JPS6350883B2 - - Google Patents

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a unidirectional transducer for surface acoustic waves.

(Prior art) Surface acoustic wave transducers are currently used in telecommunication filters, delay lines, optical IC circuits, real-time Fourier transformers,
Widely used in ghost cancellers for TVs, etc.

However, conventional transducers simply place interdigital electrodes on a piezoelectric substrate, and electrically, the main method is to excite in the same phase over the entire surface, so the excitation and reception transducers A strong standing wave is generated internally, and waves are emitted before and after the standing wave, resulting in a so-called bidirectional characteristic. This causes problems such as emitting waves to unnecessary parts and causing multiple reflections, making it impossible to obtain low loss characteristics and low ripple characteristics.

As a method of solving this problem, a method of obtaining unidirectional characteristics by appropriately varying the electrical excitation phase has been developed, and good results have been obtained. However, this method requires at least three different types of electrodes to be provided on one substrate, which causes problems when drawing out the electrodes, so a method is used in which each electrode crosses in a low coupling state. Either, or one of them must be made to meander within the converter. For the former,
There is a problem of the gap crossing of the electrodes, and the latter problem has conventionally had problems such as a blank part being formed in a part of the excitation source and the size of the excitation source being uneven.

(Problems to be Solved by the Invention) The present invention has been made in order to eliminate the drawbacks of the latter method. The purpose of this is to eliminate misalignment of the angle and phase.

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a first excitation electrode having a plurality of electrode fingers, and an electrode that intersects with the electrode fingers of the first excitation electrode. a second excitation electrode having a finger and to which a signal having a phase difference of θ (radian) with respect to the signal applied to the first excitation electrode is applied; and each of the first and second excitation electrodes. a ground electrode provided between electrode fingers, one periodic section of the arrangement of the ground electrodes is a basic section, the length of this basic section is equal to an even multiple of the wavelength λ of the surface acoustic wave, and is in the basic section. The sizes of the gaps between the electrodes are all approximately the same, and the center points of the gaps from the electrode fingers of the first excitation electrode in the basic section to the ground electrodes on the left and right sides are A and B, respectively. When the center points of the gap from the second excitation electrode to the ground electrodes on the left and right sides are respectively C and D, there is l ₁ between the length l ₁ between AC and the length l ₂ between BD. =l ₂
=l, the length y between AB has the relationship y=1/2λ, and when l=1/nλ, the relationship is approximately n=2π/(θ+(2i-1)π), where i is a positive integer. A unidirectional transducer for surface acoustic waves is provided.

(Function) According to such a configuration, when a surface acoustic wave is excited, if the wave propagation direction is X, the vibration source seen from the negative direction of X is completely canceled out, and only the positive direction component remains, resulting in a unidirectional characteristic. is obtained.

Furthermore, low ripple characteristics with sufficient directionality can be obtained near the passband, and a unidirectional converter without spurious signals can be obtained over a wide frequency range.

(Example) The configuration and operation of the unidirectional converter of the present invention will be described in detail as follows.

FIG. 1 is a transducer of the invention fabricated on a piezoelectric substrate. Electrode 1 is a meandering ground electrode, and electrode 2 is an excitation electrode having a plurality of reference electrode fingers. The electrode 8 is an excitation electrode for providing a signal whose phase is delayed by θ (radians) from the electrical signal applied to the electrode 2. Figure 2 is a cross-sectional view of the part of the transducer in Figure 1 that actually excites surface acoustic waves, and electrodes 1, 2, and 3 are electrodes 1, 2, and 3 in Figure 1, respectively.
Corresponds to 2 or 3. Here, the gaps d between each electrode are all equal, and the distance l ₁ between the center of the gap between electrodes 1 and 3 and the center of the gap between electrodes 1 and 2 is equal to the gap d between electrodes 3 and 1. It is equal to the distance l ₂ between the center and the center of the gap between the electrodes 2,1. Moreover, as shown in FIG. 2, this electrode has a distance L ₁ from the center of the wide electrode of the meandering electrode 1 to the center of the wide electrode of the next meandering electrode 1.
Or from the center of the narrower electrode of one meandering electrode 1 to the center of the narrower electrode of the next meandering electrode 1
It is periodic with L ₂ as the basic interval. Further, as shown in Fig. 2, electrodes 1 and 3 are formed so that the distance y between the centers of each gap on both sides of electrode 3 is y = λ ₀ /2, where λ ₀ is the wavelength of the elastic wave to be excited or received. has been done.

In general, when the propagation constant of an elastic wave is β, the elastic stress induced by an electrical signal whose phase is delayed by θ is the stress induced by a signal with no phase lag at a point apparently distant by (θ/β). is equal to Therefore, when the wave propagation direction is X, the apparent δ oscillation source distribution viewed from the positive direction of X is as shown in Figure 3, and the δ oscillation source distribution viewed from the negative direction of X is as shown in Figure 4. become. In both figures, the dotted line 6 is the vibration source generated by the electrode 3, and the solid line 5 is the vibration source generated by the electrode 2.
Wave whose basic interval L ₁ or L ₂ has a period of 2i (wavelength λ)
Considering, let l ₁ = l ₂ = l, l = λ ₀ /n...(1), and θ=(2/n-(2i-1))π, (where i is a positive integer)... (2) Since all the vibration sources shown in FIG. 4 are canceled out, only the positive direction component of X shown in FIG. 3 remains, and a unidirectional characteristic is obtained.

In particular, when n = 4/3 and θ = π/2, the amplitude and phase relationship of the vibration source shown in Figure 3 can be made uniform throughout, so the frequency characteristics are as shown by the solid line in Figure 5. Characteristics free of spurious components can be obtained. The solid line in Figure 5 is the forward direction of the converter (in this case
The dotted line is the characteristic for the opposite direction (the negative direction of X). The dashed-dotted line is an example of the characteristics of a conventional meandering type unidirectional converter, with points a and b.
A large spurious characteristic occurs at the point.

