv) Centrally loaded quadruple-mode resonators. This resonator has the most illustrated examples. A straight line resonator is centrally loaded with an opencircuited stepped impedance stub and two additional stepped impedance stubs at the symmetry plane of the resonator (Fig. 3(e)) to form a quadruple-mode resonator proposed by [44]. These stepped impedance stubs can be replaced by uniform impedance open-circuited stubs (Fig. 3(f)) [33]. The straight line resonator can be bent into a U-shaped, and all the stubs must be located in the same direction (Fig. 3(g)) so that the resonator can be made tuneable using varactors as described in [45]. The authors in [46] proposed the design of an open loop resonator also can be centrally loaded with Y-shaped open circuited stub and a grounding via to become a quadruple-mode resonator (Fig. 3(h)). 5. Quintuple-mode resonators Quintuple-mode resonators have five resonant modes. There are only a few quintuple-mode resonators reported. The quintuplemode resonators can be categorised into two groups. The first group is a square ring quintuple-mode resonator, first reported by [47].

This square ring resonator is located with two open-circuited stubs at its diagonal. The stubs are then further improved using T-shaped stubs (Fig. 4(a)). The second group consists of resonator with centrally loaded elements. The straight line resonator proposed by [48] is centrally loaded with an open-circuited stepped impedance stubs and another two open-circuited stubs are located at the symmetry plan of the resonator (Fig. 4(b)). The stepped impedance stub and the two open-circuited stubs of the resonator can then be replaced by an open-circuited stub and two stepped impedance stubs, respectively, and its centre is loaded with another short-circuited stub to form the resonator (Fig. 4(c)) proposed by [49]. The straight line resonator proposed by [48] can be bent into a U-shaped. The two open-circuited stubs are then replaced with two shortcircuited stubs (Fig. 4(d)) [50]. A three-section stepped impedance resonator can be centrally loaded with two open-circuited stepped impedance stubs and two open-circuited stubs are tapped at the junction of the low- and high-impedance section of the resonator (Fig. 4(e)) [51]. A straight line is centrally loaded with an opencircuited stepped impedance stub and short-circuited stubs. Then two open-circuited stubs are connected parallel to the centrally loading elements to form a quintuple-mode resonator (Fig. 4(f)) [52]. These quintuple-mode resonators are suitable for designing a wideband or ultra-wideband filter because they have sufficient amount of resonant frequencies to form the passband. The distribution of the resonant frequencies is also wide enough to cover a wider frequency range. Table 1 compares the performance of the centrally loaded multimode resonator filters discussed in Sections 2–5. Overall, the resonators filter designs reviewed in this paper have good in-band performance, sharp rejection skirt performance and good stopband performance as their advantages. As for the disadvantages, the difficulties only occur during fabrication of the resonator filter. For example, a common difficulty is to fabricate the grounding via [16–20,35], another difficulty is the fabrication of dimension of the resonator filter that is smaller than 0.15 mm [42–44,49], or filter design that uses two stacking layer of dielectric substrates [8]. 6. Overview of design methodologies using a multimode resonator approach In general, three design methodologies are employed to design a bandpass filter using multimode resonator approach. First design method uses either coupling resonators topology [11] or the

Please cite this article in press as: Lee KC, et al. A review of centrally loaded multimode microstrip resonators for bandpass filter design. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.06.006

175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211

212 213

214 215 216

fc (GHz)

FBW

IL (dB)

RL (dB)

Stopband rejection level

[12]

Dual

Duroid 6010.8 10.8, 0.635 mm

30 mm × 25 mm 0.64 g × 0.53 g

2.40

10

0.85

20

30 dB up to 4.74 GHz

[13]

Dual (Tuneable)

Duroid 6010.8 10.8, 1.27 mm

47 mm × 25 mm 0.19 g × 0.21 g

0.83–0.93

5

3

10

12 dB up to 1.05 GHz

[14]

Dual

Duroid 5880 2.2, 0.508 mm

25 mm × 40 mm 0.17 g × 0.28 g

1.53

6

1

23.4

17 dB up to 3.5 GHz

ARTICLE IN PRESS

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K.C. Lee et al. / Int. J. Electron. Commun. (AEÜ) xxx (2015) xxx–xxx

Substrate, dielectric constant, thickness

Resonator

G Model

Q4 Number of modes

Ref.

AEUE 51431 1–14

4

Please cite this article in press as: Lee KC, et al. A review of centrally loaded multimode microstrip resonators for bandpass filter design. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.06.006

Table 1 Comparison of the performance of centrally loaded multimode resonator filters.

