Abstract:
A surface acoustic wave duplexer having an antenna terminal, a transmitting terminal and a receiving terminal. The surface acoustic wave duplexer includes a transmitting SAW filter coupled between the antenna terminal and the transmitting terminal, a receiving SAW filter coupled between the antenna terminal and the receiving terminal, a common piezoelectric substrate on which both of the transmitting SAW filter and the receiving SAW filter are formed, a package covering the common piezoelectric substrate. The antenna terminal, the transmitting terminal and the receiving terminal are formed on the package, and a frequency adjusting circuit is coupled between the antenna terminal and the transmitting SAW filter or the receiving SAW filter. The frequency adjusting circuit has a capacitance element.

Description:
This nonprovisional application is a divisional of U.S. application Ser. No. 09/305,304, filed May 5, 1999 now U.S. Pat. No. 6,222,426. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a branching filter that uses an surface acoustic wave (SAW) resonance type filter used for compact mobile communication equipment for portable telephones and the like. 
     2. Description of Related Art 
     In recent years, advances have been made in the development of terminals for compact, light mobile communication equipment such as portable telephones. RF (Radio Frequency) filters are incorporated into these terminals. Surface acoustic wave (SAW) resonance type filters are used for this RF filter. 
     Accompanying the development of these terminals is a demand for the parts to be made more compact and to have higher performance. Therefore, there is also a demand for more compact, higher performance SAW resonance filters (also called SAW elements). 
       FIG. 9  is a block diagram of a structural example of a conventional portable telephone branch filter. 
     The branch filter  10  shown in  FIG. 9  comprises an antenna (ANT) terminal  11 , an LC chip  12 , a transmission filter  13 , an Rx-branching filter circuit strip line  14 , a receiving filter  15 , a transmission (Tx) terminal  16 , and a receiving (Rx) terminal  17 . The LC chip  12  is provided between the ANT terminal  11  and ground. The transmission filter  13  is connected between ANT terminal  11  and Tx terminal  16 , and the series circuit of the Rx-branching filter circuit strip line  14  and the receiving filter  15  are connected in this sequence between the ANT terminal  11  and the receiving terminal  17 . 
       FIG. 10  is a circuit structural figure of the specific circuit structure of the branching filter shown in FIG.  9 . Reference number  11  is the ANT terminal,  2 ,  3 , and  4  are branching filter circuit strip lines (inductance) (equivalent to 14 in FIG.  9 ),  13  is the transmission filter,  15  is the receiving filter,  16  is the transmission (Tx) terminal, and  17  is the receiving (Rx) terminal. 
     Conventionally, with this type of portable telephone branching filter, the transmission and receiving filters were each composed using dielectric resonators. 
       FIGS. 11 and 12  show a portable telephone branching filter and mounting aspect, respectively.  FIG. 11  is a schematic perspective view of the front surface, and  FIG. 12  is a schematic perspective view of the back surface. 
     As is clear from the structural examples shown in  FIGS. 11 and 12 , chips  13  and  15  of the transmission and receiving filters are incorporated into on-board substrate  9 . Branching filter circuit strip lines  2 ,  3 , and  4  are provided as structural elements on this on-board substrate  9 . As is also clear from  FIG. 12 , this on-board substrate  9  comprises an insulation substrate  9   a  such as a resin substrate, low temperature sinter substrate, or aluminum substrate, a metallized conductive coating pattern  9   b  provided thereon, and an insulation pattern  9   c  formed by exposing substrate  9   a.  Branching filter circuit strip lines  2 ,  3 , and  4  are formed in continuum with conductive coating pattern  9   b.    
     With this type of portable telephone branching filter, each chip of the transmission filter  13  and the receiving filter  15  is provided divided on separate piezoelectric substrates. Then, these two piezoelectric substrates are individually incorporated into one on-board substrate  9 , offering the merit of excellent insulation characteristics for both filters. 
     However, besides the piezoelectric substrate on which are provided the chips of these transmission and receiving filters  13  and  15 , an on-board substrate  9  with a space for incorporating the Rx-branching filter circuit strip line  14  and the LC chip  12  (thus, branching filter circuit strip lines  2 ,  3 , and  4  shown in  FIG. 11 ) is needed, so the on-board substrate becomes large, and the connecting wiring for the branching filter structure becomes long. Because of this, the structure of this on-board substrate  9  becomes complex, and the area occupied by the connecting wiring increases. This inhibits making the on-board substrate and thus the branching filter more compact. 
     On the other hand, a branching filter has been developed that uses a SAW resonator for the transmission filter and receiving filter (Japanese Patent Laid-open No. 6-97761). For the branching filter that uses a SAW resonance type filter disclosed in this publication, the transmission and receiving filters comprise ladder-type resonator filters with a structure similar to a serial arm SAW resonator and a parallel arm SAW resonator. With this conventional branching filter, it is possible to make the branching filter more compact to some degree, but the problem of insulation between filters has not been looked into yet.  FIG. 13  is a schematic perspective view showing a structural example of the branching filter disclosed in this Japanese Patent Laid-open No. 6-97761. This branching filter has a structure with which the structural elements are incorporated into package  20 . Specifically, inside a package construction  21 A is provided a ground layer  21 B, an impedance matching element  22 , a phase adjustment element  23 , a trap circuit  24 , a transmission SAW filter element  25 , and a receiving SAW filter element  26 . 
