JP2011205400A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a double wave of which the wavelength is a half of the wavelength of a fundamental wave from being propagated to an output-side IDT electrode in a surface acoustic wave filter including an input-side IDT electrode, the output-side IDT electrode and a grating electrode interposed between these electrodes.SOLUTION: Along a surface acoustic wave-propagating direction, a double electrode structure 31 is provided in which two grating electrode fingers 17 for which an interval between centerlines in a length direction becomes λ/4 and of which the width dimensions are equal are disposed via a gap area. A main reflection area 32 in which the double electrode structures 31 are disposed at array intervals of λ/2, and an auxiliary reflection area 33 in which the double electrode structures 31 are disposed at array intervals of 3/4λ, are provided continuously.

Description

  The present invention relates to an acoustic wave filter, for example, a (SAW: Surface Acoustic Wave) filter.

  In a transversal type elastic wave filter such as a SAW filter in which IDT (IDT: interdigital transducer) electrodes are arranged on a piezoelectric substrate as an input side electrode and an output side electrode, the electrodes are arranged in the propagation path of the elastic wave between these electrodes. In some cases, for example, a square solid electrode is disposed as a shield electrode in a part of the propagation path in order to reduce the coupling capacity between the input and output electrodes. In such a case, the propagation speed of the elastic wave is between the propagation path between the input and output electrodes and each electrode that is a grating in which a plurality of electrode fingers are arranged in a direction substantially orthogonal to the propagation direction of the elastic wave. Is different. Therefore, for example, refraction occurs when an elastic wave is output from the input side electrode to the output side electrode, and energy loss may occur in the pass frequency band. Therefore, in order to make the propagation speed of the elastic wave uniform between the propagation path and each electrode, a technique of arranging a grating type electrode in the propagation path is known.

  In addition, as the IDT electrodes on the input side and the output side described above, the electrode finger group is tapered (slant type) so that the width dimension and the separation dimension of the electrode fingers increase from one side to the other side of the pair of bus bars. May be placed in In this case, assuming that a track that is a propagation path of an elastic wave corresponding to a period unit λ composed of the width dimension and the separation dimension of the electrode finger is Tr, the period unit λ is directed from the bus bar on one side to the bus bar on the other side. Is formed from a narrow track Tr to a wide track Tr. Therefore, when the wavelength of the fundamental wave in each track Tr is λ, as in Patent Document 1, for example, λ / 8 lines (grating electrode fingers) having a width dimension of λ / 8 are formed in the propagation path between the input and output electrodes. By providing them at a distance from each other, the propagation speeds of elastic waves in this propagation path and the respective electrodes are substantially uniform. For this reason, the influence of refraction of elastic waves is reduced, and good frequency characteristics can be obtained.

However, in this Patent Document 1, in the case where a unidirectional electrode such as a DART (Distributed Acoustic Reflection Transducer) electrode is used as an input / output electrode, 2 which propagates from the input side electrode to the output side electrode together with the fundamental wave. The harmonic wave (an elastic wave whose wavelength is half that of the fundamental wave) has not been specifically studied.
Patent Document 2 describes a technique for suppressing the leaky wave from being mixed into the Rayleigh wave by adjusting the electrode period of the grating-type shield electrode, but the above-described problems have not been studied.

JP 2009-89013 A JP-A-2005-203996

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an acoustic wave filter including an input-side IDT electrode, an output-side IDT electrode, and a grating-type electrode interposed between the electrodes. An object of the present invention is to provide an elastic wave filter capable of suppressing propagation of a double wave, which is half of a fundamental wave, to an output-side IDT electrode.

The elastic wave filter of the present invention is
A pair of bus bars arranged so as to be parallel to each other along the propagation direction of the elastic wave, and an electrode finger group formed so as to extend in a comb-tooth shape from one bus bar toward the other bus bar, And an input-side IDT electrode and an output-side IDT electrode disposed on the piezoelectric substrate so as to be separated from each other in the propagation direction of the elastic wave,
A pair of grating portion bus bars provided between the input side IDT electrode and the output side IDT electrode and arranged parallel to each other along the propagation direction of the elastic wave, and connected between the pair of grating portion bus bars A grating reflector having a grating electrode finger group, and
The grating reflector is
In order to reflect the double wave whose wavelength is half of the fundamental wave toward the input-side IDT electrode, when the wavelength of the fundamental wave is λ,
The distance between the centerlines in the length direction is λ / 4, and two grating electrode fingers having the same width dimension form a pair via the gap region along the propagation direction of the elastic wave. Provided double electrode structure,
A main reflection region in which the double electrode structure is arranged at an arrangement interval of λ / 2 and an auxiliary reflection region in which the double electrode structure is arranged at an arrangement interval of 3 / 4λ are continuously provided. And

