JP2014007581A - Longitudinal coupling surface acoustic wave filter and cascade connection filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a longitudinal coupling surface acoustic wave filter that has good steepness of the rise of attenuation characteristics on a low-pass side in the vicinity of a passband, and a cascade connection filter obtained by cascade connection of the longitudinal coupling surface acoustic wave filters.SOLUTION: A longitudinal coupling surface acoustic wave filter 1 has IDT electrodes 11 including a central electrode 11a and side electrodes 11b and 11c provided on both sides of the central electrode 11a, and reflectors 12 provided on both sides of the IDT electrodes 11a to 11c, which are arranged on a piezoelectric substrate 10. When a pitch of electrode fingers 14 of the central electrode 11a is Pi1, and a pitch of electrode fingers 14 of the side electrodes 11b and 11c is Pi2, the pitches Pi1 and Pi2 satisfy a relation of 0.985 Pi1≤Pi2≤0.999 Pi1.

Description

  The present invention relates to a longitudinally coupled surface acoustic wave filter in which a plurality of IDT (interdigital transducer) electrodes are arranged on a piezoelectric substrate, and a cascaded filter in which two longitudinally coupled surface acoustic wave filters are cascaded. .

  Communication devices such as mobile phones are remarkably demanded for higher functionality and higher performance, and the types of communication devices are diversifying. Therefore, a communication band is allocated to each communication device. For this reason, the surface acoustic wave filter used in these communication devices is applicable to a wide band and has a low loss.

  As an example of the filter for the communication device, there is a longitudinally coupled surface acoustic wave filter using lithium tantalate or lithium niobate as a piezoelectric substrate. A longitudinally coupled surface acoustic wave filter has a configuration in which a plurality of IDT electrodes are arranged on a piezoelectric substrate along the propagation direction of surface acoustic waves, and reflectors are arranged on both sides of these IDT electrodes. A pass band is set using a higher-order resonance mode.

As described above, in the longitudinally coupled surface acoustic wave filter using the resonance mode, the rising characteristics of the zeroth-order and higher-order resonance modes greatly influence the out-of-band attenuation near the passband.
The inventor has proposed various longitudinally coupled surface acoustic wave filters having good characteristics (steepness) in which the insertion loss in the passband is small and the attenuation amount is rapidly increased outside the band (for example, Patent Document 1). 2). This time, a new technique capable of improving the characteristics of the longitudinally coupled surface acoustic wave filter has been found and the present invention has been achieved.

JP 2010-141727 A JP2011-23894A

  The present invention has been made under such circumstances, and an object of the present invention is to provide a longitudinally coupled surface acoustic wave filter having a good rise steepness of a low-frequency attenuation characteristic in the vicinity of the passband and the longitudinally coupled type. An object of the present invention is to provide a cascade connection type filter in which surface acoustic wave filters are cascade-connected.

The longitudinally coupled surface acoustic wave filter according to the present invention arranges IDT electrodes including a central electrode and side electrodes provided on both sides of the central electrode on a piezoelectric substrate, and reflects on both sides of these IDT electrodes. In a longitudinally coupled surface acoustic wave filter configured by arranging a vessel,
When the pitch of the electrode fingers of the central electrode is Pi1, and the pitch of the electrode fingers of the side electrode is Pi2,
These pitches Pi1 and Pi2 satisfy the relationship of 0.985Pi1 ≦ Pi2 ≦ 0.999Pi1.

The longitudinally coupled surface acoustic wave filter described above has up to x (x is a positive number and smaller than the number X of electrode fingers of the side electrode) from the reflector side of the side electrode adjacent to the reflector. If the pitch of the electrode fingers is changed to Pi2 ′ instead of Pi2, the pitch Pi2 ′ may be configured to satisfy the relationship of 0.985Pi1 ≦ Pi2 ′ ≦ 1.000Pi1 (where Pi2 ′ ≠ Pi2).
Further, a cascade connection type filter according to another invention is characterized in that it is configured by connecting two of the above-described longitudinally coupled surface acoustic wave filters in cascade.

  According to the present invention, IDT electrodes including a central electrode and side electrodes provided on both sides of the central electrode are arranged, and the pitch Pi1 of the electrode fingers of the central electrode and the pitch Pi2 of the electrode fingers of the side electrode are By satisfying the relationship of 0.985Pi1 ≦ Pi2 ≦ 0.999Pi1, it is possible to improve the steepness of the rise of the attenuation characteristic on the low band side in the vicinity of the pass band.

