Design of A Compact Bandpass Filter with Multi-Coupled Stepped Impedance Resonators Using

LTCC Technology

Wei Jiang Institute of Fiber-Optic Communication and Information

Engineering College of Information Engineering, Zhejiang University of

Technology Hangzhou, China

jiangwei512@gmail.com

Yali Qin Institute of Fiber-Optic Communication and Information

Engineering College of Information Engineering, Zhejiang University of

Technology Hangzhou, China

ylqin@zjut.edu.cn

Abstract—A multilayer LTCC bandpass filter is proposed in this paper. The proposed filter consists of two pairs of improved stepped impedance resonators (SIR’s). The improved SIR’s have realized multi-coupling which makes the filter more compact. In addition, due to the multi-coupled SIR’s, two transmission zeros are introduced to improve out-band characteristic of the filter. This filter has a central frequency of 6GHz with a bandwidth of 1GHz, the insertion loss is less than 1.5dB and the overall size is 2mm*2.2mm*0.9mm.

Keywords-bandpass filter; improved SIR; transmission zero; LTCC

I. INTRODUCTION Modern wireless communication terminals like mobile

phones and bluetooth devices trend to be more compact but possess ever-greater functionality. This demands RF sections in these products have even much smaller size and higher performance. Filter is one of the most important part in the RF section, therefore it is required to have smaller occupation and good characteristics. Low temperature co-fired ceramic (LTCC) technology is a good choice to satisfy this trend. LTCC technology offers the possibility to realize three dimensional microwave circuits in an effective way, and it is applicable to high frequencies with advantage[1].

Many researches on LTCC filters have been reported. For example, conventional SIR’s are coupled electromagnetically, which forms a signal attenuation pole[2], and this pole can be controlled by a pad above the coupled section[3]. As only one attenuation pole exists, the filters in above two papers suffer a bad out-band characteristic. In [4], additional capacitors are introduced to the SIR to minimize the filter, but the size is still not compact enough. Vertically coupled SIR’s have been reported[5], this do reduce the size of filter, but the insertion loss become larger.

In this paper, with the convenience of multilayer design introduced by LTCC technology, both vertical (broad-side) and planar (narrow-side) couplings are used in the design of filter. This coupling mechanism brings compactness and two attenuation poles to the filter.

This paper is arranged as follow: section I demonstrates the basic part of the proposed filter, this part consists two vertically coupled SIR’s, and equivalent circuit is also given; section III gives an example filter which have two basic parts introduced in section II, and the figure of frequency response is depicted for further analysis; section IV is the conclusion.

II. ANALYSIS OF IMPROVED SIR Fig. 1 shows the basic part of the proposed filter. Two

SIR’s are stacked face to face on different layers. Lines are connected to ground with via at one end and loaded with shorted capacitors at the other end. As it is known, at resonance the areas of maximum field in the wider parts of the SIR correspond to capacitive type of electrical performance and the narrow parts to inductive type[6]. Therefore, if two SIR’s are placed directly opposite, which serves to introduce additional capacitors or inductances, this will effectively reduce the resonant frequency, or for a certain frequency, this helps to reduce the size of the structure greatly.

Fig. 1 Structure of the improved SIR’s

Furthermore, loading capacitors are used in this structure for even more compact size. Also the capacitors have the effect to adjust the central frequency of the filter, which will be discussed in the following section.

Fig. 2 shows the equivalent circuit of this basic structure. For a good view, the coupling in Fig. 2 is depicted in the same plane. The proposed SIR’s can be decomposed into three parts: the first parallel coupled striplines, the second parallel coupled striplines and the loading capacitors. The last two parts have a dominant electrical field, and in the first part, magnetic field plays a leading role. So, the SIR’s got strong-defined capacitive and inductive parts.

Fig. 2 Equivalent circuit of the SIR’s

Also, the vertically coupled SIR’s have similar basic characteristics of conventional SIR, which means we can use the impedance ratio to control the central frequency and spurious frequencies[7].

In the following section, a filter consisting of two pairs of proposed vertically coupled SIR’s is designed.

III. EXAMPLE

A. Physical Structure In this section a filter with a central frequency of 6GHz is

designed, and the 3dB bandwidth is 1GHz. The insertion loss is less than 1.5dB. The substrate has a dielectric constant of 7.8. Computer software HFSS is used for simulation and optimization of filter.

Fig. 3 shows the physical structure of the proposed filter. The whole structure is distributed on four layers. Layer 1 is used for in/out port, where in/out coupling is formed between layer 1 and layer 2. Layer 4 is used to form the loading capacitors with layer 3, they are used to minimize the size of filter, and we can also adjust the size of the capacitor pad to control the frequency. On layer 2 and layer 3 there are two pairs of vertical coupled SIR’s. Using such structure, both planar and vertical couplings are realized in a compact room. Compare to the basic element in section II, only one of the loading capacitor is used in these coupled SIR’s, this layout helps to reduce size and it is more comfortable to be adjusted.

Here we make the lines connect side grounds (not shown) directly instead of using vias.

Fig.3 Structure of proposed filter, four layers are used, from top to bottom is

layer1, layer2, layer3 and layer4

The frequency response of above filter is depicted in Fig. 4. From this figure, we can find two attention poles on each side of the passband. But the central frequency is shifted a little to the right, and the insertion loss is not satisfied to the specification. So optimization is needed for a better result.

Fig. 4 Frequency response of filter

B. Analysis and Optimization In order to get a better insertion loss, the coupling

coefficient and external quality factors are the keys to be considered[8]. Here we can adjust the in/out coupling between layer 1 and layer 2 to get a larger external quality, it is a convenient way to reach a better insertion loss.

By adjust the opposite areas between the in/out port and the SIR’s, we can get the response depicted in Fig. 5.

From Fig. 5, we can find that the central frequencies and the transmission poles barely change, but the insertion loss come through a variance.

When it comes to the adjustment of the central frequency, we can simply change the value of the loading capacitors in the lower layer. Fig. 6 shows the change of the central frequency.

Fig.5 Adjustment of the in/out coupling

Fig. 6 Adjustment of the central frequency

From the frequency responses of Fig. 5 and Fig. 6, we can find some useful points. First, the lower attenuation pole barely changes with the adjustment of loading capacitors. This is because the lower pole is decided by the coupled SIR’s and the in/out coupling, When the impedance ratio and the gap between the SIR’s and in/out ports are determined, the location of the lower pole will not change. Second, we can use the loading capacitors to change the central frequency and the upper attention poles simultaneously.

With these useful points, we can optimize the designed filter step by step. The final result is shown in Fig. 7.

After optimization, we get a filter with the size of 2mm*2.2mm*0.9mm, and the insertion loss is less than 1.5dB in the passband from 5.5GHz to 6.5GHz. Two attenuation poles are located at 2.5GHz and 7.25GHz.

Fig. 7 Frequency response after optimization

IV. CONCLUSION In this paper, a compact filter with simple layout has been

proposed. The improved SIR’s which have both vertical and planar couplings are applied, this helps to make the filter more compact than the conventional one. With the in/out coupling parts and loading capacitors the optimization can be easily done in order to make the filter satisfy the specification.

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