JP3169830B2 - Surface acoustic wave filter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter, and more particularly, to an elastic surface acoustic wave filter comprising a transducer having interdigital electrode fingers arranged on a piezoelectric substrate. The present invention relates to a configuration of a bandpass or a band rejection filter formed by combining a plurality of surface acoustic wave resonators. 2. Description of the Related Art Since a surface acoustic wave filter is formed by forming a transducer having a thin conductive conductor having a simple shape on a piezoelectric substrate as described above, the filter has extremely small and stable characteristics. Has the advantage that it can be realized. The main characteristics required for this type of surface acoustic wave filter are low loss and steep frequency characteristics, that is, an abrupt change in attenuation characteristics at the boundary between a path (passing region) and a rejection (blocking region). It has characteristics. or,
In recent years, particular attention has been paid to power-resistant characteristics,
It is important to operate as a predetermined filter for a high-power signal. Conventionally, various surface acoustic wave filters have been known, but little consideration has been given to power durability. In order to improve the power durability, some electrode materials are improved. However, only those having a capacity of 0.5 watt or less are currently being developed. Accordingly, a main object of the present invention is to realize a surface acoustic wave filter having excellent power durability, that is, to realize a surface acoustic wave filter which can be used for signals having a power of several watts or more. . According to the present invention, in order to achieve the above object, three or more common electrodes are arranged on one piezoelectric substrate so as to face each other, and the three or more common electrodes are arranged in one direction. The electrode fingers are connected to the two common electrodes disposed at both ends of the electrodes in one direction and only to the other common electrode side, and to the other common electrodes on both sides in one direction. A plurality of pairs of electrode fingers are arranged by alternately inserting two of the plurality of electrode fingers into each other, and a plurality of surface acoustic wave resonances formed between two adjacent common electrodes. In addition to providing a matching circuit with an external load at the input / output terminals of the device, a filter is configured using the capacitance of the piezoelectric substrate between the common electrode and the electrode finger and the common ground constituting the surface acoustic wave resonator. . Here, at least a part of the plurality of surface acoustic wave resonators has a different resonance frequency from other surface acoustic wave resonators. In addition, it is for withstand power of more than 0.5 W and 4 W or less. According to the above configuration, as will be apparent from the following detailed description, a part (high-frequency component) of the input power is transmitted directly to the output side without being converted into acoustic energy. In addition, the mechanical stress applied to the electrode material or the substrate is reduced, and a surface acoustic wave filter that can withstand a high-power signal (for example, 2 to 4 watts) can be realized. That is, the first reason that a surface acoustic wave filter that can withstand a high-power signal (for example, 2 to 4 watts) can be realized is that a normal piezoelectric crystal is a high-quality dielectric, and thus a one-opening surface acoustic wave resonance Even if a very high voltage is induced between the electrode conductors of the resonator and the ground by a few watts of incident power, the capacitance of the piezoelectric substrate between the electrode conductors of the single-opening surface acoustic wave resonator and the ground is not destroyed. It is in. This will be described in comparison with a surface acoustic wave filter described in Japanese Patent Publication No. 58-1850 using a one-opening surface acoustic wave resonator instead of a piezoelectric substrate capacitor. The single-opening surface acoustic wave filter described in Japanese Patent Publication No. 58-1850 discloses a single-opening surface acoustic wave filter, not a piezoelectric substrate capacitor, between a connection point of two single-opening surface acoustic wave resonators connected in series and ground. The required frequency characteristics are realized by introducing a wave resonator and forming a T-type circuit. However, the present inventor has studied through experiments that a part of the energy having the frequency component of the pass band of the filter flows into the one-opening surface acoustic wave resonator and returns to the input side through the ground. It has been found that when a large amount of electric power is incident, a phenomenon occurs in which the one-port surface acoustic wave resonator is broken. This destruction is 1
This is due to the migration of the electrode material between the electrode fingers of the aperture surface acoustic wave resonator. The second reason is that the present invention uses a large number of pairs of electrode fingers for a single-opening surface acoustic wave resonator, and therefore has a large area for electrode conductors. As a result, the incident power of the filter is dispersed over a wide range and the power loss is essentially small, that is, the heat dissipation in the one-port surface acoustic wave resonator is good, and the mechanical properties generated in the electrode material or the substrate are reduced. The point is that the stress is reduced and the electrode material is resistant to destruction of the one-opening surface acoustic wave resonator due to migration of the electrode material between the electrode fingers. Further, the capacitance of the piezoelectric substrate of the present invention has a very important meaning. That is, the piezoelectric substrate capacitance is intentionally used as one of the components for synthesizing the frequency characteristics of the filter, and its importance is completely different from the so-called parasitic capacitance. Further, according to the present invention, since the one-opening surface acoustic wave resonator composed of a large number of counter electrode fingers is used, the area of the electrode finger portion is large, and the necessary capacity for grounding can be relatively easily secured. Hereinafter, the present invention will be described in detail with reference to examples. FIG. 1 is a diagram showing the configuration of an embodiment of a surface acoustic wave filter according to the present invention. In FIG. 1, reference numeral 1 denotes a signal input terminal, and 2 denotes an output terminal of a filter. The common electrodes 9-1... 9-6 are arranged in parallel on the piezoelectric substrate 8, and electrode fingers interleaved with each other are connected to each common electrode 9. The common electrode 9-1 constitutes an input terminal, and a matching circuit with the input load composed of the inductances 3 and 4 is formed between the common electrode 9-1 and the signal input terminal 1. The common electrode 9-6 forms an output terminal, and a filter output. Inductances 5 and 6 forming a matching circuit with an output load are provided between the terminal 2 and the terminal 2. In the above configuration, a one-opening SAW resonator composed of a large number of pairs of transducers is formed between two adjacent common electrodes as shown in FIG. Here, a one aperture surface acoustic wave resonator (on
e port SAW resonator is an IEE Ultrasonics Symposium Proceeding 1978, pp. 573-578 (IEEE EUtrasonics Symp. 19).
