遠東學報第二十五卷第三期

401

一種新的接地面鏤空結構，可用來改善微帶線低通與帶通濾波器

的截止頻帶表現

A New Defected Ground Structure (Dgs) And Its Applications To

Improve The Stopband Performance Of Microstrip Lowpass And

Bandpass Filters

江孟駿 大華技術學院電腦與通訊工程系助理教授

曾建中 明新科技大學電子工程所

張嘉陽 明新科技大學電子工程所

梁思明 明新科技大學電子工程所

陳安邦 明新科技大學電子工程系副教授

摘 要

一種新的接地面鏤空結構可以產生數個傳輸零點來改善截止頻帶的響應。此

種結構乃是在接地面開幾條狹縫線(slot line)，這些狹縫線的中點都是連起來的。

狹縫線的長度可以用來調整傳輸零點的位置，用以調整截止頻帶的頻寬與截止水

平。這種接地鏤空結構可以用來改善低通濾波器的截止頻帶響應，另一方面也可

以用來抑制平行耦合微帶線帶通濾波器的高階虛假響應。本論文透過設計數個電

路及模擬結果來驗證理論之可行性。

關鍵詞：微帶線、接地面鏤空結構、截止頻帶

遠東學報第二十五卷第三期

402

Meshon Jiang, Assistant Professor, Depart. of Comupter and Communication Engineering, Ta Hwa Institute of

Technology

Chien-Chung Tseng, Student, Depart. of Electronic Engineering, Minghsin University of Science and

Technology

Chia-Yang Chang, Student, Depart. of Electronic Engineering, Minghsin University of Science and

Technology

Si-Ming Leong, Student, Depart. of Electronic Engineering, Minghsin University of Science and Technology

Arn-Bun Chen, Associate Professor, Depart. of Electronic Engineering, Minghsin University of Science and

Technology

Abstract

A new defected ground structure (DGS) with plural transmission zeros is

proposed to enhance the bandgap effects. The structure is composed of three slot lines

joined together at their middle points in the ground plane. Lengths of the lines are

adjusted to generate resonant frequencies in a close proximity, and hence to control

attenuation level and bandwidth of the stopband. The DGSs are used to improve the

stopband performance of microstrip lowpass filter and parallel coupled bandpass filter.

Several circuits are designed and simulated to verified the idea.

Keyword: microstrip, DGS, stopband

遠東學報第二十五卷第三期

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I. IntroductionDefected ground structure (DGS) made of etching

defected pattern in the ground plane has been widely

investigated recently and found many viable

applications in circuit designs in microwave and

millimeter-wave regime. DGS with dumbbell-shaped

pattern is firstly proposed in [1]. It can provide a

bandgap characteristic in some frequency bands and

an excellent performance in filter applications.

Periodic DGS composed of uniform square or spiral

defected patterns possess both stopband and slow-

wave characteristics, and thus has been extensively

applied to oscillator, amplifier and antenna designs

for harmonic suppression and size reduction [2]; [4].

It is found, however, periodic DGS with uniform

defected patterns suffers from serious return losses

around cutoff frequency. Thus, various nonuniform

periodic DGSs with Chebyshev, binomial or

exponential distribution have been proposed to

improve the performance [5]; [7].

Each of all the defected ground structures

discussed above has only one transmission zero per

unit cell, which may limit their performance in wide

band application. In this paper, we try to design a

DGS with several transmission zeros using only one

unit cell. This novel DGS can be made up by using

several slot lines joined together at the middle points

in the ground plane, of which the response can be

easily adjusted by just vary the number and the

length of these slot lines. Stopband performance and

harmonic suppression capability can be both

enhanced using this new type of DGS.

II. Characteristics For Deformed

DgsStart from the dumbbell-shaped pattern DGS

proposed in [1], an LC equivalent circuit can

represent the proposed unit DGS circuit, and

theequivalent-circuit parameters are extracted based

on the circuit analysis theory.

C

LZ0

Z0

Fig 1: Equivalent circuit of the proposed DGS circuit.

Microstrip line(50 )

DGS patternon groundplane

W

g

1

Microstrip line(50 )

DGS patternon groundplanea

a

Fig 2: Schematic view of the deformed DGS circuit.

