32 GHZ DUAL-TERMINATION COUPLEDCPW BANDPASS FILTER ON HR-SI

Xi Li,1,2 Yan-Ling Shi,1,2 Yan-Fang Ding,1 Da-Wei Chen,1 andDi Shen1

1 Department of E.E., East China Normal University, Shanghai200062, China2 State Key Laboratories of Transducer Technology, China Academyof Sciences, Shanghai 200050, China; Corresponding author:ylshi@ee.ecnu.edu.cn

Received 29 October 2007

ABSTRACT: Bandpass filter is an important device in portable com-munication and other RF systems. A dual-termination coupled coplanarwaveguide (CPW) bandpass filter (BPF) has been designed by HFSS.This dual-termination coupled structure improves the coupling capaci-tances between the central metal and the matching transmission lines sothat the insertion loss can be reduced. The filter has been fabricated ona high resistivity silicon substrate (� � 1000 � cm) with 1 �m SiO2. Ithas �2.16-dB low insertion loss at peak transmission of about 32 GHz.This filter shows good s-parameter characteristics and it is attractive forRF ICs. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett50: 1705–1706, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23464

Key words: coplanar waveguide (CPW); bandpass filter (BPF); equiva-lent circuit; millimeter wave

1. INTRODUCTION

There are increasing interests in integrating filters with CMOS orMMICs on Si substrate because of low cost and compact systemconsideration [1]. Coplanar waveguide (CPW) has gained wideattention because the grounds are on the same metallization planeas the signal line that simplifies the fabricating process. However,a severe drawback is given by rather weak coupling of transmis-sion line segments in one metallization plane only. Several CPWfilters with different topologies have been proposed and investi-gated [2–6]. The asymmetrical coupled structure [2] can get lowinsertion loss but the central frequency is only about 3 GHz.Asymmetric parallel-coupled structure [3] gains better stopbandrejection but covers an area of about 10 cm2 and works at onlyseveral GHz. The dual-band ultra-wideband BPF [4] improvesworking frequency up to 10 GHz. In [5], Chan et al. uses single-termination coupled structure to fabricate CPW bandpass filter at40 GHz on an ion implanted silicon substrate. It achieves that highworking frequency on the ultra high resistivity substrate.

In this article, a 32-GHz coplanar bandpass filter with a dual-termination coupled structure has been designed by HFSS. Thisdual-termination coupled structure improves the coupling capaci-tances between the central metal and the matching transmissionlines so that the insertion loss can be reduced. This kind of filterhas been implemented on a HR (� � 1000 � cm) silicon substrateand shows good filter characteristics of �2.16-dB insertion loss at32 GHz.

2. DUAL-TERMINATION COUPLED CPW BANDPASS FILTERDESIGN PRINCIPLES

Bandpass filters at millimeter wave frequencies can be realized byresonators connected with J inverters as shown in Figure 1. Ji, i�1

are the admittance inverters given by:

J01

Y0�

Jn,n�1

Y0� ��

2

w

g0g1��1(1)

Ji,i�1

Y0�

�w

2��1

1

�gigi�1

i � 0, n

where Y0 is the characteristic admittance, w is the normalizedbandwidth, ��1 is the prototype filter cut-off frequency and gi arethe normalized impedances. To realize the J inverter with CPW,such gap is employed in the signal line of CPW as shown in Figure2. The value of electrical length � and J shown in Figure 2 aregiven by [7]:

� � tan�1�2Bb

Y0

Ba

Y0� tan�1�Ba

Y0� (2)

J

Y0� � tan��

2 tan�1�Ba

Y0��� (3)

�i � � ��i�1,i

2

�i,i�1

2 � (4)

where Ba � �Ca and Bb � �Cb, Ca, and Cb are parallel and seriescapacitances of the gap. jBn shown in Figure 1 can be realized byFigure 1 A schematic of a BPF using admittance inverter

Figure 2 Admittance inverters. (a) An ideal schematic representation.(b) An equivalent circuit. (c) CPW realization

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 6, June 2008 1705

the parallel inductance and capacitance of the resonator, can alsobe replaced by a half wavelength transmission line.

