JPH0462208B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the structure of an elastic convolver that extracts a convolution of 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. Spread spectrum communication method spreads the frequency band over a much larger frequency band (for example, 1000 times) than that required for transmitting information, and then compresses it again at the receiver, which is effective in eliminating interference waves. is remarkable. The ideally achievable interference rejection ratio is equal to the process gain given by the product of the duration T of one chip of the signal and the bandwidth W 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 wave frequency is changed for each pulse of a series of pulse trains constituting 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 will be.

In addition to the interference wave removal effect described above, a communication system that can arbitrarily change the transmission waveform has many advantages. Under these circumstances, it is strongly desired that the device (correlator) that receives the spread spectrum signal be programmable, and the elastic convolver is attracting a lot of attention 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 nitrate (LiNbO ₃ ), and obtains a convolution output of these two signals. To get this convolution signal,
It requires the intervention of some physical nonlinear effect of the medium on the surface acoustic waves. Both of the two input signals have a carrier frequency of ω, and a uniform metal thin film is used as the output detection electrode. The required length L of the metal thin film electrode must be greater than vτ, where τ (sec) is the length of one chip of the received signal and v (m/sec) is the surface acoustic wave velocity. Since v is usually 3000 to 4000 (m/sec), in order to obtain a convolution of a relatively long signal, for example, a signal with τ = 50 μsec, the length must be 15 to 20 cm.
Is required. 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. In order to cope with this problem, a curved waveguide having one or more curved portions is proposed as an output electrode for extracting a convolution signal, as described in Japanese Patent Application No. 57-073115. However, what is shown in the example is only an elastic convolver with a single channel structure, and such a structure may not be able to sufficiently suppress spurious signals called self-convolution, which will be explained below. There is.

In general, the proportion of the power converted from the two input signals to the convolution output signal due to the nonlinear effect of the substrate is very small, so most of the surface acoustic wave signal emitted from one interdigital transducer is It passes through the electrode and reaches the other input interdigital transducer installed at the opposite position to the interdigital transducer. At this time, a part of the surface acoustic wave signal is reflected by the interdigital transducer and reaches the output electrode again. Therefore, when a signal of a certain time length is input to the convolver, the leading part of the input signal reflected by the interdigital transducer located at the other end is the subsequent signal part that is still propagating in the correct direction. overlap in the output electrode area, and these two
Signal convolution occurs. This convolution signal is usually called self-convolution. The same phenomenon occurs since the surface wave signal emitted from the other interdigital electrode is also reflected by the opposite interdigital transducer. Normally, the power applied to one input converter of the convolver as a reference signal is larger than the input signal, so the magnitude of the self-convolution signal due to the above causes is due to the difference between the signal and the reference wave. A situation may also occur where the signal is on the same level as the convolution signal or even exceeds the latter.

In addition to the self-convolution mentioned above, there are some known spurious signals that occur in elastic convolvers, but they are generally smaller than self-convolution, and appropriate Suppression methods have been devised. Now, in order to suppress the above-mentioned self-convolution to a sufficient level, that is, in order to fully utilize the process gain of the convolver under any environment, it is necessary to suppress the self-convolution. The relative level to the output signal is expressed as the convolver time -
It is necessary to suppress it by more than the value given by the band product. The most effective method for suppressing this is to arrange two convolvers in parallel on the same board.
Further, one of the input interdigital transducers constituting one convolver is connected to one of the input interdigital transducers constituting the other convolver so as to have electrically opposite polarity. The input transducers installed on both sides of each output electrode are connected to a common power source (including a load). Therefore, the phase of the convolution output of the two signals applied to both input transducers is
Since the difference is 180°, the output signal can be extracted in the form of a difference signal. On the other hand, the surface acoustic wave components that are not converted into convolution signals by the output electrodes reach the transducer located on the opposite side of the output electrode in each channel, where they are converted into electrical signals and sent to one common signal. Although added to the electrical load,
Since their phases are 180° different from each other, they are electrically neutralized and no opposing waves are generated due to re-excitation. Furthermore, reflections due to electric field short-circuiting effects or mass addition effects due to electrode fingers are sufficiently small compared to reflections due to re-excitation in normal converters in which electrical matching is maintained for the purpose of minimizing conversion loss. Furthermore, by using a "split electrode" configuration for the electrode fingers, further significant improvements can be made.

