JP7429612B2 - Surface acoustic wave filters and surface acoustic wave devices - Google Patents

Description

The present invention relates to a surface acoustic wave filter and a surface acoustic wave device including the surface acoustic wave filter.

A SAW filter is known as a SAW (Surface Acoustic Wave) device, and Patent Document 1 describes an example of the SAW filter.

SAW filters often require connection of a matching circuit to match the termination impedance. Increasingly, elements constituting the matching circuit are provided as conductive patterns on a mounting board on which a SAW filter is mounted. This is aimed at reducing the number of circuit components forming the above-mentioned matching circuit outside the mounting board. Such a conductive pattern formed on the mounting board includes an inductor connected in parallel to the SAW filter, and the larger the inductance value, the larger the area occupied by the inductor on the mounting board.

JP2015-41909

A capacitor may be provided in parallel with the inductor on the input side of the SAW filter in order to reduce the inductance value of the inductor and thereby reduce the area occupied by the conductive pattern on the mounting board. As this capacitor, a comb-shaped pattern is drawn on a piezoelectric substrate forming a chip constituting the SAW filter. However, when this capacitor resonates, unnecessary ripples may appear in the characteristics of the SAW filter.

The present invention has been made in view of the above circumstances, and its purpose is to suppress the resonance of the capacitor and obtain good characteristics in a surface acoustic wave filter in which a conductive pattern forming a capacitor is provided on a piezoelectric substrate. That's true.

The surface acoustic wave filter of the present invention includes a piezoelectric substrate;
an IDT electrode provided on the piezoelectric substrate and connected to a conductive path for signal input and a conductive path for signal output;
a first comb-teeth portion that is a conductive pattern provided on the piezoelectric substrate so as to constitute the conductive path for signal input or the conductive path for signal output;
A conductive pattern is provided on the piezoelectric substrate and grounded so as to form a capacitor together with the first comb teeth, and a second conductive pattern whose teeth are formed between each tooth of the first comb teeth. The comb tooth part of
provided on each of the teeth of the first comb-teeth portion and the second comb-teeth portion, and of at least one of the first comb-teeth portion and the second comb-teeth portion. Each tooth has a plurality of narrow width parts,
provided on each of the teeth of the first comb tooth portion and the second comb tooth portion, and of at least the other comb tooth portion of the first comb tooth portion and the second comb tooth portion. a plurality of widened portions wider than the narrowed portions provided on each tooth;
The narrow width portion and the wide width portion are alternately located in the arrangement direction of each tooth of the first comb tooth portion and each tooth of the second comb tooth portion, and in a direction perpendicular to the arrangement direction. ,
Equipped with
The capacitor and the IDT electrode are connected in series when viewed from a signal input port to which the signal input conductive path is connected to a signal output port to which the signal output conductive path is connected. be done.

According to the surface acoustic wave filter of the present invention, good characteristics can be obtained by suppressing resonance of the capacitor formed of the first comb tooth portion and the second comb tooth portion.

1 is a plan view of a SAW filter according to an embodiment of the present invention. FIG. 3 is a plan view of a conductive pattern forming a capacitor constituting the SAW filter. FIG. 3 is a graph diagram showing the characteristics of the SAW filter. FIG. 3 is a graph diagram showing the characteristics of the SAW filter. FIG. 3 is a Smith chart diagram of the SAW filter. FIG. 7 is a plan view showing another example of the conductive pattern. FIG. 3 is a plan view of a diplexer including the SAW filter. FIG. 3 is a circuit diagram of the diplexer. FIG. 3 is a filter diagram showing filter characteristics of the diplexer.

A SAW filter 1 according to an embodiment of the surface acoustic wave filter of the present invention will be described with reference to FIG. 1 which is a plan view. This SAW filter 1 is a vertically coupled filter that utilizes multiple modes. 10 is a piezoelectric substrate, which is made of a piezoelectric material such as tantalum lithium oxide (LiTaO ₃ ), which is rotated by 42°, Y cut, and propagated in the X direction.

