In 2017, the first release of 5G specifications, Release-15, has been issued by 3GPP [5]. For next generation communication systems, the required data rates are at least 20 Gb/s downlink and 10Gb/s uplink [6]. For achieving the specification, number of frequency bands will increase for wider bandwidth requirement. With increasing number of bands, there will be more filters. The area of filters on RF front-end will increases and the RF front-end design will be more and more complex. For reducing system complexity and area of filter on RF front-end module, the concept of reconfigurable filter has emerged.
Before introducing reconfigurable filters, a brief introduction for filters has been given. In wireless communication systems, the filter is one of important passive components. The specifications of filter will be different due to the system requirements.
For example, a highly compact lumped-element bandpass filter has been presented for L-band receiver by using LTCC to achieve system-on-package [7]. A bandpass filter has been designed at 3 to 5 GHz for ultra-wideband system by using PCB embedding passive technology [8]. When the operating frequency increases to millimeter-wave, waveguide filters have been considered to use, including gap waveguide filter [9] and substrate integrated waveguide (SIW) filter [10]. For V-band high data rate radios, a quasi-elliptic bandpass filter has been presented [11].
For smart mobile phone systems, the size of filters is a crucial issue. Several types of filters design such as the SIR filter [12], [13], the net-type multimode filter [14], [15]
and the folded SIW filter [16] have been proposed to reduce the circuit size. However, with the evolution of systems, the filter performances, such as selectivity and loss, present more challenging issue for filter design. The quality factor of general microstrip
resonators are not good enough. Currently, the acoustic wave filters are popularly used on smart mobile phones.
The acoustic wave filters include two different technologies, which can be distinguished by direction of acoustic wave propagation. One is surface acoustic wave (SAW), the other is bulk acoustic wave (BAW). The film bulk acoustic resonator (FBAR) and solidly mounted resonator (SMR) technologies are parts of BAW [3]. The SAW/BAW components do not need DC bias and AC is used to change the deformation of piezoelectric layer. In conventional SAW/FBAR filter design, the ladder-type and lattice-type are commonly used. The transmission zeros are designed by the resonance and anti-resonance frequency of SAW/FBAR resonators and then shunt and series SAW/FBAR resonators are used to form the desired filter response [4], [18]. In [19], the thin film bulk acoustic wave (FBAR) filters have been presented for high performance and miniaturization at 5GHz. In [20], the BAW-SMR filters have been designed by combining ladder and lattice type topology for high isolation and selectivity.
The BAW filters can be used to design duplexer with matching networks. Although the processes of SAW/BAW components, IC chips or lumped passive components are different, there are some packaging techniques to integrate those components. In [21], two BAW filter chips are designed by ladder type topology and they are flip-chip mounted onto the LTCC substrate. In [22], a technique for integrating active circuitry into the lid of the wafer-scale hermetic FBAR package is presented. The literature [23]
presents the integration of a SAW filter stacked on top of a BiCMOS transceiver chip in a plastic quad flat-pack no-lead (QFN) package.
In recent years, the requirement of reconfigurable filters have emerged in 4G LTE systems [24]-[27]. In traditional multi-band multi-mode RF front-ends [26], one of the main components is the duplexer. Because each transceiving band needs one duplexer,
more than one duplexer are used in the multi-band system. For receiving part of the RF front-end module, the filters are the main components. If some filters can be merged into one reconfigurable filter, the area and complexity can be reduced drastically. The reconfigurable filter and tunable filter are generally used. Strictly speaking, the filter characteristic in this dissertation tend to tunable filter.
As is implied by the name, the reconfigurable filters are composed of filters and tunable components. The tunable components or switches are usually based on solid-state, micro-electro-mechanical (MEMS) and ferroelectric materials, such as barium strontium titanate (BST). Two types of active components in reconfigurable filters, i.e., varactor diodes [28]-[36] and RF MEMS capacitors, [37]-[40], are usually used. The RF MEMS capacitors and varactor diodes have their own advantages. In terms of quality factor and insertion loss, RF MEMS capacitors have better performance than varactor diodes. However, because of the high tuning speed, low fabrication cost, and easy integration in circuits, varactor diodes are often used [41]. The quality factor of tunable components is one of key factors for the insertion loss of reconfigurable SAW/FBAR filters. The high-Q tunable capacitors have been shown in the literature [42]-[45], including MEMS varactors [42], BST varactors [43] and RF-MEMS digital capacitors [44], [45]. The RF-MEMS digital capacitors are composed of many MEMS switched capacitors in parallel to achieve high tuning range and high capacitance density. In [44], the COMS-MEMS 3-bits digital capacitors fabricated in 0.35μm BiCOMS process are presented. The quality factors are about 150 at 2GHz for capacitance from 21.5 fF to 1.36 pF. The quality factors of MEMS digital capacitors remains constant versus capacitance. The high-Q tunable capacitors can be realized by MEMS digital capacitors and the quality factors do not change dramatically with capacitance.
