JPH0767060B2 - Single-mode two-terminal pair surface acoustic wave resonator and surface acoustic wave filter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a surface acoustic wave resonator, and more particularly to an energy trap type two-terminal surface acoustic wave resonator and a surface acoustic wave filter.

(Prior Art and Its Problems) The energy trap type two-terminal pair surface acoustic wave resonator is normally arranged on the surface acoustic wave propagation path on the surface of the piezoelectric substrate 1 for propagating the surface acoustic wave as shown in FIG. 1 (a). Two interdigital transducers (Interdigit) that convert electric signals into surface acoustic waves
al Transducer; hereinafter abbreviated as IDT), that is, IDT (2
A) and IDT (2B) are arranged in series and both sides of the IDT
The grating (lattice) reflector 3 having a periodic structure for reflecting the surface acoustic wave excited from (2A) is arranged. The input terminal 4 and the ground terminal E, and the output terminal 5 and the ground terminal E'constitute a two-terminal pair.

The intermediate electrode of the grating reflector 3 is not shown in FIG. 1 and the subsequent drawings. The number of electrode pairs of the IDTs (2A) and (2B) is arbitrarily set, but in the case of FIG. 1 (a), each pair is represented by two pairs, and the number of electrode pairs is also shown in the drawings after FIG. Is not specified but is shown as a representative example.

The material of the piezoelectric substrate 1 is usually lithium tantalate (Li T
Piezoelectric single crystal substrates such as aO ₃ ), lithium niobate (LiNbO ₃ ), and quartz are used, and aluminum (Al) and gold (Au) are used as the electrode materials for the IDTs (2A), (2B) and the grating reflector 3. Etc. are used. In addition, illustration of the piezoelectric substrate 1 is omitted from FIG.

In the conventional two-terminal-pair surface acoustic wave resonator shown in FIG. 1 (a), IDTs (2A) and (2B) are arranged in series on the surface acoustic wave propagation path (shown by an arrow). Therefore, the surface wave excited by the IDT (2A) is directly received by the IDT (2B) without passing through the reflector 3.
It exhibits a so-called transversal filter characteristic and is superimposed on the pass characteristic of the resonator. When a filter is constructed with such resonators, the IDT (2A) and the IDT (2B) are connected in series on the propagation path of the surface wave, and the coupling between the IDTs causes deterioration of the out-of-band attenuation.

FIG. 1 (b) shows the energy distribution of the longitudinal vibration mode of the resonator having the electrode configuration of FIG. 1 (a), where 6 is the basic (0th order) longitudinal mode (symmetric mode (S ₀ )) Shows the displacement distribution of vibration, and 7 shows the displacement distribution of vibration in the anharmonic first-order longitudinal mode (antisymmetric mode (a ₀ )). In FIGS. 1A and 1B, if the capacitance ratio γ of the resonator (a value proportional to the reciprocal of the difference between the resonance frequency and the anti-resonance frequency) is reduced, the pass band width becomes wider when the filter is configured. It is known to be possible. If IDT (2A) and (2B) and the number of electrode pairs are increased in order to reduce the capacitance ratio γ of the resonator, the resonator length becomes longer, so IDT (2A) and (2B) are coupled to this mode and spurious There was a drawback.

In order to improve this drawback, as shown in FIG.
Two IDTs (2C), (2
D) is arranged in parallel with the surface acoustic wave propagation direction 2
The electrode configuration of the terminal-pair surface acoustic wave resonator is proposed.
(2 terminal pair surface acoustic wave resonator filed on February 14, 1991) FIG. 2 (b) shows a longitudinal vibration mode, and FIG. 2 (c) shows a lateral direction (a right angle to the propagation direction of the surface wave). 3 types of vibration modes 9, 10, and 11) are shown. Reference numeral 9 is a basic (0th) transverse mode, 10 is a primary transverse mode, and 11 is a secondary transverse mode.

According to the electrode configuration of FIG. 2 (a), two IDTs (2C),
Since (2D) is not placed in series on the surface acoustic wave propagation path, it does not exhibit the characteristics of a so-called transversal filter, and one ID in the surface acoustic wave propagation direction.
Since T corresponds to being arranged near the center between the reflectors, it is not coupled with the primary longitudinal mode corresponding to 7 in FIG. 1 (b). That is, there is an advantage that spurious due to the primary longitudinal mode does not appear.

