JP2003115746A - Elastic surface wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a means for widening the band width of a primary-tertiary longitudinally-coupled double mode elastic surface wave filter and also reducing the insertion loss. SOLUTION: This longitudinally-coupled multiple module elastic wave filter is constituted by arranging a plurality of ID electrodes in the direction of propagation of surface waves on the main surface of a piezoelectric substrate. In a section where adjacent IDT electrodes are adjoining each other, electrode fingers are disposed at narrower pitches than those of the electrode fingers in a section excluding the section where they are adjoining each other.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter (hereinafter referred to as a SAW filter), and particularly to the first and third order.
First-third vertical coupling dual mode S using the next vertical mode
The present invention relates to a SAW filter having improved bandwidth and insertion loss of an AW filter (hereinafter referred to as a dual mode SAW filter).

[0002]

2. Description of the Related Art In recent years, SAW filters have been widely used in the field of communications and have been widely used especially in mobile phones and the like because they have excellent characteristics such as high performance, small size and mass productivity. Among them, one of the filters used in the RF section of mobile phones
One is a broadband dual-mode SAW filter that is configured by using three IDT electrodes arranged close to each other to strongly excite the first and third longitudinal modes and to utilize them.

FIG. 14 is a plan view showing the basic structure of a conventional dual-mode SAW filter. IDT electrodes 12, 1 are formed on the main surface of piezoelectric substrate 11 along the propagation direction of surface waves.
3 and 14 are arranged close to each other, and these IDT electrodes 1
Grating reflectors on both sides of 2, 13 and 14 (hereinafter,
The reflectors 15a and 15b are provided respectively. The IDT electrodes 12, 13 and 14 are formed of a pair of comb-shaped electrodes each having a plurality of electrode fingers which are inserted into each other. One of the comb-shaped electrodes of the central IDT electrode 12 is connected to the input terminal IN by wire bonding or the like. Connection and ground the other comb electrode. Further, the comb-shaped electrodes of the outer IDT electrodes 13 and 14 are connected to each other, and one of the connected electrodes is connected to the output terminal OUT,
The other side is grounded to form a dual mode SAW filter.

As shown in FIG. 15, IDT electrodes 12 and 1
The electrode finger pitches of 3 and 14 are all Lt (where the electrode period is λ, λ = 2Lt), and the reflector 15
The electrode finger pitch of a and 15b is Lr. And ID
The center-to-center spacing Ltr between adjacent electrode fingers of the T electrodes 13 and 14 and the reflectors 15a and 15b is generally set to λ / 2 in consideration of the continuity of surface acoustic waves. In addition, the pitch Lr of the reflectors 15a and 15b is such that the stop band Bs formed by them is the IDT electrodes 12 and 13,
It is generally set to include the pass band B of the dual mode SAW filter formed by the first and third modes excited by 14. With such a configuration, the Q value of the first- and third-order modes can be increased, and the filter can flatten the pass band and reduce the insertion loss.

In recent years, in order to increase the number of channels accommodated in response to the rapid spread of mobile phones, it has become necessary to increase the bandwidth of RF filters. Various improvements have been made to meet this requirement. One of them is, as shown in FIG. 15, a central IDT electrode 12 and outer IDT electrodes 1 on both sides.
Center-to-center spacing Ltt between adjacent electrode fingers 3 and 14
Is smaller than λ / 2, the dual mode SAW
It has been experimentally found that the bandwidth of the filter is increased. FIG. 16 shows the electrode finger width (line width) Ln and the space width S.
The ratio to t is 50:50 (Ln = St = λ / 4), and L
FIG. 6 is a plan view of an electrode pattern when tt is set to λ / 4, and is an electrode pattern diagram in which adjacent electrodes of a central IDT electrode and both outer IDT electrodes are in contact with each other. Since two electrode fingers are in contact with each other at the boundary between the adjacent IDT electrodes to form one electrode finger, the line width is as wide as λ / 2. Currently, in order to maximize the bandwidth of the dual mode SAW filter, the center-to-center spacing Ltt is set.
Is generally set between 0.2λ and 0.3λ, and FIG. 17 shows the dual mode S which is currently commonly used.
It is an electrode pattern of an AW filter. The wide electrode fingers are arranged at both ends of the central IDT electrode 12 'as shown in FIG.
May be arranged on the innermost side, and it is desirable that the entire electrode pattern is symmetrical with respect to the center of the central IDT electrode 12 '.

