JP3194540B2 - Surface acoustic wave filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ladder type surface acoustic wave filter used for an RF (harmonic portion) filter of a small mobile radio such as an automobile telephone and a portable telephone.

[0002] In recent years, the development of small and light mobile communication terminals such as mobile phones and mobile phones has been rapidly advanced. Along with this, there has been a demand for small and high performance components used, and a surface acoustic wave (S
AW) elements (resonators, filters, duplexers) are expected to be developed. In particular, the development of a SAW duplexer is strongly demanded because it is a device that can greatly contribute to the miniaturization of the RF unit.

[0003] This SAW duplexer is, for example, a car telephone,
When used for a filter for wireless transmission and reception, the RF section of a mobile terminal such as a mobile phone is required to have characteristics such as a small insertion loss and a large degree of suppression outside the band. In particular, a duplexer is required after the last-stage amplifier in the RF unit, and a power load of about 1 to 2 W is applied to the transmission-side filter of the duplexer, so that power durability is required.

[0004]

2. Description of the Related Art Conventionally, a dielectric duplexer has been used as a duplexer having high power durability. However, since the volume is large, there is a SAW filter capable of reducing the volume. Is generally used.

As a duplexer, a combination of a band reject type SAW filter for transmission and a transversal type SAW filter for reception is known.

Further, as a SAW filter for reducing the loss, a bandpass filter in which a SAW resonator is connected to a series arm and a parallel arm in a ladder form, and a duplexer combining this bandpass filter have been filed by the present inventors. Already.

FIG. 13 is a block diagram of a ladder-type SAW filter. FIG. 13A shows a symbolized SAW filter circuit, and FIG. 13B shows an arrangement on a substrate. The ladder type S in FIGS. 13A and 13B is shown.
The AW filter 11 includes a SAW resonator S ₁ , S ₂ , S ₃ of a serial arm and a SAW resonator of a parallel arm between the input and output on a piezoelectric substrate 12.
The resonators P ₁ , P ₂ , and P ₃ are formed in a ladder shape.

Here, FIG. 14 shows the SAW shown in FIG.
FIG. 2 shows a configuration diagram of a resonator. FIG. 14A shows SAW resonators (S _{1 to} S ₃ , P ₁ to P ₁ ) forming the series arm and the parallel arm.
P ₃ ), and the comb-shaped electrodes (13a, 13b) are aligned in a meshing state with each other. The comb electrodes 13a and 13b are formed on the piezoelectric substrate 12 by a thin film of, for example, aluminum. In this case, 14 is an electrode pair, 15 is an opening length, and 16 is a comb-shaped electrode period.

The equivalent circuit at this time is equivalent to a parallel connection of the series impedance of the resistor r ₁ , the conductance C ₁ , and the reactance L _{1 and} the impedance of the conductance C ₀ , as shown in FIG. Becomes
This is represented by a symbol in FIG. 14 (C), which is arranged as shown in FIG. 13 (A).

For example, the number of electrode pairs 14 of the parallel arms P ₁ , P ₂ , P ₃ is 50, the opening length is 150 μm, and the number of electrode pairs 14 of the serial arms S ₁ , S ₂ , S ₃ is 100. On the other hand, the opening length is set to 80 μm, and short-type reflectors (not shown) are arranged outside the respective comb-shaped electrodes 13a and 13b.

FIG. 15 is a diagram for explaining the filter characteristics of FIG. FIG. 15A shows the logarithm, opening length, and capacity ratio of the parallel arms P ₁ , P ₂ , and P ₃ and the serial arms S ₁ , S ₂ , and S ₃ . And FIG.
FIG. 5B shows the transmission characteristics of the filter.

As shown in FIGS. 15A and 15B, a SAW filter with reduced loss can be obtained.

[0013]

The ladder-type SAW filter 11 shown in FIG. 13 is generally used on the transmitting side of domestic automobiles and mobile phones.
FIG. 16 shows a graph of the accelerated life test.

In the test shown in FIG.
The operation was performed under the conditions of W and an ambient temperature of 85 ° C., and the life depends on the frequency as is clear from the drawing. That is, when power is applied to the low frequency side, all the SAW resonators (P ₁
ＰP ₃ , S ₁ ＳS ₃ ), there is no change in the comb electrodes 13 a, 13 b, and only the insertion loss of the filter increases, whereas in the case of power application on the high frequency side, the first-stage series resonator S ₁ The interdigital electrodes 13a and 13b are melted, and the function of the filter cannot be maintained.