When the Y-cut Z-propagating lithium niobate is used as a substrate, for example, the width dimension of the grating electrode fingers is formed to be wider than the gap dimension between the grating electrode fingers in the double electrode structure. preferable.
The auxiliary reflection region is preferably arranged on at least one or both of the input-side IDT electrode side and the output-side IDT electrode side in the grating reflector, or may be arranged, for example, at four or more locations. .
The electrode fingers of the input side IDT electrode, the electrode fingers of the output side IDT electrode, and the grating electrode fingers of the grating reflector have respective widths from one side to the other side in the direction orthogonal to the propagation direction of the elastic wave. It is a taper-type electrode arranged so that an array pattern consisting of dimensions and separation dimensions spreads,
The array pattern of the grating electrode fingers is arranged so that the array pattern of the IDT electrodes on one side of the input side IDT electrode and the output side IDT electrode is continuously inherited, and the IDT electrode on the other side and the grating It is connected to the array pattern of the IDT electrode on the other side at the boundary line between the reflector and
In the grating reflector, it is preferable that the double electrode structure is disposed on the IDT electrode side on the one side from the boundary line.
Each of the input side IDT electrode and the output side IDT electrode may be a DART electrode.

  In the present invention, when the wavelength of the fundamental wave of the acoustic wave propagating in the acoustic wave filter is λ, the distance between the center lines in the length direction is λ / 4, and the two width dimensions are equal. A double electrode structure in which grating electrode fingers are arranged through a gap region is provided along the propagation direction of the elastic wave, and a main reflection region in which the double electrode structure is arranged at an arrangement interval of λ / 2, and a double electrode structure Since the auxiliary reflection regions are arranged continuously at an arrangement interval of 3 / 4λ, the second harmonic wave whose wavelength is half of the fundamental wave can be reflected toward the input-side IDT electrode.

It is a top view which shows an example of the elastic wave filter which concerns on embodiment of this invention. It is the top view which expanded and showed a part of above-mentioned elastic wave filter typically. It is a schematic diagram which shows a mode that an elastic wave propagates in the above-mentioned elastic wave filter. It is a characteristic view which shows the characteristic obtained in the conventional elastic wave filter. It is a characteristic view which shows the characteristic in the elastic wave filter which changes according to the presence or absence of a grating reflector. It is a characteristic view which shows the characteristic in the elastic wave filter which changes according to the presence or absence of a grating reflector. It is a characteristic view which shows the characteristic in the elastic wave filter which changes according to the presence or absence of a grating reflector. It is a characteristic view which shows the quantity of the 2nd harmonic reflected according to the width dimension of a grating electrode finger. It is a characteristic view which shows the characteristic in the elastic wave filter which changes according to the presence or absence of a grating reflector. It is a schematic diagram which shows typically the reflective characteristic in a grating reflector. It is a characteristic view which shows the result of having simulated the reflection characteristic in a grating reflector. It is a characteristic view which shows the characteristic in the elastic wave filter which changes according to the presence or absence of a grating reflector. It is a top view which shows the other example of the elastic wave filter of this invention. It is a top view which shows the other example of the elastic wave filter of this invention.

  An elastic wave filter, such as a SAW filter, according to an embodiment of the present invention will be described with reference to FIG. The SAW filter includes, for example, tapered (slant type) input-side IDT electrodes 12 formed on the YZ plane (Y-cut Z-propagating lithium niobate) of the piezoelectric substrate 11 made of, for example, LiNbO3, and spaced apart from each other. An output-side IDT electrode 13 is provided. As will be described later, these IDT electrodes 12 and 13 are DART (Distributed Acoustic Reflection Transducer) electrodes that are unidirectional electrodes so as to propagate elastic waves from the input-side IDT electrode 12 to the output-side IDT electrode 13. Each is composed. Further, in these IDT electrodes 12 and 13, the wavelength is half of the fundamental wave and the frequency is about twice that of the fundamental wave, and the wavelength is 1/3 of the fundamental wave and the frequency is fundamental. The third harmonic wave, which is about three times, propagates from the input-side IDT electrode 12 to the output-side IDT electrode 13 similarly to the fundamental wave. These IDT electrodes 12 and 13 are made of a metal film such as aluminum. In addition, in the edge part area | region of the piezoelectric substrate 11 in the side of each IDT electrode 12 and 13, it is not illustrated for absorbing the unnecessary elastic wave which propagates to the said area | region via these IDT electrodes 12 and 13 A sound absorbing material (damper) is formed.

  In the input-side IDT electrode 12, 14a and 14b are a bus bar on one side and a bus bar on the other side, respectively, and are formed on the front side and the back side in FIG. 1 so as to be parallel to each other. The bus bar 14 a on one side is grounded, and the bus bar 14 b on the other side is connected to the input port 21. In FIG. 1, 15 is an electrode finger extending from each of the bus bars 14a, 14b of the input-side IDT electrode 12 toward the opposite bus bars 14b, 14a so as to be alternately comb-like, and 26 is one side The reflective electrode extends from the bus bar 14a toward the other bus bar 14b.

  The electrode finger 15 and the reflective electrode 26 are a pair of electrode fingers 15 and 15 formed so as to be adjacent to each other from each of the bus bars 14 a and 14 b, and a reflection extending from the bus bar 14 a so as to be adjacent to these electrode fingers 15. The electrodes 26 are arranged in a set so as to be periodically repeated along a propagation direction of the elastic wave with a predetermined arrangement pattern (period unit) λ. Therefore, in the input side IDT electrode 12, an elastic wave having the same length as the period unit λ propagates. An elastic wave having the same wavelength as the period unit λ is called a fundamental wave.

