JPH09148874A - Surface acoustic wave element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the ripple within a band which is generated due to the shape and dimension of an IDT(interdigital transducer) except three kinds of main unwanted waves of an end face reflection, a bulk wave and TTE(tripple transit echo) in a surface acoustic wave device in which the squareness ratio of the pass band width of a frequency characteristic is large and the electrode logarithms of the IDT with weight are numerous. SOLUTION: Because the unwanted waves in a surface acoustic wave device are generated due to the rates of the occupying of the electric charge concentrating in an electrode utmost end part and a very small electrode finger transposition width within the IDT, an electrode 7 for mitigation of electric charge is provided at the IDT electrode utmost end part. The side lobe composed of the very small electrode finger transposition width near output electrodes 2 and 3 side is reduced and sound absorbing material is formed in the drawing line part between the envelope of the side lobe which is far from the output electrodes 2 and 3 and a bus bar 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention particularly relates to a surface acoustic wave device having a large number of weighted electrode pairs and a large electrode finger crossing width of one wavelength (λ) or less of the operating frequency.

[0002]

2. Description of the Related Art Surface acoustic wave devices are generally used for TV band pass filters and the like. The structure of the surface acoustic wave device is such that a comb-shaped electrode finger (IDT: Inter Digital Transistor) is formed on a piezoelectric substrate.
An input electrode composed of a sd user) and an output electrode are arranged at a predetermined distance, and a desired frequency characteristic is obtained by using a surface wave propagating between the two electrodes.

FIG. 10 schematically shows a general surface acoustic wave device. One of the input / output electrodes is a normal type electrode 2 having a constant crossing width of the comb-shaped electrode fingers, and the other is a weighted type electrode 3 having different crossing widths of the electrode fingers within the electrode. Surface wave (SAW: Surface Acous
SAW filter using tic wave)
2. Desired frequency characteristics are obtained by changing the number of electrode finger pairs of the weighted type electrode 3, or the crossing widths 12 and 13 of the electrode fingers.
In the SAW filter, unnecessary waves are generated as the filter characteristics in addition to the surface dimensions corresponding to the shape and dimensions of the input / output electrodes on the piezoelectric substrate 1 and the basic design to obtain the desired filter characteristics. Becomes an obstacle. As a representative of the unnecessary wave,
(1) Edge reflection: Surface waves are generated on both sides of the IDT, and are unwanted waves that reach the edge of the piezoelectric substrate, are reflected from the edge, and reach the output electrode. (2) Bulk wave: An unwanted wave that propagates from the IDT in the thickness direction of the piezoelectric substrate, reflects from the bottom surface of the substrate, and reaches the output electrode. (3) TTE (Triple Transit Echo): There are three types of unwanted waves, such as surface waves that reach the output electrode from the input electrode, are reflected by the output electrode, return to the input electrode, and reach the output electrode again. It is known that the three types of unwanted waves are suppressed by the following method. That is, (a) end surface reflection can be suppressed by applying a sound absorbing material to the end surface of the piezoelectric substrate.
(b) Bulk waves can be suppressed by methods such as roughening the bottom surface of the substrate. (c) TTE can be suppressed by adjusting the relationship between the impedance of the split electrode composed of electrode fingers of so-called (λ / 8) width and the input / output electrode and the impedance of the input / output circuit.

However, when the required frequency characteristics become strict, the above method cannot sufficiently cope with it. Hereinafter, this point will be described. The number of electrode pairs of the weighted IDT increases as the squareness of the passband width of the frequency characteristic increases, and the ratio of the region where the crossing width of the weighted IDT is small to the entire IDT increases. As a result, the ratio of the electrode finger cross width substantially equal to the wavelength (λ) of the center frequency of the SAW filter increases. Undesired waves different from the aforementioned undesired waves exist in common in the surface acoustic wave device having a large number of electrode pairs, and they appear as ripples in the frequency band of the SAW filter, which causes inconvenience in obtaining desired frequency characteristics. As a method for suppressing surface wave spurious, which is different from the above-mentioned three types of main unnecessary waves, such as edge reflection, bulk wave, and TTE, Japanese Patent Publication No. 3-12485
And the like. However, it was not considered from the viewpoint of the width of the IDT crossing the electrode fingers.