Using the same concept as the above method, several electrodes of each phase are arranged together as shown in Fig. 6, and the distance (l) between the gaps is set as θ=π/2, i=2, and the formula (1) is given. Even if (2) is made to hold, similar good characteristics can be obtained.

Although we have mainly described the case of excitation, on the receiving side, surface acoustic waves are converted into electrical quantities by piezoelectric action in a process opposite to the excitation mechanism described above, so by adopting the above configuration, unidirectional becomes a receiving converter.

As described above, in the converter of the present invention, the relationship between the gap position between the electrodes and the phase θ is typically l=3λ ₀ /4, θ=π/2, and the relationship is l=1 ₁ =l ₂ =λ ₀ /n, the following relationship is established: n=2π/(θ+(2i-1)π) (i is a positive integer). The allowable value for θ is ±10%.

When the width of each gap is λ ₀ /8, which is the most typical example of the configuration shown in FIG. 2, which is an embodiment of the present invention, the width of each electrode finger is equal to the width of the meandering electrode. is 5λ ₀ /8, the narrower width of the meandering electrode is λ ₀ /8,
The width of the reference excitation electrode 2 is 3λ ₀ /8, and the width of the electrode 3 for adding a signal whose phase is delayed by θ is 3λ ₀ /8.
becomes. In this way, the electrode finger of each excitation electrode always has a gap on its left and right sides, and when the distances l ₁ and l ₂ are 3λ ₀ /4 as in the example in FIG. 2, the electrode finger is λ ₀ The width will not exceed /2. As shown in Figure 6, i=
2, and even when l ₁ and l ₂ exceed 3λ ₀ /4, the width of each electrode finger is does not exceed λ ₀ /2, and l ₁ ,
They are arranged for each wavelength within l ₂ . In the case of Fig. 6, where the width of each gap is λ/8, which is the most typical example, the width of each electrode finger is 5λ ₀ /8 for the meandering electrode, as in the example of Fig. 2 above. It consists of electrodes with a width of λ ₀ /8, and the widths of electrode 2 and electrode 3 are 3λ ₀ /8.

Furthermore, with the same configuration, in order to prevent leakage of the electric field in the converter section, a dielectric thin film 10 is partially inserted as shown in FIG.
Converters made by attaching a common ground electrode 11 and depositing a common ground electrode 11 thereon, and converters constructed by dividing a wide electrode part into two or three parts as shown in Fig. 8, for example, when all the gaps are In the case where the width is equal to λ ₀ /8, the width of each divided electrode finger is also λ ₀ /8.

(Effects of the Invention) As described above, according to the present invention, a unidirectional converter can be obtained that has low ripple characteristics with sufficient directionality near the passband and has no spurious characteristics in a wide frequency range.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure of a unidirectional transducer for surface acoustic waves according to the present invention. FIG. 2 is a diagram showing a cross section of the transducer and the arrangement of each electrode and gap. FIG. 3 is a diagram showing the distribution of excitation sources in the traveling direction (forward direction) of surface acoustic waves. FIG. 4 is a diagram showing the distribution of excitation sources in the opposite direction (reverse direction) to the traveling direction of surface acoustic waves. FIG. 5 is a diagram showing an example of forward and reverse frequency characteristics of the converter of the present invention and an example of forward frequency characteristics of a conventional unidirectional converter. FIG. 6 is a diagram showing an example in which electrodes of each phase are arranged together using the same concept as the basic type of FIG. 2. FIG. 7 is a diagram showing a structure provided with an electrical shielding electrode. Figure 8 shows
The figure which shows the structure when each electrode is subdivided and implemented. 1...Meandering grounding electrode, 2...Excitation electrode serving as a reference, 8...An electrode for adding a signal whose phase is delayed by θ, 4...Piezoelectric or electrostrictive substrate, 5...
... Vibration source by electrode 2, 6... Vibration source by electrode 3,
7... Forward characteristics by the converter of the present invention, 8...
Reverse characteristics by the converter of the present invention, 9... Forward characteristics by a conventional unidirectional converter, 10... Dielectric thin film, 11... Ground electrode made of metal thin film.

Claims (1)

[Scope of Claims] 1. A first excitation electrode having a plurality of electrode fingers, and a signal applied to the first excitation electrode having electrode fingers that intersect with the electrode fingers of the first excitation electrode. a second excitation electrode to which a signal having a phase different by θ (radians) from the ground is applied; and a ground electrode provided between each electrode finger of the first and second excitation electrodes, One periodic section of the electrode arrangement is a basic section, and the length of this basic section is equal to an even multiple of the wavelength λ of the surface acoustic wave, and the sizes of the inter-electrode gaps in the basic section are all approximately the same, and The center points of the gap from the electrode fingers of the first excitation electrode in the basic section to the ground electrodes on its left and right sides are A and B, respectively, and the center points of the gap from the second excitation electrode to the ground electrodes on its left and right sides are respectively A and B. When the center points of the gap leading to are respectively C and D, the length between AC l ₁ and the length between BD l ₂
There is a relationship between l ₁ = l ₂ = l, and the length y between A and B is y = 1/2λ, and when l = 1/nλ, approximately n = 2π/(θ+ (2i-1)π) A unidirectional transducer for surface acoustic waves characterized by having the following relationship. 2. The unidirectional surface acoustic wave according to claim 1, wherein the electrical phase difference θ is within π/2 (radian) ±10 degrees with θ=π/2 (radian) as the center. sex converter.