Abstract:
A ladder type filter includes series resonators connected in series between an input terminal and an output terminal, parallel resonators connected in parallel between the input terminal and the output terminal, a resonator connected in series with the series resonators between the input terminal and the output terminal, the resonator having a resonance frequency lower than resonance frequencies of the series resonators, and an inductor connected in parallel with the resonator. According to the present ladder filter, signals having frequencies away from the pass band can be suppressed by an attenuation pole formed by the inductor. it is further possible to suppress the insertion loss in the pass band by the resonator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation application of PCT/JP2008/073752 filed Dec. 26, 2008, the contents of which are herein wholly incorporated by reference. 
     
    
     FIELD 
       [0002]    A certain aspect of the present invention relates to duplexers and electronic devices. 
       BACKGROUND 
       [0003]    Recently, downsizing of radio communication terminals, which may be typically cellular phones, has progressed. For downsizing, it is studied to reduce the number of parts used in the radio communication terminals. The number of parts used in the radio communication terminals may be achieved by omitting an interstage filter provided in a transmission path or a reception path. However, it is required to improve the isolation characteristic between a transmission terminal of a duplexer and a reception terminal thereof. 
         [0004]    Japanese Patent Application Publication Nos. 2006-60747 and 2002-76879 describe improvements in the isolation characteristic of the duplexer by removal of unwanted electromagnetic couplings. 
       SUMMARY OF THE INVENTION 
       [0005]    According to an aspect of the present invention, there is provided a duplexer including: a transmission filter connected between a common terminal and a transmission terminal; a reception filter connected between the common terminal and a reception terminal; and a capacitor connected in parallel with either the transmission filter or the reception filter between at least two terminals Out of the common terminal, the transmission terminal and the reception terminal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a block diagram of a cellular phone; 
           [0007]      FIG. 2  is a block diagram of a supposed cellular phone in future; 
           [0008]      FIG. 3  is a block diagram of a conventional duplexer; 
           [0009]      FIG. 4  is a block diagram that describes the principle; 
           [0010]      FIG. 5  is a diagram that describes an amplitude and a phase difference; 
           [0011]      FIG. 6(   a ) through  6 ( d ) are diagrams that illustrate normalized power associated with the phase difference; 
           [0012]      FIG. 7  is a diagram of a duplexer in accordance with an embodiment 1; 
           [0013]      FIG. 8  is a diagram that illustrates isolation characteristics of the embodiment 1 and comparative example 1; 
           [0014]      FIG. 9  is a diagram that illustrates another duplexer in accordance with the embodiment 1; 
           [0015]      FIG. 10  is a diagram that illustrates yet another duplexer in accordance with the embodiment 1; 
           [0016]      FIG. 11  is a diagram that illustrates a duplexer in accordance with an embodiment 2; 
           [0017]      FIGS. 12(   a ) and  12 ( b ) are respectively cross-sectional and plan views of the embodiment 2; and 
           [0018]      FIG. 13  is a diagram that illustrates isolation characteristics of the embodiment 2 and comparative example 2. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    First, a description is given of the reason why the duplexer is required to have improvement in the isolation characteristic. FIG I is a diagram of an RE (Radio Frequency) block of a cellular phone. Referring to  FIG. 1 , the cellular phone has a duplexer  60 , an antenna  62 , a power amplifier  66 , interstage filters  64  and  68 , and a transceiver IC  70 . The transceiver IC has a reception circuit  52 , a transmission circuit  50 , low noise amplifiers  72  and  74 , mixers  76   a  and  76   b,  low-pass filters  78   a  and  78   b,  variable gain amplifiers  80   a  and  80   b,  a 90° hybrid circuit  82 . and an oscillator  84 . The transmission circuit  50  generates a transmission signal, The interstage filter  68  passes the transmission signal, and suppresses signals having frequencies other than the frequencies of the transmission signal. The power amplifier  66  amplifies the transmission signal. The duplexer  60  outputs the transmission signal to the antenna  62 , and does not output the transmission signal to the reception side. The antenna  62  transmits the transmission signal. 
