A LTCC-Based Ku-Band 3D Bandpass Filter Using Stepped-Impedance Hairpin Resonators

Zequan Guo1, Jianming Zhou1, Ju Zhou1

1: School of Information and Electronic, Beijing Institute of Technology Beijing 100081 China

Abstract- A new compact bandpass filter using stepped impedance (SI) hairpin resonators with multilayer configuration based on LTCC technology for Ku-band application is proposed.Two pairs of transmission zeros are achieved by introducing the cross coupling and asymmetric I/O structure, which can improve the rejection in the stop band. The compact filter is designed as a three-dimension structure which is embedded in the ceramic substrate based on the LTCC multilayer manufacturing technology. EM simulator HFSS is applied to design and optimize the proposed structure. The result shows a center frequency of 15.75 GHz with a bandwidth of 400 MHz. The insert loss is smaller than 2.1 dB, and the return loss is better than 18 dB within the passband. The overall dimension of the proposed filter is 4mm × 7.5mm × 1.056mm, about �g/2 × �g,which is suitable for the application in Ku-band integrated RF front end module.

Keywords-bandpass filter, stepped impedance hairpin resonator, asymmetric I/O structure, LTCC, Ku-band

I. INTRODUCTION

With the development of the wireless communication technology and RF application, miniaturization and high performance of the RF frond end module has become more and more important in microwave and milliwave field. Some high integrated technology such as LTCC has been used torealize light weight and high performance RF system by integrate microwave device and circuit module into one package [1]. It is precisely because of its low conductor loss, dielectric loss and reliable multilayer manufacturing technology, Low temperature co-fire ceramic (LTCC) technology allows designers to integrate varieties of MMICs, passive devices and components, such as antenna and filter [2].Among these, The band pass filter is one of the key passive components, and its size and performance decided the overall size and performance of the system.

Traditionally, due to their high performances, easily and low cost manufacturing, some planar microstrip filters areoften designed to satisfy the requirements in microwave band, such as parallel coupling filter, hairpin filter, interdigital filter and so on. However the planar microstrip filter is still too big in size due to manufacturing on a single substrate layer. To overcome this problem, a variety of filters with multilayer structure are designed and presented, which all have the advantages of compact structure and superior performance [3] [4]. However, they all require simultaneous occupancy of both the positive and negative layer of substrate, which cannot be applied to occasions where only one side is the signal layer while the opposite side is the ground plane. At the same time,

a 4 order filter can only generate a pair of transmission zero, unless additional filter order and a shift in its structure.

A new bandpass filter with multilayer configuration is proposed in this work. With reference to the previous multilayer structure, a new stripline coupled filter can be embedded in the ceramic substrate by using the LTCC multilayer manufacturing technology. At the same time, stepped-impedance (SI) hairpin resonators are implemented inthe filter to reduce the overall size of the filter. Two pairs of transmission zeros are introduced by using the cross coupling and asymmetric input/output (I/O) structure, which can greatly improve the stop-band response. In addition, a GCPW to stripline transition is designed to connect the filter inside the substrate and circuit components on the surface of substrate.The filter is designed at the center frequence of 15.75 GHz with a fraction bandwidth (FBW) of 2.54% and fabricated in a 11 layers FerryA6m substrate of 1.056mm thickness. The final design result show that the filter has the advantages of compact structure and excellent stop band selectivity.

II. STEPPED-IMPEDANCE HAIRPIN RESONATOR ANDINPUT/OUTPUT STRUCTURE

A. Stepped-Impedance Hairpin ResonatorsFigure 1(a) shows a miniaturized stepped impedance hairpin

resonator, which can be equivalent to a basis stepped-impedance resonator (SIR) that Fig.1(b) shows. A basis SIR is a cascade of a high impedance line and two low impedance line having different characteristic impedance Z1 and Z2 with corresponding electrical length �1 and �2, respectively. It is assumed that the fundamental resonance frequency f0 is fixed. Ignoring the effect of the step discontinuity, the input admittance Yin of the basis SIR from its open plane can be written as [5]:

2112

12122 tantan

tantan��

��YY

YYjYYin ��

� (1)

Then the fundamental resonance condition can be obtain when Yin =0, and it can be derived as:

K�21 tantan �� (2)Where K is the impedance ratio Z2/Z1. So the total electrical

length of the SIR can be describe as:

))tan

(tan(2)(21

1121 ����� K

T����� (3)

Therefore, from (3), it can be shown that �T can be equal to � when K is 1, which means that the SIR becomes a half-wavelength resonator. Besides, the total electrical length �T is

978-1-4673-2185-3/12/$31.00 ©2012 IEEE

below � for 0<K<1, which means that we can choose as a small value of K as possible to reduce the size of the SIR [5].