3. EXPERIMENTAL PROCEDURES

The filter is designed by HFSS with 50-� input impedance. Figure3 shows the photograph of the dual-termination coupled CPWBPF. To reduce the radiation loss, finger gap is adopted. The totallength of the filter is about �/2 with each stub finger 25-�m wide.The width of the ground lines are 500 �m respectively for good RFimpedance match and collection of power lines. Series resonatorsare formed by coupled line with coplanar structures [8]. The valuesof the equivalent capacitance and inductance depend on the gapspacing between couple lines and the width of the central line. Thewidth of the ground line is at least twice the width of signal line tobenefit the collection of power lines. The filter is fabricated on ahigh resistivity (� � 1000 � cm) silicon substrate with additional1-�m-thick top thermal oxide. After depositing a 1-�m-thickaluminum layer and patterning, the filter was completed. Charac-teristics of filters have been measured by Agilent E8363B networkanalyzer and a probe station with frequency up to 40 GHz.

4. RESULTS AND DISCUSSION

The simulated and measured results are presented in Figure 4. Forfilter on HR Si substrate, excellent RF performance has beenachieved with only �2.16 dB at peak transmission of 31.64 GHz.The measured results are very close to the ideal filter designed byHFSS. The slight 1% deviation to the designed 32-GHz centerfrequency and �1.24-dB larger loss may be induced by the contactcapacitance of the probe and substrate loss. So good agreement hasbeen achieved and this dual-termination coupled CPW bandpassfilter can fulfill the 32-GHz bandpass filter character.

In contrast, the single-termination coupled filter [5] fabricatedon the ion implanted silicon substrate, which has much higherresistivity and lower substrate loss than the Si substrate adopted inthis article, shows �3.4-dB loss at peak transmission of 40 GHz.Better RF performance is found in dual-termination coupled CPWBPF on HR-Si substrate. Consequently, the dual-termination cou-pled CPW BPF is a high performance filter at millimeter waveregime. It is also a good choice for RF circuits on Si substrate.

5. CONCLUSIONS

A dual-termination CPW structure designed by HFSS is applied torealize a wide-band, tight coupling bandpass filter on high resis-tivity (� � 1000 � cm) silicon substrate. This filter has gained lowinsertion loss of �2.16 dB at center operating frequency of 32GHz. Good agreement has been obtained between the measuredand simulated results. This dual-termination CPW BPF presents

good characteristics, so it is attractive for monolithic circuits andRF circuits on Si substrate.

ACKNOWLEDGMENT

This work is supported by Natural Science Foundation of China(Nos. 60676047, 60606010), Foundation of Shanghai Science andTechnology Committee (04QMX1419) and Shanghai-AppliedMaterials Research and Development Fund (No. 0522).

REFERENCES

1. K.T. Chan and A. Chin, High-performance microwave coplanar band-pass and bandstop filters on Si substrates, IEEE Trans MicrowaveTheory Tech 51 (2003), 2036–2040.

2. J.-S. Park, J. Kim, D. Ahn, J.-B. Lim, and T. Itoh, A design of novelasymmetrically coupled CPW bandpass filter using TEM analysis, IEEEMTT-S Dig, Baltimore, MD, (1998), 1189–1192.

3. J. Gao and L. Zhu, Asymmetric parallel-coupled CPW stages forharmonic suppressed �/4 bandpass filters, Electron Lett 40 (2004).

4. K. Li, D. Kurita, and T. Matsui, Dual-band ultra-wideband bandpassfilter, IEEE MTT-S Int (2006), 1193–1196.

5. K.T. Chan, C.Y. Chen, and A. Chin, 40-GHz colanar waveguide band-pass filters on silicon substrate, IEEE Microwave Wireless Compon Lett12 (2002), 429–431.

6. Y. Zhang and K.A. Zak, Compact, coupled strip-line broad-band band-pass filters, IEEE MTT-S Int (2006), 1189–1192.

7. D.F. Williams and S.E. Schwarz, Design and performance of coplanarwaveguide bandpass filters, IEEE Trans Microwave Theory Tech 83(1983), 558–566.

8. K. Hettak, N. Did, A.-F. Sheta, and S. Toutain, A class of noveluniplanar series resonators and their implementation in original appli-cations, Trans Microwave Theory Tech 46 (1998), 1270–1276.

© 2008 Wiley Periodicals, Inc.

Figure 3 Photograph of fabricated dual-termination coupled CPW BPF,the bright area is the metal pattern, a � 50 �m b � 125-�m-stub fingerwidth � 25-�m-stub finger distance � 25 �m. [Color figure can be viewedin the online issue, which is available at www.interscience.wiley.com]

Figure 4 Simulated and measured 32-GHz bandpass filters characteris-tics, —Measurement, - - - HFSS simulation

1706 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 6, June 2008 DOI 10.1002/mop