[0005]

DISCLOSURE OF THE INVENTION Various studies including experiments have been conducted in order to understand unnecessary waves different from the above-mentioned three types of main unnecessary waves. As a result, it was found that there are the following two causes.

(I) The electric charge is concentrated on both ends of the electrode finger of the IDT, and a large excitation is generated as compared with the design time.

(Ii) The weighted type electrode has a large number of IDT pairs, and S
The unnecessary wave becomes conspicuous when the number of microelectrode finger crossing widths less than or equal to one wavelength (λ) of the used band frequency of the AW filter is large in the IDT. In addition, the magnitude relationship between the side lobes of the weighted type electrode also influences the fact that the unwanted wave becomes noticeable.

[0008] Therefore, an object of the present invention is to reduce unnecessary waves depending on the IDT shape, which exist in addition to the above-mentioned three types of main unnecessary waves.

[0009]

Means for Solving the Problem That is, in order to solve the problem, the following means are used.

With respect to the above (i), due to the end effect due to the electric flux concentration at both ends of the weighted type electrode, unnecessary charges are distributed to the portions other than the electrode fingers to be relaxed.

Regarding the above (ii), the following will be considered as a factor in which the width of the microelectrode finger crossing contributes to the generation of the unwanted wave.
That is, since the piezoelectric substrate used for the SAW filter has sonic anisotropy, it is configured so that surface waves can be transmitted most efficiently in the input / output IDT direction. In the micro-electrode finger cross width part, the surface wave component that propagates in the direction different from the central axis direction of the surface wave propagation between the input and output IDTs becomes relatively larger than that of the electrode finger with a large cross width, which adversely affects the unwanted wave. As the number of side lobes formed by the crossing width of the microelectrode fingers in the vicinity increases, the influence increases and appears as ripples in the frequency characteristic. Therefore, in order to reduce unnecessary waves depending on the IDT shape that are present in addition to the above-mentioned three types of main unnecessary waves, it is necessary to reduce the number of the microelectrode finger crossing widths.

A first means of the present invention is to provide a charge relaxation dummy electrode outside the IDT endmost electrode finger. The second means is
A surface acoustic wave absorbing material is formed on the electrode finger lead line formed between the bus bar and the envelope of the electrode finger crossing width of the side lobe on the side opposite to the normal type electrode side with respect to the main lobe in the weighted type electrode. To do. Side lobes less than 1λ are not formed on the regular electrode side.

As another means for reducing the number of the crossing widths of the micro electrode fingers, there is also a method of determining the opening length of the weighted type electrode so that all the crossing widths of the electrode fingers of the side lobes are 1λ or more. is there.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION

Example 1 An example applied to a bandpass filter for a TVIF filter will be described as a specific example of the first means.

In FIG. 1, a piezoelectric substrate 1 is made of LiNbO ₃ (lithium niobate: LN), an input electrode is a normal type electrode 2 having 6 pairs of IDT, and an output electrode is a weighted type electrode 3, and a logarithm is 125.
It is an outline of a surface acoustic wave device designed as a pair. The number of side lobes 6 in the weighted type electrode 3 was 15, and the electrode finger crossing width of the main lobe 5 was formed so as to correspond to 28 wavelengths. In addition, the electrode configuration is such that the electrode finger width is λ / 8
A split type electrode is used. The electrode finger spacing in the IDT is even.

