JP3866709B2 - Surface acoustic wave resonator and surface acoustic wave filter - Google Patents

Description

  The present invention relates to a surface acoustic wave resonator and a ladder-type surface acoustic wave filter in which a plurality of surface acoustic wave resonators are arranged in a series arm and a parallel arm.

  As a band-pass filter, a ladder-type SAW filter using a plurality of surface acoustic wave (hereinafter referred to as SAW) resonators is known (for example, see Non-Patent Document 1).

FIG. 1 shows a configuration diagram of a conventional ladder-type SAW filter.
In the ladder-type SAW filter, series arm SAW resonators S1 and S2 are arranged between the input terminal Ti and the output terminal To on the piezoelectric substrate, and in parallel between the input and output terminals and the ground terminal G. The arm SAW resonators P1 and P2 are arranged. The SAW resonators S1, S2, P1, and P2 are generally called a one-terminal pair SAW resonator.

  FIG. 2 shows a configuration diagram of this conventional one-terminal pair SAW resonator. The one-terminal-pair SAW resonator is disposed on an SAW propagation path in order to confine the excited SAW in the IDT and an interdigital transducer (hereinafter referred to as IDT) for electrically exciting the SAW. The reflector is formed on a piezoelectric substrate. Note that when the resonance characteristic is obtained by utilizing the internal reflection of the SAW of the IDT itself, the reflector may not be provided.

In the IDT, a large number of electrodes are arranged in a comb shape at a constant period (pi) as shown in the figure.
The reflector has a large number of grating electrodes arranged at a constant period (pr) and is also called a grating reflector.
In this IDT, SAW is excited with two adjacent electrodes extending from the vertical direction as one unit.
The ladder-type SAW filter is designed so that the resonance frequency frs of the series arm SAW resonators S1 and S2 and the anti-resonance frequency fap of the parallel arm SAW resonators P1 and P2 substantially coincide with each other.

  FIG. 3A shows a pass characteristic diagram of a ladder-type SAW filter. FIG. 3B shows impedance characteristics of the series arm SAW resonators S1 and S2 and the parallel arm SAW resonators P1 and P2 alone at this time. A graph g1 in FIG. 3B is a graph of the series arm SAW resonator, and a graph g2 is a graph of the parallel arm SAW resonator. In the graph g1 of the series arm SAW resonator, the frequency at which the impedance is minimum is the resonance frequency frs, and the frequency at which the impedance is maximum is the anti-resonance frequency fas.

FIG. 4 is an explanatory diagram of frequency characteristics required for a band-pass filter such as a ladder-type SAW filter.
Here, as characteristic values, there are a desired pass bandwidth (BW1, BW2), a degree of suppression (ATT1, ATT2) at a frequency defined in the specification, a width of suppression degree BWatt1, BWatt2, and the like.
Further, the ratio of the bandwidths BW2 and BW1 (BW1 / BW2) at a certain attenuation is taken and called the squareness ratio. The squareness ratio is better as it is closer to 1, and is said to be a high squareness ratio. In the ladder-type SAW filter, the squareness ratio is substantially determined by the frequency difference between the resonance frequency fr and the antiresonance frequency fa of the SAW resonator.

  That is, the steepness of the rise from the low frequency side attenuation region to the pass region depends on the frequency difference (Δfp in FIG. 3B) between the resonance frequency frp and the antiresonance frequency fap of the parallel arm SAW resonators P1 and P2. However, the smaller the Δfp, the steep rise. Further, the steepness of the fall from the pass band to the high frequency side attenuation band depends on the frequency difference between the resonance frequency frs of the series arm SAW resonators S1 and S2 and the antiresonance frequency fas (Δfs in FIG. 3B). However, the smaller the Δfs, the sharper the fall.

  However, Δfp and Δfs are substantially determined by the electromechanical coupling coefficient of the piezoelectric substrate to be used, and do not change even when the IDT electrode pair number or the electrode crossing width is changed. In this regard, Patent Document 1 and Patent Document 2 describe SAW resonators in which Δfp and Δfs are reduced by thinning out the IDT electrode toward the outside.

Patent Document 3 describes a SAW resonator that reduces Δfp and Δfs by periodically thinning out the electrodes of the IDT. By using these thinning methods, Δfp and Δfs can be adjusted with a simple configuration, and a ladder-type SAW filter having a desired passband width and a steepness at the end of the passband is realized.
IEICE Transactions A Vol. J 76-A No.2 pp. 245-252 1993 Japanese Patent Laid-Open No. 8-23256 Japanese Patent Application No. 10-514149 JP-A-11-163664

  However, when the IDT electrodes are periodically thinned, Δfp (Δfs) becomes small and a high-angle ratio filter can be realized, but spurious is generated outside the filter passband. There was a problem. The spurious outside the pass band is generated at a plurality of locations in accordance with the period of thinning out the IDT electrodes, and often does not satisfy the attenuation characteristic outside the pass band required for the filter.

