JPS5847085B2 - Surface acoustic wave element electrode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of configuring electrodes of a surface acoustic wave utilizing element.

In order to generate a surface acoustic wave (hereinafter referred to as 5AW), generally, as shown in Fig. 1, a thin strip-shaped so-called "pass blind discharge electrode" is provided on a piezoelectric substrate and is alternately applied with positive and negative potentials.
structure is used.

That is, when an alternating potential is applied to electrodes 1 and 2 in FIG. 1, distortion occurs in the surface portion of the piezoelectric substrate, which becomes a surface wave that propagates in the direction of the arrow in the figure.

The receiving side electrode also uses the same interdigital electrode structure as the transmitting side electrode shown in FIG.

It is known that the amplitude characteristic of such an interdigital electrode is given by the real part Ga(ω) of its radiation admittance, and is given by the following equation.

k2 is the electrical/mechanical coupling coefficient of the piezoelectric substrate material, C
8 is the capacitance between the interdigital electrodes, fo is the center frequency determined by the pitch p between the electrodes, N is the number of opposing electrode pairs, ωO
=2πfo.

Here, in order to obtain the sharp selectivity characteristic required in a normal communication device, from 6→, (3), it is necessary that X be large, that is, the number N of opposing electrode pairs should be large.

However, as can be inferred from equation (2), this results in a significant decrease in the impedance of the interdigital electrodes.

This reduction in electrode impedance makes connection with an external circuit difficult.

Therefore, in order to prevent this impedance reduction, a structure 11 (Japanese Patent Laid-Open No. 48-21955) in which the electrode is folded back at the center as shown in FIG. 2 has been known.

As shown in FIG. 2, an alternating potential is applied to electrodes 3 and 5, and electrode 4 becomes a relay electrode for these electrodes 3 and 5.

Therefore, the electrodes 3 and 5 are connected in series with equal capacitance, and this capacitance is halved and the impedance is doubled.

However, in this structure, the electrodes 3 and 5 are connected at the folded portion 11.
There is a part where the electrodes directly face each other without intervening the electrode 4, and the pitch between the electrodes in this part must be equal to the same value p as in other parts, so between the electrodes 3 and 5 in the folded part, An electric field twice as strong as the

In other words, this electrode structure produces an impulse-like strain twice as strong at the central folded portion.

The frequency characteristics of the interdigital electrode structure are given by the Fourier transform of the strain distribution generated on the piezoelectric substrate, so generating such a locally strong impulse means broadening the frequency band. It becomes difficult to obtain sharp selectivity.

As a countermeasure for this, it is necessary to make necessary adjustments to the size or overlap of the electrodes (Japanese Unexamined Patent Publication No. 48-21955).
Up until now, no technical solutions have been revealed, and rather studies have been conducted from the perspective of utilizing this difference in electric field strength to contribute to the filter frequency characteristics (Japanese Patent Application Laid-Open No. 48-21955.51). -50592), but no essential solution has been found yet, leaving the problem as 1\.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide an electrode structure that eliminates local electric field concentration without reducing impedance.

In order to eliminate the above-mentioned problem of locally generated strong electric fields, the present invention effectively reduces the electrode spacing only in the portions facing the electrodes where strong electric fields occur, without changing the pitch of the electrode structure as a whole. It features an expanding structure.

That is, if the strength of the electric field is El and the strain caused in the piezoelectric substrate by this is S, then the proportionality constant is d and it is given by 5=dE (φ).

Here, the electric field strength E is proportional to the electrode spacing a, so in order to reduce the magnitude of the impulse-like strain that occurs at the center folded portion, the electrode spacing a should be doubled (2a). .

Examples of the surface acoustic wave device electrode according to the present invention will be described in detail below.

FIG. 3 shows an embodiment of the invention.

In Fig. 3, if the pitch between the electrodes is p, and the interval between the crossing parts of electrodes 3 and 4 or electrodes 5 and 4 is a, then in the conventional electrode structure, the interval between the opposing parts of electrodes 3 and 50 is also a. becomes.

In this case, a stronger electric field is generated in the opposed parts of electrodes 3 and 5 than in other parts, so in the present invention, the width of the electrodes in the opposed parts is cut in half (the dotted line area in Figure 3), and the electrodes 3 and 5 are and 5 are widened.

