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1468 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 56, NO. 6, JUNE 2008

A Microstrip Bandpass FilterWith Ultra-Wide Stopband

Ching-Wen Tang, Senior Member, IEEE, and Yen-Kuo Hsu

AbstractThis paper develops a novel microstrip bandpass filterwith ultra-wide stopband. With the assistance of open stubs andinterdigital capacitors, a very wide stopband with a sufficient re-jection level can be achieved easily. Detailed design and synthesisprocedures are provided. Moreover, the electromagnetic simulatorIE3D is used, and the prototype of the bandpass filter is fabricatedand measured. Good agreement between measured and theoret-ically predicted results demonstrates feasibility of the proposedfilter.

Index TermsBandpass filter, microstrip filter, ultra-widestopband.

I. INTRODUCTION

BANDPASS filters with high quality and compact size are

essential for designing receivers and transmitters in a mi-

crowave communication system. Planar filters with printed cir-

cuit technology are particularly attractive because of their easy

fabrication, compact size, and low cost. Due to its planar struc-

ture and easy design procedures, the parallel-coupled microstrip

bandpass filter has been one of the most commonly used filters

for the RF front end [1]. However, unwanted harmonic pass-

bands would appear and result in a narrower stopband.

Various methods have been developed to improve the stop-

band performance of a planar microwave bandpass filter. Thespurious signals can be suppressed by utilizing parallel-cou-

pled microstrip filters with over-coupled end stages [2]. The

sinusoidal rule is used to eliminate spurious harmonics by

continuously perturbing the width of the coupled lines in

the microstrip wiggly-line filter [3]. In addition, with the

stepped-impedance resonators in planar microstrip bandpass

filters [4][9], the second harmonic can shift to a higher

frequency band and a wide stopband will appear. Moreover,

harmonic suppression can be carried out as well by employing

the cross-coupling [10], the split-ring resonator [11], the open

stub and spurline [12][18], and the ground-plane aperture

[19][24], or combining the low-pass/bandstop filter with abandpass filter [25][27].

Manuscript received December 12, 2007; revised March 17, 2008. This workwas supported in part by the National Science Council, Taiwan, R.O.C., underGrant NSC 96-2628-E-194-002-MY2.

C.-W. Tang is with the Department of Communications Engineering andthe Department of Electrical Engineering, Center for TelecommunicationResearch, National Chung Cheng University, Chiayi 621, Taiwan, R.O.C.(e-mail: [email protected]).

Y.-K. Hsu is with the Department of Electrical Engineering, Na-tional Chung Cheng University, Chiayi 621, Taiwan, R.O.C. (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMTT.2008.923900

Fig. 1. Schematic diagram of the proposed ultra-wide-stopband microstripbandpass filter.

In this paper, we propose a novel microstrip bandpass filter,

as shown in Fig. 1. With the assistance of open stubs and inter-

digital capacitors, the harmonic resonance is suppressed and a

wider stopband comes up. Following are the detailed derivation

and development of this filter. In Section II, we present proce-

dures for synthesizing the ultra-wide-stopband bandpass filter.

Results of electromagnetic (EM) simulation and measurement

of the proposed filter are introduced in Section III. Section IV

then concludes this paper.

II. SYNTHESIS OF THE WIDE-STOPBAND BANDPASS FILTER

The proposed microstrip bandpass filter with an ultra-wide

stopband can be simplified as shown Fig. 2(a) because the cen-

tral frequency of the bandpass filter is only slightly influenced

by open stubs added in the resonator [17]. With symmetrical

feeding, this simplified filter is composed of two open stubs andan open-circuit coupled line to generate multiple transmission

zeros. Although the simplified structure of the bandpass filter in

this paper and that in [18] look alike, the analytical framework

utilized in [18] is not applicable to current bandpass filters. With

open stubs, the simplified schema shown in Fig. 2(a) cannot be

considered as multiple asymmetric resonators as in [18]. There-

fore, we develop a new analytic approach.

A. Theory of Filter Design

As shown in Fig. 2(b), the equivalent circuit of Fig. 2(a) can

be obtained by using the immittance inverter [28]. Therefore, the

transformed admittance inverters and of transmission

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TANG AND HSU: MICROSTRIP BANDPASS FILTER WITH ULTRA-WIDE STOPBAND 1469

Fig. 2. Simplified circuit of the proposed ultra-wide-stopband bandpass filter.(a) Schematic diagram. (b) Equivalent circuit.

lines , and the even- and odd-mode line admittances and

can be expressed as

(1)

(2)

(3)

where the s are the inverse of the s and is the corre-

sponding admittance of the symmetrical open-circuit coupledline.