Fig. 1.(a) The stepped impedance hairpin resonator; (b) Equivalent basis SIR;(c) Asymmetric I/O structure; (d) Symmetric I/O structure.

B. Input/output StructureAs shown in Figure 1, the I/O structure in Fig. 1(c) is

asymmetric and the signals at the input and output feed points are in phase while they are out of phase and the structure is symmetric in Fig. 1(d). It is observed that the asymmetric I/O structure in Fig. 1(c) would introduce a pair of transmission zeros in stop band [6]. The first zero is generated at the frequency when the length of arm (1) of the input resonator is about quarter-wavelength while the other occurs at the frequency when the length of arm (2) is about quarter-wavelength.

To prove this, a 2-order filter using different I/O structures based on the topology in Figure 1 is designed. The simulation responses are shown in Figure 2. The dashed line and solid line stand for the responses of filters with the symmetric and asymmetric I/O structure, respectively. Compared with the symmetric I/O structure, a pair of transmission zeros is introduced in stop band by using the asymmetric I/O structure, which can improve the stop band rejection well. It is also noted that the frequencies of the transmission zeros shift according to the tap position t defined in Figure 1(c). So we can obtain a desired response easily by changing the tap position t.

Fig. 2. Simulation responses for the 2-order filters with symmetric and asymmetric I/O structure.

III. FILTER DESIGN

A. Filter design and its configurationBy using the techniques described previously, a new

compact filter based on the SI hairpin resonator with an asymmetric I/O structure is proposed. It is designed at a center frequency of 15.75GHz with a FBW of 2.54%, whose transmissions zeros are at 16.25 GHz and 15.25 GHz. So the coupling coefficients and external Q value can be synthesized by the method given in [7]. Firstly, the component values of the lowpass prototype filter can be obtained as [7]: g1=0.94982, g2=1.35473, J1=�0.12333, J2=1.0181.

So, the coupling coefficients and external Q value can be calculated as:

;39.37141

;0033.01

114

;0191.02

223

;0224.021

3412

���

���

�

��

�

���

FBW

geQeQ

g

JFBWM

g

JFBWM

ggFBWMM

(4)

The configuration of the proposed filter is shown in Figure 3. The principle of SIR is employed to reduce the size of four identical hairpin resonators here. The filter topology is fabricated in a 8 layers FerryA6m substrate, which has two stripline layers separated by a middle common ground, as shown in Figure 3. The first and fourth resonators are placed on the upper stripline layer and a electric coupling M14 is generated between them. The second and third resonators are placed on the lower stripline layer and a magnetic coupling M23 is generated between them. Then the mixed coupling M12and M34 are produced via two apertures on the middle common ground. As it’s known, a pair of transmission zeros will be introduced by the cross coupling M14 because the signs of the electrical and magnetic coupling are opposite. At the same time, another pair of transmission zeros is introduced by the asymmetric I/O structure. All of these can improve the out-of-band rejection.

Fig. 3. The proposed filter configuration. (a) Perspective configuration; (b)Cross-section view.

B. Extraction of Qext and Coupling CoefficientsFirstly, in order to design the proposed filter, the relation

between the physical dimensions of the filter and the coupling coefficients should be obtained. EM simulator (HFSS) is used

to simulation the responses of the basic coupling structuresand input/output structure. Then coupling coefficients and external Q value can be determined by the responses.

For example, the coupling coefficient Ki,j between two synchronously resonators can be determined according to the following formula [7]:

21

22

21

22

, ffff

K ji ��

� (5)

In which, f1 and f2 are two resonant frequencies of the coupled structure.

Also, the external Q value which is the relation between the resonators and input/output can be written as [7]:

11090

0

Se f

fQ

��

� (6)

Where f0 is the resonant frequency and 11

090 Sf�

� is the

frequency at which the S11 phase shifts ±900 with respect to the absolute phase at f0.

The external Q value and the relations between threecoupling coefficients and three basic coupled structures areshown in Figure 4.

Fig. 4. The coupling coefficients of three coupled structures and the external Q value

Then we can find the appropriate parameters such as distant t, S14, S23 and aperture size dx, dy to achieve the synthesized coupling coefficients and external Q value from these curves shown in Figure 4. EM simulator HFSS is used to design the filter and do optimization. The optimization results will be given in the rest.

IV. DESIGN OF OVERALL FILTER

In order to connect the filter inside the substrate and the circuit components on the surface of substrate, a GCPW to stripline transition is also designed, as shown in Figure 5(a).

