JPH0923132A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress spurious in high order longitudinal and lateral modes while maintaining a Q value required as a resonator and a practically allowed range in the deterioration of an insertion loss, to improve an out-of-band attenuation variable and to reduce the deviation of group delay time in a pass band. SOLUTION: Ten or forty metallic strips for respective reflectors 13, 14, 18, 19 constituting a prestage side longitudinal coupling double mode SAW filter 3 and a post stage side longitudinal coupling double mode SAW filter 6 are prepared. A part of oscillation energy in the high order longitudinal and lateral modes of a surface acoustic wave(SAW) excited by a signal impressed between 1st and 2nd terminals 5, 8 is leaked, and while suppressing spurious in the high order modes, the SAW is closed in an IDT, so that the signal is filtered.

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 device, particularly a surface acoustic wave used as a high frequency resonator exhibiting resonance characteristics in a range of several tens of MHz to 1 GHz or a resonator filter using the resonator. Regarding the device.

[0002]

2. Description of the Related Art Conventionally, in a surface acoustic wave (SAW) resonator utilizing a wave excited by an interdigital transducer (IDT) electrode, an IDT 102 is attached to the surface of a piezoelectric substrate 101 as shown in FIG. , Reflectors with periodic metal strips on both sides 1
By disposing 03 and 103, the IDT 102
The surface acoustic waves excited by the reflectors 103, 1
It is often reflected by 03, thereby confining the vibration energy in the IDT 102 and realizing a high Q value (frequency selectivity) in many cases. At this time, the piezoelectric substrate 1
When a lithium tetraborate single crystal is used as 01, the number of periodic metal strips forming the reflectors 103 is generally more than 50 in order to increase the Q value of the resonator.

However, in this case, as shown in FIG. 14, not only the fundamental (first-order longitudinal mode) resonance mode but also the second or higher higher-order resonance mode (higher-order longitudinal mode) vibration in the SAW propagation direction of the resonator. The energy confinement has occurred, and the charge excited by the even mode vibration is
Charges that can be canceled out in the DT 102 but are excited by higher order odd mode vibrations are:
Since this cannot be offset, spurious responses appear.

In addition, the basic (1
Next-order transverse mode) Resonance mode as well as higher-order resonance modes higher than second order (higher-order transverse mode) are confined in vibration energy, and charges excited by vibrations in even-order modes are stored in the IDT 102. Although it is possible to cancel the charges, it is impossible to cancel the charges excited by the vibrations of the higher odd-order modes, so that there is a defect that a spurious response appears.

Also, as one of the SAW filters, a longitudinally coupled dual mode SAW filter has been proposed in which the waves of the first and third modes are positively used to widen the pass band of the filter. For example, as shown in FIG. 15, two longitudinally coupled dual mode SAW filters 104 are arranged in series on one piezoelectric substrate 105, and the two are cascade-connected by a filter connecting portion 109 formed of a metal strip. In the two-stage cascade connection longitudinally coupled dual mode SAW filter, two longitudinally coupled dual mode SAW filters 104 are used.
Respectively, the excited surface acoustic waves are confined in the IDT, and the passband is widened by utilizing the waves of the first and third modes generated by the acoustic coupling between the IDTs.

In this case, each longitudinally coupled dual-mode SAW filter 104 has a central IDT 106 on the surface of one piezoelectric substrate 105 and a central IDT 106 along the propagation direction of a surface acoustic wave excited or received by the outer IDTs 107, 107. I
The DT 106 and the outer IDTs 107, 107 are arranged close to each other, and the reflectors 108, 108 are provided on both sides of the DT 106 and the excited surface acoustic waves, so that the central IDT 106, the outer I
While confined in the DTs 107 and 107, the wave pass of the first and third modes generated by the acoustic coupling between the central IDT 106 and the outer IDTs 107 and 107 is used to widen the pass band of the filter.

However, even in this two-stage cascade connection vertical coupling dual mode SAW filter, the SA shown in FIG.
Similar to the W resonator, there is a defect that spurious occurs in the attenuation region of the filter characteristic due to the higher-order longitudinal mode generated in the SAW propagation direction and the higher-order transverse mode generated in the cross length direction.