  On the other hand, when the IDT electrode is thinned more toward the outer side, spurious generated outside the pass band can be suppressed to a small value, and Δfp (Δfs) is also reduced. Although it is possible to realize a filter with a high squareness ratio, this method has a poor reduction rate of Δfp (Δfs) with respect to the number of thinned electrodes (thinning rate R) with respect to the total number of electrodes of IDT, and even with the same R, The reduction rate of Δfp (Δfs) is about half that of the case where the thinning is performed periodically.

  That is, when an attempt is made to produce a filter having the same Δf (square ratio), this method requires about twice as much R as the periodic thinning method. Since the capacitance of the IDT is reduced by thinning out the electrodes, the opening length of the IDT is usually increased or the number of electrode pairs is increased in order to correct this. That is, if R is doubled, the increase in the IDT area is doubled or more, and the filter chip size cannot be reduced.

  Therefore, the present invention has been made in view of the above circumstances, and by devising the structure of the surface acoustic wave resonator used in the surface acoustic wave filter, the resonance frequency of the surface acoustic wave resonator can be reduced. It is an object of the present invention to realize a surface acoustic wave resonator capable of reducing the frequency difference between anti-resonance frequencies, suppressing spurious out of the passband, and reducing the size of the filter chip, and a surface acoustic wave filter using the surface acoustic wave resonator. .

The present invention comprises at least one interdigital transducer formed on a piezoelectric substrate, wherein the at least one interdigital transducer is composed of a predetermined number of comb electrodes, and at least three electrodes of the comb electrodes are provided. The positions of the thinned and thinned electrodes are aperiodic, and a predetermined number of comb-shaped electrodes constituting the interdigital transducer are viewed in a direction parallel to the traveling direction of the surface acoustic wave to be excited. When the region is divided into three or more divisions so that the difference between the maximum value and the minimum value of the number of electrodes in the region is 1 or less, the maximum value and the minimum of the average normalized excitation intensity of the surface acoustic wave in each division region the difference between the values is thinned comb electrodes such that 0.30 or less, the position obtained by thinning the electrode, perpendicular inter the traveling direction of the surface acoustic wave There is provided a surface acoustic wave resonator, wherein the non-mirror-symmetric to the center line and the axis of the I digital transducer.
Further, it comprises at least one interdigital transducer formed on a piezoelectric substrate, the at least one interdigital transducer is composed of a predetermined number of comb electrodes, and at least three electrodes of the comb electrodes are thinned out. The positions of the thinned electrodes are aperiodic, and a predetermined number of comb-shaped electrodes constituting the interdigital transducer are viewed in a direction parallel to the traveling direction of the surface acoustic wave to be excited. When the region is divided into three regions so that the difference between the maximum value and the minimum value of the number of electrodes is one or less, one or more thinned electrodes exist in the central region, and the electrodes in each divided region When the area is divided into three or more divisions so that the difference between the maximum value and the minimum value is one or less, the number of electrodes thinned out in each division area Ri difference two der following the large value and the minimum value, the position obtained by thinning the electrode, characterized in that non-mirror-symmetry with a symmetry axis center line of the vertical interdigital transducer in the traveling direction of the surface acoustic wave A surface acoustic wave resonator is provided.
According to this, the surface acoustic wave resonator can be made closer to the resonance frequency and the anti-resonance frequency, the surface acoustic wave filter has a higher squareness ratio, and spurious out of the passband is suppressed. A filter can be provided.

According to the present invention, in the comb-shaped electrodes constituting the IDT of the surface acoustic wave resonator, the electrodes are thinned out non-periodically, so that the resonance frequency and the anti-resonance frequency of the surface acoustic wave resonator are arbitrarily approximated. Therefore, a high-angle ratio surface acoustic wave resonator and a ladder-type SAW filter can be realized.
Further, spurious out of the pass band, which has been generated by the conventional periodic thinning method, can also be suppressed.
Furthermore, since the same ΔF reduction effect can be obtained with a thinning rate smaller than the thinning method as the outside of the conventional IDT, the element can be made smaller than the conventional one.
Similarly, when a quasi-thinning electrode is provided at a non-periodically thinned electrode position, Δf of the surface acoustic wave resonator can be reduced, spurious can be suppressed, and the size can be reduced.

  In the present invention, from the viewpoint of effectively reducing the excitation efficiency, when the total number of comb-shaped electrodes when not thinned is Ra and the number of thinned electrodes is Rm, it exists between the thinned electrodes. The number N of electrodes is preferably 1 ≦ N ≦ 0.4 × (Ra−Rm−4).

