JP2685537B2 - Surface acoustic wave device, manufacturing method thereof, adjusting method thereof, and communication device using the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a surface acoustic wave device having excellent frequency band shoulder characteristics and out-of-band characteristics when used as a filter, and a method of manufacturing and adjusting the same.

[Conventional technology]

Conventionally, as a means for improving the shoulder characteristic of the surface acoustic wave filter (the steepness of the attenuation characteristic at the filter band edge), for example, iE / E / Transaction on Microwave Theory and Technics, MTE / TE21 ( IEEE, TRANS.ON MICROWAVE THEO
RY AND TECH-NIQUES, VOL.MTT-21) No. 4, April 1973
There are technologies such as multi-strip couplers described in Mon., pp. 206-215.

[Problems to be solved by the invention]

In the above conventional technique, in order to improve the shoulder characteristics, the number of interdigital electrodes must be increased, the chip shape becomes large, and the cost increase cannot be avoided. Regarding the relationship between the shoulder characteristics and the number of electrodes, for example, iE / E / Transaction on Microwave Theory and Technics MTE / TE21 (IEEE, TRANS.O)
N MICRO-WAVE THEORY AND TECHNIQUES, VOL.MTT-21) No. 4, April 1973. Discussed at pages 162-175.

Further, in order to improve the shoulder characteristics, the number of electrode pairs must be increased in the prior art as described above, which increases the radiative conductance of the interdigital electrode, and the reflection determined by the condition with the electrical load. (RW) increases. Therefore, normally,
In order to suppress the above reflection, the aperture of the interdigital electrode must be narrowed. In that case, the influence of the diffraction effect of the surface acoustic wave increases, and the side lobe of the filter characteristic deteriorates.

An object of the present invention is to reduce the size of a surface acoustic wave device having a good shoulder characteristic as a filter and reduce the influence of the diffraction effect.

[Means for solving the problem]

In order to achieve the above object, in the present invention, a surface acoustic wave reflector is provided on at least one side of the interdigital transducer in the surface acoustic wave device.

[Action]

By providing a reflector beside the interdigital transducer, the surface acoustic wave emitted from the interdigital electrode is reflected by the reflector, and the excitation source of the surface acoustic wave is equivalent to an apparently increased number of points, which is equivalent to the chip. Shoulder properties can be improved without increasing size. Further, since the shoulder characteristic is improved without increasing the number of pairs of the interdigital electrodes, it is not necessary to make the opening of the electrode small, and the influence of the diffraction effect can be suppressed.

〔Example〕

A first embodiment of the present invention will be described with reference to FIGS.
FIG. 1 is a plan view schematically showing the first embodiment. An output interdigital electrode 2 and an input interdigital electrode 3 are arranged on a surface acoustic wave substrate 1, and a sound absorbing agent 4 is applied to the end of the substrate to suppress reflection from the end face of the substrate. Further, a short strip reflector 5 is disposed on both sides of the input interdigital transducer. The input / output interdigital transducers were of split-connect type. Next, the operation principle of this embodiment will be described with reference to FIG. FIG. 2 shows time on the horizontal axis and strength (amplitude) of the surface acoustic wave excitation source on the vertical axis, and schematically shows the impulse response viewed from a position away from the interdigital transducer.
The response shown by the solid line 6 is for the input interdigital transducer alone (therefore, the reflector is not provided), and the response shown by the broken line 7 is the response after the reflector 5 is provided. The impulse generated from the input interdigital transducer is confined between the reflectors 5, and a part of the impulse sequentially passes through the reflector 5 and reaches the output interdigital transducer 2. Therefore, as shown in FIG. 2, the impulse response train (broken line 7) has a longer response time than the impulse response train (solid line 6) before the reflector is arranged, and the above-mentioned response time length is longer. It is equivalent to a state in which a comb-shaped electrode corresponding to is arranged. Without increasing the number of interdigital electrodes,
Further, since it is not necessary to make the opening small, it is possible to obtain a small-sized surface acoustic wave device having a small shoulder effect and a good shoulder characteristic. Figure 3 shows the frequency characteristics of the attenuation of the IF filter of a domestic television receiver. Dashed line 8 is normal
The characteristics of the IF filter, the alternate long and short dash line 10, are the characteristics of the conventional IF filter in which the number of electrode pairs is increased, the shoulder characteristics of the chroma tilt portion are improved, and the aperture is narrowed to reduce reflection due to RW. Input source weighted comb-shaped electrode excitation source (impulse) logarithm
150 pairs, aperture 750 μm, the same number of pairs of normal electrodes on the output side is 15 pairs,
The opening is 1500 μm, and as the surface acoustic wave substrate 1, an X-cut 112 ° rotated Y-axis propagating LiTaO ₃ substrate is used. In the above-mentioned conventional structure, the side lobe characteristic and the trap (attenuation pole) characteristic were deteriorated as shown in FIG. The solid line 9 shows the characteristics of the filter according to the invention. The number of pairs of excitation sources of the input interdigital transducer 3 was 60, the number of openings was 1000 μm, and the number of electrodes of the reflector 5 was 20. Therefore, the electrode length on the input side including the reflector is about 80 pairs, which is about half the value of 150 pairs of the conventional filter described above. The logarithm of the output normal type interdigital transducer and the aperture are the same as the conventional one. The total length of the chip is about 3/4 compared to the conventional one
The size has been reduced to a certain level. Further, since the aperture of the input interdigital transducer is not narrowed, the diffraction effect is suppressed and good out-of-band characteristics are obtained.

