基于埋容技术的射频前端滤波器的研究

31

A Study of RF Front-End Filters withEmbedded Capacitor Technology基于埋容技术的射频前端滤波器的研究

Yunfeng Wang Lei Li

ABSTRACT As the trend in wireless communication system is toward multi-functionality and higher miniaturization at higher frequency and lower cost, to design a RF front-end modules is a challenge. Filters, baluns, Bluetooth modules, and power amplifier modules on organic substrate have been studied widely [1-�]. Among the modules, the RF filters play a most significant role. Various design and integration approaches for filters have been reported in many publications. But the conventional methods using SMDs can not meet the requirements for better performance and lower cost and size. The embedded passives are promising solutions due to its low parasitic parameters and small size, so embedded filters have been widely studied and also reported in several papers. Base on mature filter theory, better performance needs more passive components, and could not be achieved by reducing circuit orders. For example, band pass filter could be obtained by many orders of combinations of low pass and high pass filters or by many orders of LC resonators. Most researchers focused on designing different topologies and configurations of layout to reach the electrical specifications [�-8]. In this paper, the author firstly proposed that the resonators could be implemented by using only embedded capacitors by designing inductor geometries with rational design of structure, size and shape on one electrode of the embedded capacitors to realize LC resonators, so the bandpass filter (BPF) could be obtained by capacitive-coupled resonator BPF. This paper presents the physical model simulated by HFSS, and discusses the factors influencing the characteristics of the resonator and filter, such as the width, core area, etc. The most important element affecting the multi-order resonator filter is the capacitance of the coupling capacitor whose value could be changed along with the area of the top and bottom electrodes of embedded coupled capacitor. Last, a narrow band pass filter (NBPF) and a wide band pass filter (WBPF) are designed and simulated by HFSS, and the performances are figured subsequently.KEYWORDS RF front-end module; Filter; Embedded Capacitor

摘 要 无线通信系统的多功能、小型化、低成本的趋势，对射频前端模块的设计提出了很大的挑战。人们对滤波器、巴伦、蓝牙模块和功放模块的设计已经进行过大量的研究。在这些模块中，射频滤波器占有很重要的地位。许多杂志上都报道了滤波器的各种设计和集成方法。但是传统的使用贴片元件来实现的方法并不能满足高性能、低成本、小型化的要求。而埋入无源器件由于具有低寄生参数和小尺寸，所以是一个非常有前途的解决方案。许多文章也报道了埋入滤波器的研究。基于成熟的滤波器设计理论，性能越好需要的阶数就越高，进而无源元件就越多。比如，带通滤波器可以通过多阶低通和高通滤波器的联合来实现，也可以通过多阶LC谐振器的联合来实现。大多数研究者研究的重点都放在不同的拓扑结构实现的设计上。本文中的谐振器是通过在埋入电容电极上面设计不同尺寸和形状的电感的物理结构来实现的。而带通滤波器可以通过容性耦合谐振器来实现。本文通过HFSS软件对物理结构模型进行仿真，讨论了影响谐振器和滤波器性能的因素，比如：线宽、线距和中心区面积等等。多阶容性耦合带通滤波器的最大影响因素是耦合电容的容值，可以通过调节埋入耦合电容的上下极面积来调节其容值大小。最后，用HFSS设计和仿真了一个窄带滤波器和一个宽带滤波器，而且对其性能也进行了讨论。关键词 射频前端模块；滤波器；埋入电容

I Introduction

The wireless communication systems are developing towards the direction of low profile, light weight, low

cost, excellent performance and multi-functionality. SOP/SIP (system-on-package/system-in-package), especially the embedded passives technology, is considered to be one of the most challenges and exciting technology to realize the advanced micro-systems, because the

passive components take up a large real estate of a PCB, and the electrical performance and reliability are reduced by the longer interconnect and more solder joints. The embedded passives have drawn attractions to the RF front-end circuits and modules due to its low parasitic parameters, small size and high performance and reliability. A bandpass filter is by using embedded passives technology is studied in the work. In the paper,