However, the reflector 3 forms a surface acoustic wave waveguide, and if the width 8 (guide width) of the reflector 3 becomes large, the
As shown in FIG. 7C, higher-order transverse modes 10 and 11 have appeared, and there is a problem in which spurious is generated by coupling with IDTs (2C) and (2D).

Normally, the sum of the crossing widths (or crossing finger lengths) of IDTs (2C) and (2D) is formed to be approximately the same as the guide width 8 of the reflector 3, so the guide width 8 is the IDT unless otherwise specified below. (2
It is treated as almost equal to the sum of the intersection widths of C) and (2D).

FIG. 4 is an example of input impedance characteristics of the resonator having the electrode configuration shown in FIG. Crossing width =
160 μm (= 10λ ₀ ), 50 pairs of IDTs (2A) and (2B) each, electrode pitch 8 μm, number of reflector gratings 300
This is an actual measurement example of the input impedance in the case of the X-112 ° Y LiTaO ₃ substrate. That is, when the guide width = 10λ ₀ , the fourth mode by the basic (0th) transverse mode 9 in FIG.
It can be seen that there are two modes, the characteristic b in the figure and the characteristic a in FIG. 4 due to the primary transverse mode 10 in FIG. 2 (c).

Further, FIG. 9 is a characteristic diagram showing the relationship between the guide width 8 and the Q of the fundamental mode of the resonator, and FIG. 10 shows the relationship between the guide width 8 and the resonance frequencies of the transverse modes 9, 10, and 11. It is a characteristic diagram. As can be seen from FIG. 10, the guide width 8 in which the primary transverse mode 10 appears is about 6λ ₀ , and the guide width 8 in which the secondary transverse mode 11 appears is about 20λ ₀ . The appearance of the higher-order transverse mode shows a slight film thickness dependence. However, in the electrode configuration shown in FIG. 2 (a), when the guide width 8 is 6λ ₀ or more, it is always the first order or the first order and the second order. Lateral mode spurious will be mixed. In order to allow only the desired zeroth-order (basic) transverse mode 9 to exist, the guide width 8 is set to several λ ₀ or less (X−112 ° Y LiTaO ₃
In the case of, it should be 6λ ₀ or less).

However, as shown in FIG. 9, when the guide width 8 is narrowed, the surface wave propagating through the grating reflector 3 tends to have a reduced Q because the loss due to diffraction increases. When a filter is constructed with a resonator having a reduced Q, there is a drawback that the insertion loss increases.

The impedance of the resonator is determined by the capacitance determined by the IDT logarithm and the cross width. Therefore,
If the crossing width is limited to several λ ₀ or less, there is a drawback that the degree of freedom in impedance design is limited.

Next, the surface acoustic wave filter will be described.

When configuring a filter with a surface acoustic wave resonator, a method is known in which a plurality of single mode two-terminal pair surface acoustic wave resonators are connected in cascade. FIG. 6 is a configuration example of a dual mode resonator filter in which two single mode resonators having the same structure are connected in cascade. That is, the two-terminal pair surface acoustic wave resonator having the conventional electrode configuration shown in FIG. 1 has the surface acoustic wave propagation directions parallel to each other on the same substrate 1 (not shown), and is influenced by acoustic coupling with each other. They are arranged side by side and are electrically connected in cascade at negligible intervals.

As described above regarding the electrode configuration of the resonator in FIG.
Since T (2A) and IDT (2B) are arranged in series on the surface acoustic wave propagation path, the characteristics of a so-called transversal filter are superimposed and the out-of-band characteristics of the filter deteriorate. There was a drawback that spurious due to the next vertical mode appeared.

(Object of the Invention) The object of the present invention is to increase the guide width 8 to some extent in order to avoid a decrease in Q, at the same time increase the degree of freedom in impedance design, and to prevent spurious in the primary longitudinal mode and spurious in the primary transverse mode. It is to provide a single-mode two-terminal pair surface acoustic wave resonator having an electrode configuration that does not appear. Further, in a filter formed by connecting a plurality of two-terminal-pair resonators in cascade, insertion loss does not increase, the degree of freedom in impedance design is increased, and spurious in primary longitudinal mode and primary transverse mode are achieved. Another object of the present invention is to provide a surface acoustic wave filter having no spurious effect.

