JP4765396B2 - Transversal surface acoustic wave filter - Google Patents

Description

  The present invention relates to a surface acoustic wave device intended to improve insertion loss and in-band deviation in a surface acoustic wave device.

  In recent years, surface acoustic wave (hereinafter referred to as SAW) devices have been widely used in the communication field, and are often used particularly for cellular phones and the like because they have excellent characteristics such as high performance, small size, and mass productivity. Due to the increasing demand for data communication such as images, IF filters used in mobile phones are required to have low loss and small in-band deviation characteristics, and transversal SAW filters are suitable as filters that satisfy such strict specifications. .

  FIG. 5 shows a plan view of a conventional transversal SAW filter. An input IDT electrode 11 and an output IDT electrode 12 are arranged at a predetermined interval along the SAW propagation direction on the main surface of the piezoelectric substrate, and an input / output terminal is provided between the IDT electrodes 11 and 12. A shield electrode 13 for shielding a direct wave therebetween is arranged. The IDT electrodes 11 and 12 are composed of split electrodes in which two pairs of positive electrode fingers and two negative electrode fingers are paired, and one comb electrode of the IDT electrode 11 is connected to the input terminal IN and the other The comb electrode is grounded, and a transversal SAW filter is formed by connecting one comb electrode of the IDT electrode 12 to the output terminal OUT and grounding the other comb electrode.

FIG. 6 shows the pass characteristics of the transversal SAW filter. The center frequency is 40 MHz, lithium niobate is used for the piezoelectric substrate, the IDT electrode 11 has 5 pairs, and the IDT electrode 12 has 7 pairs. As shown in the figure, the pass band is largely convex near the center frequency, and it can be seen that the in-band deviation is significantly degraded.
JP-A-60-263505 M. Takeuchi and K. Yamanouchi: “New Type of SAW Reflectors and Resonators Consisting of Reflecting Elements with Positive and Negative Reflection Coefficients”, IEEE Trans.Ultrason. Ferroelec. Freq. Contr., Vol.33, No.4, pp. 369-374 (1986).

  As described above, the conventional transversal SAW filter has a problem that the in-band deviation is remarkably bad. In order to solve this problem, as shown in FIG. 7, a plurality of pairs of short-circuited reflective electrodes 14 in which adjacent electrode fingers are electrically connected to each other outside the IDT electrodes 11 and 12 are arranged to increase the reflection efficiency. The method is suitable.

  FIG. 8 shows the pass characteristics of the transversal SAW filter of FIG. The center frequency is 40 MHz, lithium niobate is used for the piezoelectric substrate, the IDT electrode 11 has 5 pairs, and the IDT electrode 12 has 7 pairs. Then, the transmission characteristics when the reflective electrode 14 is changed to 4, 6, and 8 pairs are superimposed and shown. As shown in the figure, it can be seen that when the number of reflection electrodes is increased, the convex portion near the center frequency of the pass band gradually decreases, and the in-band deviation is improved. However, on the other hand, when the number of the reflective electrodes is increased, the pass characteristic becomes a single peak, and there arises a problem that the insertion loss and the bandwidth are deteriorated.

  In order to solve the problems described above, an object of the present invention is to realize a pass characteristic with a low loss and a small in-band deviation in a SAW device.

  In order to solve the above problems, the invention according to claim 1 of the SAW device according to the present invention comprises a piezoelectric substrate, an IDT electrode formed on the piezoelectric substrate, and a reflective electrode disposed inside and outside the IDT electrode. In the surface acoustic wave device, the first to fourth floating electrodes are sequentially arranged in the reflection electrode along the propagation direction of the surface acoustic wave, and the first and third floating electrodes or the second and second floating electrodes are arranged. It is the structure which short-circuited 4 floating electrodes.

  The invention according to claim 2 is characterized in that at least one of the reflective electrodes has a structure that is horizontally reversed with respect to the other reflective electrodes.

  The invention described in claim 3 is characterized in that the piezoelectric substrate is lithium niobate.