Here, in the surface acoustic wave resonator, the number of thinned electrodes may be 2% to 22% of the total number of electrodes when not thinned.
Furthermore, in the surface acoustic wave resonator, a quasi-thinning electrode that does not contribute to excitation of the surface acoustic wave may be formed at the thinned electrode position.
In the surface acoustic wave resonator, reflectors may be arranged close to each other in the direction parallel to the traveling direction of the excited surface acoustic wave and on both sides of the interdigital transducer.

  Furthermore, in the case of forming a surface acoustic wave filter comprising a piezoelectric substrate and a plurality of surface acoustic wave resonators formed on the piezoelectric substrate and electrically connected in a ladder shape, at least one of the elastic surfaces The wave resonator may be constituted by the surface acoustic wave resonator of the present invention as described above.

  In the present invention, the excitation efficiency means a conversion coefficient defined in general mode coupling theory, and represents the excitation efficiency due to the voltage applied to the IDT (Introduction to surface acoustic wave (SAW) device simulation technology, Hashimoto (See Kenya, 1997, Realize, p216).

  The present invention will be described in detail below based on the embodiments shown in the drawings. However, this does not limit the present invention.

First, the configuration and characteristics of the SAW resonator according to the present invention will be described in comparison with a conventional one-terminal-pair surface acoustic wave resonator.
In the conventional SAW resonator of FIG. 2, an IDT (period pi) and a grating reflector (period pr) are formed on both sides of the IDT on the piezoelectric substrate by an Al thin film or the like. Pi / 2 and pr are designed to be approximately the same, and a sharp resonance is realized at a frequency f (f = v / pi) determined by the period pi of the IDT and the velocity v of the surface acoustic wave. Note that when the resonance characteristic is obtained by utilizing the internal reflection of the SAW of the IDT itself, the reflector may not be provided.

FIG. 5 shows an impedance characteristic diagram of this conventional SAW resonator. In general, a SAW resonator exhibits double resonance characteristics having a resonance frequency fr and an antiresonance frequency fa as shown in FIG. Here, Δf in the figure represents the difference between the two frequencies.
The ladder-type SAW filter has a configuration in which several surface acoustic wave resonators are connected in parallel and in series as shown in FIG. 1, but the anti-resonance frequency fap of the parallel arm SAW resonators P1 and P2 and the series arm SAW. The IDT of the resonator is designed so that the resonance frequencies frs of the resonators S1 and S2 substantially coincide (see FIG. 3).

In order to improve the squareness of the filter characteristics, it is necessary to reduce the frequency difference Δf (= fa−fr) between the antiresonance frequency fa and the resonance frequency fr of the SAW resonator as described above.
In general, a mode coupling theory is known. However, when a simulation using this theory is performed in a SAW filter and the value of the “conversion coefficient ζ” is decreased, the anti-resonance frequency fa moves to the low frequency side, and fa and fr It has been found that the frequency difference Δf is small.

FIG. 6 shows a graph of changes in the frequency difference Δf (MHz) between the resonance point and the antiresonance point with respect to the normalized conversion coefficient ζ / ζ ₀ (%). Here, an 800 MHz band SAW resonator is assumed. Note that the normalized conversion coefficient ζ / ζ ₀ is a conversion coefficient normalized by the conversion coefficient ζ ₀ in the normal design. The smaller the normalized conversion coefficient ζ / ζ ₀ is, the lower the excitation efficiency is.

  According to FIG. 6, we used mode coupling theory that if the SAW excitation efficiency in the IDT of the SAW resonator is lowered, the anti-resonance frequency fa moves to the lower frequency side and Δf becomes smaller. Found by simulation. In other words, the more the SAW excitation in the IDT is suppressed, the smaller the frequency difference Δf between the resonance point and the antiresonance point, and the squareness ratio can be increased.

  Examples of SAW resonators that can efficiently reduce the frequency difference Δf between the resonance point and the antiresonance point, suppress spurious out-of-band, and increase the squareness ratio will be described below.

FIG. 7 shows a block diagram of an embodiment of a one-terminal pair SAW resonator according to the present invention.
In FIG. 7, four pseudo-thinning electrodes (= Rm) are provided in the IDT (total number of electrodes 40 = Ra) (I1 to I4). The thinning rate R = 4/40 = 10 (%). Here, the number N of electrodes existing between the quasi-thinning electrodes is set to satisfy 1 ≦ N ≦ 0.4 × (40−4−4) = 12.8. For example, in FIG. 7, the number of electrodes Rm between the pseudo thinning electrodes I ₁ and I ₂ is 5, and the number of electrodes Rm between the pseudo thinning electrodes I ₂ and I ₃ is 11.