(Structure and Action of the Invention) First, the surface acoustic wave resonator will be described.

A single-mode two-terminal pair surface acoustic wave resonator according to claim (1) of the present invention has two grating reflectors between two opposing grating reflectors on a piezoelectric substrate. In common, three interdigital transducers are arranged in parallel at right angles to the propagation direction of the surface acoustic wave with respect to the center line of the width of the grating reflector. A single-mode two-terminal surface acoustic wave resonator having a comb-shaped transducer as an input terminal pair, which is electrically connected to the comb-shaped transducers on both sides to form an output terminal pair, in which the three comb-shaped transducers intersect each other. The sum of the widths and the guide width of the grating reflector, which is approximately equal to the sum of the intersecting widths, are set to about 20λ ₀ (λ ₀ = wavelength of the surface acoustic wave) immediately before the vibration of the second-order transverse mode among the transverse modes appears. It is characterized by increasing to Is shall.

Further, the single mode two-terminal pair surface acoustic wave resonator according to claim (3) of the present invention is provided with two grating reflectors between two opposing grating reflectors on a piezoelectric substrate. The four interdigital transducers are commonly arranged in parallel at right angles to the propagation direction of the surface acoustic wave with respect to the center line of the width of the grating reflector.
Of the two interdigital transducers, two inner interdigital transducers and an outer interdigital transducer that are symmetrical with respect to the center line are electrically connected in parallel, and one of them serves as an input terminal pair and the other interdigital transducer. A single-mode two-terminal-pair surface acoustic wave resonator configured so as to form an output terminal pair, the grating reflector having a sum of cross widths of the four interdigital transducers and substantially equal to the sum of the cross widths. And the guide width is increased up to about 20λ ₀ (λ ₀ = wavelength of surface acoustic wave) immediately before the vibration of the secondary transverse mode of the transverse modes appears. ) The following 2 according to the present invention
The terminal pair surface acoustic wave resonator will be described in detail with reference to the drawings.

FIG. 3 (a) is a first embodiment described in the claims (3) of the present invention, and is an electrode configuration diagram of a two-terminal pair surface acoustic wave resonator when four IDTs are used. Is. Third
Figures (b) and (c) show vertical and horizontal displacement distributions, respectively. Four grating reflectors 3 are arranged between the two grating reflectors 3 in parallel with the surface acoustic wave propagation direction and symmetrically with respect to the center line of the width (guide width) 8 of the grating reflector 3. The IDTs (2E, 2F, 2G, 2H) may have independent electrode configurations, but in the present embodiment, two IDTs (2E and 2F and 2G and 2H) and the ground side electrode are commonly used. , And two IDTs (2F and 2
G and 2E and 2H) are electrically connected input IDTs,
This is a configuration with an output IDT. The input terminal 14 and the ground terminal E, and the output terminal 15 and the ground terminal E'constitute a two-terminal pair.

FIG. 8 is a second embodiment of the present invention as set forth in claim 1 and is an electrode configuration diagram of a two-terminal pair surface acoustic wave resonator when three IDTs are used. In the first embodiment shown in FIG. 3 (a), IDT (2F) and IDT (2
G) is electrically connected by sharing a part of the electrode, but in this second embodiment, IDT (2F) and IDT (2G) are merged to form one IDT (2I). In addition, three IDTs (2E, 2I, 2) with the ground side electrodes of IDT (2I) and (2H) in common
It is clear that the electrode configuration of H) is used and the electrical characteristics are the same as in the case of the first embodiment. The first embodiment of FIG. 3 will be described below.

In the electrode configuration of FIG. 3 (a), each IDT (2E) (2F) ((2G) (2H) is anharmonic as in the case of FIG. 2 (a).
It is not coupled with the secondary longitudinal mode, and therefore it is obvious that the characteristic degradation due to the spurious due to the primary longitudinal mode and the characteristic degradation due to the superposition of the transversal filter characteristic, which are seen in the conventional configuration, do not occur.