FIG. 17 is a graph showing the temperature rise of the filter. The graph of FIG. 17 shows that the input power is 3.5 W,
The test was conducted at an ambient temperature of room temperature. As shown in the figure, the temperature rises rapidly as the frequency increases. In other words, the main factor of the deterioration of the power durability on the high frequency side of the SAW filter is a sharp rise in the temperature of the filter, and the temperature of the series resonator S1 in the _first stage is particularly highest.

FIG. 18 shows the temperature rise of the filter.
The figure for deteriorating is shown. FIG. 18 (A)
Surface acoustic wave resonators with different resonance frequencies for the arm and the series arm
P₁, S ₁18 (B).
Is the admittance Y of the parallel arm resonator _p(Y_p= G + jb
 g: frequency of conductance, b: frequency of susceptance)
Numerical characteristics and impedance Z of series arm resonator_S(Z_S=
r + jx, r: resistance, x: reactance)
It is a characteristic.

The susceptance b (dotted line) of the admittance Y _p of the parallel arm resonator takes the maximum value at the resonance frequency frp, where the sign changes from + to −, and becomes 0 (zero) at the anti-resonance frequency fap, and fap With the above, the sign becomes + again and gradually increases.

On the other hand, the conductance g (dot-dash line) of Y _p similarly takes the maximum value at fap, decreases rapidly beyond fap, and gradually approaches zero. Note that the conductance component g takes only a positive value.

The reactance component x of the impedance component Z _s of the series arm resonators (solid line), admittance and becomes 0 in reverse at the resonance frequency frs takes the maximum value at the antiresonance frequency fas, further from + - sign to Change, and above fas, it approaches 0 from the negative side.

The resistance r gradually increases from 0, reaches a maximum value at the anti-resonance frequency fas, and gradually decreases above that. r also takes only the value of + similarly to g.

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

The lower part of FIG. 18B shows the pass characteristics as a filter circuit in accordance with the frequency characteristics of the upper impedance and admittance. In the figure, fap ≒ frs
It has a pass band in the vicinity and becomes an attenuation region in other areas.
Also, as is clear from FIG. 3, b and x become 0 especially in the vicinity of the center frequency of the pass band.

Accordingly, r (r ₁ ) and g (C ₀ ) are closely related to out-of-band suppression of filter characteristics and insertion loss, and greatly depend on the aperture table and logarithm of the comb-shaped electrode.

On the other hand, when the temperature rises, SA in FIG.
When the input current of the W filter 11 is I, each resonator (P₁~
P_Three, S₁~ S_Three)_p1, I_p2, I_p3,
i_s1, I _s2, I_s3And

The series resonator S₁At the resonance point frs of
Shaker P₁Susceptance j is not always zero
Therefore, the parallel resonator P₁Small current i_p1, I_p2, I
_p3(I_p1> I_p2> I_p3) Flows. Therefore, a series resonator
S₁The current flowing through_p1> I_p2> I_p3First stage
Series resonator S₁Series resonator due to heat generated by the resistor r
S ₁The temperature rise is highest and the life is shortest. straight
Column resonator S₁Other than the resonance point frs of the parallel resonator P₁Flow
Current flowing through the series resonator S₁Current flowing through
And surface acoustic waves are not excited much
Therefore, the temperature rise is small and the life is long.

The present invention has been made in view of the above points, and has as its object to provide a surface acoustic wave filter that suppresses a rise in temperature and improves power durability.

[0027]

An object of the present invention is to provide a one-port surface acoustic wave resonator in which comb-shaped electrodes having a predetermined number of electrode fingers are meshed with each other and have a predetermined resonance frequency. In a ladder type surface acoustic wave filter in which one resonator is arranged in a parallel arm and a second resonator having a resonance frequency at least substantially equal to the antiresonance frequency of the first resonator is arranged in a plurality of series arms. By forming the number of pairs of electrode fingers in the engaged state of the second resonator arranged at the first stage from the input side to be greater than the number of pairs of second resonators in another stage, or by overlapping the engaged states. This problem can be solved by forming the electrode finger length to be shorter than the overlapping electrode finger lengths of other stages.

[0028]

As described above, the logarithm of the electrode fingers in the engaged state in the first-stage second resonator is formed larger than the logarithm of the other-stage second resonator.