1 is a plan view showing an example of a longitudinally coupled surface acoustic wave filter according to a first embodiment. It is a top view which shows an example of the longitudinal coupling type | mold surface acoustic wave filter concerning 2nd Embodiment. It is a top view which shows an example of the cascade connection type filter using a longitudinally coupled surface acoustic wave filter. It is the 1st explanatory view showing the simulation result concerning an example. It is the 2nd explanatory view showing the simulation result concerning an example. It is the 3rd explanatory view showing the simulation result concerning an example. It is the 1st explanatory view showing the simulation result concerning a comparative example. It is the 1st explanatory view showing the relation between the pitch of the electrode finger of a side electrode and the steepness of a guard band in the example. It is the 4th explanatory view showing the simulation result concerning an example. It is the 5th explanatory view showing the simulation result concerning an example. It is 6th explanatory drawing which shows the simulation result concerning an Example. It is the 2nd explanatory view showing the simulation result concerning a comparative example. It is the 2nd explanatory view showing the relation between the pitch of the electrode finger of a side electrode and the steepness of a guard band in the example.

[First Embodiment]
A first embodiment of a longitudinally coupled surface acoustic wave filter (hereinafter simply referred to as a filter) according to the present invention will be described with reference to FIG. The filter 1 is formed on a piezoelectric substrate 10 made of, for example, lithium tantalate (LiTaO ₃ ). In this example, the piezoelectric substrate 10 is, for example, a 36 ° to 48 ° Y cut plate, that is, when the piezoelectric substrate 10 is viewed from the X-axis direction, an upper surface (a surface on which an IDT electrode 11 described later) is formed and a Y-axis Is a substrate (36 ° rotated Y plate LiTaO ₃ ) cut so that an angle (cutting angle) θ formed by is 36 ° to 48 °. On the piezoelectric substrate 10, the surface acoustic wave propagates along the X axis of the crystal axis of the piezoelectric substrate 10.

  For example, three IDT electrodes 11 are arranged on the piezoelectric substrate 10 along the propagation direction of the surface acoustic wave, and are spaced apart in the propagation direction of the surface acoustic wave so as to sandwich the IDT electrodes 11 from both sides. Thus, two reflectors 12 (12a, 12b) are formed.

  Each of the IDT electrode 11 and the reflector 12 has a predetermined film thickness. For example, a film made of a metal such as aluminum is formed on the piezoelectric substrate 10, and then a mask layer laminated on the metal film is formed. The region other than the IDT electrode 11 and the reflector 12 is formed by a photolithography method. The IDT electrode 11 includes a pair of bus bars 13a and 13b that extend along the propagation direction of the surface acoustic wave, and a plurality of electrode fingers 14 that alternately extend in a comb-teeth shape so as to face each other from the bus bars 13a and 13b. And.

  Hereinafter, for the three IDT electrodes 11, in FIG. 1, the central IDT electrode 11 is referred to as a central electrode 11a, and the IDT electrodes 11 provided on both sides of the central electrode 11a are referred to as side electrodes 11b and 11c. In the filter 1 of this example, the center electrode 11a constitutes an input side electrode, and the side electrodes 11b and 11c constitute an output side electrode.

  The bus bar 13a on one side of the central electrode 11a which is an input side electrode is connected to the input port 15, and the bus bar 13b on the other side is grounded. The bus bar 13 a on one side of the two side electrodes 11 b and 11 c that are output side electrodes is grounded, and the bus bar 13 b on the other side is connected to the common output port 16.

  In the reflectors 12a and 12b, a large number of grating electrodes 17 arranged in parallel with the electrode fingers 14 of the IDT electrode 11 are provided along the propagation direction of the surface acoustic wave, and the multiple grating electrodes 17 are arranged on one end side thereof. And they are short-circuited to each other on the other end side. 1 to 3, the numbers of the electrode fingers 14 and the grating electrodes 17 are omitted. In these drawings, the IDT electrode 11 and the reflector 12 are painted in gray so that the region can be easily distinguished from the region on the piezoelectric substrate 10.