Circuit size

fc (GHz)

FBW

IL (dB)

RL (dB)

Stopband rejection level

[16]

Dual (Tuneable)

Duroid 5880 2.2, 1.575 mm

39 mm × 39 mm 0.30 g × 0.30 g

1.66–2.38

25

0.7

15

40 dB up to 3.5 GHz

[17]

Dual

Duroid 5880 2.2, 0.508 mm

15.8 mm × 16.5 mm 0.098 g × 0.102 g

1.35

5

3

12

20 dB up to 3.3 GHz

[18]

Dual

Duroid 6010.2 10.2, 0.635 mm

3.13 mm × 1.30 mm 0.065 g × 0.027 g

2.40

15.8

0.85

20

25 dB up to 6 GHz

ARTICLE IN PRESS

Substrate, dielectric constant, thickness

Resonator

G Model

AEUE 51431 1–14

Number of modes

Ref.

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Please cite this article in press as: Lee KC, et al. A review of centrally loaded multimode microstrip resonators for bandpass filter design. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.06.006

Table 1 (Continued)

5

fc (GHz)

FBW

IL (dB)

RL (dB)

Stopband rejection level

[28]

Triple

Not reported 2.625, no reported

33 mm × 30 mm 0.38 g × 0.35 g

2.35

15

3

11

20 dB up to 9 GHz

[29]

Triple

Duroid 5880 2.2, 0.508 mm

16.95 mm × 17.5 mm 1.38 0.107 g × 0.11 g

13.9

2.5

9.6

30 dB up to 6.8 GHz

[30]

Triple

Rogers TMM 10 9.2, 1.0 mm

30 mm × 30 mm 0.50 g × 0.50 g

30.6

1.13

22

20 dB up to 4 GHz

2.00

ARTICLE IN PRESS

Circuit size

G Model

Substrate, dielectric constant, thickness

Resonator

K.C. Lee et al. / Int. J. Electron. Commun. (AEÜ) xxx (2015) xxx–xxx

Number of modes

Ref.

AEUE 51431 1–14

6

Please cite this article in press as: Lee KC, et al. A review of centrally loaded multimode microstrip resonators for bandpass filter design. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.06.006

Table 1 (Continued)

Circuit size

[33]

Triple

Duroid 6010.8 10.8, 1.27 mm

[31]

Triple (Tri-band)

[32]

Triple

fc (GHz)

FBW

IL (dB)

RL (dB)

Stopband rejection level

9.23 mm × 3.69 mm 2.30 0.19 g × 0.076 g

16.7

0.6

21

12 dB up to 5.3 GHz

Not reported 2.55, 0.8 mm

33 mm × 13 mm 0.19 g × 0.076 g

11.517.55.71

0.80.51.2

161616

15 dB up to 3.6 GHz

Rogers TTM 10 9.2, 1.0 mm

8.0 mm × 12.84 mm 2.45 0.171 g × 0.274 g

17.74

1.16

18

20 dB up to 4.5 GHz

1.57 2.4 3.5

ARTICLE IN PRESS

Substrate, dielectric constant, thickness

Resonator

G Model

AEUE 51431 1–14

Number of modes

Ref.

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Please cite this article in press as: Lee KC, et al. A review of centrally loaded multimode microstrip resonators for bandpass filter design. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.06.006

Table 1 (Continued)

7

fc (GHz)

FBW

IL (dB)

RL (dB)

Stopband rejection level

[36]

Triple (Dual-band)

Duroid 5880 2.2, 0.508 mm

14 mm × 18 mm 0.19 g × 0.25 g

3.00 5.00

15.410.2

1.2 1.2

10 12

24 dB up to 13.5 GHz

[37]

Triple (Dual-band)

Duroid 5880 2.2, 0.508 mm

14 mm × 18 mm 0.21 g × 0.27 g

3.34 5.26

14.910.2

1.1 1.1

9.8 14

15 dB up to 13.2 GHz

[44]

Quadruple

Duroid 5880 2.2, 0.508 mm

32 mm × 23 mm 0.60 g × 0.40 g

4.37

49

1

14.5

16 dB up to 17.2 GHz

ARTICLE IN PRESS

Circuit size

G Model

Substrate, dielectric constant, thickness

Resonator

K.C. Lee et al. / Int. J. Electron. Commun. (AEÜ) xxx (2015) xxx–xxx

Number of modes

Ref.