As a result, the current flowing per electrode finger in the comb-shaped electrode of the second resonator disposed in the first stage is reduced, and the temperature rise is suppressed.

Further, the overlapping electrode finger length of the meshing state of the first stage second resonator is formed shorter than that of the other stages.
Thereby, the resistance value of the entire comb-shaped electrode in the first-stage second resonator is reduced, and the temperature rise is suppressed.

That is, it is possible to reduce the power load applied to the first-stage second resonator, reduce the temperature rise, and improve the power durability.

[0032]

FIG. 1 shows a configuration diagram of a first embodiment of the present invention. The ladder type surface acoustic wave filter (SAW
Filter) 21 _A, for example as applied to domestic transmission filter (transmission band 925 ~942 MHz), LiTaO of 36 ° Y-X propagating in the piezoelectric substrate (not shown)
₃ (Lithium tantalate) using Al-2% Cu
With a film thickness of 3000 ° and a first resonator 22 (parallel resonators P ₁ to P ₁ ).
P ₃ ) is arranged on the parallel arm, and the second resonator 23 (series resonators S ₁ , S ₂ , S ₃ ) is arranged on the serial arm.

Each of the first and second resonators 22 and 23 has a comb-shaped electrode 25 having electrode fingers 24a and 24b.
a and 25b are a pair of terminals that are engaged with each other in a meshing state (see FIG. 14A). In this case, the first resonator 22 (P ₁ to P ₃₎ has a resonance frequency frp (see FIG. 18), a second resonator _{23 (S 1, S 2,} S 3) a first resonant It has a resonance frequency frp (frp1) substantially equal to or larger than the anti-resonance frequency fap of the detector 22. Then, as a step a combination of the parallel resonator P ₁ and the series resonators S _1, are respectively a plurality of stages form.

The series resonator S ₁ arranged at the first stage from the input side has the meshed electrode fingers 24 a, 24 b.
Are formed more than the logarithms of the series resonators S ₂ and S ₃ at the other stages. Incidentally, the logarithm of the parallel resonator P ₁ to P ₃ is constant all.

FIG. 2 is a view for explaining the opening length, logarithm, and capacity ratio of FIG. In FIG. 2A, the number of logs of the parallel resonators P _{1 to} P ₃ is set to 50, and the series resonator S ₁
Are set to 150 pairs, and the logarithms of the series resonators S ₂ and S _{3 in} the other stages are set to 100 pairs.

The overlapping electrode finger length in the meshing state of the electrode fingers 24a and 24b is the so-called opening length, and the opening length (L _p1 = L _p2 = L _p3 ) of the parallel resonators P _{1 to} P ₃ is 150 μm.
m, and the opening lengths of the series resonators S ₁ , S ₂ , and S ₃ (L _s1 =
(L _s2 = L _s3 ) is set to 80 μm.

Further, the capacitance C of each of the parallel resonators P _{1 to} P _3
P and each of the series resonators S _1, S _2, volume ratio of the capacitance C _s of S ₃ a (C _p / C _s), is defined as the ratio of the product of the aperture length and the logarithm.

FIG. 3 is a graph showing the pass characteristic of the filter shown in FIG. That is, FIG. 3A shows a frequency characteristic when the logarithm of the electrode fingers 24a and 24b is set as shown in FIG. 2A.

As described above, when the logarithm of the series resonator S ₁ increases, the capacitance ratio (C _p1 / C _s1 ) of other stages (blocks) decreases. That is, in FIG. 15, impedance matching is performed with a constant capacitance ratio, but if the impedance matching of other stages does not match, both shoulders of the pass band tend to become narrow.

Therefore, the difference between the capacitance ratio (C _p1 / C _s1 ) due to the increase of the logarithm of the series resonator S ₁ and the capacitance ratio of the other stages increases, and the logarithm that does not cause impedance mismatching is obtained.
Up to 200 pairs can be tolerated.
(B) and FIG. 3 (B).

From the above, by forming the logarithm of the series resonator S ₁ more than the other series resonators S ₂ and S ₃ ,
In which the first stage of the series resonators S ₁ comb electrodes 25a, the electrode fingers 24a of 25b, the temperature rise current decreases flowing per 24b is suppressed.

Since the logarithm is increased without increasing the difference between the capacitance ratio of the other series resonators S ₂ and S ₃ , the filter characteristics of the SAW filter (11) shown in FIGS. Deterioration can be prevented.

