The 10th International Conference on Communications, Circuits and Systems
A Low-Pass Filter Design Based on Split-Ring Resonator with Defected Ground
Structure
Zhu Wei-gang, Wang Pei-zhang, Liu Han
Institute of Communications Engineering , The Army Engineering University of PLA
Nanjing, Jiangsu 210007 China
the clearance between the outer SRR and the main
transmission line. You can adjust the band-gap characteristics
of the filter by parameter adjustment.
Abstract—This paper explores an optimized low-pass filter
which is based on the split-ring resonator (SRR) with a defected
ground structure (DGS). By means of the two open stubs loaded
on the main transmission line, the stop band of the filter can be
expanded and the frequency selectivity of the filter can be
improved. Thanks to the small size and simple construction, this
type of filter can be widely applied in wireless communication
fields.
Keywords-SRR; DGS; low-pass filter; stop band; optimized
design
I.
INTRODUCTION
In the field of radio frequency circuit development, the
micro-strip filter has a very wide application because of its
small size and easiness to be integrated into active circuit
components. Some new filter design technologies emerging
in recent years such as Defected Ground Structure (DGS)[12], Split-Ring Resonator (SRR)[3-5], and Substrate Integrated
Waveguide (SIW)[6] have facilitated the development of
filters.
In DGS filter, by printing a defected pattern on the metal
grounding plate, the current distribution on the grounding
plate can be changed and the equivalent inductors and
capacitors in the transmission line is thus changed. The filter
therefore obtains the slow wave characteristics and band gap
characteristics. With the help of a single DGS cell, the central
frequency of the band gap is decided.
In addition, the SRR-based filter design has had more and
more extensive applications due to its reduced resonator size
and better resonance suppression performance. Martin and
others came up with the idea of a complementary split-ring
resonator, which is, to construct and develop a defected
ground structure which is complementary to the slip-ring
resonator and applied it to the design of micro-wave
components.
This paper studies a small-sized filter design which is
based on the SRR DGS structure. This design is able to
optimize the frequency selectivity of the filter and improve
stop band by means of adding two symmetrical open stubs.
Figure 1. Basic structure of SRR filter
Likewise, the band-gap characteristics can be obtained by
printing an SRR on the grounding plate of the micro-strip
transmission line. Fig. 2 below shows the basic structure of
the micro-strip SRR DGS filter. It can be seen from the figure
that an SRR is printed on the bottom substrate. The working
principle of the SRR is coupling a parallel resonance circuit
on the main transmission line by means of an equivalent
capacitor and thus produces the notch effect at its resonance
frequency. The characteristic of the SRR is similar to a dumb
bell structure. Since the SRR structure has elliptic filter
function which has transmission zeros, the filter thus has a
steeper transitional band and a more smooth pass band. All
this makes this SRR structure ideal for the design of low-pass
filters[7-8].
II. ANALYSIS OF THE SRR DGS-FILTER PERFORMANCE
A. Basic Structure of the Micro-strip SRR DGS Filter
As a rule, the band-gap characteristics can be achieved by
loading a slip-ring resonator on one side of the micro-strip
transmission line. Fig. 1 shows two nested slip-ring
resonators, wherein a means the length of the SRR, g stands
for its gap width, t is the clearance between two SRRs, s is
978-1-5386-9389-6/18/$31.00 ©2018 IEEE
Figure 2. Basic structure of SRR DGS filter
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The solid line shown in the figure 4 is |S21 | curve and the
broken line is |S11 | curve. It can be seen that although the
transitional band of the low-pass filter steeps rather quickly,
its stop-band is not that smooth and doesn’t have the
sufficient attenuation. For frequency ranges above 3 GHz, the
stop-band attenuation is only 5dB, which makes the SRR not
good for the low-pass filter.
From other hand, Fig. 2 shows that the 50 ohm microstrip transmission line has certain series inductance effect and
the SRR on the bottom substrate constitutes a parallel
resonance circuit. Printing the SRR on the metal bottom
substrate introduces a capacitive coupler between the main
transmission line and the parallel resonance circuit. This
structure is thus equivalent to the circuit shown in Fig. 3.
L2
L3
B. Improved SRR DGS Filter Design
In addition to the steep band-gap characteristic, the SRR
also has a pretty smooth low-pass band. On this basis,
however, the low-pass filter performance is still not satisfying
because of the poor stop-band performance.
To overcome the defect resulted from the structure shown
in filter Fig. 1 and further improve the stop-band attenuation
of this basic SRR DGS structure, it’s suggested that we add
two open stubs symmetrically at both ends of the main
transmission line to introduce a parallel capacitor to ground
and improve the stop-band performance of the low-pass filter.
