Abstract:
A filter assembly capable of unbalanced to balanced conversion. An unbalanced to balanced conversion unit is coupled to an input terminal for transforming a received unbalanced signal thereat to a balanced signal and a lattice filter coupled between the unbalanced to balanced conversion unit and two output terminals for eliminating or rejecting the noise of the balanced signal.

Description:
BACKGROUND 
   The invention relates to a filter assembly of a front-end module used in mobile communication systems, and more specifically to a filter assembly capable of balanced to unbalanced conversion. 
   Currently, dual-band, tri-band and quad-band mobile phones are available for operating in different mobile communication systems. Dual-band mobile phones switch between the operating frequencies of 900 MHz and 1800 MHz, tri-band mobile phones switch between the operating frequencies of 900 MHz, 1800 MHz and 1900 MHz, and quad-band mobile phones switch between the operating frequencies of 900 MHz, 1800 MHz, 1900 MHz and 850 MHz. The quad-band communication system is characterized by support of media applications, high speed connectivity and fast audio and image downloads. Additionally, quad-band mobile phones are compatible with all mobile communication systems. Consequently, it is desirable to have a front-end module for accommodating to the quad-band mobile communication systems. 
     FIG. 1  shows a conventional front-end module in a mobile communication system. As shown in  FIG. 1 , the front-end module  10  comprises a duplexer  12 , a power amplifier  22 , a low noise amplifier  24  and an antenna  20 . The duplexer  12  comprises two bandpass filters  14  and  16 , a 90° phase shifter  18 , an output terminal  13  coupled between the bandpass filter  14  and the power amplifier  22 , an input terminal  15  coupled between the bandpass filter  16  and the low noise amplifier  24 , and an antenna terminal  17  connected to the antenna  20 . 
   A transmitted signal is amplified by the power amplifier  22  to the duplexer  12  via the output terminal  13 . The bandpass filter  14  proceeds to allow the signal within a certain frequency band to pass therethrough and the antenna  20  then transmits the passed signal via the antenna terminal  17 . Similarly, when receiving a signal, the antenna  20  feeds the received signal to the 90° phase shifter  18  via the antenna terminal  17  and the signal is then outputted to the bandpass filter  16 , which applies the received signal to the low noise amplifier  24  via the input terminal  15 . The low noise amplifier  24  then filters the noise in the low frequency (LF) signal passing through the passband filter  16  and amplifies the signal. In order to eliminate the quality degrading disturbance in the signal passing through the bandpass filter  16 , which stems from the signal passing through the bandpass filter  14 , a 90° phase shifter  18  is typically disposed therebetween to separate the transmitted and received signals by the difference in signal phases. The signal, however, must be transformed from an unbalanced signal to a balanced signal when outputted to the antenna terminal  17  via the output terminal  13  or inputted from the antenna  17  to the input terminal  15 . Hence, an unbalanced to balanced conversion transformer (Balun) is required to be disposed before the bandpass filter  14  or after the bandpass filter  16  in the signal transmission path to reject the noise. 
   In recent years, when making filters and duplexers used in RF communication systems, the piezoelectric thin film process is typically employed in manufacturing ultrasonic components. The conventional piezoelectric thin film acoustic component can be roughly classified as a film bulk acoustic resonator (FBAR) or a solidly mounted resonator (SMR) depending on structure. SMR is supported by a Bragg reflector and FBAR is manufactured in Microelectromechanical (MEMS) surface micromachining or bulk micromachining to empty the parts below the lower electrode or supporting layer to enable the thin film structure to conform to the total reflection boundary condition of acoustic waves. 
