JP3073185B2 - Single-phase unidirectional converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface acoustic wave (SA).
W) The present invention relates to a converter and a configuration of a SAW device using the SAW converter, and in particular, to a configuration of a SAW converter having one direction.

[0002]

2. Description of the Related Art Conventionally, a transversal type SAW filter has been widely used as a high-band low-loss filter in a high-frequency region of a VHF to UHF band. However, in general, an interdigital transducer (ID
T) is known to excite the SAW evenly in both directions. However, the transversal type SAW filter has a drawback that the insertion loss is essentially increased because only one side is used. is there. Therefore,
When configuring a transversal SAW filter, an IDT having one direction is generally used. As this one-way IDT, a three-phase one-way IDT and a group-type one-way IDT are well known, but they are not practical because of difficulty in manufacturing or requiring a complicated external circuit. Was. In recent years, a transversal-type SAW filter with low loss has been put into practical use with the proposal of an internal reflection type IDT that has a simple configuration and can simplify an external circuit.

For example, Kodama et al., "Basic Design of Low-Loss SAW Filter Using Distributed Acoustic Reflection Transducer," IEICE Transactions, Vol.J69-C, No.10, pp.1297-1308 (198)
6), an internal reflection type IDT as shown in FIG. 6 is proposed, and its basic configuration is basically a pair of electrode fingers consisting of three electrode fingers 1, 2, and 3 indicated by solid lines on a piezoelectric substrate. The IDT electrode is configured as a unit, and the IDT electrode is formed by repeating this plural times. As is apparent from FIG.
When the three electrode fingers are a first electrode finger 1, a second electrode finger 2, and a third electrode finger 3 in the arrangement order from the left side in the figure, the electrode finger widths are 3λ / 8, λ, respectively. / 8 and λ /
8, a gap between the electrode fingers of the first electrode finger 1 and the second electrode finger 2, a gap between the electrode fingers of the second electrode finger 2 and the third electrode finger 3, and the first electrode finger. The gap between the electrode fingers of the first and third electrode fingers 3 is both λ / 8. Here, λ is the wavelength of the SAW propagating on the piezoelectric substrate.

Here, when the reflection characteristics in the basic unit of the internal reflection type IDT are analyzed, as shown in FIG. 6, when the edges of the electrode fingers are a to f from the left side in the figure, the acoustic reflection at each edge is shown. The vector can be represented as shown in FIG. That is, the reflections at the edges b, cd, and e function so as to cancel each other out, and the composite of the reflected waves at the remaining edges a and f becomes the total acoustic reflection. Therefore, the internal reflection type IDT shown in FIG. 6 excites the SAW forcefully toward the right side in the figure when it is used for excitation, and efficiently uses the SAW coming from the right side in the figure when used for reception. It functions as an internal reflection type one-way IDT that can receive well.

Further, Japanese Patent Application Laid-Open No. 5-308242 (Japanese Patent Application No.
“-109510)“ Surface acoustic wave transducer and surface acoustic wave device ”include an internal reflection type I as shown in FIG.
DT is disclosed, and as is apparent from FIG.
A point that the electrode finger width of the first electrode finger 1 is set to 5λ / 16 and that of the third electrode finger 3 is set to λ / 16, among the three electrode fingers 1, 2, and 3 constituting the basic unit of T; 6 in that the gap between the first and third electrode fingers 1 and 3 is set to λ / 4. The reflection characteristic of the electrode finger edge in the basic unit of the internal reflection type IDT of FIG. 8 is such that the reflections at edges b, c, d, and e cancel each other as shown in FIG. The same as that described above, but functions so that the directions of the reflection vectors at the remaining edges a and f coincide with each other. Therefore, it has the feature that the total acoustic reflection is greater than that of FIG. 6 and the one-way property is improved.

[0006]

However, in recent years, there has been a demand for a broadband filter having a lower loss characteristic as a digital portable radio or the like, and a transversal SAW using the above-mentioned conventional internal reflection type IDT has been demanded. There was a drawback that the filter could not meet this demand. An object of the present invention is to provide an internal reflection type unidirectional IDT capable of realizing a transversal type SAW filter with lower loss to solve this problem.

[0007]

In order to achieve the above-mentioned object, a single-phase unidirectional converter according to the present invention is obtained by repeating a plurality of times with an electrode finger pair consisting of three electrode fingers as a basic unit. Configured interdigital transducer (IDT)
In a surface acoustic wave (SAW) converter in which electrodes are arranged on a piezoelectric substrate, when the three electrode fingers are the first, second, and third electrode fingers in the order of arrangement, the first to second electrodes are used. 3
W1> W2 = W3 when the electrode finger widths of the electrode fingers are W1, W2 and W3, respectively, and the center distance L between the second and third electrode fingers is L ≠ λ / 4 (where λ is (Wavelength of the propagating SAW), and the relationship between the electrode finger gap g1 of the first and second electrode fingers and the electrode finger gap g2 of the second and third electrode fingers is g1> g2. And wherein the distance between the center of the first electrode finger and the midpoint between the second and third electrode fingers is substantially λ / 2, and the invention of claim 2 further comprises the IDT electrode. The distance between the excitation center and the intermediate point between the second and third electrode fingers in each basic unit is approximately λ / 8 ± nλ / 2 (n = 0,
1, 2,...).

