JPH04278707A - Surface acoustic wave resonator - Google Patents

Abstract

PURPOSE:To present a surface acoustic wave resonator which does not enlarge the device size by statisfying conditions of >=0.3% specific band width, small intra-band ripple, low out-band spurious level, and large extent of out-band attenuation. CONSTITUTION:In the surface acoustic wave resonator where plural electrode structure arrays each of which has plural IDTs interposed between a pair of reflectors are cascaded on a piezoelectric substrate 10, grounded reflectors 24, 25, 34, and 35 having plural grounded electrodes are inserted between plural IDTs of each electrode structure array, and inter-digital electrodes of IDTs are connected to grounded reflectors and are grounded.

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 resonator in which a plurality of IDTs (interdigital transducers) are connected in series on a piezoelectric substrate, and a plurality of electrode structure rows are sandwiched between a pair of reflectors. .

0002

2. Description of the Related Art In recent years, it has been proposed to use surface acoustic wave devices as front-end filters for wireless communications using the VHF band and UHF band to make wireless devices smaller and lighter. The electrical characteristics required for a front-end filter are (a) low insertion loss, (b)
(c) low out-of-band spurious levels; (d) large out-of-band attenuation; and (e) adequate bandwidth to ensure the number of channels required for the communication system. It is required that all of these conditions be satisfied.

As a conventional surface acoustic wave resonator filter, there is a resonator type filter in which interdigital electrodes are sandwiched between a pair of reflectors. Although this surface acoustic wave resonator filter has a small insertion loss, it has a narrow fractional bandwidth of about 0.05%, and improvements are required to satisfy the above conditions. was. As a conventional surface acoustic wave resonator filter, lithium tantalate (L
Using iTaO3) substrate and crystal substrate, input ID
There is one in which a T and an output IDT are sandwiched between a pair of reflectors. A vertical dual-mode bandpass filter is constructed by utilizing the two modes of vertical zero-order and vertical first-order vibrations that occur in the resonator configured by these input IDTs and output IDTs, and the fractional bandwidth is A surface acoustic wave resonator filter is known that attempts to widen the
Publication No. 4).

However, in this vertical dual-mode bandpass filter, when a crystal substrate is used as the piezoelectric substrate, the fractional bandwidth is 0 even if the standardized film thickness of the electrode is increased to 4%.
．． There was a problem in that it could only achieve up to 3% and the in-band ripple was large. Furthermore, even when lithium tantalate is used as the piezoelectric substrate, if the normalized film thickness is increased to 4%, the fractional bandwidth can be achieved up to 0.47%;
There was a problem with large in-band ripple.

As described above, conventional surface acoustic wave resonator filters are required to have a fractional bandwidth of 0.3% or more, small in-band ripples, low out-of-band spurious levels, and large out-of-band attenuation. It was difficult to use it for applications such as front-end filters for wireless communications. on the piezoelectric substrate to reduce out-of-band spurious levels.
There is a surface acoustic wave resonator in which a plurality of electrode structure rows in which a plurality of IDTs are sandwiched between a pair of reflectors are connected in series. That is, as shown in FIG. 13, the IDT 51 is placed on the piezoelectric substrate 40.
, 52, 53 sandwiched between a pair of reflectors 54, 55, and an electrode structure column 60 with IDTs 61, 62, 63 sandwiched between a pair of reflectors 64, 65 are formed. Connection IDT 52 of column 50 and connection IDT
One of the 53 electrodes is connected to the IDT 6 for connection to the electrode structure row 60.
The electrode structure row 50 and the electrode structure row 60 are connected in cascade by connecting the electrode structure row 50 and the electrode structure row 60 to one of the electrodes of the connection IDT 63 and the connection IDT 63, respectively.