As shown in FIG. 2, the IDT of a one-terminal pair SAW resonator in which all the electrodes are periodically arranged has an electrode Ia connected to the upper terminal and an electrode Ib connected to the lower terminal alternately. It is arranged. On the other hand, the quasi-thinning electrode of the present invention refers to an electrode connected to a terminal opposite to a terminal to be connected in a normal design.
A reverse polarity voltage is applied to the upper and lower terminals of the IDT. Since the polarity of the connection terminal is reversed, the surface acoustic wave is not excited from the pseudo thinning electrodes (I1 to I4). Therefore, the pseudo thinning-out electrode is connected to the terminal opposite to the terminal to be connected in the normal design as shown in FIG. 7, or even if the electrodes at positions I1 to I4 in FIG. SAW resonators having the same characteristics are formed.

By the way, since the surface acoustic wave is not excited in the region where the pseudo thinning electrode is formed or the region where the electrode is thinned, the SAW excitation efficiency of the entire IDT is lowered. This decrease in excitation efficiency results in a decrease in the conversion coefficient ζ in FIG.
FIG. 8 is an impedance characteristic diagram of the thinned SAW resonator of the present invention shown in FIG. Here, the solid line is a graph of the SAW resonator of the present invention shown in FIG. 7, and the broken line is a graph of the conventional SAW resonator shown in FIG. As a result of the reduction of the conversion coefficient ζ, the impedance characteristic of the SAW resonator shifts the antiresonance frequency fa to the low frequency side (from fa to fa ′) as shown in FIG. 8, and Δf becomes small.

  Further, from the viewpoint of reducing the filter chip size, each divided region is divided into three or more divided regions so that the number of electrodes is substantially equal when viewed in a direction parallel to the traveling direction of the SAW in which the IDT is excited. It is preferable that the comb-shaped electrodes are thinned out or quasi-thinned electrodes are provided so that the average normalized excitation strength of the SAW is substantially the same for all the divided regions.

  FIG. 9 shows the structure of a SAW resonator in which pseudo-thinning electrodes (I1 to I4) are periodically provided in a conventional SAW resonator. The total number of IDT electrodes is 40, and the number of pseudo thinning electrodes is 4. The thinning rate R = 4/40 = 10 (%), which is the same as that of the SAW resonator of the present invention shown in FIG.

  FIG. 10 shows a comparison between the impedance characteristics of the resonator shown in FIG. 9 in which pseudo-thinning electrodes are periodically provided and the impedance characteristics of the SAW resonator of the present invention shown in FIG. Since the thinning rate R is the same, it can be seen that the conversion coefficient ζ decreases by the same amount and Δf decreases by the same amount.

FIG. 11 shows a comparison of the pass characteristics of the conventional SAW resonator shown in FIG. 9 provided with pseudo thinning electrodes and the SAW resonator of the present invention shown in FIG. When viewed in a wide band, it can be seen that the conventional resonator shown in FIG. 9 provided with quasi-thinning electrodes periodically generates spurious due to the periodicity of the thinning-out positions (dashed line graph). .
On the other hand, in the SAW resonator of the present invention shown in FIG. 7, since the number of electrodes existing between the thinned electrodes is not constant, it can be seen that spurious out of the band is greatly reduced (solid line graph).
From the above results, the reduction effect of Δf is the same and the out-of-band effect is that the thinning electrodes are not periodic rather than provided periodically (the number of electrodes existing between the thinning electrodes is not constant). Since spurious can be suppressed, it can be said to be an excellent resonator structure.

  FIG. 12 shows the structure of a conventional SAW resonator in which more quasi-thinning electrodes are provided on the outer side. The total number of IDT electrodes is 40, and the number of pseudo thinning electrodes (I1 to I4) is four. The thinning rate R = 4/40 = 10 (%), which is the same as that of the SAW resonator of the present invention shown in FIG.

FIG. 13 shows a comparison diagram of the pass characteristics of the resonator shown in FIG. 12 provided with more quasi-thinning electrodes and the SAW resonator of the present invention shown in FIG. Since both resonators have no periodicity in the thinning position, no spurious is observed outside the band.
FIG. 14 shows a comparison diagram of the impedance characteristics of the resonator provided with more pseudo thinning electrodes toward the outside shown in FIG. 12 and the impedance characteristics of the SAW resonator of the present invention shown in FIG. Although the thinning rate R is the same, it can be seen that Δf of the resonator (broken line graph) provided with more pseudo thinning electrodes is not so small as the outer side shown in FIG.

