JPH0451088B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the structure of an elastic convolver that extracts a confluence of two surface acoustic wave signals propagating in opposite directions by utilizing the nonlinear effect of an acoustic propagation medium.

In recent years, spread spectrum communication methods have been attracting attention as communication methods have become more sophisticated. The spread spectrum communication method transmits information by spreading the frequency band to a much larger extent (for example, 1000 times) than the frequency band required for the original transmission, and then compressing it again at the receiver, thereby reducing interference. Remarkable removal effect. The interference rejection ratio that can ideally be achieved is equal to the process gain given by the product of the duration T of one chip of the signal and the bandwidth B of the transmitted signal. However, if the encoded waveform is now time-invariant or changes in a way that the jammer can predict, intentional jamming is possible and some or all of the process gain may be lost. For example, in a frequency hopping system, which is one type of spread spectrum communication, the carrier frequency is changed for each pulse (chip) in a series of pulses (chips) that constitute each data bit.
It is possible to demotonate the signal since the receiver knows the sequence of changes in advance. On the other hand, in order for a jammer to cause interference, it is necessary to interfere over the entire band or to estimate in advance what the carrier frequency of the next pulse (chip) will be.

As described above, in spread spectrum communication, it is desirable to have a system in which the encoding of the transmitted signal can be controlled arbitrarily, and therefore the device (correlator) that receives the spectrum signal is required to be programmable. The elastic convolver is a device that is attracting a lot of attention not only because it is programmable, but also because of its simple structure and high performance.

The elastic convolver propagates two surface acoustic wave signals in opposite directions on a piezoelectric medium such as lithium niobate (LiNbO ₃ ), and obtains a convolution output of these two signals. Obtaining a convolution signal requires the intervention of some physical nonlinear effect of the medium on the surface acoustic wave. Now, if the carrier frequencies of the two input signals are both ω, the output signal has a carrier frequency of 2ω, and a uniform metal thin film is used as the output detection electrode. The required length L of the metal thin film electrode is the length of one chip of the received signal, τ (sec),
Letting the surface acoustic wave velocity be v (m/sec), it must be greater than vτ. v is usually 3000-4000
(m/sec), so a relatively long signal, for example, τ=
In order to obtain convolution of a signal of 50 .mu.sec, a length of 15 to 20 cm is required as L. The shape of a conventionally known output electrode is a rectangle that is elongated along the propagation direction of surface acoustic waves. Therefore, in order to handle the above-mentioned signal length, the length of the piezoelectric substrate must be 15 to 20 cm or more, and therefore it is difficult to obtain a crystal substrate, and even if it is available, it is extremely expensive. Furthermore, there have been many problems in manufacturing, such as difficulty in mask manufacturing and ease of damage during manufacturing. As a method to solve this problem, as described in Japanese Patent Application Laid-open No. 58-190116, a part of the output electrode, which is a surface acoustic wave waveguide, is formed by a curved waveguide, so that the entire output electrode is formed into a U-shape or an S-shape. A waveguide-shaped structure has been proposed. For example, if a U-shaped waveguide is used, the required length of the substrate can be reduced to less than half that of a straight waveguide. Similarly, if an S-shaped waveguide having two curved waveguides is used, for example, the length can be reduced to 1/3 or less.

A more desirable condition for the output electrode is that the convolution efficiency be constant regardless of location. That is, satisfying this condition is necessary to obtain ideal correlation. However, the efficiency generally varies depending on the cut of the substrate and the propagation direction of the surface acoustic waves. That is, as is well known, the terminal open circuit voltage Vo (RMS value) of the surface acoustic wave convolver is given by the following equation, assuming that the two input acoustic power densities are P _1a /W and P _2a /W.