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments. Prior to the description of the present invention, a conventional distributed constant equivalent circuit which has been used so far to obtain the reflection characteristics at the edge of the electrode finger and a new distributed constant IDT which is used for analyzing the internal reflection type IDT according to the present invention. The distributed constant equivalent circuit will be described in some detail. In analyzing the internal reflection type IDT, it is essential to accurately grasp the reflection coefficient at the edge of the electrode finger. Until now Figure 1
As shown in FIG. 0, in order to express the energy storage effect near the edge of the electrode finger, analysis was performed using a distributed constant line model using only the parallel susceptance B as a lumped element.

Here, the line width of the electrode finger is L, the space width between adjacent electrode fingers is S, the characteristic impedance of the free surface without electrodes is Zf, and the characteristic impedance on the electrodes is Zm. Assuming that the phase velocities of the SAW propagating above are Vf and Vm, respectively, Zf Zm |
When 1 / B |, the average phase velocity of the SAW propagating on the substrate is represented by the following equation.

[0010]

(Equation 1) The reflection coefficients Γ1 and Γ2 at the edge of the electrode finger are expressed as Γ1 = γ-jδ and Γ2 = -γ-jδ, respectively.
Represents the reflection due to the parallel susceptance B, and is approximated by the following equation.

[0011]

(Equation 2) However, since the speed drop (frequency drop) due to the energy storage effect is “positive” as is well known, the parallel susceptance B should be “positive” from the above equation (1). On the other hand, δ, which is an imaginary component of the reflection coefficient, is known to be “negative” by experiments, so that the parallel susceptance B must be “negative”, Will be presented. In other words, the reflection at the edge of the electrode finger cannot accurately represent the reflection caused by the difference in the characteristic impedance between the free surface and the electrode, and the reflection caused by the parallel susceptance B. It was found that there was a large difference between Until now, in the normal type IDT in which the width of the electrode finger is λ / 4, the reflection caused by the parallel susceptance B is canceled out and can be ignored. Internal reflection type IDT
As described above, it is impossible to accurately analyze the reflection of an electrode finger line width other than λ / 4 using a conventional distributed constant line model.

In order to accurately grasp the line width dependence of the reflection at the edge of the electrode finger, the inventor of the present application has shown FIG.
We propose a new distributed parameter line model as shown in Fig. 1. That is, as a lumped constant element for expressing the behavior at the edge of the electrode finger, a series reactance X is introduced in addition to the conventional parallel susceptance B. By using this distributed constant equivalent circuit, it is possible to independently determine two events of frequency reduction and reflection due to line width.

In this model, | X | Zf Z
When m | 1 / B |, the average phase velocity V of the SAW propagating on the substrate is expressed by the following equation.

(Equation 3) The reflection coefficients Γ1 and Γ2 at the edge of the electrode finger are respectively expressed as Γ1 = γ-jδ and Γ2 = -γ-jδ, and γ and δ are respectively approximated by the following equations.

(Equation 4)

The speed drop, that is, the frequency drop is expressed by the sum of B and X, as can be seen from the third term on the right side of the equation (3). Expressed by the difference. That is, by introducing the series reactance X, the frequency drop and the reflection are independent,
It can be determined as more realistic. Hereinafter, the present invention will be described based on results of analysis using the above-described new distributed constant line model and confirmation by experiments.

According to the present invention, the conventional internal reflection type IDT has 6
Four of the reflected waves at one edge cancel each other out, and the remaining two reflected waves are used. On the other hand, two of the four reflected waves that have not been used before are used. The feature is that one-wayness is improved based on a completely new idea of actively utilizing two reflected waves.

FIG. 1 is a view showing an embodiment of an internal reflection type IDT according to the present invention, in which three electrode fingers 1, 2, and 3 shown by solid lines on a piezoelectric substrate are used as a basic unit and are used a plurality of times. IDT electrodes formed repeatedly are arranged. here,
The three electrode fingers are referred to as a first electrode finger 1, a second electrode finger 2, and a third electrode finger 3 in the arrangement order from the left side in the figure. Then, the electrode finger width W1 of the first electrode finger 1 is set to 0.334.
λ, the electrode finger width W2 of the second electrode finger 2 is 0.131λ, the electrode finger width W3 of the third electrode finger 3 is 0.131λ, and the first electrode finger 1 and the second electrode finger 2 The gap g1 between the electrode fingers is 0.166 λ, the gap between the electrode fingers between the second electrode finger 3 and the third electrode finger 3 is 0.072 λ, the first electrode finger 1 and the third electrode finger 3, and the gap g3 between the electrode fingers is set to 0.166λ. Note that the piezoelectric substrate at this time is ST cut quartz, the material of the electrode is aluminum, and the electrode thickness h is h = 1.08% λ.