[0006]

However, as shown in FIG.
In the conventional dual-mode SAW filter as shown in, since the wide electrode fingers Q1 and Q2 at both ends of the central IDT electrode 12 'are set wider than other electrode finger widths (line widths), Since the electrode period is different in that portion, the continuity of the surface wave to be excited is impaired. Therefore, there is a problem that the insertion loss of the dual mode SAW filter cannot be reduced. The present invention has been made to solve the above problems, and an object of the present invention is to provide a dual mode SAW filter having a wide bandwidth and a reduced insertion loss.

[0007]

To achieve the above object, a surface acoustic wave filter according to a first aspect of the present invention has a plurality of surface acoustic wave filters arranged on a main surface of a piezoelectric substrate along a propagation direction of surface waves. A longitudinally coupled multi-mode surface acoustic wave filter in which IDT electrodes are arranged close to each other and grating reflectors are arranged on both outer sides of these IDT electrodes, in the vicinity (adjacent section) of adjacent portions of adjacent IDT electrodes. Is a surface acoustic wave filter characterized in that electrode fingers are arranged at a pitch narrower than an electrode finger pitch (standard pitch: Lt) in a portion other than the adjacent section of the IDT electrode. According to a second aspect of the present invention, there is provided a primary-third-order longitudinally coupled dual-mode surface acoustic wave filter configured by arranging three IDT electrodes close to each other on a main surface of a piezoelectric substrate. It is the surface acoustic wave filter described. The invention according to claim 3 is characterized in that the adjacent section is widely set on the side of the IDT electrode arranged outside the IDT electrode arranged at the center, in the surface acoustic wave filter according to claim 2. is there. In the invention according to claim 4, the number of electrode fingers in the adjacent section is 4, and the ratio Lt4 / Lt between the electrode finger pitch Lt4 and the electrode finger pitch Lt other than the adjacent section is 0.8000 ≦ Lt4 / Lt ≦ 0. 9. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter is 0.911. In the invention according to claim 5, the number of electrode fingers in the adjacent section is eight, and the ratio Lt8 / Lt between the electrode finger pitch Lt8 and the electrode finger pitch Lt other than the adjacent section is 0.894.
4. The surface acoustic wave filter according to claim 1, wherein 1 ≦ Lt8 / Lt ≦ 0.9530. The invention according to claim 6 is characterized in that the width of the electrode fingers in the adjacent section is equal to the space width between the electrode fingers.
The surface acoustic wave filter according to any one of claims 1 to 5. The invention according to claim 7 is characterized in that the electrode finger pitch in the central portion of the adjacent section is the narrowest, and the electrode finger pitch is set so as to approach the electrode finger pitches other than in the adjacent section toward the outside. The surface acoustic wave filter according to claim 1.
The invention according to claim 8 is the surface acoustic wave filter according to any one of claims 1 to 5, characterized in that the widths of the electrode fingers in the adjacent section are equal to the widths of the electrode fingers in the sections other than the adjacent section.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below in detail based on the embodiments shown in the drawings. FIG. 1 is a plan view showing an electrode pattern configuration of a dual mode SAW filter according to the present invention, in which three IDT electrodes 1, 2 and 3 are arranged close to each other along a SAW propagation direction on a piezoelectric substrate (not shown). The reflectors 4a and 4b are arranged on both sides of the arrangement. The IDT electrodes 1, 2, and 3 are formed of a pair of comb-shaped electrodes each having a plurality of electrode fingers which are interleaved with each other.
One of the comb-shaped electrodes of the center IDT electrode 1 is an input terminal IN
And ground the other comb electrode. Further, the comb-shaped electrodes of the IDT electrodes 2 and 3 on the both outer sides are connected to each other, and one of the connected electrodes is connected to the output terminal OUT and the other is grounded to form a dual mode SAW filter. The dual mode SAW filter of the present invention has a central I
It is configured symmetrically with respect to the center of the DT electrode 1.

The feature of the present invention is that the central IDT electrode 1 and the IDT electrodes 2 and 3 on both outer sides are adjacent to each other (hereinafter referred to as adjacent sections), that is, in the section indicated by Li in FIG. By making the electrode finger pitch narrower than the electrode finger pitch outside the section, the bandwidth of the dual mode SAW filter becomes wider and the insertion loss is reduced.