78, pp. 573-578), a surface acoustic wave resonator in which one transducer is involved in exciting or receiving a surface acoustic wave. Generally, surface acoustic wave resonators often introduce surface acoustic wave reflectors on both sides of the transducer, but if the number of electrode fingers of the transducer is very large as shown in FIG. Even if it is not, the vibration energy is confined by the internal reflection by the electrode finger of the transducer itself, so that it becomes a resonator. In the filter shown in FIG. 1, one-opening surface acoustic wave resonators are electrically connected in series by common electrodes 9-2,..., 9-5.
Further, the surface acoustic wave excited in the transducer constituting the one-aperture surface acoustic wave resonator reaches the end of the transducer while repeating multiple reflections with other electrode fingers before reaching the end of the transducer. Leak outside the transducer. However, the energy of this leaky surface acoustic wave is very small as compared with the total energy of the surface acoustic wave repeating multiple reflections inside the transducer. In addition, the direction of the leakage is orthogonal to the direction of the electrical series connection of the single-opening surface acoustic wave resonator. Therefore, each one
No surface acoustic waves interact between the open surface acoustic wave resonators. Hereinafter, for the sake of simplicity, a one-opening surface acoustic wave resonator which is a substrate component of the filter will be described with the configuration of FIG. FIG. 3A shows an electric equivalent circuit of the resonator shown in FIG. In general SAW resonator in parallel with the capacitance Co between the electrodes, it is represented in the form of connecting the series circuit of the inductance L ₁ and the capacitor C ₁ due to the resonance. As shown in FIG. 3 (b), the impedance characteristic of such a resonator is such that the impedance is almost 0 at the resonance frequency fr,
It becomes almost infinite with fa. Therefore, the surface acoustic wave filter shown in FIG. 1 can be represented by an equivalent circuit shown in FIG. The parallel inductances 3, 6 and the series inductances 4, 5 connected to the input / output terminals 1, 2 are shown in FIG.
2 shows an external matching circuit. The equivalent circuit of FIG. 3A was used for the surface acoustic wave resonator. The capacitance indicated by the dotted line in the figure indicates the capacitance of the electrode fingers of each resonator and the electrode connecting the resonators or the input / output terminal electrode to the common ground. Here, the common ground refers to a case where the piezoelectric substrate 8 of FIG. 1 is fixed to a package or a circuit board by an adhesive or soldering.
The ground plane of the package or circuit board.
Although these capacitances are shown by dotted lines in FIG. 4, they can be arbitrarily set by increasing or decreasing the area of the electrodes connecting the resonators or the input / output terminal electrodes such as bonding pads. Further, it can be set arbitrarily by changing the thickness of the piezoelectric substrate 8. Further, these capacitors may be formed by an externally provided chip capacitor or the like as necessary. In the configuration of FIG. 1, as can be seen from the equivalent circuit of FIG. 4, at a frequency lower than the resonance frequency of each resonator,
The impedance of each resonator is approximately the capacitance between the electrodes (36, 37
.. 39) are shown in FIG. 5 (a).
In the vicinity of the resonance frequency of each resonator, each resonator is connected in series with an inductance and a capacitance due to resonance (51, 52... 5).