On the piezoelectric substrate 10, the five IDT (Interdigital Transducer) electrodes described above are formed along the propagation direction of the SAW, and for the five IDT electrodes, from one side of the piezoelectric substrate 10 to the other side, 11, 11, 12, 13, 14, and 15. Further, two reflectors 16 are provided on the piezoelectric substrate 10 so as to sandwich the IDT electrodes 11 to 15 therebetween. On the surface of the piezoelectric substrate 10, the direction in which the IDT electrodes 11 to 15 and the reflector 16 are arranged is sometimes referred to as the left-right direction, and the direction perpendicular to the left-right direction on the surface of the piezoelectric substrate 10 is sometimes referred to as the front-back direction. In the left-right direction, the IDT electrode 11 side and the IDT electrode 15 side are respectively referred to as the left side and the right side. Note that, for convenience in explaining the configuration of the SAW filter 1, the front, rear, left, and right directions are defined in this manner, but the SAW filter 1 may be used in any orientation.

The bus bars constituting each of the IDT electrodes 11 to 15 are indicated by adding A or B after the code representing the IDT electrode including the bus bar. A is the front busbar, and B is the rear busbar. That is, taking the IDT electrode 11 as an example, the front bus bar configuring the IDT electrode 11 is 11A, and the rear bus bar is 11B. In order to label each busbar in this way, busbars 11A to 15A are arranged side by side on the left and right, and bus bars 11B to 15B are arranged side by side on the left and right. In addition, electrode fingers 17 forming the IDT electrodes 11 to 15 and extending rearward and forward from the bus bars 11A to 15A and 11B to 15B, respectively, are shown.

The bus bars 12B, 14B are connected to the signal output port 21 via a conductive pattern (not shown), and the bus bars 11B, 13B, 15B are grounded. On the piezoelectric substrate 10, conductive patterns made of metal films are formed so as to be drawn out from the bus bars 11A to 15A toward the front of the piezoelectric substrate 10, respectively. The ends of the conductive patterns drawn out from one busbar, ie, busbars 11A, 13A, 15A, are directed to a position further forward than the conductive patterns drawn out from the other busbars, ie, busbars 12A, 14A. The conductive patterns drawn out from the bus bars 11A and 15A head to the right and left sides, respectively, and merge with the conductive patterns drawn out from the bus bar 13A. The conductive pattern 31 that has merged in this way is connected to the signal input port 22 and forms a conductive path for signal input. Furthermore, assuming that the conductive pattern drawn out from the bus bars 12A and 14A is 32, the conductive pattern 32 is grounded.

The conductive pattern 31 includes electrode fingers 33 extending rearward toward the front end of each conductive pattern 32. A large number of these electrode fingers 33 are provided with intervals left and right, thereby forming an IDT electrode. That is, a comb-shaped pattern (first comb-teeth portion) is formed by the large number of electrode fingers 33, and each electrode finger 33 forms a comb-teeth.

Further, each conductive pattern 32 is provided with an electrode finger 34 extending forward toward the conductive pattern 31 on its front end side. A large number of these electrode fingers 34 are provided with intervals left and right, thereby forming an IDT electrode. That is, a comb-shaped pattern (second comb-teeth portion) is formed by the large number of electrode fingers 33, and each electrode finger 34 forms a comb-teeth. The electrode finger 34 is provided so as to enter between two adjacent electrode fingers 33. When viewed in the left-right direction, the electrode fingers 33 and electrode fingers 34 are alternately and repeatedly spaced apart and arranged at regular intervals. A capacitor 30 is formed by the electrode fingers 33 and 34 arranged in this manner. The capacitor 30 plays the role of reducing the inductance value of the inductor connected in parallel to the SAW filter 1, as explained in the section on problems to be solved by the invention.

In FIG. 2, electrode fingers 33 and 34 forming the capacitor 30 are shown in an enlarged manner. Since the electrode fingers 33 and 34 have the same shape as each other except that they extend in opposite directions, the electrode finger 33 will be described as a representative example. The electrode finger 33 is configured as a widened portion 35 by bulging the portion on the way to the tip and the tip portion to form a chevron shape on the left and right, respectively. The region of the electrode finger 33 other than the widened portion 35 is defined as a narrowed portion 36 . Therefore, two narrow portions 36 and two wide portions 35 are alternately provided from the base end side to the distal end side of the electrode finger 33. Therefore, the narrow portions 36 and the wide portions 35 are alternately located when viewed in the front-rear direction (that is, in the direction perpendicular to the SAW propagation direction).

