Abstract:
A surface-acoustic-wave filter has a piezoelectric substrate; and a plurality of transversal surface-acoustic-wave (SAW) filters set on the piezoelectric substrate and respectively having at least two inter-digital transducer (IDT) electrodes for transcieving surface acoustic waves, characterized in that periods for surface acoustic waves to propagate from inputs to outputs of the SAW filters differ in the SAW filters and thereby, timings of outputs from the SAW filters are different from each other.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a surface-acoustic-wave filter using a transversal SAW filter used for a high-frequency circuit of a radio communication unit. 
     2. Related Art of the Invention 
     A surface-acoustic-wave filter has been frequently used so far as a RF- or IF-stage filter of a transceiving circuit of a communication unit. Particularly recently, a portable telephone according to a mode referred to as CDMA (Code Division Multiple Access) has been positively developed as a next-generation mobile communication system. 
     The CDMA communication mode allows the so-called RAKE reception for improving the sensitivity by synthesizing delay waves of a multipath. 
     A CDMA receiving circuit is described below by referring to the accompanying drawings. 
     FIG. 9 is an illustration showing the outline of a CDMA receiving circuit. In FIG. 9, symbol  702  denotes a demodulating section,  703   a  and  703   b  denote antennas,  704   a  and  704   b  denote filters,  705   a  and  705   b  denote amplifiers,  706   a  and  706   b  denote mixers,  707   a  and  707   b  denote transversal SAW filters, and  708  denotes a delay unit. In this case, the transversal SAW filters  707   a  and  707   b  are assumed as the same device. 
     Operations of the CDMA receiving circuit having the above configuration according to the prior art are described below. That is, a signal received by the antenna  703   a  passes through the filter  704   a  and amplifier  705   a  and is converted into an IF signal by the mixer  706   a . When assuming the signal as an IF signal “a,” the IF signal “a” passes through the transversal SAW filter  707   a  and is output to the demodulating section  702 . 
     Similarly, a signal received by the antenna  703   b  passes through the filter  704   b  and amplifier  705   b  and is converted into an IF signal by the mixer  706   b . When assuming the signal as an IF signal “b,” the IF signal “b” passes through the transversal SAW filter  707   b  and delay unit  708  and is output to the demodulating section  702 . 
     The demodulating section  702  performs RAKE reception by synthesizing the IF signal “a” and the IF signal “b” input by being delayed by the delay unit  708 . 
     Operations of the CDMA receiving circuit are described above. 
     However, the above configuration has problems that a single delay unit is required for RAKE reception and thereby, the circuit size increases. Therefore, the CDMA receiving circuit cannot be applied to a compact portable terminal unit. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve the above problems and its object is to provide a surface-acoustic-wave filter capable of reducing the number of parts for signal delay and downsizing a receiving circuit, and a communication unit using the surface-acoustic-wave filter. 
     One aspect of the present invention is a surface-acoustic-wave filter, comprising: 
     a piezoelectric substrate; and 
     a plurality of transversal surface-acoustic-wave (SAW) filters set on the piezoelectric substrate and respectively having at least two inter-digital transducer (IDT) electrodes for transceiving surface acoustic waves, 
     characterized in that periods for surface acoustic waves to propagate from inputs to outputs of the SAW filters differ in the SAW filters and thereby, timings of outputs from the SAW filters are different from each other. 
     Another aspect of the present invention is a surface-acoustic-wave filter, comprising: 
     a piezoelectric substrate; 
     a plurality of filters set on the piezoelectric substrate and respectively having at least two inter-digital transducer (IDT) electrodes for transceiving surface acoustic waves; and 
     a plurality of impedance-matching circuits connected to input sides and output sides of the filters, 
     characterized in that output timings of output-side impedance-matching circuits of the filters are different from each other. 
     Still another aspect of the present invention is a communication unit using a CDMA mode, comprising: 
     receiving means provided with; 
     a plurality of antennas, 
     a plurality of mixers for converting signals sent from the antennas into IF signals, 
     a plurality of surface-acoustic-wave (SAW) filters for passing the IF signals, and 
     a demodulating section for demodulating the IF signals, 
     wherein said surface-acoustic-wave filter is used for the SAW filters. 
     According to the above configuration, a surface-acoustic-wave filter of the present invention is characterized by having transversal SAW filters corresponding to a plurality of reception signals, making group delay times of the transversal SAW filters different from each other, and thereby providing a group delay time difference for IF signals output from the SAW filters and makes it possible to downsize a CDMA receiving circuit by including a delay-unit function in the surface-acoustic-wave filter and reducing the number of parts of the CDMA receiving circuit in accordance with the above configuration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration showing a configuration of embodiment 1 of a surface-acoustic-wave filter of the present invention; 
     FIG. 2 is an illustration showing a CDMA receiving circuit using a surface-acoustic-wave filter of the present invention; 
     FIG. 3 is an illustration showing another configuration of the embodiment 1 of a surface-acoustic-wave filter of the present invention; 
     FIG. 4 is an illustration showing still another configuration of the embodiment 1 of a surface-acoustic-wave filter of the present invention; 
     FIG. 5 is an illustration showing a CDMA receiving circuit having a configuration of embodiment 2 of a surface-acoustic-wave filter of the present invention; 
     FIG. 6 is an illustration showing impedance-matching circuits of the embodiment 2 of a surface-acoustic-wave filter of the present invention; 
     FIG. 7 is an illustration showing a configuration of a modification of the embodiment 1 of a surface-acoustic-wave filter of the present invention; 
     FIG. 8 is an illustration showing another CDMA receiving circuit using a surface-acoustic-wave filter of the present invention; and 
     FIG. 9 is a block diagram of a CDMA receiving circuit using a surface-acoustic-wave filter of the prior art. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Description of Symbols 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 101 
                 Single-crystal piezoelectric substrate 
               