Looking at the characteristics of the solid line in FIG. 3, the shoulder characteristics and the out-of-band characteristics are good, but ripples are seen in the band. This is an influence of reflection by the reflector 5.

Next, referring to FIG. 4 and FIG.
Examples will be described.

Hereinafter, the energy converted into an electric signal in the interdigital transducer is small and is ignored. FIG. 4 shows the magnitude of the entire reflected wave when the reflectors 5 are arranged on both sides of the interdigital transducer 3 (a schematic plan view is shown above and an equivalent circuit diagram is shown below) (assuming that the input is 1). A reflection coefficient Γ is obtained. Here, S is an element of the scattering matrix of the interdigital transducer and the reflector (the input port is 1 and the output port is 2), and T ₁ and T ₂ are the transmission coefficients between the reflector 5 and the interdigital electrode 3. Is. Here, the subscript a is the reflector in the previous stage, and the subscript b
The interdigital electrodes and the subscript c indicate that the values are related to the reflector in the subsequent stage. The calculation result is as follows.

Also, if the transmission coefficient T is shown for reference, Becomes Considering the vicinity of the center frequency of the filter, T ₁ = exp (± jπ / 2) = ± j- (3) T ₂ = exp (± jπ / 2) = ± j- (3 ′), The solution can be obtained by a real number equation.
Considering each scattering matrix as a real number (because it is in the vicinity of the center frequency, all phase terms of each term can be included in T ₁ and T ₂ ), and the reversibility and symmetry of each reflector and interdigital electrode ( S _{12
= S 21, S 11 = S} 22) considering, as the same structure front, the rear stage of the reflector, Then, equation (1) becomes Becomes Now, since the loss in the reflector and the interdigital transducer is sufficiently small, from the law of energy conservation, Since Eq. (8) holds, Becomes Since there is no reflection, Γ = 0 Solving the quadratic equation of Is obtained. That is, the reflection coefficients of the interdigital electrode and the reflector are determined so as to satisfy the expression (13), and the distance between the center position of the electrode at the end of the reflector and the center position of the electrode at the end of the interdigital electrode is set. (Λ is the wavelength of the surface acoustic wave at the band center frequency, and n is a natural number) (that is, It is possible to suppress the reflected wave.

Next, a second embodiment in which the present invention is applied to an IF filter of a domestic television receiver will be described. The input interdigital transducer was a normal type electrode of solid type, and a λ / 4 solid type short strip type reflector was arranged on both sides thereof. There are 15 pairs of excitation sources for normal type electrodes (31 electrodes), and the number of reflectors is
16 The interdigital electrodes on the output side have a split-connect structure with an aperture weight, and the number of pairs of excitation sources is 60.5. r _a = 0.108 and r _b = 0.054, almost satisfying equation (13). The distance between the electrode and the reflector was 3 / 4λ (center-to-center distance between each end electrode).

FIG. 5 shows the frequency characteristic of the attenuation amount of the second embodiment. The solid line 11 shows the amount of attenuation. In-band reflection characteristics are improved and ripples are reduced. As described above, in the second embodiment, a small filter having a small ripple near the center frequency and a good shoulder characteristic was obtained.

In the second embodiment, the ripple characteristic near the center frequency could be improved by defining the reflection coefficient of the reflector, but as can be seen from FIG. The degree of repression is reduced and the ripple is increased. This is because the distance between the center of the reflector and the center of the interdigital electrode is large, so the phase rotation of T ₁ and T ₂ is fast, and This is because it cannot be constant, and the reflection sharply increases when the frequency deviates from the center frequency.

