JPS62216411A - Unidirection converter for surface acoustic wave - Google Patents

Abstract

PURPOSE:To obtain a unidirection converter having no spurious by defining the section of one cycle of an array of earth electrodes as a fundamental section and equalizing the length of this fundamental section to an even-fold wavelength of a surface acoustic wave and equalizing lengths of inter-electrode gaps in the fundamental section to one another and specifying the gap and the length between electrodes CONSTITUTION:Gaps (d) between electrodes are equal to one another, and a length l1 between the center of the gap between electrodes 1 and 3 and that between electrodes 1 and 2 is equal to a length l2 between the center of the gap between electrodes 3 and 1 and that between electrodes 2 and 1. Electrodes are arranged periodically with a section L1 from the wider meandering electrode to the following wider meandering electrode 1 or a section L2 from the center of the narrower meandering electrode 1 to the following narrower meandering electrode 1 as the fundamental section. Electrodes 1 and 3 are so formed that a length (y) between centers of gaps on both dies of the electrode 3 is equal to lambda0/2 where lambda0 is the wavelength of an excited or received surface acoustic wave.

Description

DETAILED DESCRIPTION OF THE INVENTION Surface acoustic wave transducers are currently widely used in telecommunication filters, delay lines, optical IC circuits, real-time Fourier transformers, ghost cancellers for televisions, and the like.

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 inside of the excitation and reception transducer A strong standing wave is generated, 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, and it is difficult to find a way to intersect each electrode in a low-coupling state. A method must be adopted in which one of the two wires is made to meander within the converter. The former has the problem of intersecting gaps between the electrodes, and the latter has conventional problems such as a blank area appearing in a part of the excitation source and irregularities in the size of the excitation source.

In order to eliminate the drawbacks of the latter method, the present invention is characterized by devising the size and arrangement of the gap between the electrodes, and eliminating irregularities in the size and phase of the vibration sources.

By doing so, a unidirectional converter can be obtained that has low ripple characteristics with sufficient directivity near the passband and has no spurious characteristics in a wide frequency range.

The structure 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 a reference excitation electrode. The electrode 3 is an excitation electrode for providing a signal whose phase is delayed by θ (radians) from the electrical signal applied to the electrode 2. FIG. 2 is a cross-sectional view of the part that actually excites surface acoustic waves, and the electrodes 1.2J and 1.2J in FIG. 1 are respectively the electrodes 1.2.3. corresponds to Here, the gaps d between the electrodes are all equal, and the distance between the center of the gap between electrodes 1.30 and the center of the gap between electrodes 1.21 and 2. .1
is equal to the center-to-center distance of the gap between 1 and its size is e
shall be. Further, this electrode is periodic with the basic section being Ll or L2.

Generally, 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 apparently (θ/
β) is equal to the stress induced by a signal with no phase lag at a point far away. Therefore, when the direction of wave propagation is X,
The apparent S source distribution viewed from the positive direction is as shown in Figure 3, and the apparent source distribution viewed from the negative X direction is as shown in Figure 4. In both figures, the dotted line is the vibration source caused by electrode 3, and the solid line is the vibration source caused by electrode 2. Now basic section L1
Or if we consider a wave (wavelength λ0) with two periods of L2! -λo/n-・・・・・・-... (1) (where i is a positive integer) -... (2) When it becomes, all the oscillation sources shown in Figure 4 are canceled out. Therefore, only the positive direction component of X shown in FIG. 3 remains, and a unidirectional characteristic is obtained.

In particular, in the case of n=4/#, 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 positive direction of X)
The dotted line is the characteristic for the opposite direction (negative direction of X). The dashed-dotted line is an example of the characteristics of a conventional meandering type unidirectional converter, in which large spurious characteristics occur at points a and b.

Using the same concept as the above method, several electrodes of each phase are arranged together as shown in Fig. 6, and the distance i) between the gaps is set as θ-7, so that equations (1) and (2) hold. However, 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 above-mentioned excitation mechanism, so by adopting the above configuration, unidirectional reception can be achieved. Become a converter.

In this way, the converter of the present invention has the following conditions: n = 2π/(θ+(21-1)π)(if
; is a positive integer) and θ=π/2, and the allowable value of θ is ±10%.

Furthermore, a converter and a converter with the same structure were fabricated by applying a dielectric thin film 10 partially as shown in FIG. The present invention also includes a converter in which a wide electrode portion is divided into two or three parts as shown in FIG.

[Brief explanation of drawings]

Figure 1 shows the structure of a unidirectional transducer for surface acoustic waves. Figure 2 shows the cross section of the transducer and the arrangement of each electrode and gap. Figure 3 shows the distribution of excitation sources in the direction of travel (forward direction) of surface acoustic waves. Figure 4 shows the direction opposite to the traveling direction of surface acoustic waves (opposite direction).
distribution of excitation sources for FIG. 5 shows 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 an example in which the electrodes of each phase are arranged together using the same concept as the basic type shown in FIG. 2. FIG. 7 shows a structure provided with an electrical shielding electrode. FIG. 8 shows a structure in which each electrode is subdivided. 1. Meandering grounding electrode; 2. An excitation electrode serving as a reference; 3. an electrode for adding a signal whose phase is delayed by θ; 4.・--Piezoelectric or electrostrictive substrate, " 5.?
! Vibration source using electrodes, 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 the conventional unidirectional converter, 10, - Dielectric thin film, 11
．．・Ground electrode made of metal thin film

Claims (1)

[Claims] In a piezoelectric surface acoustic wave unidirectional transducer having a meandering ground electrode 1, the size of the gap between the reference excitation electrode 2 and the ground electrode 1 is such that a signal whose phase is delayed by θ (radians) is added. equal to the size of the gap between the excitation electrode 3 and the ground electrode 1, and the distance between the centers of the gap (
l) is one-nth wavelength, n=2π/(θ+(2
i-1) π), (where i is a positive integer), and θ=π/2
A unidirectional transducer for surface acoustic waves having the following relationship, and a unidirectional transducer for surface acoustic waves in which θ is within ±10° from the above condition.