Input I located at the center of the electrode structure row 50
An input bonding pad 56 to which one of the electrodes of the input IDT 51 is connected is provided on the outside of the DT 51.
An output bonding pad 66 to which one of the electrodes of the output IDT 61 is connected is provided outside the output IDT 61 located at the center of the electrode structure row 60. Also,
Connection IDTs are provided on both sides of the input bonding pad 56.
Grounding bonding pads 57 and 58 are provided to ground the other electrode of the connection IDTs 61 and 52, and grounding bonding pads 57 and 58 are provided on both sides of the output bonding pad 66 to ground the other electrode of the connection IDTs 61 and 62. 67
, 68 are provided.

[0007] A signal source 44 is inserted between the input terminal IN and the ground, together with a 50Ω termination impedance 42 that is often used in the UHF band, and a 50Ω termination impedance 46 is also connected between the output terminal OUT and the ground. has been done. Further, in order to ground the other interdigital electrode of the input IDT 52 and the other interdigital electrode of the output IDT 62, grounding bonding pads 59 and 69 are provided between the electrode structure row 50 and the electrode structure row 60. It is being This is because thin wires cannot be routed for grounding. It is also conceivable to create a two-layer wiring structure without providing the grounding bonding pads 59 and 69, but this would increase the cost and is not practical.

[0008]

As described above, in the conventional surface acoustic wave resonator, the grounding bonding pads 59 and 69, which require a large surface area, are connected to the electrode structure rows 50 and 60.
Since it is necessary to provide the space between the two, the chip size becomes large, resulting in a problem that the surface acoustic wave resonator device becomes larger.

The object of the present invention is to satisfy the following conditions: the fractional bandwidth is 0.3% or more, the in-band ripple is small, the out-of-band spurious level is low, and the out-of-band attenuation is large, and the device size can be increased. The object of the present invention is to provide a surface acoustic wave resonator that does not cause problems.

0010

[Means for Solving the Problems] The inventors of the present application have studied various conditions required when using a surface acoustic wave resonator in applications such as front-end filters for wireless communications, and have determined how to satisfy these conditions. At the same time, it was possible to realize a surface acoustic wave resonator that can be cascade-connected without providing a grounding bonding pad between electrode structure rows.

[0011] In order to use a surface acoustic wave resonator in a front-end filter, as mentioned above, (a) the insertion loss is small, (b) the in-band ripple is small,
(c) Low out-of-band spurious level; (d)
It is necessary to satisfy strict electrical characteristics such as (e) having a large amount of out-of-band attenuation and (e) having an appropriate bandwidth to secure the required number of channels.

[0012] In order to satisfy many of these electrical characteristics, it is desirable that the surface acoustic wave resonator has a large degree of freedom in design. Therefore, first, a plurality of IDTs having interdigital electrodes are provided, these IDTs are sandwiched between a pair of reflectors to form one electrode structure row, and a plurality of such electrode structure rows are connected in cascade to generate a surface acoustic wave. We decided to configure a resonator.

[0013] Furthermore, an input IDT in the electrode structure row,
Instead of providing a grounding bonding pad to connect one of the electrodes of the output IDTs, a connection reflector having multiple electrode fingers connected to one electrode of these IDTs is provided between the IDTs. did. We considered ways to reduce in-band ripple. What is in-band ripple?
This refers to the difference between the maximum and minimum insertion loss within the passband.If there is an in-band ripple, the passing power within the band will be different, which is a problem when actually using a surface acoustic wave resonator filter. The size of the in-band ripple is a very important characteristic.

Therefore, as will be described later, the inventors manufactured surface acoustic wave resonators with various electrode thicknesses, various numbers of electrode pairs, and various numbers of connected reflector electrodes, and measured their passband characteristics. As a result, the total logarithm NT of multiple IDTs in each electrode structure row is α+β(h/λ)−γ+δNt ≦NT ≦α
+β(h/λ)+γ+δNt However, h/λ is the normalized film thickness obtained by normalizing the thickness h of the electrodes of the plurality of IDTs by the wavelength λ of the surface acoustic wave, Nt is the total electrode index of the connected reflector, α =125, β=-2000, γ=3
It was concluded that it is desirable that the value be within the range of 0 and δ = -0.65.