By matching Fig. 2(b) with the generalized bandpass filter

[28], the parameters of susceptances and susceptance slopes,

and the transformed admittance inverter can be then ob-

tained as

(4)

(5)

(6)

(7)

(8)

where the s are the element values of the prototypical low-

pass filter, is the fractional bandwidth, and and are

the source and load impedances, respectively. Moreover, the ad-

mittances and can be obtained by

(9)

(10)

According to Fig. 2(b), the proposed bandpass filter is a four-

ordered one. In order to make it simpler and easier to fabricate,

the order of the proposed bandpass filter can be transformed

into a two-ordered one by setting inverters and as 0.02,

equivalent to the system impedance as 50 . Therefore, a trans-

mission line of 50 can be directly connected to the input and

output ports in this bandpass filter.

B. Generating Transmission Zeros

By using the open stub and the open-circuit coupled line

shown in Fig. 2(a) for the bandpass filter, multiple transmission

zeros can be generated so that a wide stopband will appear. As

for transmission zeros, and result from the open-stub

line and the open-circuit coupled line, respectively. The fre-

quency of transmission zeros can be represented as

(11)

(12)

where is the central frequency of the passband.

C. Transformation Into the Interdigital Capacitor

In order to make a broader bandwidth of the passband, the

open-circuit coupled line need to be transformed into the inter-

digital capacitor. First, convert the open-circuit coupled line into

the equivalent capacitors and of this interdigital capac-

itor shown on the right of Fig. 3(a) with

(13)

(14)

where and are the admittance parameters of the open-

circuit coupled line. Second, convert the capacitances of and

into finger numbers of the interdigital capacitor with the aid

of Fig. 3(b).

With mm, mm, and mm, the

interdigital capacitor designed on the substrate Rogers RO4003

is taken as an example. Fig. 3(b) indicates the tendency that the

more finger numbers there are, the wider the fractional band-

width of the passband is.

III. DESIGN OF AN ULTRA-WIDE-STOPBAND BANDPASS FILTER

Theproposedultra-wide-stopbandbandpassisfabricated.Fol-lowing are simulated and measured results described in detail.

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Fig. 3. (a) Interdigital capacitor and its equivalent circuit. (b) Relationship be-tween capacitances/fractional bandwidth and finger numbers.

Fig. 4. Transforming the uniformed transmission line into the stepped-impedance line.

A. Simplified Bandpass Filter

2 GHz, 0.01 dB, 5%, and 0.02 are selected for central fre-

quency, ripple, fractionalbandwidth, and inverter (or ) of

the simplified above-mentioned filter. Moreover, within the fre-

quencyband of 30 GHz,set transmissionzeros at 2.62, 7.86,

13.1, 18.34, 23.58, and 28.82 GHz and transmission zeros at

9.424, 18.848, and 28.272 GHz. Furthermore, electrical length

and impedance are set as 26 and 76.4 , respectively.

Consequently according to (1)(14), the electrical lengthsand and the impedances , , , and of the sim-

plified bandpass filter can be obtained as 68.7 , 19.1 , 16.85 ,

10.2 , 10.62 , and 9.81 , respectively. In addition, the uni-

formed transmission line needs to be transformed into the

stepped-impedance line, which is composed of and ,

shown in Fig. 4, with the assistance of (15) and (16) as follows:

(15)

(16)

With 100 and 15.75 being assigned to and , respec-tively, the transformed impedance and the electrical length

Fig. 5. Theoretical responses of the simplified bandpass filter. (a) Narrowbandfrequency. (b) Wideband frequency.

Fig. 6. Comparison of the simplified bandpass filters EM simulation and the-oretical prediction.

can then be derived as 57 and 6.85 , respectively. Ac-

cordingly, theoretical responses of the simplified bandpass filtercan be as shown in Fig. 5.

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TANG AND HSU: MICROSTRIP BANDPASS FILTER WITH ULTRA-WIDE STOPBAND 1471

Fig. 7. Proposed ultra-wide-stopband microstrip bandpass filter. (a) Photo-graph. (b) Narrowband responses. (c) Group delay. (d) Wideband responses.