The positional relationship between the electrode fingers 33 and 34 will be further explained. The widened portion 35 of the electrode finger 33 and the narrowed portion 36 of the electrode finger 34 are adjacent to each other in the left-right direction (that is, the SAW propagation direction), and the widened portion 35 of the electrode finger 34 and the narrowed portion 36 of the electrode finger 33 are adjacent to each other. Adjacent to each other. That is, when viewed in the left-right direction, widened portions 35 and narrowed portions 36 are alternately and repeatedly arranged.

In FIG. 2, virtual straight lines L1 and L2 are drawn to clarify the positional relationship between adjacent electrode fingers 33 and 34. More specifically, the straight lines L1 and L2 extend in the front-rear direction so as to pass through the end of the widened part 35 of the electrode finger 33 on the electrode finger 34 side and the end of the widened part 35 of the electrode finger 34 on the electrode finger 33 side, respectively. It's pulling. As shown by the straight lines L1 and L2, the widened portions 35 of the electrode fingers 33 and the widened portions 35 of the electrode fingers 34 do not overlap each other when viewed in the front-rear direction.

When the SAW propagates, the propagation speed of the SAW differs between the widened portion 35 and the narrowed portion 36 due to the difference in width thereof. As described above, the widened portions 35 and narrowed portions 36 are arranged alternately in the left-right direction and the front-back direction, so that when the SAW moves through the area where the electrode fingers 33 and 34 intersect, It is considered that as a result of the speed variation in the widened portion 35 and each narrowed portion 36, disturbance occurs in the SAW and resonance of the electrode fingers 33 and 34 is prevented.

For the purpose of preventing resonance in this way, if the size of the widest part of the widened part 35 is W1, and the size of the narrowest part of the narrow part 36 is W2, then, for example, W1/W2=1 .5 or more, and preferably W1/W2=2 or more in order to more reliably prevent resonance. Further, as described above, the widened portions 35 of the electrode fingers 33 and the widened portions 35 of the electrode fingers 34 do not overlap each other in the front-back direction. If they were to overlap, each widened portion 35 would function as an IDT electrode and these widened portions 35 would resonate, but by not overlapping, such resonance of the widened portions 35 is prevented.

A simulation experiment conducted to verify the characteristics of the SAW filter 1 will be described below. Experiments were conducted without the capacitor 30 or with the capacitance value of the capacitor 30 set to 0.3 pF, 0.6 pF, or 1.2 pF. 3 and 4 are graphs representing filter characteristics. Regarding the graphs in FIGS. 3 and 4, the horizontal axis indicates the normalized frequency, and the vertical axis indicates the S21 characteristic (unit: dB). FIG. 4 is an enlarged view of the range in which the normalized frequency ranges from 0.95 to 1.05 in the graph of FIG. FIG. 5 is a Smith chart of S11. There is no major change in the shape of the curve in the Smith chart due to the change in the capacitance value, and the position of the curve moves downward due to the increase in the capacitance value.

As shown in FIGS. 3 and 4, it was confirmed that by providing the capacitor 30, the fall of the attenuation curve on the high band side becomes steeper, and the larger the capacitance value is, the steeper the fall becomes. . Further, in the passband of the filter, no ripple occurs no matter what the capacitance value is, as in the case where the capacitor 30 is not provided. Therefore, it is considered that resonance of the capacitor 30 is prevented.

Furthermore, when the capacitance value is 0.3 pF or 0.6 pF, there is almost no deterioration in the insertion loss when the capacitor 30 is not provided, and by adjusting the matching for the SAW filter 1, The insertion loss can be fully compensated. As can be seen from the above, according to the SAW filter 1, the inductance value of the inductor of the mounting board on which the SAW filter 1 described in the background art is mounted can be suppressed, and the size of the inductor can be reduced. Moreover, the occurrence of ripples in the passband and deterioration of insertion loss are suppressed.

