JP2004007626A - Surface acoustic wave device and communication equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface acoustic wave device and a communication system solving a problem of an input/output impedance difference, and a new structure of communication equipment using the same. <P>SOLUTION: An electomechanical coupling coefficient of an interdigital electrode 2 having a smaller number of electrode pairs of input/output interdigital transducers is set larger than an electromechanical coupling coefficient of an opposite interdigital electrode. An interdigital transducer 3 having a larger number of electrode pairs is divided into tracks in a direction perpendicular to the propagation direction of a surface acoustic wave, and each divided interdigital transducer group is electrically connected in series. Each intersection portion of the interdigital transducers is also connected in series. With this, it becomes possible to improve the performance of the surface acoustic wave device of a matched filter type, and the performance of the spread spectrum communication equipment of code spread system. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device, a spread spectrum communication system, and a communication device using the same.
[0002]
2. Description of the Related Art A conventional spread spectrum communication system using a code modulation system and a surface acoustic wave device used therefor are disclosed in Japanese Society of Acoustical Sciences, Proceedings of the Acoustical Society of Japan (March 1984, page 705). , 0, the signals are subjected to phase modulation of 0 ° and 180 °, and the polarities of the interdigital electrodes of the matched filter type surface acoustic wave device corresponding thereto are inverted. As a result, a sharp output is obtained only when a signal that matches the spreading code is input, and a demodulated signal is obtained directly from the RF band. Further, since this type of surface acoustic wave must be converted into a surface acoustic wave signal corresponding to a normal input signal, the number of electrode pairs of the input IDT must be one or a small number.
[0003]
[Problems to be solved by the invention]
Since the number of pairs of input interdigital electrodes of the surface acoustic wave device is one or a few, and the number of pairs of output electrodes is large (proportional to the spreading code length), the input and output impedances are significantly different, thereby increasing the mismatch loss. In many cases.
[0004]
An object of the present invention is to provide a surface acoustic wave device and a communication system which solve the problem of the difference between input and output impedances, and a novel structure of a communication device using the same.
[0005]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, the electromechanical coupling coefficient of the interdigital transducer having the smaller number of electrode pairs of the input and output interdigital transducers is made larger than that of the opposing interdigital transducer, and the number of electrode pairs is reduced. The larger interdigital transducer is track-divided in the direction perpendicular to the direction of surface acoustic wave propagation, and the divided interdigital transducers are electrically connected in series. Can be solved by electrically connecting them in series.
[0006]
[Action]
Since the input / output interdigital electrode has the above-described structure, the difference in the input / output impedance can be reduced, so that the mismatch loss can be reduced.
[0007]
【Example】
Hereinafter, an embodiment of the present invention will be described with reference to FIGS.
[0008]
FIG. 1 schematically shows a surface acoustic wave device using the present invention. An input interdigital electrode 2 and an output interdigital electrode 3 are arranged on a surface acoustic wave substrate 1. Further, a sound absorbing material 4 is applied to suppress a reflected wave from the end face of the substrate. Signals (0 ° and −90 ° in this case) that have been subjected to phase modulation by a spreading code are input to input terminals 5 and 5 ′, and correlation signals are output to output terminals 6 and 6 ′. As the output interdigital electrodes, group-type electrodes corresponding to the diffusion codes 1, 0 and shifted by a distance (0, -wavelength / 4) corresponding to the phase of 0 ° and -90 ° are used. In the figure, the electrodes are indicated by lines, but all the electrodes and the space portions are transported at １ ／ of the surface acoustic wave wavelength at the frequency. This is performed for the purpose of suppressing the reflection due to the difference in the characteristic impedance with respect to the surface acoustic wave between the portion having the electrode and the space portion. FIG. 2 schematically shows a conventional 0 ° and 180 ° phase modulation signal and a matched filter type IDT corresponding thereto. The phase modulation signal 7 corresponds to the spreading codes 1 and 0, and the phases of 0 ° and 180 ° are applied, respectively. The electrode intersections 8 corresponding to 0 ° and the electrode intersections 9 corresponding to 180 ° are arranged as shown in the figure. As is apparent from the figure, an unnecessary intersection 10 is formed in a portion where the code changes. FIG. 3 schematically shows the communication system according to the present invention and the interdigital transducer of the surface acoustic wave device which do not cause this unnecessary intersection. The phase modulation signal 11 corresponds to the spreading codes 1 and 0, and the phases of 0 ° and −90 ° are applied, respectively. The electrode intersections 12 corresponding to 0 ° and the electrode intersections 13 corresponding to −90 ° are arranged as shown in the figure. As is clear from the figure, unnecessary intersections are not formed even in the portion where the code changes. In the figure, one code (corresponding to one cycle (wavelength)) corresponds to a signal (electrode), but usually, one code corresponds to several periods (several wavelengths) of signal (electrode). Here, for simplicity, one code corresponds to one cycle of signal. As described above, by using the first embodiment, there is no unnecessary crossing, and therefore, there is no increase in the side lobe of the correlation output, and good characteristics can be obtained.
