A novel dual mode dual band bandpass

A Novel Dual-Mode Dual-Band BandpassFilter 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

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

degen-erate 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; ezhul@ntu.edu.sg).

Digital Object Identifier 10.1109/LMWC.2009.2024826

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

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 re-sults are confirmed experimentally

II PRINCIPLE ANDANALYSIS OF THEPROPOSEDRINGFILTER

Fig 1(a) depicts the schematic of the proposed dual-mode dual-band microstrip ring resonator, where is the input and output port impedance, and are the inner and outer radii

of this ring, and is the characteristic impedance of the ring

In our design, the parallel-coupled lines are one quarter of the length of the ring, with a width of or and a spacing of As illustrated in Fig 2(a), a three-port parallel-coupled line can be treated as a capacitive impedance , a voltage trans-former with turns ratio and two parallel-connected lines at port 2 and 3 as discussed in [8] and denote the

even-1531-1309/$25.00 © 2009 IEEE

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

thickness = 1:27 mm.

and odd-mode characteristic impedances of this

parallel-cou-pled 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

each band appear at and , respectively

The two lower ones ( and ) are symmetrically located at

the low sides of the two higher ones ( and ) with respect

to In addition, there exist three transmission zeros,

, 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

admittance of the network inside the dash square in Fig

2(b) equals to 0, such that

(2)

By solving (2), all the zeros can be determined as

(3a) (3b)

Equation (3a) determines the first and third transmission zeros, and while the second zero, , is derived under

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 central position, respectively In Fig 2(b), and repre-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 de-termined Fig 2(c) plots the first and the second normalized fre-quencies of these transmission poles, and , with respect to In our design, the filter is formed on the RT/D6010 substrate with and As can be 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 be-yond the first zero An additional pole is excited by the 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

of the second passband An additional zero is stimulated 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 ANDDISCUSSION

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 resonator, needs to be reduced to 30 Fig 4(a) shows its 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

Fig 3 (a) Schematic of the dual-mode dual-band BPF using a single ring

res-onator 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

Furthermore, to be connected with two coaxial cables in the

ex-periment, 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

agree-ment 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

dual-band filters is explained and discussed via the equivalent

cir-cuit models Afterwards, a modified dual-band BPF based on

a single microstrip ring resonator with loading of eight opened

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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