JPH0629779A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To suppress the temperature rise to improve the resistance to power by forming pairs of electrode fingers in a second resonator arranged in the first stage more than those in second resonators arranged in the other stages. CONSTITUTION:First resonators 22 having a prescribed resonance frequency are arranged in a parallel arm and second resonators 23 having the antiresonance frequency of first resonators 22 are arranged in a series arm to form a surface acoustic wave filter. First and second resonators 22 and 23 are one terminal pair where comb-shaped electrodes 25a and 25b having electrode fingers 24a and 24b are matched while crossing, and a parallel resonator P1 and a series resonator S1 are combined to form a stage, and plural stages are formed. The number of pairs of electrode fingers 24a and 24b crossing each other in the series resonator S1 arranged in the first stage is larger than that in series resonators S2 and S3 in the other stages. Thus, the current flowing to each electrode finger of the comb-shaped electrode of the series resonators S1 is reduced to suppress the temperature rise.

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 as an RF (harmonic wave portion) filter of a small mobile radio such as a car phone and a mobile phone.

In recent years, the development of small and lightweight mobile communication terminals such as car phones and mobile phones has been rapidly advanced. Along with this, there is a demand for miniaturization and high performance of the components used, and the surface acoustic wave (S
AW) elements (resonators, filters, duplexers) are expected to be developed. In particular, the SAW demultiplexer is a device that can greatly contribute to the miniaturization of the RF section, and therefore its development is strongly desired.

This SAW duplexer is used, for example, in a mobile phone,
When used as a filter for wireless transmission and reception, the RF unit of a mobile terminal such as a mobile phone is required to have characteristics such as low insertion loss and high out-of-band suppression. In particular, a demultiplexer is required after the final stage amplifier of the RF section, and a power load of about 1 to 2 W is applied to the filter on the transmission side of this demultiplexer, so that power resistance is required.

[0004]

2. Description of the Related Art Conventionally, a dielectric demultiplexer has been used as a demultiplexer having high power resistance. However, since the volume is large, there is a SAW filter that can reduce the volume. Is commonly used.

As a demultiplexer, 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 SAW resonators are connected in series and parallel arms in a ladder shape, and a duplexer combining the bandpass filters are filed by the present inventors. Already done.

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

FIG. 14 shows the SAW in FIG.
The block diagram of a resonator is shown. FIG. 14 (A) shows SAW resonators (S _{1 to} S ₃ , P ₁ to which form the series arm and the parallel arm).
P ₃ ), in which the comb-shaped electrodes (13 a, 13 b) are aligned in a meshed state. The comb-shaped electrodes 13a and 13b are formed on the piezoelectric substrate 12 by a thin film such as aluminum. In this case, in the figure, 14 is an electrode pair, 15 is an aperture length, and 16 is a comb-shaped electrode period.

As shown in FIG. 14B, the equivalent circuit at this time is equivalent to _{one in which} a series impedance of a resistance r ₁ , a conductance C ₁ and a reactance L _{1 and} an impedance of a conductance C ₀ are connected in parallel. Becomes
A symbol representing this is shown in FIG. 14C, which is arranged as shown in FIG. 13A.

For example, the number of the 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 the electrode pairs 14 of the series arms S ₁ , S ₂ , S ₃ is 100. On the other hand, the opening length is 80 μm, and a short-type reflector (not shown) is arranged outside each of the comb-shaped electrodes 13a and 13b.

FIG. 15 is a diagram for explaining the filter characteristic of FIG. FIG. 15A shows the logarithm, the aperture length, and the capacity ratio of the parallel arms P ₁ , P ₂ , P ₃ and the series arms S ₁ , S ₂ , S ₃ described above. And FIG.
5 (B) shows the transmission characteristics of the filter.

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

[0013]

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

In the test shown in FIG. 16, the input power is 3.5.
The test was performed under the conditions of W and ambient temperature of 85 ° C., and the life depends on the frequency, as is clear from the figure. That is, when power is applied to the low frequency side, all SAW resonators (P ₁
To P _3, S ₁ comb electrode 13a of the to S _3), whereas increased only insertion loss of the filter without change is observed in 13b, when the power application in the high frequency side, the first stage of the series resonators S ₁ The comb-shaped electrodes 13a and 13b are blown, 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 with the ambient temperature at room temperature, and the temperature rises rapidly as the frequency becomes higher as shown in the figure. That is, a sharp rise in temperature of the SAW filter as a main factor of deterioration in power resistance on the high frequency side, and particularly the temperature of the first-stage series resonator S ₁ becomes the highest.