The stripline inside the substrate and the GCPW on the surface of the substrate are connected by a metal via. Around the via, two rows of vias are distributed to form a quasi coaxial structure, as shown in Figure 5(a). Some vias are arranged around the stripline and GCPW in order to shield the signal, as shown in Figure 5(a). So the shielding vias space (D1 and D2)should be optimized to obtain a good transmission performance. All the vias’ diameter is 0.15mm. Then the radius of the quasi coaxial (R1) and the radius of the aperture on the middle common ground (R2) are optimized to make a good impedance matching. The equation that it follows is [8]:

dRZ

r

10

2log276

� (7)

Where Z0 is characteristic impedance of the quasi coaxial structure, 50 ohms, and �r is the relative dielectric constant of 5.9 for the substrate, d is the via diameter. After optimization, The desired parameters are obtained as: D1=1mm, D2=0.9mm, R1=0.73mm, R2=0.43mm, L=0.21mm. The optimization result is shown in Figure 5(b). The insert loss better than 0.08dB and the return loss less than 24dB from 2 GHz to 30 GHz are achieved.

After the design of a single filter and the transition, the overall filter including a back to back transition can be designed and simulated by HFSS. As shown in Figure 5 (c)and (d), the overall filter model is consisted of 11 layersFerryA6m ceramic substrate with a relative dielectric of 5.9, aloss tangent of 0.002, and a thickness of 1.056mm. The filter is placed inside the lower 8 layers ceramic substrate, similar to the filter structure shown in Figure 4. And the upper 3 substrate layers constitute the coplanar waveguide and microstrip layer. For the four identical resonators, their size isL1=1.1mm, L2=0.9mm, L3=0.5mm, g=0.1mm, w1=0.15mm, w2=0.25mm. After optimization, the parameters of the filter can be obtained as: dx=0.56mm, dy=0.43mm, S23=0.2mm, S14=0.36mm, t=0.35mm.

Fig. 5. (a) A GCPW to stripline transition; (b) Response of the transition; (c)Cross-section of the overall filter; (d) Overall filter model.

The simulation result is shown in Figure 6. The insert loss is smaller than 2.1 dB and the return loss is better than 18 dB

within the passband. Four transmission zeros are obtained at the upper and lower stop band, which improve the stop band rejection well. It is observed that the size of overall filter including the transition is 4mm × 7.5mm × 1.056mm, which is about �g/2 × �g, a 50% reduction of the size compared with the conventional planar microstrip hairpin filter.

Fig. 6. Response of the overall filter

V. CONCLUSION

A compact filter using the SI hairpin resonator with the multilayer configuration is designed and presented. The filter is embedded inside the substrate by using the LTCC multilayer manufacturing technology in order to minimize its size. And It has a excellent stop band selectivity by

introducing two pairs of transmission zeros. Through this design, The filter has the advantages of compactness and high performance which can be applied to develop the wireless communication technology and the T/R module of phased arrays.

REFERENCES

[1] D. Kim, H. M. Cho, N. K. Kang, “LTCC Front-end Modules for Multi-band Applications,” IEEE, Proceedings of the 36th European Microwave Conference, pp. 541-544, Sept. , 2006..

[2] K. L. Wu and Yong Huang, “LTCC Technology and Its Applications in High Frequency Front End Modules,” IEEE, International Symposium on Antennas, propagation and EM theory, pp. 730-734, 2003..

[3] J. T. Kuo, C. F. Jou, and C. C. Chen, “Design of Microstrip Bandpass Filters with Aperture Coupled Miniaturized Hairpin Resonators,” IEEE,Asia-Pacific Microw. Conf. , vol.2, pp. 421–424, 2001.

[4] A. Djaiz and T. A. Denidni, “Experimental Investigation of a Compact Aperture-Coupled Multilayer Bandpass Filter for Wireless Systems,”IEEE, Radio and Wireless Conference, pp. 247-250, Sept. 2004.

[5] A. Djaiz and T. A. Denidni, “A New Compact Microstrip Two-Layer Bandpass Filter Using Aperture-Coupled SIR-Hairpin Resonators With Transmission Zeros,” IEEE, Trans. on Microw. Theory and Techni. , vol. 54, no. 5, pp. 1929-1936, May 2006.

[6] S. Y. Lee, C. M. Tsai, “New Cross-Coupled Filter Design Using Improved Hairpin Resonators,” IEEE, Trans. on Microw. Theory and Techni. , vol. 48, no. 12, pp. 2482-1490, Dec. 2000.

[7] J. S. Hong and M. J. Lancaster, “Microstrip filters for RF/microwave applications”, New York, Wiley, Chapter 8, 2001

[8] Y. Q. Li, J. H. Ji, Y. C. Fei and J. M. Zhou, “A LTCC Stripline Bandpass Filter For Ku-Band Receiver Module,” IEEE, International Conf. on Microw. And Millimeter Wave Techni. , vol. 1, pp. 301-303,2008.

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