[0015] Also, among the plurality of IDTs, the logarithm N1 of the central IDT formed at the central position and the sum N2 + of the logarithms of the IDTs on both sides formed at positions sandwiching the central IDT.
The ratio NC (=(N2 +N3)/N1) of N3 is p+q(h/λ)-r+sNt ≦NC ≦p+
q(h/λ)+r+sNt However, p=1.1, q
It was concluded that it is desirable to be within the ranges of =20, r=0.8, and s=-0.03.

Regarding the aperture length, when the surface acoustic wave resonator is used as a front end filter, it is desirable that the terminal impedance is 50Ω, but depending on the case, it may be set to about 50 to 500Ω. At this time, in-band ripple and insertion loss can be reduced by adjusting the maximum aperture length to best match the termination impedance.

[0017]

[Operation] According to the present invention, since one of the electrodes of the input IDT and the output IDT of the cascade-connected electrode structure row is connected to a connection reflector having a plurality of electrode fingers, the electrode structure row The surface acoustic wave resonator device can be made smaller without providing a grounding bonding pad in between.

Further, according to the present invention, by setting the total logarithm of the IDT to the electrode thickness within a predetermined range, the in-band ripple can be reduced over a wide fractional bandwidth. Furthermore, by keeping the logarithmic ratio of IDT within a predetermined range,
In-band ripple can be further reduced.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the basic structure of a surface acoustic wave resonator according to the present invention. As shown in FIG. 1, the surface of the piezoelectric substrate 10 includes three IDTs, two grounded reflectors (grounded connection reflectors) inserted between the three IDTs, and three IDTs.
Two electrode structure rows 20 and 30 are formed by a pair of reflectors sandwiching a T and two ground reflectors.

That is, as the electrode structure row 20, an input ID having N1 pairs of interdigital electrodes in the center is used.
N4 is formed on both sides of this input IDT21.
Ground reflector 24 and N5 with two grounded electrodes
A grounded reflector 25 having two grounded electrodes is formed, and a connecting IDT 22 and N3 each having N2 pairs of interdigital type electrodes on each side of the grounded reflectors 24, 25.
Connection IDT 23 with a pair of interdigital electrodes
is formed. These input IDT 21, ground reflectors 24, 25, and connection IDTs 22, 23 are sandwiched between a pair of reflectors 26, 27 to form an electrode structure row 20.

As the electrode structure array 30, an output IDT 31 having N1 pairs of interdigital type electrodes is formed in the center, and a ground reflector 34 having N4 grounded electrodes and a ground reflector 34 having N5 pairs on both sides of the output IDT 31. A grounded reflector 35 having a grounded electrode is formed, and a connecting IDT 32 having N2 pairs of interdigital type electrodes and a connecting IDT 33 having N3 pairs of interdigital type electrodes are formed on both sides of the grounded reflectors 34 and 35. is formed. These output IDTs 31, ground reflectors 34, 3
5. Connecting IDTs 32 and 33 to a pair of reflectors 36 and 37
An electrode structure row 30 is formed by sandwiching the electrode structure rows 30 between them.

The input bonding pad 28 is formed outside the input IDT 21 of the electrode structure row 20 and is connected to the interdigital electrode outside the input IDT 21. Interdigital electrodes inside the input IDT are commonly connected to ground reflectors 24 and 25 on both sides. Grounding bonding pads 29a, 29b, 29c, and 29d are formed on both sides of the input bonding pad 28. The grounding bonding pad 29a is a connection ID.
The ground bonding pad 29c is connected to the interdigital electrode on the outside of T22, and the grounding bonding pad 29c is connected to the electrode of the grounding reflector 24. Grounding bonding pad 29b
is connected to the interdigital electrode on the outside of the connection IDT 23, and the grounding bonding pad 29d is connected to the electrode of the grounding reflector 25.