  Here, as shown in FIG. 1, in the central electrode 11a, the pitch is the distance between the centers of the two electrode fingers 14, 14 extending adjacent to each other from the bus bar 13 (the bus bar 13a or the bus bar 13b) on one side. Let Pi1 be the length dimension of (arrangement interval). In the side electrodes 11b and 11c, the length dimension of the pitch between the two electrode fingers 14 and 14 extending adjacent to each other from the bus bar 13 (the bus bar 13a or the bus bar 13b) on one side is defined as Pi2. In addition, Pr1 shown in the figure is the dimension (pitch) between the centers of the grating electrodes 17 and 17 for one wavelength in each reflector 12a and 12b.

In the conventional longitudinally coupled surface acoustic wave filter, the pitch Pi1 of the electrode fingers 14 of the central electrode 11a and the pitch Pi2 of the electrode fingers 14 of both side electrodes 11b and 11c are equal to each other, and Pi1 = Pi2. The IDT electrode 11 (the center electrode 11a and the side electrodes 11b and 11c) was formed.
On the other hand, as shown in FIG. 1, in the filter 1 of the present embodiment, the pitch Pi2 of the electrode fingers 14 in both side electrodes 11b and 11c is a value within the range of 0.985Pi1 ≦ Pi2 ≦ 0.999Pi1. It is set to become.

Next, the reason why the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c is made smaller than the pitch Pi1 of the center electrode 11a as described above will be described based on a simulation result described later.
In the 3IDT type filter 1 having the central IDT electrode 11a and the side electrodes 11b and 11c provided on both sides thereof, when a frequency signal is input from the input port 15, the central electrode 11a converts the electrical signal into a mechanical signal. As a result, a surface acoustic wave is generated and propagates in the right and left directions. The surface acoustic waves that propagate to the left and right reach the reflectors 12a and 12b via the side electrodes 11b and 11c, respectively, are reflected by the reflectors 12a and 12b, and then pass through the side electrodes 11b and 11c again. And return to the central electrode 11a.

  In this way, the surface acoustic wave propagating through the region where the central electrode 11a and the side electrodes 11b and 11c are provided while reflecting between the reflectors 12a and 12b is a frequency signal input to the input port 15 having a predetermined frequency signal. Resonance occurs when the frequency is reached, and the 0th order resonance mode (0th order mode) is excited. When the frequency of the input frequency signal is further increased, a higher-order, for example, second-order resonance mode (second-order mode) is excited, and a passband is set between these modes.

  In FIG. 4, the frequency characteristic of the filter 1 when the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c is Pi2 = 0.996Pi1 is shown by a solid line (Example 1-1), and when Pi2 = Pi1 The frequency characteristics of the conventional filter are indicated by thin lines (reference examples: the same in FIGS. 5 to 7 and FIGS. 9 to 12). As will be described later, the filter of the reference example is configured in the same manner as the filter 1 of FIG. 1 except that the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c is equal to the pitch Pi1 of the center electrode 11a.

  According to the results shown in FIG. 4, the frequency characteristics of the filter 1 of Example 1-1 are substantially the same as the passband of the reference example filter, while the attenuation band on the low frequency side changes from the passband to the passband. The rise of the attenuation characteristic is steep. As a result, the width of the transition band on the low frequency side (for example, the frequency range in which the attenuation changes from −2.5 dB to −15 dB, hereinafter referred to as the guard bandwidth) is narrowed.

  Thus, it is not clear why the guard band width is narrowed by reducing the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c with respect to the pitch Pi1 of the electrode fingers 14 of the central electrode 11a. As the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c is reduced, it becomes more prominent. 5 and FIG. 6, the frequency characteristics of the filter 1 when Pi2 is Pi2 = 0.994Pi1 and 0.992Pi1 are shown by solid lines (Examples 1-2 and 1-3). In these embodiments as well, the guard band width on the low frequency side is narrower than in the reference examples indicated by thin lines, and reducing Pi2 relative to Pi1 raises the attenuation characteristics from the attenuation band to the passband. It can be confirmed that this is an effective method of sharpening.

  Further, as is clear from the comparison with the reference example, as the value of Pi2 in Examples 1-1 to 1-3 decreases from 0.996 Pi1 → 0.994 Pi1 → 0.992 Pi1, the guard band width is increased. Becomes gradually narrower, and it can be confirmed that the steepness of this region is improved.

