Surface acoustic wave filter

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to an elastic surface wave filter, and more particularly to a ladder type elastic surface wave filter that can be applied to an RF (radio frequency portion) filter of a small mobile wireless device such as a car phone or a mobile phone.

【0002】

One example of the current specifications for automobiles and mobile phones in Japan is the transmission band in the range of ± 8.5MHz, centered on 933.5MHz. In terms of specific bandwidth, it is about 2%.

【0003】

The surface acoustic wave filter needs to have characteristics that satisfy the above specifications. Specifically, the passband width is as wide as 2% or more in terms of the specific bandwidth, and the loss is as low as 1.5 to 2 dB or less. In other words, it is necessary that the degree of suppression is as high as 20 dB to 30 dB or more.

【0004】

In order to satisfy this requirement, the surface acoustic wave filter uses an elastic surface wave element as a resonator instead of the conventional transversal type, and a resonator type in which this is configured as a ladder type is desired.

【0005】

[Conventional technology]

FIG. 70 shows an equivalent circuit of the surface acoustic wave filter 1 described in Japanese Patent Application Laid-Open No. 52-19044. In this filter 1, the surface acoustic wave resonator 3 is arranged on the series arm 2, the surface acoustic wave resonator 5 is arranged on the parallel arm 4, and the equivalent parallel capacitance C of the resonator 5 of the parallel arm 4 is arranged._OBThe series arm 2 resonator 3 equivalent parallel capacitance C_OAIt is a larger configuration. This filter 1 has the characteristics shown by line 6 in FIG.

【0006】

[Problems to be Solved by the Invention]

In the above filter 1, the equivalent parallel capacitance C will be described later._OBWhen is large, the degree of suppression can be increased as shown by arrow 7. But this capacity C_OBAs shown by arrow 8, the passband becomes narrower and the loss increases as shown by arrow 9, and the characteristics become as shown by line 10.

【0007】

If the degree of suppression is set to 20 dB or more, the pass bandwidth will be 1% or less in terms of the specific bandwidth, and the above specifications of the automobile mobile phone cannot be satisfied.

【0008】

Therefore, an object of the present invention is to provide a surface acoustic wave filter capable of simultaneously achieving an increase in the pass band and an improvement in the degree of suppression outside the pass band.

【0009】

[Means for solving problems]

The invention according to claim 1 has a bandpass characteristic in which a plurality of parallel arms and series arms composed of surface acoustic wave resonators are connected in a ladder shape and formed on one piezoelectric substrate. It is a surface acoustic wave filter characterized in that an inductance is connected in series to each of a plurality of parallel arms. Since a plurality of parallel arms and series arms are formed on one piezoelectric substrate, the resonance frequency of the surface acoustic wave resonator can be accurately controlled, and the passband width can be expanded and the suppression degree outside the passband can be improved at the same time. Can be achieved.

【0010】

The invention according to claim 2 is a surface acoustic wave filter according to claim 1, wherein each surface acoustic wave resonator has a propagation path that does not overlap with each other.

【0011】

【0012】

BEST MODE FOR CARRYING OUT THE INVENTION

First, one of the basic principles of the present invention will be described. FIG. 1 shows the principle configuration of the surface acoustic wave filter 20 that can be used in the present invention. Reference numeral 21 denotes a first one-terminal surface acoustic wave resonator, which has a predetermined resonance frequency frp and is arranged on the parallel arm 22. Reference numeral 23 denotes a second one-terminal surface acoustic wave resonator, which has a resonance frequency frp that substantially matches the antiresonance frequency flap of the first resonator 21, and is arranged on the series arm 24. Reference numeral 25 is an inductance, which is added in series with the first resonator 21 and arranged on the parallel arm 22.

【0013】

The principle that a circuit having a one-terminal pair acoustic wave resonator on a series arm and a parallel arm has filter characteristics is as follows. This principle is also necessary for explaining the principle of the present invention, and will be described in detail here.

【0014】

To evaluate whether or not the resonant circuit exhibits filter characteristics, the method using image parameters is easy to understand. This method is described in detail in "Theory and Design of Filters" by Yanagisawa et al. (Sanpo Publishing: Electronics Selection, 1974).

【0015】

The principle will be described below based on this.

【0016】

Figure 2 shows a basic ladder circuit showing filter characteristics. In the figure, the shaded black boxes are surface acoustic wave resonators 30 and 31.

【0017】

Now, for the sake of brevity, we assume that the surface acoustic wave resonator is a reactance circuit with no resistance, the impedance of the resonator 30 of the series arm is Z = jx, and the admittance of the resonator 31 of the parallel arm is Y. Let = jb.

【0018】

According to the image parameter method, the input side voltage and current are V respectively.₁, I₁, Output side V₂, I₂Then (see Figure 2), [0019]

[Number 1]

<img file="JPP3429751B2_D0001.tif" />The image transmission amount γ (complex number) defined in is important. That is, [0020]

[Number 2]

<img file="JPP3429751B2_D0002.tif" />In the equation of, if the value represented by this equation is an imaginary number, the entire two-terminal pair circuit of FIG. 2 shows a passing characteristic, and if it is a real number, it shows an attenuation characteristic. Here, the symbols A, B, C, and D are four-terminal constants when the entire circuit in FIG. 2 is represented by the F matrix, and each of them is represented by the above-mentioned x and b as follows.

【0021】

A = 1 B = jx C = jb D = 1-bx ... (3) Therefore, Eq. (2) becomes the following Eq.

【0022】

[Number 3]

<img file="JPP3429751B2_D0003.tif" />From Eq. (4), when 0 &lt;bx &lt;1, that is, when b and x have the same sign and small values, the entire circuit in Fig. 2 shows passage characteristics, and when bx &lt;0 or bx&gt; 1, that is, b and x. It can be seen that when is different sign or the bx product is a large value, the attenuation characteristic is exhibited.

【0023】

Here, in order to further qualitatively know the frequency characteristics of b and x, the frequency characteristics of the impedance and admittance of the surface acoustic wave resonator are investigated.

【0024】

The one-terminal surface acoustic wave resonator consists of a comb-shaped electrode 40 as shown in Fig. 3 (A) (described on pages 76 to 98 of the November 29, 1976 issue of Nikkei Electronics).

【0025】

41 is the electrode pair, 42 is the opening length (intersection width), and 43 is the comb-shaped electrode period. This comb-shaped electrode is generally represented by an equivalent circuit 45 as shown in FIG. 3 (B), ignoring the resistance component. C here_OIs the capacitance of the comb electrode, C₁, L₁Is an equivalent constant.

【0026】

This equivalent circuit 45 is hereinafter represented by the symbol 46 shown in FIG. 3 (C).

【0027】

Fig. 4 (A) and (B) qualitatively show the frequency dependence of impedance and admittance when the comb-shaped electrodes are represented by the equivalent circuit as shown in Fig. 3 (b).

【0028】

The characteristics in the figure are double resonance characteristics with two resonance frequencies fr and fa, similar to a quartz resonator. Here, fr is called a resonance frequency and fa is called an antiresonance frequency. When resonators having such double resonance characteristics are arranged on the series arm and the parallel arm, respectively, and the antiresonance frequency fap of the parallel arm is substantially matched with the resonance frequency frs of the series arm, a band having that as the center frequency is obtained. A circuit showing pass-type filter characteristics can be configured. The reason is that, as shown in the figure of the frequency characteristic of imitation in FIG. 5 (A), 0 &lt;bx &lt;1 is satisfied in the vicinity of the center frequency where fap frs, and the passing region is obtained from the above condition, and the center. This is because bx&gt; 1 in the frequency region slightly distant from the frequency and bx &lt;0 in the region far away from the frequency, both of which are in the attenuation region.

【0029】

Therefore, the surface acoustic wave filter 1 having the configuration shown in FIG. 1 qualitatively has the filter characteristics shown by the center line 47 in FIG. 5 (B).

【0030】

[Passband determinant] Next, the bandwidth determinant in such a resonator type surface acoustic wave filter will be considered.

【0031】

As can be seen from FIG. 5, the bandwidth is mainly determined by the difference between the resonance frequency fr and the antiresonance frequency fa in each resonator. If this difference is large, the bandwidth is wide and the bandwidth is wide, and if this difference is small, the bandwidth is narrow. Here, fr and fa can be determined from the following equation using the equivalent circuit constants in Fig. 3 (B).

【0032】

[Number 4]

<img file="JPP3429751B2_D0004.tif" />Specific bandwidth (Δf / f₀Since) is mainly determined by the difference between fr and fa, it is expressed as the following equation using equations (6) and (7).

【0033】

Δf / f₀ = 2 (fa-fr) / (fa + fr) 2 / (4γ + 1) ... (8) As is clear from the above equation, γ (capacity ratio) is an important factor that determines the specific bandwidth. However, this value is almost determined by the type of the substrate material on which the comb-shaped electrode is formed, as described in Japanese Patent Application Laid-Open No. 52-19044. For example, in ST-cut quartz with a small electromechanical coupling coefficient of the material, γ is 1300 or more, while 36 ° Ycut-x propagation LiTaO with a large electromechanical coupling coefficient.₃Then, γ is the value at the 15th place. The specific bandwidth is 0.04% for ST-cut quartz and 36 ° Ycut-X propagation LiTaO from equation (8).₃Then it becomes 3.3%. Therefore, once the substrate material is determined, the bandwidth is almost determined.

【0034】

Then, in order to increase the degree of out-of-band suppression, as described in Japanese Patent Application Laid-Open No. 52-19044, the equivalent parallel capacitance C_OBIf is set to large, the bandwidth will become narrower and narrower.