               
                   
                 102, 103 
                 Transversal SAW filter 
               
               
                   
                 104, 105, 106, 107 
                 IDT electrode 
               
               
                   
                 201 
                 SAW filter package 
               
               
                   
                 202 
                 Demodulating section 
               
               
                   
                 203a, 203b, 203c 
                 Antenna 
               
               
                   
                 204a, 204b, 204c 
                 Filter 
               
               
                   
                 205a, 205b, 205c 
                 Amplifier 
               
               
                   
                 206a, 206b, 206c 
                 Mixer 
               
               
                   
                 301 
                 Strip electrode 
               
               
                   
                 401 
                 Metallic film 
               
               
                   
                 501a, 501b, 502a, 502b 
                 Impedance-matching circuit 
               
               
                   
                 601a, 601b, 601c, 601d 
                 Capacitor 
               
               
                   
                 602a, 602b, 602c, 602d 
                 Capacitor 
               
               
                   
                 702 
                 Demodulating section 
               
               
                   
                 703a, 703b 
                 Antenna 
               
               
                   
                 704a, 704b 
                 Filter 
               
               
                   
                 705a, 705b 
                 Amplifier 
               
               
                   
                 706a, 706b 
                 Mixer 
               
               
                   
                 707a, 707b 
                 Transversal SAW filter 
               
               
                   
                 708 
                 Delay unit 
               
               
                   
                   
               
             
          
         
       