  From the above results, the number of pseudo-thinning electrodes provided on the entire IDT is non-periodic, i.e., the number of electrodes existing between the thinning electrodes is not constant, rather than providing more on the outside of the IDT. (SAW resonator in FIG. 7) can suppress out-of-band spurious and can effectively reduce Δf, so that it can be seen that the resonator structure is excellent in manufacturing a high-angle ratio filter.

Further, since the SAW resonator of the present invention shown in FIG. 7 has R = 10%, the reduction rate of the capacitance of the IDT due to thinning is about 20%. The increase rate of the IDT area for correcting this is 1 / (1-0.2) = 1.25 (times).
On the other hand, in order to produce the same Δf reduction effect as that of the SAW resonator of the present invention shown in FIG. 7 with the resonator provided with more pseudo thinning electrodes on the outer side shown in FIG. Needs to be approximately doubled (R = 20%). At this time, the increase rate of the IDT area is 1 / (1-0.4) = 1.67 (times). Therefore, the SAW resonator of FIG. 7 of the present invention can realize a smaller SAW filter than the conventional SAW resonator of FIG.

Next, the electrode thinning rate will be described.
Here, the thinning rate R is defined as the ratio of the thinned electrode number Rm to the total electrode number Ra of the comb-shaped electrodes constituting one IDT. That is, R = (Rm / Ra) × 100 (%). Therefore, when the total number of IDT electrodes is 100 and the number of thinned electrodes is 10, the thinning rate R = 10%. In addition, when a pseudo thinning electrode is provided, the number of electrodes is Rm.

  From the viewpoint of satisfying characteristics such as the squareness ratio required for the SAW filter, the thinning rate R is preferably in the range of 2% ≦ R ≦ 22%. This is because if the thinning rate R is increased beyond this range, the bandwidth is reduced and the anti-resonance Q value is deteriorated, which is not practical. Further, when the thinning rate R is smaller than 2%, there is almost no change in Δf and no improvement in the squareness ratio is observed.

In FIG. 15, as another embodiment of the present invention, the difference between the maximum value and the minimum value of the number of pseudo thinning electrodes in the three regions A, B, and C of the IDT is two or less and exists between the thinning electrodes. The block diagram of the resonator with which the number of electrodes is not constant is shown.
Here, a case where the IDT is divided into three regions is shown, but the present invention is not limited to this. The IDT may be divided into four or more divisions, and pseudo thinning in each divided region with any number of divisions of three or more. What is necessary is just to make it the difference of the maximum value of electrode number and the minimum value become two or less.

  In FIG. 15, the maximum value of the pseudo thinning electrode in each divided region is 3, the minimum value is 1, and the difference is 2. In this way, when the difference between the maximum value and the minimum value of the thinning number in the divided area is set to 2 or less and the number of electrodes existing between the thinned electrodes is not constant, the configuration shown in FIG. Similar to the solid line graph, resonance / filter characteristics with a high squareness ratio and no out-of-band spurious can be obtained.

  As shown in FIG. 15, when the IDT is divided into regions, when viewed from another viewpoint, if the pseudo thinned-out electrodes are arranged so that the parameter “average normalized excitation intensity” is substantially equal for all the divided regions. Thus, a resonance / filter characteristic having a high squareness ratio and almost no out-of-band spurious can be obtained as shown in FIG.

    Here, the “average normalized excitation strength” of SAW in each divided region when the IDT is divided into regions is defined with reference to FIG. FIG. 16 shows an example in which an IDT composed of 20 electrodes is divided into three so as to have an approximately equal number of electrodes. Regions A, B, and C are composed of seven, six, and seven electrodes, respectively. In FIG. 16, only one pseudo thinning electrode I1, I2, I3 is provided in each region A, B, C. If the intensity of SAW excited between two adjacent electrodes that are not pseudo thinning electrodes is defined as “1” as “normalized excitation strength”, the pseudo thinning electrode is not provided in the resonator of FIG. If this is the case, the total value of the normalized excitation strength in each region is 6.5, 6, and 6.5, respectively.

  When pseudo thinning electrodes are provided as shown in FIG. 16, since SAW is not excited between the thinning electrodes I1, I2, and I3 and the electrodes on both sides thereof, normalization between the thinning electrodes I1, I2, and I3 and the electrodes on both sides thereof is performed. The excitation intensity is both “0”. Accordingly, assuming that pseudo thinning electrodes are provided as shown in FIG. 16, the total values of the normalized excitation strength in each region are 4.5, 4, and 4.5, respectively. This is the same when a pseudo thinning electrode is provided, or when so-called thinning is performed when no electrode is provided at the position of the pseudo thinning electrode.

