JPH07135444A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To provide a dual mode resonance filter in which a resonance level due to undesired multiple reflection between reflectors is suppressed and a spurious level is low with high performance by differentiating the distances between input output digital transducers and between reflectors adjacent them. CONSTITUTION:A input transducer 11 and an output transducer 12 are arranged side by side on a piezoelectric substrate and let a center distance 15 between electrode fingers closest to each other be L1, then a reflector 13a is arranged to the left side of the transducer 11 and a reflector 13b is arranged to the right side of the transducer 12 respectively and they are made close to the input and output transducers. In this case, let the center distance 14a between closest electrode fingers of the transducer 11 and the reflector 13a be L2a and let the center distance 14b between closest electrode fingers of the transducer 12 and the reflector 13b be L2b, then the center distances 14a, 14b are set differently. A phase shift is produced in surface acoustic waves excited in two directions and the resonance level 23 is suppressed.

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 resonance type surface acoustic wave filter in which an input / output IDT (interdigital transducer) and reflectors are arranged on both sides of the input / output IDT on a piezoelectric substrate. It is about.

[0002]

2. Description of the Related Art In a conventional surface acoustic wave filter, as shown in the surface structure of FIG. 3, an input IDT 11 of a λ / 4 (λ is an electrode arrangement period) single electrode and an output IDT 12 of the same are arranged on a piezoelectric substrate 10. The reflectors 13a and 13b are arranged adjacent to each other, and the reflectors 13a and 13b are arranged adjacent to the input / output IDTs 11 and 12, respectively.

If N.epsilon., Which is the product of the number N of electrodes of the input / output IDT and the acoustic impedance discontinuity coefficient .epsilon. Of the metal film forming the IDT, is 0.55 or more, resonance of three different longitudinal modes is obtained. Is excited. Such a vehicle is based on the 1993 IEICE Transactions (A), Vo1. J76-A,
2, pp. 219-226.

That is, the input IDT 11 and the output IDT 1
There are three modes of resonance, a resonance 21 due to multiple reflection between the reflector 2 and the internal resonance 22 of the IDT itself, and a resonance 23 due to multiple reflection between the reflectors 13a and 13b.

Then, the distance 15 (L1) between the centers of the electrode fingers of the input / output IDTs 11 and 12 which are closest to each other,
The triple mode surface acoustic wave filter shown in FIG. 4A when the input / output IDT and the distance 14 (L2) between the centers of the electrode fingers of the reflectors 13a and 13b that are closest to each other satisfy the following expression. Is realized.

(N / 2-4 / 24) λ ≦ L1 ≦ (n / 2-2 / 24) λ ... (1) (n / 2-2 / 24) λ ≦ L2 ≦ (n / 2 + 7 / 24) λ ... (2) In the above equation, n is a natural number.

Further, when the above L1 (15) satisfies the condition of (n / 2/1/24) λ≤L1≤ (n / 2 + 7/24) λ ... (3), FIG. The dual mode surface acoustic wave filter shown can be realized. The applicant of the present application proposes and discloses these techniques in Japanese Patent Application No. 5-176847.

In FIGS. 4A and 4B, 21-
23 corresponds to the resonance modes 21 to 23 shown in FIG. 3, respectively.

The dual mode filter having the characteristics shown in FIG. 4B lowers the level of the resonance 23 due to the multiple reflection between the reflectors depending on the condition of the distance 15 (L1) between the input and output IDTs, and Although two modes of the resonance 21 due to multiple reflections between them and the internal resonance 22 of the IDT itself are used, the level due to the resonance 23 is not sufficiently suppressed and exists as a spurious.

[0010]

As described above, in the conventional surface acoustic wave filter, when the double mode filter is constructed, the resonance 23 between the reflectors exists as a spurious on the low frequency side, which is preferable. There is a drawback that the filter characteristics cannot be obtained.

Therefore, the present invention has been made to solve the above-mentioned drawbacks of the prior art, and its object is to obtain a surface acoustic wave having good filter characteristics and utilizing double mode resonance. To provide a filter.

[0012]

According to the present invention, a piezoelectric substrate, input / output IDTs (interdigital transducers) provided adjacent to each other on the piezoelectric substrate, and both sides of these input / output IDTs are provided. The first adjacent to each other
A surface acoustic wave filter including a second reflector and a center-to-center distance between electrodes of the input IDT and the first resonator that are closest to each other, the output IDT, and the second resonator. The elastic surface is characterized in that the distances between the centers of the electrodes closest to each other are different from each other, and a filter is obtained.

[0013]

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

FIG. 1A is a diagram showing a planar structure of an embodiment of the present invention, and the same portions as those in FIG. 3 are designated by the same reference numerals. Input IDT 11 and output IDT 12 are provided on the piezoelectric substrate 10.
Are arranged adjacent to each other, and the distance 15 between the centers of the electrode fingers closest to each other is L1.