  According to the first aspect of the present invention, the first and third floating electrodes are sequentially arranged inside and outside the IDT electrode along the propagation direction of the surface acoustic wave. By disposing a reflective electrode in which the electrodes or the second and fourth floating electrodes are short-circuited, a SAW device with a low loss and a small in-band deviation can be realized.

  According to the invention described in claim 2, the insertion loss is further improved as compared with the SAW device according to claim 1 by adopting a structure in which at least one of the reflection electrodes is horizontally reversed with respect to the other reflection electrodes. be able to.

  According to the third aspect of the present invention, wide band pass characteristics can be realized by using lithium niobate for the piezoelectric substrate.

  Hereinafter, the present invention will be described in detail based on the embodiments shown in the drawings. FIG. 1 shows a SAW device according to the present invention. An input IDT electrode 1 and an output IDT electrode 2 are arranged at a predetermined interval along the SAW propagation direction on the main surface of the piezoelectric substrate, and between the input / output terminals between the IDT electrodes 1 and 2. A shield electrode 3 for shielding the direct wave is arranged. The IDT electrodes 1 and 2 are composed of split electrodes in which two pairs of positive electrode fingers and two negative electrode fingers are paired. One comb electrode of the IDT electrode 1 is connected to the input terminal IN and the other one is connected. The comb electrode is grounded, one comb electrode of the IDT electrode 2 is connected to the output terminal OUT, and the other comb electrode is grounded. The reason why the excitation electrodes are split electrodes is to prevent reflections between the excitation electrodes from overlapping and to obtain a symmetrical transmission response. The reflective electrode 4 is disposed outside the IDT electrodes 1 and 2 and inside the IDT electrode 2. FIG. 1 (b) shows an enlarged view of the reflective electrode 4. The reflective electrode 4 is composed of four floating electrodes. The floating electrodes O1 and O2 function as open gratings, and the floating electrodes S1 and S2 are connected to each other. It is made to function as a short-circuit grating by electrically short-circuiting.

The reflective electrode is the same as PNR (Positive and Nagative Reflectivity) disclosed in Patent Document 1 and Non-Patent Document 1. According to Patent Document 1 and Non-Patent Document 1, by placing the PNR outside the IDT electrode of the SAW resonator, mode conversion to a bulk wave is reduced as compared with a conventional open type or short type reflective electrode. It is disclosed that the reflection efficiency can be increased. On the other hand, in the present invention, the PNR is disposed both outside the IDT electrode and inside the IDT electrode, thereby improving the reflection efficiency and improving the insertion loss and in-band deviation.

FIG. 2 shows the pass characteristics of the transversal SAW filter of the present invention. The center frequency is 40 MHz, lithium niobate is used for the piezoelectric substrate, the IDT electrode 1 has 5 pairs, and the IDT electrode 2 has 7 pairs. 5 pairs of reflective electrodes 4 are arranged outside the IDT electrodes 1 and 2, and one reflective electrode 4 is arranged inside the IDT electrode 2. For comparison, the characteristic shown in FIG. 6 is the pass characteristic shown in FIG. 6 and the characteristic shown in conventional 2 is that five pairs of reflective electrodes are arranged only outside the IDT electrode, and the reflection inside the IDT electrode is reflected. The pass characteristic of the transversal SAW filter when no electrode is disposed is shown.

From FIG. 2, it can be seen that the pass characteristic of the transversal SAW filter of the present invention is smaller in the convex portion near the center frequency of the pass band than in the conventional characteristic 1, and the in-band deviation is greatly improved. I understand. Further, the shape of the pass band is a square, and low loss and wide band characteristics are obtained. Specifically, in the transversal SAW filter of the present invention, characteristics with an insertion loss of 10.4 (dB) and an in-band deviation of 0.7 (dB) were obtained. Further, when comparing the characteristics of the present invention with the characteristics of Conventional 2, the characteristics of Conventional 2 have an insertion loss of 11.0 (dB) and an in-band deviation of 1.1 (dB). The insertion loss and in-band deviation were better. Thus, it was confirmed that the insertion loss and the in-band deviation can be improved by disposing the reflective electrode on the outside and inside of the IDT electrode.