[0009]
FIG. 4 schematically shows a part of the surface acoustic wave device according to the second embodiment. In the first embodiment, since a group type electrode is used, a meandering (turning) electrode is formed, so that an increase in loss due to conductor resistance there is a problem. Therefore, in this embodiment, an interdigital electrode is formed without using a meander electrode. Further, in the present embodiment, the interdigital transducer is adapted to a system in which the modulation is performed in both the phases of -90 ° and + 90 ° when the spreading codes 1 and 0 change. The intersection 14 corresponding to the code 1 is followed by the intersection 15 having the -90 ° phase corresponding to the code 0, and the intersection 16 having the + 90 ° phase corresponding to the code 1 is arranged next. By appropriately combining the phases of −90 ° and + 90 ° in this way, it is possible to suppress the background output in the absence of the correlation output and to set the center of the signal spectrum at the carrier frequency. For example, when the code 1 is fixed to 0 ° and the code 0 is fixed to 90 °, a large correlation output is generated for the carrier of the carrier signal, and the correlation characteristic is degraded. Further, when a code change (from 0 to 1 or from 1 to 0) is simply applied to −90 °, the entire spectrum is shifted because the entire code length becomes short. This problem is solved, for example, by applying a code change to a phase modulation as shown in FIG. First, assume that the first phase state is (1). Next, a phase difference of + 90 ° is given in accordance with the code change, and the phase is set to (2). At this time, the state of the electrode is the same as the structure that changes from the intersection 15 to the intersection 16. Similarly, the phases of (3) and (4) are set corresponding to the code change. Next, when a code change occurs, the same structure as the structure changing from the crossing 14 to the crossing 15 is used, and a phase difference of -90 ° is given to set the state of (5). Similarly, the phase states of (6) and (7) are set, and similarly, the phases are changed in the same order in the case of the phase state from (1). As a result, almost no correlation occurs for the carrier carrier, and the center of the spectrum coincides with the carrier frequency. As described above, according to the present embodiment, it is possible to suppress an increase in loss due to the meander electrode, to suppress the correlation output with the carrier, and to make the center of the spectrum coincide with the carrier frequency.
[0010]
Next, a third embodiment will be described with reference to FIG. Using the first and second embodiments, it was shown that a good correlation output can be obtained by using 90 ° phase modulation without causing unnecessary crossing. However, in that system, the system on the transmitting side also needs to be changed. Therefore, in the present embodiment, a structure of an interdigital transducer corresponding to the phase modulation of 0 ° and 180 ° and having no side lobe rise of the correlation output will be described. In FIG. 7A, an intersection 19 that is bent obliquely is arranged between an intersection 17 corresponding to 0 ° and an intersection 18 corresponding to 180 °. Although unnecessary intersections occur in ordinary electrodes, the aperture is formed in a direction different from the main propagation direction, so that output by surface acoustic waves from the main propagation axis direction does not occur, and the side of the correlation output does not occur. No lobe rise occurs. Similarly, in FIG. 9B, the unnecessary intersection itself can be removed by changing the polarity of the electrode at the unnecessary intersection 19 '. As described above, according to the present embodiment, it is possible to suppress an increase in the side lobe of the correlation output without changing the output communication method on the transmission side.