As explained above, the configuration in which two channels of convolvers are arranged in parallel and the outputs generated at both output electrodes are extracted differentially is an extremely effective method in terms of suppressing self-convolution.
In order to extract the output as a complete difference signal, it is necessary that the surface wave propagation times of both convolvers be equal to each other.

Although it is theoretically possible to juxtapose convolvers having curved waveguides and having the same surface wave propagation time, it is extremely difficult in practice. For example, in the case of the U-shaped waveguide exemplified in the above-mentioned Japanese Patent Application No. 57-073115, a U-shaped waveguide with a smaller (or larger) radius of curvature is provided inside (or outside), and In terms of design, it is possible to equalize the surface wave propagation times by adjusting the length of the straight portion. However, in order to do this, it is essential to know in advance the sound velocity of the surface waves propagating in any direction on the piezoelectric substrate with extremely high accuracy. The sound speed characteristics of waves are
It varies slightly and also depends on the thickness of the conductive film that serves as a guide, so it is extremely difficult to precisely match the surface wave propagation times of both convolvers.

An object of the present invention is to provide an elastic convolver that is easy to manufacture and has excellent spurious, particularly self-convolution signal suppression characteristics, despite using a curved waveguide.

According to the present invention, the first and second interdigital transducers that excite surface acoustic wave signals on the piezoelectric substrate and the surface acoustic wave signals from both the transducers are propagated in opposite directions and are superimposed, Two elastic convolvers equipped with output electrodes for extracting convolution signals of the two signals by utilizing the nonlinear effect of the substrate are arranged side by side, and the first (or second) interdigital transducers of both convolvers are connected to each other. In the same phase, the second (or first) interdigital transducer is excited in the opposite phase, and the output signal is taken out as a differential voltage between the output electrodes of both convolvers, and the output electrodes are all connected to an even number of curved sections. An elastic convolver is obtained, which is characterized by comprising an elastic surface acoustic wave waveguide including:

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to drawings showing embodiments. The figure shows an embodiment of an elastic convolver according to the invention. In the figure, reference numeral 1 denotes a piezoelectric substrate, and as will be described later, a material having a large figure of merit M representing the strength of nonlinear interaction, such as LiNbO ₃ , is a desirable material. On the surface of the piezoelectric substrate, interdigital transducers 2 and 3 for converting electrical signals into surface acoustic wave signals, and output electrodes 4 for extracting these surface acoustic wave signals are formed. In this embodiment, the beam width of the surface acoustic waves excited by the interdigital transducers 2 and 3 is sufficiently small, approximately several times the wavelength of the surface acoustic waves at the operating frequency, so that the beam width is directly guided to the output electrode as a waveguide. However, a surface acoustic wave is excited by a relatively wide interdigital transducer, compressed by a horn-type beam width compressor or a beam width compressor using a multi-strip coupler, and then the output electrode is used as a conductor. It may be made to match the width of the wave path. The reason why such beam width compression of the surface acoustic wave signal is necessary is as follows. That is, as is well known, the terminal open circuit voltage V of the surface acoustic wave convolver.
(RMS value) is the two-input acoustic power density P ₁ a/W,
It is proportional to the product of P ₂ a/W and is expressed by the following formula.

Vo=M/W(P ₁ a・P ₂ a) ^1/2 (1) Here, M is the figure of merit of the nonlinear substrate material,
P ₁ a and P ₂ a are the input acoustic powers, and W is the beam width. As is clear from the above formula, if W is made smaller,
The convolution output voltage Vo can be increased.

Now, two surface acoustic wave signals excited by both interdigital transducers 2 and 3 and whose carrier angular frequencies are ω are incident on the S-shaped output electrode 4 from mutually opposite end faces. Since this electrode 4 also functions as a waveguide for surface acoustic waves, the surface waves propagate along this waveguide, and both surface wave signals 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 4.