The output bonding pad 38 is formed outside the output IDT 31 of the electrode structure array 30 and is connected to the interdigital type electrode outside the output IDT 31. Interdigital electrodes inside the output IDT are commonly connected to ground reflectors 34 and 35 on both sides. Grounding bonding pads 39a, 39b, 39c, and 39d are formed on both sides of the output bonding pad 38. The grounding bonding pad 39a is a connection ID.
A grounding bonding pad 39 is connected to the electrode of the grounding reflector 34. The grounding bonding pad 39b is
The grounding bonding pad 39d is connected to an interdigital electrode on the outside of the connection IDT 33, and the grounding bonding pad 39d is connected to an electrode of the grounding reflector 35.

The interdigital type electrode inside the IDT 22 for connection of the electrode structure row 20 is connected to the IDT for connection of the electrode structure row 30.
By connecting the interdigital type electrode inside the DT 32 and connecting the interdigital type electrode inside the connecting IDT 23 of the electrode structure row 20 to the interdigital type electrode inside the connecting IDT 33 of the electrode structure row 30, the electrode The structure row 20 and the electrode structure row 30 are connected in cascade.

Note that the input IDT 21 of the electrode structure row 20
Although the interdigital electrodes inside the electrode structure array 30 and the interdigital electrodes inside the output IDT 31 of the electrode structure row 30 may be commonly connected, it is preferable in terms of characteristics that they be separated as in this embodiment. Between the input terminal IN and ground, there is a UHF
A signal source 14 is inserted together with a termination impedance 12 of 50Ω, which is often used in the band, and a termination impedance 16 of 50Ω is also connected between the output terminal OUT and ground.

In this embodiment, the piezoelectric substrate 10 has an angle of 45°.
Rotating X plate Z propagation Lithium tetraborate (Li2 B4 O7
) using a substrate. The propagation speed of surface acoustic waves is 3440 m
/second. In order to propagate the surface acoustic waves in the Z-axis direction of the piezoelectric substrate 10, the propagation direction of the surface acoustic waves in the electrode structure rows 20, 30 is set to the Z-axis of the piezoelectric substrate 10, which is a 45° rotated X-plate and Z-propagation lithium tetraborate substrate. direction.

[0027] IDT21, 22, 23 in this embodiment
, 31, 32, and 33 were formed of an aluminum layer or an aluminum alloy layer. However, since the lithium tetraborate substrate, which is a piezoelectric substrate, dissolves in an aluminum etching solution, the electrodes were formed by a lift-off method. IDT21, 22, 23
, 31, 32, and 33 are ultimately determined so that the image impedance of the filter matches 50Ω, but initially, through preliminary consideration, it is determined that the wavelength λ of the surface acoustic wave The normalized maximum crossing width W/λ was set to 170. Weighting for transverse mode suppression was done using general cosine apotization.

The reflectors of the ground reflectors 24, 25, 34, 35 and the reflectors 26, 27, 36, 37 formed gratings as metal strips (electrode fingers) made of an aluminum layer or an aluminum alloy. Reflector 26, 2
The number of metal strips 7, 36, and 37 was set to 100 each in consideration of the surface acoustic wave reflectance per metal strip.

In the surface acoustic wave resonator configured as described above, the normalized film thickness of the aluminum layer of the interdigital electrode (the film thickness h normalized by the wavelength λ of the surface acoustic wave) h/λ, Logarithm N1 of input IDT 21 or output IDT 31 and connection IDT 22, 23 or connection IDT
Logarithm ratio NC of sum N2 +N3 of logarithms of 32 and 33 (
=(N2 +N3)/N1) and IDT21-23
Or the total logarithm NT of 31 to 33 (=N1+N2 +N3
) and the total number of electrodes Nt of the ground reflectors 24, 25 or 34, 35 were varied, and the characteristics of the surface acoustic wave resonator filter were measured. The measurement results are shown in FIGS. 2 to 9.