For example, as the piezoelectric substrate 105, 45 degrees X
A cut, Z-propagating lithium tetraborate single crystal plate is used, and the crossover length is 15λ in the central portion on the piezoelectric substrate 105.
Central IDT 106 consisting of 60 pairs of metal strips
And a crossover length of 1 is formed on both sides of the central IDT 106.
Forming outer IDTs 107, 107 composed of 40 pairs of metal strips each of which has a size of 5λ.
When the reflectors 108, 108 each composed of 60 metal strips are formed outside the T107, 107, as shown in FIG.
As shown in FIG. 6, there is a defect that spurious is generated in the attenuation region of the filter characteristic.

Therefore, in order to solve such a defect,
Conventionally, for example, in the cross length direction with respect to the IDT,
There has been a method of weighting to cancel the charge in the higher-order transverse mode and to deteriorate the Q value of the higher-order longitudinal mode to suppress spurious. In addition, a method of suppressing the occurrence of higher-order transverse modes by setting the crossover length of the IDT short has also been used. However, weighting or shortening the crossover length increases the input / output impedance, making it impossible to achieve the required matching with an external circuit, which impairs the flatness of the passband characteristics of the filter. There was a defect. Further, the method of shortening the crossover length is effective in suppressing higher-order transverse mode spurious, but has a defect that it cannot suppress higher-order longitudinal mode spurious.

[0010]

As described above, in the method of weighting or shortening the crossover length,
There is a defect that the input / output impedance increases and the required matching with an external circuit cannot be realized, which impairs the flatness of the pass band characteristic of the filter. Further, the method of shortening the crossover length is effective in suppressing higher-order transverse mode spurious, but has a defect that it cannot suppress higher-order longitudinal mode spurious. In view of the above circumstances, the present invention has been made to eliminate the drawbacks of the surface acoustic wave device using the lithium tetraborate single crystal piezoelectric substrate as described above. It is an object of the present invention to provide a surface acoustic wave device that suppresses spurious noise caused by the above.

[0011]

In order to achieve the above object, a surface acoustic wave device according to the present invention comprises, in claim 1, at least one interdigital transducer (IDT) on a lithium tetraborate single crystal piezoelectric substrate. In a surface acoustic wave device in which electrodes are arranged and reflectors are provided on both sides of the electrodes to substantially confine the vibration energy of excited surface acoustic waves (SAW) in the IDT, the number of periodic metal strips constituting the reflectors. The feature is that the number is 40 or less. A second aspect of the present invention is characterized in that, in the surface acoustic wave device according to the first aspect, the number of periodic metal strips forming the reflector is 10 or more. A third aspect of the present invention is characterized in that, in the surface acoustic wave device according to any of the first and second aspects, a lithium tetraborate single crystal piezoelectric substrate is used as the piezoelectric substrate.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The surface acoustic wave device according to the present invention will be described in detail below with reference to the embodiments shown in the drawings. FIG. 1 shows an embodiment of a surface acoustic wave device according to the present invention, which is a two-stage cascade connection longitudinally coupled dual mode SAW.
It is a block diagram of a filter. The two-stage cascade connection vertical-coupling dual-mode SAW filter shown in this figure includes a piezoelectric substrate 2 formed in a thin plate shape and a front-stage vertical coupling two formed on the upper side (upper side in the figure) of the piezoelectric substrate 2. A heavy mode SAW filter 3, a first terminal 5 connected to the front side longitudinally coupled dual mode SAW filter 3 by a wiring 4, and a rear side formed on the lower side (lower side in the figure) of the piezoelectric substrate 2. A longitudinally coupled dual mode SAW filter 6, a second terminal 8 connected to the latter-stage longitudinally coupled dual mode SAW filter 6 by a wiring 7, and a filter connecting portion 9 formed in a central portion of the piezoelectric substrate 2. An elastic surface excited by a signal applied between the first terminal 5 and the second terminal 8 by the front-stage side longitudinally coupled dual-mode SAW filter 3 and the rear-stage longitudinally coupled double-mode SAW filter 6. wave The signals are filtered by leaking a part of the vibrational energy of the higher-order longitudinal mode and the higher-order transverse mode, respectively, and confining the surface acoustic wave in the IDT while suppressing spurious due to the higher-order mode. .

