JPH10163792A - Structure of dual mode SAW filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve attenuation inclination by setting the adjacent inter- electrode finger intervals between IDT on both sides of a primary-tertiary DMS filter and IDT at the center to be within a specified range. SOLUTION: Primary-tertiary modes along the direction of a surface wave are excited by providing three IDT2, IDT3 and IDT4 on the main face of a piezoelectric substrate 1 and reflectors 5 and 5 on both sides so as to constitute the primary-tertiary DMS filter using the two modes. When the center interval G1 of the adjacent electrode fingers of IDT2 and IDT3 by sandwiching the electrode finger 3a whose width is λ/2 is set to be G1=5/4 and the center interval of the adjacent electrode fingers of IDT2 and IDT4 by sandwiching the electrode finger 4a is set to be G2, G2 is set to be (0.84+n)λ<G2((1.0+n)λ. Thus, they are asymmetrically constituted with the center shaft of center IDT as a reference. Consequently, attenuation inclination can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resonator type surface acoustic wave filter (hereinafter referred to as "SAW filter"), and more particularly to a 1-3 order longitudinally coupled double mode SAW having a steep attenuation gradient.
Regarding filters.

[0002]

2. Description of the Related Art In recent years, SAW filters have been widely used in wireless devices such as mobile phones, because of their excellent miniaturization, high frequency, mass productivity, and the like. Particularly in recent PHS, cordless telephones, etc., the first IF filter is required to have a higher frequency and a wider band.
An AW filter is optimal. As a means for widening the bandwidth of a resonator type SAW filter, a first-order / third-order longitudinally-coupled dual-mode SAW filter (hereinafter, referred to as “first-order / third-order”) mode using a first-order mode and a third-order mode
1st-3rd order DMS filters).
FIG. 5 (a) is a schematic plan view showing one example of this, in which three interdigital transducers (hereinafter referred to as IDTs) 12 are arranged on the main surface of a rectangular piezoelectric substrate 1 along the propagation direction of surface waves. 13 and 14 are arranged, and grating-type reflectors 15 and 15 are arranged on both sides thereof. IDT12
14 to 14 are constituted by a pair of comb-shaped electrodes each having a plurality of electrode fingers interposed therebetween. IDT1
One of the comb electrodes 2 to 14 is connected to a ground potential terminal, and the other comb electrode is electrically connected to an input terminal or an output terminal.

[0005] Each IDT shown in FIG.
DT, and the width of each electrode finger and the space between the electrode fingers are generally set to 波長 of the wavelength λ of a desired center frequency. Regarding the broadening of the first- and third-order DMS filters, see IEICE Transactions A, Vol.J76-ANo.2, p.
As described in FIGS. 5 and 6 under “Band-Bandwidth Dual-Mode SAW Filter”, the bandwidth can be continuously changed by changing the interval L between the input and output IDTs, and the maximum bandwidth can be obtained. Is described as being the case when L = λ / 4. Generally, an electrode finger 1 at the end of an adjacent IDT
The center-to-center distance L between the electrodes 7 and 18 is defined as L as shown in FIG. 5 (b). When this L is set to λ / 4 according to the above-mentioned document, the width of the electrode fingers 17 and 18 usually becomes To λ
Therefore, the adjacent electrode fingers 17 and 18 come into contact with each other. Therefore, as shown in FIG. 5C, the electrode fingers 17 and 18 are integrated to form the electrode finger 16. Therefore, the electrode finger width of the electrode finger 16 is λ / 2. In this specification, when describing an electrode pattern that provides the maximum bandwidth, the center distance L between the electrode fingers is not expressed as λ / 4, but as shown in FIG.
As shown in (c), the center-to-center spacing between the adjacent electrode fingers with the electrode finger 16 interposed therebetween is taken and described as 5 / 4λ.