【0035】

This will be explained in detail with reference to FIG. As is clear from the above explanation of the principle, if the admittance value is increased while the fr and fa of the parallel resonator are fixed (to increase the admittance value, the opening length of the comb-shaped electrode or the opening length of the comb-shaped electrode is kept constant while γ is kept constant. Increase the logarithm and capacitance C₀As shown in Fig. 6 (A), the bx product increases negatively outside the band, so the amount of attenuation increases and the characteristics improve, but the bx product increases positively near the center frequency. The area of bx&gt; 1 is expanded, and as a result, the passing area of 0 &lt;bx &lt;1 is narrowed and sufficient bandwidth cannot be obtained. This situation is represented by the arrow in Fig. 6 (B).

【0036】

[Improvement of passband width] As one means to solve the above points, widen the difference between fr and fa of either the resonator of the series arm or the resonator of the parallel arm, at least one of them, and its It is necessary to satisfy the two conditions of increasing the impedance value or admittance value. The reason for increasing the impedance value and admittance value is to increase the out-of-band attenuation amount. If this can be achieved, the out-of-band attenuation can be improved without widening or narrowing the passband.

【0037】

First, as a method of widening the difference between fr and fa of the resonator, which is the condition of, it is effective to add an inductance L in series with the one-terminal vs. surface acoustic wave resonator. Figures 7 (A) and 7 (B) show the frequency changes in impedance and admittance when 8nH is connected in series with a surface acoustic wave resonator as L. The constants of the equivalent circuit of the surface acoustic wave resonator used in the calculation are shown in the figure.

【0038】

In FIG. 7 (A), line 50 shows the impedance characteristics before adding L. Line 51 shows the impedance characteristics after adding L.

【0039】

Figure 7 (B), line 52 shows the admittance characteristics before adding L. Line 53 shows the admittance property after L is added.

【0040】

From Fig. 7 (A), it can be seen that the distance between fr and fa is widened by adding L. In this case, it was expanded by about 30MHz. The reason for this is that, as is clear from the frequency characteristics of the impedance in Fig. (A), when L is added in series, the impedance of the original resonator is raised to the + side by ωL, and as a result, fr becomes fr'. Because it changed to. At this time, fa hardly moves. Admittance, which is the reciprocal of impedance, also changes as shown in Fig. (A) for the same reason. In this case as well, it can be clearly seen that fr has changed to fr'.

【0041】

As for the next condition, the admittance value is increased by adding L as is clear from Fig. 7 (B). However, as shown in Fig. 7 (A), the impedance value is rather small outside the band. Therefore, when this method is applied to the resonance circuit of the series arm, a method of increasing the impedance value is further required. This can be solved by connecting a plurality of the same surface acoustic wave resonators in series.

【0042】

In FIG. 8, line 55 shows the impedance characteristics of one resonator. Line 56 shows the impedance characteristics of the resonant portion when n resonators are connected in series.

【0043】

As shown in FIG. 8, by connecting n resonators, the impedance value of the resonator portion becomes n times. On the other hand, regarding the difference between fa and fr, the expansion of the resonance frequency when L is connected is fr , which is slightly narrower than fr'in the case of one resonator, but fa and fr are smaller than when L is not connected. The difference between fa and fr can be increased by increasing the value of L if necessary.

【0044】

As a second means of expanding the passband, instead of making the anti-resonance frequency fap of the parallel arm resonator and the series arm resonance frequency frs approximately match as shown in FIG. 44, a method of making frs&gt; fap is conceivable. Be done.

【0045】

However, when frs&gt; fap, as shown in FIG. 44, bx &lt;0 near the center frequency, the above-mentioned passing region condition is not satisfied, and there is a risk that loss and ripple will increase.

【0046】

However, by controlling the magnitude of Δf by setting frs-fap = Δf, it is possible to substantially prevent an increase in loss and an increase in ripple and realize an expansion of the pass band. In addition, by expanding Δf, it is possible to improve the degree of out-of-band suppression at the same time. Details will be described later in Example 11.

【0047】

Hereinafter, the contents of the present invention will be described with reference to specific examples. Most of the examples were performed by simulation. Therefore, first, the simulation used in the present invention will be briefly described, and in order to prove the validity of the simulation, a comparison with an experiment will be shown.

【0048】

The equivalent circuit shown in Fig. 3 (B) can simply simulate the characteristics of a one-terminal vs. surface acoustic wave resonator, but changes in the logarithm, aperture length, electrode film thickness, etc. of the comb-shaped electrodes that make up the resonator, and the reflector. It is difficult to accurately simulate the effects of. Therefore, based on the Smith equivalent circuit already developed by the inventors, a method of expressing this by a transfer matrix was used and applied to a resonator (O.Ikata et al .: 1990 ULTRASONIC SYMPOSIUM Proceedings, vol.1, pp83). See -86, (1990)., Which is referred to as Reference (1)).

【0049】

Figure 9 (A) shows the simulation results when a one-terminal pair acoustic wave resonator is placed on the parallel arm.

【0050】

In Fig. 9 (B), a one-terminal surface acoustic wave resonator consisting of a comb-shaped electrode made of Al-2% Cu material and a thickness of 1600 Å is placed on the parallel arm, and a 3 mm long bond is placed on this resonator. The result of the experiment when the wire (L = 1.5nH) is connected is shown.

【0051】

Comparing FIGS. 9 (A) and 9 (B), the resonance point due to the change in aperture length (fr in the figure).₁, fr₂, fr₃It can be seen that the experimental values and the calculated values are in good agreement with each other for the movement of (shown in) and the amount of attenuation near the resonance point.

【0052】

FIG. 10 (A) shows the result of the simulation when the resonator is arranged on the series arm. Since the bonding pad used in the experiment described later was rather large, the simulation considers a 0.5pF capacitor as its stray capacitance.

【0053】

FIG. 10B shows the results of an experiment when a resonator is connected to the series arm.

【0054】

To compare FIGS. 10 (A) and 10 (B), the antiresonance frequency fa₁, fa₂, fa₃It can be seen that the point that does not depend on the aperture length and the change in the amount of attenuation near the antiresonance frequency are in good agreement with the experiment.

【0055】

Therefore, it is clear that the filter characteristics when these are combined are in good agreement with the experiment, and the following examples are performed by simulation.

【0056】

[Example 1] FIG. 11 shows a surface acoustic wave filter 60 according to a first embodiment of the present invention.

【0057】

Currently, to give one example in the specifications of domestic automobiles and mobile phones, the center frequency is 933.5MHz, the range of ± 8.5MHz is the transmission band of mobile devices, and 878.5MHz, which is -55MHz away from it, is the center. There is a specification that the reception band is in the range of ± 8.5MHz as the frequency.

【0058】

This embodiment is designed to be suitable for the transmission side filter of the above-mentioned mobile device. The same applies to other examples described later.

【0059】

One terminal vs. surface acoustic wave resonator R on series arm 61₂And R₄Is arranged.

【0060】

One terminal vs. surface acoustic wave resonator R for parallel arms 62, 63, 64 respectively₁, R₃, R₅Is arranged. L₁, L₂, L₃Is the inductance, each resonator R₁, R₃, R₅It is connected to and arranged on parallel arms 62, 63, 64. Resonator R₁~ R₅Has a comb-shaped electrode structure shown in FIG. 3 (A). The logarithm is 100 and the opening length is 80 μm. The material is Al-2% Cu and the film thickness is 3,000 angstroms.

【0061】

In addition, the period of the comb-shaped electrode is appropriately determined, and each resonator R in the parallel arms 62, 63, 64₁, R₃, R₅The resonance frequency of is 912 MHz, and the anti-resonance frequency is 934 MHz. Each resonator R in the series arm 61₂, R₄The resonance frequency of is 934MHz, and the antiresonance frequency is 962MHz. Inductance L₁, L₂, L₃Are both 4nH.

【0062】

The surface acoustic wave filter 60 having the above configuration has the passing characteristics shown by line 65 in FIG. When the inductances L are 2nH and 6nH, the passing characteristics of the filter 60 in FIG. 11 are as shown by lines 66 and 67 in FIG. 12, respectively.

【0063】

The L dependence on the passband based on FIG. 12 is shown by line 70 in FIG. 13 (A). Here, the frequency width at the level of the attenuation amount -3 dB lower than the minimum insertion loss was defined as the pass bandwidth.

【0064】

Similarly, based on FIG. 12, the L dependence on the degree of normal out-of-band suppression is shown by line 71 in FIG. 13 (B).

【0065】

As can be seen from FIG. 12, if L is made too large, the suppression region on the 55 MHz low frequency side from the center frequency cannot be sufficiently taken. Therefore, L is set to 4nH as described above. The value of L is appropriately selected according to the specifications of the filter.

【0066】

The passing characteristics of the filter 1 having the conventional configuration shown in FIG. 70 are as shown by the middle line 68 in FIG.

【0067】

In FIG. 12, when the pass characteristic (line 65) of the filter 60 of the present embodiment is compared with the pass characteristic (line 68) of the conventional filter, the filter 60 of the present embodiment has an arrow 75 as compared with the conventional filter. It can be seen that the passband width is wide as shown by arrow 76, the degree of suppression outside the passband is high as shown by arrow 76, and the loss is low as shown by arrow 77.

【0068】

14 and 15 show a surface acoustic wave filter device 80 that realizes the surface acoustic wave filter 60 of FIG.

【0069】

81 is a ceramic package, 82 is a filter tip, and 83 is a lid that functions as a ground. The ceramic package 81 is made of alumina ceramic and measures 5.5 x 4 mm.²The height of is as small as 1.5 mm. This ceramic package 81 has an Au electrode terminal 84_-1~84_-6Is formed. Filter chip 82 is LiTaO₃Made of 2 x 1.5 mm in size²The thickness of is 0.5 mm.

【0070】

Resonator R having a comb-shaped electrode structure with a logarithm of 100, an aperture length of 80 μm, a material of Al-2% Cu, and a film thickness of 3,000 Å on the surface of this filter chip 82.₁~ R₅However, they are staggered so as not to share the propagation path of surface acoustic waves with each other.