By the way, in the SAW filter 1, the above-mentioned widened portions 35 and narrowed width portions 36 of the electrode fingers 33 and 34 are not limited to two each, but a larger number of widened portions 35 and narrowed width portions 36 may be provided. The widened portions 35 and the narrowed portions 36 may be alternately and repeatedly provided when viewed in the front-rear direction. Further, in FIG. 6, for the electrode finger 33, a narrow part 36, a wide part 35, and a narrow part 36 are provided in order in the front-rear direction, and for the electrode finger 34, a wide part 35, a narrow part 36, and a narrow part 36 are provided in the front-rear direction. An example in which widened portions 35 are provided in sequence is shown. In this way, even if the electrode fingers 33 and 34 include two of the widened part 35 and the narrowed part 36 and only one of the other, it is considered that the progress of the SAW can be sufficiently disturbed. That is, when providing the widened portions 35 and the narrowed portions 36 alternately in the front-rear direction on each electrode finger, it is not limited to providing a plurality of them.

Note that, as mentioned above, it is considered that it is sufficient that the SAW is disturbed due to the difference in the width of each part of the electrode fingers 33 and 34, so the shape of the widened part 35 is not limited to the shape shown in the figure, but may be, for example, a rectangular shape. It may be a shape. Furthermore, the electrode fingers 33 and 34 are not limited to being formed in the above-mentioned orientation, but may be formed, for example, to face in the left-right direction (that is, the extending direction is the left-right direction).

Although not shown in FIG. 1, for example, between the signal output port 21 and the IDTs 12 and 14, a conductive pattern drawn out from the bus bars 12B and 14B and formed on the piezoelectric substrate 10 is used as a conductive path for signal output. intervene. Further, a conductive pattern is formed on the piezoelectric substrate 10 so as to be drawn out from the bus bars 11B, 13B, and 15B, and the conductive pattern is grounded. Instead of providing the electrode fingers 33 and 34 at the positions described above, the electrode fingers 33 and 34 may be formed on the conductive pattern connected to the signal output port 21 and the conductive pattern grounded, respectively. That is, the capacitor 30 may be provided on the output side of the SAW filter 1 instead of on the input side of the SAW filter 1. However, the matching on the output side of the SAW filter 1 is normally performed by a device connected to the subsequent stage of the SAW filter 1, and the matching device provided on the mounting board described in the section of the problem to be solved by the invention is used to perform matching on the output side of the SAW filter 1. The effect of reducing the size of the inductor cannot be obtained. That is, from the viewpoint of practicality, it is preferable to provide the capacitor 30 on the input side of the SAW filter 1.

Next, the diplexer 4, which is a SAW device to which the SAW filter 1 is applied, will be described with reference to the plan view of FIG. 7 and the circuit diagram of FIG. 8. The diplexer 4 includes a first filter section 5A and a second filter section 5B, and these filter sections have a common input side and separate output sides.

The first filter section 5A includes SAW resonators 51A to 54A and a SAW filter 1 (for convenience, 1A), and the second filter section 5B includes SAW resonators 51B to 55B and a SAW filter 1 (for convenience, 1B). ). A conductive pattern 61 that connects these SAW resonators and SAW filters to each other is formed on the piezoelectric substrate 10. Therefore, the first filter section 5A and the second filter section 5B are formed on a common piezoelectric substrate 10. In the piezoelectric substrate 10, a first filter section 5A is formed from the left and right centers to the right end, and a second filter section 5B is formed from the left and right centers to the left end. In FIG. 7, a conductive pattern 61 is shown with a large number of dots, and the conductive patterns 31 and 32 described in FIG. 1 form part of this conductive pattern 61.

Note that the SAW resonators 51A to 54A and 51B to 55B are formed by an IDT electrode and a reflector placed across the IDT electrode from the left and right sides, but the detailed configuration is omitted in FIG. One IDT and one reflector are each shown as square frames. Furthermore, as with the SAW resonators, detailed configurations of the SAW filters 1A and 1B are not shown in FIG. 7.