Further, in the electrode configuration of FIG. 3 (a), the IDTs are arranged symmetrically with respect to the center line of the surface acoustic wave waveguide in the direction perpendicular to the surface acoustic wave propagation direction, and Since the two IDTs at different positions are electrically connected to each other, the input / output IDT is not coupled to the primary transverse mode 10.

FIG. 5 is an example of input impedance characteristics of the resonator having the configuration shown in FIG. 3 (a), using an X-112 ° Y LiTaO ₃ substrate,
DT 50 pairs, 300 reflector electrodes, guide width 8 10
It is an actual measurement value when λ ₀ is set. As is apparent from this figure, the spurious a of the first-order transverse mode shown in FIG. 4 is suppressed cleanly, and only the characteristic b by the basic (0th-order) transverse mode 9 is obtained.

Therefore, as can be seen from FIG. 10, in the case of realizing a single-mode two-terminal pair surface acoustic wave resonator, in the configuration of FIG. 2 (a), the guide width 8 is set just before the primary transverse mode 10 appears (about 6λ). ₀ ) or less, but with the configuration of FIG. 3 (a), it can be expanded to just before the appearance of the secondary transverse mode 11 (and 20λ ₀ ), and the Q of the resonator is increased. be able to.

Next, the surface acoustic wave filter will be described.

FIG. 7 shows an example of the electrode configuration of the surface acoustic wave filter of the present invention. The two single mode two-terminal pair surface acoustic wave resonators of the first embodiment of the present invention shown in FIG. 3 are the same. The surface acoustic wave propagation directions are parallel to each other on the substrate 1 (not shown), and they are arranged in parallel at intervals such that the influence of acoustic coupling can be ignored, and are electrically connected in cascade. Of course, it is also effective to configure the two two-terminal pair surface acoustic wave resonators of the second embodiment of the present invention shown in FIG. 8 in the same manner.

In the embodiment shown in FIG. 7, two resonators are used, but it is obvious that the number of resonators may be increased in order to further improve the damping characteristic of the filter.

A feature of the filter according to this configuration is that the single mode two-terminal pair surface acoustic wave resonator according to the present invention is used as the constituent resonator. Therefore, the advantage of the resonator becomes the advantage of the filter as it is.

That is, as compared with the conventional configuration of FIG. 6, in the configuration of the present invention, the primary longitudinal mode that deteriorates the characteristics does not appear, so
Since the number of electrode pairs of one IDT (2E to 2H) is increased until just before the second-order longitudinal mode appears, the capacitance ratio of the resonator can be reduced, so that the pass band width of the filter can be widened. Further, since the IDTs are not arranged in series on the surface acoustic wave propagation path, the desired out-of-band attenuation can be secured without the transversal filter characteristics of the input / output IDTs being superimposed. Further, even if the guide width of the IDT is lengthened just before the appearance of the secondary transverse mode, spurious due to the primary transverse mode does not appear, so that the Q of the resonator is suppressed from being lowered and the insertion loss of the filter is prevented from increasing. Furthermore, considering the fact that the IDT logarithm can be increased, the degree of freedom in impedance design can be increased.

(Effects of the Invention) As described in detail above, the single-mode two-terminal pair surface acoustic wave resonator according to the present invention suppresses the primary longitudinal mode and the primary transverse mode that cause spurious and characteristic deterioration. Single mode, that is, both vertical and horizontal modes are basic (0
The effective utilization range of the following) mode is expanded at the same time extends from a few lambda ₀ the guide width as a single-mode resonator to 10 several lambda ₀ can be improved Q, the degree of freedom of impedance design Has a remarkable effect of increasing.

Further, the surface acoustic wave filter according to the present invention is a filter that has no spurious and characteristic deterioration due to the first-order longitudinal mode and the first-order transverse mode, has a small insertion loss, has a wide bandwidth and has a high degree of freedom in impedance design. It has the remarkable effect of being configurable.