It is represented as shown in FIG. 5B in a form close to 4). In the vicinity of the anti-resonance frequency of each resonator, each resonator is connected in parallel with the capacitance between the electrodes and the inductance due to resonance (65, 66 ... 68)
5 (c). At a frequency sufficiently higher than the anti-resonance frequency, the resonator has only the capacitance between the electrodes again, and is represented as shown in FIG. In addition, 40,
41 ... 45, 55, 56 ... 60, 69, 70 ... 74 indicate the equivalent capacitance between the electrode conductors of the transducer and the common ground. Next, it will be described that the configuration of FIG. 1 is a bandpass filter or a band rejection filter. The case where the pass band as a filter is set to a resonance frequency or a frequency lower than the resonance frequency will be described. In general, in such a configuration, it is preferable that the pass band of the filter is set near the resonance frequency. In this band, FIGS. 5A and 5B are simplified,
It is approximately represented as shown in FIG. That is, since the surface acoustic wave resonator can be represented only by the capacitance with respect to the common ground, the external circuit can always match the input / output load. At frequencies lower than the pass band, the equivalent circuit of the filter is represented by the capacitance of the piezoelectric substrate and the capacitance Co between the electrodes of the single-opening surface acoustic wave resonator as shown in FIG. Here, the equivalent circuit of the surface acoustic wave resonator is the electrostatic capacitance between the electrodes.
What is represented by Co is clear from FIG. Therefore, in a frequency band lower than the pass band, the impedance of both the capacitance of the piezoelectric substrate and the capacitance Co between the electrodes of the surface acoustic wave resonator is large, so that the filter is in an attenuation region. At frequencies higher than the normal band, first, near the anti-resonance frequency, it is represented as shown in FIG. 5C. Since the impedance of the surface acoustic wave resonator becomes almost infinite due to anti-resonance, the filter is in an attenuation range. . Further, in a frequency band higher than the anti-resonance frequency, the effect of the capacitance of the piezoelectric substrate becomes relatively large. Therefore, the equivalent circuit of the filter is expressed only by the capacitance of the piezoelectric substrate as shown in FIG. Area. In the above configuration, since the pass band is near the resonance frequency of each resonator and the attenuation band is near the anti-resonance frequency, a band where a very steep rise characteristic is required on the high band side of the pass band is required. This configuration is suitable for a pass filter or a band rejection filter. FIG. 6 shows an example of the frequency characteristic of the filter according to this configuration. The surface acoustic wave resonator used a multi-pair transducer having a logarithm of 400. The piezoelectric substrate used was a 36 ° rotation Y-cut X-propagating LiTaO ₃ , and 20 surface acoustic wave resonators and electrode patterns and boding pads for connecting the resonators in series were formed on the substrate surface. The substrate thickness was set to 0.35 mm so that the electrode finger with respect to the common ground, the connection electrode portion, and the capacity of the padding pad became a desired value. The frequency characteristics are 825 to 845 MHz pass band, 870
A transmission filter of a mobile phone having an attenuation range of 890890 MHz is assumed. The design was based on the following design concept in order to obtain a steep rising characteristic on the high frequency side and to widen the attenuation range. The common electrodes corresponding to 9-1, 9-2,..., 9-6 in FIG. 1 were formed on the piezoelectric substrate, thereby forming 20 one-opening surface acoustic wave resonators. Further, the impedance characteristics of the single-aperture surface acoustic wave resonator shown in FIG. 3B are sequentially changed from the input side of the filter to each of two adjacent single-aperture surface acoustic wave resonators, and the resonance characteristics of 10 groups are obtained. It was a container.
The resonance frequency fr and the anti-resonance frequency fa were slightly shifted from each other between the resonators of each group, and these were gradually increased from the input side to the output side of the filter. Also, a considerably large value of the piezoelectric substrate capacitance is required between the electrode conductor of each one-opening surface acoustic wave resonator and the common ground,
This capacity was ensured by making the thickness of the piezoelectric substrate relatively thin, 0.35 mm. With these designs, a steep rising characteristic on the high frequency side and a wide band attenuation area are realized. As the external matching circuit, as shown in FIG. 1, the input and output of each of the series and the parallel were used. As shown in FIG. 6, the pass band (825 to 845 MHz)
z) Loss less than 0.5dB, attenuation 3 (870-890MHz) loss 3
A characteristic of 0 dB or more was obtained. Further, the rising characteristics are very steep, and realize characteristics that are difficult to realize with a conventional surface acoustic wave filter. Further, the present filter is characterized in that the handling power (passing power), which has recently become one of the parameters indicating the goodness of the filter, is extremely large. According to the experimental result, it passed the passing power more than 30dBm. The reason why the passing power is very large is that the 36 ° rotation Y-cut X-propagating LiTaO ₃ used as the piezoelectric substrate is a high-quality dielectric crystal having an extremely small tan δ, and is not in contact with the electrode conductor of the surface acoustic wave resonator. The reason is that despite the fact that a very high applied voltage is generated by the incident power of several watts in the piezoelectric substrate capacitance formed between the grounds, the piezoelectric substrate capacitance is not destroyed. In general, when a large power is incident on a surface acoustic wave filter, the incident frequency is reduced due to the nonlinear effect of elastic vibration.