  In addition, the arrangement pattern of the electrode fingers 15 and the reflective electrodes 26 is such that the width of the electrode fingers 15 and the reflective electrodes 26 increases so that the period unit λ increases from the rear bus bar 14b to the front bus bar 14a in FIG. In addition, the distance between the electrode fingers 15 and 15 and the distance between the electrode fingers 15 and the reflective electrode 26 is formed so as to gradually widen. Accordingly, when the track that is the propagation path of the elastic wave is Tr, in a direction orthogonal to the propagation direction of the elastic wave, the cycle unit λ is narrow from Tr1 to Tr2 that extends from the rear bus bar 14b to the front bus bar 14a. Will be formed. In FIG. 1, the width dimensions of the electrode fingers 15 and the reflective electrodes 26 are drawn as constant widths for simplification of illustration.

  In this example, the width dimension of the reflective electrode 26 and the dimension between the straight lines passing through the centers of the electrode fingers 15 in the adjacent electrode fingers 15 and 15 are 3λ / 8 and λ / 4, respectively. Moreover, the width dimension of the electrode finger 15 and the separation distance between the electrode fingers 15 and 15 are each λ / 8.

  The output-side IDT electrode 13 is configured as a unidirectional electrode similar to the input-side IDT electrode 12, and specifically, as shown in FIG. 1, the one-side bus bar 14c and the other-side bus bar 14d. It has. The bus bar 14c on one side is disposed on the front side in FIG. 1 and connected to the output port 22, and the bus bar 14d on the other side is disposed on the back side and grounded. Similarly to the input-side IDT electrode 12, the output-side IDT electrode 13 has a constant cycle unit λ along the elastic wave propagation direction, and the cycle unit from the back-side bus bar 14d toward the front-side bus bar 14c. The electrode fingers 15 and the reflective electrodes 26 are arranged so that λ becomes an array pattern extending from Tr1 to Tr2. The electrode fingers 15 and the reflection electrodes 26 of the output side IDT electrode 13 are also formed to have the same width and spacing as the arrangement pattern of the input side IDT electrode 12 described above.

  Between the input-side IDT electrode 12 and the output-side IDT electrode 13, a grounded grating reflector 16 is formed of a metal such as aluminum, for example, and the grating reflector 16 extends along the propagation direction of the elastic wave. And a pair of grating section bus bars 18a, 18b arranged so as to be parallel to each other. One grating section bus bar 18a is formed on the front side in FIG. 1, and the other grating section bus bar 18b is disposed on the back side. A plurality of grating electrode fingers 17 extending in a direction substantially orthogonal to the propagation direction of the elastic wave are formed between the grating portion bus bars 18a and 18b, like the electrode fingers 15 described above. The grating electrode finger 17 has one end side and the other end side in the length direction connected to the grating portion bus bars 18a and 18b, respectively.

  Next, the specific layout of the grating electrode fingers 17 and the reason why the grating electrode fingers 17 are arranged in this way will be described in detail. The grating electrode finger 17 has a width dimension and a separation dimension so that the elastic waves from the track Tr1 to the track Tr2 described above propagate from the grating portion bus bar 18b on the back side in FIG. 1 toward the bus bar 18a on the near side. The arrangement pattern consisting of is configured to gradually spread. Further, the arrangement pattern of the grating electrode fingers 17 is such that the arrangement pattern of the electrode fingers 15 and the reflection electrodes 26 on the output IDT electrode 13 is extended from the output IDT electrode 13 side toward the input IDT electrode 12 as it is. The boundary line 20 between the grating reflector 16 and the input-side IDT electrode 12 is connected to the array pattern on the input-side IDT electrode 12. That is, the grating electrode finger 17 has a distance of λ / 4 (or λ / 2) between the centerlines in the length direction, and the electrode finger 15 in the IDT electrode 13 adjacent to the grating reflector 16 and The gaps between the grating electrode fingers 17 adjacent to the electrode fingers 15 are each λ / 8. The arrangement pattern of the grating electrode fingers 17 is cut at the boundary line 20 so that a gap region having a width dimension of λ / 8 is formed between the bus bars 18a and 18b with the input-side IDT electrode 12. It is in a missing state. Therefore, the double electrode structure 31 to be described later has a layout in which only one grating electrode finger 17 is unavoidably arranged in the boundary line 20 without the two grating electrode fingers 17 and 17 being one set. A region (track Tr) is formed.

  Here, in order to simplify the description of the layout of the grating electrode fingers 17 and the characteristics of the elastic wave propagating in the grating reflector 16, first, one track Tr in the tracks Tr1 to Tr2 is specifically shown in FIG. A description will be given with reference to FIG. 2 while focusing on the track Tr in a region close to the grating portion bus bar 18a on the front side of the middle side. Therefore, in the grating reflector 16, an elastic wave having substantially the same characteristics as those described with reference to the track Tr in FIG. 2 propagates from the tracks Tr1 to Tr2. In FIG. 1 and FIG. 2, the number of grating electrode fingers 17 is simplified and drawn.