     In this way, the conventional branching filter disclosed in Japanese Patent Laid-open No. 6-97761 is structured to house in a single package  20  the transmission SAW filter element  25 , the receiving SAW filter element  26 , an LC chip, and an Rx-branching filter circuit strip line. 
     However, in this case, particularly because the transmission SAW filter element  25 , the receiving SAW filter element  26 , the LC chip (phase adjustment element  23 ), and the Rx-branching filter circuit strip line (impedance matching element  22 ) are housed within the same package  20 , there are problems including a degradation of the insulation characteristics between the transmission area and receiving area, and a degradation of the branching filter characteristics as an interaction works between the connecting wires. 
     SUMMARY OF THE INVENTION 
     Thus, an object of the present invention is to provide a branching filter that uses a SAW resonance type filter that is capable of being made more compact as well as having a higher performance level. 
     Another object of the present invention is to provide branching filter that uses a SAW resonance type filter with a structure with which the transmission SAW filter and receiving SAW filter can be placed in one chip. 
     To achieve these objects, the branching filter of the present invention comprises the unique structure described below. Specifically, the branching filter of the present invention comprises a SAW resonator. The SAW resonator comprises a transmission SAW filter linked between an antenna terminal and transmission terminal, a receiving SAW filter with different bandpass characteristics from the above-mentioned transmission SAW filter linked between the above-mentioned antenna terminal and receiving terminal, and a composite circuit of a frequency adjusting LC circuit and branching filter circuit (also called a strip line for a branching filter circuit) linked between the above-mentioned antenna terminal and the above-mentioned transmission and receiving SAW filters. Also, with the present invention, this branching filter circuit is composed from a serial arm SAW resonator. Specifically, all or part of this branching filter circuit is structured as a serial arm SAW resonator. 
     For a preferred embodiment of the present invention, it is desirable to have a structure, between the antenna terminal and transmission SAW filter, with a frequency adjusting LC circuit connected to the antenna terminal and a Tx-branching filter circuit strip line connected as a branching filter circuit between the above-mentioned LC circuit and transmission SAW filter for the composite circuit. It is also acceptable to have a structure with only a frequency adjusting LC circuit connected between the antenna terminal and transmission SAW filter for the composite circuit. 
     For a preferred embodiment of the present invention, it is desirable to have a structure, between the antenna terminal and receiving SAW filter, with the frequency adjusting LC circuit described above connected to the antenna terminal and an Rx-branching filter circuit strip line connected as a branching filter circuit between the aforementioned LC circuit and receiving SAW filter as the composite circuit. 
     For a preferred embodiment of the present invention, it is desirable to form a single piezoelectric substrate shared by the transmission SAW filter and receiving SAW filter. 
     With a preferred embodiment of the present invention, it is desirable to form a single piezoelectric shared by the transmission SAW filter, the receiving SAW filter, and the branching filter circuit. 
     With another preferred embodiment of the present invention, it is desirable to form a single piezoelectric substrate shared by the transmission SAW filter, the frequency characteristics adjusting LC element, the Rx-branching filter circuit strip line, and the receiving SAW filter. 
     With yet another preferred embodiment of the present invention, it is desirable to form a single piezoelectric substrate shared by the transmission SAW filter, the receiving SAW filter, and the branching filter circuit, and to provide a frequency adjusting LC element outside of the piezoelectric substrate. 
     With yet another preferred embodiment of the present invention, it is desirable to provide on the on-board substrate a piezoelectric substrate on which in some cases the transmission SAW filter and receiving SAW filter are formed together with the branching filter circuit and/or frequency adjusting LC element. 
     Also, for this preferred embodiment, it is desirable to form a single combined SAW resonator from a first level (stage) serial arm SAW resonator on the antenna terminal side and a serial arm SAW resonator of the branching filter circuit for both or only one of the transmission SAW filter and the receiving SAW filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the present invention will be better understood from the following description taken in connection with the accompanying drawings, in which; 