That is, the temperature rise at the time of power application can be reduced without impairing the characteristics of the ladder-type bandpass filter, so that the power durability can be improved. In particular, power is applied near the resonance point of the resonator connected to the series arm. It is possible to improve the power durability when it is performed.

Next, FIG. 4 shows a diagram for explaining the aperture length and the like in the second embodiment of the present invention, and FIG. 5 shows a graph of the pass characteristic of the filter of FIG. Figure 4 SAW filter in (a), the series resonators S ₁ of the logarithm of the electrode finger while the series resonator S _2, greater than S ₃ 120 pairs of other stages, the following stages of the series resonators S _2, sequentially reducing the number of electrode fingers of the S _3, 110 pairs, which was set to 100 pairs and the others are the same as in FIG.

The capacity ratio at this time is sequentially C_p1/ C_s1= 0.78
1, C_p2/ C_s2= 0.852, C_p3/ C _s3= 0.938,
A graph of the pass characteristic of the filter is shown in FIG.

That is, by sequentially providing a difference between the capacitance ratio of the first stage and the capacitance ratio of the next stage and thereafter, each electrode finger 24
a, 24b are reduced, and the power durability is improved without deteriorating the filter characteristics.

Similarly, in FIG. 4B, the series resonators S ₁ to S ₁
Sequentially 140 vs logarithm of S _3, 120 pairs, and 100 pairs, Figure 4
(C) shows the graph of the pass characteristic of the filter when the logarithm is 150 pairs, 125 pairs, and 100 pairs in sequence.
And impedance matching is performed with the allowable ranges shown in FIGS. 4 (C) and 5 (C).

Next, FIG. 6 shows the structure of the third embodiment of the present invention.
A diagram is shown. SAW filter 21 of FIG._BIs the parallel resonance
Bowl P₁~ P_ThreeAnd series resonator S₁~ S_ThreeElectrode finger 24
a, 24b with the same logarithm, and the series resonator S₁Opening
Length L_s1Only the other series resonator S _Two, S_ThreeOpening length L_s2,
L_s3(L_s2= L_s3) Longer than that shown in Figure 1
Is the same as

Here, FIG. 7 explains the opening length and the like in FIG.
FIG. 8 shows a graph of the pass characteristic of the filter of FIG.
Show rough. As shown in FIG. 7A, the parallel resonator P₁
~ P _ThreeLogarithm of 50 pairs and aperture length of 150 μm (L_p1= L_p2
= L_p3) And the series resonator S _Two, S_ThreeLogarithm of 100,
The opening length is 80 μm (L_s2= L_s3)
Shaker S₁Logarithm of 100 pairs, opening length L_s1Was set to 60 μm
Things. The pass characteristic of the filter in this case is shown in FIG.
It is shown in (A).

In FIG. 7B, the series resonator S
FIG. 8 (B) shows the pass characteristics of the filter when the opening length L _{s1 of 1} is 40 μm.

As described above, the opening length L of only the series resonator S ₁ is obtained.
By shortening _s1 , the resistance value of the entire comb-shaped electrodes 25a and 25b is reduced, the temperature rise is suppressed, and the power durability can be improved.

Note that the opening length L _s1 of the series resonator S ₁ is 40
In the case of μm, the difference between the capacitance ratio and the capacitance ratio of the series resonators S ₂ and S ₃ at other stages becomes large, the insertion loss is slightly deteriorated, and the bandwidth is narrowed. Therefore, considering the impedance matching, the aperture length L _s1 is allowed up to 60 μm.

Next, FIG. 9 shows a diagram for explaining the aperture length and the like in the fourth embodiment of the present invention, and FIG. 10 shows a graph of the pass characteristic of the filter of FIG.

A fourth embodiment of the present invention shown in FIG.
In this embodiment, the opening length L _s1 of the series resonator S ₁ is made shorter than the series resonators S ₂ , S ₃ of the other stages, and the opening length L _s2 , L of the series resonators S ₂ , S _{3. s3} is sequentially formed longer, and the improvement of the power durability is the same as that of the third embodiment.

Therefore, as shown in FIG. 9A, the logarithm of the parallel resonators P _{1 to} P ₃ is 50 pairs, and the aperture length is 150 μm (L
_p1 = _Lp2 = _Lp3 ). Further, the series resonators S _{1 to} S ₃
Is 100 pairs, the opening length is sequentially L _s1 = 70 μm,
FIG. 10 (A) shows the filter passing characteristics in which the filter is formed as long as L _s2 = 75 μm and L _s3 = 80 μm.