FIGS. 2 and 3 show the normalized film thickness h/λ and the logarithmic ratio N.
It shows the change in in-band ripple when the total logarithm NT is changed while C is kept constant. In Figure 2, the normalized film thickness h/λ is 0.01, and the logarithmic ratio NC of IDT is 1.
．． 2 and the total number of electrodes Nt of the ground reflector is 20, and the change in the magnitude of the in-band ripple is shown when the total logarithm NT is changed from about 70 pairs to about 140 pairs. As is clear from Fig. 2, the in-band ripple is the total logarithm NT
is 1.5 dB or less in the range of about 80 to 110 pairs.

FIG. 3 shows the total logarithm NT when the normalized film thickness h/λ is 0.025, the logarithmic ratio NC of the IDT is 1.14, and the total number of electrodes Nt of the ground reflector is 20. This shows the change in the magnitude of the in-band ripple when changing from about 40 pairs to about 100 pairs. As is clear from FIG. 3, the in-band ripple is 1.5 dB or less when the total logarithm NT is in the range of approximately 60 to 90 pairs.

FIG. 4 shows the change in in-band ripple when the normalized film thickness h/λ and the total logarithm NT are kept constant and the logarithm ratio NC is varied. In other words, when the normalized film thickness h/λ is 0.025 and the total logarithm NT is 73.5 pairs, the magnitude of the in-band ripple when the logarithm ratio NC is varied from 0.2 to 2.8 is Figure 4 shows the change in height. As is clear from FIG. 4, the in-band ripple becomes 1.5 dB or less when the logarithmic ratio NC is in the range of about 0.6 to 1.8.

FIGS. 5 to 7 show the distribution of in-band ripples when the normalized film thickness h/λ is kept constant and the total logarithm NT and logarithmic ratio NC are varied. The points where the in-band ripple is 1.5 dB or less are indicated by ◎, the points where the in-band ripple is in the range of 1.5 to 3 dB are indicated by ○, and the points where in-band ripple is 3 dB or more are indicated by ×. . Figure 5
shows the band ripple distribution when the normalized film thickness h/λ is 0.01 and the total number of electrodes Nt of the ground reflector is 20. The area where the in-band ripple is 3 dB or less (◎ and ○) is
Logarithm ratio NC when total logarithm NT is in the range of about 95 to 140 pairs
is in the range of approximately 0.3 to 1.4, and this region includes a region (◎) in which the in-band ripple is 1.5 dB or less.

FIG. 6 shows the distribution of in-band ripples when the normalized film thickness h/λ is 0.02 and the total number of electrodes Nt of the ground reflector is 20. The region where the in-band ripple is 3 dB or less (◎ and ○) is the region where the total logarithm NT is in the range of about 70 to 115 pairs and the logarithm ratio NC is in the range of about 0.3 to 1.9. includes a region (◎) where the in-band ripple is 1.5 dB or less.

FIG. 7 shows the band ripple distribution when the normalized film thickness h/λ is 0.03 and the total number of electrodes Nt of the ground reflector is 20. The region where the in-band ripple is 3 dB or less (◎ and ○) is the region where the total logarithm NT is in the range of about 50 to 80 pairs and the logarithm ratio NC is in the range of about 0.5 to 2.2,
This region includes a region (◎) in which the in-band ripple is 1.5 dB or less.

FIGS. 8 and 9 are graphs summarizing the measurement results shown in FIGS. 2 to 7. Figure 8 shows the normalized film thickness h/λ
The horizontal axis represents the total logarithm NT, and the vertical axis represents the region where the in-band ripple is less than or equal to a predetermined value. As is clear from FIG. 8, the region where the in-band ripple is equal to or less than a predetermined value is a range in which the total logarithm NT is a function of the normalized film thickness h/λ expressed by the following equation.