         FIG. 1  is a block diagram that gives a summary explanation of a structural example of the branching filter using a SAW resonator type filter of the present invention; 
         FIG. 2  is a circuit block diagram for explaining a specific structural example of the branching filter using a SAW resonator type filter of the present invention; 
         FIG. 3  is a circuit block diagram for explaining another specific structural example of the branching filter using a SAW resonance type filter of the present invention; 
         FIG. 4  (including FIGS.  4 (A) through  4 (C)) is a schematic oblique diagram for explaining an aspect of the branching filter using a SAW resonance type filter of the present invention; 
         FIG. 5  is a block diagram for explaining the function of each structural element of the branching filter when the transmission operation is performed for the branching filter using a SAW resonance type filter of the present invention; 
         FIG. 6  is a block diagram for explaining the function of each structural element of the branching filter when the receive operation is performed for the branching filter using a SAW resonance type filter of the present invention; 
         FIG. 7  is a figure that provides an explanation of the impedance of the branch filter using a SAW resonance type filter of the present invention; 
         FIG. 8  (including FIGS.  8 (A) and  8 (B)) is a figure that provides an explanation of the serial arm SAW resonator and its LC equivalent circuit used for the branching filter using a SAW resonance type filter of the present invention; 
         FIG. 9  is a block diagram that gives a summary explanation of a structural example of a conventional branching filter using a SAW resonance type filter; 
         FIG. 10  is a figure that explains a specific structural example of a conventional branching filter using a SAW resonance type filter; 
         FIG. 11  is a schematic perspective view seen from the front side for explaining an aspect of a conventional branching filter using a SAW resonance type filter; 
         FIG. 12  is a schematic perspective view seen from the back side for explaining an aspect of a conventional branching filter using a SAW resonance type filter; and 
         FIG. 13  is a schematic perspective view seen from the front side for explaining another aspect of a conventional branching filter using a SAW resonance type filter. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings, a detailed explanation will be given to preferred embodiments of the branching filter of the present invention. In the drawings, the structural elements are simply shown in summary form to make the invention easier to understand.  FIG. 1  is a block diagram that schematically shows a structural example of the branching filter using a SAW resonator type filter of the present invention. 
     The branching filter  100  in the structural example shown in  FIG. 1  comprises an antenna terminal  102 , a transmission terminal  104 , and a receiving terminal  106 . Also, this branching filter  100  comprises a transmission SAW filter  140  linked or connected between the antenna terminal  102  and the transmission terminal  104  and a receiving SAW filter  150  linked or connected between this antenna terminal  102  and the receiving terminal  106 . This transmission SAW filter  140  and receiving SAW filter  150  have different bandpass characteristics from each other. Furthermore, the branching filter  100  comprises a composite circuit  160  made from a frequency adjusting LC circuit  108  and a branching filter circuit  110  between this antenna terminal  102  and each of the transmission SAW filter  140  and the receiving SAW filter  150 . These transmission and receiving SAW filters  140  and  150 , the frequency adjusting LC circuit  108 , and the branching filter circuit  110  form a branching filter circuit that uses a SAW resonator type filter. 
     Also, with the present invention, as will be described later with reference to  FIGS. 2 and 3 , part of this branching filter circuit  110  is constructed from a serial arm SAW resonator. 
     Preferably, the branching filter circuit  110  comprises a transmission side Tx-branching filter circuit strip line  120  and a receiving side Rx-branching filter circuit strip line  130 . However, the Tx-branching filter circuit strip line  120  is not absolutely necessary, so it can be used as appropriate for specific designs. 
     Therefore, it is acceptable to construct the composite circuit  160 , between the antenna terminal  102  and the transmission SAW filter  140 , with the frequency adjusting LC circuit  108  connected to the antenna terminal  102  and the Tx-branching filter circuit strip line  120  connected between the aforementioned LC circuit  108  and the transmission SAW filter  140 . And/or, this composite circuit  160  can be constructed only with the frequency adjusting LC circuit  108  connected between the antenna terminal  102  and the transmission SAW filter  140 . 
     On the other hand, this composite circuit  160  is preferably constructed between the antenna terminal  102  and the receiving SAW filter  150  with the frequency adjusting LC circuit  108  described above connected to the antenna terminal  102  and the Rx-branching filter circuit strip line  130  connected between the aforementioned LC circuit  108  and the receiving SAW filter  150 . 
     A specific example of the branching filter  100  described above will be explained with reference to  FIGS. 2 and 3  in a case when the branching filter  100  comprises the Tx-branching filter circuit strip line  120  and Rx-branching filter circuit strip line  130 .  FIG. 2  is a circuit diagram that shows a specific structural example of the branching filter  100  using a SAW resonator of the present invention.  FIG. 3  is a circuit diagram that shows another specific structural example of the branching filter  100  of the present invention. 
