Dual-Mode Dual-Band Filter

IEEE MICROW A VE AND WIRELESS COMPONENTS LETTERS,VOL.20,NO.2,FEBRUARY 201073

Dual-Mode Dual-Band Filter With Band Notch Structures

Youngje Sung

Abstract—A novel approach for designing dual-mode dual-band bandpass ?lters is proposed and experimentally studied.By em-bedding a pair of slits in the square patch,the dual-band response is realized and one transmission zero is introduced between the two passbands.Two degenerate modes are achieved by loading with a small square perturbation,which can be placed at the diagonal line.There are three transmission zeros.The slits control the ?rst and third transmission zeros.On the other hand,the right crossed slots control the ?rst and the second transmission zeros.Index Terms—Dual-mode dual-band ?lter.

I.I NTRODUCTION

R

ECENT development in wireless communication sys-tems has created a need for RF circuits with a dual-band operation.Various con?gurations have been proposed for realizing a dual-band ?lter [1]–[4].In [1],a dual-band was implemented as a combination of two inpidual ?lters with two speci?c single passbands.By cascading a wideband passband ?lter and a stopband ?lter,a dual-band bandpass ?lter (BPF)was achieved [2],but these solutions suffer from high insertion loss and large overall size.The stepped impedance resonator (SIR)has been used in dual-band design [3],[4].In this case,the second resonance property is dominated by the impedance

ratio

of the SIR.Dual-mode ?lters have been widely used in wireless commu-nications systems because of their advantages in applications requiring high quality microwave s with features such as small size and low loss.As is well known,there are several commonly used dual-mode microstrip resonators including the circular ring [5],square loop [6],and square patch [7].

In this letter,a dual-mode dual-band with a pair of slits is pro-posed.The dual-band property derives from the slits applied to the square and the dual-mode property in the two passbands that is induced by a perturbation positioned at the center of the diag-onal line.The right angle crossed slots control the ?rst resonance and lower the resonant frequency,which makes the square patch resonator compact.On the other hand,the right angle crossed slots only slightly perturb the second resonance and keep the second resonant frequency unchanged [7].

Manuscript received May 28,2009;revised October 18,2009.First published January 26,2010;current version published February 10,2010.

The author is with the Department of Electronics Engineering,Kyonggi Uni-versity,Suwon 443-760,Korea (e-mail:yjsung@kgu.ac.kr).

Color versions of one or more of the ?gures in this letter are available online at 7161e50d581b6bd97f19ea8a.

Digital Object Identi?er

10.1109/LMWC.2009.2038434

Fig.1.Con?guration of the proposed dual-band dual-mode ?lter.

II.C ONFIGURATION

The basic con?guration investigated in this letter is shown in Fig.1.The ?lter proposed here is fabricated on a commer-cially available substrate

with

and .The width of the feed line is chosen to be 1.2mm,which cor-responds to the characteristic impedance of

50.The ?lter is fed by a microstrip line by gap coupling where the gap for the feed structure is denoted

by .It is found from the simulated

results that the dimensions

(

and )of the T-shaped feed structure can be adjusted for good impedance matching.Simu-lation is carried out using HFSS.

The outer side length of the proposed ?lter is denoted

by ;it has slits—with the length and width

of

and ,respec-tively—that are located in front of the I/O

port,apart from its edge.The slot situated at the center is a combination of the right cross slots with the

length and

width [7]and the square slots

that are located

at

from the I/O port.The side length of the square slot is denoted by .While a structure without slits was already reported in [8],the proposed ?lter failed to realize

dual-mode property at the second resonant

frequency

even though it was designed for dual-mode dual-band application.Fig.2presents the simulated results that compare the values

of

for square patch ?lters with and without slits,respec-tively.The solid line represents

the

property of a conven-tional square patch ?lter with the side length of 16mm;the

dotted line stands for

the

property of a square patch ?lter to which a pair of slits are added.A conventional square patch ?lter exhibits single resonance property,while one with slits,as demonstrated in Fig.2,forms a transmission zero around 2.7GHz,separating single passband into two bands.By prop-erly setting the length of the slit to be 10mm (approximately a quarter-wavelength at 2.7GHz),the slit resonates,acting as a notch and trapping some of the power that was transmitted

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74IEEE MICROWA VE AND WIRELESS COMPONENTS LETTERS,VOL.20,NO.2,FEBRUARY

2010Fig.2.Simulated results (S )comparing the properties of square patch ?lters

with and without slits.

from input port to output port [9].The slot located at the center,

whose length in one side

is ,leads each of the bands to have

dual-mode property.Accordingly,the proposed ?lter has dual-

band dual-mode property.