In this embodiment, the input side IDT of the weighted electrode 3
A charge relaxation electrode 7 was provided that extends from the bus bar at the end to the SAW propagation path direction and the input electrode side, and forms an angle of about 30 degrees with the IDT having a length of 3λ. Further, the charge relaxation electrode 7 is SAW
In order to correct the sonic velocity phase due to the formation on the propagation path, the sonic velocity correction electrode 8 and the charge relaxation electrode 7 were not formed SA
Formed in the W propagation path region. The ground electrode 4 formed as a shield between the input and output has a "dogleg" shape that folds back along the SAW propagation path center axis in order to keep the same distance from the charge relaxation electrode 7.
And There is also a slight size reduction effect due to this "doglegged" shape.

As a result, the electric charge concentrated at the outermost end of the weighted type electrode 3 is relaxed by the electric charge relaxation electrode 7 of the present invention, and SAW.
The ripple appearing in the filter band was reduced by about 0.5 dB and the desired frequency characteristics could be obtained.

Embodiment 2 The following is the same as Embodiment 1 as a specific example of the second means.
An example of using the SAW filter will be described with reference to FIG.

In the weighted type electrode 3 of the SAW filter, the number of side lobes at which the electrode finger cross width is equal to or less than 1λ is 10
Individual. In this embodiment, the weighted type electrode 3 on the input electrode 2 side
Delete 10 side lobes of the crossing width of the micro-electrode fingers inside,
A sound absorbing material 10 was applied and formed between the bus bar 11 and the side lobe envelope 6 in the side lobe 6 region on the side opposite to the input side electrode finger. That is, the weighted type electrode 3 has a laterally asymmetrical configuration with respect to the main lobe 5.

As a result, the unwanted wave generated from the crossing width of the microelectrode fingers on the side opposite to the normal type electrode 2 is absorbed by the sound absorbing material 10 formed outside the side lobe envelope 6 of the weighted type electrode 3 and the input electrode The unnecessary wave generated from the crossing width of the microelectrode fingers on the side was suppressed by deleting the crossing width of the microelectrode fingers. As an adverse effect of the present invention, since one side lobe is deleted and the weighted type electrode is made asymmetric,
Ripple occurred in the band, but it was small for unwanted waves targeted by the present invention and characteristically about 0.
A 7W improved SAW filter could be obtained. Also,
According to the present embodiment, the number of electrode finger pairs of the weighted type electrode can be reduced from 125 pairs to 90 pairs, so that the element can be downsized and the cost can be reduced at the same time. Furthermore, the effect of shortening the group delay time by shortening the input-output distance and shortening the ripple cycle was also obtained.

Since the method according to the present embodiment is effective in relation to the side lobe region where the electrode finger crossing width of the side lobe shows a saturation tendency, when the number of side lobes in the region is 3 or more. Especially effective.

Embodiment 3 As another embodiment, in the case of Embodiment 2 in which the side lobes of the width of the microelectrode fingers on the side of the normal type electrode 2 are electrically cut off by providing slits in the bus bar in the structure of FIG. However, the object of the present invention has been achieved.

Embodiment 4 The object of the present invention has been achieved even when the structure shown in FIG. 4 in which the above-described Embodiments 1 and 2 are combined is used as another embodiment.

As another embodiment, as shown in FIGS. 5 and 6, the bus bar 11 and the electrode finger crossing between the bus bar 11 of the weighted type electrode 3 on the opposite side of the normal type electrode 2 and the envelope 6 of the electrode finger crossing width Part 5,
When the lead wire connecting 6 and 6 is the entire surface electrode 14. Further, when the charge relaxation electrode 7 is provided at both ends of the weighted type electrode 3 as shown in FIGS. 7 and 8, and when the charge relaxation electrode 7 is directed to the outside of the bus bar 11 as shown in FIG. However, the object of the present invention has been achieved. Further, even when the aperture length of the weighted electrode is set so that all electrode finger crossing widths including side lobes are 1λ or more, the unwanted wave targeted by the present invention is suppressed.

[0025]

The present invention has a large squareness ratio of frequency characteristics,
To provide a surface acoustic wave device in which the in-band ripple generated in a SAW filter having a large number of IDT electrode pairs of weighted electrodes is reduced. Further, according to the second means of the present invention, it is possible to exert an effect in achieving cost reduction.