     In the structural example shown in  FIG. 2 , the transmission SAW filter  140  is constructed as a ladder-type filter made from a two layer structure of a serial arm resonator and a parallel arm resonator. Specifically, the serial arm, connected between the Tx-branching filter circuit strip line  120  and the transmission terminal  104 , comprises a first level (first) serial arm resonator (TS 1 )  140   a  from the Tx-branching filter circuit strip line  120  side and a second level (second) serial arm resonator (TS 2 )  140   b.  The parallel arm comprises a first layer (first) parallel arm resonator (TS 3 )  140   c  connected between the first layer and second layer serial arm resonators  140   a  and  140   b  connection points and earth and a second layer (second) parallel arm resonator (TS 4 )  140   d  connected between the transmission terminal  104  and earth. 
     In comparison, the receiving SAW filter  150  is constructed as a ladder-type filter made from a three layer structure serial arm resonator and parallel arm resonator. Specifically, the serial arm, connected between the Rx-branching filter circuit strip line  130  and the receiving terminal  106 , comprises a first layer (first) serial arm resonator (RS 1 )  150   a  from the Rx-branching filter circuit strip line  130  side, a second layer (second) serial arm resonator (RS 2 )  150   b,  and a third layer (third) serial arm resonator (RS 3 )  150   c.  The parallel arm comprises a first layer parallel arm resonator (RP 1 )  150   d  connected between the connection point of first layer and second layer serial arm resonators  150   a  and  150   b  and earth, a second layer (second) parallel arm resonator (RP 2 )  150   e  connected between the connection point of second and third serial arm resonators  150   b  and  150   c  and earth, and a third layer (third) parallel arm resonator (RP 3 )  150   f  connected between receiving terminal  106  and earth. 
     With the structural example shown in  FIG. 2 , from the perspective of making the branching filter and therefore the SAW resonator filter more compact, the branching filter circuit strip lines  120  and  130  are respectively composed from the serial arm resonators (TxS and RxS)  120   a  and  130   a.    
     In  FIG. 2 , the frequency adjusting LC circuit  108  comprises a capacitor component  108   a  and an inductor component  108   b  which exist between the antenna terminal  102  and the branching filter circuit  110 , and therefore between the Tx-branching filter circuit strip line  120  and the Rx-branching filter circuit strip line  130 . The capacitance of this capacitor component  108   a  is C ANT , and the inductance of the inductor component  108   b  is L ANT . 
     With the present invention, as shown by the structural example shown in  FIG. 2 , it is also acceptable to provide the serial arm resonators  120   a  and  130   a  for branching filter circuit strip lines described above and the transmission and receiving SAW filter first level serial arm resonators  140   a  and  150   a  individually. 
     However, to make the resonator filter more compact, it is also acceptable to combine these two transmission side serial arm resonators  120   a  and  140   a  to construct a single composite or combined resonator. Similarly, it is also acceptable to combine these two receiving side serial arm resonators  130   a  and  150   a  to construct a single composite or combined resonator.  FIG. 3  shows a structural example with these serial arm resonators  120   a  and  140   a  combined into a composite resonator  142  and serial arm resonators  130   a  and  150   a  combined into a composite resonator  152 . The other structural elements shown in  FIG. 3  are constructed in the same manner as the structural example shown in FIG.  2 . 
     However, with the present invention, as has already been explained, the goal is to achieve a more compact branching filter with a higher performance level by combining the transmission and receiving filters into one chip. To do this, in addition to the mechanism of the circuit structure described above, if possible, it is also necessary to have a mechanism for surface mounting of the structural elements that form the branching filter. 
       FIG. 4  (including FIGS.  4 (A) through  4 (C)) is a schematic perspective view that explains a structural example seen from the perspective of an aspect of the branching filter of the present invention. 
     FIG.  4 (A) shows an example of the transmission SAW filter  140  and the receiving SAW filter  150  formed together on one piezoelectric substrate  170 . Then, this piezoelectric substrate  170  is incorporated into the package on-board substrate  180 . A resin substrate, low temperature sinter substrate, or aluminum substrate can be used as this on-board substrate  180 . It is also possible to use a multi-layer substrate for this on-board substrate. In this case, it is possible to provide the frequency adjusting LC circuit and branching filter circuit strip line outside the piezoelectric substrate  170 , one example being on-board substrate  180 . In FIG.  4 (A), Tx-in and Tx-out are transmission input terminals and output terminals, and Rx-in and Rx-out are receiving input terminals and output terminals. Transmission output terminal and receiving input terminal Tx-out and Rx-in are connected to the antenna terminal  102  ( FIG. 1 ) in on-board substrate  180 . On the other hand, transmission input terminal and receiving output terminal Tx-in and Rx-out correspond respectively to the transmission terminal  104  and the receiving terminal  106  shown in FIG.  1 . 