These theoretical values are converted to dimensions of thewide-stopband bandpass filter, which is designed on the sub-

strate Rogers RO4003 whose dielectric constant, loss tangent,

and thickness are 3.38, 0.0027, and 0.508 mm, respectively.

Fig. 6 compares EM simulation and theoretical prediction of the

wide-stopband microstrip bandpass filter. In terms of EM sim-

ulation, the greatest harmonic appears at 20 GHz, which makes

the stopband . Moreover, the insertion loss is greater than

28 dB within the stopband.

B. Utilizing Open Stubs and Interdigital Capacitors

In order to achieve a wider passband and stopband simulta-

neously, multiple open stubs and the interdigital capacitor are

employed. Multiple open-stub lines, shown via dashed circles

in Fig. 1, are added to the simplified above-mentioned bandpass

filter; moreover, the open-circuit coupled lines need to be trans-

formed into the interdigital capacitors, shown via dotted circles

in Fig. 1.

The central frequency, fractional bandwidth, and ripple of this

newly developed filter, shown in Fig. 1, are set as 1 GHz, 18%,

and 0.01 dB, respectively. Moreover, its dimensions aremm, mm, mm, mm,

mm, mm, and mm, mm,

mm, mm, mm, and mm.

Fig. 7 presents a photograph of the fabricated filter and com-

parison of EM simulation and measurement. Fig. 7(b) indicates

that the measured insertion loss is less than 0.9 dB, while the

measured return loss is greater than 15.2 dB within the pass-

band (0.931.11 GHz). Fig. 7(c) points out that the measured

group delay is less than 2 ns within the passband. Moreover,

because the transmission zero only appears in a higher pass-

band skirt, a greater group delay appears in the higher passband.

Fig. 7(d) shows that the greatest harmonic appears at 22 GHz,

which makes the stopband . Moreover, the measured inser-tion loss is greater than 26 dB within the stopband. Furthermore,

these measured results are the same as the one enclosed in a con-

ductive box.

To further extend the stopband, the patterned ground plane

is used at the bottom of the proposed bandpass filter. With the

aid of the patterned ground plane, the impedance , shown in

Fig. 2(a), increases so that there is a broader stopband, which

is extended to 50 GHz. Unfortunately, even with a quite broad

stopband, the patterned ground plane is not appropriate for a

packaged circuit due to the serious radiation effect in the higher

frequency band.

IV. CONCLUSION

A new approach to generate an ultra-wide stopband in the

microstrip bandpass filter has been proposed. Methods for filter

design and analysis have been introduced. The fabricated filter

has demonstrated the potential for harmonic suppression with

the assistance of multiple open stubs and interdigital capacitors.

Moreover, excellent agreement between theoretical and mea-

sured results has validated the proposed structure.

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Ching-Wen Tang (S02M03SM07) receivedthe B.S. degree in electronic engineering fromChung Yuan Christian University, Chungli, Taiwan,R.O.C., in 1991, and the M.S. and Ph.D. degrees incommunication engineering from National ChiaoTung University, Hsinchu, Taiwan, R.O.C., in 1996

and 2002, respectively.In 1997, he joinedthe RF Communication Systems

Technology Department, Computer and Communi-cation Laboratories, Industrial Technology ResearchInstitute (ITRI), Hsinchu, Taiwan, R.O.C., as an RF

Engineer, where he developed low-temperature co-fired ceramic (LTCC) multi-layer-circuit (MLC) RF components. In 2001, he joined Phycomp Taiwan Ltd.,Kaohsiung, Taiwan, R.O.C., as a Project Manager, where he continues to de-velop LTCC components and modules. Since February 2003, he has been withthe Department of Communications Engineering, National Chung Cheng Uni-versity, Chiayi, Taiwan, R.O.C., where he is currently an Associate Professor.He also holds a joint appointment with theDepartment of Electrical Engineeringand Center for Telecommunication Research, National Chung Cheng Univer-

sity. His research interests include microwave and millimeter-wave planar-typeand multilayered circuit design, and the analysis and design of thin-film com-ponents.

Yen-Kuo Hsu was born in Kaohsiung, Taiwan,R.O.C., in 1984. He received the B.S. degree in

electrical engineering from the National Chin-Yi In-stitute of Technology, Taichung, Taiwan, R.O.C., in2006, and the M.S. degree in electrical engineeringfrom National Chung Cheng University, Chiayi,Taiwan, R.O.C., in 2008.

He is currently with National Chung Cheng Uni-versity. His research interests include the design andanalysis of RF and microwave circuits.

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