The front side longitudinally coupled dual-mode SAW filter 3 has a predetermined number of teeth, for example 60 pairs, whose crossover length is a predetermined wavelength multiple of the signal frequency to be selected, for example, 15λ. It is composed of two comb-teeth-shaped metal strips, and is arranged on the central upper side (in the figure, upper central side) of the piezoelectric substrate 2, and a signal is input between the first terminal 5 and the second terminal 8. At this time, a central IDT 10 that is excited to generate or receive a surface acoustic wave and a predetermined pair of teeth, for example, 40 pairs of teeth having a crossover length of a predetermined wavelength times a signal frequency to be selected, for example, 15λ are formed. Two outer IDTs 1 each of which is composed of two comb-tooth-shaped metal strips and is arranged on the piezoelectric substrate 2 outside the central IDT 10 (in the propagation direction of surface acoustic waves).
1, 12 and a predetermined pair, for example, 40 or less metal strips formed in a length of a predetermined wavelength times the signal frequency to be selected, for example, 15λ, and the central portion of each of these metal strips are electrically connected. On the piezoelectric substrate 2 outside the outer IDTs 11 and 12 (propagation direction of surface acoustic waves).
And two reflectors 13 and 14 arranged at the same position.

Then, when a signal is input between the first terminal 5 and the second terminal 8, the latter-stage longitudinally coupled dual mode S electrically connected by the filter connecting portion 9.
The central IDT 10 and the central IDT 10 are operated in cooperation with the AW filter 6 so as to leak a part of vibrational energy of the higher-order longitudinal mode and higher-order transverse mode with respect to a predetermined frequency component in the signal and suppress spurious due to the higher-order mode. The signals of the first and third modes of the predetermined frequency component are confined in the outer IDTs 11 and 12 to filter the signal.

Also, the latter-stage vertical-coupling dual-mode SAW
The filter 6 is composed of two comb-teeth-shaped metal strips having a predetermined pair of teeth, for example 60 pairs of teeth, each having a crossover length of a predetermined wavelength times the signal frequency to be selected, for example, a crossover length of 15λ. The piezoelectric substrate 2 is arranged on the lower central side (lower central side in the figure) and when a signal is input between the first terminal 5 and the second terminal 8, it is excited to generate or receive a surface acoustic wave. A central IDT 15 and two comb-teeth-shaped metal strips having a predetermined pair, for example 40 pairs of teeth, formed to have a crossover length of a predetermined wavelength times the signal frequency to be selected, for example, 15λ. On the piezoelectric substrate 2, the two outer IDTs 16 and 17 arranged outside the central IDT 15 (in the propagation direction of the surface acoustic wave) and the length thereof is a predetermined wavelength times the signal frequency to be selected, for example, 1 A predetermined pair, for example, 40 or less metal strips formed in a length of 5λ and a metal strip electrically connecting the central portions of these metal strips, and on the piezoelectric substrate 2, the outer sides The two reflectors 18 and 19 are provided outside the IDTs 16 and 17 (in the propagation direction of the surface acoustic wave).

When a signal is input between the first terminal 5 and the second terminal 8, the front-stage side longitudinally coupled dual mode S electrically connected by the filter connecting portion 9.
In cooperation with the AW filter 3, a part of the vibration energy of the higher-order longitudinal mode and the higher-order transverse mode with respect to a predetermined frequency component in the signal is leaked, and spurious due to the higher-order mode is suppressed, while the central IDT 15 and The signals of the first and third modes of the predetermined frequency component are confined in the outer IDTs 16 and 17 to filter the signal.