【0071】

Also, on the surface of the filter chip 82, there are two signal line terminals 85 as bonding terminals._-1,85_-2And three ground terminals 85_-3,85_-4,85_-5Is formed. 86_-1~86_-5Is a bonding wire, made of Al or Au, with a diameter of 25 μmφ, each terminal 84_-1~84_-5And terminal 85_-1~85_-5It is bonded to and connected to. Of these, wire 86_-1,86_-2Each constitutes a part 61a and 61b of the series arm 61 in FIG. Wire 86_-3Is the ground electrode terminal 84_-3And 85_-3Connected to and wire 86_-4Is another ground electrode terminal 84_-4And 85_-4Connected to and wire 86_-5Is another ground electrode terminal 84_-5And 85_-5It is connected between. This wire 86_-3~86_-5Both have a long length of 2.0 mm.

【0072】

Thus, thin and long wires have an inductance component according to the theory of high frequencies.

【0073】

According to the theoretical formula of the aerial ribbon inductor (Kuraishi: Science and Engineering Course, "Example Circumferential Microwave Circuit", described on page 199 of Tokyo Denki University Press), the above wire 86_-3,86_-4,86_-5The inductance of is about 1nH.

【0074】

This was not enough to obtain an inductance of 4nH, and the ceramic package and L on the filter chip as shown in FIGS. 40 and 41 described later were used.

【0075】

In this way, the inductance L in FIG.₁, L₂, L₃To configure.

【0076】

[Example 2] FIG. 16 shows a surface acoustic wave filter 90 according to a second embodiment of the present invention.

【0077】

In the figure, the same reference numerals are given to the parts corresponding to the components shown in FIG.

【0078】

Resonator R in series arm 61₂Aperture length A_SIs 80 μm.

【0079】

The parallel arm 62 has a resonator R1A and an inductance L.₁Are connected in series and arranged.

【0080】

Resonator R1A has an aperture length of A_PIs 120 μm. Aperture length A_PIs the opening length A_SLonger, opening length A_SIt is 1.5 times that of. Resonator R₂And the logarithm N of R1A_P, N_SAre both 100 and equal.

【0081】

This filter 90 has the passing characteristics shown by line 91 in FIG.

【0082】

Comparing this pass characteristic with the pass characteristic of the filter 60 of FIG. 11 shown by line 65, it can be seen that the degree of suppression outside the pass band is improved as shown by arrow 92 without changing the pass bandwidth.

【0083】

FIG. 18 shows the aperture length dependence of the pass characteristics of the filter of the configuration of FIG.

【0084】

FIG. 6A shows the opening lengths (A) of the respective series arm resonators when L is added as shown in FIG. 16 and when L is not added as shown in FIG. 42._S) With respect to the opening length of the parallel arm resonator (A)_P) Ratio A_P/ A_SAnd the relationship of the degree of out-of-band suppression. The out-of-band suppression degree is as shown by line 92 when an inductance L of 4 nH is added, and is as shown by line 93 when an inductance L is not added.

【0085】

In addition, Fig. 18 (B) shows A._P/ A_SAnd the passband bandwidth are shown. The pass bandwidth is as shown by line 95 when the inductance L of 4 nH is added, and is as shown by line 96 when the inductance L is not added.

【0086】

The following can be seen from FIGS. 18 (A) and 18 (B).

【0087】

Aperture length A of resonator R1A in parallel arm 62_PThe series arm 61 in the resonator R₂Aperture length A_SThe longer the length, the greater the degree of out-of-band suppression.

【0088】

Inductance L on parallel arm 62₁By adding, the opening length A of the resonator R1A is compared with the case where there is no inductance._PThe effect of the increase is large, and there is almost no deterioration in bandwidth.

【0089】

From the above, it can be seen that the filter 90 of the above embodiment has a pass characteristic in which the passband suppression degree is increased without narrowing the passband width as compared with the filter 60 of FIG.

【0090】

[Example 3] FIG. 19 shows a surface acoustic wave filter 100 according to a third embodiment of the present invention.

【0091】

In the figure, the parts corresponding to the components shown in FIGS. 11 and 16 are designated by the same reference numerals.

【0092】

Resonator R of series arm 61₂Logarithm N_SIs 100. The parallel arm 62 has a resonator R1B and an inductance L.₁Are connected in series and arranged. Resonator R1B has a logarithmic N_PIs 150 and the above resonator R₂Logarithm N_SMore than that, 1.5 times that. Resonator R₂And R1A opening length A_S, A_PAre both 80 μm and are equal.

【0093】

This filter 100 has the passing characteristics shown by line 101 in FIG. Comparing this pass characteristic with the pass characteristic of the filter 60 of FIG. 11 shown by line 65, it can be seen that the degree of suppression outside the pass band is improved as shown by arrow 102 without narrowing the pass bandwidth.

【0094】

Further, it can be seen that the loss deterioration is small as compared with the passing characteristics of the filter 90 of FIG. 16 shown by the middle line 91 of FIG.

【0095】

Therefore, the filter 100 of the present embodiment has an increased degree of suppression outside the passband without narrowing the passband width as compared with the filter 11 of FIG. 11, and has less loss deterioration than the filter 90 of FIG. Has pass characteristics.

【0096】

[Example 4] FIG. 21 shows a surface acoustic wave filter 110 according to a fourth embodiment of the present invention. In this embodiment, the anti-resonance frequency f of the resonance circuit of the series arm_aAnd resonance frequency f_rThe passage characteristics are improved by widening the difference with.

【0097】

In the figure, the parts corresponding to the components shown in FIG. 11 are designated by the same reference numerals. The same resonator R in the part between the parallel arms 62 and 63 of the series arms 61₂Are connected in series, and an inductance L of 3nH is connected in series with this._SIs added. Similarly, in the series arm 61, in the part between the parallel arms 63 and 64, the same resonator R₄Are connected in series, and an inductance L of 3nH is connected in series with this._SIs added. Only one resonator R position is arranged on the parallel arm 62. Similarly, on the parallel arm 63, one resonator R₃Only are arranged. Similarly, the parallel arm 64 has one resonator R₄Only are arranged.

【0098】

This filter 110 has the passing characteristics shown by line 111 in FIG.

【0099】

Where the inductance L_SAnd one resonator R₂, R₄The effect of the addition of

【0100】

From the filter 110 in FIG. 21, the inductance L_SAnd one resonator R₂, R₄The passage characteristics when and are deleted are as shown by line 68 (see FIG. 12). The above inductance L_SWas added to increase the passband width as indicated by arrow 112 and the degree of out-of-band suppression as indicated by arrow 113. Looking at the passband in particular, the expansion to the high frequency side was particularly large, and the bandwidth of about 15 MHz expanded to the high frequency side. The passage characteristics are as shown by line 114. In this state, the degree of out-of-band suppression is not sufficient. So one resonator R₂, R₄Was added.

【0101】

This one resonator R₂, R₄With the addition of, the degree of out-of-band suppression was improved by about 5 dB, as shown by arrow 115, without narrowing the passband width, resulting in the pass characteristic shown by line 111. Comparing line 111 with line 68, the loss is also improved compared to line 68, as shown by arrow 116.

【0102】

The resonator R of the series arm 61₂, R₄May be three or more each.

【0103】

Further, as shown by the alternate long and short dash line in FIG. 21, inductance may be inserted into the parallel arms 62 to 64.

【0104】

[Example 5] FIG. 21 shows a surface acoustic wave filter 120 according to a fifth embodiment of the present invention.

【0105】

In the figure, the same parts as those shown in FIG. 11 are designated by the same reference numerals, and the description thereof will be omitted.

【0106】

Inductance L of parallel arm 62₁The inductance value of is 4nH. Inductance L of another parallel arm 63₂The inductance value of is 5.5 nH. Inductance L of yet another parallel arm 64₃The inductance value of is 7nH.

【0107】

In this way, the inductance L of each parallel arm 62 to 64₁~ L₃By making the inductance values of the filters 120 different, the filter 120 has the passing characteristics shown by the line 121 in FIG. 24.

【0108】

Here, Indantax L₁~ L₃Compare with the passing characteristics of the filter 60 in Fig. 11, where all the inductance values of are equal to 4nH.

【0109】

This filter 60 has the passage characteristics (see FIG. 12) shown by line 65 in FIG. 24.

【0110】

The pass characteristic of the filter 120 of this embodiment is higher than the pass characteristic of the filter 60 in that the degree of suppression outside the pass band is increased as shown by arrow 122 without narrowing the pass bandwidth. Looking at the frequency side lower than the pass band, the filter 60 had only one attenuation pole 123 near 902 MHz, while attenuation poles 124 and 125 were generated at two locations, 875 MHz and 892 MHz. As a result, the frequency band 126 between the two attenuation poles 124 and 125 becomes the blocking region 127.

【0111】

[Example 6] FIG. 25 shows a surface acoustic wave filter 130 according to a sixth embodiment of the present invention. In this embodiment, the loss is reduced.

【0112】

In the figure, the same reference numerals are given to the parts corresponding to the components shown in FIG. 11, and the description thereof will be omitted.

【0113】

As shown in FIG. 26, the first surface acoustic wave resonator R1B of the parallel arm 62 has an excitation electrode 131 and reflectors 132 and 133 on both sides thereof. The reflectors 132 and 133 have the following equation for the distance d between the centers of the excitation electrode 131 and the reflectors 132 and 133. d = (n + β) λ ... (Here, n is an appropriate integer, β is a real number of 1 or less, and λ is the period of the comb-shaped electrode corresponding to the resonance frequency), and it is arranged at the position when β = 0.4.

【0114】

The logarithm of the reflectors 132 and 133 is 50. The resonator R1B with a reflector is represented by a symbol with "*" added as shown in FIG. The resonators R3B and R5B of the other parallel arms 63 and 64 are also configured to have a reflector in the same manner as the above-mentioned resonator R1B.

【0115】

The filter 130 having the above configuration has the passing characteristics shown by the middle line 134 in FIG. 27. As shown by the arrow 135, the insertion loss in the normal band is reduced in this pass characteristic as compared with the pass characteristic of the filter 60 in FIG. 11 (shown by line 65).