Here, in each divided region, the ratio of the total value of the normalized excitation strength after provision of the pseudo thinning electrode to the total value of the normalized excitation strength before thinning is defined as “average normalized excitation strength”. In the case of FIG. 16, the average normalized excitation strengths in the divided areas A, B, and C are 0.69, 0.67, and 0.69, respectively, and are substantially equal for all the divided areas.
As described above, even when the pseudo-thinning electrode is provided or the comb-shaped electrode is thinned out so that the average normalized excitation strength of the SAW is substantially the same in all the divided regions, spurious is hardly generated at a high squareness ratio. No SAW resonator / SAW filter can be provided.

  Note that it is not necessary to strictly match the number of electrodes in the divided area, and the divided areas are divided into substantially equal numbers, and thinning is performed so that the difference between the maximum value and the minimum value of the thinned number in the divided area is 2 or less. If the number of electrodes existing between the thinned electrodes is not constant, filter characteristics with a high squareness ratio can be obtained.

  Here, the number N of electrodes existing between the thinning electrodes is preferably in the range of 1 ≦ N ≦ 0.4 × (Ra−Rm−4) as described above. This is because when N outside this range is set, the region excited by SAW and the region not excited are not sparse, and the excitation efficiency of the entire IDT cannot be effectively reduced. That is, when N outside this range is set, the reduction rate of Δf with respect to the thinning rate is deteriorated, and it is difficult to realize a required small filter.

An embodiment of a specific configuration of a surface acoustic wave filter using the surface acoustic wave resonator according to the present invention will be described below.
FIG. 17 shows a configuration diagram of an embodiment of a ladder-type SAW filter using the surface acoustic wave resonator of the present invention.
This is a SAW filter having two series arm SAW resonators S1 and S2 and two parallel arm SAW resonators P1 and P2 as in the conventional ladder type SAW filter shown in FIG.
However, it differs from the conventional ladder-type SAW filter shown in FIG. 1 in that a SAW resonator having a quasi-thinning electrode as shown in FIG. 7 is used.

  The ladder-type SAW filter of the present invention is not limited to that shown in FIG. 17, but the SAW resonator of the present invention of FIG. 7 is used only for the series arm SAW resonator, and the parallel arm SAW resonator is shown in FIG. A conventional SAW resonator as described above may be used. Conversely, the SAW resonator of FIG. 7 may be used only for the parallel arm SAW resonator. Further, the number of SAW resonators of the series arm and the parallel arm is not limited to that shown in FIG. 17 and can be any number of three or more according to required performance, specifications, etc. The number of SAW resonators and parallel arm SAW resonators may not be the same.

<First embodiment>
First, a ladder-type SAW filter using the SAW resonator shown in FIG. 7 of the present invention will be described only for the series arm SAW resonator.
Here, a ladder-type SAW filter in which four SAW resonators are formed in series and two in parallel on a 42 ° Y cut-X propagation LiTaO ₃ substrate is used.

The SAW resonators connected in series have the configuration as shown in FIG. 7, IDT period pi = 4.60 μm, IDT aperture length = 133 μm, IDT logarithm 116 pairs, reflector period pr = 2.30 μm, reflector The number of electrodes is 160, and the IDT electrode thinning rate R is 12.5%.
The number N of electrodes existing between the pseudo thinning electrodes is set to satisfy 1 ≦ N ≦ 79.6. Here, Ra = 232 and Rm = 29.

  The SAW resonators connected in parallel have the configuration as shown in FIG. 2, the IDT period pi = 4.80 μm, the IDT aperture length = 120 μm, the IDT logarithmic pair 78, the reflector period pr = 2.40 μm, the reflector The number of electrodes is 120.

  FIG. 18 shows a SAW filter according to the first embodiment of the present invention, a filter using a conventional periodically thinned resonator (R = 12.5%) as shown in FIG. The comparison figure of the frequency characteristic of the SAW filter (FIG. 1) comprised only with the conventional resonators is shown. A thin solid line is a SAW filter of the present invention, a broken line is a filter using a periodic thinning resonator, and a thick solid line is a characteristic diagram of a conventional SAW filter (FIG. 1).

  According to FIG. 18, both the thin solid line SAW filter of the present invention and the filter using the broken line periodic thinning resonator have a small frequency difference Δf between the resonance point and the antiresonance point of the series arm SAW resonator. Therefore, it can be seen that the falling from the pass band to the attenuation area on the high frequency side becomes steep and the squareness ratio is improved.