     FIG. 2A  is a schematic diagram of a film bulk acoustic resonator (FBAR) in a stacked crystal filter (SCF) arrangement. As depicted in  FIG. 2A , the FBAR  30  comprises an input electrode  32 , an output electrode  34 , a grounded electrode  36 , an upper piezoelectric layer  31 , and a lower piezoelectric layer  33 . The input electrode  32  receives a signal from the input terminal  35  and the upper piezoelectric layer  31  then generates a bulk acoustic wave to the lower piezoelectric layer  33  in response to the signal excitation, thereby a resonance is generated between the input electrode  32  and the output electrode  34 . The output electrode  34  then outputs the signal to an output terminal  37 . The FBAR  30  is only used for unbalanced signal transmission since the input and output signals share the grounded electrode  36 . 
   In order to compensate for the defect of the FBAR  30 , another FBAR in a SCF arrangement is provided as shown in  FIG. 2B . The FBAR  40  comprises an input electrode  42 , two output electrodes  46  and  48 , a grounded electrode  44 , an upper piezoelectric layer  41 , a lower piezoelectric layer  43  and a dielectric layer  50 . While the transmission principle is the same as the described operation, the FBAR  40 , however, is capable of both balanced and unbalanced signal transmission due to the insulation of the dielectric layer  50 . Thus, the unbalanced signal only exists at the input terminal  45  of the input electrode  42  and the grounded terminal of the grounded electrode  44  whereas the balanced signal is outputted to the output terminals  47  and  49  by the output electrodes  46  and  48 . 
     FIG. 2C  shows a conventional FBAR in a coupled resonator filter (CFR) arrangement. The FBAR  60  of  FIG. 2C  comprises an input electrode  62 , two output electrodes  66  and  68 , a grounded electrode  64 , an upper piezoelectric layer  61 , a lower piezoelectric layer  63 , a plurality of first coupling layers  72  and a plurality of second coupling layers  74 . The operating principle thereof is the same as the previously described operation because the FBAR  60  is similar to the FBAR  40  except that the dielectric layer  5 Q is replaced with the interleaved first coupling layer  72  and the second coupling layer  74 . The first coupling layer  72  and the second coupling layer  74  comprise different material having different acoustic impedance but the same thickness of a quarter-wavelength. Thus, the unbalanced signal only exists at the input terminal  65  of the input electrode  62  and the grounded terminal of the grounded electrode  64  whereas the balanced signal is outputted to the output terminals  67  and  69  by the output electrodes  66  and  68 . 
     FIG. 3A  shows a schematic diagram of the frequency response of the FBAR  40  in  FIG. 2B . As shown in  FIG. 3A , there are three resonant modes in the frequency response of FBAR  40 . Generally speaking, there is only a passband in a common bandpass filter and the operating band of the quad-band mobile communication system is from 850 MHz to 1900 MHz, thus the modes outside the 1400 MHz to 1800 MHz frequency range cannot reject noise effectively, resulting in signal quality degradation. Hence, the FBAR  40  and FBAR  60  cannot meet the low noise requirement of the mobile communication systems. 
     FIG. 2D  shows a solidly mounted resonator (SMR) in SCF arrangement and  FIG. 3B  is the frequency response thereof. As shown in  FIG. 2D , the SMR  80  comprises an input electrode  82 , two output electrodes  86  and  88 , a grounded electrode  84 , an upper piezoelectric layer  81 , a lower piezoelectric layer  83 , a dielectric layer  90 , a plurality of first reflective layers  92 , a plurality of second reflective layers  94  and a substrate  96 . The operating principle of the SMR  80  is substantially similar to the FBAR  40  except that a plurality of first and second reflective layers  92  and  94  are disposed under the lower surface of the output electrode  88  for support, wherein the interleaved first reflective layer  92  and the second reflective layer  94  formed on the substrate  96  are acoustic reflectors and made of different materials having different acoustic impedance. The thickness of the first reflective layer  92  and the second reflective layer  94  is a quarter acoustic wavelength and thus when the acoustic wave progresses to the first and second reflective layers  92  and  94 , a Bragg reflection which approximates a total reflection is formed and the resonant energy is then maintained in the SMR  80 . The unbalanced signal only exists at the input terminal  85  of the input electrode  82  and the grounded terminal of the grounded electrode  84  whereas the balanced signal is outputted to the output terminals  87  and  99  by the output electrodes  86  and  88 .  FIG. 3B  shows a frequency response curve  140  of the SMR  80  when the first reflective layer  92  and the second reflective layer  94  are made of tungsten (W) and SiO 2  respectively, and a frequency response curve  130  when the first reflective layer  92  and the second reflective layer  94  are made of AlN and SiO 2  respectively. Because the acoustic impedance ratio between AlN and SiO2 is smaller than that between W and SiO 2 , a narrower reflection bandwidth having better noise rejection performance in the bandpass filter is obtained. The reflectivity of the reflective layers with smaller acoustic impedance ratio, however, is worse, thus there exists a need for more layers. Consequently, the propagation route of the acoustic wave is increased and the Q value thereof is also degraded due to the transmission loss. 
