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Compact Microstrip Lowpass Filter Based on Defected Ground

Structure and Compensated Microstrip Line

JiaLin Li , JianXin Chen , Quan Xue , JianPeng Wang , Wei Shao , and LiangJin Xue

Institute of Applied Physics, University of Electronic Science and Technology of China, ChengDu

610054, P. R. China

Department of Electronic Engineering, City University of Hong Kong, Tat Chee 83, Kowloon, Hong Kong

Abstract An improved defected ground structure (DGS)with compensated microstrip line is investigated for lowpassfilter (LPF) applications. With this structure, the basic resonantelement exhibits the elliptic-function lowpass responses. The useof introduced resonant elements allows sharp cutoff frequencyresponse and high harmonic suppressions together with smallsize to be obtained with less number of periodic structures. An

equivalent lumped L-C circuit model is presented and itscorresponding L-C parameters are also extracted by usingparametric relationships. Based on the equivalent circuit model,a 3-pole LPF, using 3 DGS units cascaded, is optimally designedand implemented; measurements show good consistency withcalculations.

Index Terms Low-pass filters, microstrip, microwavefilters, resonators.

I. INTRODUCTIONIn modern wireless communication systems, compact size

and high performance filters are commonly required to reduce

the cost and enhance system performances. Recently, the

defected ground structure (DGS) for microstrip lines [1]-[4] or

coplanar waveguide (CPW) [5]-[6], such as various photonicbandgap (PBG) structures [7]-[8], has become one of the most

interesting areas of reseach owing to their extensive

applicability in microwave circuits. DGS, which is realized by

etching off a defected pattern from the backside metallic

ground plane and has periodic structures, has been known as

providing rejection of certain frequency band, namely,

bandgap effects. Therefore, a direct application of such

frequency selective characteristics is in microwave filters [1]-

[9]; many passive and active microwave circuits have been

developed by using DGS or PBG patterns to suppress

harmonics and/or realize the compact size [1]-[10].

In this paper, we report a recent investigation into

microstrip periodic structures with resonant elements in theground plane for lowpass filter (LPF) applications. The

introduced etched defect pattern is an improved configuration

from [1]-[3] and can effectively disturb the shield current

distribution in the ground plane of microstrip line. This

disturbance greatly changes the characteristics of the

microstrip line such as line inductance L and capacitance C.

With this structure, the basic resonant element exhibits the

elliptic-function lowpass responses; moreover, owing to the

ZP

Zc c

L

L0

Z0 0 Zc c Z0 0

Z ZP

L0

C0 C0

(a)

-90

-75

-60

-45

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-15

0

0 1 2 3 4 5 6 7 8Frequency (GHz)

Magnitude(dB)

(b)

Fig. 1. (a) Circuit model for a transmission line with periodicallyloaded lumped elements. (b) Typical frequency responses with oneelliptic-function model unit.

increased equivalent inductance, capacitance and slow-wave

effects, the required area for the investigated DGS is much

smaller than that of the dumbbell-shaped DGS [1]-[3] for the

same resonant frequency. An equivalent lumped L-Cnetwork

has been proposed to model the introduced DGS unit; and its

corresponding L-C parameters have also been extracted.Comparison between EM-simulations on the DGS circuits and

circuit simulations on its equivalent networks has been shown

the validity of the proposed equivalent circuit model. The use

of proposed resonant elements allows larger attenuation in the

stopband and higher harmonic suppressions to be obtained

with less number of periodic structures as compared to the

conventional DGS. Also, by using the proposed equivalent

circuit model, a harmonic rejection LPF has been optimally

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designed and implemented. Simulated and experimental

results for the fabricated filter at 3GHz cutoff frequency are

presented to demonstrate the idea.