【0116】

Where Ripple r_PIs generated by arranging the reflectors 132 and 133 on both sides of the excitation electrode 131 of the parallel arm as shown in FIG.

【0117】

Here, the reason why the arrangement positions of the reflectors 132 and 133 are determined as described above will be described.

【0118】

In the above equation, change β from 0 to 0.5 and ripple r_PThe effect on the width of is as shown by the center line 140 in FIG. In the figure, point 141 is the point where the ripple width is the smallest, and β at this time is 0.4. For this reason, β is set to 0.4.

【0119】

FIG. 29 shows a surface acoustic wave filter device 150 that realizes the filter 130 of FIG. 25. In the figure, the same reference numerals are given to the parts corresponding to the components shown in FIG. 14, and the description thereof will be omitted. 132,133,151,152,153,154 are reflectors, respectively.

【0120】

Next, a modified example of the first one-terminal pair surface acoustic wave resonator will be described.

【0121】

FIG. 30 shows one modification. This resonator R₁Ba has a configuration in which comb-shaped electrodes 160 and 161 having a short-circuit electrical load are arranged as reflectors on both sides of the excitation electrode 131.

【0122】

FIG. 31 shows another modified example. This resonator R₁B_{b b}Is a configuration in which strip array type electrodes 165 and 166 are arranged as reflectors on both sides of the excitation electrode 131.

【0123】

[Example 7] FIG. 32 shows a surface acoustic wave filter 170 according to a seventh embodiment of the present invention. In this embodiment, the loss is reduced as in the sixth embodiment, and the same reference numerals are given to the parts corresponding to the components shown in FIG. 21 in the drawings, and the description thereof will be omitted.

【0124】

The filter 170 has βs on both sides of the excitation electrode 131 as shown in FIG. 26, respectively, for the first surface acoustic wave resonators R1B, R3B, and R5B of the parallel arms 62, 63, 64 in the fital 110 of FIG. It is a configuration in which the reflectors 132 and 133 are arranged at the position determined by 0.4. According to this filter 170, a pass characteristic with less loss in the pass band and suppressed ripple can be obtained as compared with the characteristic shown by the middle line 111 in FIG.

【0125】

[Example 8] In this example, the ripple r in FIG. 27_PThe purpose is to remove.

【0126】

First, a means for effectively removing the ripple that appears when the reflector is added will be described.

【0127】

The inventors investigated the relationship between the frequency position where ripples appeared and the electrode film thickness by simulation. In the simulation, the effect of increasing the film thickness was replaced by increasing the ratio of the acoustic impedance under the electrode (Zm) to the acoustic impedance of the free surface (Zo). This is because, as described in Ref. (1), an increase in electrode film thickness is an increase in mass, which is considered to be directly proportional to an increase in the amount of discontinuity in acoustic impedance. Q = Zo / Zm = Vo / Vm = 1 + k²/ 2 + α (t) ... (9) (Vo, Vm: speed of sound on free surface and under electrodes, k²: Electromechanical coupling coefficient), and α (t) was changed as a parameter proportional to the film thickness t.

【0128】

When placed in this way, the center frequency fo of the filter is fo = 2fo'/ (1 + Q) ... (10) This is consistent with the well-known experimental fact that as the film thickness increases, the center frequency fo'moves to the lower frequency side when there is no discontinuity in acoustic impedance. As a result of the simulation, when α (t) is increased, that is, when the electrode film thickness is increased, the ripple r_PAs shown by the arrow 180 in FIG. 33, it was found that the frequency position where the above appears moves to the high frequency side of the pass band and finally falls into the attenuation pole on the high frequency side. This is schematically shown in FIG. 33.

【0129】

In addition, in FIG. 33, another ripple r_SIs caused by the reflector of the series arm resonator.

【0130】

FIG. 34 shows the passage characteristics when α (t) = 0.08 and the ripple generated from the reflector of the resonator of the parallel arm falls into the attenuation pole on the high frequency side. Therefore, in the figure, the ripple disappears from the pass band, and the insertion loss is considerably reduced. In this figure, the center of the pass band has moved to the low frequency side according to Eq. (10), so in order to correct this, the resonance frequencies of the resonators of the series arm and parallel arm are set so that the center frequency is 932 MHz. It shifts to the high frequency side by 15MHz.

【0131】

In order to see this in correspondence with the actual film thickness, we made a prototype chip and investigated its passage characteristics.

【0132】

Lines 185, 186, and 187 in FIGS. 35 (A), (B), and (C) show the passage characteristics when the film thickness is 2000 Å, 3000 Å, and 4000 Å, respectively.

【0133】

The center frequency changes by changing the film thickness, but in order to correct this, the period of the comb-shaped electrode is changed so that the center frequency does not fluctuate so much.

【0134】

As is clear from Fig. 35, the ripple r of the parallel arm resonator that appeared in the band at 2000 Å._P, And out-of-band series arm ripple r_SHowever, at 3000 Å, it moves to the high frequency side and r_P', r_S'And r_PAs a result of being buried in the attenuation pole on the high frequency side,'has good characteristics with no ripple in the band. This result is qualitatively in good agreement with the simulation result.

【0135】

However, when the film thickness is increased, loss deterioration due to bulk waves that cannot be calculated by simulation (Ebata et al .: "LiTaO"₃Surface Acoustic Wave Resonator on Substrate and Its Application to Oscillator for VTR , Journal of Electronics and Communication Engineers, vol.J66-C, No.1, pp23-30,1988) and loss improvement due to resistance loss. The balance is also an important factor.

【0136】

Therefore, Fig. 36 (A) plots the change in the minimum insertion loss when the film thickness is changed.

【0137】

In the figure, line 190 shows the loss due to the bulk wave, and line 191 shows the loss due to the resistance loss. Line 192 is the experimental value. As can be seen from the figure, the insertion loss is about 2500 Å, and the effects of both are balanced, and from about 3500 Å, the increase in loss due to bulk waves becomes dominant and begins to deteriorate.

【0138】

Line 193 in FIG. 36 (B) is a ripple r when the film thicknesses of the excitation electrode 131 and the reflectors 132 and 133 in FIG. 26 are changed._PPassband center frequency f at the frequency position of₀Shows the change for.

【0139】

Comprehensively judging FIGS. 36 (A) and 36 (B), a film thickness of 2600 Å to 4000 Å is appropriate because it does not form ripple in the band and there is little loss deterioration. This is determined by the electrode period λ of the parallel arm resonator, which is almost determined by the center frequency of the filter._PWhen standardized by (at 932 MHz, it is 4.4 μm, see Fig. 26), it becomes 0.06 to 0.09.

【0140】

This example is based on the above study results.

【0141】

FIG. 37 shows the first terminal vs. surface acoustic wave resonator 200 of the surface acoustic wave filter of the present invention.

【0142】

In the figure, 201 excitation electrodes 202 and 203 are reflectors, each made of Al or a mixture of Al mixed with several% dissimilar metals by weight, and have a film thickness of t.₁Is 0.06 to 0.09 times the thickness of the electrode period λp.

【0143】

The pass characteristics of the elastic surface wave filter in which this resonator 200 is applied to the resonators R1B, R3B, and R5B in FIGS. 25 and 32 are as shown by line 205 in FIG. 38, and ripple appears in the pass band. Not.

【0144】

When the product is made of the above Al alloy, the power resistance characteristics are improved as compared with the case where the product is made of Al. The dissimilar metals to be mixed are Cu, Ti and the like.

【0145】

FIG. 39 shows a resonator 210 which is a modification of the resonator. 211 is an excitation electrode and 212,213 are reflectors. These are made by Au.

【0146】

Since this phenomenon occurs due to the influence of the mass addition effect, the ratio to the density of the optimum film thickness value range Al becomes smaller than the above value. Since Al density / Au density = 2.7 / 18.9 = 0.143, the optimum film thickness t₂Is multiplied by 0.143 and the electrode period λ_PThe thickness is 0.0086 to 0.013 times that of.

【0147】

The pass characteristics of the elastic surface wave filter in which this resonator 210 is applied to the resonators R1B, R3B, and R5B in FIGS. 25 and 32 are also as shown in FIG. 38, and no ripple appears in the pass band.

【0148】

[Example 9] In this embodiment, the inductance L in FIG. 11₁, L₂, L₃Is another example of realizing.

【0149】

In FIG. 40, the same reference numerals are given to the parts corresponding to the components shown in FIG. 14, and the description thereof will be omitted.

【0150】

220 and 221 are zigzag microstrip lines, each with terminal 84_-3And 84_-5It is more extended and formed on the ceramic package 81. The tip of each microstrip line 220,221 is connected to ground. The pattern width of each microstrip line 220,221 is 100 μm, and the length between the microstrip line and ground is 0.5 mm. Assuming that the relative permittivity of the ceramic package 81 is 9, the inductance value of the above microstrip lines 220 and 221 is 2 nH from the theoretical formula of the ribbon inductor.

【0151】

[Example 10] In this example, the inductance L in FIG. 11₁, L₂, L₃Is yet another example of realizing.

【0152】

In FIG. 41, the same reference numerals are given to the parts corresponding to the components shown in FIG. 14, and the description thereof will be omitted.

【0153】

230 and 231 are zigzag microstrip lines, each with resonator R₁, R₂It is more extended and formed on the filter chip 82. Terminal 85 at the tip of each microstrip line 230,231_-3,85_-5Is formed. Each microstrip line 230,231 is 3000 Å thick, 60 μm wide and about 2 mm long. Filter tip (LiTaO₃Assuming that the relative permittivity of) 82 is 44, the inductance value of the microstrip lines 230 and 231 is 2.2 nH according to the theoretical formula.

【0154】

The inductance is the bonding wire 86._-3It can also be formed by appropriately combining the microstrip line 220 on the ceramic package 81 and the microstrip line 230 on the filter chip 82.

