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Novel Trisection Cross-coupled Filter Based on Mixed Split-ring Resonators

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Progress In Electromagnetics Research Symposium, Hangzhou, China, March 24-28, 2008 37

Novel Trisection Cross-coupled Filter Based on Mixed Split-ring

Resonators
Bian Wu1 , Zheng-Zheng Hou2 , and Chang-Hong Liang1
1
National Laboratory of Antennas and Microwave Technology, Xidian University, Xi’an 710071, China
2
College of Information and Electronic Engineering, Zhejiang Gongshang University
Hangzhou 310018, China

Abstract— This paper presents a novel trisection cross-coupled filter by using defected split-
ring resonator combined with the conventional microstrip split-ring resonators. Defected split-
ring can be used as an independent resonator, which has a coupling with two microstrip split-ring
resonators, considering the existing coupling between the two microstrip rings, a trisection cross-
coupled filter with a pair of transmission zeros is then obtained. Such a practical filter which
operates at 1.4 GHz is successfully designed and fabricated, compared with the conventional
trisection microstrip split-ring filter, this filter has a more compact structure and a symmetrical
frequency response with a pair of transmission zeros.

1. INTRODUCTION
Recent development in wireless communication system has created a need of bandpass filter with
low insert loss, high out-of-band suppression and compact size. Cross-coupled filters are widely in-
vestigated because they introduce one or more additional coupling between nonadjacent resonators
and creates finite transmission zeros out of band [1, 2]. Microstrip open-loop or split-ring is one of
the most popular resonant units that be used, it is equivalent to a half-wavelength resonator, the
coupling between two split-rings can be electric, magnetic or mixed depending on the orientation
of the resonators, and the input/output coupling can be realized by strip-gap or tapped line. This
structure can be easily applied to the exact filter design using classical theories of coupled resonator
circuits [3–5]. In recent years, there has been an increasing interest in the planar filter design with
defected ground structure due to its compact size and band-gap property [6–8].
In this paper, both microstrip split-ring and defected split-ring are applied to the design of a
mixed split-ring cross-coupled filter. First, the resonant property of defected split-ring is studied
and compared with that of the microstrip split-ring. Then the defected split-ring is loaded at
the bottom of the two-section microstrip split-ring filter, which have a mutual coupling with each
other, and a bandpass filter with a pair of transmission zeros is obtained. At last, such a kind of
trisection mixed split-ring cross-coupled filter which has a center frequency of 1.4 GHz is fabricated,
the experimental results agree well with the simulation that validates the presented method.
2. RESONANT PROPERTY OF DEFECTED SPLIT-RING CELL
In the previous work, the defected split-ring has been proposed and used to construct lowpass filter

Microstrip Split-ring Feed Structure Split-ring DGS Feed Structure

(a) (b)

Figure 1: (a) Microstrip split-ring unit, (b) Defected split-ring unit.

and suppress the harmonic effect. Here the defected split-ring is considered as an independent
resonant unit, and its resonant property is studied.
38 PIERS Proceedings, Hangzhou, China, March 24-28, 2008

As shown in Fig. 1, the defected split-ring is etched at the bottom of the microstrip substrate
with a relative dielectric constant of 2.65 and a thickness of 1 mm. The length and width of the ring
are a = 23 mm and b = 12 mm, respectively, and the split-gap is g = 1.2 mm, each narrow side of
the split-ring is fed by a microstrip T-shaped branch, which has a length of L = 11 mm and width
of t = 1.4 mm, and is connected with the 50 ohm conductor at the end. The difference between
(a) and (b) is that, microstrip split-ring has a capacitive gap with the T-shaped branch, but the
defected split-ring is overlapped with the T-shaped branch in two different sides of substrate.
The two different split-ring units are then analyzed with the 3D simulation software Ansoft
HFSS, we found that they have the similar resonant properties. As illustrated in Fig. 2, the
defected split-ring has a resonant peak at about 2 GHz. For the same magnitude of S21, defected
split-ring has a much wider passband than that of the microstrip split-ring.