Here, in analyzing the reflection characteristics in the basic unit of the internal reflection type IDT, when the edge of the electrode finger is af from the left side in the figure, the acoustic reflection vector at each edge is expressed as shown in FIG. be able to. That is, it functions so that the directions of the reflection vectors at the edges a, b, c and f coincide with each other. Also, the remaining edges d and e
The direction of the reflected wave is opposite to the direction of the above four reflected waves, but the total acoustic reflection intensity of all reflected waves including this is the same as that of the conventional internal reflection type IDT. It is significantly larger than that. As a result, the transversal type SAW filter having a very low insertion loss can be formed by using the IDT having this configuration, which has greater unidirectionality than that of the conventional internal reflection type IDT.

FIG. 3 shows an internal reflection type ID using the configuration of FIG.
The directionality at the center of the stop band of T is compared with that of the conventional internal reflection type IDT shown in FIG. 6 by an experiment, and the horizontal axis is the number of basic units constituting the IDT.
The vertical axis shows the directionality in each case. As is clear from the figure, it was confirmed that the use of the internal reflection type IDT of the present invention exhibited a greater directionality.

The inventor performed analysis on various samples by simulation, not only in the case of FIG. 1 but also in FIG.
In order to obtain the distribution of acoustic reflection vectors as shown in
It was found that the configuration should be such that the following conditions were satisfied, and it was confirmed that the results of this analysis agreed very well with the experimental results. The condition is that the electrode finger widths W1, W2 and W3 of the first to third electrode fingers are W1> W2 = W
3 and the distance L between the centers of the second and third electrode fingers is L ≠ λ / 4 (L = 0.203 in the example of FIG. 1).
λ), and the relationship between the electrode finger gap g1 of the first and second electrode fingers and the electrode finger gap g2 of the second and third electrode fingers is g1> g2. The distance between the center of one electrode finger 1 and the reflection center located at the midpoint between the second and third electrode fingers 2 and 3 is substantially equal to λ / 2. Further, the distance between the excitation center and the midpoint between the second and third electrode fingers, ie, one of the reflection centers, is approximately λ / 8 ± nλ / 2 (n = 0, 1,
2,...), It is possible to obtain a greater unidirectionality. Here, the excitation center means a position where the Fourier lowest order component of the surface potential distribution is maximized as shown in FIG.

The center of the excitation tends to be shifted from the center of the second electrode finger 2 in the direction of the first electrode finger 1, and it has been confirmed that the excitation center approaches the wider one of the gaps g1 and g2. Therefore, by adjusting these two gaps, the distance from the reflection center can be made closer to λ / 8 ± nλ / 2. Incidentally, in the example of FIG. 1, the distance between the excitation center and the reflection center was 0.122λ. As is clear from FIG. 2, the smaller the combined component of the reflected waves at the edges d and e, the greater the directionality is obtained.
The angle of the acoustic reflection vector at e should be large. And for that, the gap g
It was also confirmed that 2 should be as small as possible.

Based on these findings, for example, the first
The electrode finger width W1 of the electrode finger 1 is 0.281λ, the electrode finger width W2 of the second electrode finger 2 is 0.129λ, and the third electrode finger 3
The electrode finger width W3 is 0.129λ, the gap g1 between the first electrode finger 1 and the second electrode finger 2 is 0.199λ, the second electrode finger 2 and the third electrode finger are The gap g2 between the electrode fingers of the first and third electrode fingers 1 and 3 is set to 0.072λ, and the gap g3 between the electrode fingers of the first and third electrode fingers 3 is set to 0.1.
With the configuration of 99, the distance between the excitation center and the reflection center becomes closer to 0.124λ and λ / 8, and a larger unidirectionality can be obtained as compared with the case of FIG.
Although the present invention has been described above with reference to the example using the ST-cut quartz substrate, the present invention is not limited to this, and can be applied to a single-phase unidirectional converter using another piezoelectric substrate. X-112 @ Y LiTaO3 substrate and 45
It has been confirmed that the reflection of SAW propagating on a substrate such as a ゜ XZ Li2B4O7 substrate is particularly effective because mechanical reflection is more dominant than electrical reflection. For example, FIG. 5 shows a unidirectional ID using four reflected waves among reflections at six edges using the two piezoelectric substrates.
FIG. 9 shows an example in which the dimensions of the electrode fingers are set by performing vector analysis to obtain T. FIG. As is clear from this result, a large unidirectionality can be obtained even on a piezoelectric substrate other than quartz under the above-described conditions.