  As schematically shown in FIG. 2, the grating electrode finger 17 is composed of a pair of two grating electrode fingers 17 and 17 so that the distance between the centerlines in the length direction is λ / 4. It is arranged in multiple places. When the set of these two adjacent grating electrode fingers 17 and 17 is called a double electrode structure 31, in this example, the gap dimension between the grating electrode fingers 17 and 17 in the double electrode structure 31 is, for example, 3 / 40λ. It has become. In addition, the width dimensions of the grating electrode fingers 17 in the double electrode structure 31 are equal to each other, and in this example, each is 7 / 40λ. Accordingly, if the width dimension and the gap dimension of the grating electrode finger 17 are L (line) and S (space), respectively, the line occupation ratio η (= L / (L + S)) is 0.7 in this example.

  Here, the arrangement period of the double electrode structures 31 and 31 adjacent to each other, that is, the distance between the left edges of the left grating electrode fingers 17 and 17 in each of the double electrode structures 31 and 31 in FIG. In other words, the grating reflector 16 is formed with a main reflection region 32 on the center side where a plurality of double electrode structures 31 are continuously arranged at an arrangement interval of λ / 2. Therefore, in this main reflection area 32, an even number of grating electrode fingers 17 are arranged via a gap area of 3 / 40λ.

  Further, on both sides of the main reflection region 32 in the propagation direction of the elastic wave, the double electrode structure is arranged so that the arrangement interval between the end portions of the main reflection region 32 and the double electrode structure 31 is 3 / 4λ. 31 and 31 are arranged to form auxiliary reflection regions 33 and 33. The gap dimension between these regions 32 and 33 is, for example, 13 / 40λ. Accordingly, in this example, since the auxiliary reflector regions 33 are arranged at two locations in the grating reflector 16, the even number of grating electrode fingers 17 are passed through the gap region of 3 / 40λ along the propagation direction of the elastic wave. In this example, the third grating electrode fingers 17 and 17 (see FIG. 2) are removed from the right and left sides of the grating reflector 16 in this example to form a boundary between the two regions 32 and 33. It can be said that it has adopted. 2, the acoustic wave filter in one track Tr is schematically shown on the upper side as described above, and the right region and grating in the input-side IDT electrode 12 are shown on the lower side in FIG. A part of the left region of the reflector 16 (region surrounded by a one-dot chain line in FIG. 2) is enlarged.

Next, the reason why the grating electrode fingers 17 are arranged in this way in the grating reflector 16 will be described with reference to FIG. First, elastic waves generated in this filter will be described.
As described above, in the DART electrode which is a unidirectional electrode, both the fundamental wave and the second and third harmonics are excited and propagate from the input side IDT electrode 12 to the output side IDT electrode 13 and to the output port 22. It will be received. With respect to the frequency characteristics received from the output port 22 when this DART electrode is used, the center frequency of the fundamental wave is set to 140 MHz, and the IDT electrodes 12 and 13 are replaced with the above described grating reflector 16 as a shield electrode. FIG. 4 shows characteristics obtained when a plate-shaped solid electrode is arranged. As can be seen from FIG. 4, the second and third harmonics appear as unnecessary waves at positions approximately twice and three times the center frequency of 140 MHz of the fundamental wave.

  Here, a state in which a fundamental wave (wavelength λ) is generated in the IDT electrodes 12 and 13 is schematically shown in FIG. In the region where the amplitude of the fundamental wave is positive, “+” charges are generated in the electrodes (electrode fingers 15 and 15 and the reflection electrode 26), and in the region where the amplitude of the elastic wave is negative, “−” is generated in the electrode. Considering the case where charges are generated, it can be seen that the total amount of charges generated at these electrodes does not become zero within the range of one period unit λ. Accordingly, the fundamental waves excited by these electrodes propagate, for example, from the input side IDT electrode 12 toward the output side IDT electrode 13 without canceling each other. The acoustic wave is actually excited in the intersecting region between the electrodes, but is described here as being excited by these electrodes in order to simplify the explanation.

On the other hand, if the wavelength of the second harmonic is λ ₂ (λ ₂ = λ / 2), as shown in FIG. 3 (b), the total amount of charges does not become zero within one cycle for this second harmonic as well. Similarly, it propagates toward the output-side IDT electrode 13. The same applies to the third harmonic wave. Thus, the propagation of the second harmonic and the third harmonic along with the fundamental wave is a phenomenon peculiar to a unidirectional electrode such as a DART electrode.

  At this time, the 3rd harmonic usually has lower excitation efficiency than the 2nd harmonic and is often attenuated by the matching circuit. Further, in this third harmonic wave, the wavelength specific film thickness is three times that of the fundamental wave, and therefore the amount of reflection at each of the grating electrode fingers 17 is larger than that of the fundamental wave and the second harmonic wave. For this reason, there is often no problem with the third harmonic wave, but the attenuation characteristic of the second harmonic wave may be deteriorated as an unnecessary wave. Accordingly, the layout of the grating electrode finger 17 capable of obtaining a large amount of attenuation with respect to the second harmonic wave was examined as follows.