The reflectors 15, 15 shown in FIG.
It has the function of reflecting the leaky surface waves from 2, 14
By confining the energy of the surface acoustic wave excited at T12 to T14 between the reflectors 15, 15, the Q value of the primary mode and the tertiary mode is increased. At this time, 2
The next mode is naturally excited, but since the displacement distribution of the mode is distributed antisymmetrically with respect to the center of the IDT 12 in the propagation direction of the surface wave, the generated charge similarly has a different sign with respect to the center of the IDT 12. , And the input and output IDTs are canceled out and substantially not excited, so that only the first mode and the third mode can be used. As a means for improving the cut-off characteristic of the pass band and increasing the amount of attenuation outside the band, it is also generally practiced to use a two-stage cascaded primary-tertiary DMS filter as shown in FIG. ing. The filtering characteristic of FIG. 7 is the two-stage cascade connection type DMS having the electrode configuration shown in FIG.
This is an example of a filter in which a 36 ° YX Li
Using TaO3, frequency F ₀ = 110.592 MHz,
Pass bandwidth B = 2 MHz, central IDT 66.5 pairs,
G1 and G2 are 5 / 4λ, and IDTs 3 and 4 are each 33.33.
It is a filtering characteristic when 5 pairs and 30 reflectors are used.

Japanese Patent Application Laid-Open No. Hei 8-191229 discloses a means for suppressing a spurious appearing in a high-frequency attenuation band and improving an attenuation near a high frequency side in a longitudinally coupled dual mode SAW filter. FIG. 8A shows an electrode pattern. In a longitudinally coupled dual mode SAW filter using a first-order mode and a third-order mode, an innermost electrode finger facing a central input IDT 22 and output IDTs 23 on both sides is used. When the center-to-center spacings are L1 and L2, one is L1 = nλ / 2, and the other is L2 = (2n−1) λ / 4 (n = 1, 2, 3
..) describes that a filter having a large amount of high-frequency attenuation near the passband can be realized. FIG.
(B) is a diagram showing the filtering characteristics at that time.

[0006]

However, as shown in FIG. 5 (c), in an IDT pattern in which the interval between adjacent electrode fingers is set to 5 / 4λ and the bandwidth is maximized, 3 dB is used. There is a problem that the ratio B30 / B3 of the bandwidth (B3) to the 30 dB bandwidth (B30), that is, the attenuation gradient is degraded by spurious components generated on the high frequency side of the passband, and does not satisfy the requirements for PHS and cordless telephones. This is the attenuation slope obtained from FIG.
It can also be understood from the fact that No. 3 is deteriorated by spurious components on the high frequency side. Also, as shown in FIG. 8 described above, the center-to-center spacing of the innermost electrode fingers facing the center IDT and both-side IDTs is approximately L1 = nλ / 2 for one, and L2 = (2n
-1) Even in a first-order / third-order DMS filter with λ / 4, there is often no situation where the requirements cannot be satisfied due to manufacturing variations because the attenuation gradient has no margin for the required conditions, and the yield is not improved. was there. The present invention has been made to solve the above problem, and has as its object to provide a first-order to third-order DMS filter having a steep attenuation gradient. As can be seen from the figure, the attenuation gradient, that is, the ratio of the 3 dB bandwidth (B3) to the 30 dB bandwidth (B30) is deteriorated by spurious components on the high frequency side.

[0007]

According to a first aspect of the present invention, there is provided an IDT comprising three IDTs on a piezoelectric substrate and reflectors on both sides of the IDTs in the direction of propagation of the surface acoustic wave. In the primary-tertiary-order longitudinally coupled dual-mode SAW filter constructed as described above, when the center distance G1 between the electrode fingers of the center IDT and one of the adjacent IDTs is 5λ / 4, the other IT
The distance G2 between the center of the electrode finger and DT is (0.84 + n) λ
<G2 <(1.0 + n) λ, the central IDT
A double mode SAW filter characterized by being asymmetrically configured with respect to a central axis of the filter.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1A is a schematic electrode pattern showing one embodiment of a dual mode SAW filter according to the present invention.
By disposing the reflectors 5, 5 on both sides of the DTs 2, 3, and 4 along the propagation direction of the surface wave, the first and third modes along the direction of the surface wave are excited. A first-third-order DMS filter using two modes is configured. FIG.
(B) is an enlarged view of the IDT portion of (a) of the same figure, and the distance G1 between the centers of the adjacent electrode fingers between IDT2 and IDT3 across the electrode finger 3a having a width of λ / 2 is represented by G1. = 5 / 4λ, and the distance between the centers of the adjacent electrode fingers of the IDTs 2 and 4 across the electrode finger 4a is G2.

