Abstract:
A surface acoustic wave (SAW) filter includes a first SAW resonator, a second SAW resonator connected in series to the first SAW resonator at a first node, a third SAW resonator connected in series to the second SAW resonator at a second node, a fourth SAW resonator connected in series to the third SAW resonator at a third node, a fifth SAW resonator connected between the first node and a ground, a sixth SAW resonator connected between the third node and a ground, and a first capacitance element having a capacitance and connected between the second node and a ground. This SAW filter has a sharp attenuation characteristic at a high frequency area of a pass band, thereby widening the pass band and reducing a loss at the pass band.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a surface acoustic wave filter used in a communication device and a device using the filter. BACKGROUND ART  
         [0002]      FIG. 15  shows conventional ladder type surface acoustic wave (SAW) filter  1001  disclosed in Japanese Patent Laid-Open Publication No. 6-152317. Series resonators  23 ,  24 ,  25  and  26  are connected between input terminal  21  and output terminal  22  in series in this order from input terminal  21  to output terminal  22 . One end of parallel resonator  27  is connected with a point between series resonators  23  and  24 . One end of parallel resonator  28  is connected with a point between series resonators  25  and  26 . Other end of parallel resonator  27  is connected with a ground via inductance element  29  providing an inductance. Other end of parallel resonator  28  is connected with a ground via inductance element  30  providing an inductance.  
         [0003]     SAW filter  1001  has characteristics denoted by line  202  in  FIGS. 2 and 3 . As shown by line  202 , SAW filter  1001  has attenuation bands at both sides of a pass band. The filter, such as SAW filter  1001 , is required to have characteristics reducing a loss in the pass band and shifting sharply to the attenuation bands.  
       SUMMARY OF THE INVENTION  
       [0004]     A surface acoustic wave (SAW) filter includes a first SAW resonator, a second SAW resonator connected in series to the first SAW resonator at a first node, a third SAW resonator connected in series to the second SAW resonator at a second node, a fourth SAW resonator connected in series to the third SAW resonator at a third node, a fifth SAW resonator connected between the first node and a ground, a sixth SAW resonator connected between the third node and a ground, and a first capacitance element having a capacitance and connected between the second node and a ground.  
         [0005]     This SAW filter has a sharp attenuation characteristic at a high frequency area of a pass band, thereby widening the pass band and reducing a loss at the pass band. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a circuit diagram of a surface acoustic wave (SAW) filter in accordance with an exemplary embodiment of the present invention.  
         [0007]      FIG. 2  shows frequency characteristics of the SAW filter in accordance with the embodiment.  
         [0008]      FIG. 3  shows frequency characteristics of the SAW filter in accordance with the embodiment.  
         [0009]      FIG. 4  is a sectional view of the SAW filter in accordance with the embodiment.  
         [0010]      FIG. 5  shows an equivalent circuit of the SAW filter in accordance with the embodiment.  
         [0011]      FIG. 6  shows an equivalent circuit of the SAW filter in accordance with the embodiment.  
         [0012]      FIG. 7  is a plan view of the SAW filter in accordance with the embodiment.  
         [0013]      FIG. 8  is a plan view of a capacitance element of the SAW filter in accordance with the embodiment.  
         [0014]      FIG. 9  is a plan view of another capacitance element of the SAW filter in accordance with the embodiment.  
         [0015]      FIG. 10  is a plan view of a further capacitance element of the SAW filter in accordance with the embodiment.  
         [0016]      FIG. 11  is a plan view of another SAW filter in accordance with the embodiment.  
         [0017]      FIG. 12  is a plan view of a further SAW filter in accordance with the embodiment.  
         [0018]      FIG. 13  is a circuit diagram of a still further SAW filter in accordance with the embodiment.  
         [0019]      FIG. 14  is a block diagram of a device including the SAW filter in accordance with the embodiment.  