     FIG.  4 (B) shows a structural example of the transmission SAW filter  140 , the receiving SAW filter  150 , and the branching filter circuit  110  being formed on a single common piezoelectric substrate  170 . When the Tx-branching filter circuit strip line  120  and the Rx-branching filter circuit strip line  130  are contained in the branching filter circuit strip line  110 , both can be provided on this piezoelectric substrate  170 . Or, when only the Tx-branching filter circuit strip line  120  is contained in the branching filter circuit strip line  110 , it is acceptable to provide only this Tx-branching filter circuit strip line  120  on the piezoelectric substrate  170 . Also, in FIG.  4 (B), the required wiring and input terminals and output terminals are not illustrated, and the Tx-branching filter circuit strip line  120  is shown by a dotted line while the Rx-branching filter circuit strip line  130  is shown by a solid line. In this structural example, the frequency adjusting LC element  108  can be provided outside the piezoelectric substrate  170 . 
     FIG.  4 (C) shows a structural example in which the transmission SAW filter  140 , the frequency characteristics adjusting LC element  108 , the Rx-branching filter circuit strip line  130 , and the receiving SAW filter  150  are all formed on a single common piezoelectric substrate  170 . When the Tx-branching filter circuit strip line  120  is contained in the branching filter circuit strip line  110 , this Tx-branching filter circuit strip line  120  can be provided on the piezoelectric substrate  170 . Also, in this FIG.  4 (C), required wiring and input terminals and output terminals are not illustrated, and this Tx-branching filter circuit strip line  120  is shown as a dotted line. 
     In this way, the transmission SAW filter  140  and the receiving SAW filter  150 , or in some cases, the branching filter circuit  110  and/or the frequency adjusting LC element  108  are formed together on the piezoelectric substrate  170  (the piezoelectric substrate shown on any of FIGS.  4 (A) through  4 (C)), and this can be provided on the on-board substrate  180 . 
     The Tx-branching filter circuit strip line  120  and the Rx-branching filter circuit strip line  130  formed on this piezoelectric substrate  170  are each composed from a serial arm SAW resonator. Also, the structural elements provided outside the piezoelectric substrate  170  (in the structural example in FIG.  4 (A), the branching filter circuit and the frequency adjusting LC element, or in the structural example in FIG.  4 (B), the frequency adjusting LC element) are provided in a package that houses the on-board substrate  180 . Or, though not illustrated, the package can be formed with a multi-layer structure, and the structural elements provided outside the piezoelectric substrate  170  can be provided on the intermediate layer or the upper layer (including the package lid). By using a structure like those of the structural examples shown in FIGS.  4 (A) through (C), it is possible to make branching filters more compact and give them a higher performance level. 
     Next, explanation will be given to an example of operation of a branching filter using a SAW resonator of the present invention.  FIG. 5  is a structural diagram that shows this branching filter by individual function during a transmission operation.  FIG. 6  is a structural diagram that shows this branching filter by individual function during a receive operation.  FIG. 7  provides an explanation of the impedance of this branching filter. 
     The branching filter  100  handles transmission and receiving by one antenna  200 . To do this, the transmission system and receiving system are directly connected to the antenna. Therefore, the performance of this branching filter  100  is largely related to the performance of portable telephones. 
     As shown in  FIG. 5 , when the branching filter  100  is used for transmission, transmission signals from a power amplifier  210  are sent to the transmission filter  140  via the transmission terminal  104 . The frequency band of these transmission signals are restricted by the transmission filter  140 , are sent to the antenna  200  via the antenna terminal  102 , and transmission signals are sent from here. In this case, the receiving system  220  that contains the Rx-branching filter circuit strip line  130  and the receiving filter  150  is viewed as a load circuit together with the antenna  200 . 
     Also, as shown in  FIG. 6 , when the branching filter  100  is used for receiving, signals received by the antenna  200  are sent to the receiving filter  150  via the antenna terminal  102 . With the receiving filter  150 , the frequency band of the received signals is restricted, and these are sent to the receiving circuit  220  via the receiving terminal  106 . In this case, the transmission system  230  that includes the Tx-branching filter circuit strip line  120  and the transmission filter  140  is viewed as a load circuit together with the antenna  200 . 
     In view of these points, the necessary conditions for a branching filter to function as a high performance branching filter are as follows. 
     The input impedance on the side of the Rx that includes a branching filter circuit when using the branching filter for transmission ( FIG. 5 ) is Zr. This Zr is shown as  400  in FIG.  7 . This Zr must satisfy the conditions of the following approximate expressions (1-1) and (1-2). 
       Zr*Z   ANT /( Zr+Z   ANT )≈50  (1-1)
 
