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8/3/2019 th1f-5_DGS_LPF
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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
-30
-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)
-50
-40
-30
-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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-30
-20
-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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EM results
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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.
-50
-40
-30
-20-10
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-
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[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
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[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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EM results