The main part of the present invention will be described with reference to the drawings.
FIG. 2A is an enlarged plan view showing the adjacent portion between the central IDT electrode 12 ′ and the outer IDT electrode 14 ′ of the electrode pattern of the conventional dual mode SAW filter shown in FIG. In FIG. 6B, the white electrode fingers in the cross-sectional view are, for example, the (+) side, and the patterned electrode fingers are the (−) side.
As shown in FIG. 2A, 3 from f1 to f3 are symmetrical with respect to the center of the wide electrode finger f2 of the central IDT electrode 12 ′.
Electrode fingers, select the electrode fingers fa on either side of the electrode finger row.
This section is defined as L4 from the right end position to the left end position of the electrode finger fb of the IDT electrode 14 '. Here, adjacent sections are selected symmetrically from the center of the wide electrode finger f2 in which the continuity of the surface acoustic wave is broken. Further, the electrode finger pitch other than the adjacent section is Lt. Instead of the three electrode fingers f1, f2, f3 in the adjacent section L4, four electrode fingers F1, F2, F3, F4 are newly added as shown in FIG. 2C.
Are formed with a uniform electrode finger pitch Lt4. As a matter of course, Lt4 <Lt. FIG. 2D shows the cross-sectional view of FIG. 2C and its polarity. Thus, the adjacent section L4
Is replaced by four electrode fingers with an even electrode finger pitch,
The continuity of surface acoustic waves is improved.

In the above embodiment, instead of the three electrode fingers in the adjacent section of the IDT electrodes shown in FIG. 17, four electrode fingers having a newly formed line-space ratio of 1: 1 are replaced. It is an example. This is as shown in FIG.
As shown in (a), the basic (FIG. 15) continuous electrode finger pitches Lt and Ltt are configured to be equal, and the surface wave can be continuously propagated. As shown in the figure, by arranging the adjacent IDT electrodes close to each other, the pass band can be widened, but the continuity of the surface wave is impaired. Therefore,
When the offset amount Lof is the amount by which the electrode finger pitch is narrowed (Lt-Ltt), the offset amount Lof is not concentrated in one place and one wide electrode finger is arranged, but the narrow pitch is evenly distributed in the adjacent section L4. The present invention is characterized in that the continuity of the surface wave is improved by disposing the electrode fingers evenly.

The filter characteristic shown by the broken line in FIG. 3 is the passband characteristic when an RF filter for PDC (low frequency RF filter of dual filter for PDC) is prototyped using the conventional electrode pattern (FIG. 17). Then, 39 ° Y-X LiTaO ₃ was used for the piezoelectric substrate, and the center frequency was set to 8
77.5MHz, bandwidth 15MHz, electrode finger to space ratio is 50:50, electrode finger center spacing Ltt is λ / 4, center IDT electrode 12 'is 28.5 pairs, IDTs on both outer sides
The number of pairs of electrodes 13 'and 14' is 19.5 pairs, and the crossover length W is 3
This is the case where the number of 3λ (λ is the electrode period) and the number of reflectors 15′a and 15′b are 90, respectively. On the other hand, the filter characteristic indicated by the solid line is the four electrodes formed by the three electrode fingers existing in the adjacent section L4 at the uniform electrode finger pitch Lt4 as shown in FIGS. 2C and 2D. It is a passband characteristic when it is replaced with a finger and a space. The pitch ratio α = Lt4 / Lt of the newly formed four electrode fingers is α = 0.8773. Further, when the present invention is applied, not only the range of 0.2λ ≦ Ltt ≦ 0.3λ which has been generally used as the electrode finger spacing Ltt of the reference dual-mode SAW filter but also 0.1λ ≦ Ltt. It was found that the pass band can be widened within the range of ≤0.3λ. Therefore, in this example, the range that can be set as the pitch ratio α of the electrode fingers is 0.8000 ≦ α ≦ 0.9111. As is clear from FIG. 3, the dual-mode SAW filter according to the first embodiment has been found to have a wider bandwidth and a lower insertion loss than the conventional filter.