α+β(h/λ)−γ+δNt ≦NT ≦α
+β(h/λ)+γ+δNt However, α, β, γ,
δ is a constant. As is clear from FIG. 8, the localization number of the function indicating the region where the in-band ripple becomes 3 dB is α = 125 and β = -200.
0, γ=30, and δ=−0.65. Note that in order to achieve even smaller in-band ripples, γ may be made smaller. FIG. 8 shows the range of γ=20 and the range of γ=10. γ=
If it is set to 20, the in-band ripple will be 1.5 dB or less,
If γ=10, the in-band ripple can be further reduced. Note that as the total number of electrodes Nt of the ground reflector increases, the line of γ=0 shifts downward in FIG. In addition to the case where Nt = 0 (dash-dotted line), the case where Nt = 10 (dash-dotted line) and the case where Nt = 20 (dash-dotted line) are shown in FIG.

FIG. 9 shows a region where the in-band ripple is below a predetermined value, with the horizontal axis representing the normalized film thickness h/λ and the vertical axis representing the logarithmic ratio NC. As is clear from the figure, the region where the in-band ripple is less than a predetermined value is the range where the total logarithm NT is a function of the normalized film thickness h/λ expressed by the following equation. p+q(h/λ)−r+sNt ≦NC ≦p
+q(h/λ)+r+sNt However, p, q, r,
s is a constant.

As is clear from FIG. 9, the constant of the function indicating the region where the in-band ripple is 3 dB or less is p=1.
1, q=20, r=0.8, and s=-0.03. Note that in order to realize even smaller in-band ripples, r may be made smaller. FIG. 9 shows the range of r=0.6 and the range of r=0.2. When r=0.6, the in-band ripple is 1.
5 dB or less, and by setting r=0.2, the in-band ripple can be further reduced.

Note that as the total number of electrodes Nt of the ground reflector increases, the line of γ=0 shifts downward in FIG. In addition to the case where Nt = 0 (dashed line), the case where Nt = 10 (dashed two-dot line) and the case where Nt = 20 (dashed three-dot line) are shown in FIG. In addition, in order to realize a surface acoustic wave resonator filter with small in-band ripples, the normalized film thickness h/λ is within the range of 0.005 to 0.04, as shown in FIGS. 8 and 9. This is desirable.

FIGS. 10 and 11 show the shapes of in-band ripples in specific examples of surface acoustic wave resonator filters. The solid line shows the insertion loss. The horizontal axis is the frequency fo (= normalized frequency f/fo normalized at 386.56 MHz) corresponding to the electrode period of the IDT. As shown in FIG. 1, the surface acoustic wave resonator filter of this embodiment has two rows of electrode structures of the same configuration connected in cascade, and the various constants are as follows.

Piezoelectric substrate = 45° rotation
λ=0.025 Aperture length of each IDT 21, 22, 23 (31, 32, 33) = 1300 μm Weighting of each IDT 21, 22, 23 (31, 32, 33) = cosine apotization total logarithm NT = 73.5 pairs Logarithm ratio NC = 1.14 Logarithm N1 of IDT21 (31) = 33.5 IDT22
Logarithm N2 of (32) = 24.5 Logarithm N3 of IDT23 (33) = 25.5 Each ground reflector 24, 25 (34,
35) Number of strip electrodes = 10 each reflector 26,
Number of strip electrodes of 27 (36, 37) = 100 The magnitude of the in-band ripple at this time was as small as 0.8 dB.

FIG. 11 shows the characteristics of a surface acoustic wave resonator filter as a comparative example of the present invention. Each constant of the surface acoustic wave resonator filter of this comparative example is as follows. Piezoelectric substrate = 45° rotated
λ = 0.024 Aperture length of each IDT 21, 22, 23 (31, 32, 33) = 1300 μm Weighting of each IDT 21, 22, 23 (31, 32, 33) = cosine apotization total logarithm NT = 120.5 pairs Logarithm ratio NC = 1.43 Logarithm N1 of IDT21 (31) = 49.5 IDT22
Logarithm N2 of (32) = 35.5 Logarithm N3 of IDT23 (33) = 35.5 Each ground reflector 24, 25 (34,
Number of strip electrodes in 35) = 10 Number of strip electrodes in each reflector 26, 27 (36, 37) = 100 The magnitude of the in-band ripple at this time was as large as 4.6 dB.