0
a=23 mm
b=12 mm
g=1.2 mm
-10 t=1.4 mm
L=11 mm
S21 (dB)

s=0.5 mm
-20 w=2.8 mm

Microstrip split ring

-30
Defected split ring

-40
1.6 1.8 2.0 2.2 2.4
Frequency (GHz)

Figure 2: Comparison of resonance between microstrip and defected split-ring.

3. MIXED SPLIT-RING CROSS-COUPLED FILTER

Considering the resonant property of defected split-ring, they can be used to design a novel compact
filter combined with the conventional microstrip split-ring. Fig. 3(a) shows a simple two-pole
coupled resonator split-ring filter, when a defected split-ring with the same size is etched at the
bottom of the microstrip split-ring, a novel trisection filter is formed as depicted in Fig. 3(b). The
main transmission passage is from one microstrip split-ring to the bottom defected split-ring, then
to the another above split-ring. The cross-coupled filter is obtained due to the additional coulpling
between the two microstrip split-rings.
23
1.2
1.4

12
10
23
2.8 1.4
1.1

1.4 1.8 12

(a) (b)

Figure 3: (a) Two-pole microstrip split-ring filter, (b) Trisection mixed split-ring filter.

The simulation frequency responses of the two different split-ring filter are compared in Fig. 4. It
can be found that, by adding the defected split-ring at a proper location, a trisection cross-coupled
Progress In Electromagnetics Research Symposium, Hangzhou, China, March 24-28, 2008 39

filter with good performance and a pair of transmission zeros is achieved, which has a much wider
bandwidth as well as low return loss. By adjusting the distance between two split-ring resonators,
the locations of the out-of-band transmission zeros will shift at the same time.
0

-10

-20

S parameters (dB)
-30

-40

-50 S21 of mixed split-ring

S11 of mixed split-ring
-60 S21 of microstrip split-ring
S11 of microstrip split-ring
-70
0. 6 0 .8 1.0 1. 2 1 .4 1. 6 1 .8 2. 0 2 .2 2. 4
Frequency (GHz)

Figure 4: Frequency responses of mixed split-ring filter and microstrip split-ring filter.

4. MEASUREMENT RESULTS
To validate this method, a practical mixed split-ring cross-coupled filter which operates at 1.4 GHz
is fabricated on the RF printed circuit board. The substrate has a permittivity of 2.65 and a
height of 1 mm. Fig. 5 shows the photographs of the fabricated mixed split-ring cross-coupled

Figure 5: Photographs of the fabricated mixed split-ring cross-coupled filter.

-10 S11

-20
Magnitude (dB)

-30

-40

-50 S21
Measurement
-60 Simulation

-70
0.5 1.0 1.5 2.0 2.5
Frequency (GHz)
Figure 6: Comparison of frequency responses between EM simulation and measurement.

filter, and the measured results are depicted and compared with the simulation in Fig. 6. It shows
this compact filter has a low insertion loss of no more than 1 dB around the center frequency, and
40 PIERS Proceedings, Hangzhou, China, March 24-28, 2008

a relative bandwidth of 0.14. Moreover, the measured transmission zeros appear at f 1 = 0.7 GHz
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and f 2 = 1.9 GHz, the experiment and simulation agree well with each other.
5. CONCLUSIONS
The design method of mixed split-ring cross-coupled filter is proposed in this paper. The defected
split-ring is found to have the similar resonant property as the microstrip split-ring, by combin-
ing the two kinds of split-rings, a novel trisection mixed split-ring filter is successfully designed.
Compared with the conventional trisection microstrip split-ring filter, the new filter has a broad
bandwidth, compact size and a pair of out-of-band transmission zeros, which improve the filter
performance at various aspects. This kind of filter may find its applications in the miniaturized
planar microstrip circuit design.
ACKNOWLEDGMENT
This work is supported by the NSFC under project No. 60501023.
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