  As described above, the transversal SAW filter according to the present invention has a low-loss and low in-band deviation compared with a conventional SAW device by disposing a PNR type reflection electrode outside and inside the electrode. Realized. In addition, wide bandwidth characteristics were obtained because lithium niobate was used for the piezoelectric substrate. In FIG. 1, the reflective electrode 4 is arranged only inside the IDT electrode 2, but the reflective electrode 4 may be arranged inside both the IDT electrodes 1 and 2.

  FIG. 3 shows a modification of the SAW device according to the present invention. The difference from FIG. 1 is that a PNR type reflection electrode 5 arranged inside the IDT electrode 2 is arranged outside the IDT electrodes 1 and 2. The structure is the same as that shown in FIG. 1 except that the PNR type reflection electrode 4 is reversed from side to side.

FIG. 4 shows the pass characteristics of the transversal SAW filter of FIG. 3. The center frequency is 40 MHz, lithium niobate is used for the piezoelectric substrate, the IDT electrode 1 has 5 pairs, and the IDT electrode 2 has 7 pairs. 5 pairs of reflective electrodes 4 are arranged outside the IDT electrodes 1 and 2, and one reflective electrode 5 that is horizontally reversed is arranged inside the IDT electrode 2. For comparison, the characteristic indicated by the broken line in FIG. 4 indicates the pass characteristic of the transversal SAW filter in FIG. From the figure, the transversal SAW filter of FIG. 3 indicated by the solid line has an insertion loss of 10.0 (dB) and an in-band deviation of 0.8 (dB), which is almost the same as the transversal SAW filter of FIG. It was confirmed that the insertion loss can be further improved while maintaining the same in-band deviation.

  In addition, although the example which arrange | positioned the PNR type | mold reflective electrode reversed left and right inside the IDT electrode was demonstrated in FIG. 3, even if the reflection electrode arrange | positioned outside the IDT electrode is reversed right and left, it is effective in the improvement of insertion loss. It was confirmed by experiment.

  In the above, an example in which lithium niobate capable of realizing broadband characteristics is used for the piezoelectric substrate has been described. However, the present invention is not limited to this, and quartz, lithium tantalate, lithium tetraborate, langasite is used for the piezoelectric substrate. Needless to say, the present invention can also be applied when used in the above. Needless to say, the present invention can also be applied to a resonator type or a mode coupled filter other than the transversal SAW filter.

It is a figure explaining the transversal SAW filter which concerns on this invention, (a) shows a top view, (b) shows a PNR type reflective electrode. The comparison of the pass characteristic of this invention and the conventional transversal SAW filter is shown. The modification of the transversal SAW filter which concerns on this invention is shown. The pass characteristic of the modification of the transversal SAW filter which concerns on this invention is shown. The top view of the conventional transversal SAW filter is shown. The pass characteristic of the conventional transversal SAW filter is shown. The top view of the transversal SAW filter which has arrange | positioned the conventional short type reflective electrode is shown. The transmission characteristic when the logarithm of the reflective electrode of the transversal SAW filter which has arrange | positioned the conventional short-circuited reflective electrode is changed is shown.

Explanation of symbols

1, 2: IDT electrode 3: Shield electrode 4, 5: Reflective electrode O1, O2: Open type floating electrode S1, S2: Short-circuit type floating electrode

Claims (3)

In a transversal surface acoustic wave filter comprising a piezoelectric substrate, an IDT electrode formed on the piezoelectric substrate, and reflective electrodes disposed inside and outside the IDT electrode,
Each of the reflective electrodes, the first through fourth floating electrodes sequentially arranged along the propagation direction of a surface acoustic wave, and short-circuits the first and third floating electrodes are the second and fourth floating electrode structure having an open, or the first and third floating electrode opening, the second and transversal type surface acoustic wave filter characterized in that the fourth is either a structure in which short-circuited floating electrodes are . 2. The transversal surface acoustic wave filter according to claim 1, wherein at least one of the reflective electrodes has a left-right inverted structure with respect to another reflective electrode. The transversal surface acoustic wave filter according to claim 1, wherein the piezoelectric substrate is lithium niobate.

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