The above conductive pattern 61 includes an input terminal 62 formed at the center front end of the piezoelectric substrate 10 in the left-right direction, and output terminals 63A and 63B formed at the right rear end and left rear end of the piezoelectric substrate 10, respectively. including. The input terminal 62 is shared by the first filter sections 5A and 5B, and is connected to the signal input port 22. The output terminals 63A and 63B constitute a first filter section 5A and a second filter section 5B, respectively, and the conductive pattern 61 forms conductive paths 66A and 66B from the input terminal 62 to the output terminals 63A and 63B, respectively. There is. Therefore, the conductive paths 66A and 66B are conductive paths specific to the first filter section 5A and the second filter section 5B, respectively. Output terminals 63A and 63B are connected to signal output ports 21A and 21B, respectively. These signal output ports 21A and 21B correspond to the signal output port 21 shown in FIG. Further, as shown in FIG. 7, a plurality of locations of the conductive pattern 61 are configured as ground terminals.

An inductor 71 is shown between the signal input port 22 and the input terminal 62 and provided in parallel with the first filter section 5A and the second filter section 5B. The inductor 71 is the matching inductor described in the background art section.

To explain the configuration of the first filter section 5A, SAW resonators 51A and 52A, which are series arms, are provided in order toward the output side, and the SAW filter 1A is provided between these SAW resonators 51A and 52A. SAW resonators 53A and 54A, which are parallel arms, are provided between the SAW resonator 51A and the SAW filter 1A, and on the downstream side of the SAW resonator 52A, respectively.

To explain the configuration of the second filter section 5B, SAW resonators 51B and 52B, which are series arms, are provided in order toward the output side, and the SAW filter 1B is provided between these SAW resonators 51B and 52B. Then, SAW resonators 53B, 54B, and 55B, which are parallel arms, are provided between the SAW resonator 51B and the SAW filter 1B, between the SAW filter 1B and the SAW resonator 52B, and on the downstream side of the SAW resonator 52B, respectively. It will be done.

A capacitor 65 is formed by a conductive pattern 61 between the front stage of the SAW resonator 51B and the rear stage of the SAW resonator 53B. The capacitor 65 includes a comb-shaped pattern formed on the piezoelectric substrate 10 similarly to the capacitor 30, but as shown in FIG. 7, the comb teeth are oriented horizontally (left and right) and the width of each tooth is uniform. . In other words, the capacitor 65 is not provided with the widened portion 35 and the narrowed portion 36 .

Although matching can be adjusted by adjusting the inductance value of the inductor 71, the matching of both the first filter section 5A and the second filter section 5B will be changed. In the diplexer 4, capacitors 30 are individually provided in the conductive paths 66A and 66B. By appropriately setting the capacitance value of each capacitor 30, the matching between the first filter section 5A and the second filter section 5B can be individually adjusted to achieve appropriate matching.

By the way, resonance occurs in the capacitor 65 due to the propagation of longitudinal waves. Since the tooth orientation of the capacitor 65 is not the original orientation (front-to-back) of the IDT electrode on the piezoelectric substrate 10, the Q value is low, and some ripples occur due to resonance. In some cases, ripples that occur when the teeth of a capacitor are formed horizontally can be effectively used, but as mentioned in the section on problems to be solved by the invention, they often become a nuisance. Regarding the capacitor 30 described above, the comb teeth face in the front-rear direction due to the space for drawing the pattern, but as described above, resonance is prevented and ripples are prevented from occurring. That is, according to the configuration of the capacitor 30 including the widened part 35 and the narrowed part 36, since resonance is prevented, the degree of freedom in the direction in which the capacitor is provided is increased, and the capacitor can be provided in a limited space. There is an effect.

Note that if installation space can be secured, the capacitor 30 is not limited to being provided at the above-mentioned position, but may be provided closer to the signal input port 22 than the resonators 51A and 51B, for example. By the way, although it has been explained that the capacitor 30 is provided in both the first filter section 5A and the second filter section 5B, the capacitor 30 may be provided in only one of them.