AEUE 51431 1–14

8

Please cite this article in press as: Lee KC, et al. A review of centrally loaded multimode microstrip resonators for bandpass filter design. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.06.006

Table 1 (Continued)

Circuit size

fc (GHz)

FBW

IL (dB)

RL (dB)

Stopband rejection level

[33]

Quadruple

Duroid 6010.8 10.8, 1.27 mm

9.23 mm × 5.83 mm 0.19 g × 0.12 g

2.30

27

0.7

18

12 dB up to 5.1 GHz

[45]

Quadruple

F4B-2 2.65, 0.5 mm

20 mm × 34 mm 0.60 g × 0.40 g

2.50

40

0.8

11.1

20 dB up to 6 GHz

[48]

Quadruple

Not reported 2.55, 0.8 mm

14.8 mm × 10.0 mm 0.514 g × 0.312 g

7.10

117

3

11

20 dB up to 16.8 GHz

ARTICLE IN PRESS

Substrate, dielectric constant, thickness

Resonator

G Model

AEUE 51431 1–14

Number of modes

Ref.

K.C. Lee et al. / Int. J. Electron. Commun. (AEÜ) xxx (2015) xxx–xxx

Please cite this article in press as: Lee KC, et al. A review of centrally loaded multimode microstrip resonators for bandpass filter design. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.06.006

Table 1 (Continued)

9

fc (GHz)

FBW

IL (dB)

RL (dB)

Stopband rejection level

[49]

Quadruple

Duroid 5880 2.2, 0.508 mm

32 mm × 17 mm 0.60 g × 0.32 g

4.08

64

1

14

40 dB up to 11.5 GHz

[50]

Quadruple

Duroid 6010 10.5, 0.635 mm

17.5 mm × 7.0 mm 0.514 g × 0.312 g

6.85

109.5

2

13

25 dB up to 17.5 GHz

[51]

Quadruple

Duroid 5880 2.2, 0.508 mm

130 mm × 69 mm 1.46 g × 0.77 g

2.45

40

1.35

15

70 dB up to 5.75 GHz

fc = centre frequency. IL = insertion loss. RL = return loss. FBW = fractional bandwidth.

ARTICLE IN PRESS

Circuit size

G Model

Substrate, dielectric constant, thickness

Resonator

K.C. Lee et al. / Int. J. Electron. Commun. (AEÜ) xxx (2015) xxx–xxx

Number of modes

Ref.

AEUE 51431 1–14

10

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Table 1 (Continued)

G Model

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11

Fig. 3. Quadruple-mode resonators: (a) coupled-ring resonator [40], (b) periodic stepped impedance concentric ring resonator [41], (c) five-sections stepped impedance resonator [42], (d) three-section stepped impedance resonator with embedded stubs [43], (e) centrally loaded open-loop resonator [44], (f) [33], (g) [45], and (h) [46].

Fig. 4. Quintuple-mode resonators from various reference: (a) [47], (b) [48], (c) [49], (d) [50], (e) [51], and (f) [52].

231

immittance inverter [53]. A dual-mode resonator filter [11,19] is normally designed using this method because the two resonant frequencies can be designed to match the separation of the resonant frequencies from two coupled single-mode resonators [19]. Each of the resonant frequencies on the dual-mode resonator can be taken as one resonant frequency from one single-mode resonator. Other parameters can be obtained from the design curve of the coupling coefficient values of the two adjacent resonators and the external quality factor values of the filter design. Second design method deals with distributing the fundamental resonant frequencies of the resonator according to the insertion loss function of the Chebyshev pole in the passband [27]. The resonant frequencies of the resonator can be derived using odd and even mode analysis and can be adjusted to the desired frequencies. The desired pole frequencies, fn , are calculated [27] where

232

fn = fc

233

xn = cos

217 218 219 220 221 222 223 224 225 226 227 228 229 230

234 235 236 237



1 + xn ×

FBW 2



(1)

 2k + 1 − 2n  2k

 ,

n = 1 to k

(2)

Here, fc is the centre frequency of the passband. FBW is the fractional bandwidth of the passband. k is the number of pole. Once the resonant frequencies are located at the pole frequencies, suitable feed structures can then be incorporated to form a good filter

passband. The quintuple-mode resonator filters designed by [52] have the poles distribution which closely reassembles the Chebyshev pole frequency distribution. Third design method, so-called 3 dB matching method, is more common for higher order resonator filter design, for examples, the quadruple-mode resonators [41,45] and quintuple-mode resonators [50,51] filter designs. The lowest and the highest resonant frequencies of the resonator are located close to or at the 3-dB passband frequencies. The rest of the resonant frequencies are distributed in between these two 3-dB frequencies. By distributing the fundamental resonant frequencies within the 3-dB frequencies, the required passband response can be obtained using suitable feed structures.