The present invention is not limited to the above-mentioned embodiments, and various modifications are possible. For example, in the above embodiment, the electrode structure row 20,
Ground reflectors 24, 25 or 34, 35 are provided on both sides of the 30 input IDTs 21 or output IDTs 31, but if the total number of electrodes is within a predetermined range, a ground reflector is provided only on one side as shown in FIG. 24 or 34 may be provided. In the above embodiment, the input terminal I is used as an unbalanced input/output.
N, connected to the output terminal OUT, but the ground reflector 24,
25, 34, and 35 may be balanced input/output without being grounded.

Furthermore, in the above embodiment, the ground reflectors 24, 2
The electrodes 5, 34, and 35 were connected to the grounding bonding pads 29c, 29d, or 39c, 39d, respectively.As shown in FIG.
9a and 29b, 29c and 29d, 39a and 39b, 39
It may be grounded by connecting c and 39d, respectively. It is better not to connect the ground bonding pads 29a, 29b and the ground bonding pads 29c, 29d. Similarly, it is better not to connect the ground bonding pads 39a, 39b and the ground bonding pads 39c, 39d.

Furthermore, in the above embodiment, two electrode structure rows are cascaded to form a surface acoustic wave resonator, but three or more electrode structure rows may be cascaded to form a surface acoustic wave resonator. Good too. By cascading a plurality of electrode structure rows, it is possible to realize a surface acoustic wave resonator filter with a low out-of-band spurious level and a large out-of-band attenuation. Furthermore, in the above embodiment, the IDT is weighted by cosine apotization or linear apotization in order to suppress the transverse mode, but other weighting methods such as cosine square apotization or modified cosine apotization may be used. When the weighting method is changed, a surface acoustic wave resonator filter with similar characteristics can be realized by increasing or decreasing the number of interdigital electrodes of each IDT.

[0047]

[Effects of the Invention] As described above, the present invention provides a
In a surface acoustic wave resonator in which a plurality of electrode structure rows sandwiched between a pair of reflectors are cascade-connected on a piezoelectric substrate, an electrode structure is connected to one electrode of the IDT between the plurality of IDTs in each electrode structure row. Since it is characterized by the insertion of a connecting reflector with multiple electrode fingers, the fractional bandwidth is 0.3% or more, the in-band ripple is small, the out-of-band spurious level is low, and the band It is possible to realize a surface acoustic wave resonator that satisfies the condition of large external attenuation and does not cause an increase in device size.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the basic structure of a surface acoustic wave resonator according to an embodiment of the present invention.

FIG. 2 is a graph showing changes in in-band ripple when the normalized film thickness h/λ and the logarithmic ratio NC of the surface acoustic wave resonator are kept constant and the total logarithm NT is changed.

FIG. 3 is a graph showing changes in in-band ripple when the normalized film thickness h/λ and the logarithmic ratio NC of the surface acoustic wave resonator are kept constant and the total logarithm NT is changed.

FIG. 4 is a graph showing changes in in-band ripple when the normalized film thickness h/λ of the surface acoustic wave resonator and the total logarithm NT are kept constant and the logarithm ratio NC is changed.

FIG. 5 is a graph showing the distribution of in-band ripples when the normalized film thickness h/λ of the surface acoustic wave resonator is kept constant and the total logarithm NT and the logarithmic ratio NC are varied.

FIG. 6 is a graph showing the distribution of in-band ripples when the normalized film thickness h/λ of the surface acoustic wave resonator is kept constant and the total logarithm NT and the logarithm ratio NC are varied.

FIG. 7 is a graph showing the distribution of in-band ripples when the normalized film thickness h/λ of the surface acoustic wave resonator is kept constant and the total logarithm NT and the logarithmic ratio NC are varied.

FIG. 8 is a graph summarizing the measurement results shown in FIGS. 2 to 7 and showing a region of the total logarithm NT in which the in-band ripple of the surface acoustic wave resonator is equal to or less than a predetermined value.