32

Vol. 3 No.3 / Mar. 2009

the author firstly proposed the RF front-end bandpass filter designed with only embedded capacitor. The bandpass filters are composed of 2-order LC resonators coupled by the capacitors. The resonator is implemented by designing spiral inductor geometries on the top electrode of embedded capacitor material, and one end of the resonator is connected to the bottom electrode by a micro-via. NBPF has a size of 2mm x 1.9mm. It exhibits a 3 dB bandwidth of 140MHz, an insertion loss of 2.68dB and return loss of 31.8dB. WBPF has the same size with NBPF, but has a different coupling capacitance between two resonators. It exhibits an insertion loss of 15.1dB, and band width of 500MHz, and a center frequency of 2.4GHz. All the filters’ performance are simulated by using 3D EM simulator.

II Design

a)ResonatorThe resonator of the filter is implemented by a panar spiral inductor geometry on the top electrode to form an inductor, and one end of the inductor is connected to the bottom electrode through a via. Fig1 (a) shows the typical resonator layout, (b) shows the cross section of the embedded capacitor, and the material specification is listed in Table 1, Metal layers are assumed perfect conductor material, which is PEC for simplicity.

Table1. Specifications of Embedded Capacitor

As shown from the Fig 1, w is the inductor width, s is the space between the winding, and d is the inner diameter of the spiral inductor. These planar spiral inductors are designed with rectangular shapes for finding out optimal geometry. Width and core area of the inductors varies for finding out the most optimal performance. The inductance and the capacitance between the spiral inductor and the bottom electrode comprise of the inductance and capacitance of the resonator, and the mutual inductance and parasitic capacitance among the inductor turns could be ignored because of weak electromagnetic coupling.

b)Multi-orderresonatorAs shown in Fig 3, the bandpass performance of one resonator is not good enough to meet the requirements

of bandpass filter, therefore two or more resonators should be coupled by the embedded capacitors to obtain the filter property. The layout and structure of a 2-order resonator filter is shown in Fig 2. The resonators are coupled by embedded capacitor. As shown in Fig 2, the top electrode of coupled capacitor is connected to the first resonator, and the bottom one is connected to the second resonator through micro-via. Fig 4 shows the insertion loss and return loss of a 2-order resonator filter. The bandpass performance has been improved greatly.

Fig.2. Layout of 2-order resonator filter

The factor impacting on bandpass performance of the filter most significant is the value of the embedded coupling capacitor. Whether the value is higher or lower, the performance of filter will go to the bad owing to overcoupling or under coupling.

c)NBPFandWBPFNBPF and WBPF have been designed using 2-order resonators coupled by embedded capacitors. The dimension of each resonator is 2 by 0.9mm2, and the one of filter is 2 by 1.9 mm2. The electrical characteristic of resonator and filter are simulated by Ansoft 3-D full wave electromagnetic software HFSS. The insertion loss and the return loss of these filters are specified in Fig 8 and Fig 9. The embedded coupling capacitances are 1.41pF and 1.5pF, respectively. Width, thickness, and spacing of the conductor lines are designed with 100um, 18um, and 100um to meet the requirements of conventional PCB standard process.

III Results and Discussion

Fig.1. (a) Layout of resonator (b) Cross section

基于埋容技术的射频前端滤波器的研究

33

The electrical characteristic of resonator and filter are simulated by Ansoft 3-D full wave electromagnetic software HFSS. Fig 3 shows the insertion loss and return loss of single resonator. The resonator exhibits bandpass feature, but the curve out-of-band drops too slowly to be used for BPF, so the BPF must be composed by more than one resonator.

