JPH0537290A - Interdigital transducer and surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To obtain the same reflected wave cancelling effect as the double electrode type by applying an absorbent on a specific part of partial electrodes where internal reflect occurs out of plural electrodes and absorbing the surface acoustic wave propagated from another electrode by this absorbent. CONSTITUTION:An absorbent 1 is applied on the part where internal reflection occurs, for example, the electrode part in a range of oblique lines of an apodize electrode group 10, and electrode oscillation due to a reflected wave is suppressed by the absorbent 1. High polymer materials like silicone rubber or a non-volatile adhesive is used as the absorbent 1, and its application quantity is a minimum required for suppression of electrode oscillation. Therefore, electrode oscillation due to the surface acoustic wave propagated in the direction opposite to the direction of an arrow is suppressed by the absorbent 1 when a voltage is applied to the electrode group 10. Consequently, internal reflection is relaxed, and the same reflected wave cancelling effect as a double electrode type comb shaped type electrodes (IDT) is realized with the single type electrode group (apodize electrode group).

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 filter, and more particularly, to a surface acoustic wave interdigital transducer (IDT: interdigital transducer) weighted by an apodizing method.

[0002]

2. Description of the Related Art A surface acoustic wave filter has a four-terminal structure in which an IDT for exciting and receiving surface waves is provided on a piezoelectric substrate. The IDT has a constant band-pass filter characteristic even if the crossing width of a plurality of electrodes facing each other is equal to each other, but the IDT has a certain bandpass characteristic.
A so-called weighting for giving frequency characteristics to itself is generally performed.

Various methods have been proposed for weighting the IDT, but in particular, as shown in FIG. 2, a method of forming a so-called apodized electrode group 20 by changing the crossing width of each electrode (apodizing method) is used. It is most often used because it can be weighted easily and accurately.

[0004]

By the way, the surface acoustic wave excited by the IDT propagates in two directions, that is, the direction toward the receiving IDT shown in FIG. 2 and the opposite direction. The surface acoustic wave propagating in the opposite direction is internally reflected at another specific part of the electrode, for example, a part near point A in FIG.
When this internal reflected wave is received by the receiving IDT, it appears as ripples in the pass band or group delay distortion, which adversely affects the filter characteristics.

Therefore, in the case of designing a filter that requires high-precision characteristics, a so-called double electrode type IDT in which the reflection of the surface acoustic wave is canceled by intersecting the electrodes every two sheets as shown in FIG. Was used.

However, the double-electrode type IDT has a smaller electrode width than the single-electrode type IDT such as the apodized type and is difficult to process, and the interval between the electrodes is λ / 8 (λ is the applied voltage). Therefore, when the frequency is about several hundred [MH _Z ], it is more difficult to process the electrode.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide a single electrode type IDT which can obtain a reflected wave canceling effect similar to the double electrode type IDT.

Another object of the present invention is to provide a surface acoustic wave filter having a small ripple in the pass band and a flat group delay characteristic.

[0009]

According to the IDT of the present invention, a plurality of electrodes, which respectively intersect and oppose each other, are arranged in parallel on the surface of a piezoelectric substrate, and the crossing widths of the electrodes are arranged unevenly. When a voltage is applied, the surface acoustic waves are propagated in the direction of electrodes adjacent to each other, and in a part where internal reflection occurs in a specific part of the partial electrode, a sound absorbing agent is applied to the part where internal reflection occurs in the partial electrode, By absorbing the surface acoustic wave propagating from another electrode with this sound absorbing agent, the generation of the internal reflected wave is significantly reduced.

Further, the surface acoustic wave filter of the present invention has two I's for exciting and receiving surface acoustic waves on the piezoelectric substrate.
A DT is provided, and at least one of these IDTs for excitation is provided with a plurality of electrodes facing each other, and the crossing widths of the electrodes are unevenly arranged. A voltage is applied to each electrode. Then, in the case where surface acoustic waves are propagated in the direction of electrodes adjacent to each other to cause internal reflection at a specific portion of a part of the electrode, a sound absorbing agent is applied to a portion of the IDT where internal reflection is caused, and The surface acoustic wave propagating from the electrodes is absorbed by the sound absorbing agent.

[0011]

When a voltage is applied to the IDT, surface acoustic waves propagate in the direction of adjacent electrodes, that is, in both directions. In the case of an IDT consisting of a so-called apodized electrode group, internal reflection occurs at a specific part of the electrode. Occurs. Moreover, the electrode that causes internal reflection and its part differ depending on the desired frequency characteristic, that is, the degree of weighting. In the IDT of the present invention,
A sound absorbing agent is applied to the electrode at the site where the internal reflection occurs, and thereby, the electrode vibration due to the surface acoustic wave propagating from the other electrode is significantly suppressed, and the internal reflection is alleviated.