Further, the filter connecting portion 9 is formed in the central portion on the piezoelectric substrate 2 and each outer IDT 11, 12 constituting the front side longitudinally coupled dual mode SAW filter 3.
Two metal strips 20 and 21 for respectively connecting the comb-teeth metal strips of No. 1 and the comb-teeth metal strips of the outer IDTs 16 and 17 constituting the latter-stage side longitudinally coupled dual-mode SAW filter 6 and the piezoelectric element. A metal strip 22 formed in the central portion of the substrate 2 and connecting the central portions of the metal strips 20 and 21;
Each metal strip 20, 21, 22 constitutes the front side longitudinally coupled dual mode SAW filter 3 and each outer side I.
The DTs 11 and 12 and the outer IDTs 16 and 17 constituting the latter-stage side longitudinally coupled dual mode SAW filter 6 are respectively,
These are vertically connected dual mode SA connected electrically before
The W filter 3 and the post-stage vertical-coupling dual-mode SAW filter 6 are operated in cooperation with each other to filter the signal.

In this case, in the two-stage cascade connection longitudinally coupled double mode SAW filter according to this embodiment, the reflectors 13 and 1 constituting the preceding stage vertically coupled dual mode SAW filter 3 are arranged.
The number of metal strips of 4 is 40 or less,
The number of metal strips of each of the reflectors 18 and 19 forming the rear side longitudinally coupled dual mode SAW filter 6 is set to 40 or less, thereby leaking a part of the vibration energy of the higher order longitudinal mode and the higher order transverse mode, Suppresses spurious due to higher modes.

For example, a 45 ° X-cut, Z-propagating lithium tetraborate single crystal plate is used as the piezoelectric substrate 2, and 60 pairs of comb-shaped metal having a crossover length of 15λ in the central portion of the piezoelectric substrate 2. Two central IDT1s composed of strips
Four outer IDTs 11, 12, 1 formed of 40 pairs of comb-like metal strips forming 0, 15 and on each side of each of these central IDTs 10, 15 with a crossover length of 15λ.
6, 17 to form each of these outer IDTs 11, 12, 1
Reflectors 13, 14, 1 formed on the outside of 6, 17
The number of metal strips of 8 and 19 is 40 and 30 respectively.
When two, two, ten, and zero two-stage cascade connection longitudinally coupled dual-mode SAW filters were actually created and the filter characteristics were measured, the filter characteristics shown in FIGS. 2 to 6 were obtained. I was able to.

As is clear from the filter characteristics of the two-stage cascade connection longitudinally coupled dual mode SAW filters shown in FIGS. 2 to 6, the number of metal strips forming each of the reflectors 13, 14, 18, 19 is determined. By reducing the number to less than 40, it is possible to suppress spurious due to the high-order longitudinal mode and the high-order transverse mode that appear in the attenuation region of the filter characteristic.

Further, each of the above-mentioned two-stage cascade connection longitudinally coupled dual mode SAW filters, that is, each of the reflectors 13 and 14,
The number of metal strips of 18 and 19 is 40, 30,
The pass band characteristics and group delay time (Delay) characteristics of each of the two-stage cascade-connected longitudinally coupled dual-mode SAW filters each having 20 pieces, 10 pieces, and 0 pieces were measured, and are shown in FIGS. 7 to 11, respectively. It was possible to obtain pass band characteristics and group delay time characteristics.

As is clear from the pass band characteristics and the group delay time characteristics shown in FIGS. 7 to 11, the conventional S
In the case of an AW device, that is, a SAW device in which each reflector comprises 60 metal strips, the deviation of the group delay time cannot be 1 μs or less, whereas the reflectors 13, 14, 18, 19 are In each two-stage cascade connection longitudinally coupled dual mode SAW filter according to the present invention in which the number of metal strips to be configured is less than 40, the group delay time deviation within the pass band can be set to 1 μs or less.

However, as is clear from the filter characteristics shown in FIGS. 2 to 6, the respective reflectors 13, 14, 18, 19 are shown.
As the number of the metal strips constituting the element is smaller, spurious can be suppressed and the out-of-band attenuation amount can be improved, but at the same time, the Q value of the resonator deteriorates, and the insertion loss tends to deteriorate. For each reflector 1
It is desirable to set the number of metal strips forming 3, 14, 18, and 19 to 10 or more.