   SUMMARY 
   In view of the above, the invention provides a filter assembly capable of unbalanced to balanced conversion and better stopband noise rejection. 
   According to one aspect of the invention, the filter assembly used in a front-end module of the mobile communication system comprises an unbalanced to balanced conversion unit coupled to an input terminal for transforming a received unbalanced signal thereat to a balanced signal and a lattice filter coupled between the unbalanced to balanced conversion unit and two output terminals for eliminating or rejecting the noise of the balanced signal. 
   The lattice filter can be used as an acoustic component, thus, filters such as FBARs in Stacked Crystal Filter (SCF) or Coupling Resonator Filter (CRF) arrangements, or SMRs can be adopted with the lattice filter of the filter assembly in the invention for eliminating the extra resonant modes of the unbalanced to balanced conversion unit to obtain a filter assembly with high Q value, low insertion loss and high frequency stop band attenuation. 
   Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the following detailed description and the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention, and in which: 
       FIG. 1  is a schematic diagram of a conventional front-end module in a mobile communication system. 
       FIG. 2A  is a schematic diagram of a thin film bulk acoustic resonator (FBAR) in stacked crystal filter (SCF) arrangement. 
       FIG. 2B  is a schematic diagram of another thin film bulk acoustic resonator (FBAR) in stacked crystal filter (SCF) arrangement. 
       FIG. 2C  is a schematic diagram of a thin film bulk acoustic resonator (FBAR) in coupling resonator filter (RCF) arrangement. 
       FIG. 2D  is a schematic diagram of a solidly mounted resonator (SMR) in stacked crystal filter (SCF) arrangement. 
       FIG. 3A  is the frequency response of the FBAR in  FIG. 2B . 
       FIG. 3B  is the frequency response of the FBAR in  FIG. 2D . 
       FIG. 4A  is a schematic diagram of an acoustic component according to an embodiment of the invention. 
       FIG. 4B  is the frequency response of the acoustic component in  FIG. 4A . 
       FIG. 5A  is a schematic diagram of a filter assembly according to an embodiment of the invention. 
       FIG. 5B  is the frequency response of the filter assembly in  FIG. 5A . 
   

   DETAILED DESCRIPTION 
   The invention solves the problem that present a front-end module for a mobile communication system is not capable of unbalance to balanced conversion using only the piezoelectric film acoustic component or a filter assembly. Only the principle and operation of the main components of the invention are described in the following, and details of other related art components such as FBAR, SMR, SCF and CRF are therefore omitted. 
   With reference to  FIGS. 4A and 4B ,  FIG. 4A  is an acoustic component according to an embodiment of the invention and  FIG. 4B  shows the frequency response thereof. As shown in  FIGS. 4A and 4B , a lattice filter  150  comprising four single port FBARs is adopted as an acoustic component in this embodiment, wherein a first resonator  152  and a second resonator  154  are disposed according to the signal transmission direction and the third resonator  156  and the fourth resonator  158  are disposed perpendicular to the signal transmission direction. Furthermore, the electrode area of the first resonator  152  is the same as that of the second resonator  154  and greater than that of the third resonator  156  which is the same as the electrode area of the fourth resonator  158 . The greater the electrode area ratio between the electrode area of the first resonator  152  and that of the third resonator  156 , the narrower the bandwidth of the frequency response of the lattice filter  150 . 