A reflector 13a is provided on the left side of the input IDT 11, and a reflector 13b is provided on the right side of the output IDT 12.
Are arranged respectively. Therefore, the input / output IDTs 11 and 1 are
Therefore, the reflectors 13a and 13b are arranged on both sides of 2 in proximity to the input / output IDT.

Let L2a be the distance 14a between the centers of the electrode fingers of the input IDT 11 and the reflector 13a which are closest to each other.
If the distance 14b between the centers of the electrode fingers closest to each other between the output IDT 12 and the reflector 13b is L2b, these two center distances L2a and L2b are set to different values.

In this example as well, the value of L1 is preset so as to satisfy the above expression (3) so that it functions as a dual mode filter. However, the above two distances L2a and L2b are used. By setting and differently from each other,
Due to the asymmetry of both distances, the resonance 23 due to the multiple reflection between the reflectors is weakened among the characteristics shown by the broken line in FIG. 1B, and a good spurious-free 2 shown by the solid line in FIG.
A dual mode filter characteristic is obtained.

In FIG. 1 (B), 21 is an input / output I.
22 indicates resonance due to multiple reflection between DTs, and 22 indicates IDT
It shows its own internal resonance.

FIG. 2 shows the simulation characteristics of these filters. FIG. 2A is a characteristic diagram of a conventional structure in which L2a = L2b = 0.7λ. (B) is L2a = 0.5λ,
L2b = 0.7λ (or L2a = 0.7λ, L2b = 0.5
FIG. 6 is a characteristic diagram according to the present invention when λ) is set.

In the characteristic of FIG. 2B, the level of the resonance 23 between the reflectors existing at the lowest frequency side is suppressed by about 5 dB compared with the characteristic of the conventional example of FIG. 2A, and the spurious is improved. You can see that it is done. Further, the resonance 21 due to the multiple reflection between other IDTs and the internal resonance 22 of the IDT itself hardly change.

Distance 14a between input / output IDT and reflector
Since (L2a) and 14b (L2b) are different from each other, the resonance 23 between the reflectors (see FIG. 3) is caused by the input IDT1.
The propagation distance until the surface wave excited from 1 in both directions is reflected by the reflector 13a and received by the output IDT 12 is different from the propagation distance until reflected by the reflector 13a and received by the output IDT 12. There will be a gap.

Due to this phase shift, when a dual mode filter is constructed, the level of the resonance 23 between the reflectors existing as spurious on the low frequency side can be suppressed as shown in FIG. 1 (B).

Further, the resonance 21 due to the multiple reflection between the input and output IDTs and the internal resonance 22 of the IDT itself are N.ε ≧ 0.
Due to the state of energy confinement inside the IDT due to the above-described relationship of 55, it is difficult for that energy to reach the reflector, and therefore resonance due to this energy
The effects on 1 and 22 are almost negligible.

[0024]

As described above, according to the present invention, since the distance between the input / output IDT and the adjacent reflector is different from each other, the resonance level due to multiple reflection between unnecessary reflectors is suppressed. This has the effect of enabling a high-performance dual-mode resonance filter with low spurious levels.

[Brief description of drawings]

FIG. 1A is a diagram showing a plan configuration of an embodiment of the present invention,
(B) is a diagram showing the filter characteristic.

FIG. 2A is a simulation characteristic diagram of a conventional filter, and FIG. 2B is a simulation characteristic diagram of the filter of the present invention.

FIG. 3 is a diagram showing a planar configuration of a conventional surface acoustic wave filter.

4A is a characteristic diagram of a triple mode filter, FIG.
FIG. 4 is a characteristic diagram of a dual mode filter.

[Explanation of symbols]

 10 piezoelectric substrate 11 input IDT 12 output IDT 13a, 13b reflector 21 resonance between IDTs 22 resonance of IDT itself 23 resonance between reflectors

Claims (3)

[Claims] 1. A piezoelectric substrate, an input / output IDT (interdigital transducer) provided on the piezoelectric substrate so as to be adjacent to each other, and first and second electrodes provided at both sides of the input / output IDT so as to be adjacent to each other. A surface acoustic wave filter including a second reflector, wherein a center-to-center distance between electrodes of the input IDT and the first resonator closest to each other, the output IDT and the second resonator, The surface acoustic wave filter is characterized in that the distances between the centers of the electrodes closest to each other are different from each other. 2. The elastic surface according to claim 1, wherein resonance caused by multiple reflection between the first and second reflectors is suppressed by changing two center distances of the two. Wave filter. 3. A resonance due to multiple reflection between the input IDT and the output IDT and an internal resonance of the IDT itself.
The surface acoustic wave filter according to claim 2, having one resonance mode.