[Brief description of drawings]

FIG. 1 is a diagram showing an electrode configuration example of a conventional 2-terminal pair surface acoustic wave resonator, FIG. 2 is a diagram showing an electrode configuration example of the previously proposed 2-terminal pair surface acoustic wave resonator, and FIG. FIG. 4 shows a second example of the electrode configuration of the first embodiment of the terminal-paired surface acoustic wave resonator.
FIG. 6 is a diagram showing an example of the input impedance characteristic of the two-terminal pair surface acoustic wave resonator, and FIG. 5 is a diagram showing an example of the input impedance characteristic of the two-terminal pair surface acoustic wave resonator according to the configuration of FIG. 3 of the present invention.
FIG. 7 is a diagram showing an example of an electrode structure of a conventional dual mode surface acoustic wave filter, FIG. 7 is a diagram showing an example of electrode structure of a double mode surface acoustic wave filter of the present invention, and FIG. 8 is a second example of a resonator according to the present invention. FIG. 9 is a characteristic diagram showing the relationship between the guide width and Q, and FIG. 10 is a characteristic diagram showing the relationship between the guide width and the resonance frequency of the transverse mode. 1 …… Piezoelectric substrate, 2A, 2B, 2C, 2D, 2E, 2F, 2G, 2H, 2I …… ID
T, 3 …… Grating reflector, 4,12,14 …… Input terminal, 5,13,15 …… Output terminal, 6,7 …… Vibration displacement distribution in longitudinal mode, 8 …… Guide width, 9,10 , 11 …… Transverse mode vibration displacement distribution.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Shimizu 1-1-1, Kojima-cho, Chofu-shi, Tokyo (72) Inventor Yuji Suzuki 1-7-14, Kutagawa, Kofu-shi, Yamanashi (56) References 63-285018 (JP, A)

Claims (4)

[Claims] Claim: What is claimed is: 1. On a piezoelectric substrate, three interdigital transducers sharing two grating reflectors between two grating reflectors facing each other, wherein three interdigital transducers are perpendicular to the propagation direction of the surface acoustic wave. Of the three interdigital transducers are arranged in parallel symmetrically with respect to the center line of the width of the intermediary device, and the interdigital transducers in the center are used as input terminal pairs to electrically connect the interdigital transducers on both sides to output. In a single-mode two-terminal-pair surface acoustic wave resonator as a terminal pair, the sum of the cross widths of the three interdigital transducers and the guide width of the grating reflector substantially equal to the sum of the cross widths are set to A single-mode two-terminal-pair surface acoustic wave resonator characterized in that it is increased to about 20λ ₀ (λ ₀ = wavelength of surface acoustic wave) just before vibration of the second-order transverse mode among transverse modes. 2. A plurality of single-mode two-terminal-pair surface acoustic wave resonators according to claim 1, the propagation directions of surface acoustic waves are all parallel on the same piezoelectric substrate, and the influence of acoustic coupling is neglected. A surface acoustic wave filter characterized in that the surface acoustic wave filters are arranged side by side at an interval as much as possible and are electrically connected in cascade. 3. On a piezoelectric substrate, four interdigital transducers sharing two grating reflectors between two opposing grating reflectors are provided with four interdigital transducers at right angles to the propagation direction of surface acoustic waves. Of the four interdigital transducers arranged symmetrically with respect to the center line of the width of the vessel and symmetrical with respect to the central line of the four interdigital transducers and an outer interdigital transducer. In a single mode two-terminal-pair surface acoustic wave resonator, each of which is electrically connected in parallel and one of which is an input terminal pair and the other of which is an output terminal pair. The sum of the cross widths and the guide width of the grating reflector which is approximately equal to the sum of the cross widths are set to about 20λ ₀ (λ ₀ = the surface acoustic wave wavelength just before the occurrence of the second-order transverse mode vibration among the transverse modes. ) Up to Single-mode two-terminal-pair surface acoustic wave resonators, wherein the door. 4. A plurality of single-mode two-terminal pair surface acoustic wave resonators according to claim 3, the propagation directions of surface acoustic waves are all parallel on the same piezoelectric substrate, and the influence of acoustic coupling is neglected. A surface acoustic wave filter characterized in that the surface acoustic wave filters are arranged side by side at an interval as much as possible and are electrically connected in cascade.