Similarly, in FIG. 9B, the opening length is set to L _s1 = 6.
9 μm, L _s2 = 70 μm, and L _s3 = 80 μm.
In (c), the opening length is L _s1 = 50 μm, L _s2 = 65 μm,
L _s3 = 80 μm, and the opening length is L _s1 = 4 in FIG.
0 μm, L _s2 = 60 μm, L _s3 = 80 μm, and the pass characteristic of each filter is shown in FIG.
It is shown in (D).

In this case, the capacitance ratios of FIGS. 9A to 9D are sequentially increased. If the capacitance ratio of the third stage, 0.938, is a conventional SAW filter, the larger the difference, the narrower the bandwidth. , The insertion loss increases. In this case, FIG.
Until there is no deterioration of characteristics.

Next, FIG. 11 shows a diagram for explaining a fifth embodiment of the present invention. Fifth embodiment of the present invention shown in FIG. 11, as well as more than the first stage of the series resonators S series resonator S ₂ of the logarithm of the other stage _1, S _3, aperture length L _s1
Are formed short.

[0059] Thus, comb electrode 25a of the series resonators S _1, to reduce the overall resistance of 25b, and reduces the current flowing to the electrode fingers 24a, 24b, to suppress the temperature rise, electric power resistance Can be improved.

For example, as shown in FIG. 11A, the logarithm of the parallel resonators P _{1 to} P ₃ is 50 pairs, and the aperture length is 150 μm.
(L _p1 = L _p2 = L _p3 ), the logarithm of the series resonators S ₂ and S ₃ is 100 pairs, the aperture length is 80 μm (L _s2 = L _s3 ), and the logarithm of the series resonator S ₁ is 150 pairs, opening length L _s1 is 5
It was set to 0 μm. FIG. 11B shows the pass characteristic of the filter in this case.

In this case, the capacitance ratio of the series resonator S ₁ is the same as that of the other stages, and the characteristic deterioration is prevented. However, the aperture length and the logarithm may be changed as long as the difference is allowed.

Next, FIG. 12 shows a diagram for explaining a sixth embodiment of the present invention. Sixth embodiment of the present invention shown in FIG. 12 is configured to sequentially formed long aperture length of the first stage of the series resonators S ₁ to S _3, which was successively reduced log, the principle of power durability improvement by this Is the same as in the fifth embodiment.

For example, as shown in FIG. 12A, the logarithm of the parallel resonators P _{1 to} P ₃ is 50 pairs, and the aperture length is 150 μm.
(L _p1 = L _p2 = L _p3 ). In addition, the series resonators S ₁ to
Sequentially 150 vs logarithm of S _3, 115 pairs, 100 pairs the short form, the aperture length _{sequence, L s1 = 70μm, L s2} = 75μm,
FIG. 12 (B) shows the filter pass characteristics at this time, which is formed as long as L _s3 = 80 μm.

In this case, the capacitance ratio of the series resonator S ₁ is the same as that of the other stages, and the characteristic deterioration is prevented. However, the aperture length and the logarithm may be changed as long as the difference is allowed.

Although not shown, the first-stage series resonator S
₁ is formed shorter than the opening lengths L _s2 , L _s3 (L _s2 = L _s3 ) of the series resonators S ₂ , S ₃ of the other stages, and the opening length L _s1 of the series resonators S ₁ -S ₃ The same effect can be obtained even if the logarithm is sequentially reduced. Also, conversely with (the logarithm of S _2, S ₃ the same) series resonator series resonator S ₂ of the logarithm of the other stages S _1, S ₃ logarithmic form more than, aperture length L _s1 ~L
The same effect can be obtained even if _s3 is formed sequentially longer.

[0066]

As described above, according to the present invention, the logarithm of the first-stage second resonator is formed to be larger than that of the first-stage second resonator, or the opening length is made shorter. The current flowing through each electrode finger of the comb-shaped electrode is reduced, the resistance value of the entire comb-shaped electrode is reduced, the temperature rise is suppressed, and the power durability can be improved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a first embodiment of the present invention.

