Proceedings of Asia-Pacific Microwave Conference 2007

Compact LTCC Tn-Band Filter Design

Kengyi Huang, Tsenchieh Chiu and Hann-Biau Wu*Department of Electrical Engineering

National Central UniversityJhongli City, Taoyuan County 32001, Taiwan

tfdc.easongmsa.hinet.net*Chung Shan Institute of Science and Technology

Abstract-A compact multi-band bandpass filter design with low-temperature co-fired ceramic (LTCC) technology is proposed inthis paper. Using divided combline resonators with grounded andcoupled capacitors, dual-band (2.4 GHz and 5.5 GHz) and tri-band (2, 4 and 7 GHz) bandpass filters are obtained. Moreover,while the number of the pass bands increase, the volume of thefilter remains almost unchanged. The theoretical analysis of theprototype filters is described in the paper. The measurement andlayouts of fabricated components are also presented.

Keywords-component; coupled line, tri-band bandpass filter,dual-band bandpass filter, low-temperature co-fired ceramic(LTCC).

I. INTRODUCTION

Development of compact multi-band devices has been animportant research issue, especially for wireless applicationsbased on widely separated frequency bands. Many multi-bandcomponents, such as antennas, filters, low noise amplifiers,and power amplifiers (PA) [1], transceivers [2] have beendesigned in the past. Of all the components, filters playimportant roles since it usually determines the exact pass-bandrange.

There have many multi-band filter designs in the literaturesand most of them are dual-band ones. In [3], two differentfilters are simply parallel connected without the considerationof compactness. In [4], the filter are constructed usingcascaded 1/4 X coupled line resonators. To achievecompactness, designs using low-temperature co-fired ceramic(LTCC) have been proposed [5]. In addition to the highpermittivity and low loss, LTCC with multi-layered substratesoffers great freedom in 3-dimensional circuit componentdesign, which is often crucial to circuit miniaturization.

In this paper, we propose a novel filter design for multi-band bandpass filter, as shown in Fig. 1. The design scheme isevolved from traditional single-band combline filter (shown inFig. 2) without increasing the size significantly. All comblinesand capacitors are realized using LTCC technology with theceramic dielectric constant er = 34.

II. COMBLINE FILTER DESIGN

Figure 2 shows the conventional circuit topology for thesecond-order combline bandpass filter. The filter is composedof a serial capacitor Cs, two grounded capacitors Cp, and the

Figure 1. Proposed topology for tri-band bandpass filter.

Figure 2. Architecture ofthe second-order combline bandpass filter.

edge-coupled combline. Cs and combline can be replacedwith equivalent circuits, as shown in Fig. 3. For Cs, theequivalent circuit is expressed as

1-4244-0749-4/07/$20.00 w2007 IEEE.

0, L

WI

J = C 12.

For combline, the equation are given by

YOO = Yo + JCL tan 0,

YOe = Yo - JCL tan 0,

Figure 3. Parallel network connection of the second-order bandpass filter.

where the Y and Yoe are odd- and even-mode admittances,and 0 is the electrical length of the edge-coupled combline.To combine the parallel part in Fig. 3, Cs is shunted with

CP and Jc is merged with JCL, an equivalent circuit forcombline bandpass filter is obtained in Fig. 4. The mergedadmittance JM and capacitor CT are described as

M JCL JC = 2 cot -CS

CT =CS + CP

Figure 5 indicates the generalize bandpass filter using Jimmittance inverters. Comparing Fig. 4 with Fig 5, theadmittance inverters, susceptance, and its slope parameter are

derived by [6, 7]

YAb

A go91

J12 JM =A

YAb

J23-=\I g2g3

B(W)=WCT -YO cot6,

bC 0 dB(wc)2 dc =o

T YO 0

where the gi's are the element values of the prototype lowpassfilter and the A is the fractional bandwidth, and Ys and YLare the admittance of source and load transmission lines,respectively. In order to realize the combline bandpass filter inLTCC process, the values of the ZOO and Zoe of the edge-coupled combline are obtained by tuning the value of coupledspacing S, line-width W and length L using computer softwareAdvanced Design System (ADS). The 3-D structure of the 2.4GHz LTCC filter is shown in Fig. 6. The photograph of theLTCC filter is shown in Fig. 7. Figure. 8 shows the measuredand simulated results of second-order LTCC bandpass filter.This LTCC filter is designed at 2.4 GHz. The insertion loss is1.9 dB at 2.4 GHz. The design parameters Cs, Cp, ZOO,ZOe and L are 0.8 pF, 5 pF, 15 ohm, 27 ohm, and 54mil,respectively.

Figure 4. Equivalent circuit of the second-order bandpass filter.

Ys B0L

Figure 5. Parallel network connection of the generalized bandpass filter.

Figure 6. 3-D structure of second-order combline bandpass filter on HFSS

Do , Oe

Jo1 1 im 2

YSE | +90 CT +900 CT +90 Y

1 0 0 4

This filter is fabricated in the substrate with the size is 2.5mm X 2.0 mm X 0.8 mm. Its dielectric constant and losstangent are 34 and 0.002, respectively. The results arevalidated using High frequency structure Simulator (HFSS),the measured results is obtained by network analyzer AgilentE5071B with TRL calibration.

Figure 7. Photograph ofthe second-order bandpass LTCC filter.

Frequency ( GHz )

easily implemented in LTCC process. Thus, the total size ofproposed dual-band filter is still 2.5 mm X 2.0 mm X 0.8 mm.

Figure 10. Simulated results of dual-band combline bandpass filter.

Figure 8. Measured and simulated results of second-order comblinebandpass filter.

