A novel dual mode dual band bandpass

IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 19, NO. 8, AUGUST 2009

497

A Novel Dual-Mode Dual-Band Bandpass
Filter Based on a Single Ring Resonator
Sha Luo, Student Member, IEEE, and Lei Zhu, Senior Member, IEEE

Abstract—A dual-mode dual-band bandpass filter with two
transmission poles in both passbands using a single ring resonator
is proposed. Two excited ports are placed at the 135 -separated
positions along the ring resonator and coupled with the ring
via parallel-coupled lines, leading to synchronous excitation of
two transmission poles in dual passbands. After the principle
of this initial filter is described, an improved ring resonator
with periodic loading of open-circuited stubs is constructed and
studied to achieve compact size and adjustable spacing between
the two passbands. Finally, a dual-band ring resonator filter with
center frequencies at 2.4 and 5.8 GHz is designed and fabricated.
Measured results verify the design principle.
Index Terms—Bandpass filter (BPF), dual-band, dual-mode,
open-circuited stubs, ring resonator.

I. INTRODUCTION

M

ICROSTRIP ring resonators have been widely used in
designing microwave components, such as antennas,
bandpass filters (BPFs), baluns, couplers, mixers and oscillators
[1]. In 1972, Wolff firstly reported that there were two degenerate modes coexisting at the two resonant frequencies [2].

These two modes can be split by disturbing the symmetry of a
ring resonator so that the two transmission poles in the primary
passband can be excited. To meet the requirement in the recent
development of advanced multi-band wireless systems, there is
high demand to explore various dual-band BPFs. In particular,
the dual-band filters based on the dual-mode ring resonator
[3]–[7] have been attracting much attention in the recent years
due to their compact size and good roll-off skirt. In this aspect,
a dual-band filter is constructed in [3] using the first and second
resonant modes of a stepped-impedance ring resonator, but it
fails to generate two transmission poles in the second passband.
In [4]–[7], two dissimilar ring resonators with different shapes
or diameters are properly formed in a single- or two-layer
substrate. In this case, the dual passbands with two poles in
each individual band are realized by virtue to two different sets
of two degenerate modes in two individual ring resonators.
To our best knowledge, there has been no reported work that
implements a dual-band filter with two transmission poles in
both passbands using a single ring resonator.
In this paper, a novel dual-mode dual-band BPF with two
transmission poles in two passbands is designed based on a

Manuscript received March 01, 2009; revised April 13, 2009. First published
July 28, 2009; current version published August 07, 2009.
The authors are with the School of Electrical and Electronic Engineering,
Nanyang Technological University, Singapore 639798 (email: luos0002@ntu.
edu.sg; ).
Digital Object Identifier 10.1109/LMWC.2009.2024826

Fig. 1. Proposed dual-mode dual-band BPF using a single uniform ring resonator. (a) Schematic. (b) S-parameters versus electrical length ( ) with Z =

30
, Z = 73
, Z = 108
and Z
= 30
.

single microstrip ring resonator on a single-layer substrate. As
shown in Fig. 1(a), the two excited ports are placed along the
ring with a separation of 135 and they are capacitively coupled
to this ring via parallel-coupled lines. The remaining parts of
this work describe the principle of the proposed ring resonator
dual-band filter and demonstrate its dual-band performance via
an equivalent circuit model. Finally, a compact dual-BPF with
periodically loading of opened stubs is designed for 2.4/5.8 GHz
wireless local area network applications, and the predicted results are confirmed experimentally.
II. PRINCIPLE AND ANALYSIS OF THE PROPOSED RING FILTER
Fig. 1(a) depicts the schematic of the proposed dual-mode
dual-band microstrip ring resonator, where
is the input and
and
are the inner and outer radii
output port impedance,
is the characteristic impedance of the ring.
of this ring, and
In our design, the parallel-coupled lines are one quarter of the
and a spacing of
length of the ring, with a width of or
. As illustrated in Fig. 2(a), a three-port parallel-coupled line
, a voltage transcan be treated as a capacitive impedance

former with turns ratio
and two parallel-connected lines at
and
denote the evenport 2 and 3 as discussed in [8].

1531-1309/$25.00 © 2009 IEEE


498

IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 19, NO. 8, AUGUST 2009

Fig. 2. (a) Equivalent-circuit diagram of three-port parallel-coupled lines. (b)
Complete equivalent-circuit model for the filter in Fig. 1(a). (c) Normalized
frequencies of the two poles (f =f and f =f ) in the first passband
versus spacing (s) with Z = 30
, Z = 73
. Substrate: " = 10:8 and
thickness = 1:27 mm.

and odd-mode characteristic impedances of this parallel-coupled line, while is their effective electrical length. Follow the
work in [8], the relationship between all the element parameters
of the two networks in Fig. 2(a) can be derived as
(1a)
(1b)
As such, the equivalent-circuit model of the filter in Fig. 1(a)
can be derived as shown in Fig. 2(b). Fig. 1(b) plots its simulated
-parameters versus electrical length
. As can be observed,
the first and second passbands with two transmission poles at

and
, respectively.
each band appear at
and
) are symmetrically located at
The two lower ones (
the low sides of the two higher ones (
and
) with respect
. In addition, there exist three transmission zeros,
to
,
and
, between the two passbands. We can analyze
this proposed ring resonator filter based on Fig. 2(b).
According to the transmission theory, transmission zeros of
this ring filter occur at the frequencies where the overall mutual
of the network inside the dash square in Fig.
admittance
2(b) equals to 0, such that
(2)
and
where
By solving (2), all the zeros can be determined as

.