The improved SRR DGS is shown in Fig. 5
C2
L1
C1
Figure 3. Equivalent circuit of basic SRR DGS filter
According to Fig. 3, the impedance of the parallel circuit
L1-C1 is as follows:
Ζ1 =
1
𝑗𝜔𝐶1 +
(1)
1
𝑗𝜔𝐿1
Wherein, the impedance of the single capacitor C2 is as
follows:
Ζ2 =
1
(2)
𝑗𝜔𝐶2
When Z1+Z2=0, the parallel branch resonates and its
resonance frequency is as follows:
𝑓𝑠 =
1
Figure 5. Dimensions of SRR DGS filter
(3)
2𝜋√𝐿1 (𝐶1 +𝐶2)
Its equivalent circuit is similar to what’s shown in Fig. 3,
wherein the 50 ohm micro-strip transmission line has certain
series inductance effect which is represented by L2 and L3;
the two equivalent capacitors on the two loaded open stubs
are represented by C3 and C4, which is the biggest difference
from the structure given in Fig. 2. The SRR on the bottom
substrate constitutes a parallel resonance circuit, which is
represented by the parallel resonance circuit of L1 and C1.
Printing the SRR on the metal bottom substrate introduces a
capacitive coupler C2 between the main transmission line and
the parallel resonance circuit.
When the parallel circuit resonates, a zero position
appears at the transmission function.
The filter designed in this paper has a cut-off frequency of
2.5 GHz. Its dimentional size is shown in Fig. 2, wherein L1
stands for the width of this resonator, T is the separation
distance of the two SRR, g is the gap width of the SRR, and
W2 is the width of the main transmission line. The dielectric
substrate thickness of the micro-strip is 1.5 mm and its
dielectric constant is 2.65. We simulated the structure with
the HFSS software and obtained the following transmission
characteristic curves, see Fig 4
XY Plot 1
0.00
L2
L3
HFSSDesign1
Curve Info
dB(S(1,1))
Setup1 : Sweep
dB(S(2,1))
Setup1 : Sweep
-5.00
C2
-10.00
C3
-15.00
C4
Y1
-20.00
L1
C1
-25.00
-30.00
-35.00
r
-40.00
1.00
2.00
3.00
Freq [GHz]
4.00
5.00
6.00
Figure 6. Equivalent circuit of SRR DGS
Figure 4. Simulation curves of the basic SRR DGS filter
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The biggest difference of this equivalent circuit from that
shown in Figure 3 is that there are two capacitors C3 and C4
connected in parallel in the main transmission line. These
capacitors filter out the high-frequency component, which
thus largely increase the stop-band performance of the lowpass filter.
C. Simulation Results of the SRR DGS-based Filter
Based on this structure in Fig5, still we took micro-strip
substrate dielectric constant 2.65, which was 1.5 mm thick
and the width of the main transmission line was 4 mm. We
used the HFSS software to simulate the structure in Figure 5.
It can be seen from Figure 7 that after open stubs were added
symmetrically at both ends of the main transmission line, the
stop-band performance of the S11 curve was largely
improved and more smooth with better attenuation, which
makes it more ideal for the design of the low-pass filter.
One can see from the figure that the broken line is the
|S11 | curve, the return wave loss of which within the pass
band with a cut-off frequency of 2.5 GHz is less than -12dB,
while the out-of-band rejection within the very wide stopband is more than 10 dB. This cell structure has shown better
performance than that in Figure 2, but is still unsuitable for
the low-pass filter due to insufficient performance
improvement and thus requires further improvement of the
structure in Figure 5
XY Plot 2
0.00
Figure 9. Structure of fifth-level cascade SRR DGS filter
The equivalent circuit of these structures are shown in
Figure 10 and Fig 11 below:
L2
L2
C3
C7
L4
C5
L8
C8
C10
L5
L6
L1
C6
C1
C4
L8
C11
L9
L11
C10
C5
L12
L14
L15
C14
C9
L7
C12
C17
C13
L10
L13
C16
C18
C15
It can be seen from the figure that the 5-level cascaded
SRR DGS equivalent circuit is more complicated than the 3level cascaded SRR DGS, and the actual circuit size of the
former is increased accordingly. In practical application, we
can properly add the number of cascades according to the
actual performance requirements to achieve expected
performance.
Y1
-30.00
E. Simulation Result of the Five-level Cascade SRR DGS
Filter
It can be seen from the figure 12 and fig 13 below that
adding two open stubs does not influence the smoothness and
side band steepness of the low frequency pass band of the
low-pass filter. The introduction of the parallel capacitor to
ground which suppresses the high frequency and largely
improves the bandwidth of the stop band. In the design of the
cascade low-pass filter, the cell structure is identical. The
optimization was made through the adjustment of the length,
width and clearance distance of the cell structures on the
parallel open stubs. The simulation results is given in Figure
12 and Fig.13.