 Zr≈∞   (1-2)
 
     The input impedance on the side of the Tx that includes a branching filter circuit when using the branching filter for receiving ( FIG. 6 ) is Zt. This Zt is shown as  300  in FIG.  7 . This Zt must satisfy the conditions of the following approximate expressions (2-1) and (2-2).
 
 Zt*Zr /( Zt+Zr )≈50  (2-1)
 
 Zt≈∞   (2-2)
 
     For portable telephones, the transmission band is 890 to 915 MHz and the receiving band is 935 to 960 MHz. With the transmission filter  140  in the transmission system  230  shown in  FIG. 6 , it is possible to set the end frequency to the receiving band of 930 to 960 MHz using the serial arm SAW resonator of this filter, so the transmission filter  140  in this case can satisfy the input impedance approximate expression (2-1). However, for the transmission system  230 , it is not possible to set the end frequency of the serial arm SAW resonator of this filter to the transmission band of 890-915 MHz. Because of this, it is not possible to satisfy the input impedance approximate expressions (1-1) and (1-2). 
     FIG.  8 (A) is a circuit diagram showing the serial arm SAW resonator used for the branching filter of the present invention, and FIG.  8 (B) is an LC equivalent circuit diagram of this resonator. 
     Thus, to compare the impedance characteristics during transmission for a branching filter of a conventional structure ( FIG. 9 ) and those for the branching filter of the present invention (FIG.  5 ), a simulation was performed. The branching filters used as the subject of this simulation were GSM method branching filters that use a portable telephone type SAW resonator. This GSM method branching filter does not comprise the structural elements shown by  120   a  and inductor  108   b  in  FIG. 2 , but rather has a structure comprising the Rx-branching filter circuit strip line (for the conventional branching filter) or in place of this the serial arm SAW resonator  130   a  (for the branching filter of the present invention). Also, of the frequency band 890 to 960 MHz, the simulation was performed at 890, 915, 935, and 960 MHz. 
     The GSM method branching filter transmission filter  13  of conventional methods and transmission filter  140  of the present invention used as subjects both have the same structure as the transmission filter shown by  140  in FIG.  2 . Similarly, the receiving filters  15  and  150  both have the same structure as the receiving filter shown by  150  in FIG.  2 . Table 1 shows the intersection length (shown as D (μm) in table 1) and electrode logarithm (shown by M in table 1) of the SAW resonator that composes the transmission and receiving filters of these branching filters. (Please refer to the attached table 1.) In table 1, the SAW resonators  140   a,    140   b,    140   c,  and  140   d  that compose the transmission filter  140  shown in  FIG. 2  are shown as TS 1 , TS 2 , TS 3 , and TS 4 . Also, the serial arm SAW resonators  150   a,    150   b,  and  150   c  that compose the receiving filter  150  in  FIG. 2  are shown as RS 1 , RS 2 , and RS 3 . Also, parallel arm SAW resonators  150   d,    150   e,  and  150   f  are shown as RP 1 , RP 2 , and RP 3 . Furthermore, with the branching filter of the present invention used as a subject for simulation, the Rx-branching filter circuit strip line  130  ( FIG. 1 ) has been substituted by the serial arm SAW resonator  130   a,  so this serial arm SAW resonator  130   a  is shown as RxS. Note that the Tx-branching filter circuit strip line  120  and the serial arm SAW resonator  120   a  that should be substituted for this (shown as TxS in  FIG. 2 ) have been omitted. 
     Furthermore, for the conventional branching filter (shown as  10  in  FIG. 9 ) that is the subject of simulation, the transmission filter  13  is incorporated into a single piezoelectric substrate, receiving filter  15  is incorporated into another single piezoelectric substrate, and the Rx-branching filter circuit strip line  14  and the LC chip  12  are provided on a multi-layer substrate (on-board substrate) for which these transmission and receiving filters  13  and  15  are incorporated on the above-mentioned piezoelectric substrates. 
     Table 2 shows branching filter circuit impedance values for the desired parameters and obtained simulation results for the specific type of branching filter circuit. In Table 2, code items I and II indicate a branching filter of a conventional structure. Code items III, IV, and V indicate a branching filter of the present invention. (Please refer to the attached Table 2.) 
     In this Table 2, with the conventional branching filter  1 , the structure is such that the branching filter circuit is the strip line, only the Rx-branching filter circuit strip line (strip line length (LR)=40 mm) (shown as  14  in  FIG. 9 ) is provided, without providing the Tx-branching filter circuit strip line (strip line length (LT)=0 mm), and there is also no frequency adjusting LC chip (shown as  12  in  FIG. 9 ) provided. Therefore, the input terminal of the transmission filter  13  and the input terminal of the Rx-branching filter circuit strip line  14  are directly connected to the antenna terminal  11 . 
     Also, with the conventional branching filter II, the structure is such that the branching filter circuit is the strip line, neither the Tx-branching filter circuit strip line nor the Rx-branching filter circuit strip line is provided (strip line length (LT, LR)=0 mm), and there is also no frequency adjusting LC chip (shown as  12  in  FIG. 9 ) provided. Also, the input terminals of the transmission filter  13  and the receiving filter  15  are directly connected to the antenna terminal  11 . 
     The three types of branching filter of the present invention III, IV, and V that are subjects of simulation are not provided with a Tx-branching filter circuit strip line  120  or a serial arm SAW resonator (TxS)  120   a  ( FIG. 2 ) in the circuit structure shown in FIG.  1 . Therefore, the input terminal of the transmission filter  140  is directly connected to the frequency adjusting LC element  108 . Furthermore, these branching filters III, IV, and V have a structure with which instead of providing the Rx-branching filter circuit  130 , the serial arm SAW resonator (RxS)  130   a  is provided. Then, as has already been explained in reference to  FIGS. 2 and 3 , these branching filters III, IV, and V are structured with the serial arm SAW resonator  130   a,  and the first level serial arm SAW resonator  150   a  of the receiving filter  150  combined as a composite resonator  152 . Based on such conditions, the branching filter III comprises capacitor component C ANT    4 (capacitance=10 pF) and inductor L ANT  (inductance=7 nH) as the externally mounted frequency adjusting LC element  108 . Also, the branching filter IV comprises not capacitor component C ANT  but only inductor L ANT  (inductance=7 nH) as the externally mounted frequency adjusting LC element  108 . Similarly, the branching filter V comprises not capacitor component C ANT  but only inductor L ANT  (inductance=10 nH) as the externally mounted frequency adjusting LC element  108 . In this way, the branching filter of the present invention is structured with the goal of improving frequency characteristics using the frequency characteristics adjusting LC element  108 . Impedance values for 890, 915, 935, and 960 MHz are shown for the transmission and receiving filters of the conventional technology and the present invention. 
     Of the conventional branching filters and that of the present invention, Table 3 shows real number and imaginary number values for input impedance Zt  300  of the transmission filter  140  and input impedance Zr  400  of the receiving filter  150  as shown in  FIG. 7  for specified branching filters, specifically branching filters II (conventional) and IV (the present invention) of Table 2. Then, input impedance values for 890, 900, 915, 935, and 960 MHz are shown for the transmission and receiving filters. (Please refer to the attached Table 3.) 
     By comparing input impedance Zt and Zr of branching filters II and IV in Table 3, it became clear that the receiving filter transmission band impedance is large for the branching filter of the present invention. Looking specifically, for the receiving filter of Table 1, when frequency f is 890 MHz, the input impedance Zr for branching filter II has a real number of 0.0127 and an imaginary number of −1.098. In comparison, the input impedance Zr for branching filter IV has a real number value of 3.54 and an imaginary number value of 23.20. In this way, the input impedance of the branching filter of the present invention is significantly larger than that of the conventional branching filter. Thus, with the branching filter of the present invention, we can see that there is a significant improvement in frequency characteristics. This improvement is also clear from the impedance characteristics results shown in Table 2. 
     Also, from the impedance characteristics results of table 2, we can see that the transmission frequency band is 890 to 915 MHz and the receiving frequency band is 935 to 960 MHz. 
     As has already been explained, the branching filter of the present invention which was provided for the above-mentioned impedance characteristics simulation has a structure made more compact by including a receiving filter first level serial arm SAW resonator in the Rx-branching filter circuit strip line. Here, we will explain the input impedance at f=900 MHz which is the central frequency of the transmission band which is of the most interest in terms of portable telephone quality. 
     When we see the transmission filter side from the point C  500  shown in  FIG. 7 , the combined impedance Z IN  (Tr) is given by the following equation (3).
 