Fig.1 shows the equivalent circuit of the proposed

DGS circuit, and the series capacitance C and

inductance L can be represented respectively as:

22010

1

c

c

gZC

(1)

遠東學報第二十五卷第三期

404

Table I. Extracted l and c values for the dgs in Fig 1

1 (mm) fc (GHz) f0 (GHz) C (pF) L (nH)

4.4 3.57 5.28 0.375 2.42

3.75 3.16 4.34 0.568 2.366

2.5 2.95 3.76 0.864 2.074

1.25 3.04 3.69 1.106 1.682

0 5.83 6.74 0.811 0.687

Table II. Extracted l and c values for different w

values

w (mm) fc (GHz) f0 (GHz) C (pF) L (nH)

4.4 3.02 3.61 1.227 1.584

3 3.51 4.23 0.995 1.423

2 3.96 4.75 0.92 1.22

1 4.69 5.52 0.89 0.93

Table III. Extracted L and C values for different bent

angles

(degree) fc (GHz) f0 (GHz) C (pF) L (nH)

0o 5.83 6.74 0.811 0.688

20o 5.88 6.79 0.814 0.675

40o 6.04 6.86 0.91 0.591

60o 6.37 6.95 1.303 0.403

CfL

2

0241

(2)

Where g1 is given by the prototype value of the

Butterworth-type low-pass filter, c is the cutoff

frequency of the low-pass filter and 0 is the

resonance angular frequency of the parallel LC

resonator. The dumbbell-shaped pattern DGS in [1] is

deformed as plotted in Fig 2. The length of the

narrow etched area in backside metallic ground plane

is increased. In order to analyze the characteristics of

the deformed DGS, S-parameters corresponding to

different dimensions are simulated using 3-D HFSS.

The substrate for simulations was an RT/Duroid 6010

with 50-mil-thick and a dielectric constantr of 10.2.

The L and C values extracted the EM simulation

results are listed in Table I. It can be seen from table I

that increasing the length of the narrow gap can

effectively increase the effective capacitance C, and

the value of L decrease relatively due to shrinking

wide gap area. However, both L and C would

decrease as 1 approach 0, which means a uniform

slot line. If the values of , g and 1 for the DGS

dimensions are fixed to be 5mm, 1.25mm and 0.2mm

respectively, the extracted L/C values corresponding

to different w values are also given in Table. II, of

which the results reveal that larger wide gap area

corresponds to larger effective inductance L. The

deformed DGS in Fig.2 is bent at the middle by an

angle as plotted in Fig. 4. DGSs with same dimension

but different bent angles are investigated, and Fig. 4

show the EM simulation results for the bent DGS

with 1 = w = 2mm, 2 = 3mm, g = 0.2mm. It can be

shown that bent angle has little influence on f0, but

larger will cause narrower stopband bandwidth.

w

g

1

2

1

gw

Fig 3: Schematic of the deformed DGS bent at the

遠東學報第二十五卷第三期

405

middle point.

-40

0-45

|S|,

|S|(

dB)

-15

11

-35

-30

21 -20

-25

0

-10

-5

Frequency(GHz)21 3 54 6 1087 9

2

1

= 0= 30= 60o

o

o

wg

Fig 4: Simulated S-parameters for the bent DGS with

different bent angles, where 1 = w = 2mm, 2

= 3mm, g = 0.2mm.

If we let w = g = 0.2mm and = 2 = 5mm, the

DGS becomes a uniform bent slot line, of which the

extracted L/C values for different bent angles are

listed in Table III. The results show that larger bent

angle will bring about larger effective capacitance

and lower effective inductance.

III. Characteristic For Central

Jointed DGSBased on the discussion in section II, it is an

intuitive idea that several bent slot lines can be joined

together at the central point to form a DGS with

multi- transmission zeros. In order to verify this idea,

we join two uniform slot lines with different lengths

as plot in Fig. 5 to form the DGS. When a =

7.05mm, b = 7.758mm, = 45o, the simulated

S-parameters for individual and combined one are

shown in Fig. 6.

Microstrip line(50 )

DGS patternon groundplane

slot a

ab

slot b

Fig 5: Schematic of the DGS with two joint slot lines.

-40

-450

|S11|,

|S21|(

dB)

-30

-35

-15

-20

-25

0

-5

-10

Frequency(GHz)721 3 54 6 108 9

Combined

f = 5.0 GHz

f = 4.5 GHz

0a

0b

a

slot a

b

slot b

Fig 6: Simulated S-parameters for the DGS using slot

a, slot b and combined one respectively.

The self- transmission zero frequency for slot a

and slot b are f0a = 5GHz and f0b = 4.5GHz

respectively. It can be shown that the combined one

has widen stopband bandwidth and two divergent

transmission zeros located near f0a and f0b. The

resonant frequencies f0a and f0b are directly related

toa and b, thus the stopband bandwidth and

attenuation level can be adjusted by simply tuning

the lengths of slot lines. When fixing f0b = 4.5GHz

and varying f0a from 4.9GHz to 4.6GHz, it can be

seen that the minimum of the attenuation level occurs

as the distance between f0a and f0b equals to a specific

value. Influence of the divergent angleis also

investigated. The divergent anglefor the DGS with

f0b = 4.5GHz and f0a = 4.75GHz is varied from 45o to

18o, and the simulated S-parameters is plot in Fig 8.