Generally, the electrode spacing a and the electrode width at the intersection are designed to be equal, so there is a relationship of p-2a.

Here, since only the opposing portions of electrodes 3 and 5 are cut by half of the electrode width, the interval between these portions is equal to p.

As a result, the electrode spacing can be locally widened without significantly changing the average interelectrode pitch of the interdigital electrodes as a whole, and the amplitude of large impulses generated in the center can be reduced.

Next, in SAW devices using interdigital electrode structures, there is a phenomenon called triple transit echo (TTE).

This is a SAW that is multiplely reflected between the transmitter and receiver electrodes.
is detected, and is an unnecessary wave that degrades the S/N of the necessary received signals.

As a countermeasure against unnecessary waves, a split electrode structure shown in FIG. 4 is known.

This is because the basic structure called the solid electrode structure in Figure 3 has (1/2) p (p; pitch) increments, whereas this split electrode structure has (1/4) increments. It is characterized by its location.

The present invention can also be applied to this split electrode structure, and FIG. 4 shows an embodiment to which this is applied.

In the button shown in FIG. 4, only the opposed portions of the electrodes 6 and 8 in the center folded portion 11 have a solid electrode structure, and the same technical idea as shown in FIG. 3 is applied.

However, in the split electrode structure shown in Fig. 4, the electrode spacing between electrodes 6 and 7 and electrode 8 button and 7 is (1/4).
Since the distance between the electrodes 6 and 8 in the folded portion 11 is (1/2)p, the distance between the electrodes 6 and 8 may be (1/2)p.

Therefore, the relationship between the electrode dimensions is as shown in FIG.

Another embodiment in which the present invention is applied to a split electrode structure is shown in FIG.

As shown in Fig. 5, the central folded part 11 is also 1 inch with a split electrode structure, and only the electrodes of the folded opposing parts are shifted by (1/8)p so that the interval is (1/2)p. There is.

This creates a portion with a spacing of (1/8)p, but this does not pose a problem since the electrodes are at the same potential.

FIG. 6 shows still another embodiment in which the present invention is applied to a split electrode.

In FIG. 6, the electrode spacing is all equal to (1/4)p, but only in the opposing portions of electrodes 9 and 10, the intersecting length of the electrodes is half the length W of the other portions.

As a result, the energy of strong impulses locally generated in the center folded portion 11 can be effectively halved.

As described above, the surface acoustic wave element electrode of the present invention has been improved by reducing the input/output impedance of the surface acoustic wave element electrode, whereas conventionally, in order to obtain sharp selectivity characteristics, it was unavoidable to lower the input/output impedance. Since the amplitude-frequency characteristics can be easily realized without impedance reduction, the contribution to this technical field, which is entering the period of practical application, is significant.

[Brief explanation of drawings]

Figure 1 is a basic structural diagram of a comb-shaped electrode, which is a conventional surface acoustic wave element electrode, Figure 2 is a diagram of a conventional electrode structure that prevents a drop in impedance of the element electrode, and Figure 3 is a solid type electrode according to the present invention. The explanatory diagrams of the structure and FIGS. 4 to 6 are respectively explanatory diagrams of a split type electrode for triple transit eco-measures according to the present invention. 1.2,3,5; Electrode to which a potential is applied from the outside, 4,
7; Relay electrode, 6, 8, 9, 10; Split type electrode to which a potential is applied from the outside, 11; Folded opposing part, a
; electrode spacing, p; inter-electrode pitch.

Claims (1)

[Claims] 1. The first comb-shaped electrode and the second comb-shaped electrode are connected to the third comb-shaped electrode.
In a surface acoustic wave element electrode having a folded structure that is electrically coupled via a comb-shaped electrode and arranged in parallel in the propagation direction of a surface acoustic wave, the first comb-shaped electrode and the second comb-shaped electrode
A surface acoustic wave element electrode characterized in that only the comb-teeth spacing of the opposing portion with the comb-teeth-shaped electrode is enlarged. 2 The first comb-shaped electrode and the second comb-shaped electrode
In a surface acoustic wave element electrode having a folded structure that is electrically coupled via a comb-shaped electrode and arranged in parallel in the propagation direction of a surface acoustic wave, the first comb-shaped electrode and the second comb-shaped electrode
A surface acoustic wave element electrode characterized in that only the length of the comb teeth of the portion facing the comb-shaped electrode is shortened.