The filter characteristic shown by the broken line in FIG. 4 is a pass band characteristic when an RF filter for AMPS was prototyped using the conventional electrode pattern (FIG. 17), and 39 ° Y-X LiTaO was formed on the piezoelectric substrate. ₃ , the center frequency is set to 881.5 MHz and the bandwidth is set to 25 MHz, the line width to space ratio is 50:50, the electrode finger center-to-center spacing Ltt is 0.3λ, and the logarithm of the center IDT electrode 12 ′ is 19.5. Pair, the number of pairs of IDT electrodes 13 'and 14' on both sides is 13.5 pairs, the crossover length W is 45λ (λ is the electrode period), and reflectors 15'a and 15'b
2 is a filter characteristic in the case where two dual-mode SAW filters each having 120 are connected in cascade. On the other hand, the passband characteristic shown by the solid line is shown in FIG.
As shown in (d), the filter characteristics are obtained when two-stage cascade connection of a dual-mode SAW filter in which the adjacent section L4 is replaced with four electrode fingers having a uniform electrode finger pitch Lt4 and a space. Pitch ratio of newly formed four electrode fingers α = L
t4 / Lt is α = 0.89. It can be confirmed from FIG. 4 that the effect of the present invention is to increase the bandwidth and reduce the insertion loss.

FIG. 5 is an example showing a second embodiment according to the present invention. FIG. 5 (a) is a central IDT electrode 12 'of the conventional dual mode SAW filter shown in FIG. The plan view of the electrode pattern which expanded and showed the principal part with IDT electrode 14 'of the outer side, the same figure (b) is the sectional drawing. Figure 5
As shown in (a), with respect to the center of the wide electrode finger f4 of the central IDT electrode 12 ', 7 from f1 to f7 are symmetrically arranged.
Electrode fingers, select the electrode fingers fa on either side of the electrode finger row.
The section from the right end position to the left end position of the electrode finger fb of the IDT electrode 14 ′ is L8. The adjacent section L8 is selected symmetrically with respect to the center of the wide electrode finger f4. Instead of the seven electrode fingers in the adjacent section L8, eight electrode fingers F1 and F having a uniform electrode finger pitch Lt8 are used.
2, F3, F4, F5, F6, F7, F8 are formed.
FIG. 5D shows the cross-sectional view of FIG. 5C and its polarity. In this way, by replacing the electrodes in the adjacent section L8 including the wide area with electrode finger rows of equal pitch, the continuity of the surface acoustic wave in the adjacent section L8 is improved and the insertion loss of the filter is improved. There is expected.

The filter characteristic shown by the broken line in FIG. 6 is the characteristic of the filter when the RF filter for PDC was prototyped using the conventional electrode pattern (FIG. 17), and the constants shown in FIG. 3 were used. ing. As shown in FIGS. 5 (c) and 5 (d), the filter characteristic indicated by the solid line in the figure is such that one electrode finger is added to the adjacent section L8 where the IDT electrodes are adjacent to each other and the uniform electrode finger pitch Lt8 is 8 The passband characteristics when replaced with a row of electrode fingers are shown. The electrode finger pitch ratio α = Lt8 / Lt is 0.9235. As described above, when the inter-electrode finger spacing Ltt of the reference dual-mode SAW filter is 0.1λ ≦ Ltt ≦ 0.3λ, the settable range of the electrode finger pitch ratio α is 0.8941 ≦ Lt8 / L.
It becomes t <= 0.9530. As is apparent from FIG. 6, compared with the conventional dual mode SAW filter (broken line), the dual mode SAW filter (solid line) of the second embodiment has a wider filter characteristic pass band and reduced insertion loss. I understand. Also, comparing FIGS. 3 and 6, it was found that the filter replaced with eight electrode fingers had a wider pass band and smaller insertion loss than the filter with four electrodes.
It is speculated that this is because the continuity of the surface wave is further improved.