   For instance, as the curve  160  of  FIG. 4B  shows, when the electrode area ratio between the electrode area of the first resonator  152  and that of the third resonator  156  is 1.2, there are one central passband having a center frequency of 1600 MHz, and two noise stop bands having boundary frequencies of 1500 MHz and 1700 MHz, in the frequency response of lattice filter  150 . When the electrode area ratio between the electrode area of the first resonator  152  and that of the third resonator  156  is adjusted to 1.02, as the curve  170  of  FIG. 4B  shows, there are one central passband having a center frequency of 1600 MHz, and two noise stop bands having boundary frequencies of 1300 MHz and 1900 MHz. It can be derived from above that the center frequency does not vary when the corresponding resonator area ratio of the lattice filter  150  is reduced, but the bandwidth of the central passband widens, wherein the boundary frequencies move towards the each side of the frequency response diagram. With reference to  FIGS. 5A and 5B ,  FIG. 5A  is a filter assembly according to an embodiment of the invention and  FIG. 5B  shows the frequency response thereof. As shown in  FIGS. 5A and 5B , the filter assembly  200  of the embodiment comprises an unbalanced to balanced conversion unit  210 , a lattice filter  150 , an input terminal  155  and two output terminals  157  and  159 . 
   In this embodiment, the unbalanced to balanced conversion unit  210  can be a bandpass filter, a FBAR in SCF or CRF arrangements or a SMR. Those skilled in the art will be familiar with various ways of implementing the unbalanced to balanced conversion unit  210  with components capable of unbalanced to balanced conversion. 
   With reference to  FIG. 5A , the lattice filter  150  comprises a first resonator  152  and a second resonator  154  coupled in series with the output terminal of the unbalanced to balanced conversion unit  210 , a third resonator  156  coupled in parallel with the unbalanced to balanced conversion unit  210  and the second resonator  154 , between the output terminals  157  and  159 , and a fourth resonator  158  coupled in parallel with the unbalanced to balanced conversion unit  210  and the first resonator  154  between the output terminals  157  and  159 . 
   When an unbalanced signal is inputted to the filter assembly  200  via the input terminal  155 , the frequency response of the output signal of the filter assembly  200  at terminals  157  and  159  is equal to that of the unbalanced to balanced conversion unit  210  plus that of the lattice filter  150  because the filter assembly  200  employs an unbalanced to balanced conversion unit  210  coupled in series with a lattice filter  150 . 
   As the frequency response curve  100  of  FIG. 3A  shows, there are one central passband having a center frequency of 1600 MHz, and two noise pass bands having boundary frequencies of 1200 MHz and 2100 MHz, in the frequency response of the unbalanced to balanced conversion unit  210 . With the ability of the lattice filter  150  to adjust bandwidth as previously described, the center frequency of the lattice filter  150  can be adjusted to the center frequency of the unbalanced to balanced conversion unit  210  and the electrode area ratio between the electrode area of the first resonator  152  and that of the third resonator  156  (or the second and fourth resonators  154  and  158 ) of the lattice filter  150  is then adjusted until the boundary frequencies of the frequency response of the lattice filter  150  is the same as the center frequency of the noise passbands of the unbalanced to balanced conversion unit  210 . Thus the two noise passbands of the unbalanced to balanced conversion unit  210  are counteracted whereas the central passband of the lattice filter  150  is greater than that of the unbalanced to balanced conversion  210 . Frequency response curve  250  of the balanced signal outputted by the filter assembly  200  as shown in  FIG. 5B  is then obtained. 
   From the forgoing discussion, it can be seen that the invention not only solves the problem of present filters such as FBARs in SCF or RCF arrangements, or SMRs which extra resonant modes thereof allow noise to pass through, but also has the advantage of low insertion loss, high frequency stop band attenuation with bulk acoustic resonators having a high Q value. 
   While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.