[Brief description of the drawings]

FIG. 1 is a schematic view of a surface acoustic wave device for explaining an embodiment of the invention.

FIG. 2 is a schematic view of a surface acoustic wave device for explaining an embodiment of the invention.

FIG. 3 is a schematic view of a surface acoustic wave device for explaining an embodiment of the invention.

FIG. 4 is a schematic view of a surface acoustic wave device for explaining an embodiment of the invention.

FIG. 5 is a schematic view of a surface acoustic wave device for explaining an embodiment of the invention.

FIG. 6 is a schematic view of a surface acoustic wave device for explaining an embodiment of the invention.

FIG. 7 is a schematic view of a surface acoustic wave device for explaining an embodiment of the invention.

FIG. 8 is a schematic view of a surface acoustic wave device for explaining an embodiment of the invention.

FIG. 9 is a schematic view of a surface acoustic wave device for explaining an embodiment of the invention.

FIG. 10 is a schematic view of a surface acoustic wave device for explaining a conventional example.

[Explanation of symbols]

1 ----- Piezoelectric substrate 2 ----- Regular type electrode 3 ----- Weighted type electrode 4 ----- Ground electrode 5 ----- Main lobe envelope 6 ---- -Side lobe envelope 7 ----- Electrode for charge relaxation 8 ----- Electrode for sound velocity correction 9 ----- λ / 4 electrode 10 ----- Sound absorbing material 11 ----- Bus bar 12, 13 ----- Crossing width of electrode fingers

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Takashi Shiba, Inventor Takashi Shiba, 292 Yoshida-cho, Totsuka-ku, Yokohama, Kanagawa, Ltd. Multimedia system development headquarters, Hitachi, Ltd. 292, Machi, Ltd. Hitachi, Ltd. Multimedia system development headquarters

Claims (5)

[Claims] 1. An input / output electrode composed of a comb-shaped electrode is provided on a piezoelectric substrate, and at least one of the electrodes is composed of a weighted type electrode, and the crossing width of the electrode fingers of the weighted type electrode is a wavelength of a use center frequency. In a surface acoustic wave device having an electrode finger crossing width of 1λ or less with respect to (λ), a charge relaxation dummy electrode is provided on at least one of the left and right electrode finger extreme ends of the weighted type electrode. Surface acoustic wave device. 2. The surface acoustic wave device according to claim 1, wherein a cross width of a plurality of side lobes formed at both ends of a main lobe of a graphic pattern formed from the cross widths of electrode fingers in a weighted type electrode is defined. When the side lobe is given a number N from the side closer to the main lobe to be S (N), and the side lobe number (N) is increased as the distance from the main lobe increases, the side lobe ratio (S (N + 1) / S ( The surface acoustic wave device is characterized in that (N)) changes from substantially constant to increasing, and the number of side lobes that become substantially constant is 3 or more. 3. A surface acoustic wave device, wherein at least one of the input and output electrodes is a weighted electrode, and the number of side lobes is different on the left and right with respect to the main lobe, and the main lobe is on the other electrode side. A surface acoustic wave device characterized in that it is formed close to. 4. At least one of the input and output electrodes of the surface acoustic wave device is a weighted type electrode, except for the side lobes whose electrode finger cross width is 1λ or less, the remaining side lobes are centered on the main lobe. The surface acoustic wave device is characterized in that the side lobes having the same number and the electrode finger crossing width of 1λ or less are formed only on the opposite side to the normal type electrode. 5. An electrode finger crossing width on the side closer to the normal type electrode, wherein at least one of the input and output electrodes of the surface acoustic wave device is a weighted type electrode and the left and right side lobes are configured to have the same number as the main lobe. The surface acoustic wave device is characterized in that the side lobe of 1 λ or less is electrically separated from the main lobe by providing a slit in the bus bar.