Further, as shown in the figure, a convolver similar to the elastic convolver described above, which is composed of input transducers 5, 6 and an output electrode 7, is arranged in parallel. However, each electrode finger of the interdigital transducer 6 is connected to an extraction electrode so as to excite a surface wave whose phase is 180° different from that of the transducer 3. Further, the length of the elastic convolver, that is, the effective surface wave propagation distance between both input transducers, is set to be equal between both channels. Therefore, an input signal is input to each set of input converters 2 and 5, and 3 and 6 of both convolvers.
When E ₁ and E ₂ are applied, the convolution output signals appearing at output electrodes 4 and 7 are out of phase by 180°. Therefore, output can be efficiently extracted by differentially extracting the output from both output electrodes.
As shown in the figure, a specific configuration method is as follows: tap electrodes 9 and 1 are provided at both ends of the load resistor 8 and each output electrode.
0 should be electrically connected. As already mentioned, such a two-channel structure can effectively suppress self-convolution, which is the most serious spurious that can occur in an elastic convolver.

However, in this case, in order to extract the output efficiently, it is necessary that the phases of the convolution output signals obtained at the output electrodes of both channels differ ideally or by approximately 180 degrees.
For this purpose, it is necessary to set the surface wave propagation time in both channels of the convolver to be equal. However, since the speed of surface waves generally varies depending on the cut of the piezoelectric substrate crystal and the propagation direction, a two-channel converter having curved portions with different geometric shapes and equal surface wave propagation times is fabricated. In practice, it is extremely difficult. On the other hand, in the structure having two curved waveguide output electrodes as shown in the above embodiment, the time T ₁ T ₂ ' for propagating through the curved lengths L ₁ and L ₂ of one waveguide 4 is longer than that of the other waveguide 4. The length of the bending part of the waveguide 7
By making the propagation times T ₃ and T ₄ the same through L ₃ and L ₄ and their respective shapes, they can always be made equal regardless of the in-plane anisotropy of the sound velocity in the substrate crystal.

That is, T ₁ =T ₃ , T ₂ =T ₄ (2). In this example, each waveguide has two bends, but the structure using an even number of bends based on the above principle reduces the propagation time difference between both channels based on the in-plane anisotropy of the sound speed. It can be made small enough to be practically acceptable.

The advantage of using a curved waveguide is that it is possible to obtain signal convolution with a long duration using a substrate of relatively short length, and the S-shaped waveguide shown in this example , it is shortened to less than 1/3 compared to the conventionally known linear type. A more desirable condition for the output electrodes 4 and 7 is that the convolution efficiency be constant regardless of location. Therefore, it is necessary that the right side of equation (1) be constant at all positions of the waveguide electrodes 4 and 7. However, the figure of merit M generally varies depending on the cut of the substrate and the propagation direction of the surface acoustic waves. Therefore, in order to correct this, it is sufficient to change the width W of the electrodes 4 and 7 along the propagation direction and keep M/W constant. For example, cut board 1 in a Y-cut.
LiNbO ₃ , interdigital transducers 2, 3 and 5,
7 is the propagation direction of the surface acoustic wave excited by Z
If it is an axis, then M for a surface wave propagating in the Z direction is approximately three times that for propagating in the X direction. Therefore, it is desirable that the electrode width of the curved waveguide electrode section 8 be approximately 1/3 of the electrode width of the straight section.

As explained above, according to the present invention, since a curved waveguide is used, a relatively short substrate can be used.
It is possible to obtain convolution of long-duration signals, and at the same time, it is easy to achieve the exact matching of surface wave propagation times between both channels, which is necessary for self-convolution suppression by double channels, making it possible to achieve industrial The value is extremely great.

[Brief explanation of the drawing]

The figure shows one embodiment of an elastic convolver according to the invention. 1 is a piezoelectric substrate, 2, 3, 5, and 6 are interdigital transducers for input, and 4 and 7 are electrodes for taking out convolution signals.

Claims (1)

[Claims] 1. The first part that excites a surface acoustic wave signal on the piezoelectric substrate.
and a second interdigital transducer, and the surface acoustic wave signals from both of the transducers are propagated in opposite directions and are superimposed, and a convolution signal of the two signals is extracted using the nonlinear effect of the substrate. Two elastic convolvers equipped with output electrodes are placed side by side, and the first (or second) interdigital transducers of both convolvers are in phase, and the second (or first) interdigital transducers are out of phase. In an elastic convolver, which is excited and the output signal is taken out as a differential voltage between the output electrodes of both convolvers,
An elastic convolver characterized in that each of the output electrodes is comprised of a folded surface acoustic wave waveguide including an even number of curved sections.