 Z   IN ( Tr )= Zt*Zr /( Zt+Zr )  (3) 
 
     In this case, the input impedance of the transmission filter  140  and the receiving filter  150  at f=900 MHz is as follows based on Table 3.
 
 Zt ( 900 )=0.863 −j 0.626  (4)
 
 Zr ( 900 )=0.0175 −j 0.934  (5)
 
     Therefore, impedance Z IN  (Tr) ( 900 ) on the transmission filter side from the point C  500  shown in  FIG. 7  is given by the following equation (6).
 
 Z   IN ( Tr )( 900 )=0.2409 −j 0.501  (6)
 
If this Z IN  (Tr) ( 900 ) undergoes impedance correction only by the inductance L ANT    108   b  of the frequency adjusting LC element  108  in the structural example shown in  FIG. 2  for the present invention, then the value of the inductance L ANT  is as follows.
 
  L   ANT =4.4( nH )  (7)
 
In this case, when the characteristics impedance is not the desired value, an impedance matching circuit must be introduced.
 
     In reality, with this type of portable telephone, the optimum characteristics are demanded not only for frequency f=900 MHz but for the transmission band (890 to 915 MHz). These optimum characteristics are normally determined using simulation. The branching filters IV and V shown in Table 2 show the results of adjusting impedance for transmission band 890 to 915 MHz using only the inductor L ANT    108   b.  Also, the branching filter III shown in Table 2 shows the results of adjusting impedance for the same kind of transmission band using the inductor L ANT    108   b  and the capacitor C ANT    108   a.  One combination of these inductor L ANT  and capacitor C ANT  values is L ANT =7.0 nH and C ANT =10.0 pF. 
     In this way, as is clear from the results shown in Table 2, with the structure of the branching filter of the present invention, by incorporating the transmission filter  140  and the receiving filter  150  on a single common piezoelectric substrate and by providing an externally mounted frequency adjusting LC circuit  108 , it became clear that it is possible to improve frequency characteristics, particularly passband characteristics. 
     From the results of Table 3, it can be found that the impedance for the transmission band of the receiving filter  150  is large using the branching filter circuit strip line serial arm SAW resonator  130   a.  This improvement in frequency characteristics depends on the impedance seen on the filter side from the point C  500  shown in FIG.  7 . Specifically, it is assumed that this is caused by introducing the branching filter circuit strip line serial arm SAW resonator  130   a  (RxS) on the input terminal of the receiving filter  150 . Inconsistencies in impedance values due to the introduction of this branching filter circuit strip line serial arm SAW resonator  130  a (RxS) are adjusted with an externally mounted frequency adjusting LC element  108 . This frequency adjusting LC element is made into chip form, and by providing this LC chip on the on and receiving filter package on-board substrate or by providing it on a piezoelectric substrate on which is created a transmission and receiving filter, it is possible to make a branching filter comprising a SAW resonator generally more compact and higher in performance. 
     It will be clear to those in the industry that it is possible to make many changes and variations of the present invention without straying from the main point of the present invention and without being restricted by the preferred embodiments described above. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 TRANSMISSION 
                 TS1 
                 TS2 
                 TS3 
                 TS4 
               