【0155】

[Example 11] FIG. 42 shows a surface acoustic wave filter 240 according to the eleventh embodiment of the present invention. FIG. 43 shows a configuration that embodies this.

【0156】

For convenience of explanation, first, an outline of this embodiment and a basic configuration of this embodiment will be described.

【0157】

Outline of this embodiment In this embodiment, the resonance frequency frs of the resonator of the series arm is appropriately set higher than the anti-resonance frequency fap of the resonator of the parallel arm to expand the passing bandwidth, and Δffrs-fap is set. The configuration is set within a range that does not significantly deteriorate the loss in the pass band.

【0158】

Basic configuration of this embodiment In each of the above examples, fap = frs is an indispensable condition for forming the pass band of the filter. However, as long as this condition is observed, there will be an upper limit on the pass band. Therefore, in order to increase the passband width, we considered setting fap &lt;frs as shown in Fig. 44.

【0159】

Then, as is clear from the figure, bx &lt;0 in the range of fap &lt;f &lt;frs, which may be an attenuation region from the above theory. However, in reality, if the magnitude of Δf (= frs-fap) is limited as described below, the value of the bx product remains a very small value, so no attenuation occurs and there is virtually no problem as a pass band. It turned out that there was no.

【0160】

FIG. 45 shows the passage characteristics of the ladder type filter when Δf = frs-fap is increased from zero.

【0161】

As an experimental condition, the piezoelectric substrate is LiTaO with an electromechanical coupling coefficient of 0.05.₃The Al electrode for the comb-shaped electrode used the condition of a film thickness of 3000 Å.

【0162】

The electrode configuration is based on a ladder-type connection of a parallel resonator and a series resonator as shown in FIG. 42, which are connected vertically in two stages, and in parallel to make the input side and output side symmetrical. The resonator is connected to the last stage. The reason why the ladder type circuit is multi-staged is to increase the out-of-band suppression degree to a practical value.

【0163】

However, since the loss in the pass band also increases due to the increase in the number of stages, the number of stages in the number of stages is adjusted according to the specific filter specifications. This example is given as one configuration example that realizes a loss of 2 dB or less and an out-of-band suppression degree of 20 dB or more. The design conditions for the comb-shaped electrode are that the resonators of the series-parallel arms have an opening length of 180 μm and a logarithm of 50 pairs. Since the logarithm and aperture length conditions of the series-parallel resonator are the same, the ratio P = Cp / Cs of each capacitance is 1.

【0164】

In FIG. 45, (A) is the case of Δf = 0, that is, the case of the above-described embodiment.

【0165】

Figure (B) shows the case of Δf = 10MHz. Compared with Fig. (A), the minimum insertion loss of the passband is hardly deteriorated, and the passband (the bandwidth that guarantees a loss of 2.5 dB or less) is improved from 22 MHz to 40 MHz. The bandwidth is improved more than the increase in Δf because the loss recovery on the low frequency side of the passband can be seen.

【0166】

In addition, the degree of out-of-band suppression is also improved. In FIGS. 45 (A) and 45 (B), the out-of-band suppression degree (shown in the figure) on the high frequency side is improved from 19 dB to 20 dB. As described above, the expansion of Δf is a technique that not only has an effect on the bandwidth expansion but also improves the out-of-band suppression at the same time.

【0167】

Increasing Δf in this way improves the characteristics, but the amount of increase is limited.

【0168】

FIG. 45 (C) is a passage characteristic diagram when Δf = 19 MHz. Loss deterioration begins to be seen on the slightly high frequency side in the pass band. In this case, it is about 2.5 dB. This also caused an increase in in-band ripple, which was about 1.0 dB of the ripple specification limit in this example. Further increases in Δf resulted in loss degradation and an increase in in-band ripple. Therefore, Δf = 19MHz is the limit when increasing Δf. Further, the out-of-band suppression degree at this time is about 21 dB as shown in FIG. 45 (C), which is an improvement of 2 dB as compared with the above-mentioned FIG. 45 (A).

【0169】

At this time, what kind of value the above-mentioned bx product is was investigated by taking the case of Δf = 19 MHz in FIG. 45 (C) as an example.

【0170】

First, the surface acoustic wave resonators that make up the parallel arms of FIG. 42 and the surface acoustic wave resonators that make up the series arms are individually manufactured, and in parallel with the circuit configuration shown in FIGS. 46 (A) and 46 (B). The arm resonator measured admittance, and the series arm resonator measured impedance. The measurement was performed using a network analyzer, and each S21 was measured. Then, the values were substituted into the equations shown in FIGS. 46 (A) and 46 (B) to obtain the impedance Zp and the admittance Yp.

【0171】

As a result, the frequency characteristics shown in FIG. 47 were obtained. This characteristic is the value of admittance, the value of only the imaginary part of impedance, that is, the value of b or x.

【0172】

When the value of the bx product is calculated from these, the frequency characteristics are as shown in Fig. 48.

【0173】

From the figure, it can be seen that the bx product is negative and takes a small value in the range of fap &lt;f &lt;frs.

【0174】

Maximum absolute value of bx product | bx_max| As described later [0175]

[Number 5]

<img file="JPP3429751B2_D0005.tif" />It was given at the time of, and was 0.06 in this example. That is | bx_maxWhen the | value is less than this value, it can be seen that both the deterioration of the insertion loss and the in-band ripple described above can be suppressed as small as 1 dB or less.

【0176】

If Δf&gt; 19MHz, | bx_max| The value also increases, and both loss deterioration and in-band ripple are 1 dB or more, which is not practical.

【0177】

Therefore | bx_max| The value serves as an index of the upper limit of characteristic deterioration and determines the permissible value of Δf.

【0178】

It will be further generalized and described in detail below.

【0179】

Fig. 49 shows the equivalent circuit diagram when the surface acoustic wave resonator is approximated by the LC double resonance circuit as in Fig. 3 and assembled in the ladder type filter as shown in Fig. 2.

【0180】

Assuming that the impedance of the surface acoustic wave resonator of the series arm is Zs and the admittance of the surface acoustic wave resonator of the parallel arm is Yp, [0181]

[Number 6]

<img file="JPP3429751B2_D0006.tif" />Will be. here, [0182]

[Number 7]

<img file="JPP3429751B2_D0007.tif" />Is.

【0183】

Finding the bx product from equations (11) and (12) [0184]

[Number 8]

<img file="JPP3429751B2_D0008.tif" />Will be.

【0185】

The angular frequency ω that gives the extremum to bx in equation (13) is obtained from (bx) / ω = 0, which is [0186]

[Number 9]

<img file="JPP3429751B2_D0009.tif" />Will be.

【0187】

The value obtained by substituting this into Eq. (13) is the maximum value of the absolute value of the bx product in the passband. If you ask for this [0188]

[Number 10]

<img file="JPP3429751B2_D0010.tif" />Will be.

【0189】

here, Δω = ωrs-ωap = 2π Δf ... (16) Is.

【0190】

Fig. 50 is obtained by plotting Eq. (15) with P = Cop / Cos as the parameter as the relationship between bxmax and Δf / frs.

【0191】

In the figure, if the condition that the permissible value of the product of bx obtained earlier is 0.06 or less is illustrated, it becomes a region as shown by a diagonal line.

【0192】

Therefore, the permissible value α of Δf / frs, which differs depending on P = Cop / Cos, can be determined, which is | bx in Eq. (15)._maxWith | = 0.06, the following equation is obtained.

【0193】

[Number 11]

<img file="JPP3429751B2_D0011.tif" />The capacitance ratio γ depends on the substrate material, and according to experiments, 36 ° Y-cut propagation LiTaO₃It was about 15.

【0194】

Therefore, Eq. (17) is [0195]

[Number 12]

<img file="JPP3429751B2_D0012.tif" />Will be.

【0196】

When P = 1, α = 0.02, and in the case of the example of FIG. 45 with frs = 948MHz, Δf = 19MHz, and it can be confirmed that Eq. (18) holds.

【0197】

The effect of increasing Δf is effective for a piezoelectric substrate material having a small capacitance ratio γ, that is, a substrate material having a large electromechanical coupling coefficient, and Eq. (17) was obtained for such a piezoelectric substrate material.

【0198】

γ value is electromechanical coupling coefficient k²Since it is proportional to the reciprocal of, LiTaO of 36 ° Y cut X propagation₃Gamma value and k²Other materials with high electromechanical coupling coefficient using a value of = 0.05 64 ° Y cut X propagation LiNbO₃(k²= 0.11) and 41 ° Y cut X propagation LiNbO₃(k²When the γ value of = 0.17) is calculated, the former is 6.8 and the latter is 4.4. In addition, these k²For the value of, refer to the literature (Applications for Piezoelectric Leaky Surface Wave: K. Yamanouchi and M. Takeuchi, 1990 ULTRASONICS SYMPOSIUM Proceedings, pp11-18, 1990).

【0199】

In addition, FIG. 51 shows the capacitance ratio γ and the electromechanical coupling coefficient k.²Show the relationship with.

【0200】

The relationship in the figure is 36 ° Y cut propagation LiTaO₃K²Using the value of and γ value, [0201]

[Number 13]

<img file="JPP3429751B2_D0013.tif" />It is what was sought as.

【0202】

From the relationship shown in Fig. 51, X-propagation LiTaO with 64 Y ° cut and 41 ° Y cut₃The γ value of can be obtained, and γ = 6.8 and 4.4, respectively, as described above.

【0203】

Configuration of Example 11 Here, the configuration of the embodiment shown in FIGS. 42 and 43 will be described.

【0204】

241 is 36 ° Y-LiTaO₃This is a piezoelectric substrate with a size of 1.5 x 2 x 0.5 mm.

【0205】

Parallel arm resonators (Rp) in order from the input side₁), Series arm resonator (Rs)₁), Parallel arm resonator (Rp)₂), Series arm resonator (Rs)₂), Parallel arm resonator (Rp)₃) Are arranged in this order. Each resonator has a structure with reflectors 242 (short-circuit type) on both sides. Each resonator has an opening length of 180 μm, a logarithm of electrode fingers of 50 pairs, and a reflector of 50 pairs.