The third embodiment remedies the above-mentioned drawbacks. Equation (13) is
If r _a and r _b are sufficiently smaller than 1, r _b ≈ 2r _a − (14). Further, the MEL (reflection determined by the geometrical shape of the electrodes) of the solid type short strip reflector and the solid type interdigital transducer is expressed by the following equation.

Here, Z _r is the ratio of the propagation impedance of the surface acoustic wave at the place where the electrode is present and where it is not, and N is the number of electrodes. In general, Z _r is sufficiently close to 1, and thus Z _r = 1 + ε (ε << 1) − (16). Equation (15) is It is expressed as When | Γ | is sufficiently smaller than 1 (N is small), it is expressed as | Γ | ≈N ₆ − (18). Applying the result of (18) to the case of the above-mentioned reflector (indicated by subscript a) and interdigital electrode (indicated by subscript b), and using equation (14), N _b ε ≈ 2N _a ε N _b ≈ The relationship of 2N _a − (19) is obtained. That is, when the number of electrodes is sufficiently small, the ratio of the number of electrodes of the reflector to the number of interdigital electrodes is
If it is 1: 2, the total reflection can be suppressed. Eg 1
To suppress the reflection of the interdigital electrodes of the pair of excitation sources, a reflector consisting of one electrode should be placed on both sides. (N is a natural number).
Therefore, if n = 1, for example, the reflected wave can be suppressed sufficiently in a wide band. This electrode group (two electrode fingers of a solid type interdigital electrode and one reflector electrode (one set)) is used as a basic structure, and a plurality of them are arranged in the surface wave propagation direction to obtain a desired frequency characteristic. FIG. 6 shows the structure. A portion 13 surrounded by a broken line has a basic configuration. The split-type electrode d provided between the basic components is for making the reflector section a short-strip type (removing the RW portion of the reflector section). The split type is used to suppress the MEL component there. Therefore, the interdigital electrode 12 has a structure in which a reflector is arranged inside.

As a third embodiment, a case where the surface acoustic wave device having the input interdigital transducer having the above structure is applied to the IF filter of a domestic television receiver will be described. A comb-shaped electrode in which five basic components were connected was used as an input normal type comb-shaped electrode, and an aperture-weighted split connect type was used as the output comb-shaped electrode. The aperture of the input normal type electrode was 1500 μm, the number of pairs of excitation sources of the output interdigital electrode was 120 pairs, and the aperture was 1000 μm. The attenuation frequency characteristics of this filter are shown in Fig. 7 by the solid line 14
Indicated by The ripples are reduced because the reflected waves are suppressed in a wide band. As described above, by using the third embodiment, it is possible to obtain a small-sized surface acoustic wave filter having excellent shoulder characteristics in which ripples are suppressed in a wide band.

Next, a fourth embodiment will be described with reference to FIGS. 8 and 9. FIG. 8 schematically shows the fourth embodiment, in which reflectors 5 are provided on both sides of the input (output) interdigital transducer 15 facing the output (input) interdigital transducer 2, as in the above-mentioned respective embodiments. It is provided. By arranging the reflectors in this way, energy can be trapped between the reflectors and the shoulder characteristics can be improved. At the same time, however, Q rises at the frequency at the band edge, so at that frequency the group The delay time increases. If flatness of the group delay time is required, it is necessary to advance the group delay time characteristic of the interdigital transducer 15 in advance. FIG. 9 shows the group delay characteristics of the fourth embodiment. The broken line 16 is the characteristic before the reflector is arranged, and the solid line 17 is the characteristic after the reflector is arranged. The group delay is constant up to the band edge frequency. As described above, according to the fourth embodiment, it is possible to obtain a small-sized high performance filter having a sharp shoulder characteristic and a constant group delay.

Next, a fifth embodiment will be described with reference to FIGS. As can be seen from FIG. 9, the group delay cannot be made completely constant in the fourth embodiment, and ripples occur within the band. This is because the reflector is composed of a large number of electrodes, energy is trapped in the reflector, and the total impulse response does not have a symmetrical characteristic on the time axis. Therefore, when the flatness of the group delay time is required, a perfect reflection surface is required.

FIG. 10 schematically shows a fifth embodiment, in which one end of the input (output) interdigital transducer 18 facing the output (input) interdigital transducer 2 has a split-connect type interdigital transducer reflection of 0.5 pair excitation source. Is equipped with a container. In order to obtain a high reflection coefficient, the capacitance of the electrode is canceled by using the inductance 20.
In this way, the interdigital transducer type reflector can obtain a high reflection coefficient even with a small number of logarithms, and is therefore effective for ensuring the flatness of the group delay. Besides, it is possible to use the end face of the substrate as a means for obtaining a perfect reflection surface.
It is considered more effective to do the above because of problems such as chipping. FIG. 11 is a solid line showing the group delay time characteristics of the fifth embodiment.
Shown at 21. It can be seen that the in-band flatness is improved compared to the fourth embodiment. As described above, when the fifth embodiment is used, the group delay characteristic can be further improved.