         [0020]      FIG. 15  is a circuit diagram of a conventional SAW filter. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]      FIG. 1  is a circuit diagram of surface acoustic wave (SAW) filter  101  in accordance with an exemplary embodiment of the present invention. Series resonators  3 - 6  are connected in series in this order from input terminal  1  to output terminal  2  between input terminal  1  and output terminal  2 . One end of parallel resonator  7  is connected to a node between series resonators  3  and  4 . One end of parallel resonator  8  is connected to a node between series resonators  5  and  6 . Other end of parallel resonator  7  is connected with a ground via inductance element  9  having an inductance. Other end of parallel resonator  8  is connected with a ground via inductance element  10  having an inductance. Capacitance element  11  having a capacitance is connected between a ground and series resonators  4  and  5 .  
         [0022]      FIGS. 2 and 3  show frequency characteristics of SAW filter  101  in accordance with the embodiment.  
         [0023]     As shown by line  201  in  FIG. 2 , SAW filter  101  of the embodiment has a loss at a pass band from  1 . 92 GHz to  1 . 98 GHz less than a loss (shown by line  202 ) of conventional SAW filter  1001  shown in  FIG. 15 . SAW filter  101  has an attenuation at both sides of the pass band more than a attenuation (shown by line  202 ) of conventional SAW filter  1001 , and the attenuation of SAW filter  101  is sharper than the attenuation (shown by line  202 ) of SAW filter  1001 . SAW filter  101  of the embodiment has a larger bandwidth and a small loss than conventional SAW filter  1001 .  
         [0024]      FIG. 4  is a sectional view of SAW filter  101  in accordance with the embodiment. Inductance elements  9  and  10  are formed of wires connected with piezoelectric board  51  having resonators  3 - 8  provided thereon.  FIG. 5  is an equivalent circuit of a portion of SAW filter  101  shown in  FIG. 1 . In general, respective self parallel resonances of series resonators  3 - 6  greatly contribute to attenuation at high frequencies in of the pass band of SAW filter  101 . Additionally, in general, respective self series resonances of parallel resonators  7  and  8  greatly contribute to attenuation at low frequencies in the pass band of SAW filter  101 . In general, respective self series resonances of series resonators  3 - 6  and respective parallel resonances of parallel resonators  7  and  8  greatly contribute to characteristics in the pass band of SAW filter  101 . Attenuation characteristic has been conventionally designed by controlling these resonators. A ratio of the series resonance to the parallel resonance is determined substantially by conditions of the piezoelectric board having these resonators thereon.  
         [0025]      FIG. 6  is an equivalent circuit around an attenuation pole at high frequencies of SAW filter  101  having the equivalent circuit shown in  FIG. 5 . In this equivalent circuit, series resonators  4  and  5  correspond to parallel resonant circuits  12  and  13 , respectively, and parallel resonators  7  and  8  correspond to capacitors  14  and  15 , respectively. The equivalent circuit including capacitance element  11  shown in  FIG. 6  is approximated to an elliptic function type band-pass filter around the attenuation pole at high frequencies.  
         [0026]     As a result, SAW filter  101  has a large, sharp attenuation at high frequencies, thus having a wide bandwidth and a small loss, as shown by line  201  in  FIG. 3 .  
         [0027]     For example, the capacitance of capacitance element  11  is determined to be 0.1 pF, and series resonators  3 - 6  and parallel resonators  7  and  8  are designed appropriately, thus reducing a loss in the pass band (from 1.92 GHz to 1.98 GHz) from −0.9 dB (shown by line  202 ) to −0.8 dB (shown by line  201 ), as shown in  FIG. 3 .  
         [0028]     Capacitance element  11  may be formed on the piezoelectric board similarly to series resonators  3 - 6  and parallel resonators  7  and  8 . A method of the forming will be described.  
         [0029]      FIG. 7  is a plan view of SAW filter  101  in accordance with the embodiment. Input terminal  1 , output terminal  2 , series resonators  3 - 6 , parallel resonators  7  and  8 , ground electrodes  16 - 18  and capacitance element  11  are formed on piezoelectric board  51 . Capacitance element  11  is composed of interdigital electrodes  141  extending from ground electrode  18  and node  52  between series resonators  4  and  5 , respectively, and facing each other.  