In order to obtain a square characteristic similar to that of the present invention by thinning out the outside of the conventional IDT, it is necessary to thin out the electrodes of the series arm SAW resonator by about 25%. At this time, in order to correct the decrease in the capacitance of the IDT, the opening length of the IDT of the series arm SAW resonator needs to be expanded to 200 μm. This is an enlargement of the area of 35751 μm ² compared to the SAW resonator (opening length = 133 μm) according to the present invention. Since four SAW resonators are used for the series arm, the area expansion of the entire SAW filter is 143005 μm ² . Therefore, it is difficult to reduce the size of the conventional SAW resonator, and the SAW filter of the present invention can be reduced in size as compared with the method of thinning more toward the outside of the conventional IDT.

FIG. 19 shows a broadband SAW filter according to the first embodiment of the present invention and a filter using a conventional periodically thinned resonator (R = 12.5%) as shown in FIG. The comparison figure of the frequency characteristic measured by is shown.
In the conventional filter using a periodically thinned resonator (broken line), a plurality of spurs are generated outside the pass band, but in the SAW filter (solid line) of the present invention, this spurious is suppressed. I understand. Therefore, the SAW filter of the present invention can realize out-of-band attenuation characteristics better than the conventional periodic thinning method.

<Second embodiment>
A ladder-type SAW filter using the SAW resonator shown in FIG. 7 of the present invention will be described only for the parallel arm SAW resonator.
Here, a ladder-type SAW filter in which four SAW resonators are formed in series and two in parallel on a 42 ° Y cut-X propagation LiTaO ₃ substrate is used.

The SAW resonators connected in series are configured as shown in FIG. 2, IDT period pi = 4.60 μm, IDT aperture length = 100 μm, IDT logarithm 116 pairs, reflector period pr = 2.30 μm, reflector The number of electrodes is assumed to be 160.
The SAW resonators connected in parallel are configured as shown in FIG. 7, IDT period pi = 4.76 μm, IDT aperture length = 160 μm, IDT logarithm 78 pairs, reflector period pr = 2.38 μm, reflector The number of electrodes is 120, and the electrode thinning rate R of IDT is 12.5%.
The number N of electrodes existing between the pseudo thinning electrodes is set so as to satisfy 1 ≦ N ≦ 53.2. Here, Ra = 156 and Rm = 19.

  FIG. 20 shows a SAW filter according to a second embodiment of the present invention, a filter using a conventional periodically thinned resonator (R = 12.5%) as shown in FIG. The comparison figure of the frequency characteristic of the SAW filter (FIG. 1) comprised only with the conventional resonators is shown. A thin solid line is a SAW filter of the present invention, a broken line is a filter using a periodic thinning resonator, and a thick solid line is a characteristic diagram of a conventional SAW filter (FIG. 1).

  According to FIG. 20, both the thin solid line SAW filter of the present invention and the filter using the broken line periodic thinning resonator have a small frequency difference Δf between the resonance point and the antiresonance point of the parallel arm SAW resonator. Therefore, it can be seen that the rising from the low frequency side attenuation region to the pass region becomes steep and the squareness ratio is improved.

In order to obtain a square characteristic similar to that of the present invention by thinning out the outside of the conventional IDT, it is necessary to thin out the electrodes of the parallel arm SAW resonator by about 25%. At this time, in order to correct the decrease in the capacitance of the IDT, the IDT opening length of the parallel arm SAW resonator needs to be expanded to 240 μm. This is an enlargement of 29702 μm ² compared to the SAW resonator according to the present invention (aperture length = 160 μm). Since two SAW resonators are used for the parallel arm, the area expansion of the entire SAW filter is 59404 μm ² . Therefore, it is difficult to reduce the size of the conventional SAW resonator, and the SAW filter of the present invention can be reduced in size as compared with the method of thinning more toward the outside of the conventional IDT.

  FIG. 21 shows a wide band measurement of a SAW filter according to the second embodiment of the present invention and a filter using a conventional periodically thinned resonator (R = 12.5%) as shown in FIG. 9 as a parallel arm. A comparison diagram of the frequency characteristics is shown. In the conventional filter using a periodically thinned resonator (broken line), a plurality of spurs are generated outside the pass band, but in the SAW filter (solid line) of the present invention, this spurious is suppressed. I understand. Therefore, the SAW filter of the present invention can realize out-of-band attenuation characteristics better than the conventional periodic thinning method.