III.D UAL -M ODE D UAL -B AND B ANDPASS F ILTER

Fig.3shows the simulated results of the proposed ?lter with

different

sizes

of the slits.The length of the square in one side is set at 16mm;the

length

and

width of the T-shaped feed structure are 14mm and 0.2mm,respectively,

and the

gap

is 0.1mm.As for the slot located at the

center,

,,

and .In Fig.3(a),the

length

of is altered while

?xing at 0.3mm;in Fig.3(b),the

length

of is changed

with ?xed at 10.5mm.The simulated re-

sults show that both of the resonant frequencies of the proposed

?lter,

and ,tend to drop as the length of the

slit

becomes greater.Also,an increase in the slit’s

size

causes a greater split between modes at the two resonant frequencies

(i.e.,

and

)and thereby widens the bandwidth.

As increases from 9.5

to 10.5mm,the 3dB bandwidth goes up from 7.2%to 8.2%

at

and from 4%to 5.3%

at .Likewise,

as increases from

0.2mm to 0.6mm,the 3dB bandwidth goes up from 6.8%to

11.9%

at

and from 3.9%to 7%

at .With the slit growing bigger,the ?rst and third transmission zeros change,while the

second transmission zero shows relatively less or no movement.

Fig.4shows the simulated results of the proposed ?lter with

different .The dimension of the ?lter

is

,and the other dimensions are the same as above.It

is found from the simulated result that the location of the ?rst

transmission zero is decreased as the

position increases.But,

the second and the third transmission zeros still remains un-

changed.

Fig.5shows the simulated results of the proposed ?lter with

different

lengths of the slot.The dimension of the ?lter

is

,

,and the other dimen-

sions are the same as above.

As increases,the third transmis-

sion zero remains unchanged but the ?rst and second transmis-

sion zeros do move,making it possible to adjust the resonance

frequency while not

altering .Fig.6illustrates changes in

?lter properties by ,

where and other

parame-Fig.3.Simulated frequency response (S )of the proposed ?lter with different (a)slit length and (b)

width.Fig.4.Simulated frequency response (S )of the ?lter with different c .ters are set as described above;an increase in perturbation

size leads to a greater split between modes

at

and .IV .S IMULATED AND M EASURED R ESULTS Fig.7shows the simulated and measured frequency response for the fabricated ?lter.To prove validity of the proposed struc-

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SUNG:DUAL-MODE DUAL-BAND FILTER WITH BAND NOTCH STRUCTURES

75

Fig.5.Simulated frequency response (S )of the ?lter with different s

.

Fig.6.Simulated frequency response (S )of the ?lter with different p .

ture,a dual-mode dual-band ?lter is designed and fabricated.The side length of the square is selected to be 16mm.The pa-rameters of the slit are chosen to

as

,,

and

.The other parameter values

are:

and .The impedance matching condition can be op-timized

when

,,

and .The two passbands are centered at 2.21and 2.81GHz,with the fractional bandwith of 10.4%and 8.2%.The measured min-imum insertion losses of the two passbands are 1.1and 1.6dB,respectively.Three transmission zeros are realized at 1.69,2.45,and 3.13GHz.They are close to the passband edges and can greatly improve the selectivity of the ?lter and the isolation be-tween the two passbands.Theoretical simulation result agrees well with measured result.The conductor and dielectric losses,which are not considered in the simulation,contribute to the small deviation between simulation and

measurements.

Fig.7.Simulated and measured results of the proposed ?lter.

V .C ONCLUSION

A novel dual-mode dual-band using a pair of slits has been proposed,which has good dual-passband performance at 2.21and 2.81GHz.The dual-passband characteristics are generated by slits.By a small square slot in the center as a perturbation,the symmetry of the structure can be perturbed,which makes it possible for excitation of two near-degenerate orthogonal modes for dual-mode operation.By the slits and the right crossed slots,three transmission zeros are easily controlled,which means the change of the bandwidth.It is well suited to compact and low-cost circuit applications at microwave frequencies.

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