Vo=M/W(P _1a・P _2a ) ^1/2 (1) Here, M is the nonlinear figure of merit of the substrate material, P _1a ,
P _2a is the input acoustic power, and W is the beam width of the surface acoustic wave, that is, the width of the output electrode. The figure of merit M usually differs depending on the direction of sound wave propagation on the piezoelectric substrate, so in order to make Vo constant at all locations on the output electrode, for example, W should be varied along the waveguide. It is sufficient to keep M/W constant.
For example, in the above-mentioned Japanese Patent Application No. 57-073115, the substrate is Y
The case of a U-shaped waveguide using cut LiNbO ₃ and having the Z-axis in the direction of the straight propagation path is explained. In this case, M for surface waves propagating in the Z direction is about three times that for propagating in the X direction, so it is desirable that the electrode width at the center of the curved waveguide be about 1/3 of the electrode width at the straight section. It is said that It is desirable that the waveguide width W is as narrow as possible in order to increase the output voltage.
It is usually selected to be about three times the surface acoustic wave wavelength. Therefore, the width at the center of the curved waveguide is approximately one wavelength. On the other hand, from the viewpoint of propagation path radiation loss of surface acoustic waves, it is required that the bending radius of the waveguide be as large as possible, and this requirement becomes particularly strong as the waveguide width becomes narrower. Therefore, in order to suppress radiation loss below a certain value, the area of the convolver substrate must necessarily be increased.

SUMMARY OF THE INVENTION An object of the present invention is to provide an elastic convolver which improves the above-mentioned drawbacks, and which has a small surface acoustic wave propagation path radiation loss despite using a waveguide section with a narrow waveguide width and a small bending radius. .

The elastic convolver of the present invention includes first and second interdigital transducers that excite surface acoustic wave signals on a piezoelectric substrate, and a surface acoustic wave signal that propagates in opposite directions to the piezoelectric substrate. In an elastic convolver equipped with an output electrode consisting of a straight waveguide and a curved waveguide, the convolution signal of the two signals is taken out using the nonlinear effect of the curved waveguide. It is characterized by being larger than the width of.

The present invention will be described below with reference to drawings showing embodiments. FIG. 1 shows one embodiment of an elastic convolver according to the invention. In the figure, 1 is a Y-cut LiNbO ₃ substrate. Desirable characteristics for a substrate are that both the electromechanical coupling coefficient K ² and the nonlinear figure of merit M are large, and the Y cut
The LiNbO ₃ substrate (Z direction propagation) has almost all of the above characteristics. On the surface of the substrate 1, there are interdigital transducers 2 and 3 for converting electrical signals into surface wave signals, and the beam width of the surface acoustic waves is narrowed in the direction in which the surface acoustic waves propagate, that is, in the direction of the X-axis. compressed beam compressors 4 and 5, further compressed 2
An output electrode 6 is formed for guiding the surface acoustic wave beam and extracting the convolution of these two signals. The output electrode 6 consists of straight parts 7, 8 and a curved part 9, as shown in the figure. All of these patterns (interdigital transducer beam compressors, output electrodes, etc.) consist of electrically conductive thin films and are typically fabricated using photolithographic techniques.
The manufacturing process is well known to those skilled in the art and will therefore be omitted.

Now, the beam widths of the two surface acoustic wave signals having the same angular frequency ω excited by the interdigital electrodes 2 and 3 are compressed by the beam width compressors 4 and 5, respectively. Although a horn type compressor is used as the beam width compressor in this embodiment, other types such as a compressor using a multi-strip coupler may also be used. Furthermore, the beam width compressor can be omitted by using a curved interdigital transducer that excites spherical surface acoustic waves, or by using a chirp interdigital transducer that directly excites narrow beam width surface acoustic waves efficiently. It is. The reason why it is necessary to narrow the surface acoustic wave beam in this way is to improve the convolution efficiency, as explained above, but on the other hand, the frequency dispersion of the propagation velocity becomes more pronounced as the beam width becomes smaller. In consideration of this, the beam width is usually selected to be 2 to 3 times the surface acoustic wave wavelength.