         [0020]    The antenna  62  receives a reception signal. The duplexer  60  outputs the reception signal to the reception side, and does not output the reception signal to the transmission side. The low noise amplifiers  72  and  74  amplify the reception signal. The interstage filter  64  passes the reception signal and suppresses signals having frequencies other than those of the reception signal. The interstage filter  64  converts an unbalanced signal to balanced signals. The 90° hybrid circuit  82  shifts the phase of an oscillation signal output by the oscillator  84  by 90° and applies the 90° phase shifted oscillation signals to the mixers  76   a  and  76   b , respectively. The mixers  76   a  and  76   b  mix the reception signals with the oscillation signals, The low pass filters  78   a  and  78   b  pass down-converted reception signals and suppress carrier signals. The reception circuit  52  processes the received signals. 
         [0021]    For downsizing and cost reduction of the cellular phones, a circuit as illustrated in  FIG. 2  is desired. Referring to  FIG. 2 , the interstage filters  64  an  68  and the low noise amplifier  72  illustrated in  FIG. 1  are removed, and the duplexer  60  performs the conversion to the balanced signals, The removal of the interstage filters  64  and  68  causes the duplexer  60  to take over the functions of the interstage filters. That is, the filter in the duplexer  60  is required to have suppression performance corresponding to two filters. Particularly, it is required to improve the isolation characteristic of the duplexer. 
         [0022]      FIG. 3  is a block diagram of a duplexer. In a duplexer  100 , a transmission filter  10  is connected between a common terminal Ant and a transmission terminal Tx, A reception filter  20  is provided between the common terminal Ant and a reception terminal Rx. A matching circuit  30  is provided between the transmission filter  10  and the common terminal Ant and between the reception filter  20  and the common terminal Ant. The transmission circuit  50  and the reception circuit  52  are connected to the transmission terminal Tx and the reception terminal Rx, respectively. 
         [0023]    As indicated by a dashed line  102 , the transmission filter  10  passes the signals in the transmission band among the signals that are input via the transmission terminal Tx towards the common terminal Ant, and suppresses signals having other frequencies. As indicated by a dashed line  104 , the reception filter  20  passes the signals in the reception band among the signals that are input via the common terminal Ant, and suppresses signals having other frequencies, The matching circuit  30  establishes impedance matching that causes the transmission signal passing through the transmission filter  10  to be output from the common terminal Ant without leaking to the reception filter  20  side. As described above, ideally, the transmission signal input to the transmission terminal Tx is output to the common terminal Ant via the transmission filter  10  and the matching circuit  30 , and is not output to the reception terminal Rx. However, as indicated by a signal  110  in  FIG. 3 , some of the power of the transmission signal passes through the matching circuit  30  and the reception filter  20  and is output to the reception terminal Rx. The power of the signal input to the transmission terminal Tx is very higher than the power of the reception signal input to the common terminal Ant. It is thus required to reduce the ratio of the transmission signal output to the reception terminal Rx to a very small value. The ratio of power leak to the reception terminal Rx to the power of the transmission signal input to the transmission terminal Tx is referred to as isolation between the transmission terminal and the reception terminal. 
         [0024]    The principle of improvement of the isolation characteristic is described below.  FIG. 4  is a block diagram for describing the principle of improving the isolation characteristic. Referring to  FIG. 4 , a capacitor  40  is connected in parallel with the reception filter  20  between the transmission terminal Tx and the reception terminal Rx. The other structures are the same as those in  FIG. 3  and a description thereof is omitted here. Since a capacitive coupling between the transmission terminal Tx and the reception terminal Rx is made, part of the transmission signal reaches the reception terminal Rx from the transmission terminal Tx via the capacitor  40 , as indicated as a signal  112  in  FIG. 4 . The capacitor  40  couples the transmission terminal Tx and the reception terminal Rx with each other at RF frequencies (for example, 800 MHz˜2.5 GHz used in cellular phones), and disconnects these terminals from each other in DC. 
         [0025]      FIG. 5  is a diagram that illustrates a relationship between the signal  110  and the signal  112 , It is assumed that the amplitude of the signal  112 . normalized by the amplitude of the signal  110  is denoted as A, and the phase difference between the signals  110  and  112  is denoted as P. Solid lines illustrated in  FIGS. 6(   a ) through  6 ( d ) indicate normalized power associated with the phase difference P, the normalized power being obtained by normalizing (during one cycle) the power of the signals  110  and  112  after combining by the power of the signal  110  (dashed lines) before combining. That is, the normalized power that is less than 1 indicates that the isolation characteristic is improved by the signal  112 .  FIGS. 6(   a ) through  6 ( d ) respectively show cases where the amplitude A of the signal  112  is 1.0, 0.75, 0.5 and 0.25. 