FIG. 7 is a sectional view showing the conventional example (FIG. 17) in the upper stage and the form of the third embodiment in the lower stage. The same number of electrode fingers 4 on the left and right in the drawing centering on the wide electrode finger f5
Select a total of nine electrodes, and select the electrode fingers f on both sides of the electrode finger row.
The section from the left end position of 1 to the right end position of the electrode finger f9 is L10. Electrode finger f1 in the adjacent section L10
Instead of the nine electrode fingers F9 to f9, ten electrode fingers F1 to F10 are newly formed. As shown in the lower part of FIG. 7, the electrode finger pitch is symmetrically selected from the left and right of the adjacent section L10 as pitches p1, p2, and p3, and p1>p2> p.
The electrode finger pitch of 3 and the central portion is the narrowest, and the electrode finger pitch is set so as to approach the electrode finger pitches other than the adjacent section as going outward. Specifically, it is set to be V-shaped, U-shaped, or inverted trapezoidal (p1> p2 = p3). It is presumed that the continuity of the surface acoustic wave is not impaired by setting the change in the pitch pi of the adjacent section L10 as small as possible with respect to the electrode pitch Lt other than the adjacent section L10.

The filter characteristic shown by the broken line in FIG. 8 is the RF filter for PDC (high frequency RF filter of PDC dual filter) using the electrode pattern of the conventional example (FIG. 17).
The pass band characteristics in the case where the prototype was manufactured, 39 ° Y-X LiTaO ₃ was used for the piezoelectric substrate, and the center frequency was 1489.
0MHz, bandwidth 24MHz, line to space ratio 60:40, electrode finger center spacing Ltt 0.3λ,
22.5 pairs of IDT electrodes 12 'in the center, IDTs on both sides
The number of pairs of electrodes 13 'and 14' is 15.5 pairs and the crossover length W is 30λ.
(Λ is the electrode period) and filter characteristics when the number of reflectors 15′a and 15′b is 90, respectively. On the other hand, as shown in the lower part of FIG. 7, the pass band characteristic shown by the solid line is formed by forming nine electrode fingers in the adjacent section L10 at the electrode finger pitch pi (i = 1, 2, 3). It is a pass band characteristic when replacing with 10 electrode fingers. The electrode finger pitches p1, p2, p3 are expressed as pi = αi · Lt by using the electrode finger pitch ratio αi with respect to the original electrode finger pitch Lt. In the example of FIG. 8, the pitch ratio αi is set to 0.9667, 0.9333, and 0.9333 in an inverted trapezoidal shape, respectively. In addition to this, the change in pitch is V-shaped (linear expression, | a (x-b) |),
A U-shape (quadratic expression, a (x-b) ² ) or the like is considered. As is clear from FIG. 8, it was found that the dual-mode SAW filter of the present invention has a wider bandwidth and a smaller insertion loss than the conventional filter.

FIG. 9 is a diagram showing the form of the fourth embodiment. FIG. 9 (a) is a reference example of the prior art (FIG. 17), and FIGS. 9 (b) and 9 (c) are the fourth embodiment. It is sectional drawing and the top view which show the structure of an example electrode pattern. In the above-described embodiments, the adjacent section is determined symmetrically around the wide electrode finger as the adjacent section in which the number of electrode fingers is increased by one, but in this example, the center of the adjacent section is selected from the wide center to the outer side. This is an example in which the IDT electrode is shifted toward the IDT electrode to include many electrode fingers of the outer IDT electrode. As shown in FIG. 9A, a section from the right end position of the electrode finger fa of the central IDT electrode 12 ′ to the left end position of the electrode finger fb of the outer IDT electrode 14 ′ is L4. In this adjacent section L4, there are wide electrode fingers f1 and electrode fingers f2 and f3 having an electrode finger pitch Lt,
Instead of these three electrode fingers f1, f2, f3, four electrode fingers F1, F2, F3 having an even electrode finger pitch Lt4,
Form F4. As a matter of course, Lt4 <Lt, and the electrode finger pitch ratio is α4 = Lt4 / Lt <1.

The filter characteristic shown by the broken line in FIG. 10 is the pass band characteristic of the dual mode SAW filter constituted by four equal electrode finger pitches according to the present invention shown in FIGS. 2 (c) and 2 (d). , The constants shown in FIG. 4 were used. The filter characteristic shown by the solid line in the figure is that of the dual mode SAW filter using the fourth embodiment, and
As shown in (c), the pass band characteristic is obtained when three electrode fingers existing in the adjacent section L4 are removed, and one electrode finger is added to form four electrode fingers at an equal pitch. Ripple occurs in the passband due to the mismatching of the termination impedance, but the passband can be made flat by setting the number of electrode pairs appropriately.
Adjacent sections where the electrode finger pitch is narrowed as in this example do not necessarily have to be symmetrically selected about the wide electrode finger, and may be biased toward the outer IDT electrode.