It is a block diagram of the conventional ladder type SAW filter. It is a block diagram of the conventional one terminal pair SAW resonator. It is a pass characteristic figure of the conventional ladder type SAW filter. It is explanatory drawing of the frequency characteristic calculated | required by a band pass filter. It is an impedance characteristic figure of the conventional SAW resonator. It is a graph of the change of frequency difference (DELTA) f with respect to a normalization conversion coefficient. It is a block diagram of one Example of one terminal pair SAW resonator of this invention. It is an impedance characteristic view of the SAW resonator of the present invention. FIG. 6 is a configuration diagram of a conventional SAW resonator in which a pseudo thinning electrode is periodically provided. It is the impedance characteristic figure of the conventional and SAW resonator of this invention. It is a comparison figure of the passage characteristic of the conventional and SAW resonator of this invention. It is a block diagram of a conventional SAW resonator in which a larger number of pseudo thinning electrodes are provided on the outer side. It is a comparison figure of the passage characteristic of the conventional and SAW resonator of this invention. It is a comparison figure of the impedance characteristic of the SAW resonator of the past and this invention. It is a block diagram of one Example of the SAW resonator of this invention. In the SAW resonator of this invention, it is a block diagram for description of an average normalization excitation intensity | strength. It is a block diagram of one Example of the ladder type SAW filter using the SAW resonator of this invention. It is a comparison figure of the frequency characteristic of the SAW filter of the past and the 1st example of this invention. It is a comparison figure of the frequency characteristic of the SAW filter of the past and the 1st example of this invention. It is a comparison figure of the frequency characteristic of the SAW filter of the past and the 2nd example of this invention. It is a comparison figure of the frequency characteristic of the SAW filter of the past and the 2nd example of this invention.

Explanation of symbols

1 IDT
2 Reflector S1 Series Arm SAW Resonator S2 Series Arm SAW Resonator P1 Parallel Arm SAW Resonator P2 Parallel Arm SAW Resonators I1 to I4 Pseudo Thinned-out Electrode A Divided Area B Divided Area C Divided Area

Claims (8)

Consisting of at least one interdigital transducer formed on a piezoelectric substrate,
The at least one interdigital transducer is composed of a predetermined number of comb electrodes, and at least three electrodes of the comb electrodes are thinned, and the positions of the thinned electrodes are aperiodic;
When a predetermined number of comb-shaped electrodes constituting the interdigital transducer are viewed in a direction parallel to the traveling direction of the excited surface acoustic wave, the difference between the maximum value and the minimum value of the number of electrodes in each divided region is 1. Comb electrodes so that the difference between the maximum value and the minimum value of the average normalized excitation intensity of the surface acoustic wave in each divided region is 0.30 or less when the region is divided into three or more so as to be less than this The surface acoustic wave resonator is characterized in that the position where the electrodes are thinned and the electrode is thinned is not mirror-symmetrical about the center line of the interdigital transducer perpendicular to the traveling direction of the surface acoustic wave. 2. The surface acoustic wave resonator according to claim 1, wherein the difference between the maximum value and the minimum value of the number of electrodes thinned out in each divided region is two or less. Consisting of at least one interdigital transducer formed on a piezoelectric substrate,
The at least one interdigital transducer is composed of a predetermined number of comb electrodes, and at least three electrodes of the comb electrodes are thinned, and the positions of the thinned electrodes are aperiodic;
When a predetermined number of comb-shaped electrodes constituting the interdigital transducer are viewed in a direction parallel to the traveling direction of the excited surface acoustic wave, the difference between the maximum value and the minimum value of the number of electrodes in each divided region is 1. When the area is divided into three areas or less, one or more thinned electrodes exist in the central area, and the difference between the maximum value and the minimum value of the number of electrodes in each divided area is one or less. when area division as three or more divisions becomes, Ri difference two der less between the maximum value and the minimum value of the decimated number of electrodes in each divided region, thinned out electrode position, a surface acoustic A surface acoustic wave resonator characterized by not being mirror-symmetric with respect to the center line of an interdigital transducer perpendicular to the wave traveling direction . 2. The surface acoustic wave resonator according to claim 1, wherein the number of thinned electrodes is 2% to 22% of the total number of electrodes when not thinned. When the total number of comb-shaped electrodes when not thinned is Ra and the number of thinned electrodes is Rm, the number N of electrodes existing between the thinned electrodes is 1 ≦ N ≦ 0.4 × (Ra The surface acoustic wave resonator according to claim 1, wherein the surface acoustic wave resonator is −Rm−4). 2. The surface acoustic wave resonator according to claim 1, wherein a quasi-thinned electrode that does not contribute to excitation of the surface acoustic wave is formed at the position of the thinned electrode. The surface acoustic wave resonator according to claim 1, wherein reflectors are arranged close to each other on both sides of the interdigital transducer in a direction parallel to a traveling direction of the excited surface acoustic wave. A piezoelectric substrate and a plurality of surface acoustic wave resonators formed on the piezoelectric substrate and electrically connected in a ladder shape, wherein at least one of the surface acoustic wave resonators is defined in claims 1 to 7. A surface acoustic wave filter comprising the surface acoustic wave resonator according to claim 1.

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