         [0026]    According to  FIG. 6(   a ), for an amplitude A of 1 the isolation characteristic is improved within the range of 180±60° in the phase difference P According to  FIG. 6(   b ), for an amplitude A of 0.75, the isolation characteristic is improved within the range of approximately 180±65′ in the phase difference P. According to  FIG. 6(   c ), for an amplitude A of 0.5, the isolation characteristic is improved within the range of approximately 180±75° in the phase difference P. According to  FIG. 6(   d ), for an amplitude A of 0.25, the isolation characteristic is improved within the range of approximately 180±85° in the phase difference. 
         [0027]    As described above, when the phase difference P is 180°, the isolation characteristic has the greatest improvement. When the phase difference P is not 180° but is close to 180°, the isolation characteristic is improved. In the case where the amplitude A is 1 as illustrated in  FIG. 6(   a ), the isolation characteristic is improved within the range of ⅓ (120°) of the total phase difference)(360°). As illustrated in  FIG. 6(   b ), in the case where the amplitude A is 0.25, the isolation characteristic is improved within the range of approximately ½ (170°) of the total phase difference (360°), As described above, the addition of the capacitor  40  is capable of improving the isolation characteristic with a probability of at least ½˜⅓. 
         [0028]    The amplitude A may be controlled by the capacitance value of the capacitor  40 . In order to improve the isolation characteristic considerably; it is preferable that the amplitude A is 1. In order to enlarge the range of the phase difference P in which the isolation characteristic is improved, it is preferable that the amplitude A is smaller than 1. 
         [0029]    The phase difference P results from the difference between the phase of the signal  112  that shifts by the capacitor  40  and the phase of the signal  110  that shifts by the transmission filter  10 , the matching circuit  30  and the reception filter  20 . The phase of the signal  112  that shifts by the capacitor  40  is small because the capacitance value is small. The phase of the signal  110  that shifts by the transmission filter  10 . the matching circuit  30  and the reception filter  20  is comparatively large. Thus, there is a high possibility that the phase difference P may be within the ranges in which the isolation characteristic is improved illustrated in  FIGS. 6(   a ) through  6 ( d ) without a particular phase control. From a viewpoint of the isolation characteristic, it is preferable to connect a phase shift circuit that shifts the phase of the transmission band to the transmission filter  10  or the reception filter  20  in series and the phase difference P is made close to 180°. However, from viewpoints of downsizing and cost reduction, it is preferable that the phase shift circuit is not provided. As described above, the isolation characteristic can be improved simply by connecting the capacitance between the terminals. 
         [0030]    Embodiments are described below with reference to the drawings. 
       Embodiment 1 
       [0031]      FIG. 7  is a diagram of a circuit configuration of a duplexer in accordance with an embodiment 1. The transmission filter  10  and the reception filter  20  are ladder type filters. in the transmission filter  10  series resonators S 1  and parallel resonators P 1  are arranged in the form of a ladder. Similarly, in the reception filter  20 , series resonators S 2  and parallel resonators P 2  are arranged in the form of a ladder. The transmission filter  10  and the reception filter  20  used in a simulation are respectively ladder type filters in which six stages are connected. The series resonators S 1  and S 2  and the parallel resonators P 1  and P 2  use piezoelectric thin-film resonators (FBAR: Film Bulk Acoustic wave Resonator). Instead, these resonators may be surface acoustic wave (SAW: Surface Acoustic Wave) resonators or SMRs (Solidly Mounted Resonators). 
         [0032]    The matching circuit  30  is connected between the common terminal Ant and the transmission filter  10  and between the common terminal Ant and the reception filter  20 . The matching circuit  30  has an inductor  32  connected between the common terminal Ant and ground. The capacitor  40  is connected in parallel with the transmission filter  10  and the reception filter  20  between the transmission terminal Tx and the reception terminal Rx. 