Next, a sixth embodiment will be described with reference to FIG. The above-mentioned reflector is for improving the shoulder characteristic at the filter band edge, but if the reflection coefficient of the reflector is too large or too small, the flatness of the in-band characteristic of the attenuation is deteriorated. Further, if the reflected wave phase of the reflector deviates from the optimum value, the Q value other than the desired frequency rises, causing deterioration of shoulder characteristics and deterioration of in-band flatness of attenuation. FIG. 12 shows a method of adjusting the reflection coefficient of the reflector in such a case. That is, the reflector 5
A protective film such as a resist film is formed only on a necessary portion (a portion desired to be left) and etching is performed, and trimming for removing the unnecessary portion 22 is sequentially performed from the end portion to adjust the reflection coefficient. The use of this embodiment can improve the yield because the deterioration of the frequency characteristics due to the deviation of the reflection characteristics can be improved.

The seventh embodiment relates to a method of adjusting the reflected wave phase of the reflector. FIG. 13 shows a method of adjusting the reflector of the seventh embodiment. The reflector 5 is used as a short strip electrode,
Sequentially, an open strip portion is formed at the cutting point e by a trimming technique such as a laser cutter and an ultrasonic cutter to adjust the phase of the reflected wave. The use of this embodiment is effective in improving the yield.

The eighth embodiment relates to a method of adjusting frequency characteristics of a communication device using a surface acoustic wave device of the present invention having a interdigital transducer as a circuit. FIG. 14 schematically shows the eighth embodiment. Next to the input (output) interdigital transducer 18 facing the output (input) interdigital transducer 2, a interdigital transducer 19 is arranged. A variable inductance 23 is arranged in the external circuit of the surface acoustic wave device, and the interdigital transducer type reflector is arranged.
Electrically connect to 19. The reflection characteristics of the interdigital transducer can be adjusted by changing the matching inductance value. Therefore, after the surface acoustic wave device of the present invention is installed in the communication device, the value of the variable inductance is adjusted.
The use of this embodiment is effective in that the frequency characteristic can be adjusted in a state where it is mounted on a communication device.

Next, an embodiment in which the present invention is used in a communication device will be described. FIG. 15 is a block diagram of the television receiver of the ninth embodiment. The IF output of the tuner 24 is connected to the surface acoustic wave device 25 of the present invention, the output of the surface acoustic wave device 25 is divided into two systems, one system is input to the video detection circuit 26, and the other system is the audio detection circuit. It has been entered in 27. The characteristics of the surface acoustic wave device 25 of this embodiment are as shown in FIG.
The resolution of the screen is improved by the increase of the chroma level, and the sound level is attenuated by using the technique of the present invention. Therefore, good characteristics without interference such as cross color and 920 KHz beat are obtained.

Next, a tenth embodiment will be described with reference to FIGS. FIG. 16 is a block diagram of the satellite broadcast receiver of the tenth embodiment. The signal down-converted in the outdoor unit 28 is sent to the indoor unit 29 via a cable. Further, in the indoor unit 29, it is down-converted by the oscillation signal from the oscillator 30 and sent to the surface acoustic wave device 31 of the present invention which is the second IF filter. The signal passed through the second IF filter is detected by the PLL synchronous detection circuit 32 and sent to the video signal processing circuit 33 and the audio signal processing circuit 34, respectively.

FIG. 17 shows the attenuation frequency characteristics of the second IF filter of the tenth embodiment. The broken line 36 shows the characteristic of the conventional surface acoustic wave device, and the solid line 35 shows the characteristic of the surface acoustic wave device of the present invention. Since the characteristic of the surface acoustic wave device of the present invention faithfully reproduces the signal band, the signal is not distorted, and the differential gain characteristic (DG),
Since the differential phase characteristic (DP) was improved and the characteristic at the band edge became steeper, the amount of noise in that part was reduced and S / N could be improved.