As described above, in this embodiment, the respective reflectors 1 constituting the front-stage side longitudinally coupled double-mode SAW filter 3 and the rear-stage side longitudinally coupled double-mode SAW filter 6 are formed.
The number of metal strips 3, 14, 18 and 19 is set to 10 to 40, and these front side longitudinally coupled dual mode SAWs are used.
The high-order longitudinal mode and the high-order transverse mode of the surface acoustic wave excited by the signal applied between the first terminal 5 and the second terminal 8 by the filter 3 and the post-stage longitudinally coupled dual-mode SAW filter 6, respectively. Since the surface acoustic wave is confined in the IDT while leaking a part of the vibration energy and suppressing the spurious due to the higher-order mode, the Q value necessary for the resonator is obtained. While suppressing degradation of insertion loss within a practically permissible range, spurious due to higher-order longitudinal modes and higher-order transverse modes can be suppressed, and out-of-band attenuation can be improved. The delay time deviation can be reduced.

FIG. 12 is a structural diagram of a SAW resonator showing another embodiment of the surface acoustic wave device according to the present invention. The SAW resonator shown in this figure is a thin plate-shaped piezoelectric substrate 50 composed of a 45 ° X-cut, Z-propagation lithium tetraborate single crystal plate and the like, and is arranged outside this piezoelectric substrate 50.
First and second terminals 51, 5 for fetching signals to be resonated
2 and two comb-shaped metal strips having a predetermined pair, for example 60 pairs of teeth, formed to have a cross length of a predetermined wavelength times the signal frequency to be selected, for example, 15λ. An IDT 53 which is arranged in the central portion (the central portion in the drawing) of the substrate 50 and which is excited when a signal is input between the first terminal 51 and the second terminal 52 to generate or receive a surface acoustic wave. Electrically connecting a predetermined pair, for example, 10 to 40 metal strips and a central portion of each of these metal strips, the length of which is a predetermined wavelength multiple of the signal frequency to be selected, for example, 15λ. Two reflectors 54 and 55, which are formed by connecting metal strips and are arranged outside the IDT 53 (in the propagation direction of surface acoustic waves) on the piezoelectric substrate 50, are provided. You.

When a signal including a signal component to be resonated is input between the first and second terminals 51 and 52, the IDT 53 is driven by the signal applied between the first terminal 51 and the second terminal 52. It is excited to generate a surface acoustic wave, and at the same time, the reflectors 54 and 55 cause a part of the vibrational energy of the higher-order longitudinal mode and the higher-order transverse mode of the surface acoustic wave to leak to prevent spurious due to the higher-order mode. While suppressing, the vibration energy is confined in the IDT 53, and the resonance operation for the signal is performed.

As described above, also in this embodiment, the number of metal strips constituting each reflector 54, 55 is 40.
Since the number is equal to or less than this, as in the above-described embodiment, the Q value required for the resonator is maintained and the deterioration of the insertion loss is maintained within a practically allowable range while the high order longitudinal mode and the high order transverse mode are used. Spurious can be suppressed and out-of-band attenuation can be improved.

Further, in each of the above-mentioned embodiments, the surface acoustic wave device according to the present invention has been described by taking the two-stage cascade connection longitudinally coupled dual mode SAW filter and SAW resonator as examples. Are not limited to these, and other devices utilizing surface acoustic waves, such as 1
Suppression due to higher-order transverse modes and higher-order longitudinal modes is suppressed even when applied to longitudinally coupled dual-mode SAW filters other than the third-third order, laterally coupled dual-mode SAW filters, SAW resonators, SAW resonator filters, etc. Has the same effect as

[0029]

As described above, according to the present invention, when the lithium tetraborate single crystal piezoelectric substrate is used, spurious due to the higher-order longitudinal mode and the higher-order transverse mode are suppressed, and the group delay time deviation is suppressed. It exerts a remarkable effect in realizing a small number of surface acoustic wave devices.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a two-stage cascade connection longitudinally coupled dual mode SAW filter showing an embodiment of a surface acoustic wave device according to the present invention.