  FIG. 8 shows the result of plotting the guard bandwidth [MHz] against the value of Pi2 / Pi1. The results of Examples 1-1 to 1-3 are plotted with black diamonds. A white circle plot indicates a reference example. It can be confirmed that the guard band width can be narrowed as the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c decreases. It can also be seen that the effect of narrowing the guard band width by decreasing the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c gradually decreases with the decrease of Pi2.

  Therefore, FIG. 7 shows the result of further reducing the pitch of the electrode fingers 14 of the side electrodes 11b and 11c to Pi2 = 0.980Pi1 (Comparative Example 1-1). Thus, it was confirmed that when the value of Pi2 is 2% smaller than Pi1, the guard band width is further narrowed. However, looking at the frequency characteristics of FIG. 7, in Comparative Example 1-1, the amount of attenuation in the low frequency region in the pass band increases, and the pass characteristics deteriorate. This is because, in the longitudinally coupled surface acoustic wave filter 1 in which the pass band is formed by exciting the resonance mode in the region extending from the central electrode 11a to the side electrodes 11b and 11c, if the Pi2 value becomes too small, the resonance occurs. It is presumed that this is because the mode state changes.

  Thus, the preferred range of Pi is 0.985 Pi1 ≦ Pi2 ≦ from the tendency that the effect of narrowing the guard band width decreases with the decrease of Pi2 and the phenomenon that the pass characteristic of the filter 1 deteriorates when Pi2 becomes too small. In the range of 0.999 Pi1, more preferably in the range of 0.990 Pi1 ≦ Pi2 ≦ 0.999Pi1, the attenuation characteristic from the attenuation band to the pass band is maintained while maintaining the pass characteristic of the filter 1 that is not problematic in practice. It has been confirmed that the rise can be made steep.

  Taking into account variations in etching processing during photolithography, the pitch of the electrode fingers 14 of the side electrodes 11b and 11c with respect to the central electrode 11a is in a ratio of 0.985 to 0.999. It can be specified from the average value of the pitches Pi1 and Pi2 of each electrode finger 14. Further, the pitches Pi1 and Pi2 of the electrode fingers 14 after etching may be specified based on the pitch of the electrode fingers drawn on the mask substrate used in photolithography.

  The filter 1 according to the first embodiment has the following effects. The IDT electrodes 11 including the central electrode 11a and the side electrodes 11b and 11c provided on both sides of the central electrode 11a are arranged, the pitch Pi1 of the electrode fingers 14 of the central electrode 11a, and the electrode fingers of the side electrodes 11b and 11c. When the pitch Pi2 of 14 satisfies the relationship of 0.985Pi1 ≦ Pi2 ≦ 0.999Pi1, it is possible to improve the steepness of the rise of the attenuation characteristic on the low band side in the vicinity of the pass band.

[Second Embodiment]
As schematically shown in FIG. 2, in the filter 1 of the second embodiment, among the pitches Pi <b> 2 of the electrode fingers of the side electrodes 11 b and 11 c, x pieces from the outside (the electrode fingers 14 of the side electrodes 11 b and 11 c). The pitch of the electrode fingers 14 of x <X is changed to Pi2, and Pi2 ′ (Pi2 ′ ≠ Pi2) is set. In FIG. 2, the same reference numerals as those shown in FIG. 1 are given to the constituent elements common to the first embodiment.

  As shown in FIG. 2, in this filter 1a, among the electrode fingers 14 of the side electrodes 11b and 11c, x (x is a positive number from the reflector 12a and 12b side, and the number X of electrode fingers of the side electrodes X If the pitch of the electrode fingers 14 up to (P2) is Pi2 ′, the pitch Pi2 ′ is different from the pitch Pi2 of the electrode fingers 14 (x + 1) or more away from the reflector 12a side.

  In other words, in each of the side electrodes 11b and 11c, the pitch Pi2 ′ of the electrode fingers 14 on the end side (reflector 12 side) when viewed from the center electrode 11a is larger than the pitch Pi2 of the electrode fingers 14 close to the center electrode 11a. Each is set to be smaller or larger by x.

  For example, in FIG. 9, in the side electrodes 11b and 11c where the total number of electrode fingers 14 is X = 26 (13 pairs when two are counted as one pair), three from the reflectors 12a and 12b ( The frequency characteristics of the filter 1a in which the pitch Pi2 ′ of the electrode fingers 14 with x = 3 and the number of pitches 2) are larger than the pitch Pi2 of the other electrode fingers 14 are shown (Example 2-1). Specifically, each pitch is Pi2 = 0.994Pi1, Pi2 '= 1.00Pi1.