〔The invention's effect〕

As described above, according to the present invention, a wide band having steep shoulder characteristics and improved performance such as side lobe characteristics without increasing the chip size and without reducing the opening length of the interdigital transducer. It is possible to obtain various filters, and the effect of reducing the cost of the device can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of the first embodiment of the present invention, FIG. 2 is an impulse response diagram, FIG. 3 is an attenuation amount frequency characteristic diagram of the first embodiment, and FIG. 4 is a reflected wave of the second embodiment. FIG. 5 is an explanatory diagram of the analysis method, FIG. 5 is an attenuation frequency characteristic diagram of the second embodiment, and FIG.
FIG. 7 is a schematic diagram of the embodiment, FIG. 7 is a characteristic diagram of the attenuation amount frequency of the third embodiment, FIG. 8 is a schematic diagram of the fourth embodiment, and FIG. 9 is a group delay time frequency characteristic diagram of the fourth embodiment. FIG. 10 is a schematic diagram of the fifth embodiment, FIG. 11 is a group delay time frequency characteristic diagram of the fifth embodiment,
FIG. 12 is a drawing for explaining the manufacturing method of the sixth embodiment, FIG. 13 is a drawing for explaining the manufacturing method of the seventh embodiment, FIG. 14 is a drawing for explaining the adjusting method of the communication device of the eighth embodiment, and FIG. FIG. 16 is a block diagram of the television receiver of the ninth embodiment, FIG. 16 is a block diagram of the satellite broadcast receiver of the tenth embodiment, and FIG. 17 is an attenuation frequency characteristic diagram of the surface acoustic wave filter of the tenth embodiment. . 2,3,12,15,18 …… Interdigital electrodes 5,19 …… Reflector, 13 …… Basic configuration 20 …… Inductance, 22 …… Unnecessary part 23 …… Variable inductance 25 …… Elastic meta-wave device , 31 …… Surface acoustic wave device

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Jun Yamada 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Inside the Home Appliance Research Laboratory, Hitachi, Ltd. (56) References JP-A-56-10725 (JP, A) JP-A-SHO 55-49021 (JP, A) JP-A 53-140928 (JP, A) JP-A 51-72257 (JP, A) JP-A 49-14064 (JP, A) JP-A 53-123051 (JP, A) A)

Claims (10)

(57) [Claims] 1. An interdigital transducer for input and an interdigital transducer for output are provided on a surface acoustic wave substrate, and at least one interdigital transducer of the interdigital transducer for input and the interdigital transducer for output. With reflectors of the same structure on both sides of λ, the distance between the end electrode center of the interdigital electrode and the end electrode center of the reflector, where λ is the surface acoustic wave wavelength at the center frequency of the filter and n is a natural number. Where λ / 4 + nλ / 2 on both sides, the reflection coefficient of the reflector is ra, and the reflection coefficient of the interdigital transducer is rb The two-port filter type surface acoustic wave device is characterized in that the structure of the reflector and the interdigital transducer is defined so as to satisfy the above condition. 2. A set of the interdigital electrode and the reflector according to claim 1 is used as a basic structure, a plurality of sets are arranged on a surface acoustic wave propagation path, and each interdigital electrode is electrically connected in parallel. A surface acoustic wave device characterized by the above. 3. The surface acoustic wave device according to claim 2, wherein the relation between the reflection coefficient ra of the reflector and the reflection coefficient rb of the interdigital electrode satisfies rb≈2ra. 4. The basic structure according to claim 3, wherein a solid type interdigital electrode composed of two electrode fingers and one electrode finger having the same width as the electrode fingers of each interdigital electrode arranged on both sides thereof. The surface acoustic wave device according to claim 3, wherein the surface acoustic wave device is formed of a reflector. 5. The surface acoustic wave device according to claim 1, wherein the reflector is a interdigital electrode type having a small number of electrode pairs. 6. The surface acoustic wave device according to claim 1, wherein the reflector provided in the surface acoustic wave device is trimmed in order from the end to change the reflection coefficient of the reflector or the phase of the reflected wave. A method for manufacturing a surface acoustic wave device, characterized in that a desired frequency characteristic is obtained. 7. A communication device using the surface acoustic wave device according to claim 1 as a filter in its own circuit. 8. A communication device, wherein an IF filter between a tuner IF output of a television receiver and a circuit under test is the surface acoustic wave device according to claim 7. 9. An indoor unit of a satellite broadcast receiver
A communication device using the surface acoustic wave device according to claim 7 as a 2IF filter. 10. The surface acoustic wave device according to claim 5, wherein a circuit composed of a variable inductance element is connected to the interdigital transducer reflector, and the inductance value is sequentially changed to obtain a reflection coefficient or reflection of the reflector. A method of adjusting a surface acoustic wave device, characterized in that the wave phase is changed so that the surface acoustic wave device obtains a desired frequency characteristic.