JPS6161574B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave device (hereinafter, surface acoustic wave is abbreviated as SAW) using split interdigital electrodes.

Generally, the interdigital electrode used in the transducer of a SAW device has a configuration in which a set of interdigitated electrodes consisting of a large number of electrode fingers are inserted into each other, and the capacitance of the transducer is set between each opposing electrode finger. If the dielectric constant of the piezoelectric substrate is large, it becomes relatively large. Therefore, when incorporating a SAW device into a circuit, impedance matching between the front and rear circuits may be difficult. Therefore, the interdigital electrode is divided into multiple parts along the SAW propagation direction, and the A method has been proposed to reduce the capacity by connecting. According to this method, in order to prevent deterioration of frequency characteristics due to division, the capacitance of each divided electrode is made equal, and in most cases, the electrode pattern is configured symmetrically (including point symmetry). . Such symmetrically arranged interdigital electrodes provide sufficiently good frequency characteristics when focusing only on the electrodes.

However, such interdigital electrode 2
are constructed on the surface wave substrate 1 as shown in FIG. 1, other necessary parts (for example, the other interdigital electrode 3, shield electrode 4, etc.) are constructed as appropriate, and a package (not shown) is applied. did
When the SAW device is incorporated into a circuit, when observing the frequency characteristics, as shown by the broken line in FIG. 5, the characteristics within the pass band are good, but a phenomenon occurs in which the amount of attenuation of the attenuation pole becomes small. Therefore, the inventor conducted various experiments to investigate the cause of the deterioration of the attenuation peak, and in one of the experiments, the frequency characteristics were calculated by intentionally changing the capacitance of the divided partial electrodes. However, the calculation results shown in Figure 4 were obtained. In Figure 4, all data are based on an example of dividing into two parts, curve A is the capacitance of the two partial electrodes shifted by 20%, and curve B is the capacitance of the two partial electrodes shifted by 10%.
The curve C shows the shifted one, and the curve C shows the equal one.
Inferring from this result, the above-mentioned deterioration of the attenuation pole is
This is thought to be due to the fact that even though the electrode patterns are arranged symmetrically in advance, the capacitance becomes non-uniform due to the effects of the back electrode, conductive adhesive, shield electrode, etc., due to mounting.

The present invention has been made in view of the conventional technical situation described above, and it is possible to reduce the capacitance while keeping the attenuation amount of the attenuation pole of the frequency characteristic sufficiently large.
The purpose is to provide a SAW device, and its gist is that the capacitances of a plurality of divided partial electrodes of one interdigital electrode are sequentially reduced to C _{1 ,} C 2 , C ₂ ,
......Cn, then C ₁ ≦C ₂ ≦...≦Cn (however, C ₁
=C ₂ =...=Cn is excluded), so that
The reason lies in the structure of the pattern of one electrode.

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

FIG. 2 is a schematic diagram showing the capacity of each part when the SAW device is installed, for explaining the present invention.
Corresponding to the SAW device shown in FIG. 1, 1 is a surface wave substrate, 2 is a split type interdigital electrode, 3 is the other interdigital electrode placed on the same propagation path at an appropriate distance from electrode 2, and 4 is a surface wave substrate. This is a shield electrode placed between both electrodes 2 and 3. 5,
6, 7, and 8 are partial electrodes of the split electrode 2, the partial electrode 5 adjacent to the other electrode 3 is composed of a pair of interdigitated comb-shaped electrodes 9, 10, and the partial electrode 6 is similarly composed of a pair of comb-shaped electrodes 9, 10 inserted into each other. The comb electrodes 10 and 11 of the partial electrode 5 and the comb electrode 10 of the partial electrode 6 are electrically connected in series. Similarly, the partial electrodes 7 and 8 are composed of comb-teeth electrodes 11 and 12 and comb-teeth electrodes 12 and 13, respectively. The capacitance due to the partial electrode 5 is the capacitance formed between only the pair of comb-shaped electrodes 9 and 10, and is assumed to be C ₁ . Similarly, the capacitances due to the partial electrodes 6, 7, 8 are calculated as C ₂ , C ₃ ,
Let it be C ₄ . These capacitances C ₁ , C ₂ , C ₃ , C ₄ are C ₁
≦C ₂ ≦C ₃ ≦C ₄ (excluding C ₁ = C ₂ = C ₃ = C ₄ )
The pattern of the electrode 2 is selected so that the following relationship is established. That is, the electrode 2 has the logarithm of the partial electrodes 5, 6, 7, and 8 as shown in FIG.
The capacitances C ₁ , C ₂ , C ₃ , and C ₄ are formed so as to satisfy the above relationship by increasing the number of pairs, 7 pairs, 5 pairs, and 3 pairs sequentially from the other electrode 3 side.

On the other hand, stray capacitances are formed between the partial electrodes 5, 6, 7, 8 and the shield electrode 4 and the other electrode 3, as shown in _FIG . They are added to C ₂ , C ₃ , and C ₄ respectively. Such stray capacitance becomes larger as it is formed in the partial electrode closer to the shield electrode 4 and the other electrode 3. Between each pair of comb-teeth electrodes 9 and 10, 1
The capacitances appearing between 0 and 11, between 11 and 12, and between 12 and 13 are the capacitance C ₁ due to the partial electrodes mentioned above,
Since it is the sum of C ₂ , C ₃ , C ₄ and the above stray capacitance,
The capacitances between each pair of comb-shaped electrodes in the mounted state are substantially equal to each other. Therefore, when voltage V is applied between terminals 14 and 15, a voltage of approximately V/4 is applied to each of partial electrodes 5, 6, 7, and 8, and the voltages applied between each adjacent electrode finger are equal. . Therefore, the frequency characteristics will not deteriorate due to division.

When the SAW device shown in FIG. 1 is constructed using the electrode pattern shown in FIG. 3, the amount of attenuation of the attenuation pole can be made sufficiently large, as shown by the solid line in FIG.

In the above embodiment, a ground electrode may be provided on the back surface of the surface wave substrate 1. Further, although the above embodiments show examples of regular electrodes, the present invention can also be applied to weighted electrodes, and may also be applied to the other interdigital electrode.

Since the present invention adopts the configuration as described above, it has the effect that the capacitance can be reduced by division and that the amount of attenuation of the attenuation pole in the frequency characteristic can be made sufficiently large.

[Brief explanation of the drawing]

Fig. 1 is a plan view showing a general SAW device, Fig. 2 is a schematic diagram for explaining the present invention, Fig. 3 is a pattern diagram of a segmented interdigital electrode according to the SAW device of the present invention, and Fig. 4 is a schematic diagram for explaining the present invention. FIG. 5 is a frequency characteristic diagram for explaining the present invention. FIG. 5 is a frequency characteristic diagram of a conventional SAW device and a SAW device of the present invention.

Claims (1)

[Scope of Claims] 1. One interdigital electrode divided into a plurality of parts in the surface acoustic wave propagation direction and electrically connected to each other in series, and the other interdigital electrode arranged at an appropriate distance from this electrode. In the surface acoustic wave device, capacitances due to the plurality of partial electrodes of the one electrode are sequentially C ₁ and C ₂ from the other electrode side.
......Cn, then C ₁ ≦C ₂ ≦...≦Cn (however, C ₁
=C ₂ =...=Cn is excluded), so that
A surface acoustic wave device characterized in that the pattern of the one electrode is configured.