7. A design example of a multimode resonator bandpass filter This section demonstrates the design of a multimode resonator bandpass filter using the third method described in the preceding section. A quadruple-mode resonator is designed, as shown in Fig. 5(a). The open-loop U-shaped resonator is centrally loaded with an open-circuited Y-shaped stub and a grounded via. This resonator structure is used because it is a continuation of our former work [46]. This type of centrally loaded resonator is used for the filter

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Fig. 5. (a) Layout of the quadruple-mode resonator structure. Solid grey colour circle indicates grounding via. (b) Distribution of the fundamental resonant frequencies (f1 , f2 , f3 and f4 ) of the quadruple-mode resonator by varying the length of microstrip line 3 and the length of microstrip line 4 .

Fig. 6. (a) Final structure of the resonator with the input/output signal feed structure. (All dimensions are in millimetres. Solid grey circle indicates grounding via.) (b) Fractional bandwidth of the passband by varying the tap position, t.

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design so that second harmonics of the passband will only appear at three times of the centre frequency of the first passband. The filter has a centre frequency (fc ) of 2.0 GHz and a 3-dB fractional bandwidth of 37.5%. The two 3-dB frequencies of the passband are 1.625 GHz and 2.375 GHz to obtain a passband bandwidth of 0.75 GHz. The resonant frequencies of the resonator must be 0.055 GHz higher than these 3-dB frequencies. This is due to the coupling of the resonator to the feed structures will shift the resonant frequencies of the resonator down to 0.055 GHz. Hence, the lowest and highest fundamental resonant frequencies of resonator must be set at 1.68 GHz and 2.43 GHz before coupled to the feed structures. First, the width of all the microstrip lines, W1 , W2 , W3 , and W4 are fixed at 1.0 mm, 0.5 mm, 0.5 mm and 2.0 mm, respectively in order to simplify the design process. The diameter of the grounded via is 0.52 mm. The length of the microstrip line 1 is 1 /4, where 1 is the guided wavelength of the microstrip line 1 at 2 GHz. 1 can be calculated using the following equation:

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fc =

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c √ 41 ε1

(3)

where c is the speed of light in free space. ε1 is the effective dielectric constant for the microstrip line 1 . Hence the length of 1 is 14.35 mm. The length of microstrip line 2 is kept as short as possible for the ease of design, and hence it is chosen to be 0.50 mm. To

observe the four fundamental resonant frequencies of resonator, the length of microstrip line 3 is allowed to vary from 5.5 mm to 8.0 mm and the length of microstrip line 4 varies from 2.0 mm to 4.0 mm. The four fundamental resonant frequencies of resonator are labelled as f1 , f2 , f3 and f4 . When 3 and 4 are equal to 7.0 mm and 3.0 mm, respectively, the resonator has the following four resonant frequencies (as shown in Fig. 5(b)): f1 = 1.68 GHz, f2 = 2.00 GHz, f3 = 2.29 GHz and f4 = 2.43 GHz. The 4 value is chosen because its will give a bandwidth of 0.75 GHz. The bandwidth will be shifted to the centre frequency of 2 GHz when the resonator is coupled to the input and output feed structures. Using the layout in Fig. 6(a), the tap position of the feed structure, t is varied to determine the final fractional bandwidth of the passband of the filter. The required fractional bandwidth is 37.5% and hence the tap position, t, is 10.55 mm (see Fig. 6(b)). The final layout of the resonator (Fig. 6(a)) is simulated, fabricated and measured. The photograph of the realised filter is shown in Fig. 7(a). The final size of the resonator is 10.00 mm × 10.35 mm. The size of the whole filter is 24.50 mm × 20.00 mm. The simulated and measured results of the filter are recorded in Fig. 7(b) for a better comparison. The measured centre frequency of the passband is 2.02 GHz with a fractional bandwidth of 0.372. The measured insertion loss and return loss for the passband are 1.0 dB and 13.5 dB, respectively. The filter stopband shows a rejection better than 20 dB from 2.47 GHz to 5.10 GHz. The first harmonic of the passband occurs at 5.10 GHz which is 2.55 times the centre frequency of the

Please cite this article in press as: Lee KC, et al. A review of centrally loaded multimode microstrip resonators for bandpass filter design. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.06.006

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Fig. 7. (a) Photograph of the fabricated quadruple-mode resonator filter. (b) Simulated and measured results of the quadruple-mode resonator filter.

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filter passband. The measured results show a good agreement with the simulated results. This design method is suitable for the design of the filter using the proposed resonator.