         [0033]    The isolation characteristic was simulated in such a way that the pass band of the transmission filter  10  was set to 1920 MHz˜1980 MHz, and the pass band of the reception filter  20  was set to 2110˜2170 MHz, supposing duplexers used in W-CDMA (Wideband Code Division Multiple Access). In the embodiment 1, the ratio Cpt/Cst between the capacitance Cpt of parallel resonators P 1  of the transmission filter  10  and the capacitance Cst of the series resonators S 1  thereof is set equal to 0.3. The ratio Cpr/Csr between the capacitance Cpr of the parallel resonators P 2  of the reception filter  20  and the capacitance Csr of the series resonators S 2  thereof is set equal to 0.5. The capacitance of the capacitor  40  is set to 1.5 fF, and the inductance of the inductor  32  is set to 6 nH, In a comparative example 1, the capacitor  40  is not connected. 
         [0034]      FIG. 8  is a diagram illustrating simulation results of the isolation characteristic of the embodiment 1 (solid line) and the isolation characteristic of the comparative example 1 (dashed line) of the comparative example that does not have the capacitor  40 . As illustrated in  FIG. 8 , the embodiment 1 is capable of improving the isolation characteristic by 3˜9 dB over the whole transmission range, as compared with the comparative example 1. 
         [0035]      FIG. 9  illustrates another example of the embodiment 1. As illustrated in  FIG. 9 , the capacitor  40  may be connected in parallel with the reception filter  20  between the common terminal Ant and the reception terminal Rx. In the example of  FIG. 9 , some of the power of the transmission signal applied to the common terminal Ant reaches the reception terminal Rx via the capacitor  40 . This signal is capable of canceling the transmission signal that leaks from the reception filter  20  and reaches the reception terminal Rx. Thus, the isolation characteristic can be improved. 
         [0036]      FIG. 10  illustrates yet another example of the embodiment 1. As illustrated in  FIG. 10 , the capacitor  40  may be connected in parallel with the transmission filter  10  between the common terminal Ant and the transmission terminal Tx. In the example illustrated in  FIG. 10 , some of the power of the reception signal applied to the common terminal Am reaches the transmission terminal Tx via the capacitor  40 . This signal is capable of canceling the reception signal that leaks from the transmission filter  10  and reaches the reception terminal Rx. Thus, it is possible to improve the isolation characteristic from the reception terminal Rx to the transmission terminal Tx. 
         [0037]    As illustrated in  FIGS. 7 ,  9  and  10 , the capacitor  40  may be connected in parallel with the transmission filter  10  or the reception filter  20  between at least two of the common terminal Ant, the transmission terminal Tx and the reception terminal Rx. Since the transmission signal input to the transmission terminal Tx is very greater than the reception signal input to the common terminal Ant, the isolation from the transmission terminal Tx to the reception terminal Rx is particularly likely to raise a problem. Thus, one end of the capacitor  40  is preferably connected to the reception terminal Rx. 
         [0038]    When the transmission signal passes through both the transmission filter  10  and the reception filter  20 , the phase of the transmission signal shifts greatly and the phase difference P is capable of becoming close to 180°. As illustrated in  FIG. 7 , the capacitor  40  is preferably connected between the transmission terminal Tx and the reception terminal Rx. Thus, downsizing may be achieved without the phase shift circuit or the like. 
       Embodiment 2 
       [0039]    An embodiment 2 is an example configured to have a plurality of capacitors and a microstrip line is provided between the capacitors,  FIG. 11  is a diagram of a circuit configuration of the embodiment 2. As illustrated in  FIG. 11 , a duplexer of the embodiment 2 has multiple capacitors  40 . A microstrip line  42  is connected between the capacitors  40 . The other structures are the same as those illustrated in  FIG. 7  of the embodiment 1, and a description thereof is omitted here. 
         [0040]      FIG. 12(   a ) is a cross-sectional view of the duplexer of the embodiment 2, and  FIG. 12(   b ) is a plan view of a second layer  91 . Referring to  FIG. 12(   a ), a package  90  has a plurality of ceramic layers, which may be composed of a first layer  92  and the second layer  91 . The first layer  92  defines a cavity  96  for sealing filter chips  12  and  22 , A lid  93  is fixed to the top of the first layer  92 , and the filter chips  12  and  22  are thus sealed. An interconnection  97  is formed on the surface of the second layer  91 , The filter chips  12  and  22  are flip-chip mounted on the interconnection via bumps  95 , Foot pads  94  are formed on the lower surface of the second layer  91 . Vias  98  that pass through the second layer  91  and are full of a metal are formed. The transmission filter  10  is formed on the filter chip  12 , and the reception filter  20  is formed on the filter chip  22 . 
         [0041]    Referring to  FIG. 12(   b ), the filter chips  12  and  22  and the inductor  32  (inductor chip) are flip-chip mounted on the upper surface of the second layer  91 . One end of the transmission filter  10  of the filter chip  12 , one end of the reception filter  20  of the filter chip  22 , and one end of the inductor  32  are connected to the foot pad  94  of the common terminal Ant via a line Lant of the interconnection  97  and a via Vain of one of the vias  98 . The other end of the inductor  32  is connected to a ground foot pad via a line Lg of the interconnection  97  and a via Vg of one of the vias  98 . The other end of the transmission filter  10  of the filter chip  1  is connected to the foot pad  94  of the transmission terminal Tx via a line Lt of the interconnection  97  and a via Vt of one of the vias  98 . The other end of the reception filter  20  of the filter chip  22  is connected to the foot pad  94  of the reception terminal Rx via a line Lr of the interconnection  97  and a via Vi of one of the vias  98 . The microstrip line  42  is spaced apart from the lines Lt and Lr, and these spaces form the capacitors  40 . 
         [0042]    The duplexer of the embodiment 2 and a comparative example having the microstrip line were manufactured and the isolation characteristics thereof were measured. The ratio Cpt/Cst between the capacitance of the parallel resonators P 1  of the manufactured transmission filter  10  and the capacitance Cst of the series resonators S 1  thereof was equal to 0.3. The ratio Cpr/Csr between the capacitance Cpr of the parallel resonators P 2  of the manufactured reception filter  20  and the capacitance Csr of the series resonators S 2  was equal to 0.8, The characteristic impedance of the microstrip line  42  was  26  Ω, and the spaces between the microstrip line  42  and the lines Lt and Lr that form the capacitors  40  were approximately 100 μm. Glass ceramic was used for the second layer  91 . 
         [0043]      FIG. 13  is a diagram that illustrates a frequency dependence of isolation in the embodiment 2 (solid line) and the comparative example 2 (dashed line). The isolation in the transmission band in the example 2 is improved by 4 dB, as compared with the comparative example 2, As described above, even in the embodiment 2, the isolation characteristic can be improved. 
         [0044]    It is required that the amplitude A of the signal after passing through the capacitor  40  is approximately equal to the amplitude of the transmission signal that leaks from the reception filter  20 . Thus, the capacitance value of the capacitor  40  is as very small as fF. Thus, as in the case of the embodiment 2 it is preferable that the capacitors  40  are each formed by two lines formed on the same surface of the insulation layer (second layer  91 ) in the package  90 . It is thus possible to realize the capacitances having small capacitance values. 
         [0045]    It is preferable that the capacitance is realized by connecting multiple capacitors  40  in series. It is thus possible to realize the capacitor between the terminals having a small capacitance value. 
         [0046]    The lines Lant, Lt and Lr are spaced. apart from each other in order to suppress the interference between the lines. When at lest two of the lines Lant, Lt and hr are coupled via a capacitance, a line for connecting the at least two of the lines Lant. Lt and Lr is provided. The two capacitors  40  are provided in the vicinities of the vias Vant, Vt and Vr, and two capacitors  40  are connected by the microstrip line  42 . That is, the two capacitors  40  are formed by two of the lines Lant, Lt and Lr formed on the surface of the insulation layer (second layer  91 ) and both ends of the microstrip line  42 . It is thus possible to reduce the area for arranging the capacitors  40 . The microstrip line  42  may transmit a signal for suppressing isolation. The characteristic impedance of the microstrip line  42  is not limited specifically as long as the above signal is transmitted. 
         [0047]    The duplexers of the embodiments 1 and 2 may be used for electronic devices such as cellular phones that have been described with reference to  FIG. 2 . 
         [0048]    The embodiments of the present invention have been described. The present invention is not limited to these specific embodiments but may be varied or changed within the scope of the claimed invention.