[0011]
Next, a fourth embodiment will be described with reference to FIG. As shown in the first embodiment, the number of electrode pairs of the input interdigital transducer 2 is usually one, and the number of output interdigital transducers needs to be proportional to the spreading code. Further, usually, a material having good temperature characteristics such as ST-quartz is used as the surface acoustic wave substrate 1, and their electromechanical coupling coefficients are generally small. Therefore, the impedance of the input interdigital transducer has a considerably large value, and a large mismatch loss occurs. In order to solve this problem, in this embodiment, the thin film 20 having a high electromechanical coupling coefficient is formed below the input interdigital transducer 21. The thin film 20 is not formed under the output interdigital electrode 22. This makes it possible to lower the input impedance. As described above, by using this embodiment, an increase in loss due to mismatch can be suppressed.
[0012]
In the fourth embodiment, since the coupling coefficient is increased, a thin film must be formed, and the drawing and the process become complicated. In the fifth embodiment, in order to solve this problem, the input and output surface wave modes are changed. FIG. 8 shows the situation. The electrode material of the input interdigital transducer 23 was Au, and the electrode material of the output interdigital transducer 24 was Al. As a result, the mode becomes a love wave mode on the input side, and the coupling coefficient increases. As described above, according to the present embodiment, the mismatch can be suppressed without complicating the process.
[0013]
The fifth embodiment is a relatively easy process because there is no thin film formation step, but different electrodes must be formed for input and output, which is more complicated than the conventional process. In the sixth embodiment, in order to use the same electrode material for input and output, the structure shown in FIG. 9 was adopted. In the figure, the same parts as those in FIG. 1 are indicated by the same numbers. The output interdigital transducer 3 is divided into a number of tracks 25 in a direction perpendicular to the main propagation axis, each of which is electrically connected in series. As a result, the impedance of the output interdigital transducer 3 is increased, and the impedance difference between the input and output interdigital transducers 2, 3 is eliminated, so that the loss can be reduced. As described above, according to this embodiment, it is possible to suppress an increase in loss due to mismatch without complicating the process as compared with the related art.
[0014]
The sixth embodiment has the advantage that the same process as the conventional one can be used. However, as can be seen from FIG. 9, since an electrode gap or the like is required for each track, the aperture loss is increased by that amount. Since the aperture of each track becomes narrow, the characteristic deterioration due to the diffraction effect also increases. In the seventh embodiment, in order to improve this point, each intersecting portion 26 of the output interdigital electrodes is connected in series. In the figure, the crossing portion 27 corresponding to the code 1 and the crossing portion 28 corresponding to the code 0 are connected in series with their polarities reversed. As a result, the impedance of the output interdigital electrodes increases, and the difference between the input and output impedances can be reduced. As described above, according to the present embodiment, it is possible to reduce the mismatch loss without causing an increase in aperture loss and deterioration in characteristics due to a diffraction effect.
[0015]
FIG. 10 is a system block diagram of a wireless modem (modem) using the surface acoustic wave device of the present invention. The spread spectrum signal input from the antenna 29 is amplified by the first-stage amplifier 30 and converted into a correlation output with a spread code by the matched filter type surface acoustic wave device 31 of the present invention. Code frequency) 32, further converted into a square wave signal by a limiter half-wave rectifier circuit 33 and output to an output terminal 37. In the transmitting section, the square information code input from the input terminal 38 is a signal output from the phase modulation circuit 35 for modulating the phase of the oscillator section (carrier frequency) 36 in accordance with the code or a change in the code. Is amplified by the amplifier 34 and output from the antenna 29. In the present embodiment, wireless is shown as an example, but it goes without saying that a similar effect can be obtained in a wired system. As described above, in the present embodiment, since the high-frequency signal can be directly demodulated, higher-speed data transmission / reception is possible.