  According to the frequency characteristics shown in FIG. 9, the filter 1 of Example 2-1 has substantially the same passband as the filter of the reference example, but the guard band width on the low frequency side is narrowed, and the attenuation band The rise of the attenuation characteristic from to the passband became steep.

  Next, the value of Pi2 is fixed to Pi2 = 0.994Pi1, and the frequency characteristics of the filter 1a when Pi2 'is smaller than Pi2 = 0.993Pi1, 0.988Pi1 and Pi2 are shown in FIGS. 10 and 11, respectively. (Examples 2-2 and 2-3). In these filters 1a, the same tendency as in Example 2-1 was observed, in which the passbands of the reference example and the passband were almost the same, but the guard band width on the low frequency side was narrower than that of the reference example. .

  FIG. 13 shows the result of plotting the guard bandwidth [MHz] against the value of Pi2 / Pi1, and the black diamond plots show the results of Examples 2-1 to 2-3. A white circle plot shows the result of the reference example, and a white diamond plot shows the result of Example 1-2 (corresponding to Pi2 ′ = Pi2).

According to the result shown in FIG. 13, it can be confirmed that the guard band width can be narrowed as the value of Pi2 ′ decreases. At this time, the trend line connecting Examples 2-1 to 2-3 is shifted in the direction in which the guard band width is narrower than the plot (white diamond) of Example 1-2. Accordingly, by changing the pitch of the electrode fingers 14 at positions close to the reflectors 12a and 12b, the electrode finger 14 on the central electrode 11a side can be obtained in order to obtain an effect that the rise of the attenuation characteristic from the attenuation band to the pass band is sharp. It is considered that the influence of the pitch Pi2 ′ of the electrode fingers on the reflectors 12a and 12b side is greater than that of the pitch.
FIG. 13 also shows that the effect of narrowing the guard bandwidth by changing the pitch Pi2 ′ of the electrode fingers 14 on the reflectors 12a and 12b side is gradually decreasing with the decrease of Pi2 ′.

  Next, in Comparative Example 2-1 (FIG. 12) in which the value of Pi2 ′ was further reduced and Pi2 ′ = 0.880 Pi1 (fixed at Pi2 = 0.994 Pi1), the guard band width was further reduced, but the low The attenuation in the frequency domain increased and the pass characteristics deteriorated. Even if the number of electrode fingers 14 for changing Pi2 ′ is three, if the value of Pi2 ′ becomes close to 11.5% smaller than the value of Pi2, the state of the resonance mode changes and the frequency characteristics deteriorate. It is presumed that this will be done.

  Thus, the preferred range of Pi2 ′ is 0.985 Pi1 because the effect of narrowing the guard band width decreases with the decrease of Pi2 ′, and the pass characteristic of the filter 1a deteriorates when Pi2 ′ becomes too small. ≦ Pi2 ′ ≦ 1.000Pi1, more preferably 0.990Pi1 ≦ Pi2 ′ ≦ 1.000Pi1. Within this range, it has been confirmed that it is possible to make the rise of the attenuation characteristic from the attenuation band to the pass band steep while maintaining the pass characteristic of the filter 1a having no practical problem.

  Further, the effect of steepening the rise of the attenuation characteristic by changing the pitch Pi2 ′ of the electrode fingers 14 on the reflectors 12a and 12b side is to have a pitch Pi2 ′ of at least one (electrode index x = 2) as the number of pitches. Change it. In addition, the upper limit of the number of pitches for changing Pi2 'includes a case where the pitch Pi2' of the other electrode fingers 14 is Pi2 ≠ Pi2 'while leaving at least one pitch Pi2 on the central electrode 11a side. As a more preferable example, the pitch Pi2 ′ of the electrode fingers 14 on the reflectors 12a, 12b side ((x / X) ≧ 0.5) from the center of the side electrodes 11b, 11c is set to Pi2 ≠ Pi2 ′. be able to.

  The filter 1a according to the second embodiment has the following effects. From each reflector side 12a, 12b in the side electrodes 11b, 11c adjacent to the reflectors 12a, 12b to x (x: a positive number and a value smaller than the number X of the electrode fingers 14 of the side electrodes 11b, 11c) When the pitch of the electrode fingers 14 is Pi2 ′ instead of Pi2, the pitch Pi2 ′ satisfies the relationship 0.985Pi1 ≦ Pi2 ′ ≦ 1.000Pi1 (where Pi2 ′ ≠ Pi2), The rise of the attenuation characteristic on the low frequency side in the vicinity can be made steeper.