The two surface acoustic wave signals that have passed through the beam width compressors 4 and 5 enter the U-shaped convolution signal extraction electrode 6 from opposite end faces. Since this electrode 6 also functions as a waveguide for surface waves, the surface waves propagate along this waveguide, and both surface waves overlap each other. Then, due to the nonlinear effect of the substrate material on the surface waves, a convolution signal having a carrier angular frequency of 2ω is generated at the electrode 6. As the reference potential, a ground electrode 10 arranged in parallel along the output electrode 6 is usually selected. In the embodiment, the ground electrode 1
0 is provided only on one side of the electrode 6, but it may be provided on both sides, or may be provided at a position facing the electrode 6 on the back surface of the substrate.

Now, as already mentioned, the nonlinear figure of merit M generally varies depending on the cut of the substrate and the direction of surface wave propagation. The dependence of the nonlinear figure of merit M on the propagation direction (θ) of the Y-cut LiNbO ₃ used in this example is
As shown in the figure. Here, θ is the angle from the X axis. Furthermore, the figure also shows the propagation direction dependence of the electromechanical coupling coefficient K ² for surface acoustic waves (Rayleigh waves). This K ² is 2 when the propagation speeds of surface acoustic waves (Rayleigh waves) propagating on the free surface of the substrate and the surface of the conductive thin film are Vf and Vm, respectively.
It is given by (Vf−Vm)/Vf. Therefore, in a waveguide using the piezoelectric short-circuit effect as shown in this embodiment, the larger K ² is, the more the energy of the surface acoustic wave is confined under the electrode.

As shown in the embodiment of FIG. 1, the output electrode has a structure in which the width W of the curved waveguide portion 9 is wider than that of the straight portions 7 and 8. To be more precise, M/W
The width W is varied along the waveguide so that the width W is constant. Therefore, as is clear from equation (1), the convolution output Vo at each point of the waveguide
remains constant regardless of location. Furthermore, it is known that when the radius of bending is fixed to a certain value, the radiation loss of the surface acoustic waves propagating there is reduced as the width W becomes larger and the degree of energy confinement of the surface waves becomes larger. In this embodiment, W
is large and the electromechanical coupling coefficient ^K2 is larger as shown in Figure 2, so the degree of energy confinement under the electrode becomes more significant, the radiation loss becomes smaller, and the performance of the convolver is improved. It can be seen that no deterioration occurs. Furthermore, the radiation loss of a curved waveguide becomes smaller as the radius of the curve increases. Therefore, assuming a certain radiation loss is allowed, the present structure allows the use of a waveguide with a smaller radius and reduces the required substrate area.

As shown in Fig. 2, the nonlinear figure of merit M and the electromechanical coupling coefficient K ² have similar dependence on the propagation direction θ of the surface acoustic wave, that is, both M and K ² change as the propagation moves from the X-axis to the Y-axis. There is a tendency to increase. Although it has not been theoretically proven that this tendency necessarily holds true for all piezoelectric materials and cuts, it seems that it almost holds true empirically.

In the above embodiments, the shape of the waveguide serving as the output electrode was a U-shape with one curved waveguide, but the shape is not necessarily limited to this, and structures having two or more curved waveguides may also be used. good. Furthermore, it is clear that the rights of this patent extend to double channel convolvers in which two waveguides are installed in parallel as a structural method to reduce self-convolution, which is one of the harmful spurious signals of elastic convolvers. be.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of an elastic convolver according to the present invention, in which 1 is a Y-cut lithium nanoboard 2, 3 is an input interdigital transducer;
4 and 5 are surface acoustic wave beam width compressors 6 are waveguides and convolution signal extraction electrodes; 10
is the grounding electrode. FIG. 2 is a graph showing the propagation angle dependence of the electromechanical coupling coefficient K ² and the nonlinear figure of merit M of Y-cut lithium niobate.

Claims (1)

[Claims] 1 A piezoelectric substrate is provided with first and second interdigital transducers that excite surface acoustic wave signals, and the surface acoustic wave signals from both of the transducers are propagated in opposite directions and utilize the nonlinear effect of the substrate. In the elastic convolver, the width of the curved waveguide is made larger than the width of the straight waveguide. An elastique convolver characterized by: