Ａ Method for Improving Out-of-Band Characteristics of a Wideband Bandpass Filter in an LTCC Substrate

1. Introduction

Recently, compact wireless modules using low temperature co-fired ceramic (LTCC) technology are widely used for the wireless systemssuch as mobile phones, Bluetooth, and wireless local-area networks(Lin et al.,2004;Wang et al.,2005). Fig.1 shows the general structure of the compact wireless modules using the LTCC technology. The modules consist of an LTCC substrate, integrated circuits, chip components, a shield, and passive components embedded in the LTCC substrate (e.g., the bandpass filter, coupler and balun). The embedded components in the LTCC substrate are fabricated by using the multilayer structures based on thin ceramic sheets and conductor patterns. It becomes possible to produce very compact modules compared to those with a general printed circuit board substrate, because a number of passive components can be embedded in the substrate. To adapt this technology for ultra wideband (UWB) wireless systems,a wideband bandpass filter in the LTCC substrate is one of the most important technology because it decreases the influences from other wireless systems. Various wideband bandpass filters have already been presented(Ishida & Araki, 2004;Saitou et al.,2005;Li et al., 2005;Zhu et al.,2005;Horii et al.,2006;

Figure 1.

A general structure of the compact wireless module using the LTCC technology.

Yamamoto et al.,2007;Shaman & Hong,2007;Tanii et al.,2008 ; Sun & Zhu,2009). However,these approaches cannot be easily applied for the embedded components in the LTCC substrate. Therefore we have studiedthe compact wideband bandpass filters based on the LTCC technology(Oshima et al.,2008; Oshima et al.,2010).

In this study, we proposea method for improving out-of-band characteristics ofa wideband bandpass filter.It is suitable for the compact UWB wireless modules using the LTCC technology.The UWB systems assume the band group 1 (3.168-4.752GHz) of the multiband orthogonal frequency-division multiplexing systems(Ghorashiet al.,2004).

Section 2 describes a wideband filter using the LTCC technology. We also point out that the filter is desired to improve the attenuation characteristics. Section 3 explains a method for improving out-of-band characteristics. In Section 4 and Section 5, we indicate the LTCC structure of the presented filter, the simulated results, and the experimental results. Finally, the conclusion of this studyis summarized in Section 6.

2.Bandpass filter for UWB systems using the low-frequency band

Fig.2 shows the schematic of the wideband bandpass filter for UWB systems using the low-frequency band (Oshima et al.,2010). Resonator 2 is the resonator which has a wide passband and creates attenuation poles near the passband. Resonator 1 and Resonator 3 are tap-feed resonators. The capacitors of C₂ and C₃ are coupling capacitors between the resonators. The capacitors of C₄ and C₅ are used for the impedance matching and they also improve out-of-band characteristics in the high-frequencyregion. The capacitor C₆ is used to shorten the length of the strip line. In this study, the length of Resonator 3 differs from that of Resonator 1in order to create two attenuation poles at the high-frequency region. The attenuation pole created by Resonator 1 is given by

$$\frac{{\text{jZ}}_{\text{A}}{Z}_{\text{B}}}{{\text{Z}}_{\text{A}}\text{cot}{\theta }_{\text{B}}{\text{-Z}}_{\text{B}}\text{tan}{\theta }_{\text{A}}}=0.$$ E1

The attenuation pole created by Resonator 3 is also given by

$$\frac{{\text{jZ}}_{\text{E}}{\text{Z}}_{\text{F}}}{{\text{Z}}_{\text{E}}\text{cot}{\theta }_{\text{F}}{\text{-Z}}_{\text{F}}\text{tan}{\theta }_{\text{E}}}=0.$$ E2

The attenuation poles created by Resonator 2 are given by

$$\frac{{\text{Y}}_{\text{0}}{\text{(Y}}_{\text{1}}{\text{-Y}}_{\text{2}}\text{)}}{{\text{(Y}}_{\text{0}}{\text{+Y}}_{\text{1}}{\text{)(Y}}_{\text{0}}{\text{+Y}}_{\text{2}}\text{)}}=0$$ E3

where,

$${\text{Y}}_0=0.02$$ E4

$${\text{Y}}_1=\frac{\text{1-2}\omega {\text{C}}_{\text{1}}{\text{Z}}_{\text{D}}\text{tan}{\theta }_{\text{D}}}{{\text{jZ}}_{\text{D}}\text{tan}{\theta }_{\text{D}}}$$ E5

$${\text{Y}}_2=\frac{\text{jtan}{\theta }_{\text{D}}+{\text{Y}}_{\text{3}}{\text{Z}}_{\text{D}}}{{\text{Z}}_{\text{D}}+{\text{jY}}_{\text{3}}{\text{Z}}_{\text{D}}^{\text{2}}\text{tan}{\theta }_{\text{D}}}$$ E6

$${\text{Y}}_3=\frac{\text{j(}\omega {\text{C}}_{\text{6}}{\text{Z}}_{\text{C}}+\text{tan}{\theta }_{\text{C}})}{{\text{2Z}}_{\text{C}}\text{(1-}\omega {\text{C}}_{\text{6}}{\text{Z}}_{\text{C}}\text{tan}{\theta }_{\text{C}})}.$$ E7

The circuit parameters of the bandpass filter are decided by means of adjusting the parameters in consideration of the conditions for the attenuation poles. This adjustment is carried out by a commercial circuit simulator(ADS, Agilent Technologies, Inc.). Table 1 shows the parameters of the filter. Here, the reference frequency for the electrical length is 4.0 GHz. In this study, we use the physical strip line model in the circuit simulation because this model can simulate the losses of the conductor and the LTCC substrate. Fig.3 shows the physical strip line model. The relative permittivity of the LTCC substrate and the dielectric loss tangent of the substrate are 7.1 and 0.005, respectively. The conductor in the substrate is silver. Fig.4 indicates the results of the circuit simulation. The filter produces good attenuation performances near the passband due to attenuation poles (f₁ and f₂) which are created by Resonator 2. The filter achieves good spurious suppression up to 10 GHz due to the attenuation poles(f₃ and f₄). They are created by Resonator 1 and Resonator 3. The input impedance of the filter is also 50 ohm in the wide passband.However this filter is desired to improve the attenuation characteristics because the filter cannot create a number of attenuation poles at the frequency region lower than the passband and has the spurious responses at the frequency band higher than 10 GHz.

Figure 2.

Schematic of the UWB bandpass filter for the low-frequency band.

C1	1.0pF	ZA	46.3 ohm	$${\theta }_A$$	43.9deg.
C2	0.75pF	ZB	46.3 ohm	$${\theta }_B$$	26.9deg.
C3	0.75pF	ZC	46.3 ohm	$${\theta }_C$$	50.9 deg.
C4	0.5pF	ZD	38.6 ohm	$${\theta }_D$$	15.7deg.
C5	0.5pF	ZE	46.3 ohm	$${\theta }_E$$	38.4 deg.
C6	2.7pF	ZF	46.3 ohm	$${\theta }_F$$	26.9 deg.

Table 1.

Parameters for the schematic shown in Fig.2.

Figure 3.

Physicalmodel for a stripline.

Figure 4.

Simulated results of the filter shown in Fig.2.

3. A method for improving out-of-band characteristics

In order to improve the spuriousresponses, the lowpass filter is very useful(Kurita &Li,2007.). Fig.5 shows the schematic of the lowpass filter. This filter consists of a stripline and a capacitor, which is suitable for the embedded components in the LTCC substrate (Ohwada et al.,2002.). Fig.6 indicates the simulated results of the lowpass filter by the circuit simulator. Where, Z_S and $${\theta }_S$$ are 46.3ohm and 33 deg.(@4GHz), respectively. In Case A, the capacitor C_a is 0.27 pF. In Case B, the capacitor is 0.34 pF. This lowpass filter can attenuate the frequency region which is higher than 10 GHz. The attenuation characteristics of the filter can be controlled by the value ofthe capacitor C_a. However, the lowpass filter can not attenuate the low-frequency band.

Figure 5.

Schematic of the lowpass filter.

Figure 6.

Simulated results of the lowpass filter shown in Fig.5.

The input/output coupled filter can create attenuation poles (Shaman& Hong. 2007). It is useful for improving the attenuation performances near the passband. However this filter requires the quarter-wavelength coupled line and has the third harmonic.

For improving the out-of-band characteristics of the filter, we propose a method using lowpass filters which consist of the coupling structure. Fig.7 shows the schematic of the filter using the presented method. This circuit adds the lowpass filters at the input/output portsof the filter shown in Fig.2. And a part of the stripline of the lowpass filters is the coupling structure. Table2 shows the parameters of the lowpass filters shown in Fig.7. In Table2, the reference frequency for the electrical length is 4.0 GHz. Z_So and Z_Se are odd- and even-mode characteristic impedances. Fig.8 indicates thesimulated results of the filter. We can confirm that the filter has an additional attenuation pole (f₅) at the low-frequency band and suppresses the second and third harmonics. Fig.9 shows the characteristics of the filter, when the coupling condition of the stripline is varied. It is confirmed that the attenuation pole (f₅) is controlled by the coupling stripline. This method uses the weak coupling condition. Therefore it has little effect on the passbandand the attenuation poles near the passband.The filter keeps a high attenuation level in the high-frequency region. Note that thelocations of attenuationpoles especially in the high-frequency region are varied by the coupling condition.

Figure 7.

Schematic of the bandpass filter.

Ca1	0.27pF	ZS1	46.3 ohm	ZSo	45.7 ohm		10 deg.
Ca2	0.25pF	ZSe	45.9 ohm		18 deg.		5 deg.

Table 2.

Parameters of thelowpass filters shown in Fig.7.

Figure 8.

Simulated results of the filter shown in Fig.7.

Figure 9.

Simulated results of the filter shown in Fig.7, when the coupling condition of the stripline is varied.

4. LTCC structure

Fig.10, Fig.11 and Fig.12 indicate the LTCC structure of the filter.The filter is obtained by means of modifying the structure based on the basic circuit shown in Fig.7, taking into consideration the various parasitic effects caused by the three-dimensional LTCC structure.The filter consists of the three conductor layers inserted into the middle portion of the LTCC substrate, with the ground planes on the top and bottom layers. The conductor thickness is 8 um. The diameter ofvia holes is 0.1 mm. The ground planes are connected by the via holes. The via hole between the coupled line adjusts the coupling condition.The dimensions of the bandpass filter are 6.2 x 2.7 x 0.366 mm³, and this size could be fabricated into the LTCC substrate for wireless modules. Fig.13 shows the simulated results using a commercial electromagnetic simulator (HFSS Ansys Inc.).The filter has the wide passband and suppresses second and third harmonics. The filter also has an additional attenuation pole at the low-frequencyregion.

Figure 10.

Three-dimensional structure of the filter.

Figure 11.

Cross sectional structure of the filter.

Figure 12.

Top view of the filter.

Figure 13.

Simulated results by the electromagnetic simulator.

5. Experiments

We verify the effectiveness of the presented method by experiments.Fig. 14 indicates the LTCC structureforthe evaluation of the embedded filter. Thedimensions of the LTCC

Figure 14.

Structure of the LTCC substrate for evaluation.

substrate are 8.0 x 5.0 x0.63 mm³. The presented filter (6.2 x 2.7 x 0.366 mm³) is fabricated in the substrate. In order to connect the SMA connectors for the evaluation, thetoplayerof the LTCC substrate hastheelectrodes for RF signals and a ground plane. The feedlines between the filter and theinput/output ports consist of a via hole, a stripline, and the electrode ofthe top layer. These feed lines are designed 50 ohm.Fig. 15 shows a photograph of the LTCC substrate. The prototype which is connected to the SMA connectors is measured by a vector network analyzer(N5230A PNA-L, Agilent Technologies Inc).Fig.16 and Fig.17 indicate the measured results. It is confirmed that the filter suppresses the spurious responses less than 20 dB up to 16 GHz and has an additional attenuation pole in the low-frequency region. In addition, the insertion loss is less than 3.0dB and the group delay is within 1 ns in the wide passband.

Figure 15.

Photograph of the prototype.

Figure 16.

Measured results of the filter shown in Fig.15.

Figure 17.

Measured group delay of the filter shown in Fig.15.

6. Conclusion

In this study, we propose a method for improving out-of-band characteristics for the wideband filter in the LTCC substrate.This method uses the lowpass filters with the coupling structure, which are set at input and output ports of the bandpass filter. This method is very useful for the compact wireless modules because additional compact circuits can suppress spurious responses and can add an attenuation pole in the low-frequency band. The fabricated UWB bandpass filter for the low-frequency bandachieves the insertion loss less than 3.0 dB and the group delay within 1ns in the wide passband. The filter also suppresses spurious responses up to 16 GHz and has the good attenuation performances in the low-frequency region.