2 is a two-stage cascade connection vertical coupling dual mode SA shown in FIG.
In the W filter, it is a graph showing an example of filter characteristics when the number of metal strips forming each reflector is 40.

FIG. 3 is a two-stage cascade connection vertical coupling dual mode SA shown in FIG.
In the W filter, it is a graph showing an example of filter characteristics when the number of metal strips forming each reflector is 30.

4 is a two-stage cascade connection vertical coupling dual mode SA shown in FIG.
In the W filter, it is a graph showing an example of filter characteristics when the number of metal strips forming each reflector is 20.

5 is a two-stage cascade connection vertical coupling dual mode SA shown in FIG.
In the W filter, it is a graph showing an example of filter characteristics when the number of metal strips forming each reflector is 10.

6 is a two-stage cascade connection vertical coupling dual mode SA shown in FIG.
In the W filter, it is a graph showing an example of filter characteristics when the number of metal strips forming each reflector is zero.

7 is a two-stage cascade connection vertical coupling dual mode SA shown in FIG.
7 is a graph showing pass band characteristics and group delay time characteristics when the number of metal strips forming each reflector is set to 40 in the W filter.

8 is a two-stage cascade connection vertical coupling dual mode SA shown in FIG.
7 is a graph showing pass band characteristics and group delay time characteristics when the number of metal strips forming each reflector is 30 in the W filter.

9 is a two-stage cascade connection vertical coupling dual mode SA shown in FIG.
7 is a graph showing pass band characteristics and group delay time characteristics when the number of metal strips forming each reflector is 20 in the W filter.

10 is a two-stage cascade connection vertical coupling dual mode S shown in FIG.
7 is a graph showing pass band characteristics and group delay time characteristics when the number of metal strips forming each reflector is 10 in an AW filter.

11 is a two-stage cascade connection vertical coupling dual mode S shown in FIG.
7 is a graph showing pass band characteristics and group delay time characteristics when the number of metal strips forming each reflector is set to 0 in the AW filter.

FIG. 12 is a configuration diagram of a SAW resonator showing another embodiment of the surface acoustic wave device according to the present invention.

FIG. 13 is a configuration diagram showing an example of a conventionally known SAW resonator.

14 is a schematic diagram showing an example of a basic longitudinal mode, a high-order longitudinal mode, a basic transverse mode, and a high-order transverse mode generated in the SAW resonator shown in FIG.

FIG. 15 is a configuration diagram showing an example of a conventionally known two-stage cascade connection vertical-coupling dual-mode SAW filter.

16 is a graph showing an example of filter characteristics of the two-stage cascade connection vertical coupling dual mode SAW filter shown in FIG.

[Explanation of symbols]

2 piezoelectric substrate, 3 front-stage side longitudinally coupled dual-mode SAW filter, 4 wiring, 5 first terminal, 6 rear-stage side longitudinally coupled dual-mode SAW filter, 7 wiring, 8 second terminal, 9 filter connecting portion, 10 central IDT , 1
1, 12 outer IDT, 13, 14 reflector, 15
Central IDT, 16, 17 Outer IDT, 18, 19
Reflector, 21, 21, 22 metal strip, 5
0 piezoelectric substrate, 51 first terminal, 52 second terminal,
53 IDT, 54, 55 reflector.

Claims (3)

[Claims] 1. At least one interdigital transducer (IDT) electrode is arranged on a piezoelectric substrate, and reflectors are further provided on both sides of the electrode, and an excited surface acoustic wave (SA) is generated.
In the surface acoustic wave device for substantially confining the vibration energy of W) in the IDT, the number of periodic metal strips constituting the reflector is 4
A surface acoustic wave device characterized in that the number is 0 or less. 2. The surface acoustic wave device according to claim 1, wherein the number of periodic metal strips forming the reflector is one.
A surface acoustic wave device characterized in that the number is 0 or more. 3. The surface acoustic wave device according to claim 1, wherein a lithium tetraborate single crystal piezoelectric substrate is used as the piezoelectric substrate.