FIG. 11 (a) shows a conventional reference (FIG. 17).
It is sectional drawing which shows the electrode pattern of the dual mode SAW filter of (b) is sectional drawing of the electrode pattern which shows the form of the 5th Example which concerns on this invention. L3 is a section from the left end position of the adjacent electrode fingers f1 centering on the wide electrode finger of FIG. 11A to the right end position of the electrode finger f3.
The three electrode fingers f1, f2, f3 in this adjacent section L3
Instead of four electrode fingers F1, F2, F3, F4
To form. The line width of either electrode finger is the original line width L.
The electrode finger f1 is left as F1 and the electrode finger f3 is left as F4. Then, instead of the electrode finger f2, two electrode fingers F2 and F3 having a line width Ln are arranged at equal pitches. Therefore, the space S2 between the electrode fingers becomes narrower than the reference space width St. That is, S2 can be obtained from the following equation. 3S2 = 2St + (Ltt-λ / 4) ... (1) In other words, the conventional offsets that were concentrated in one place were dispersed in three places to improve the continuity of surface waves. Is intended.

The filter characteristic shown by the broken line in FIG. 12 is a pass band characteristic when an RF filter for AMPS was prototyped using a conventional (FIG. 17) electrode pattern, and 39 ° Y-X LiTaO was formed on the piezoelectric substrate. ₃ , the center frequency is set to 881.5 MHz, the bandwidth is set to 25 MHz, the electrode finger / space ratio is 50:50, and the electrode finger center-to-center spacing Ltt is 0.3.
λ, the number of pairs of IDT electrodes 12 'in the center is 19.5 pairs, I on both sides
The number of pairs of DT electrodes 13 'and 14' is 13.5 pairs, and the crossover length W is 45.
Two double-mode SAW filters each having λ (where λ is the electrode period) and 120 reflectors 15'a and 15'b are provided.
It is the pass band characteristic of the filter connected in cascade. On the other hand, the filter characteristic shown by the solid line is the reference electrode finger spacing L.
As shown in FIG. 11 (b), the passband characteristics of a filter in which two double-mode SAW filters in which the electrode finger pitch L't4 of the adjacent section L3 is 0.425λ are cascaded are set after setting tt to 0.275λ. is there. Space S2 is 0.175
becomes λ. It is clear from FIG. 12 that the effect of the present invention is to increase the bandwidth and reduce the insertion loss. Further, FIG. 13 is a diagram showing the filter characteristics in the attenuation range, and the broken line is the conventional (FIG. 17)
The characteristics of the filter, and the solid line shows the characteristics of the filter of the present invention. It can be seen that the characteristics of the filter of the present invention are slightly improved on the low frequency side.

[0022]

Since the present invention is configured as described above, the dual mode SAW filter according to the present invention exhibits the excellent effects of widening the pass band width and reducing the insertion loss.

[Brief description of drawings]

FIG. 1 is a plan view showing a configuration of an electrode pattern of a dual mode SAW filter according to the present invention.

2 (a) and 2 (b) are enlarged plan views and cross-sectional views of a main part of a conventional dual-mode SAW filter that serves as a reference;
(C) and (d) are an enlarged plan view and a sectional view of an essential part of the first embodiment according to the present invention.

FIG. 3 is a pass band characteristic of an RF filter for PDC manufactured by using the electrode pattern according to the first embodiment of the present invention.

FIG. 4 is a passband characteristic of an AMPS RF filter prototyped using the electrode pattern of the first embodiment according to the present invention.

5 (a) and 5 (b) are enlarged plan views and a cross-sectional view of an essential part of a conventional dual-mode SAW filter serving as a reference,
(C) and (d) are an enlarged plan view and a sectional view of an essential part of a second embodiment according to the present invention.

FIG. 6 is a pass band characteristic of an RF filter for PDC manufactured by using the electrode pattern of the second embodiment.

FIG. 7 is an enlarged cross-sectional view of an essential part of a conventional dual-mode SAW filter serving as a reference, and a lower stage is an enlarged cross-sectional view of an essential part of a third embodiment according to the present invention.

FIG. 8 is a pass band characteristic of an RF filter for PDC manufactured by using the electrode pattern of the third embodiment.