II. CIRCUIT MODEL AND ITS IMPLEMENTATIONIn our study, a circuit model for a transmission line with

periodically loaded lumped elements is adopted as shown in

Fig. 1(a), where Z is the impedance of lumped elements, Zc

and c are the characteristic impedance and propagation

constant of transmission line with the periodP; andZo and o

are the feed line`s characteristic impedance and propagation

constant, respectively.

The series impedance Z can result from different type of

reactive element; a single inductor or a parallel LCresonator

may be the simplest topology. However, a filter with its

attenuation poles at finite frequencies, namely high selectivity,

would be preferable owing to the ever-increasing demand for

currently expanding communication systems within finite

spectrum resources. And the criterion can be fulfilled by

employing filters with elliptic-function responses. Thus, herewe consider this type of filters and it is contrived by using

microstrip DGS patterns. Fig. 1(b) illustrates the typical

frequency responses of the elliptic-function filters; the

transmission zero close to the passband and sharp cutoff

frequency characteristics have effectively improved the

frequency selectivity. By changing the inductance L, L0 or

capacitance C0, the frequency responses can be changed

easily. Here we optimize the values ofL, L0 and C0 being

3nH, 1.5nH and 1pF, respetively; a LPF with its cutoff

frequency at 3GHz and transmission zero adjacent to 4GHz is

implemented, seen Fig. 1(b).

To realize the above structures in microstrip DGS patterns,

we investigate a square open-loop with a slot in middlesection. Fig. 2(a) shows the square open-loop etched off on the

backside metallic ground plane. The DGS shape with its

dimensions is illustrated in Fig. 2(b). And Fig. 3 shows the

presented equivalent circuit model; whereL0 and C0 denote

the inductance and capacitance on the narrow slot DGS region

with its width d, whereas L1 and C1 describe the influence

resulting from the fringing field around the open-loop. For

more accurately modeling the DGS section, a capacitance C2

should be considered as a part of the equivalent circuit

models, which is due to the relatively large fringing field

distribution at the discontinuity area with its separation g. In

our study, considering the transmission line and the dielectric

substrate are lossless; and actually, the losses are so small asto may be ignored. The equivalent network of the DGS unit

may be described asZDGS, as shown in Fig. 3.

In order to derive the equivalent network parameters, the S-

parameters of a DGS unit at the reference plane should be

calculated using EM-simulation. And then, by using the

relationship between the S-parameter and ABCD-matrix, the

equivalent network parameters can be extracted [1].

(a)

(b)

Fig. 2. (a) 3-Dimensional view of the investigated DGS unitsection. (b) The DGS shape with its dimensions.

L0

C0

Z0 00 Z0L1 L1

C1 C1C2

ZDGS Fig. 3. The presented equivalent circuit model of a DGS unit.

To confirm the validity of the presented equivalent model, a

DGS unit, shown in Fig. 4(a), has been simulated using

Ensemble, a full-wave EM-simulator based on the Method ofMoment (MoM). The substrate for simulation has a relative

dielectric constant r of 9.6 and a thickness H of 0.8mm; the

dimensions of the DGS section, shown in Fig. 2(b), are as

follows: a=7.0mm, b1=3.2mm, b2=5.8mm, d=0.2mm, and

g=0.2mm. The corresponding equivalent network is illustrated

in Fig. 4(b); where ZDGS is depicted in Fig. 3. By extracting

the values of lumped L-C elements, the equivalent network

Ground Plane

Microstrip Line

DGS Section in the

Ground Plane

Dielectric Substrate

a

a

d

b1

g

b2

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(a)

p: 10mmw: 1.2mm

ZDGS

p: 10mmw: 1.2mm

(b)

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-20

-10

0

0 1 2 3 4 5 6 7 8

Frequency (GHz)

Magnitude(dB)

(c)

Fig. 4. (a) A DGS unit for modeling. (b) Extracted equivalentnetwork. (c) Comparison between EM-simulations with one DGSunit and circuit simulations on its equivalent model.

parameters are:L0=2.6283nH, C0=0.115pF, L1=4.6012nH,

C1=0.4452pF, and C2=31.2453pF. As shown in Fig. 4(c), the

simulation results, using EM and extracted equivalent network

method respectively, are illustrated a good consistency

between them.