(3a)
(3b)

Equation (3a) determines the first and third transmission
and
while the second zero,
, is derived under
zeros,
in (3b).
Under the even- and odd-mode excitations at two ports, the
symmetrical plane in Fig. 2(b) becomes perfect magnetic wall
(M.W.) and electric wall (E.W.). Thus, its bisection becomes
a one-port network with open- and short-circuited ends at the
and
reprecentral position, respectively. In Fig. 2(b),
sent the two input admittances at the port, looking into the left
and right sides. Under the even- and odd-mode resonances, i.e.,
and
,
,
and
,
, can be determined. Fig. 2(c) plots the first and the second normalized freand
, with
quencies of these transmission poles,
respect to . In our design, the filter is formed on the RT/D6010
and
. As can be
substrate with
found in Fig. 2(c), when increases from 0.1 mm to 0.5 mm,
gradually moves towards
. This means that the
first and second poles in the first passband or third and fourth

poles in the second passband become close to each other as the
coupling degree of the parallel-coupled lines is reduced.
Next, a modified ring resonator with periodic loading of eight
identical opened stubs, that have a width of and a length of ,
is constructed as displayed in Fig. 3(a) to make up a size-reduced
and dual-passband controllable dual-band fiter. Fig. 3(b) plots
,
the normalized frequencies of the transmission zeros,
,
and
, and poles,
,
,
,
and
, versus normalized stub length
. Herein,
is the second zero without stubs and
is the electrical length of the stubs. As increases from 0
to 1.0, the first and second poles are simultaneously reduced. At
the same time, the first zero moves closely to the right side of the
first passband and the second zero works a certain distance beis excited by the
yond the first zero. An additional pole
opened stubs. With the increment of , the third and fourth
poles move close to each other and merge to one pole around
. The fifth pole quickly moves towards the third and
fourth poles, and it forms the second passband together with the
merged pole. The third zero always stays close to the left side
is stimulated
of the second passband. An additional zero

when
. From
to 1.0,
moves towards to
the second passband and locates at its right side. Furthermore,
as the stubs are stretched, the ratio between center frequencies
of the first and second passbands is gradually reduced from 3.0
to 2.3.
III. RESULTS AND DISCUSSION
Based on the above analysis, a modified dual-mode dual-band
BPF is designed and implemented. The center frequencies of the
two passbands are designated at 2.4 and 5.8 GHz. To get a 12%
fractional bandwidth for the first passband, is chosen as 73 ,
is 108 and
is 30 . Meanwhile,
is selected
to achieve the center frequencies ratio of 2.42 that is required in
the design of a 2.4/5.8-GHz dual-band filter. To achieve good
impedance matching in the second passband of the fixed ring
needs to be reduced to 30 . Fig. 4(a) shows its
resonator,
layout with all the dimensions denoted. In our final design, the
two stubs placed at the two feeding points are slightly shortened
to compensate for the unexpected effects caused by T-junctions.


LUO AND ZHU: NOVEL DUAL-MODE DUAL-BAND BPF

Fig. 3. (a) Schematic of the dual-mode dual-band BPF using a single ring resonator with eight periodically-loaded open stubs. (b) Normalized frequencies
of transmission poles (f =f , f =f , f =f , f =f and f =f ) and

zeros (f =f , f =f , f =f and f =f ) versus normalized stub length
(t =  = ), with Z = 30
, Z = 73
, Z = 108
and Z = 30
.

Furthermore, to be connected with two coaxial cables in the experiment, two transmission line transformers with a width of 2.0
mm and a length of 13.2 mm are installed at its two feeding lines
to transform 30 into 50 . Fig. 4(b) plots the simulated results
from the ADS fullwave simulator [9] and the measured results of
a fabricated filter circuit. Both of them are in reasonable agreement with each other. Visibly, the two expected transmission
poles exist in both of the first and the second passbands at the
required center frequencies of 2.4 and 5.8 GHz. From Fig. 4(b),
the measured insertion losses in the two passbands are lower
than 1.4 and 3.2 dB, respectively, whereas the measured return
losses in the dual passbands are both higher than 20 dB. With
the help of the second transmission zero, the isolation between
these two passbands is better than 10 dB from 2.55 to 5.52 GHz.
IV. CONCLUSION
In this paper, microstrip dual-mode ring resonator BPFs with
uniform and periodically stub-loaded configurations have been
presented and implemented. The principle of the proposed dualband filters is explained and discussed via the equivalent circuit models. Afterwards, a modified dual-band BPF based on
a single microstrip ring resonator with loading of eight opened

499

Fig. 4. Modified dual-mode dual-band BPF for fabrication and measurement.
(a) Layout. (b) Simulated and measured frequency responses.

stubs is designed and fabricated. Our experiment has verified
that a dual-band filter with two poles in both passbands can be
constructed using a single ring resonator.
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