It can be shown that as the divergent angledecrease,

遠東學報第二十五卷第三期

406

the two transmission zero would repel each other and

cause degraded attenuation capability in the passband.

This would be the ruling limitation for the number of

joint slot lines that one DGS can use.

IV. Design of Wide Stobband DgsBased on the discussion in section III, we try to

design DGS composed of three central joint slot lines

with uniform and nonuniform slot lines respectively.

Their schematic views are plot in Fig 9. They are

21|S

|(dB

)

-30

Frequency (GHz)

-354 5 6

-25

-20

-10

-15

-5

0

f = 4.7GHzf = 4.6GHz

f = 4.9GHzf = 4.8GHzf = 4.75GHz0a

0a

0a

0a

0a

Fig 7: Simulated S-parameters for the DGS with

different distances between f0a and f0b.

21|S

|(dB

)

-30

Frequency (GHz)

-354 5 6

b

= 45= 30= 18

-25

-20

-10

-15

-5 o

o

0o

slot a

a

slot b

Fig 8: Simulated S-parameters for the DGS with

divergent angles

(a)

slot cslot b

c

b

a

slot a

2c

(b)

g

wa

wb

1c

wc

2b

2a

1a

1b

Fig 9: Schematic views of two types of DGS with

three slot lines.

Designed on an RT/Duroid 6010 with 50-mil-thick

and a dielectric constant r of 10.2. The stopband

center frequency is set to be 5GHz. For the DGS plot

in Fig. 9(a), the attenuation poles in the stopband can

be tuned simply by varying the lengths of slot line a,

|S11

|,|S

21|(

dB)

-30

Frequency (GHz)4

-35g

5 6

slot aslot c

-25

-20

b

c a

slot b

-10

-15

-511|S |(dB)

0

3 slots(Mea.)

3 slots(Sim.)

1 slot

Fig 10: Comparison of simulated and measured S-

parameters between the DGSs with three

joint slot lines and the one has only one slot

line. (a = 6.604 mm, b = 6.856mm, c =

7.778mm, g = 0.2mm, = 45o)

遠東學報第二十五卷第三期

407

|S11

|,|S

21|(

dB)

Frequency (GHz)

g-30

-354 5

-251c

-20wc

2b

2c1a

2a

wb

wa1b

-10

-15

-5 |S |(dB)11

6

0

1 slot3 slots(Sim.)3 slots(Mea.)

Fig 11: Comparison of simulated and measured

S-parameters between the DGSs with three

joint slot lines and the one has only one slot

line. (2a = 3.19 mm, 2b = 2.73mm, 2c =

2.4mm, g = 0.2mm, = 45o)

b and c Before joined together, their lengths are

trimmed to located attenuation poles at 4.5GHz,

5.0GHz and 5.5 GHz individually. After jointed

together at the central points, their lengths are fine

tuned to let the attenuation level better than –15dB

through the stopband frequency. The simulation and

measured results compared with the results of DGS

with only one slot line are shown in Fig 10 and the

circuit dimensions are listed in the figure caption.

The simulation and measured results agree very well.

It can be seen that the DGS has three

transmission zeros and the stopband bandwidth can

be more than three times that of the DGS with only

one slot line at the attenuation level equals to be

–15dB. Based on the analysis in section II, the

uniform slot lines can be substituted by nonuniform

slot lines for miniature circuit size. Consider the DGS

shown in Fig. 9(b), square wide areas are used, and

wa, wb, wc, 1a, 1b and 1c are fixed to be 2mm.

Length 2a, 2b, and

(a)

(b)

Fig 12: Layouts of the lowpass filters. (a) Composed

of DGS unit cells with only one slot. (lb =

6.856mm, la = lc = 0, g = 0.2mm) (b)

Composed of DGS unit cells with three slots.

(la = 6.604 mm, lb = 6.856mm, lc = 7.778mm,

g = 0.2mm)

-26.15 dB

0.58 GHz

11|S | (dB)|S | (dB)21

2111

|S|,

|S|(

dB)

Frequency(GHz)

10

0

-10

-20

-30

-40

-50

10987654321-60

0

(a)

遠東學報第二十五卷第三期

408

1.79 GHz

-26.15 dB

0-60

1 2 3 4 5 6 7 8 9 10

-50

-40

-30

-20

-10

0

10

Frequency(GHz)

|S|,

|S|(

dB)

1121

21|S | (dB)|S | (dB)11

(b)

Fig 13: (a) Frequency responses of the lowpass filter

in Fig. 12 (a). (b) Frequency responses of

the lowpass filter in Fig. 12(b).