For comparison, Fig. 5(a) shows a conventional dumbbell

DGS unit [1]-[3]; where the feed lines with its width 1.2mm

and length 10mm are the same as the investigated DGS unit;

(a)

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-10

0

0 1 2 3 4 5 6 7 8

Frequency (GHz)

Magnitude(dB)

(b)

Fig. 5. A comparison between the conventional DGS andinvestigated DGS. (a) Conventional DGS with dumbbell shape. (b)Simulated frequency responses.

both frequency responses are plotted in Fig. 5(b). As shown in

Fig. 5(b), with the same resonant frequency, the investigated

DGS can provide better frequency responses in the passband

and steeper cutoff frequency responses in the stopband than

that of the traditional DGS; meanwhile, the size for proposedDGS unit is only 7mm by 7mm, whereas it is 7.7mm by

16.6mm for traditional DGS unit. Thus the introduced DGS

has not only high selectivity but also compact size.

III. OPTIMIZATION AND DESIGN DGSLPFSBased on the equivalent circuit model mentioned above, a

3-pole lowpass filter, using 3 DGS units cascaded, has been

optimally designed, shown in Fig. 6(a); where the feed lines

are set to the 50 characteristic impedance micrstrip line with

its width W=0.76mm and length P=10mm for input/output

matching. Hence, in this case the values of equivalent lumped

L-C elements should be optimally varied since the L-C parameters in Fig. 4(b) are extracted on the basis of 40-

microstrip feed lines (1.2mm for this type of substrate).

p: 10mmw: 0.76mm

ZDGS

p: 3.0mmw: 1.2mm

ZDGS

p: 3.0mmw: 1.2mm

ZDGS

p: 10mmw: 0.76mm

(a)

(b)

Fig. 6. (a) Schematic of the optimized LPF using 3 DGS unitscascaded equivalent circuits. (b) Layout of the optimized LPF with 3DGS units.

17mm

1.2mm

10mm

27mm

10mmr: 9.6H: 0.8mm

1.2mm7.7mm

7.7mm

10mm

27.7mm

27mm0.6mm

47mm

17mm

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After optimally designing, an equivalent lumped L-C

elliptic-function LPF has been realized; the filter has its cutoff

frequency at 3GHz and the first transmission zero at about

4GHz. The optimized parameters forZDGS, referred to Fig. 3,

are as follows: L0=4.06441nH, C0=1.14937pF,

L1=1.85713nH, C1=0.34196pF, and C2=35.3635pF; the

separation between two adjacent resonators is 3.0mm, and the

width of compensated microstrip line is 1.2mm. Using the unit

DGS pattern seen in Fig. 2(b), we have implemented and

optimized the 3-pole DGS LPF. Fig. 6(b) shows the layout of

the optimized LPF with 3 DGS units cascaded; the overall

length including the 50-microstrip feed lines is 47mm.

Simulations on the optimized LPF are plotted in Fig. 7. As

shown in Fig. 7, both circuit and EM simulations on the DGS

LPF are demonstrated the optimum performances in the

passband and the stopband.

-70-60-50-40-30

-20-10

0

0 1 2 3 4 5 6 7 8Frequency (GHz)

Magnitude(dB)

Fig. 7. Circuit and EM simulations on the optimized LPF.

IV.EXPERIMENTAL RESULTS

The optimized DGS LPF was fabricated on a substrate with

a relative dielectric constant r of 9.6 and a thickness H of

0.8mm. Measurements were carried out on an HP8722Dnetwork analyzer. Fig. 8 illustrates the measurements on the

fabricated DGS LPF. One can see from Fig. 7 and Fig. 8, the

experimental results show good consistency with simulated

ones. The fabricated LPF has a 3dB cutoff frequency at

3.12GHz and suppression levels are 37dB approximately from

3.85 to 6.9GHz; the insertion loss in the passband is about

0.85dB. The conductor loss and non-ideal microstrip/coaxial

line transitions contribute to the higher insertion loss in the

measurement than that in the simulation.