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are described below by referring to the accompanying drawings. 
     Embodiment 1 
     FIG. 1 is an illustration showing a configuration of a surface-acoustic-wave filter of embodiment 1 of the present invention. In FIG. 1, symbol  101  denotes a single-crystal piezoelectric substrate. A surface acoustic wave is excited by forming an electrode pattern on the substrate. Symbol  104  denotes a wave-transmission-side IDT electrode and  105  denotes a wave-reception-side IDT electrode. The IDT electrodes  104  and  105  are set separately from each other by a SAW propagation distance W 1  to constitute a transversal SAW filter  102 . Similarly, a wave-transmission-side IDT electrode  106  and a wave-reception-side IDT electrode  107  on the single-crystal piezoelectric substrate  101  are set separately from each other by a SAW propagation distance W 2  (W 2 &gt;W 1 ) to constitute a transversal SAW filter  103 . The transversal SAW filters  102  and  103  are stored in one package to constitute a SAW filter package  201 . 
     It is preferable that the SAW  102  and SAW  103  substantially have the same frequency characteristic. 
     FIG. 2 is an illustration showing the outline of a CDMA receiving circuit including a SAW filter package  201 . In FIG. 2, symbol  202  denotes a demodulating section,  203   a  and  203   b  denote antennas,  204   a  and  204   b  denote filters,  205   a  and  205   b  denote amplifiers, and  206   a  and  206   b  denote mixers. Moreover, symbols  102  and  103  in the SAW filter  201  denote transversal SAW filters shown in FIG.  1 . 
     A signal received by the antenna  203   a  passes through the filter  204   a  and amplifier  205   a  and is converted into an IF signal by the mixer  206   a . When assuming the IF signal as an IF signal “a,” the IF signal “a” is input to the transversal SAW filter  102  in the SAW filter package  201 . 
     Similarly, a signal received by the antenna  203   b  passes through the filter  204   b  and amplifier  205   b  and is converted into an IF signal by the mixer  206   b . When assuming the IF signal as an IF signal “b,” the IF signal “b” is input to the transversal SAW filter  103  in the SAW filter package  201 . 
     Operations of the surface-acoustic-wave filter of the embodiment 1 of the present invention are described below by referring to FIG.  1 . 
     As described for the prior art, a CDMA receiver can perform RAKE reception in accordance with a method for receiving multipaths by using two receiving antennas. To improve the RAKE reception function, it is necessary to cause a group-delay-time difference between reception signals of the two systems. 
     The IF signal “a” is converted into a surface acoustic wave by the wave-transmission-side IDT electrode  104 , propagates on the piezoelectric substrate  101  and enters the wave-reception-side IDT electrode  105  separate by the distance W 1 , and is reconverted into an electrical signal and output. At the same time, an IF signal is filtered by comb shapes of the IDT electrodes  104  and  105 . 
     Similarly, the IF signal “b” is converted into a surface acoustic wave by the wave-transmission-side IDT electrode  106 , propagates on the piezoelectric substrate  101  and enters the wave-reception-side IDT electrode  107  separate by the distance W 2 , and is reconverted into an electrical signal and output. At the same time, an IF signal is filtered by comb shapes of the IDT electrodes  106  and  107 . 
     In this case, the IF signals “a” and “b” are input to the demodulating section  202  with a time difference equivalent to the distance difference W 2 −W 1  when the signals propagate as surface acoustic waves. 
     In this case, a narrow-band CDMA communication mode is considered as an applied example of this embodiment. In the case of the narrow-band CDMA communication mode, radio waves are transmitted at a chip rate of 1.2288 Mcps. Therefore, to perform RAKE reception, a group delay time difference of 0.8 μsec or more is necessary between receiving systems. 
     For this embodiment, a case of using ST-quartz as the piezoelectric substrate  101  is considered. When assuming the acoustic velocity for a surface acoustic wave to propagate on the ST-quartz as 3159 m/s, the delay of 0.8 μsec can be realized by setting the SAW propagation distance difference W 2 −W 1  to approx. 2.5 mm. Because generally a SAW filter for narrow-band CDMA has a design which needs a long electrode size in the propagation direction, sizes of a filter are not greatly changed due to the difference of 2.5 mm. 
     Moreover, because a chip rate is 4.096 Mcps in the case of a wide-band CDMA communication mode, a necessary group delay time reaches 0.24 μsec or more and the SAW propagation distance difference W 2 −W 1  further decreases up to approx. 0.77 mm. 
     As described above, according to this embodiment, it is possible to generate a group delay time difference between reception signals serving as filter outputs by setting a difference between SAW propagation distances of SAW filters for filtering reception signals of a plurality of systems. 
     As shown in FIG. 7, even if a dummy electrode D is added and thereby propagation paths are the same (W 1 =W 2 ), the influence due to addition of the dummy electrode is actually small and actual propagation distances of surface acoustic waves are still different (W 1 &lt;W 2 ). Therefore, it is possible to produce a group delay time difference. 
     Moreover, in the case of this embodiment, it is described to make the SAW propagation distance W 2  larger than the SAW propagation distance W 1  as a method for setting a group delay time difference between reception signals of two systems. When producing a group delay time difference by changing SAW speeds in a SAW propagation path, SAW propagation distances ate not always set so as to be W 2 &gt;W 1 . A method for making the propagation speeds different from each other is optional. 
     For example, in the case of this embodiment, it is also permitted to set a metallic strip electrode  301  or a metallic film  401  to at least either of the SAW propagation path of the transversal SAW filter  102  (between the wave-transmission-side IDT electrode  104  and the wave-reception-side IDT electrode  105 ) and that of the transversal SAW filter  103  (between the wave-transmission-side IDT electrode  106  and wave-reception-side IDT electrode  107 ). 
     In this case, the speed of an SAW propagating in a SAW propagation path is lowered by the action of the strip electrode  301  or metallic film  401  and as a result, it is, possible to realize the same group delay time at a small propagation distance compared to the case in which no strip electrode is used and further downsize a surface-acoustic-wave filter. 