             
          
           
               
                 FILTER 
                 D (μm) 
                 M 
                 D (μm) 
                 M 
                 D (μm) 
                 M 
                 D (μm) 
                 M 
               
               
                   
               
               
                   
                 85 
                 90 
                 42.5 
                 90 
                 84 
                 86 
                 60 
                 60 
               
               
                   
               
             
          
           
               
                 RECEIVING 
                 SERIAL 
                 RxS 
                 RS1 
                 RS2 
                 RS3 
               
             
          
           
               
                 FILTER 
                 ARM 
                 D (μm) 
                 M 
                 D (μm) 
                 M 
                 D (μm) 
                 M 
                 D (μm) 
                 M 
               
               
                   
               
               
                   
                   
                 124 
                 90 
                 124 
                 90 
                 62 
                 90 
                 62 
                 90 
               
               
                   
               
             
          
           
               
                   
                 PARALLEL 
                   
                 RP1 
                 RP2 
                 RP3 
               
             
          
           
               
                   
                 ARM 
                   
                 D (μm) 
                 M 
                 D (μm) 
                 M 
                 D (μm) 
                 M 
               
               
                   
               
               
                   
                   
                   
                 102 
                 120 
                 102 
                 120 
                 76 
                 80 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 BRANCHING FILTER 
                   
                   
               
               
                   
                 CIRCUIT FREQUENCY 
                 TRANSMISSION 
                 RECEIVING 
               
               
                   
                 ADJUSTING LC ELEMENT 
                 FILTER 
                 FILTER 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 I 
                 LT = 0 (mm) 
                 LR = 40 (mm) 
                 1.22 
                 1.17 
                 35.7 
                 36.8 
                 31.6 
                 58.5 
                 3.11 
                 3.04 
               
               
                 II 
                 LT = 0 (mm) 
                 LR = 0 (mm) 
                 3.56 
                 3.21 
                 37.9 
                 28.6 
                 33.0 
                 55.6 
                 3.11 
                 2.32 
               
               
                 III 
                 LT ANT  = 7 (nH) 
                 C ANT  = 7 (pF) 
                 1.28 
                 1.28 
                 36.6 
                 35.0 
                 34.1 
                 59.0 
                 3.28 
                 3.20 
               
               
                 IV 
                 LT ANT  = 7 (nH) 
                   
                 1.30 
                 1.32 
                 34.7 
                 33.2 
                 35.1 
                 58.5 
                 3.74 
                 4.0  
               
               
                 V 
                 LT ANT  = 10 (nH) 
                   
                 1.37 
                 1.08 
                 36.1 
                 29.2 
                 35.5 
                 54.7 
                 3.10 
                 3.70 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 TRANSMISSION FILTER 
                 RECEIVING FILTER 
               
             
          
           
               
                   
                 FREQUENCY (MHz) 
                 890 
                 900 
                 915 
                 935 
                 960 
                 890 
                 900 
                 915 
                 935 
                 960 
               
               
                   
                   
               
             
          
           
               
                 II 
                 REAL NUMBER 
                 1.283 
                 0.8627 
                 1.345 
                 2.313 
                 0.0831 
                 0.0127 
                 0.0175 
                 0.0320 
                 0.606 
                 0.7414 
               
               
                   
                 IMAGINARY NUMBER 
                 −0.816 
                 −0.6256 
                 0.5287 
                 0.8715 
                 −4.017 
                 −1.098 
                 −0.934 
                 −0.654 
                 −0.017 
                 1.263 
               
               
                 IV 
                 REAL NUMBER 
                 1.283 
                 0.8627 
                 1.345 
                 2.313 
                 0.0831 
                 3.540 
                 4.7507 
                 0.435 
                 0.875 
                 0.2421 
               
               
                   
                 IMAGINARY NUMBER 
                 −0.816 
                 −0.6256 
                 0.5287 
                 0.8715 
                 −4.017 
                 23.20 
                   
                   
                 0.0479 
                 1.150