【0206】

Only the cycle of the comb-shaped electrode finger is changed between the parallel arm resonator and the series arm resonator. The period of the parallel arm resonator is λp = 4.39 μm (the pattern width and the gap are 1: 1 so the pattern width is λp / 4 1.1 μm), and the period of the series resonator is λs = 4.16 μm (similarly, the pattern width). Is λs / 4 = 1.04 μm).

【0207】

In each period, the resonance frequency (frp, frs) of each resonator becomes a predetermined value (frp = 893MHz, frs = 942MHz). λs = V_m/ frs and λp = V_m/ frp It was decided more. Where V_mIs a 36 ° Y-cut X propagation LiTaO when the electrode film thickness is 3000 Å.₃The speed of sound of the surface wave of a crystal, experimentally V_mIt was calculated as = 3920 m / s.

【0208】

The surface acoustic wave filter 240 having the above configuration has the wide band and low loss passage characteristics shown in FIG. 45 (C). It should be noted that Δf = 19 MHz.

【0209】

In FIG. 43, when only λp is changed to 4.35 μm, Δf becomes 10 MHz, and the characteristics shown in FIG. 45 (B) can be obtained. The electrode material is an Al-Cu alloy, the film thickness is 3000 Å, and the surface waves are arranged so as to propagate in the X-axis direction of the piezoelectric substrate 241.

【0210】

Next, an example in the case of using another piezoelectric substrate will be described.

【0211】

64 ° Y Cut X Propagation LiNbO₃In the case of, γ = 6.8 and Eq. (17) is [0212]

[Number 14]

<img file="JPP3429751B2_D0014.tif" />Will be.

【0213】

41 ° Y Cut X Propagation LiNbO₃In the case of, γ = 4.4, [0214]

[Number 15]

<img file="JPP3429751B2_D0015.tif" />Will be.

【0215】

The smaller the γ value, that is, the larger the electromechanical coupling coefficient of the substrate, the larger α becomes, and even if Δf is widened, characteristic deterioration is unlikely to occur.

【0216】

[Example 12] FIG. 52 shows a circuit configuration diagram of a surface acoustic wave filter 250 according to a twelfth embodiment of the present invention.

【0217】

FIG. 53 shows a structure embodying the surface acoustic wave filter having the circuit configuration of FIG. 52.

【0218】

54 and 55 show the characteristics of the surface acoustic wave filters of FIGS. 52 and 53.

【0219】

Outline of the example For convenience of explanation, first, an outline of this embodiment will be described.

【0220】

The surface acoustic wave filter of this embodiment is a ladder type surface acoustic wave filter in which elastic surface wave resonators are connected in series and parallel, and a plurality of the surface acoustic wave resonators are multistaged. The image impedance between them is matched to reduce the loss at each connection point.

【0221】

This makes it possible to reduce the insertion loss in the pass band.

【0222】

Thinking process until the completion of the invention Next, the thinking process until the completion of the present invention will be described.

【0223】

As shown in FIGS. 56 (A) and 56 (B), a bandpass characteristic can be obtained by connecting at least one series arm resonator and a parallel arm resonator in a ladder type. The ladder-type connection between the series arm resonator and the parallel arm resonator is the unit interval of the filter.

【0224】

At this time, it is desirable that the resonance frequency of the series-arm resonator and the anti-resonance frequency of the parallel resonator match, or that the former has a higher frequency than the latter in terms of expanding the passing bandwidth. There are two types of unit intervals in FIGS. 56 (A) and 56 (B), depending on whether the input / output ends are series arms, and those in which these are connected in multiple stages are shown in FIGS. 57 (A), (B), It is classified into three types as shown in (C).

【0225】

Fig. 57 (A) shows the case where one of the input / output sides is a series arm and the other is a parallel arm (asymmetric type), and (B) shows the case where both input / output ends are parallel arms (symmetric type). ) Is a case where both input / output ends are series arms (symmetric type).

【0226】

When the number of stages is increased in this way, both the insertion loss and the out-of-band suppression degree are n times the unit interval, and although the insertion loss generally becomes worse, the out-of-band suppression degree is improved. This multi-stage is especially effective when the loss in the unit interval is close to zero.

【0227】

However, if impedance matching in the passband between unit intervals is not appropriate, the insertion loss will be worse than the theoretical n times.

【0228】

This is because if impedance matching is not appropriate, power reflection occurs at the boundary of the unit interval (each boundary from line 1-1'to n-n' in FIG. 57), resulting in an increase in loss.

【0229】

If the power reflection between unit intervals is Г, the loss is also n10log (Г). Therefore, it is important to suppress the increase in insertion loss as much as possible by matching the impedance between the unit intervals and suppressing the power reflection at the boundary.

【0230】

Next, a method of impedance matching between unit intervals will be described.

【0231】

As shown in FIG. 58, when connecting circuits having two different 4-terminal constants (four constants A, B, C, D of the F matrix) for impedance matching, from the boundary b-b'. It is sufficient to set that the image impedances of each circuit side are equal to each other.

【0232】

As shown in Fig. 58, the image impedance Zi1 seen on the circuit 1 side is the 4-terminal constant A of the circuit 1.₁, B₁, C₁, D₁It is expressed as the following equation using.

【0233】

[Number 16]

<img file="JPP3429751B2_D0016.tif" />Similarly, the image impedance Zi2 when viewed from the circuit 2 side is expressed by the following equation.

【0234】

[Number 17]

<img file="JPP3429751B2_D0017.tif" />These image impedances are load resistance (pure resistance) R₀It is decided regardless of.

【0235】

If equations (21) and (22) are set to be equal, the impedance matching condition as shown in the following equation can be obtained.

【0236】

D₁B₁/ C₁A₁= A₂B₂/ C₂D₂ ...(twenty three) FIG. 59 shows a case where the above-mentioned impedance matching condition is applied to the unit interval of the ladder type circuit.

【0237】

FIG. 59 (A) shows a connection method with poor impedance matching, which does not satisfy the condition of Eq. (23).

【0238】

The reflectance coefficient Г when viewed from the boundary b-b'to the right is [0239]

[Number 18]

<img file="JPP3429751B2_D0018.tif" />Will be. Z_sY_pIn an actual element, Г also does not become 0 because the pass band does not become completely 0.

【0240】

In comparison, in Fig. 59 (B) or Fig. 59 (C), the boundary b-b'satisfies the condition of Eq. (23), so the reflection becomes 0 and no loss occurs.

【0241】

For example, in the case of Fig. 59 (B), the image impedance seen from the boundary b-b'on the left side is calculated from Eq. (21). [0242]

[Number 19]

<img file="JPP3429751B2_D0019.tif" />Will be. It can be seen that the image impedance Zi2 seen from the boundary b-b'on the right side is also equal to Zi1 when calculated from Eq. (22).

【0243】

Therefore, impedance matching is achieved, and the reflectance coefficient at the boundary becomes 0.

【0244】

Figure 59 (C) also proves that impedance matching is achieved.

【0245】

Next, consider a method of connecting unit intervals in multiple stages using the connection methods shown in FIGS. 59 (B) and 59 (C).

【0246】

FIG. 60 (A) shows a circuit in which the connection methods of FIGS. 59 (B) and 59 (C) are alternately repeated to connect the unit intervals in n (&gt; 2) stages. If such a connection method is used, power reflection does not occur in each unit section no matter how many stages are connected for the reason described above.

【0247】

In the configuration of FIG. 60 (A), if the resonators of the parallel arms adjacent to each other or the resonators of the series arms are added together, it becomes equivalent to that of FIG. 60 (B).

【0248】

As a result, it can be seen that only the arm closest to the input / output end has an impedance or admittance value that is half the magnitude of the inner arm.

【0249】

When this principle is applied to the three types of multi-stage methods shown in FIG. 57, the methods shown in FIGS. 61 (A), (B), and (C) can be obtained as connection methods for impedance matching, respectively.

【0250】

FIG. 61 (A) shows the matched connection method corresponding to FIG. 57 (A), in which one of the input / output ends is a series arm and the other is a parallel arm. In this case, the impedance value of the series arm resonator at the end is half the impedance value of the inner series arm resonator, and the admittance value of the parallel arm resonator at the other end is also the inner parallel arm resonance. It is half the admittance value of the vessel.

【0251】

Similarly, FIG. 61 (B) is the matched connection method of FIG. 57 (B), and FIG. 61 (C) is the matched connection method of FIG. 57 (C).

【0252】

In the case of FIG. 61 (B), both ends are parallel arms, and the admittance value of the parallel arm resonators at both ends is half the admittance value of the parallel arm resonator inside.

【0253】

In the case of FIG. 61 (C), both ends are in series arms, and the impedance value of the series arm resonators at both ends is half the impedance value of the series arm resonator inside.

【0254】

Configuration of Example 12 Next, a twelfth embodiment of the present invention based on the above concept will be described.

【0255】

FIG. 52 shows the basic configuration of the surface acoustic wave filter 250 according to the twelfth embodiment of the present invention.

【0256】

The surface acoustic wave filter is embodied as shown in FIG. 53.

【0257】

Three series arm resonators (Rs)₁, Rs₂, Rs₃) And three parallel arm resonators (Rp)₁, Rp₂, Rp₃), And they are connected as shown in Fig. 52. All of these six resonators have the same aperture length (90 μm) and the same number of electrode finger pairs (100 pairs). Further, each resonator has a short-circuit type reflector as shown in the figure on both sides to increase Q. The logarithm of the reflector is about 100 pairs.

【0258】

Series arm resonator (Rs)₁~ Rs₃) Are all electrode finger periods (λs) of the same length, and λs = 4.19 μm. Also, a parallel arm resonator (Rp)₁~ Rp₃) Is a different period λp = 4.38 μm.

【0259】

As a comparison target, FIG. 62 shows a comparative example for this configuration.