2c can be used to determine the locations of the three

transmission zeros respectively. Similar to the

approach used for the case with uniform joint slot

line, the DGS with attenuation level better than

–15dB was simulated and fabricated, and the results

are plot in Fig. 11. It can be shown that the DGS

(a)

11|S | (dB)|S | (dB)21

2111

|S|,

|S|(

dB)

Frequency(GHz)

10

0

-10

-20

-30

-40

-50

-60

-7015131197531

f0 02f 3f0 04f 5f0 06f

(b)

Fig 14: (a) Layout of the parallel coupled microstrip

bnadpass filter. (b) Frequency responses of (a).

possess three transmission zeros and bandwidth

three times that of only one slot line. The occupied

area for the DGS in Fig.9(a) is 141.837mm2, while

that in Fig.9(b) is 77.528mm2. Thus using the DGS

with nonuniform slot lines can greatly reduce the

circuit size compared with the uniform one.

V. ApplicationsLowpass filters can be formed by duplicating the

DGS cells in a line as shown in Fig. 12. The

frequency responses of them simulated by HFSS are

plot in Fig. 13. It can be found that the lowpass filter

composed of DGS cells with three central joint slot

lines can have a wider stopband bandwidth more than

three times that of the one with only one slot line at a

attenuation level of -26.15dB.

The novel DGS can also be applied to suppress the

unwanted spurious responses of microstrip bandpass

filters. Fig. 14 depict the layout of parallel coupled

microstrip bandpass filter center at f0 = 2.45GHz and

its simulated frequency responses. This kind of

bandpass filter suffers from unwanted resonances

(spurious responses) at multiples of fundamental

frequency f0. Supposed we aim at suppressing the

spurious at 4 f0 (around 9GHz). First, the dimensions

of the DGS cells must be determined. Two DGS cells

centered at 9GHz are designed. Their layouts and

responses are depicted in Fig. 15.

If the parallel coupled bandpass filter in Fig. 14(a)

is equipped with DGS cells of the type in Fig. 15(a)

both at input and output lines, one can get the

frequency responses as plot in Fig. 16(b). It can be

shown that the spurious resonance at 4f0 is obviously

suppressed to -11.35dB and the response at f0 is

遠東學報第二十五卷第三期

409

(a)

(b)

21|S | (dB)|S | (dB)11

Frequency(GHz)

|S|,

|S|(

dB)

1121

10

0

-10

-20

-30

-40

-5015131197531

(c)

1 3 5 7 9 11 13 15-50

-40

-30

-20

-10

0

10

2111

|S|,

|S|(

dB)

Frequency(GHz)

11|S | (dB)|S | (dB)21

(d)

Fig 15: (a) DGS cell with single slot. (b) DGS cell

with three slot lines. (c) Frequency responses

of (a). (d) Frequency responses of (b).

almost invariant. If the DGS cells in Fig. 16(a) are

substituted with the type in Fig. 15(b), an even better

performance can be obtained. Based on the simulated

frequency responses in Fig. 17(b), the unwanted

resonance at 4f0 can be further suppressed below

-26.5dB.

VI. ConclusionIn this paper, the improved DGSs with multi-

transmission zeros and wider stopband bandwidth are

presented. It is found that the transmission zero of the

DGS can be diverged by joining several slot lines at

the central points just below the microstrip line. The

lengths of these slot lines can be used to adjust the

locations of the individual attenuation poles, thus the

stopband bandwidth and attenuation level can be

(a)

21|S

|(dB

)

Frequency(GHz)

10

0

-10

-20

-30

-40

-50

-60

-7015131197531

-11.35 dB

Fig 16: (a) Layout of the parallel coupled bandpass

filter equipped with DGS cells. (b)

Frequency responses of (a).

(a)

遠東學報第二十五卷第三期

410

1 3 5 7 9 11 13 15-70

-60

-50

-40

-30

-20

-10

0

10

Frequency(GHz)

|S|(

dB)

21

-26.5 dB

Fig 17: (a) Layout of the parallel coupled bandpass

filter equipped with DGS cells. (b)

Frequency responses of (a).

determined directly. This novel DGS can be used to

design compact lowpass filters with enhanced

stopband performances. Besides, the unwanted

spurious responses of microstrip bandpass filters can

also be effectively suppressed by the proposed DGS.

Several circuits are designed and simulated to

validate these ideas.

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and T. Itoh, A design of the low-pass filter using

the novel microstrip defected ground structure.

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[2]Y. T. Lee, J. S. Lim, J. S. Park, D. Ahn and S. Nam,

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[3]J. S. Lim, J. S. Park, Y. T. Lee, D. Ahn and S. Nam,

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[4]I. Chang, B. Lee, Design of defected ground

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