III. DUAL-BAND AND TRI-BAND FILTER DESIGN

In section 111, we will demonstrate dual-band and tri-bandcombline bandpass filter design examples and simulationresults. Figure 9 shows the proposed topology of dual-bandbandpass filter. This design is based on the second-ordercombline filter, as shown in Fig.2. Two additional capacitorsare added to form a virtual grounding plane for frequencyband-2 in Fig. 10, of the dual-band filter. Notice that the totalelectric length of the combline is the same as the one of thesingle-band bandpass filter in Fig. 2. In the design, the originalcombline is divided into two sections of equal length. Due tothe same electric length of comblines, the lump capacitors are

The simulation result is shown in the Fig. 10, frequencypass-band 1 (fl) and pass-band 2 (f2) are at 2.4GHz and 5.5GHz, respectively. The insertion loss are less than 1.5 and 1.3dB at 2.2 and 5.4 GHz. The ratio of f2 /If is about 2.3because of the virtual ground plane-I cut the combline intotwo equal-length sections. The corresponding parameters ofdual-band filter Cs. Cp, Cl, ZOol, ZOel , Z002, ZOe2, andtotal combline length L are 1.1 pF, 1 pF, 8.5 pF, 24 ohm,29.5 ohm, 21 ohm, 22.5 ohm and 54 mil, respectively. Fig. 11

depicts the circuit structure of tri-band bandpass filter, itincludes four additional capacitors for two virtual groundingplanes. Frequency pass-band 1, 2 and 3 are at 2, 4, and 7 GHzrespectively, as shown in Fig. 12. The insertion loss are lessthan 1.8, 1.65, and 1.3 dB at 2, 4 and 7 GHz.The corresponding parameters of tri-band filter Cs, CP,

Figure 9. Proposed topology for dual-band bandpass filter.

5, Sg _

'I WE I i

<-/. \\ /<.35

X 2 324 5 7FrequentV GHi

C1 and C2, L1, L2 and L3 are 0.9 pF, 2 pF, 5.1 pF, 13 pF,23 mil, 23 mil and 8 mil, respectively. The comblineimpedance parameters ZOI1, ZOeln Z002, ZOe2 ZOOZOe3 are 30, 23, 30, 28, 30 and 28 ohm, respectively. Thesimulation result shows the pass-bands are controllable by thevirtual ground planes and with good band selectivity in-between bands simultaneously. Even with many additionalcomponents, the volume of the tri-band filter remains the same

as the ones of the single- and dual-band filters, since theelectrical length of the combline remains the same.

Figure 11. Virtual ground of the proposed topology for tri-band bandpassfilter.

IV. CONCLUSIONIn the paper, a circuit topology is proposed for dual- and

tri-band LTCC filter. The conventional LTCC single and dual-band filter are composed of capacitor and comblines, and thesize of the filter is mainly determined by the electrical lengthof the combline.

In the proposed designs, no matter for single, dual- or tri-band filters, the length for the combline remains the same.Thus, except for additional capacitors, the volume for themulti-band filters remains the same as it is for single bandfilter. The filter structure and corresponding circuit and full-wave simulation results have been shown in the paper. Themeasurement results will be presented in the conference.

Figure13. 3-D structure of tn-band combline bandpass filter on HFSS.F~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...............................

_...............................

0

-6

-10

-15

-40

-45

1 2 3 4 5

Frequency ( GHz )6 7 8

ACKNOWLEDGMENT

The authors would like to thank Walsin TechnologyCorporation for the assistance to LTCC filters fabrication.

REFERENCES

[1] S. F. R. Chang, W. L. Chen, S. C. Chang, C. K. Tu, C. L. Wei, C.H.Chien, C. H. Tsai, J. Chen, and A. Chen, "A dual-band RF transceiverfor multistandard WLAN applications," IEEE Trans. Microw. TheoryTech., vol. 53, no. 3, pp. 1048-1055, Mar. 2005.

[2] M. S. Tong, M. W. Yang, Q. S. Cao, H. S. Kim, Y. L. Lu, Y. C. Chen,and T. G. Chang, "Design and analysis of integrated-circuit packageantenna (ICPA) for dual-band wireless transceivers," Int. J. RF Microw.Comput.-Aided Eng., vol. 16, no. 3, pp. 250-258, May 2006.

[3] H. Miyake, S. Kitazawa, T. Ishizaki, T. Yamada, and Y. Nagatomi, "Aminiaturized monolithic dual band filter using ceramic laminationtechnique for dual mode portable telephones," in IEEE MTT-S Int.Microw. Symp. Dig., 1997, pp. 789-792.

[4] S. F. Chang, Y. H. Jeng, and J. L. Chen, "Dual-band step-impedancebandpass filter for multimode wireless LANs," Electron. Lett., vol. 40,no. 1, pp. 38-39, Jan. 2004.

[5] C. W. Tang, S. F. Youand I. C. Liu, "Design of a dual-band bandpassfilter with low-temperature co-fired ceramic technology," IEEE Trans.Microw. Theory Tech., vol. 54, no. 8, pp. 3327-3332, Aug. 2006.

[6] G. L. Matthael, L. Young, and E. M. Jones, Microwave Filters,Impedance-Matching Network, and Coupling Structures. Norwood, MA:Artech House, 1980.

[7] J. S. G. Hong and M. J. Lancaster, Microstrip Filters for RF/MicrowaveApplications. New York, NY: Wiley, 2001.

11T II T IIBand- Band-2 Band-3

I~~~~~~~~~~ ~ I

2

- 1-S11S21

Figure 12. Simulated results of tri-band combline bandpass filter.

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