  First, with respect to the width dimension and the separation dimension of the grating electrode finger 17 in the double electrode structure 31, the dimensions similar to the electrode finger 15 in the IDT electrodes 12 and 13 (width dimension of the grating electrode finger 17: λ / 8, the grating electrode finger 17, The distance between 17 was set to (λ / 8) (line occupancy η (L / (L + S) = 0.5)), and how the second and third harmonics propagated was examined. As a result, with respect to the fundamental wave, as shown by a solid line in FIG. 5, the influence of diffraction and refraction is reduced as compared with the case where a plate-shaped solid electrode is formed instead of the grating reflector 16 (dotted line in FIG. 5). As a result, the energy loss and ripple in the pass frequency band of the fundamental wave are reduced, and the attenuation gradient (attenuation characteristic) on the high frequency side, which has been degraded by the diffraction effect, has been improved. FIG. 5 (b) is an enlarged view of a part of FIG. 5 (a).

  On the other hand, with respect to the second harmonic and the third harmonic, as shown in FIG. 6, the average attenuation is increased by about 5 dB as compared with the case where the solid electrode is formed between the IDT electrodes 12 and 13 (input-side IDT electrode 12 The amount of reflection to the side is large). In order to examine the reason, a simulation was performed for one track Tr as shown in FIG. As a result, as shown in FIG. 7, the response in the main lobe of the second harmonic wave is lacking due to the stopband characteristic of reflection by the double electrode structure 31 as compared with the case where the solid electrode is arranged without providing the grating reflector 16. It was found that the amount of attenuation increases. FIG. 6 (b) is an enlarged view of a part of FIG. 6 (a).

  That is, as shown in FIG. 3C, in the grating reflector 16, the interval between the grating electrode fingers 17 is set to λ / 4, and the width dimension of the two adjacent grating electrode fingers 17 and 17 is set. Are equal to each other. Therefore, for the fundamental wave, the difference in wavelength reflected by the grating electrode fingers 17 and 17 adjacent to each other is λ / 2 (= λ / 4 + λ / 4) and the phases are reversed, and the amplitude intensity of these reflected waves is equal. Therefore, the reflection coefficients between the grating electrode fingers 17 and 17 are equal to each other, so that these reflected waves cancel each other and apparent reflection does not occur.

On the other hand, for the second harmonic, as shown in FIG. 2D, the difference in wavelength reflected by the adjacent grating electrode fingers 17, 17 is λ ₂ (λ ₂ : wavelength of the _second harmonic, λ ₂ = λ / 2). For this reason, when viewed from the second harmonic, since the grating electrode fingers 17 are arranged in a regular form with a period that is half the wavelength of the second harmonic, they are generated in the grating electrode fingers 17 and 17 adjacent to each other. The reflected waves are amplified in phase with each other and are reflected to the input-side IDT electrode 12 to suppress propagation to the output-side IDT electrode 13. Accordingly, the provision of the grating reflector 16 causes reflection of the second harmonic and the third harmonic without disturbing the propagation to the output-side IDT electrode 13 as shown in FIG. It can be seen that transmission to the output port 22 is suppressed.

  The amount of reduction of the main lobe of the second harmonic at this time is about 5.5 dB, which is the same result as FIG. In addition, the dimension (opening length) between the electrode fingers 15 and 15 of the IDT electrodes 12 and 13 intersecting each other in each simulation described above is set to have an appropriate impedance, and the IDT electrodes 12 and 13 are the same. Thinning-out weighting is also performed for excitation and reflection. The same applies to the following simulations. Moreover, this FIG. 3 has shown typically the side surface which looked at the elastic wave filter from the side.

Therefore, it is considered that a filter having a characteristic superior to that of FIG. 6 described above can be obtained by increasing the reflection amount of the second harmonic wave in the grating reflector 16. Therefore, it was confirmed how the reflection amount of the second harmonic wave changes by changing the width dimension of the grating electrode finger 17. Here, when changing the width dimension of the grating electrode finger 17, the separation distance between the grating electrode fingers 17 and 17 in one double electrode structure 31 is set so that the fundamental wave is not reflected to the input-side IDT electrode 12 side. Was kept at λ / 4, and the above-described line occupancy η (L / (L + S)) was changed so that the gap dimension between the grating electrode fingers 17 and 17 was shortened. This result is shown in FIG. 8 as the reflectivity κ ′ ₁₂ (κ ′ ₁₂ : coupling coefficient between modes) of the second harmonic. From FIG. 8, it can be seen that the reflectivity (reflection amount) of the second harmonic wave increases by increasing the line occupation ratio η, that is, by increasing the width dimension of the grating electrode finger 17.