FIG. 2 is a diagram for explaining an opening length, a logarithm, and a capacitance ratio of FIG. 1;

FIG. 3 is a graph of a pass characteristic of the filter of FIG. 1;

FIG. 4 is a diagram for explaining an opening length and the like in a second embodiment of the present invention.

FIG. 5 is a graph of the pass characteristic of the filter of FIG.

FIG. 6 is a configuration diagram of a third embodiment of the present invention.

FIG. 7 is a diagram for explaining an opening length and the like in FIG. 6;

FIG. 8 is a graph of a pass characteristic of the filter of FIG. 7;

FIG. 9 is a diagram for explaining an opening length and the like in a fourth embodiment of the present invention.

FIG. 10 is a graph showing the pass characteristic of the filter of FIG. 9;

FIG. 11 is a diagram for explaining a fifth embodiment of the present invention.

FIG. 12 is a diagram for explaining a sixth embodiment of the present invention.

FIG. 13 is a configuration diagram of a ladder-type SAW filter.

14 is a configuration diagram of the SAW resonator in FIG.

FIG. 15 is a diagram for explaining the filter characteristics of FIG.

FIG. 16 is a graph of an accelerated life test.

FIG. 17 is a graph of temperature rise of a filter.

FIG. 18 is a diagram for explaining temperature rise and deterioration of a filter.

[Explanation of symbols]

21 _A , 21 _B SAW filter 22 First resonator 23 Second resonator 24 a, 24 b Electrode finger 25 a, 25 b Comb-shaped electrode

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Motoharu Taniguchi 1015 Uedanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture Inside Fujitsu Limited (56) References JP-A-4-253414 (JP, A) JP-A-1-260911 (JP, a) JP flat 3-205908 (JP, a) JP flat 5-22074 (JP, a) JP Akira 63-132515 (JP, a) (58 ) investigated the field (Int.Cl. ⁷ , DB name) H03H 9/64 H03H 9/25

Claims (6)

(57) [Claims] 1. A one-port surface acoustic wave resonator in which comb electrodes (25a, 25b) having a predetermined number of electrode fingers (24a, 24b) are meshed and matched with each other, and have a predetermined resonance frequency ( frp) (22)
In a parallel arm, and a second resonator (23) having a resonance frequency (frs) at least substantially equal to the anti-resonance frequency (fap) of the first resonator in a series arm in a ladder type. In the surface acoustic wave filter, the second resonator (S ₁ ) arranged at the first stage from the input side
The meshing state of the electrode fingers (24a, 24b) the logarithm of the other stage a second resonator (S _2, S ₃₎ SAW filter, which comprises more forming the logarithm of the. 2. A second resonator (S ₁₎ of said at following stages second resonator (S _2, S ₃₎ the engagement state of the electrode fingers (24a, 24b) of the first stage the logarithm of 2. The surface acoustic wave filter according to claim 1, wherein the number of the surface acoustic wave filters is reduced in order. 3. A one-port surface acoustic wave resonator in which comb-shaped electrodes (25a, 25b) each having a predetermined number of electrode fingers (24a, 24b) are engaged with each other in a meshing state, and have a predetermined number of resonance frequencies. (Frp) of the first resonator (2
2), the anti-resonance frequency (fap
), A ladder-type surface acoustic wave filter in which a plurality of second resonators (23) having a resonance frequency (frs) substantially at least equal to that of the second arm are arranged in the series arm. Resonator (S ₁ )
At the overlapping electrode fingers (24a,
24b) Change the length of the second resonator (S ₂ , S ₃ ) of another stage
A surface acoustic wave filter formed to be shorter than the length of the overlapping electrode fingers (24a, 24b). Wherein said first-stage second resonator (S ₁₎ of said at following stages second resonator (S _2, S ₃₎ the engagement overlapping electrode fingers (24a, 24b) of the state of the length To
4. The surface acoustic wave filter according to claim 3, wherein the surface acoustic wave filter is formed to be sequentially longer. 5. The logarithm of the engaged electrode fingers (24a, 24b) in the first stage second resonator (S ₁ ) is determined by the logarithm of the other stage second resonator (S ₂ , S ₃ ). The surface acoustic wave filter according to claim 3, wherein the surface acoustic wave filter is formed to have more than a logarithm. 6. The meshed electrode fingers (24a, 24a) at a stage subsequent to the first stage second resonator (S ₁ ).
6. The surface acoustic wave filter according to claim 5, wherein the logarithm of b) is sequentially reduced.