FIG. 2 shows the attenuation gradient S30 of the first-order / third-order DMS filter using the electrode finger center distance G1 as a parameter and the electrode finger center distance G2 as a parameter, as shown in FIG. (= B30 / B3) is an example of a result obtained by simulation. As is clear from this figure, the attenuation gradient shows periodicity while changing in a complicated manner with respect to the electrode finger center distance G2. JP-A-8-19
The innermost electrode finger center distance L2 = λ / 4 disclosed in Japanese Patent No. 1229 corresponds to G1 = 5λ / 4 in the description of the present specification, and L1 = λ / 2 corresponds to G2 = λ. Shown by arrows inside. There is a G2 value at which the attenuation gradient is improved from the G2 = λ value (L1 = λ / 2) disclosed in the above-mentioned publication. That is, the value of G2 is (0.84 + n) λ ~
It is clear from FIG. 2 that the value of the attenuation gradient S30 is smaller than the broken line B in the range of (1 + n) λ. FIG.
Represents the attenuation gradient S40 (= B
40 / B3) is a diagram showing a change, and shows almost the same tendency as in the case of S30.

FIG. 4 shows that a 36 ° YX LiTa piezoelectric substrate is used.
Using O3, frequency F ₀ = 110.592 MHz, pass bandwidth B = 2 MHz, central IDT 42.5 pairs, G1
= 5/4 [lambda], G2 = 9/10 [lambda], 23.5 pairs of IDTs 3 and 4, and 30 reflectors each. Although the filtering characteristics of FIG. 8B based on the conventional design are different from the center frequency, the bandwidth, the number of electrode pairs, the number of reflectors,
Comparing the attenuation gradients of the two filtering characteristics, it can be seen that the attenuation gradient is improved as shown in the above simulation results.

Although the case where LiTaO3 is used for the piezoelectric substrate has been described above, it is needless to say that the present invention can be applied to other piezoelectric materials, such as quartz, LiNbO3, and LBO.

[0012]

Since the present invention is constructed as described above, the distance between the adjacent electrode fingers of the IDT on both sides of the first-order and third-order DMS filters and the center IDT is set as described above. It is possible to improve the damping gradient, which is excellent when used in a mobile phone or the like.

[Brief description of the drawings]

FIG. 1A is a diagram showing an embodiment of an electrode pattern of a primary-tertiary DMS filter according to the present invention, and FIG. 1B is an enlarged view of an IDT thereof.

FIG. 2 is a diagram showing a relationship between an electrode finger center distance G2 and an attenuation gradient S30.

FIG. 3 is a diagram showing a relationship between an electrode finger center distance G2 and a damping gradient S40.

FIG. 4 is a diagram illustrating filtering characteristics using an IDT pattern according to the present invention.

FIG. 5A is a diagram showing an electrode pattern of a conventional primary-tertiary DMS filter, and FIGS. 5B and 5C are enlarged views thereof.

FIG. 6 is a two-stage cascaded DMS filter of a conventional primary-tertiary DMS filter.

FIG. 7 is a diagram showing filtering characteristics of the configuration of FIG. 6;

FIG. 8A is an electrode pattern diagram of JP-A-8-191229, and FIG. 8B is a diagram of its filtering characteristics.

[Explanation of symbols]

 1. Piezoelectric substrate 2, 3, 4 ... IDT 3a, 4a ... Electrode finger 5: Reflector G1, G2 ... Electrode finger center spacing S30, S40 ... Decay slope

Claims (1)

[Claims] 1. A primary-third arrangement comprising three IDTs arranged on a piezoelectric substrate and reflectors arranged on both sides thereof in the direction of propagation of surface waves.
In the next-order longitudinally-coupled dual-mode SAW filter, the center I
The distance G between the centers of the electrode fingers between DT and one of the adjacent IDTs
When 1 is set to 5λ / 4, the distance G2 between the center of the electrode finger and the other IDT is set to (0.84 + n) λ <G2 <(1.0+
n) The dual mode SA is characterized in that it is asymmetrical with respect to the center axis of the central IDT by using λ.
Structure of W filter.