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-30

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0

0 1 2 3 4 5 6 7 8

Frequency (GHz)

Magnitude(

dB)

Fig. 8. Measured performances on the fabricated LPF.

V.CONCLUSION

In this paper, we have investigated a square open-loop DGS

pattern for microstrip lowpass filter applications. And an

equivalent lumped L-Cnetwork has been presented to model

the introduced DGS unit; by using parametric relationships,

the values of lumpedL-Celements for the DGS unit have also

been extracted. Based on the equivalent circuit model, a 3-

pole LPF has been optimally designed and then implementedon the microstrip line. For demonstration, the filter has been

fabricated and the measurements show good consistency with

the simulations. The compact size, sharp cutoff frequency

response and high harmonic suppressions would make the

introduced DGS pattern to meet the requirements of modern

wireless communication systems.

REFERENCES

[1] D. Ahn, J.-S. Kim, C.-S. Kim, J. Qian, Y. X. Qian and T. Itoh, A design of the low-pass filter using the novel microstripdefected ground structure, IEEE Trans. Microwave Theory &Tech., vol. 49, no. 1, pp. 86-92, January 2001.

[2] J.-S. Lim, C.-S. Kim, Y.-T. Lee, D. Ahn and S. Nam, Designof lowpass filters using defected ground structure andcompensated microstrip line, Electronics Letters, vol. 38, no.22, pp. 1357-1358, October 2002.

[3] H. W. Liu, Z.-F. Li, X.-W. Sun and J.-F. Mao, An improved 1-D periodic defected ground structure for microstrip line, IEEEmicrowave and Wireless Component Letters, vol. 14, no. 4, pp.180-182, April 2004.

[4] C. Caloz and T. Itoh, A super-compact super-broadband tapereduniplanar PBG structure for microwave and millimeter-waveapplications, 2002 IEEE MTT-S Int. Microwave Symp. Dig.,

pp. 1157-1160, June 2002.[5] J.-S. Lim, C.-S. Kim, Y.-T. Lee, D. Ahn and S. Nam, A spiral-

shaped defected ground structure for coplanar waveguide,IEEE microwave and Wireless Component Letters, vol. 12, no.

9, pp. 330-332, September 2002.[6] F. Martin, F. Falcone, J. Bonache, R. Marques and M. Sorolla,

Miniaturized coplanar waveguide stop band filters based onmultiple tuned split ring resonators, IEEE microwave andWireless Component Letters, vol. 13, no. 12, pp. 511-513,December 2003.

[7] Q. Xue, K. M. Shum and C. H. Chan, Novel 1-D microstripPBG cells,IEEE microwave and Wireless Component Letters,vol. 10, no. 10, pp. 403-405, October 2000.

[8] X. S. Rao, L. F. Chen, C. Y. Tan, J. Liu and C. K. Ong, Designof one-dimensional microstrip bandstop filters with continuous

patterns based on Fourier transform,Electronics Letters, vol.39, no. 1, pp. 64-65, January 2003.

[9] J.-S. Lim, Y.-T. Lee, C.-S. Kim, D. Ahn and S. Nam, Avertically periodic defected ground structure and its application

in reducing the size of microwave circuits, IEEE microwaveand Wireless Component Letters, vol. 12, no. 12, pp. 479-481,December 2002.

[10] K. M. Shum, Q. Xue and C. H. Chan, A novel microstrip ringhybrid incorporating a PBG cell, IEEE microwave andWireless Component Letters, vol. 11, no. 6, pp. 258-260, June2001.

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