-35.00
-40.00
-45.00
5.00
L9
C9
Figure 11.Equivalent circuit of five-level cascade SRR DGS
HFSSDesign1
4.00
C1
L4
-25.00
Freq [GHz]
L3
C2
-20.00
3.00
L7
Figure 10. Equivalent circuit of three-level cascade SRR DGS
-15.00
2.00
L6
C4
L1
-10.00
1.00
L5
C6
C3
Curve Info
dB(S(1,1))
Setup1 : Sweep
g1='0.5mm' L='9mm' t='1mm' w1='3mm' X1='40mm' Y1='40mm'
dB(S(2,1))
Setup1 : Sweep
g1='0.5mm' L='9mm' t='1mm' w1='3mm' X1='40mm' Y1='40mm'
-5.00
L3
C2
6.00
Figure 7. Simulation curves of the SRR DGS filter with loaded open stubs
D. Cascaded SRR DGS Filter Design
To further improve the out-of-band rejection of the filter,
an improved cascaded SRR DGS with the same cell structure
is applied
This cascaded structure design is able to produce a
translation zero point outside the band in order to improve the
frequency selectivity of the SRR DGS filter and in the
meantime, the indicators of the filter inside the band will not
be negatively influenced.
Figure 8 and Figure 9 show three and fifth-level cascade
SRR DGS filters whose cell structure are the same as the
dimensions in Figure 5.
XY Plot 6
0.00
HFSSDesign1
Curve Info
dB(S(1,1))
Setup1 : Sweep
g='0.6mm'
dB(S(2,1))
Setup1 : Sweep
g='0.6mm'
-10.00
Y1
-20.00
-30.00
-40.00
-50.00
-60.00
1.00
1.50
2.00
2.50
3.00
Freq [GHz]
3.50
4.00
4.50
5.00
Figure 12. Simulation curves of the three-level cascade SRR DGS filter
Figure 8. Structure of three-level cascade SRR DGS filter
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XY Plot 6
0.00
a transmission zero point, which can largely improve the
frequency selectivity. This method is able to improve the
filter application.
HFSSDesign1
Curve Info
dB(S(1,1))
Setup1 : Sweep
w1='2.5mm'
dB(S(2,1))
Setup1 : Sweep
w1='2.5mm'
-12.50
-25.00
Y1
-37.50
REFERENCES
-50.00
-62.50
[1]
-75.00
-87.50
1.00
1.50
2.00
2.50
3.00
Freq [GHz]
3.50
4.00
4.50
5.00
[2]
Figure 13. Simulation curves of the fifth-level cascade SRR DGS filter
It can be seen from the simulation results in Figures13
that the insert losses of the filter within the band are almost
the same. The return loss is close to 20 dB. The transitional
band of the filter is steeper and its stop-band attenuation
exceeds 50 dB. This proves that the SRR DGS filter is a lowpass filter with better performance.
[3]
[4]
[5]
III. CONCLUSION
This paper explores an SRR DGS with smooth low pass
band and steep band-gap characteristics and makes analysis
of the relations between its structural dimensions and the
equivalent circuit. Besides, this paper provides a small-sized
SRR DGS filter design. This type of filter has a compact
structure and is thus easy to be developed. The poor
frequency selectivity of the basic SRR DGS filter makes it
impossible to be widely applied in the field of wireless
communications. To improve the situation, the present paper
provides a loaded open stub method and uses cascade
structure to optimize the filter design. This method generates
[6]
[7]
[8]
C.M.Tsai, S.Y.Lee, and C.C.Tsai, “Performance of a planar filter using
a feed structure”, IEEE Tran. Microw. Theory&Tech.,
2002,50(10),2362-2367
C.S.Kim, J.I.Park, A.Dal, “A novel 1-D periodic defected ground
structure for Planar Circuits.”, IEEE Microwave Guided Wave
Lett,2000,10(4),131-133
P.Gay-Balmaz, O.J.F.Martin, “Electromagnetic resonances in
individual and coupled split-ring resonators”, Journal of Applied
Physics,2002,92(5),2929-2936
H.Y.Yao, L.W.Li, Q.Wu, “Macroscopic performance analysis of metal
materials synthesized from macroscopic 2-D isotropic cross split-ring
resonator array”, Progress in Electromagnetics Research,2005,PIER
51,197-217
J.Bonache, F.Martin, F.Falcone, “Application of complementary splitring resonators to the design of compact narrow band-pass structures in
micro-strip technology”, Microwave and Optical Technology
Letters,2005,46(5),508-512
Dominic Deslandes, W.Ke, “Integrated micro-strip and rectangular
waveguide in planar form”, IEEE Microwave Wireless Components
Letters,2001,11(2),67-70
D.Ahn, J.S.Park, C.S.Kim, “A Design of the Low-Pass Filter Using the
novel Micro-strip Defected Ground Structure”, IEEE Trans. Microwave
Theory Tech.,1999,49(1),86-93
A.Abdel-Rahman, A.K.Verma, A.Boutejdar, “Compact Stub Type
Micro-strip Bandpass Filter Using Defected Ground Plane”, IEEE
Microwave and Wireless Components Letters,2004,14(4),136-138
125
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