【0260】

Impedance Z for both FIGS. 52 and 62_sThe design conditions for the one-terminal surface acoustic wave resonator of the series arm indicated by (1) are an aperture length of 90 μm and a logarithm of 100 pairs. Admittance Y_pThe same conditions apply to the one-terminal pair surface acoustic wave resonator of the parallel arm shown by.

【0261】

Piezoelectric substrate crystal is 36 ° Y cut X propagation LiTaO₃A comb-shaped pattern of 3000 Å Al alloy film is formed on it as a surface acoustic wave resonator.

【0262】

In FIG. 54, the solid line 251 shows the characteristics of the filter 250 in FIG. 53. The broken line 252 shows the characteristics of the filter of the comparative example of FIG. 62. It can be seen that the filter 250 of this embodiment has a lower loss than both. Especially, the improvement at both ends of the pass band is great.

【0263】

Next, in the filter of the comparative example of FIG. 62, the admittance Y of the unit interval (3)_pAdmittance Y by reducing the logarithm from 100 pairs to 80 pairs only for the parallel resonator represented by_pThe passage characteristics when the value of is reduced are shown by line 253 in FIG. 55. Similarly, it can be seen that the insertion loss is improved. Therefore, even if the admittance value at the end is not halved, it can be said that it is effective, though not sufficient, to reduce it from the admittance value inside. The same applies to the impedance value.

【0264】

As described above, an example for the basic form shown in FIG. 61 (A) has been shown, and this has the same effect even if a large number of unit intervals are added in the central portion.

【0265】

[Example 13] FIG. 63 is a surface acoustic wave filter 260 according to a thirteenth embodiment of the present invention.

【0266】

This surface acoustic wave filter is based on the configuration method shown in FIG. 61 (B). This surface acoustic wave filter 260 provides a loss reduction effect similar to that shown by line 251 in FIG.

【0267】

[Example 14] FIG. 64 is a surface acoustic wave filter 270 according to the 14th embodiment of the present invention.

【0268】

This surface acoustic wave filter is based on the configuration method shown in FIG. 61 (C). This surface acoustic wave filter 270 also provides a loss reduction effect similar to that shown by line 251 in FIG.

【0269】

[Example 15] Next, the surface acoustic wave filter according to the fifteenth embodiment of the present invention will be described with reference to FIGS. 65 and 66.

【0270】

Outline of the example For convenience of explanation, first, an outline of this embodiment will be described.

【0271】

In this embodiment, we focus on the resistance and conductance of the comb-shaped electrode as the cause of determining the insertion loss in the pass band, reduce the resistance for the resonator in a series arrangement, and reduce the resistance of the parallel arm resonator. On the other hand, by reducing the conductance component, the insertion loss of the filter characteristics when these are assembled in a ladder shape is reduced.

【0272】

Next, the background and the like of the present invention will be described.

【0273】

Background of the present invention FIG. 65 shows the basic configuration of a ladder type filter circuit in which two surface acoustic wave resonators having different resonance frequencies (frs, frp) are arranged on the series arm and the parallel arm, respectively.

【0274】

Here, the admittance of the parallel arm resonator, Y_p= g + j b ... (26) g: Conductance b: Susceptance And.

【0275】

Also, the impedance of the series arm resonator, Z_s= r + j · x ... (27) r: resistance x: reactance And.

【0276】

Assuming in this way, the frequency characteristics of g, b, r, and x are as shown in FIG.

【0277】

Admittance Y of parallel arm resonator_pThe susceptibility component b (dotted line in Fig. 69) takes the maximum value at the resonance frequency frp, where the sign is changed from + to-, and it becomes 0 (zero) at the antiresonance frequency fap, and the sign becomes + again at the fap and above. It gradually increases.

【0278】

Meanwhile, Y_pThe conductance component g (dashed line in FIG. 69) also takes the maximum value at fap, decreases sharply when it exceeds fap, and gradually approaches 0.

【0279】

The conductance component g takes only a + value.

【0280】

Impedance of series arm resonator Z_sThe reactance component x (solid line in Fig. 69) is the opposite of admittance and becomes 0 at the resonance frequency frs, takes the maximum value at the antiresonance frequency fas, and further changes the sign from + to-, and from one side above fas. It approaches 0.

【0281】

In addition, the resistance component r gradually increases from 0, reaches the maximum value at the antiresonance frequency fas, and gradually decreases above that. Like g, r takes only a + value.

【0282】

Here, in order to create the filter characteristics, it is a condition that the anti-resonance frequency fap of the parallel resonator and the resonance frequency frs of the series resonator are substantially the same or the latter is slightly larger.

【0283】

The lower part of FIG. 69 shows the passing characteristics of the filter circuit according to the frequency characteristics of the impedance and admittance above. The pass band is taken near fap frs, and the other areas are the attenuation region. As is clear from the figure, b and x are 0 in the passband, especially near the center frequency.

【0284】

Therefore, the pass characteristics as a filter for S21 are determined only by r and g. [0285]

[Number 20]

<img file="JPP3429751B2_D0020.tif" />Will be.

【0286】

Here, since r&gt; 0, g&gt; 0, in Eq. (28), S21 becomes smaller than 1 as both r and g increase, and the insertion loss represented by 20log | S21 | also increases.

【0287】

Therefore, the closer r and g are to 0, the smaller the insertion loss.

【0288】

Next, what part of the comb-shaped electrode forming the surface acoustic wave resonator will be described for r and g.

【0289】

Here, in Fig. 3 (B), r₁Is also taken into consideration.

【0290】

r₁Is the sum of the electrical resistance of the comb-shaped electrode and the energy loss that leaks from each end of the comb-shaped electrode finger into the substrate as a bulk wave as the acoustic resistance.

【0291】

Now, since the resistance due to bulk wave radiation hardly depends on the shape of the comb-shaped electrode, the electrical resistance r of the comb-shaped electrode₁Is proportional to. Especially near the center frequency of x = 0, r = r₁Will be. The admittance conductance g of the parallel arm resonator is the conductance 1 / r of the electrical resistance of the comb electrode.₁Is proportional to.

【0292】

Now, the resistivity of the electrode finger of the comb-shaped electrode is ρ_{o o}, The width of the electrode finger is W, the film thickness is t, and the opening length of the series arm resonator is l._s, Logarithm N_sThen r = l_s Ρ_{o o}/ (N_s W t) ... (29) Will be.

【0293】

Also, the opening length of the parallel arm resonator is l_p, Logarithm N_pThen, when using the same substrate and the same metal film, ρo, W, and t are almost the same. g = N_p W t / (l_p Ρ_{o o}) ... (30) Will be.

【0294】

Therefore, the increase in insertion loss in Eq. (28) is r + 50r g + 2500g = l_s Ρ_{o o}/ (N_s W t) +50 (l_s/ l_p) (N_p/ N_s) +2500 N_p W t / (l_p Ρ_{o o}) ... (31) Will be.

【0295】

From equation (31), the series arm resonator has an aperture length of l._sIs short, logarithmic N_sThe more the parallel resonator is, the longer the aperture length is._pIs long, logarithmic N_pIt can be seen that the smaller the amount, the more effective the loss reduction is. Especially l_s/ l_p&lt;1, N_p/ N_s&lt;1, in other words, the opening length of the series arm resonator is shorter than that of the parallel arm resonator, and the logarithm of the series arm resonator is more effective than that of the parallel arm resonator. ..

【0296】

Here, the reason for this will be described.

【0297】

In equation (31) above, r = r_s(r_s: Electrical resistance of series arm resonator) and g = 1 / r_p(r_p: Electrical resistance of parallel arms) r + 50r g + 2500g = r_s+50 (r_s/ r_p) + 2500 (1 / r_p) Will be. Therefore, (r_s/ r_p) &lt;1, i.e. r_s&lt;r_pIf so, the increase in insertion loss can be suppressed.

【0298】

In this case l_sIf it is narrowed too much, a loss due to surface wave diffraction will appear, and conversely l_pIf it is made too long, the Q of the parallel resonator will decrease due to the increase in resistance, and the degree of out-of-band suppression on the low frequency side will deteriorate, so the size is limited.

【0299】

Furthermore, the film thickness of the metal film forming the comb-shaped electrode is t for the series arm._s, T for parallel arms_pThen, Eq. (31) becomes as follows.

【0300】

r + 50r g + 2500g = l_s Ρ_{o o}/ (N_s W t) +50 (l_s/ l_p) (N_p/ N_s) ( t_p/ t_s) + 2500 N_p W t_p/ (l_p Ρ_{o o}) ... (32) Therefore, t_p/ t_sSimilarly, the loss can be reduced.

【0301】

In addition to this, it is also possible to make a filter by arranging resonators consisting of two types of metal films with different resistivity (ρos, ρop) on the series arm and the parallel arm so that ρos / ρop &lt;1. , When actually making an element, it is not practical in consideration of mass productivity and the like.

【0302】

Configuration of Example 15 Next, an embodiment adopting the above concept will be described.

【0303】

FIG. 65 shows the circuit configuration of the surface acoustic wave filter 280 according to the fifteenth embodiment of the present invention.

【0304】

FIG. 66 shows a structure embodying the circuit configuration of FIG. 65. The piezoelectric substrate 241 used was 36 ° Y cut XLiTaO.₃The electrode material is an Al film of 3000 Å.

【0305】

In the comparative example, the opening length of the comb-shaped electrode is l for both the series arm and the parallel arm._s= l_p= 90 μm, logarithmic N_p= N_s= 100 pairs, whereas in this example, the series arm is l_s= 45 μm, N_s= 200 pairs, parallel arms l_p= 180 μm, N_p= 50 pairs. l_p&gt; l_sAnd N_s&gt; N_pIs. Also l_s/ l_p= 0.25 and N_p/ N_s= 0.25.

【0306】

At this time, the capacitance C of the comb-shaped electrode, which is shaped by the product of the logarithm and the aperture length, is C._OI tried not to change.