[0022]

Since the present invention is constructed as described above, it can be used for transversal SAW.
The configuration of the filter has a remarkable effect in minimizing the insertion loss in the pass band and realizing a filter exhibiting good filter characteristics. In addition, by introducing a new distributed parameter line model, it is possible to easily and accurately analyze the reflection caused by the electrode finger edge, and to obtain a single-phase unidirectional converter.

[0023]

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a basic unit of an internal reflection type IDT according to the present invention.

FIG. 2 is a diagram showing an acoustic reflection vector of FIG. 1;

3 is a graph showing experimental values obtained by measuring the directivity at the center of a stop band of the internal reflection type IDT of FIG. 1;

FIG. 4 is a diagram illustrating an excitation center.

FIG. 5 is a diagram showing a configuration using another piezoelectric substrate.

FIG. 6 is a diagram showing a configuration of a basic unit of a conventional internal reflection type IDT.

FIG. 7 is a view showing an acoustic reflection vector of FIG. 6;

FIG. 8 is a diagram showing another configuration of a basic unit of a conventional internal reflection type IDT.

FIG. 9 is a diagram showing the acoustic reflection vector of FIG. 8;

FIG. 10 is a diagram illustrating a conventional distributed constant line model.

FIG. 11 is a diagram illustrating a new distributed constant line model.

[Explanation of symbols]

 Reference Signs List 1 1st electrode finger 2 2nd electrode finger 3 3rd electrode finger W1 1st electrode finger width W2 2nd electrode finger Electrode finger width W3 ... Electrode finger width of third electrode finger g1 ... Gap between first and second electrode fingers g2 ... Gap between second and third electrode fingers g3 ... Gap between third and first electrode fingers

Continuation of front page (56) References JP-A-9-107260 (JP, A) P. Abbott, C .; S. Hartmann, D .; C. Maloch a; "Matching of Single-Phase Uniquely SAW Transducers and a Demonstrations Using a Low-Loss EWC / SPUD filtering. 1990 IEEE ULTRASON ICS Symposium, Vol. 1 (1990) p. 49-54V. B. Chvets, P .; G. FIG. Ivanov, V .; M. Makarov, V .; S. Orlov; "LOW-LOS S FILTERS USING NEW SPUDT Struct Ures." Proceedings. 1997 IEEE ULTRASONIC S SYMPOSIUM, VOL. 1 (Accepted 1998.6.8) p. 69-72 Hirota, Kazuhiro; "Study on SAW Device Equivalent Circuit Based on Mode Coupling Theory," Dentsu Talk Session, Tohoku University, Vol. 63, No. 1, Tohoku University Research Institute of Electrical Communication (July 26, Heisei 26) P. 43-44 (58) Fields investigated (Int.Cl. ⁷ , DB name) H03H 9/08 H03H 9/145 H03H 9/25 H03H 9/42-9/76

Claims (5)

(57) [Claims] 1. A surface acoustic wave (SAW) converter in which an interdigital transducer (IDT) electrode formed by repeating an electrode finger pair consisting of three electrode fingers as a basic unit a plurality of times is disposed on a piezoelectric substrate. In the above, when the three electrode fingers are first, second, and third electrode fingers in the arrangement order, respectively, the electrode finger widths W1, W2, and W3 of the first to third electrode fingers are set to W1>. W2 = W3, and the center distance L between the second and third electrode fingers is L 電極 λ / 4 (λ
Is the wavelength of the SAW propagating on the substrate) and
The relationship between the inter-electrode-finger gap g1 of the first and second electrode fingers and the inter-electrode-finger gap g2 of the second and third electrode fingers is g1> g2. A single-phase unidirectional converter characterized in that the distance from the intermediate point between the second and third electrode fingers is approximately λ / 2. 2. The distance between the excitation center in each basic unit of the IDT electrode and the midpoint between the second and third electrode fingers is approximately λ / 8 ± nλ / 2 (n = 0, 1, 2). , ...)
The single-phase unidirectional converter according to claim 1, wherein 3. The single-phase unidirectional converter according to claim 1, wherein the piezoelectric substrate is a quartz substrate. 4. The method according to claim 1, wherein the piezoelectric substrate is X-112 @ Y LiT.
3. The single-phase unidirectional converter according to claim 1, wherein the single-phase unidirectional converter is an aO3 substrate. 5. The single-phase unidirectional converter according to claim 1, wherein the piezoelectric substrate is a Li2B4O7 substrate.