FIG. 9 is a graph similar to FIG. 3 showing the results obtained by the actual measurement test of the characteristics of the first filter section 5A. This actual measurement test was conducted with and without the capacitor 30 described above. The slope of the attenuation curve was slightly steeper when the capacitor 30 was provided than when the capacitor 30 was not provided. Note that the insertion loss is made equal to each other due to matching by the inductor 71. In other words, it was confirmed that when the capacitor 30 is provided, the attenuation characteristics can be improved while suppressing the deterioration of the insertion loss.

The embodiments disclosed this time should be considered to be illustrative in all respects and not restrictive. For example, the number and shape of IDT electrodes forming the filter, the shape of each conductive pattern, the number and connection of SAW resonators forming the diplexer, etc. can be changed as appropriate. The embodiments described above may be omitted, replaced, modified, or combined in various forms without departing from the scope and spirit of the appended claims.

1 SAW filter 10 Piezoelectric substrate 11 to 15 IDT
30 capacitors 31, 32, conductive patterns 33, 34 electrode fingers 35 widened part 36 narrowed part

Claims (6)

a piezoelectric substrate;
an IDT electrode provided on the piezoelectric substrate and connected to a conductive path for signal input and a conductive path for signal output;
a first comb-teeth portion that is a conductive pattern provided on the piezoelectric substrate so as to constitute the conductive path for signal input or the conductive path for signal output;
A conductive pattern is provided on the piezoelectric substrate and grounded so as to form a capacitor together with the first comb teeth, and a second conductive pattern whose teeth are formed between each tooth of the first comb teeth. The comb tooth part of
provided on each of the teeth of the first comb-teeth portion and the second comb-teeth portion, and of at least one of the first comb-teeth portion and the second comb-teeth portion. Each tooth has a plurality of narrow width parts,
provided on each of the teeth of the first comb tooth portion and the second comb tooth portion, and of at least the other comb tooth portion of the first comb tooth portion and the second comb tooth portion. a plurality of widened portions wider than the narrowed portions provided on each tooth;
The narrow width portion and the wide width portion are alternately located in the arrangement direction of each tooth of the first comb tooth portion and each tooth of the second comb tooth portion, and in a direction perpendicular to the arrangement direction. ,
Equipped with
The capacitor and the IDT electrode are connected in series when viewed from a signal input port to which the signal input conductive path is connected to a signal output port to which the signal output conductive path is connected. surface acoustic wave filter. The widened portion of the first comb tooth portion and the widened portion of the second comb tooth portion do not overlap when viewed in the vertical direction,
Regarding the first comb-teeth part and the second comb-teeth part adjacent to each other, the end of the widened part of the first comb-teeth part on the second comb-teeth part side in the arrangement direction of each of the teeth, 2. The surface acoustic wave filter according to claim 1, wherein the end of the widened portion of the second comb tooth portion on the first comb tooth portion side is located at a different position . 3. The surface acoustic wave filter according to claim 1, wherein a plurality of said IDT electrodes are provided, and the arrangement direction of said IDT electrodes is the arrangement direction of each tooth of said capacitor. 4. The surface acoustic wave filter according to claim 1, wherein the first comb tooth portion constitutes a conductive path for signal input. The elasticity according to any one of claims 1 to 4, wherein a plurality of the narrow portions and the widened portions are provided on each tooth of the first comb tooth portion and each tooth of the second comb tooth portion. surface wave filter. a first filter section including a first surface acoustic wave filter and a first resonator that are connected to each other;
a second filter section including a second surface acoustic wave filter and a second resonator connected to each other;
The first filter section and the second filter section are provided on the common piezoelectric substrate,
The piezoelectric substrate is provided with an input terminal common to the first filter section and the second filter section,
At least one of the first surface acoustic wave filter and the second surface acoustic wave filter is the surface acoustic wave filter according to any one of claims 1 to 5,
The capacitor is a surface acoustic wave device provided in a conductive path specific to the first filter section or a conductive path specific to the second filter section formed on the piezoelectric substrate.

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