Further, by using such an IDT for excitation, surface acoustic waves due to internal reflection do not reach the receiving IDT, and the occurrence of ripples and group delay distortion is suppressed.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing the electrode structure of the IDT of the present invention, which is composed of a so-called apodize type electrode group 10 like the conventional one.

In this embodiment, the sound absorbing agent 1 is applied to a part of the electrode group 10 where internal reflection occurs in a part of the electrodes. For the sound absorbing agent 1, for example, a polymer material such as silicon rubber or a non-volatile adhesive is used, and its application amount is set to the minimum necessary amount for suppressing electrode vibration. Moreover, the application site is, for example, as shown in FIGS.
The electrode parts in the shaded area in (c), preferably the electrode parts in the shaded area in FIG. 1 (b).

With this configuration, the vibration of the electrodes due to the surface acoustic waves propagating in the direction opposite to the direction of the arrow when a voltage is applied to the electrode group 10 is significantly suppressed by the sound absorbing agent 1.
Therefore, the internal reflection is remarkably relaxed as compared with the conventional one, and the reflected wave canceling effect similar to that in the case of using the double electrode type IDT can be realized by the single type electrode group (apodized type electrode group). .

Next, a case where a surface acoustic wave filter is constructed by using the IDT will be described. In this case, the IDT is used as an exciting IDT for the surface acoustic wave filter. By doing so, the surface acoustic wave in which the internal reflection is suppressed is received by the receiving IDT, and it is possible to realize a surface acoustic wave filter having a small ripple in the pass band and a flat group delay characteristic. it can. If the IDT is also used for the receiving IDT, the above effect becomes more remarkable.

The hatched portions in FIGS. 1A, 1B and 1C correspond to point A in FIG.
The site where this internal reflection occurs changes depending on the desired frequency characteristic, that is, the degree of weighting by the apotize method. Therefore, when the electrode pattern of the apodized electrode group 10 is changed, the electrode portion to which the sound absorbing agent 1 is applied is also changed, and the application range is not necessarily limited to the application range shown in each drawing of FIG.

[0019]

As described above, the IDT according to the present invention.
In this case, a sound absorbing agent was applied to a specific part of some of the electrodes that causes internal reflection, and the surface acoustic waves propagating from other electrodes were absorbed by this sound absorbing agent. A reflected wave canceling effect similar to that of the IDT can be obtained. Therefore, even if the frequency used is high, electrode processing becomes easy, and the manufacturing cost can be significantly reduced.

Further, in the surface acoustic wave filter of the present invention,
Since at least the excitation IDT is composed of the above IDT,
It has an effect that the ripple in the pass band is small and the group delay characteristic is flat.

[Brief description of drawings]

FIG. 1 is an electrode structure diagram of an IDT according to an embodiment of the present invention, in which hatched areas in (a), (b), and (c) are views showing portions of the electrode to which a sound absorbing agent is applied. is there.

FIG. 2 is a view showing a conventional electrode structure of this type of IDT and a site where internal reflection occurs.

FIG. 3 is an electrode structure diagram of a double electrode type IDT.

[Explanation of symbols]

1 ... Sound absorbing agent, 10, 20 ... Apodized IDT electrode.

Claims (2)

[Claims] 1. An interdigital transducer comprising a plurality of electrodes facing each other in parallel on the surface of a piezoelectric substrate, and the electrodes having a non-uniform crossing width, wherein a voltage is applied to each electrode. When a surface acoustic wave is propagated in the direction of electrodes adjacent to each other to cause internal reflection at a specific part of the partial electrode, a sound absorbing agent is applied to the part causing internal reflection of the partial electrode, and another electrode is applied. An interdigital transducer characterized in that the surface acoustic wave propagating from the surface acoustic wave is absorbed by the sound absorbing agent. 2. Two interdigital transducers for exciting and receiving a surface acoustic wave are provided on a piezoelectric substrate, and at least one of these interdigital transducers for exciting is provided with a plurality of electrodes that cross each other. Along with this, the cross widths of the electrodes are arranged unevenly, and when a voltage is applied to each electrode, a surface acoustic wave propagates in the direction of the electrodes adjacent to each other, and some parts of the electrodes internally In the case of causing reflection, a sound absorbing agent is applied to a part of the interdigital transducer where internal reflection occurs, and the surface acoustic wave propagating from another electrode is absorbed by this sound absorbing agent. A surface acoustic wave filter characterized by the above.