  FIG. 3 shows an example in which the filter 1 (1a) according to the first and second embodiments is cascade-connected to form a cascade-connected filter 1b. On the piezoelectric substrate 10, the two filters 1 (1a) described in the first and second embodiments are arranged at the front and rear, and these filters 1 (1a) are arranged on the side electrodes 11b and 11c. Opposing bus bars 13b, 13a are connected to each other. Here, the side electrodes 11 b and 11 c are short-circuited by the connection line 18. When the input port 15 is connected to the front filter 1 (1a), since the side electrodes 11b and 11c are connected as described above, the rear filter 1 (1a) is connected. The output port 16 is provided in the central electrode 11a.

Next, simulation conditions and results thereof will be briefly described for the simulation described in each of the above embodiments.
<Example 1-1> (FIG. 4)
(Simulation conditions)
Piezoelectric substrate 10: 36 ° Y-cut X propagation LiTaO ₃
Center electrode 11a pitch Pi1: 2.099 [μm]
Side electrode 11b, 11c pitch Pi2: 2.091 [μm] (0.996 Pi1)
Reflector 12 pitch Pr1: 2.138 [μm]
Number of pairs of electrode fingers 14 of the central electrode 11a (logarithm): 22 pairs (electrode index: 44)
Number of pairs of electrode fingers 14 of the side electrodes 11b and 11c: 13 pairs each (electrode index: 26 pieces each)
Cross width D of electrode fingers 14: 47 [μm]
Number of grating electrodes 17 of reflector 12: 100 each
Film thickness of IDT electrode 11: 190 [nm]

<Reference Examples> (FIGS. 4-7, 9-12)
(Simulation conditions)
The conditions are the same as in Example 1-1 except that the pitch Pi2 of the side electrodes 11b and 11c is Pi2 = Pi1 (2.099 [μm]).
(simulation result)
By making the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c smaller than the pitch Pi1 of the electrode fingers 14 of the central electrode 11a, the steepness of rising on the low frequency side in the pass frequency band is improved as compared with the reference example. . In addition, the passband width and attenuation were similar to those in the reference example.

<Example 1-2> (FIG. 5)
(Simulation conditions)
Except that the pitch Pi2 of the side electrodes 11b and 11c is Pi2 = 0.994 Pi1 (2.086 [μm]), the conditions are the same as those in Example 1-1.
(simulation result)
By making the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c smaller than the pitch Pi1 of the electrode fingers 14 of the central electrode 11a, the steepness of rising on the low frequency side in the pass frequency band is improved as compared with the reference example. . In addition, the passband width and attenuation were similar to those in the reference example.

<Example 1-3> (FIG. 6)
(Simulation conditions)
Except that the pitch Pi2 of the side electrodes 11b and 11c is Pi2 = 0.922 Pi1 (2.083 [μm]), the conditions are the same as those in Example 1-1.
(simulation result)
By making the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c smaller than the pitch Pi1 of the electrode fingers 14 of the central electrode 11a, the steepness of rising on the low frequency side in the pass frequency band is improved as compared with the reference example. . In addition, the passband width and attenuation were similar to those in the reference example.

<Comparative Example 1-1> (FIG. 7)
(Simulation conditions)
Except that the pitch Pi2 between the side electrodes 11b and 11c is Pi2 = 0.980 Pi1 (2.057 [μm]), the conditions are the same as those in Example 1-1.
(simulation result)
Also by making the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c significantly smaller than the pitch Pi1 of the electrode fingers 14 of the center electrode 11a, the steepness of the rise on the low frequency side in the pass frequency band is improved over the reference example. It was done. However, the shoulder on the low frequency side of the pass band is rounded, and the pass characteristics are deteriorated.