Fig .3. Insertion loss and return loss of single resonator

Fig 4 shows the performance of 2-order resonator. The insertion loss gets higher, and the return loss lower. Fig 5 shows a comparison of insertion loss and return loss of one resonator with 1.5 turn and 1.3 by 0.3mm2 core area spiral inductor. These inductors have different width varying from 60 to 120 micrometers. As shown in Fig 6, the width is larger, the insertion loss is lower and the return loss is higher. Fig 6 shows a comparison of insertion loss and return loss of one resonator with 1.5 turn and 0.1 mm width spiral inductor. These inductors have different core areas varying from 1.3 by 0.2 mm2 to 1.3 by 0.5 mm2. As shown in Fig 6, these resonators with different core areas, nearly have the same maximal insertion loss and minimum return loss. Nevertheless, resonance frequency decreases along with the increase of the core areas. That is because the larger core areas enhance the capacitance of the resonator, and the resonance frequency descend.

Fig 7 shows a comparison of insertion loss and return loss of 2-order resonator filter with different coupling capacitor values. The different capacitor values are

obtained by changing the areas of the top and bottom electrodes. As shown in Fig 7, if the capacitance is larger, there will be over coupled phenomenon, just like 1.5 pF, and if the capacitance is smaller, there will be under coupled phenomenon, just like 0.3 pF. So the appropriate capacitance is essential for good bandpass filter performance.

Fig.7. Comparison of insertion loss and return loss of 2-order resonator filter with different coupling capacitance

Fig 8 shows simulated performance characteristics of NBPF. The BPF has insertion loss of 2.68dB, and return loss of 31.8dB at the frequencies ranged from 2.14 GHz to 2.64 GHz. In the low frequency band of 0.1GHz~0.82GHz, the insertion loss is below 40dB.

Fig.8. Insertion loss and return loss of NBPF

Fig 9 shows simulated performance characteristics of

Fig.�. Insertion loss and return loss of 2-order resonator

Fig.�. Comparison of insertion loss and return loss of a singleresonator

with different widths

Fig.�. Comparison of insertion loss and return loss of a single resonator

with different core area

3�

Vol. 3 No.3 / Mar. 2009

WBPF. It exhibits a center frequency of 2.4GHz, a bandwidth of 500MHz, and an insertion loss of 15.1 dB.

Fig.9. Insertion loss and return loss of WBPF

V Conclusions

The bandpass filter using only embedded capacitor has proposed, designed and simulated by HFSS.

Through comparison and analysis of resonators, it has been found that width and core area of resonator are important design parameters to affect performance and characteristics of the resonator, and the value of coupled embedded capacitor is a significant parameter for the multi-order resonator filter. WLAN and WBPF are designed using 2-order resonators, and these filters are useful for future RF front-end modules with multi-functionalities, low profile and smaller size.

REFERENCES[1] Chang-Sheng Chen. Embedded Capacitors Technology in 2.4

GHz Power Amplifier with Multi-layer Printed Wiring Board (PWB) Process. int’l symposium on Electronic Material and Packaging, 2002.

[2] Ching-Liang Weng. Embedded Passive Technology for Bluetooth Application in Multi-layer Printed Wiring Board (PWB).Electronic Components and Technology Conference, 2004.

[3] Mekita F. Davis. Integrated RF Architectures in Fully-Organic SOP Technology. IEEE Transacton on Advanced Packaging.

[4] Dalmia,S. Design of inductors in organic substrate for 1-3GHz wireless applications. IEEE MTT-S International, 2002

[5] Seung J.Lee. Fully embedded High Q Passives and Band Pass Filters for Low Cost Organic RF SOP Applications. Electronic Components and Technology Conference, 2007.

[6] Greg Brzezina. A Miniature LTCC Bandpass Filter Using Novel Resonators for GPS Applications. Proceedings of the 37th European Microwave Conference, 2007.

[7] Mohamadou Baba. An Efficient Methodology for Design and Implementation of Embedded Bandpass filters for RF/Wireless Applications. EPTC , 2007.

[8] Joong Keun Lee. Design of Bandpass Filter for 900MHz ZigBee Application Using LTCC High Q Inductor. APMC, 2005.

作者简介

王云峰 男，深圳先进技术研究院光电子集成技术研究室在读博士生，主要从事微波射频电子系统无源和有源器件埋入技术的研究。

李 磊 作者简介见本期封2页。