  Therefore, as shown in FIG. 3E, the case where the line occupancy η is set to, for example, 0.9 (when the main reflection region 32 described above is arranged in the entire grating reflector 16) is described above. As shown in FIG. 7, a simulation was performed for one track Tr. As shown in FIG. 9, the stopband width of reflection was widened, and the main lobe of the second harmonic wave was largely missing, resulting in an increase in attenuation. Therefore, by arranging the grating electrode fingers 17 having this width dimension from the tracks Tr1 to Tr2, the peak level of the second harmonic wave is reduced by about 10 dB compared to the case where a solid electrode is arranged instead of the grating reflector 16. It is thought that.

  However, in the graph of FIG. 9, the peak level of the second harmonic main lobe is greatly reduced, but square peaks remain at both ends of the main lobe, and a flat characteristic is not obtained. Therefore, in the present invention, in order to reduce the square peak, the grating electrode fingers 17 are arranged so that the line occupancy η approaches 0.9 in this example so that the line occupancy η approaches 0.7. In addition to the main reflection region 32 in which the structures 31 are arranged at an arrangement interval of λ / 2, an auxiliary reflection region 33 in which the double electrode structures 31 are arranged at an arrangement interval of 3 / 4λ is provided.

That is, by arranging the auxiliary reflection region 33, as shown in FIG. 3F, for example, when viewed from an arbitrary electrode finger 15 in the input side IDT electrode 12, it is reflected twice from each region 32, 33. The waves are 180 ° out of phase with each other and have opposite reflection coefficients. In other words, it can be said that by providing the auxiliary reflection region 33, the reflection characteristics of the second harmonic shown in FIG. 10 are weighted. Therefore, by arranging the auxiliary reflection region 33 in which the reflection characteristics are weighted as described above in the grating reflector 16, as shown in FIG. 12, the square peak of the second harmonic main lobe is substantially flat. Therefore, it can be reduced by 15 dB or more than the case where the solid electrode is arranged. FIG. 11 shows the result of simulation of the reflection characteristics shown in FIG. In FIG. 11, the horizontal axis represents frequency, and the vertical axis represents (mechanical) reflection characteristics of the grating reflector 16 as return loss. When the return loss is 0 dB, total reflection is applied to the input IDT electrode 12. It means that all the elastic waves come back. Further, in FIG. 11, the characteristic in which the double electrode structures 31 are all arranged at an arrangement interval of λ / 2 is shown by a solid line, and the double electrode structure 31 at an arrangement interval of λ / 2 has a double electrode with an arrangement interval of 3 / 4λ The characteristics when the structures 31 are mixed are shown by dotted lines. From FIG. 11, the double electrode structure 31 having an arrangement interval of λ / 2 is arranged by arranging the double electrode structure 31 having an arrangement interval of 3 / 4λ together with the double electrode structure 31 having an arrangement interval of λ / 2. Compared to FIG. 5, the peak reflectance is slightly reduced, but the width of the main lobe is widened, and the frequency range reflected on the input-side IDT electrode 12 is widened.
Similarly to the second harmonic described above, propagation of the third harmonic wave to the output-side IDT electrode 13 is also suppressed by reflection.

  Subsequently, the configuration and characteristics of the entire filter in which the grating electrode fingers 17 are arranged from Tr1 to Tr2 in the layout almost the same as the track Tr described above will be described. In the area close to the bus bar 14b on the back side of the track Tr, as shown in FIG. 1, in addition to the auxiliary reflection area 33, the main reflection area 32, and the auxiliary reflection area 33 described above, these areas 32, 33 are provided. The main reflection region 32 is arranged in the same layout so as to be adjacent to the left side (input side IDT electrode 12 side). That is, the main reflection region 32 in which a plurality of double electrode structures 31 are arranged is formed so that the arrangement interval between the auxiliary reflection region 33 and the grating electrode finger 17 on the input-side IDT electrode 12 side is 3 / 4λ. Yes. Then, the front end portion of the grating electrode finger 17 extending from the grating portion bus bar 18b on the back side is cut so that a gap region having a width dimension of λ / 8 is formed between the boundary line 20 and the input side IDT electrode 12. It is missing.

  Therefore, as described above, in each of the double electrode structures 31, the two grating electrode fingers 17, 17 are not one set in the boundary line 20, and only one grating electrode finger 17 is unavoidably arranged. A region (track Tr) having a layout is formed. In other words, the grating reflector 16 is closer to the output-side IDT electrode 13 than the boundary line 20, more specifically, from the position farther to the right in FIG. A double electrode structure 31 is formed. Therefore, in the track Tr, the reflected waves of the fundamental waves do not cancel each other at the boundary line 20 and may be reflected slightly toward the input-side IDT electrode 12. However, since the double electrode structures 31 are arranged at a plurality of locations inside the grating reflector 16, the reflection of the fundamental wave is suppressed when heading over the boundary line 20 toward the output IDT electrode 13 side, so that almost the entire amount is obtained. Energy propagates toward the output-side IDT electrode 13. Therefore, in this filter, the reflection of the fundamental wave to the input-side IDT electrode 12 is suppressed from the tracks Tr1 to Tr2, and the propagation of the double wave to the output-side IDT electrode 13 is suppressed.