<Example 2-1> (FIG. 9)
(Simulation conditions)
Except that the pitch Pi2 ′ of the three electrode fingers 14 (logarithm 1.5) on the reflectors 12a, 12b side of each side electrode 11b, 11c is 1.00Pi1 (2.099 [μm]). The conditions are the same as in Example 1-2.
(simulation result)
While the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c is made smaller than the pitch Pi1 of the electrode fingers 14 of the center electrode 11a, the pitch Pi2 'of the three electrode fingers 14 adjacent to the reflectors 12a and 12b is set to the center. By making it larger than the pitch Pi2 on the part side (Pi2 <Pi2 ′ = Pi1), the steepness of the rise on the low frequency side in the pass frequency band was improved as compared with the reference example. In addition, the passband width and attenuation were similar to those in the reference example.

<Example 2-2> (FIG. 10)
(Simulation conditions)
Except that the pitch Pi2 ′ of the three electrode fingers 14 (logarithm 1.5) on the reflectors 12a, 12b side of each side electrode 11b, 11c is 0.993 Pi1 (2.084 [μm]). The conditions are the same as in Example 1-2.
(simulation result)
While the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c is made smaller than the pitch Pi1 of the electrode fingers 14 of the center electrode 11a, the pitch Pi2 'of the three electrode fingers 14 adjacent to the reflectors 12a and 12b is set to the center. Also by making the pitch Pi2 smaller than the part side pitch Pi2 (Pi2> Pi2 ′), the steepness of rising on the low frequency side in the pass frequency band is improved as compared with the reference example. In addition, the passband width and attenuation were similar to those in the reference example.

<Example 2-3> (FIG. 11)
(Simulation conditions)
Except that the pitch Pi2 ′ of the three electrode fingers 14 (logarithm 1.5) on the reflectors 12a, 12b side of each side electrode 11b, 11c is 0.988 Pi1 (2.074 [μm]). The conditions are the same as in Example 1-2.
(simulation result)
While the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c is made smaller than the pitch Pi1 of the electrode fingers 14 of the center electrode 11a, the pitch Pi2 'of the three electrode fingers 14 adjacent to the reflectors 12a and 12b is set to the center. By making it smaller than the pitch Pi2 on the part side (Pi2> Pi2 ′), the steepness of the rise on the low frequency side in the pass frequency band is improved as compared with the reference example. In addition, the passband width and attenuation were similar to those in the reference example.

<Comparative Example 2-1> (FIG. 12)
(Simulation conditions)
Except that the pitch Pi2 ′ of the three electrode fingers 14 (logarithm 1.5) on the side of the reflectors 12a and 12b of the side electrodes 11b and 11c is 0.880 Pi1 (1.847 [μm]). The conditions are the same as in Example 1-2.
(simulation result)
While the pitch Pi2 of the electrode fingers 14 of the side electrodes 11b and 11c is made smaller than the pitch Pi1 of the electrode fingers 14 of the center electrode 11a, the pitch Pi2 'of the three electrode fingers 14 adjacent to the reflectors 12a and 12b is set to the center. By making the pitch Pi2 significantly smaller than the part side pitch Pi2 (Pi2> Pi2 ′), the steepness of rising on the low frequency side in the pass frequency band is improved as compared with the reference example. However, the shoulder on the low frequency side of the pass band is rounded, and the pass characteristics are deteriorated.

1, 1a, 1b
Filter 10 Piezoelectric substrate 11 IDT electrode 11a Center electrode 11b, 11c
Side electrodes 12, 12a, 12b
Reflector

Claims (3)

A longitudinally coupled surface acoustic wave formed by arranging IDT electrodes including a central electrode and side electrodes provided on both sides of the central electrode on a piezoelectric substrate and arranging reflectors on both sides of these IDT electrodes. In the filter,
When the pitch of the electrode fingers of the central electrode is Pi1, and the pitch of the electrode fingers of the side electrode is Pi2,
A longitudinally coupled surface acoustic wave filter characterized in that the pitches Pi1 and Pi2 satisfy a relationship of 0.985Pi1 ≦ Pi2 ≦ 0.999Pi1. Pi2 is replaced with Pi2 by changing the pitch of electrode fingers up to x (x: a positive number, which is smaller than the number X of electrode fingers of the side electrode) from the reflector side in the side electrode adjacent to the reflector. 'Then
2. The longitudinally coupled surface acoustic wave filter according to claim 1, wherein the pitch Pi2 ′ satisfies a relationship of 0.985Pi1 ≦ Pi2 ′ ≦ 1.000Pi1 (where Pi2 ′ ≠ Pi2).   A cascade-connected filter comprising two cascade-coupled surface acoustic wave filters according to claim 1 connected in cascade.

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