     Moreover, instead of setting either of a strip electrode and a metallic film on a SAW propagation path as described above, it is also possible to set both the strip electrode and the metallic film on the path. Furthermore, when setting a strip electrode or a metallic film to SAW propagation paths of a plurality of reception systems, it is possible to make speeds of SAWs propagating through SAW propagation paths different from each other by making shapes of strip electrodes or metallic films set to SAW propagation paths different from each other, and further precisely produce a group delay time difference between reception signals. 
     Moreover, it is also permitted to make SAW propagation distances different from each other and simultaneously make propagation speeds different from each other by using a strip electrode or the like. Thereby, it is possible to realize a further-precise group delay time. 
     Furthermore, this embodiment is described by assuming that the transversal SAW filters  102  and  103  are formed on the same piezoelectric substrate. However, it is also permitted to form these SAW filters on piezoelectric substrates different from each other and also in this case, the same advantage is obtained. 
     Furthermore, this embodiment is described by storing the transversal SAW filters  102  and  103  in one package. However, the same advantage is also obtained by forming these filters in different packages. 
     Furthermore, this embodiment is described by assuming that a CDMA receiving circuit has two receiving systems. However, as shown in FIG. 8, even when the circuit has three receiving systems or more, the same advantage is obtained by constituting transversal SAW filters equal to the number of receiving systems and making group delay times of SAW propagation paths different from each other in accordance with the above method (method of making SAW propagation distances different from each other or method of changing speeds of SAWs in SAW propagation paths). 
     Moreover, in the case of this embodiment, it is preferable that the transversal SAW filters  102  and  103  have almost the same frequency characteristic. That is, in the case of the present invention, it is preferable that weighting functions of the transversal SAW filters  102  and  103  are almost the same. For example, it is preferable that the weighting function of the wave-transmission-side IDT electrode  104  constituting the transversal SAW filter  102  and that of the wave-transmission-side IDT electrode  106  constituting the transversal SAW filter  103  are almost the same and moreover, the weighting function of the wave-reception-side IDT electrode  105  constituting the transversal SAW filter  102  and that of the wave-reception-side IDT electrode  107  constituting the transversal SAW filter  103  are almost the same. 
     Furthermore, it is possible to realize a more compact communication unit by mounting the surface-acoustic-wave filter described for this embodiment on a communication unit. 
     Embodiment 2 
     FIG. 5 is an illustration showing a configuration of the surface-acoustic-wave filter of the embodiment 2 of the present invention. In FIG. 5, a component same as or corresponding to that in FIGS. 1,  2 , and  7  is provided with the same symbol and its overlapped description is omitted. Moreover, in FIG. 5, symbols  501   a ,  501   b ,  502   a , and  502   b  denote impedance-matching circuits. The impedance-matching circuit  501   a  matches the impedance of a mixer  206   a  with that of a transversal SAW filter  102  and the impedance-matching circuit  502   a  matches the impedance of a transversal SAW filter  102  with that of a demodulating section  202 . 
     Similarly, the impedance-matching circuit  501   b  matches the impedance of the mixer  206   b  with that of a transversal SAW filter  103  and the impedance-matching circuit  502   b  matches the impedance of the transversal SAW filter  103  with that of the demodulating section  202 . 
     FIG. 6 shows specific configurations of the impedance-matching circuits  501   a ,  502   a ,  501   b , and  502   b . As shown in FIG. 6, the impedance-matching circuits  501   a  and  502   a  are such that connection sides with external circuits are grounded through capacitors  601   a  and  601   b  and connection sides with the transversal SAW filter  102  is connected in series with coils  602   a  and  602   b . Moreover, the impedance-matching circuits  501   b  and  502   b  are such that connection sides with the transversal SAW filter  103  are grounded through coils  602   c  and  602   d  and connection sides with external circuits are connected in series with capacitors  601   c  and  601   d.    
     In the case of the surface-acoustic-wave filter of this embodiment having the above impedance-matching circuits, because configurations of the impedance-matching circuits are different from each other, it is possible to provide a group delay time difference for IF signals “a” and “b” output from the transversal SAW filters  102  and  103  to the demodulating section and thereby, the same advantage as the embodiment 1 can be obtained. 
     This embodiment 2 is described by assuming that the impedance-matching circuits  501   a ,  502   a ,  501   b , and  502   b  have the configurations shown in FIG.  6 . These configurations are not restricted to FIG.  6 . 
     Moreover, for this embodiment 2, it is permitted to use conventional transversal SAW filters  707   a  and  707   b  having a delay output effect instead of the transversal SAW filters  102  and  103  described for the embodiment 1. When using the filters of the embodiment 1, an advantage is obtained that a group delay time can be increased or precisely adjusted compared to a conventional case. 
     The embodiment 2 is described by assuming that a CDMA receiving circuit has two systems of reception circuits. However, when the receiving circuit has three receiving systems or more, it is possible to provide a group delay time difference for the reception signal of each system by forming transversal SAW filters equal to the number of systems and making configurations of impedance-matching circuits to be connected with the SAW filters different from each other. 
     Moreover, the embodiment 2 is described by assuming that a surface-acoustic-wave filter for performing filtering uses a transversal SAW filter. However, the same advantage can be obtained also by using another type of filter such as a coaxial filter or dielectric laminated filter. 
     Furthermore, it is possible to realize a more compact communication unit by mounting the surface-acoustic-wave filter described for the embodiment 2 on a communication unit. 
     Furthermore, each embodiment is described by assuming that a piezoelectric substrate of the present invention uses a single-crystal piezoelectric substrate. However, it is also permitted to use polycrystal or semiconductor for the piezoelectric substrate. It is permitted to use any piezoelectric substrate as long as the substrate can transfer a surface acoustic wave. 
     As described above, according to the present invention, RAKE reception of a CDMA receiving circuit can be realized without using a single delay unit. Therefore, it is possible to easily downsize the CDMA receiving circuit.