【0307】

The solid line 281 in FIG. 66 is the characteristic of this embodiment, and the broken line 282 is the characteristic of the comparative example. In the past, the loss was 2.5 dB, but it became 2.0 dB in this example, and it was improved by 0.5 dB or more in this example. That is, the insertion loss of the filter was improved by 25% in terms of dB.

【0308】

Further, in the case of this embodiment, by increasing the logarithm of the series arm resonator, the power resistance is also improved, and the maximum applicable power is improved by 20%.

【0309】

In the case of the above embodiment, l_sDiffraction loss begins to appear below = 30 μm, l_pThese values were the limits because out-of-band degradation on the low frequency side began to occur at = 300 μm and above.

【0310】

As described above, it is clear that the insertion loss of the passband is improved by lowering the electric resistance of the series arm and increasing the electric resistance of the parallel arm (lowering the conductance).

【0311】

Further, the film thickness of the parallel arm resonator may be made thinner than the film thickness of the series arm resonator. With this configuration as well, the loss of the pass band can be reduced as in the case of the above embodiment.

【0312】

[Effect of the invention]

As described above, according to the present invention, since a plurality of parallel arms and series arms are formed on one piezoelectric substrate, the resonance frequency of the surface acoustic wave resonator can be accurately controlled, and a filter for widening the bandwidth can be obtained. Frequency control is possible.

[Simple explanation of drawings]

[Figure 1]

It is a principle diagram of the surface acoustic wave filter of this invention.

[Figure 2]

It is a figure which shows the basic structure of the filter circuit using a resonator.

[Fig. 3]

It is a figure which shows the structure of a 1-terminal vs. surface acoustic wave resonator, its equivalent circuit, and its symbol.

[Fig. 4]

It is a figure which shows the frequency characteristic of the impedance and admittance of a one-terminal vs. surface acoustic wave resonator.

[Fig. 5]

It is a figure which shows the imitation characteristic of the surface acoustic wave resonator in the vicinity of a resonance frequency, and the filter characteristic of the filter of FIG. 1 which connects them.

[Fig. 6]

It is a figure explaining the conventional surface acoustic wave filter.

[Fig. 7]

It is a figure which shows the effect when the inductance is added in series to a resonator.

[Fig. 8]

It is a figure which shows the effect when n one-terminal vs. surface acoustic wave resonators are connected in series.

[Fig. 9]

It is a figure which shows the aperture length dependence of the passage characteristic of a parallel arm resonator.

[Fig. 10]

It is a figure which shows the aperture length dependence of the passage characteristic of a series arm resonator.

[Fig. 11]

It is a circuit diagram of the 1st Example of the surface acoustic wave filter of this invention.

[Fig. 12]

It is a figure which shows the passing characteristic of the filter of FIG.

[Fig. 13]

It is a figure which shows the effect of the inductance addition to a parallel arm resonator.

[Fig. 14]

It is a top view which shows the structure of the surface acoustic wave filter of FIG. 11 with the lid removed.

[Fig. 15]

FIG. 41 is a cross-sectional view taken along the line XV-XV.

[Fig. 16]

It is a figure which shows the 2nd Example of the surface acoustic wave filter of this invention.

[Fig. 17]

It is a figure which shows the passing characteristic of the filter of FIG.

[Fig. 18]

Aperture length of parallel arm resonator (A)_P) And the opening length of the series arm resonator (A)_S) Ratio (A_P/ A_S) Is shown in the figure showing the increasing effect.

[Fig. 19]

It is a figure which shows the 3rd Example of the surface acoustic wave filter of this invention.

[Fig. 20]

It is a figure which shows the passing characteristic of the filter of FIG.

[Fig. 21]

It is a figure which shows the 4th Example of the surface acoustic wave filter of this invention.

[Fig. 22]

It is a figure which shows the passing characteristic of the filter of FIG.

[Fig. 23]

It is a figure which shows the 5th Example of the surface acoustic wave filter of this invention.

[Fig. 24]

It is a figure which shows the passing characteristic of the filter of FIG.

[Fig. 25]

It is a circuit diagram of the 6th Example of the surface acoustic wave filter of this invention.

[Fig. 26]

FIG. 25 is a diagram showing a first one-terminal pair elastic surface resonator.

[Fig. 27]

It is a figure which shows the passing characteristic of the filter of FIG.

[Fig. 28]

It is a figure which shows the influence on the ripple width by β of a reflector installation position d = (n + β) · λ.

[Fig. 29]

It is a top view which shows the structure of the surface acoustic wave filter of FIG. 25 with the lid removed.

[Fig. 30]

It is a figure which shows the modification example of one of the 1st terminal vs. surface acoustic wave resonators in FIG. 25.

[Fig. 31]

It is a figure which shows another modification of the 1st terminal vs. surface acoustic wave resonator in FIG. 25.

[Fig. 32]

It is a figure which shows the 7th Example of the elastic surface wave filter of this invention.

[Fig. 33]

It is a figure which shows the effect of the electrode film thickness (t) on the ripple generation position.

[Fig. 34]

Ripple due to the reflector of the parallel arm cavity (r_PIt is a figure which shows the state when) falls to a high frequency attenuation pole.

[Fig. 35]

It is a figure which shows the film thickness dependence of the passing characteristic of a resonator type filter.

[Fig. 36]

It is a figure which shows the result of the experiment of the film thickness dependence of the insertion loss and the ripple generation position.

[Fig. 37]

It is a figure which shows the 1st terminal vs. surface acoustic wave resonator of the 8th Example of the surface acoustic wave filter of this invention.

[Fig. 38]

It is a figure which shows the passing characteristic of the surface acoustic wave filter to which the resonator of FIG. 37 is applied.

[Fig. 39]

It is a figure which shows the modification of the 1st terminal vs. surface acoustic wave resonator of the 8th Example of the surface acoustic wave filter of this invention.

[Fig. 40]

It is a figure which shows another example which realizes the inductance of the surface acoustic wave filter of FIG.

[Fig. 41]

It is a figure which shows still another example which realizes the inductance of the surface acoustic wave filter of FIG.

[Fig. 42]

It is a circuit diagram of the eleventh embodiment of the surface acoustic wave filter of this invention.

[Fig. 43]

It is a figure which shows the structure which embodied the circuit of FIG. 42.

[Fig. 44]

It is a figure which shows the imitation characteristic of the surface acoustic wave resonator when fap &lt;frp.

[Fig. 45]

It is a figure explaining the change of the passing characteristic of a ladder type filter when Δffrs-fap is increased from zero.

[Fig. 46]

It is a figure explaining the characteristic measurement method of the surface acoustic wave resonator.

[Fig. 47]

It is a figure which shows the characteristic of the admittance and impedance of each surface acoustic wave resonator of a parallel arm and a series arm.

[Fig. 48]

It is a figure which shows the frequency dependence of the bx product.

[Fig. 49]

It is the figure which represented a part of the circuit of FIG. 42 by the equivalent circuit of L and C.

[Fig. 50]

| bx_maxIt is a figure which shows the relationship between | and Δf / frs.

[Fig. 51]

k²It is a figure which shows the relationship between γ and γ.

[Fig. 52]

It is a circuit diagram of the twelfth embodiment of the surface acoustic wave filter of this invention.

[Fig. 53]

It is a figure which shows the structure which embodied the circuit of FIG. 52.

[Fig. 54]

It is a figure which shows the characteristic of the surface acoustic wave filter of FIG. 53.

[Fig. 55]

In the filter of FIG. 63, the output side Y_pIt is a figure which shows the characteristic when is reduced.

[Fig. 56]

It is a circuit diagram of a unit interval in which each surface acoustic wave resonator is made into a ladder type.

[Fig. 57]

It is a circuit diagram of a circuit formed by connecting the unit interval of FIG. 56 in multiple stages (n stages).

[Fig. 58]

It is a figure which shows the connection of two 4-terminal circuits and the boundary between them.

[Fig. 59]

It is a figure which shows the connection between unit intervals.

[Fig. 60]

It is a figure explaining the method of connecting a unit interval in n (&gt; 2) stages.

[Fig. 61]

It is a figure explaining the construction method of the ladder type circuit of this Example.

[Fig. 62]

It is a circuit diagram of a conventional surface acoustic wave filter.

[Fig. 63]

It is a circuit diagram of the thirteenth embodiment of the surface acoustic wave filter of this invention.

[Fig. 64]

It is a circuit diagram of the 14th Example of the surface acoustic wave filter of this invention.

[Fig. 65]

It is a circuit diagram of the fifteenth embodiment of the surface acoustic wave filter of this invention.

[Fig. 66]

It is a figure which shows the structure which embodied the circuit of FIG. 65.

[Fig. 67]

It is a figure which shows the characteristic of the filter of FIG.

[Fig. 68]

It is a figure which shows the ladder type filter circuit which arranged the surface acoustic wave resonators with different resonance frequencies on the parallel arm and the series arm.

[Fig. 69]

Admittance of parallel arm resonators (Y)_p) Frequency characteristics and series arm resonator impedance (Z)_s) Are shown in correspondence with each other.

[Fig. 70]

It is a figure which shows an example of the conventional surface acoustic wave filter.

[Fig. 71]

It is a figure which shows the passing characteristic of the filter of FIG. 70.

[Explanation of symbols]

60,90,100,110,120,130,240,250,260,270,280 Surface acoustic wave filter 80,150 Surface acoustic wave filter device 81 ceramic package 82 filter tip 83 lid 84_-1~85_-6 Electrode terminal 85_-1~85_-5 Terminal 86_-1~86_-5 Bonding wire 124,125 Attenuating pole 127 Blocking area 131,201,211 Excitation electrode 132,133,160,161,166,167,202,203,212,213,242 Reflector 220,221,230,231 Microstrip line 241 36 ° Y cut X propagation LiTaO₃Board (chip) Rs₁, Rs₂ Series arm resonator Rp₁~ Rp₃ Parallel arm resonator