9A is an enlarged cross-sectional view of an essential part of a conventional dual-mode SAW filter as a reference, and FIGS. 9B and 9C are enlarged sectional views of an essential part of a fourth embodiment according to the present invention. FIG.

10 is a passband characteristic of a filter prototyped using the electrode pattern of the fourth embodiment, and a broken line is a passband characteristic of a filter prototyped using the electrode pattern of the first embodiment shown in FIG. is there.

FIG. 11A is a conventional dual-mode SAW serving as a reference.
FIG. 7B is an enlarged cross-sectional view of the main part of the filter, and FIG. 9B is an enlarged cross-sectional view of the main part of the fifth embodiment according to the present invention.

FIG. 12 is a passband characteristic of a filter prototyped using the electrode pattern of the fifth embodiment of the present invention, and a broken line is a passband characteristic when a conventional electrode pattern is used.

FIG. 13 shows the attenuation band characteristic of a filter prototyped using the electrode pattern of the fifth embodiment of the present invention, and the broken line shows the attenuation band characteristic when the conventional electrode pattern is used.

FIG. 14 is a plan view of a basic electrode pattern showing a configuration of a conventional dual mode SAW filter.

FIG. 15 is a diagram showing dimensions of respective parts of a basic electrode pattern showing a configuration of a conventional dual mode SAW filter.

FIG. 16 is an electrode pattern diagram showing the configuration of a conventional dual-mode SAW filter configured to expand the pass band width.

FIG. 17 is an electrode pattern diagram showing a configuration of a conventional dual-mode SAW filter adapted to put FIG. 16 into practical use.

FIG. 18A is a cross-sectional view showing a basic electrode pattern of a conventional dual-mode SAW filter, a cross-sectional view of the electrode pattern when the bandwidth is expanded, and dimensions of each part.

[Explanation of symbols]

1, 2, 3 ... IDT electrodes 4a, 4b ... Reflectors Li, L3, L4, L8, L10 ... Adjoining adjacent sections fa, fb, f1, f2f3, f4, f5, f6 of two IDT electrodes, f
7, f8, f9 ... Electrode fingers F1, F2, F3, F4, F5, F6, F7, F8, F
9, F10 ... Electrode fingers p1, p2, p3 ... Electrode pitch S2 ... Space

Claims (8)

[Claims] 1. A longitudinally coupled multimode in which a plurality of IDT electrodes are arranged close to each other along the propagation direction of a surface wave on the main surface of a piezoelectric substrate, and grating reflectors are arranged on both outer sides of these IDT electrodes. In the surface acoustic wave filter, the electrode fingers are arranged at a pitch narrower than the electrode finger pitch (Lt) in a portion other than the adjacent section of the IDT electrode near the adjacent section (adjacent section) of the adjacent IDT electrodes. A surface acoustic wave filter characterized in that 2. The elastic member according to claim 1, which is a primary-third-order longitudinally coupled dual-mode surface acoustic wave filter constituted by arranging three IDT electrodes close to each other on a main surface of a piezoelectric substrate. Surface wave filter. 3. The surface acoustic wave filter according to claim 2, wherein the adjacent section is widely set on the side of the IDT electrode arranged outside the centrally arranged IDT electrode. 4. The number of electrode fingers in the adjacent section is four,
The ratio Lt4 / Lt between the electrode finger pitch Lt4 and the electrode finger pitch Lt other than the adjacent section is 0.8000 ≦ Lt4 / Lt.
4. The relationship according to claim 1, wherein .ltoreq.0.9111.
The surface acoustic wave filter described. 5. The number of electrode fingers in the adjacent section is 8,
The ratio Lt8 / Lt between the electrode finger pitch Lt8 and the electrode finger pitch Lt other than the adjacent section is 0.8941 ≦ Lt8 / Lt.
≤ 0.9530, wherein:
The surface acoustic wave filter described. 6. The width of the electrode fingers in the adjacent section and the space width between the electrode fingers are equal to each other.
The surface acoustic wave filter described. 7. The electrode finger pitch in the central portion of the adjacent section is the narrowest, and the electrode finger pitch is set so as to approach the electrode finger pitches other than in the adjacent section toward the outside. The surface acoustic wave filter according to 1. 8. The surface acoustic wave filter according to claim 1, wherein the width of the electrode fingers in the adjacent section is equal to the width of the electrode fingers in the sections other than the adjacent section.