         [0030]      FIGS. 8-10  are plan views of other capacitance elements  111 - 113 . In capacitance element  111  shown in  FIG. 8 , portions  111 C and  111 D protrude from respective long sides of electrodes  111 A and  111 B, respectively. Electrode  111 A extends from node  52  between series resonators  4  and  5 . Electrode  111 B extends from ground electrode  18  and face electrode  111 A. In capacitance element  112  shown in  FIG. 9 , electrodes  112 C and  112 D protrude from electrodes  112 A and  112 B, respectively, and extend in parallel to each other. Electrode  112 A extends from node  52  between series resonators  4  and  5 . Electrode  112 B extends from ground electrode  18 . In capacitance element  113  shown in  FIG. 10 , toothed portions  113 C and  113 D are formed at electrodes  113 A and  113 B. Electrode  113 A extends from node  52  between series resonators  4  and  5 . Electrode  113 B extends from ground electrode  18  and face electrode  113 A.  
         [0031]      FIG. 11  is a plan view of another SAW filter  102  in accordance with the embodiment. Interdigital electrodes  142 , differently interdigital electrodes  11  forming capacitance element  141  shown in  FIG. 10 , are formed perpendicularly to series resonators  3 - 6  and parallel resonators  7  and  8 . The number of fingers and intervals of interdigital electrodes  141  and  142  are determined by the capacitances. Since directions of interdigital electrodes  142  is different from directions of interdigital electrodes of the resonators by  90  degrees, interdigital electrodes  142  do not resonate with surface acoustic waves of the resonators. As a result, vibrations of the resonators do not influence capacitance element  11  regardless of the capacitance of capacitance element  11 .  
         [0032]      FIG. 12  is a plan view of further SAW filter  103  in accordance with the embodiment. Ground electrodes  16 - 18  are connected with ground electrode  40  surrounding elements of SAW filter  103  on piezoelectric board  51 . This structure prevents the electrodes from breaking due to short-circuit between the electrodes caused by electric charges accumulating at piezoelectric board  51  during forming of the elements on piezoelectric board  51 . Ground electrode  40  functions as a mark when a wafer of a piezoelectric board having plural SAW filters formed thereon is divided into the SAW filters. After the wafer is divided into the SAW filters, ground electrode  40  is eliminated, thereby not influencing characteristics of SAW filter  103 .  
         [0033]      FIG. 13  is a circuit diagram of SAW filter  104  in accordance with the embodiment. Capacitance element  19  is connected between a ground and node  61  between series resonators  4  and  5 . Capacitance element  20  is connected between a ground and node  62  between series resonators  5  and  6 . In SAW filter  104 , parallel resonators  7  and  8  forming an equivalent elliptic function band-pass filter may be designed flexibly at high frequencies of an attenuation band. Capacitance elements  19  and  20  may have the same structure as capacitance element  11  shown in  FIGS. 7-12 .  
         [0034]      FIG. 14  is a block diagram of a device including SAW filters  101 - 104  and other elements, such as antenna  41  and amplifiers  43  and  44 , in accordance with the embodiment. Duplexer  42  including the SAW filter connected with antenna  41  is connected with power amplifier  43  for sending and power amplifier  44  for receiving. The device shown in  FIG. 14  may be a portable telephone including rechargeable battery  145  for supplying electric power to duplexer  42  and amplifiers  43  and  44 . For example, in the cases that battery capacity W of battery  145  is 580 mAh, an effect of reduction of a loss at the pass band of the SAW filter by 0.1 dB will be described.  
         [0035]     Efficiency η(%) of the portable telephone is calculated by the following equation: 
 
η=10 (Pout/10) /(1000× V×I )×100 
 
 where a voltage of battery  145  is V(V), a consumption current is I(A), and an output voltage of radio wave from antenna  41  is Pout(dBm). 
 
         [0036]     In the case that the voltage V=3(V) and the efficiency η=40(%) are constant, when Pout is 33(dBm), the consumption current of battery  145  is reduced by approximately 38.73(mA) from that of Pout of 33.1(dBm). If the portable telephone makes a call for 120 minutes with battery  145 , a duration T of call in tha case that the consumption current is reduced by 38.73(mA) is expressed as the following equation: 
 
 T= 120×(580+38.73)/580≈128 
 
         [0037]     Thus, when a loss at the pass band of the SAW filter is reduced by 0.1 dB, the duration of call of the portable telephone increases by eight minutes.  
       INDUSTRIAL APPLICABILITY  
       [0038]     A surface acoustic wave (SAW) filter of the present invention has sharp attenuation characteristics at high frequencies in a pass band, thereby having a wide pass band and a small loss at the pass band.