FIG. 9 is a graph summarizing the measurement results shown in FIGS. 2 to 7 and showing a region of the logarithmic ratio NC in which the in-band ripple of the surface acoustic wave resonator is equal to or less than a predetermined value.

FIG. 10 is a graph showing insertion loss characteristics of an example of a surface acoustic wave resonator according to the present invention.

FIG. 11 is a graph showing insertion loss characteristics of a comparative example of a surface acoustic wave resonator.

FIG. 12 is a perspective view showing the basic structure of a surface acoustic wave resonator according to an embodiment of the present invention.

FIG. 13 is a perspective view showing the basic structure of a conventional surface acoustic wave resonator.

[Explanation of symbols]

10... Piezoelectric substrate 12, 16... Termination impedance 14... Signal source 20, 30... Electrode structure row 21... IDT for input 31... IDT for output 22, 23, 32, 33... IDT for connection 24, 25, 3
4, 35...Ground reflector 26, 27, 36, 37...Reflector 28...Input bonding pad 38...Output bonding pad 29a, 29b, 29c, 29d, 39a, 39b, 3
9c, 39d... Grounding bonding pad 40... Piezoelectric substrate 42, 46... Termination impedance 44... Signal source 50, 60... Electrode structure row 51... Input IDT 61... Output IDT 52, 53, 62, 63... Connection IDT 54 ,55,6
4, 65...Reflector 56...Input bonding pad 66...Output bonding pad 57, 58, 59, 67, 68, 69...Grounding bonding pad

Claims (4)

[Claims] 1. In a surface acoustic wave resonator in which a plurality of electrode structure rows in which a plurality of IDTs are sandwiched between a pair of reflectors are cascade-connected on a piezoelectric substrate, a plurality of IDTs in each electrode structure row are connected in series. , a surface acoustic wave resonator characterized in that a ground reflector having a plurality of electrode fingers connected to one electrode of the IDT is inserted. 2. The surface acoustic wave resonator according to claim 1, wherein the piezoelectric substrate is a lithium tetraborate single crystal,
The total logarithm NT of the plurality of IDTs in each electrode structure row is
α+β(h/λ)−γ+δNt ≦NT ≦α+β
(h/λ)+γ+δNt However, h/λ is the above I
The normalized film thickness obtained by normalizing the thickness h of the DT electrode by the wavelength λ of the surface acoustic wave, Nt is the total number of electrodes in the connected reflector, α = 125, β = -2000, γ = 3
0, δ=−0.65. 3. The surface acoustic wave resonator according to claim 2, wherein the plurality of IDTs include a central IDT formed at a central position and IDTs on both sides formed at positions sandwiching the central IDT. Yes, the logarithm N1 of the central IDT
and the ratio N of the sum N2 +N3 of the logarithms of the surrounding IDTs
C is p+q(h/λ)-r+sNt ≦N
C ≦p+q(h/λ)+r+sNt However, p=
1.1, q=20, r=0.8, and s=-0.03. 4. The surface acoustic wave resonator according to claim 1, wherein the surface acoustic wave resonator includes two rows of the electrode structures, and one of the electrodes of the IDT in the center of one electrode structure row is connected to an input terminal. one end of the IDT, the other electrode of the IDT is connected to the electrode of the adjacent connection reflector, one of the electrodes of the IDT in the center of the other electrode structure row is connected to one end of the output terminal, and the IDT the other of the electrodes of the ground reflector is connected to the electrode of the adjacent ground reflector;
The other of the electrodes of the IDTs on both sides sandwiching the DT and the other electrode of the IDTs on both sides sandwiching the center IDT of the other electrode structure row are commonly connected, and the IDTs on both sides of the one electrode structure row are connected in common.
A surface acoustic wave resonator, characterized in that one of the electrodes of the DT is grounded, and one of the electrodes of the IDTs on both sides of the other electrode structure row is grounded.