JPS6174409A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To obtain a surface acoustic wave excellent in characteristic by selecting a center distance of interdigital electrodes of dummy electrodes as lambda0/2j(lambda0 is the wavelength of a surface acoustic wave at the center frequency) and the center distance between the closest electrode fingers of the dummy electrode and the input/output blocks as lambda0/4. CONSTITUTION:The electrode finger number of the block of input and output electrode fiber blocks and of the dummy electrodes is selected as, e.g., four. Then the distance (d) of the closest electrode fingers between the dummy electrode and the input/output block is selected as, e.g., 3/4lambda0, where lambda0 is the wavelength of a surface acoustic wave at the center frequency of the surface acoustic wave device. The line width and interval of the electrode fingers of the input/ output/dummy electrodes are selected all as 1/4lambda0. Through the setting above, a surface acoustic wave stimulated from the input electrode 21 passes through the dummy electrode 25 and reaches the output comb electrode 23 and since the phase of the reflected wave of the electrodes 25, 21 is inverted, they are cancelled and reflection is eliminated. Further, in using the dummy electrode as a common ground electrode 26, the grounding is unified effectively.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a surface acoustic wave device, and more particularly to a surface acoustic wave device with good characteristics that are less susceptible to secondary effects.

Conventional structure and its problems Devices using surface acoustic waves usually have comb-shaped electrode transducers on the input side and output side on a substrate made of piezoelectric material, and generate elastic waves excited by the transducer on the input side. The surface wave propagates to the above-mentioned converter on the output side, and is detected again as an electric signal by the converter on the output side. At this time, the center frequency, band, and delay time of the surface acoustic wave device can be set by designing the pitch and number of electrode fingers of the comb-shaped electrode transducer, and the distance between the input and output transducers. However, due to the secondary effect of the electrode fingers, surface acoustic waves are reflected at the electrode fingers, making it often difficult to achieve desired characteristics. This type of problem is most noticeable in high-frequency devices where the thickness of the electrode fingers cannot be ignored compared to the wavelength of the surface acoustic wave, and has a large effect on narrow-band devices with a large number of electrode fingers. In addition, since the propagation loss of surface acoustic waves increases in the high frequency region, the number of electrode fingers increases in narrowband devices, and the distance between the input and output comb-shaped electrode transducers becomes longer, resulting in an increase in propagation loss. there were. In order to prevent this increase in propagation loss, the structure shown in Fig. 1 was considered, in which the electrode fingers of the input comb-shaped electrode transducer were twisted around, and the output comb-shaped electrode transducer was installed in that area. Since the reflection from the fingers is large and the entire device becomes a single surface acoustic wave resonator, there is little freedom in design such as frequency characteristics, making it difficult to achieve desired characteristics.

OBJECTS OF THE INVENTION An object of the present invention is to provide a novel surface acoustic wave device that eliminates such conventional problems, and will be explained below with reference to the drawings.

Structure of the Invention The present invention has a thinned-out structure in which the input-side and output-side comb-shaped electrode transducers are each divided into a plurality of blocks, and the output-side electrode fingers are arranged at the thinned-out positions of the input-side comb-shaped electrode transducers. It is constructed by arranging blocks. At this time, the electrode index of the blocks on the input side and the output side that are adjacent to each other is made approximately equal, and dummy electrodes with a number approximately equal to the above electrode index are placed between them, and the electrode fingers of the dummy electrode and the input side This is realized by setting the center-to-center distance of the closest electrode fingers on the output side to the center frequency of the surface acoustic wave device. Here, n is an integer.

DESCRIPTION OF EMBODIMENTS Next, one implementation of the present invention will be described. First, in order to clarify the difference between the structure and the conventional example, the present invention shown in FIG. 2 will be explained in detail in comparison with the conventional example shown in FIG.

FIG. 1 is a schematic diagram showing the structure of a conventional surface acoustic wave device, in which a plurality of comb-shaped electrode transducers 12 (indicated in black in the figure) on the input side are constructed on a piezoelectric substrate 11. It is divided into blocks. Here, each block is composed of two pairs, that is, four electrode fingers. The electrodes of each block on the input side are collectively connected to an input terminal 14, and the other terminal 15 is grounded. Comb type electrode converter 13 on the output side
(shown in white here) was similarly divided into a plurality of blocks, inserted between blocks on the input side, and an electrical signal was taken out from the output terminal 16. The other terminal 17 on the output side is grounded.

Conventionally, as the overall electrode index increased, so-called secondary effects such as reflection due to discontinuity of acoustic impedance at the end of the electrode fingers and electrical re-excitation due to the piezoelectric effect occurred, worsening the characteristics. . According to the present invention, such secondary effects can be effectively removed and high-performance surface acoustic waves can be realized.

FIG. 2 is a schematic diagram showing an embodiment of the present invention.

Here, the number of electrode fingers in each block of electrode fingers on the input side and the output side and the dummy electrodes arranged between them is all four. These do not need to be completely the same number, but it is sufficient as long as they are approximately the same number. The distance d between the nearest electrode fingers between the dummy electrode and the blocks on the input side and the output side was set to %λ0 here.

Input side, output side.

And the line width and spacing width of the electrode fingers of the dummy electrode are all %λ
It is 0. With this setting, the input side comb-shaped electrode 21
The surface acoustic wave excited by passes through the dummy electrode 26,
The reflected waves reach the comb-shaped electrode 23 on the output side, but since one phase of the reflected waves from the dummy electrode 25 and the comb-shaped electrode 21 are reversed, they cancel each other out and the reflection is eliminated. Furthermore, by using this dummy electrode as a common grounding electrode 26, grounding can be effectively carried out all at once as shown in FIG.

Effects of the Invention Since the present invention eliminates the reflection at the electrode finger, the triple reflected wave (TTK) due to the electrode finger reflection within the passband is approximately 1 sd.
B was reduced, and ripples in the amplitude characteristics and phase characteristics within the passband were significantly improved.

Outside the passband, due to the Nund effect of the common ground electrode,
The capacitive coupling between input and output has been reduced, and the guaranteed attenuation in the high range has been improved by 10 dB or more.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a conventional surface acoustic wave device, and FIG. 2 is a partial schematic diagram illustrating an embodiment of the present invention. 21... Input side comb-shaped electrode, 23... Output side comb-shaped electrode, 26... Dummy electrode, 26...
...Ground electrode. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 □, A;-

Claims (2)

[Claims] (1) The comb-shaped electrode transducers on the input side and the output side each have a thinned-out structure in which the electrode fingers are divided into a plurality of blocks, and the electrode fingers of the comb-shaped electrode transducer on the input side are placed in the space where the electrode fingers are thinned out. A block of electrode fingers on the output side is arranged, and between the electrode fingers on the input side and the output side, dummy electrodes having a number approximately equal to the electrode index of the block of electrode fingers are arranged, and the electrodes of the dummy electrode are arranged. The distance between the centers of the fingers is λ_0/
2 (λ_0 is the wavelength of the surface acoustic wave at the center frequency of the surface acoustic wave device), and the distance between the centers of the dummy electrode and the nearest electrode fingers of the input and output blocks is (n/2+1/4). ) λ_0 (n is an integer). (2) The surface acoustic wave device according to claim 1, wherein a dummy electrode is used as an electrical connection means for common ground electrodes on the input side and the output side.