According to the above-described embodiment, when the wavelength of the fundamental wave is λ, the distance between the centerlines in the length direction is λ / 4, and the two grating electrode fingers 17 having the same width dimension are used. Using the double electrode structure 31 in which the double electrode structure 31 is arranged through the gap region, the main reflection region 32 in which the double electrode structure 31 is arranged at the arrangement interval of λ / 2, and the arrangement interval of the double electrode structure 31 at 3 / 4λ. And the auxiliary reflection region 33 arranged in the above are continuously provided. Therefore, the fundamental wave is reflected from the input-side IDT electrode 12 while being propagated from the input-side IDT electrode 12 to the output-side IDT electrode 13, and the double wave is reflected toward the input-side IDT electrode 12. Can be made. Therefore, good filter characteristics with reduced harmonic components such as second harmonics can be obtained.
At this time, in order to suppress the propagation of higher harmonic components such as the second harmonic wave, a grating reflector 16 that has been conventionally used to align the propagation speed of the elastic wave between the IDT electrodes 12 and 13 is used. Therefore, for example, an increase in the size of the filter and an increase in manufacturing cost can be suppressed.

Further, since the auxiliary reflection regions 33 are provided on substantially both sides of the grating reflector 16, as described above, the reflection characteristics of the grating reflector 16 are adjusted so that the peak shape of the second harmonic main lobe is flat. Can be weighted. Furthermore, since the line occupancy η described above is increased by increasing the width dimension of the grating electrode finger 17, the harmonic component can be further reduced.
In addition, since the grating reflector 16 is disposed between the IDT electrodes 12 and 13, the propagation speed of the elastic wave in the filter can be made uniform. Therefore, it is possible to improve the energy loss and the ripple in the pass frequency band by suppressing the influence of diffraction and refraction of the elastic wave, and a good characteristic can be obtained with respect to the attenuation gradient (attenuation characteristic) on the high frequency side. Further, in the IDT electrodes 12 and 13 and the grating reflector 16, the electrode fingers 15 and the grating electrode fingers 17 are arranged so that the connecting portion of the period unit λ is only the boundary line 20, so that the fundamental wave is diffracted. Refraction and reflection can be suppressed. Furthermore, since the line 20 is provided between the input-side IDT electrode 12 and the grating reflector 16, there is an advantage that the layout of the grating electrode finger 17 can be easily designed.

  In the above-described example, the auxiliary reflection regions 33 and 33 are arranged on both sides of the main reflection region 32. However, the position is adjusted according to the peak shape of the generated second harmonic main lobe, for example, the center of the grating reflector 16 The auxiliary reflection region 33 may be provided at the position. Further, the auxiliary reflection region 33 is arranged at an arrangement interval of 3 / 4λ. For example, the material of the piezoelectric substrate 11, the propagation direction of the elastic wave on the piezoelectric substrate 11, the IDT electrodes 12, 13 and the grating reflector are used. In order to make the peak shape of the generated second harmonic main lobe flat according to, for example, the constituent material and thickness dimension of 16, the width dimension of the electrode finger 15 and the grating electrode finger 17, and the shape of the end, for example, They may be arranged so as to be shifted within a range of 3 / 4λ × ± 5%. Further, the main reflection region 32 may be arranged so as to be shifted from the arrangement interval of λ / 2 within the same range. Further, the above-described line occupation ratio η may be set individually for each double electrode structure 31.

  Here, for example, with respect to the array pattern of the grating electrode fingers 17, the region close to the input-side IDT electrode 12 is arranged so that the array pattern of the input-side IDT electrode 12 is continuously inherited, and the output-side IDT electrode 13. When the line 20 described above is formed at the center position of the grating reflector 16, for example, the line 20 is arranged so that the arrangement pattern of the output IDT electrodes 13 is continuously inherited. The second harmonics reflected toward the input-side IDT electrode 12 in the right and left regions may cancel each other out. Therefore, the line 20 is disposed so as to be formed between the input-side IDT electrode 12 and the grating reflector 16 or between the grating reflector 16 and the output-side IDT electrode 13. Therefore, in the double electrode structure 31, the area where the two grating electrode fingers 17 and 17 are inevitably arranged and only one is unavoidably arranged is between the grating reflector 16 and the input-side IDT electrode 12. Alternatively, it is between the grating reflector 16 and the output side IDT electrode 13.

  In addition, the distance between each IDT electrode 12, 13 and the grating reflector 16 is set to λ / 8, and the arrangement pattern of each of the electrode finger 15 and the grating electrode finger 17 is continuously inherited. Although the IDT electrodes 12 and 13 and the grating reflector 16 are disposed, the separation dimension may be set to λ / 8 or more, for example. Even in that case, the double electrode structure 31 suppresses the reflection of the fundamental wave to the input-side IDT electrode 12 and suppresses the propagation of the double wave to the output-side IDT electrode 13. Therefore, in the area where the two grating electrode fingers 17 and 17 are unavoidably arranged in the double electrode structure 31 described above and only one is unavoidably arranged, as shown in FIG. By removing the double electrode structure 31 where the tip of the grating electrode finger 17 extending from the grating portion bus bar 18b is cut out along the boundary line 20, the double electrode structure 31 extends across all the tracks Tr1 to Tr2. Only one may be arranged.