In designing a reconfigurable filter, one of the critical issues is to adjust the center frequency but keep certain filter response unchanged. Two types of bandwidth are often mentioned: constant fractional bandwidth (CFBW) and constant absolute bandwidth (CABW) responses [28], [29]. For CFBW reconfigurable filters, the coupling coefficients and external quality factors are required to maintain constant over the entire tuning range. For CABW reconfigurable filters, the coupling coefficients become lower while the external quality factors become higher with increased center frequency. For achieving the CFBW and CABW response, several methods have been presented. In [28], either CFBW or CABW reconfigurable filters can be achieved with different lumped capacitance by tuning electric coupling coefficients, which needs additional area. In addition, the reflection coefficient |S11| becomes worse when the center frequency of filter is tuned lower. At the lowest band of the reconfigurable filter, it is even larger than -10 dB and hence the band will not be suitable.
Reconfigurable filters with other special features are also proposed, including a reconfigurable bandstop filter with constant bandwidth [30], a planar reconfigurable filter with adjustable center frequency and fixed out-of-band rejection response [31], a dual-band reconfigurable filter with independently controllable passbands of two frequency bands [32], etc.
For achieving the constant coupling coefficient, the dual-mode reconfigurable filters have been presented. The concept of dual-mode filter has been used in designing reconfigurable filters previously [32], [33], [46]. However, the external quality factors are not designed appropriately to meet the requirement value when changing the center frequency of the reconfigurable filters. Although the filter responses still exist, the return losses degrade over the center frequency tuning range because the external quality factor is not the desired value. In [34], tunable structures for coupling
coefficients and external quality factors have been designed to achieve wideband reconfigurable filters. The constant 3dB-bandwidth of the reconfigurable filter with CABW response is 300MHz and the tunable fractional bandwidth of 12-95% can be achieved.
For most of the microstrip reconfigurable filters in the literatures [33], [47], [48], the insertion losses are difficult to reduce because of the low quality factor of microstrip line resonators. To increase the quality factor of resonators, the cavity resonators or substrate integrated waveguide (SIW) resonators have been used to improve the insertion loss [35], [49], [50]. But the size is too large to meet the use of mobile devices.
For achieving compact size, reconfigurable filters are designed in organic substrate [36]
or ferrite LTCC package [52]. For achieving better performance, filter has been designed by combining SAW resonators and transmission lines [51], [52], but the size of transmission line is still too large. In [53], [54], the filter and reconfigurable filter are designed by SAW resonators and lumped elements.
Based on the requirements of compact size, low loss and high selectivity, the SAW/
BAW reconfigurable filters have attracted a lot of attention. For the high selectivity of SAW/ FBAR filters, the electromechanical coupling coefficient (kt
2) of piezoelectric material is small. For single narrow band and high selectivity filter response, the SAW/
FBAR resonators with small kt
2are appropriate. But for reconfigurable filters, the small kt2SAW/FBAR resonators might be unsuitable. In order to increase the tuning range of a reconfigurable filter, the high kt
2 SAW/ FBAR resonators have been exploited [55]-[57].
In [55], the LiNbO3 is the most often used piezoelectric material of SAW resonators and the kt
2 on acoustic wave is limited. They used other acoustic modes to realize a larger coupling coefficient. The maximum coupling coefficient kt2 of an SH0-mode plate wave is 54%. For FBAR resonators, the piezoelectric material is the major factors of
electromechanical coupling coefficient (kt
2). The ZnO and AlN are generally used and their kt2 are about 8.5% and 6.1%, respectively. The lead zirconate titanate (PZT) is another widely used piezoelectric ceramic material, with higher kt
2 of 15% to 26% [58].
Based on the above considerations, the reconfigurable or switchable filters by SAW/ FBAR resonators with tunable components or switches have been reported [59]-[63]. The switchable filters by SAW/FBAR resonators are usually designed by switches to change between two specific bands [59]. The reconfigurable ladder-type SAW/FBAR filters by diode varactors are shown in [60]. It achieves a wide tuning range by using high kt
2 SH0-mode plate wave resonators. Two varactors are used to change the resonance and anti-resonance frequencies of one SAW resonator and the two kinds of capacitance need to be designed. In the presented design method, the only anti-resonance is used and one varactor is used for one SAW/FBAR resonator. The bandwidth-tunable SAW filters are designed by BST varactors and fabricated together on a lithium tantalate wafer [61]. The fractional bandwidth of filter is about 0.32% to 0.62%, which is too small to use. Recently, the switchable filters based on BST-on-Si composite FBAR have been presented [62], [63]. These filters are quite application limited; e.g, not designed to be continuously tunable at the center frequency. In [62], the switchable FBAR filter is for tuning ON and OFF stages. In [63], the center frequency of the reconfigurable BST FBAR filter is tuned by controlling the ON/ OFF stage of BST FBARs. The number of FBAR is increased with increasing tunable bands.
As mentioned before, the tuning mechanism of reconfigurable filters are mainly controlled by extra tunable components. As adding extra components, the loss increase and the unwanted response can also be induced. For improving the problems, the tunable film bulk acoustic resonator (TFBAR), published in US patent [64], has been presented. The basic traditional FBAR includes a top electrode, a general piezoelectric
layer and a bottom electrode. The TFBAR contains a semiconducting piezoelectric layer between the top electrode and general piezoelectric layers. The semiconducting piezoelectric layer is a piezoelectric layer with a controlled doping concentration. The neutral region and depletion region in semiconducting piezoelectric layer are induced by a DC biasing voltage. When the DC voltage is varied, the thickness of neutral region and depletion region can change to affect the thickness ratio of the general and semiconducting piezoelectric layers. The equivalent capacitance, inductance and resistance vary, so that the resonance frequency of TFBAR can be adjusted.