[0016]
FIG. 11 shows a communication system between computers including the modem unit of the present invention. The antennas 39, 39 'and the modem sections 40, 40' of the present invention are connected to computers 41, 41 '. The connection to the computer can be RS-232C serial connection, GP-IB parallel connection, Ethernet (registered trademark) LAN connection, or the like. As described above, the system of the present embodiment can cope with a case where the information rate is high, and even if it is wireless, it is a spread spectrum communication, so it is less affected by fading and the like, and has high reliability. A system that does not require wire connection is possible. For example, if the present system is applied to a laptop computer and further connected to a public line, any large computer can be operated anywhere.
[0017]
【The invention's effect】
As described above, according to the present invention, the rise of the correlation signal side lobe at the time of direct demodulation in code spread type spread spectrum communication can be suppressed, and a low-loss system can be realized. In addition, the performance of the code spread type spread spectrum communication apparatus can be improved.
[Brief description of the drawings]
FIG. 1 is a schematic view of a matched filter type surface acoustic wave device according to the present invention.
FIG. 2 is a schematic view of a part of a conventional matched filter type surface acoustic wave device.
FIG. 3 is a schematic view of a part of the first embodiment of the present invention.
FIG. 4 is a schematic view of a part of a second embodiment of the present invention.
FIG. 5 is an explanatory diagram of a phase modulation communication system according to a second embodiment of the present invention.
FIG. 6 is a schematic view of a part of a third embodiment of the present invention.
FIG. 7 is a schematic view of a surface acoustic wave device according to a fourth embodiment of the present invention.
FIG. 8 is a schematic view of a surface acoustic wave device according to a fifth embodiment of the present invention.
FIG. 9 is a schematic view of a surface acoustic wave device according to a sixth embodiment of the present invention.
FIG. 10 is a schematic view of a surface acoustic wave device according to a seventh embodiment of the present invention.
FIG. 11 is a diagram showing system blocks of a modem according to an eighth embodiment of the present invention.
FIG. 12 is a diagram showing a communication system between computers according to a ninth embodiment of the present invention.
[Explanation of symbols]
1 .... Surface acoustic wave substrate,
2. Input interdigital electrode,
3. Output interdigital electrode,
7, 11 ... phase modulation signal,
31, 35: The surface acoustic wave device of the present invention.

Claims (3)

An interdigital transducer for converting an electric signal into a surface acoustic wave is arranged on a substrate having a low electromechanical coupling coefficient, and a correlation signal having a short response time is output when a signal corresponding to a certain 1,0 signal code is input. In a matched filter type surface acoustic wave device,
The interdigital transducer is composed of an input / output interdigital transducer having a small number of electrode pairs and an input / output interdigital transducer having a large number of electrode pairs,
A piezoelectric film having a higher electromechanical coupling coefficient than the substrate having a lower electromechanical coupling coefficient is formed on the input / output interdigital transducer having a small number of electrode pairs, and the input / output interdigital transducer having a large number of electrode pairs is formed by the low input / output interdigital transducer. A surface acoustic wave device formed on an electromechanical coupling coefficient substrate. A substrate having a low electromechanical coupling coefficient;
A first input / output interdigital transducer formed on the substrate having a low electromechanical coupling coefficient;
A second input / output interdigital transducer formed on the substrate having a low electromechanical coupling coefficient with a piezoelectric film having a large electromechanical coupling coefficient than the substrate;
Wherein the number of electrode pairs of the first input / output interdigital transducer is greater than the number of electrode pairs of the second input / output interdigital transducer. A communication device using the surface acoustic wave device according to claim 1.

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