  In the above-described example, the grating reflector 16 is formed so that the peak of the second harmonic main lobe is substantially flat for a certain track Tr. However, the frequency characteristics obtained by combining the characteristics of the tracks Tr are good. The arrangement of the grating reflector 16 and the IDT electrodes 12 and 13 may be optimized so that Furthermore, although the width dimension of the grating electrode finger 17 is formed wider than the gap dimension between the grating electrode fingers 17 and 17 in the double electrode structure 31, the width dimension of the grating electrode finger 17 depends on the material of the piezoelectric substrate 11 and the cutting. Depending on the orientation and the material of the grating electrode finger 17, various settings may be made so that the amount of reflection of the second harmonic increases.

  As the IDT electrodes 12 and 13, the electrode fingers 15 may be weighted. Furthermore, in the example described above, the separation dimension between the center position of the input-side IDT electrode 12 in the propagation direction of the elastic wave and the center position of the output-side IDT electrode 13 in the propagation direction of the elastic wave is set for each track Tr. The number of electrode fingers 15 in each IDT electrode 12 and 13 is set to be equal in each track Tr, but the number of electrode fingers 15 in each IDT electrode 12 and 13 is set as shown in FIG. In order to make them equal in each track Tr, a separation dimension between the center position of the input-side IDT electrode 12 in the propagation direction of the elastic wave and the center position of the output-side IDT electrode 13 in the propagation direction of the elastic wave is determined by each track Tr. May be different. In this example as well, the IDT electrodes 12 and 13 and the grating reflector 16 are formed in a tapered shape as in FIG. 1 described above, but in FIG. In FIG. 14, the electrode finger 15 is drawn in a curved line, but whether the electrode finger 15 is linear or curved is a matter of design adjustment. The IDT electrodes 12 and 13) in FIG. 1 are slightly curved.

  Further, the electrode fingers 15 may be orthogonal to the bus bar 14 as shown in FIG. 2 so that the elastic wave of a certain track propagates without arranging the electrode fingers 15 in a tapered shape. In addition to the DART electrode, the IDT electrodes 12 and 13 may be electrodes that propagate a double wave together with the fundamental wave, such as an EWC-SPUD (Electronic Width Controlled-SPUDT) electrode. An electrode through which a double wave propagates may be provided only at least on the input side IDT electrode 12 among the IDT electrodes 12 and 13.

11 Piezoelectric substrate 12 Input side IDT electrode 13 Output side IDT electrode 16 Grating reflector 17 Grating electrode finger 31 Double electrode structure 32 Main reflection area 33 Auxiliary reflection area

Claims (5)

A pair of bus bars arranged so as to be parallel to each other along the propagation direction of the elastic wave, and an electrode finger group formed so as to extend in a comb-tooth shape from one bus bar toward the other bus bar, And an input-side IDT electrode and an output-side IDT electrode disposed on the piezoelectric substrate so as to be separated from each other in the propagation direction of the elastic wave,
A pair of grating portion bus bars provided between the input side IDT electrode and the output side IDT electrode and arranged parallel to each other along the propagation direction of the elastic wave, and connected between the pair of grating portion bus bars A grating reflector having a grating electrode finger group, and
The grating reflector is
In order to reflect the double wave whose wavelength is half of the fundamental wave toward the input-side IDT electrode, when the wavelength of the fundamental wave is λ,
The distance between the centerlines in the length direction is λ / 4, and two grating electrode fingers having the same width dimension form a pair via the gap region along the propagation direction of the elastic wave. Provided double electrode structure,
A main reflection region in which the double electrode structure is arranged at an arrangement interval of λ / 2 and an auxiliary reflection region in which the double electrode structure is arranged at an arrangement interval of 3 / 4λ are continuously provided. An elastic wave filter.   2. The acoustic wave filter according to claim 1, wherein the width dimension of the grating electrode fingers is formed to be wider than the gap dimension between the grating electrode fingers in the double electrode structure.   The elastic wave filter according to claim 1, wherein the auxiliary reflection region is disposed on at least one of the input-side IDT electrode side and the output-side IDT electrode side in the grating reflector. The electrode fingers of the input side IDT electrode, the electrode fingers of the output side IDT electrode, and the grating electrode fingers of the grating reflector have respective widths from one side to the other side in the direction orthogonal to the propagation direction of the elastic wave. It is a taper-type electrode arranged so that an array pattern consisting of dimensions and separation dimensions spreads,
The array pattern of the grating electrode fingers is arranged so that the array pattern of the IDT electrodes on one side of the input side IDT electrode and the output side IDT electrode is continuously inherited, and the IDT electrode on the other side and the grating It is connected to the array pattern of the IDT electrode on the other side at the boundary line between the reflector and
4. The acoustic wave filter according to claim 1, wherein the grating reflector has the double electrode structure disposed on the IDT electrode side on one side of the boundary line. .   5. The acoustic wave filter according to claim 1, wherein each of the input-side IDT electrode and the output-side IDT electrode is a DART electrode.

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