A study of RF front-end filters with embedded capacitor technology

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 [1-5]. 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 [6-8]. 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.