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8. Conclusion

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The development of the multimode resonators in the past 10 years is reviewed in this paper. These multimode resonators are used for designing filter with wider bandwidth since the resonators exhibit multiple fundamental resonant frequencies. The centrally loaded open-loop resonator is used because the resonator structure can be easily analysed using the odd- and even-mode analysis. The resonator filter can be designed using one of the three common filter design methods as discussed in this paper. A fourth-order bandpass resonator filter is designed using a quadruple mode resonator to illustrate the design principles. References [1] Matthaei GL, Young L, Jones EMT. Microstrip filters, impedance-matching networks, and coupling structures. Norwood, MA: Artech House; 1980. [2] Matsuo M, Yabuki H, Makimoto M. Dual-mode stepped-impedance ring resonator for bandpass filter applications. IEEE Trans Microw Theory Tech 2001;4(7):1235–40. [3] Tu W-H, Chang K. Compact microstrip bandstop filter using open stub and spurline. IEEE Microw Wirel Compon Lett 2005;15(4):268–70. [4] Djoumessi EE, Wu K. Multilayer dual-mode dual-passband filter. IEEE Microw Wirel Compon Lett 2009;19(1):21–3. [5] Fu S, Wu B, Chen J, Sun S, Liang C. Novel second-order dual-mode dual-band filters using capacitance loaded square loop resonators. IEEE Trans Microw Theory Tech 2012;60(3):477–83. [6] Liu J-C, Wang J-W, Zeng B-H, Chang D-C. CPW-fed dual-mode doublesquare-ring resonators for quad-band filters. IEEE Microw Wirel Compon Lett 2010;20(3):142–4. [7] Baik J-W, Zhu L, Kim Y-S. Dual-mode dual-band bandpass filter using balun structure for single substrate configuration. IEEE Microw Wirel Compon Lett 2010;20(11):613–5. [8] Chen J-X, Yum TY, Li J-L, Xue Q. Dual-mode dual-band bandpass filter using stacked-loop structure. IEEE Microw Wirel Compon Lett 2006;16(9):502–4. [9] Wu G-L, Mu W, Dai X-W, Jiao Y-C. Design of novel dual-band bandpass filter with microstrip meander-loop resonator and CSRR DGS. Prog Electromagn Res 2008;78:17–24. [10] Allen CA, Leong KMKH, Itoh T. Dual-mode composite-right/left-handed transmission line ring resonator. Electron Lett 2006;42(2):96–7. [11] Liao C-K, Chi P-L, Chang C-Y. Microstrip realization of generalized Chebyshev filters with box-like coupling schemes. IEEE Trans Microw Theory Tech 2007;55(1):147–53. [12] Wang J, Ge L, Wang K, Wu W. Compact microstrip dual-mode dual-band bandpass filter with wide stopband. Electron Lett 2011;47(4):263–5. [13] Tang W, Hong J-S, Chun Y-H. Compact tunable microstrip bandpass filters with asymmetrical frequency response. In: Proceedings of the 38th European microwave conference. 2008. p. 599–602. [14] Zhang X-S, Zhao Y-J, Deng H-W, Zhang L, Chen W. High selectivity dualmode bandpass filter with source-loaded coupling. Prog Electromagn Res Lett 2010;18:187–94.

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Please cite this article in press as: Lee KC, et al. A review of centrally loaded multimode microstrip resonators for bandpass filter design. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.06.006

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Ker Chia Lee received PhD degree from Swinburne University of Technology (Sarawak Campus) in 2014. She also had received her Master of Engineering by research from the same university in 2010. Her research interests are in design and development of radio frequency and microwave filters and tuneable filters for communication.

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Hieng Tiong Su received the B. Eng degree in electrical and electronic engineering from the University of Liverpool, Liverpool, U.K., in 1994, and Ph.D. degree from the University of Birmingham, Birmingham, U.K., in 2001. From 2001 to 2006, he was a Research Fellow with the Electronic and Electrical Engineering Department, University of Birmingham, where he was involved with the design of novel superconducting delay lines, and tunable microwave filters using ferroelectric materials and MEMS switches. Since 2006, he joined Swinburne University of Technology (Sarawak Campus) Malaysia as a full-time lecturer. Dr. Su current research interests include compact microwave filters, tunable microwave devices, RF-MEMS and micromachined devices.

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Manas Kumar Haldar received the PhD degree from the University of Cambridge, UK. He was with the surface acoustic wave research group at the University of Oxford. He then joined the Electrical Engineering Department of the National University of Singapore. In 2006, he joined the Swinburne University of Technology, Sarawak Campus. Manas has 32 years of teaching experience and has been a senior member of IEEE for 22 years. His current research interests are in RF filters, semiconductor lasers, smart grids and e-voting.

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