2f _T, generating harmonics of such 3f _T against f _T. This filter is a nonlinear effect is also very small, relative to 30 dBm (1watt) more passing power, 2f _T, harmonics such as 3f _T could not be observed in the normal spectrum analyzer. As described above, in the configuration of FIG. 1, the case where the steep attenuation region exists on the high frequency side of the pass band has been described. If the attenuation band exists on the lower side of the pass band, the anti-resonance frequency of the surface acoustic wave resonator may be set to the lower side of the pass band, and matching may be performed in the pass band by an external circuit. As a result of the computer simulation, the loss in the pass band slightly increases as compared with the case where the attenuation band exists on the high frequency side, but a similar filter can be used. FIG. 7 shows another embodiment of the surface acoustic wave filter according to the present invention. FIG. 7 shows a case where a plurality of filters 7-1 and 7-2 are provided to improve isolation between input and output.
And a ground electrode 13 is inserted between them, and they are connected by wire bonding, a crossover pattern, or the like.
Also, in FIG. 7, the same effect can be obtained by dividing the filter into a plurality of chips, connecting them to each other after mounting them in the same package or in another package. According to the present invention, the power durability of a surface acoustic wave filter can be improved to the order of watts.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a configuration of an embodiment of a surface acoustic wave filter according to the present invention. FIG. 2 is a configuration diagram of a one-opening surface acoustic wave resonator constituting the filters of FIGS. 1 and 7; FIG. 3 is a diagram showing an equivalent circuit and characteristics of the single-aperture surface acoustic wave resonator of FIG. FIG. 4 is a diagram showing an equivalent circuit of the filter of FIG. FIG. 5 is a circuit diagram obtained by rewriting FIG. 4 according to a frequency. FIG. 6 is a frequency characteristic diagram of one embodiment of the present invention. FIG. 7 is a diagram showing a configuration of one embodiment of a surface acoustic wave filter according to the present invention. [Description of Signs] 1, 2,... Signal input terminal and output terminal, 3, 4, 5, 6
... inductance, 8 ... piezoelectric substrate, 9 ... common electrode, 7
... Transducer for 1 aperture SAW resonator, 10 ...
1: dielectric capacitance between electrode fingers of the aperture surface acoustic wave transducer; 11... Equivalent resonance inductance of the 1 aperture SAW resonator; 12... Equivalent resonance capacitance of the 1 aperture SAW resonator;

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Junji Sumioka               32, Miyukicho, Kodaira-shi, Tokyo Hitachi Electronics Co., Ltd.               Inside the Koganei Plant (72) Inventor Yoshikatsu Ishida               1410 Inada, Katsuta, Ibaraki Pref.               Hitachi, Ltd. Tokai Plant                (56) References JP-A-51-3833 (JP, A)                 JP-A-51-92147 (JP, A)                 JP-A-59-4216 (JP, A)                 JP-A-58-131810 (JP, A)                 Kazuo Okuno, Yoichi Kobayashi `` Surface acoustic wave vibration               Bandpass filter using a transducer "Nihon Acoustics               Transactions, 3-3-8, May 1979,               PP. 359-360

Claims (1)

(57) [Claims] An input terminal, an output terminal, N (N ≧ 3) surface acoustic wave resonators electrically connected in series between the input terminal and the output terminal, the input terminal and the N elastic surfaces A first matching circuit for an input load provided between the surface acoustic wave resonator and a second matching between an output load provided between the output terminal and the N surface acoustic wave resonators. A surface acoustic wave filter that outputs only a signal in a desired frequency band among the signals input from the input terminal from the output terminal, wherein the N surface acoustic wave resonators include one piezoelectric element. N + 1 common electrodes formed on the surface of the conductive substrate, and a large number of pairs of N electrode finger groups formed between adjacent common electrodes,
N-1 common electrodes other than the two common electrodes disposed at both ends are connected to two electrode finger groups on two substantially parallel sides thereof, and are shared by two surface acoustic wave resonators. A common ground is formed on the back of the piezoelectric substrate,
A surface acoustic wave filter, wherein a capacitance is formed between each of the electrode finger groups and each part of the common electrode and the common ground via a piezoelectric substrate. 2. The surface acoustic wave device according to claim 1, wherein a part of the N surface acoustic wave resonators and another surface acoustic wave resonator have different resonance frequencies. Wave filter. 3. The surface acoustic wave filter is larger than 0.5 W and 4
3. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter is used for withstand power of W or less. 4. 3. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter is used for withstand power of 2 W or more and 4 W or less. 4. 5. The piezoelectric substrate has a 36 ° rotation Y-cut X-propagation Li
The surface acoustic wave filter according to claim 1 to 4, characterized in that a TaO _3. 6. The surface acoustic wave filter according to any one of claims 1 to 5, wherein the surface acoustic wave filter is a transmission filter.