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

The susceptance component b (dotted line) of the admittance Y _p of the parallel arm resonator takes a maximum value at the resonance frequency frp, where the sign is changed from + to − and becomes 0 (zero) at the anti-resonance frequency fap. As a result, the sign becomes + again and increases little by little.

On the other hand, the conductance component g (dashed-dotted line) of Y _p similarly takes the maximum value at fap, sharply decreases when it exceeds fap, and gradually approaches 0. The conductance component g takes only a positive value.

The reactance component x (solid line) of the impedance component Z _s of the series arm resonator is 0 at the resonance frequency frs, which is the opposite of the admittance, takes the maximum value at the anti-resonance frequency fas, and the sign is changed from + to −. Change it, and if it is fas or higher, it approaches 0 from the minus side.

Further, the resistance component r gradually increases from 0, reaches the maximum value at the antiresonance frequency fas, and gradually decreases when the antiresonance frequency fas is exceeded. Similarly to g, r takes only a positive value.

Here, in order to create the filter characteristic, 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.

The pass characteristic of the filter circuit is shown in the lower part of FIG. 18B in accordance with the frequency characteristics of the impedance and admittance. In the figure, fap ≈ frs
It takes a pass band in the vicinity and becomes an attenuation region in other areas.
Further, as is clear from the figure, b and x are 0 especially in the vicinity of the center frequency of the pass band.

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

On the other hand, when the temperature rises, SA in FIG.
The input current of the W filter 11 is I, each resonator (P₁~
P₃, S₁~ S₃) Current i_p1, I_p2, I_p3，
i_s1, I _s2, I_s3And

Series resonator S₁At the resonance point frs of
Shake P₁Susceptance j of is not necessarily zero
Therefore, the parallel resonator P₁Little current i_p1, I_p2, I
_p3(I_p1> I_p2> I_p3) Flows. Therefore, the series resonator
S₁The current flowing through is i_p1> I_p2> I_p3Because,
Series resonator S₁Series resonator due to heat generated by resistance r
S ₁Has the highest temperature rise and the shortest life. straight
Row resonator S₁Parallel resonator P except the resonance point frs of₁Flow
The series current S₁Current flowing through
Less and the surface acoustic waves were not excited much
Therefore, the temperature rise is small and the life is long.

Therefore, the present invention has been made in view of the above points, and an object of the present invention is to provide a surface acoustic wave filter which suppresses a temperature rise and improves power resistance.

[0027]

The above object is a one-terminal pair surface acoustic wave resonator in which comb-shaped electrodes having a predetermined number of electrode fingers are matched with each other in an intermeshing state and have a predetermined resonance frequency. A ladder-type surface acoustic wave filter in which one resonator is arranged in parallel arms and a plurality of second resonators having a resonance frequency at least approximately equal to the anti-resonance frequency of the first resonator are arranged in series arms. , The number of pairs of electrode fingers in the meshed state of the second resonator arranged at the first stage from the input side is larger than that of the second resonators in the other stages, or the overlapping state of the meshed states. The problem is solved by forming the electrode finger length to be shorter than the overlapping electrode finger length of the other stage.

[0028]

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

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

Further, the overlapping electrode finger lengths of the first stage second resonator in the meshed state are formed shorter than those of the other stages.
As a result, 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 resistance.

[0032]

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

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

The series resonator S ₁ arranged in the first stage from the input side has the electrode fingers 24a, 24b in the engaged state.
Is formed more than the logarithm of the series resonators S ₂ and S ₃ of the other stages. The logarithm of the parallel resonators P _{1 to} P ₃ is constant.

Here, FIG. 2 shows a diagram for explaining the aperture length, the logarithm, and the capacitance ratio of FIG. In FIG. 2A, the number of pairs of parallel resonators P _{1 to} P ₃ is 50, and the series resonator S _{1 is}
The number of pairs of the series resonators S ₂ and S ₃ is 100, and the number of pairs of the series resonators S ₂ and S ₃ is 100.

The overlapping electrode finger lengths of the electrode fingers 24a and 24b in the meshed state are so-called aperture lengths, and the aperture lengths (L _p1 = L _p2 = L _p3 ) of the parallel resonators P _{1 to} P ₃ are 150 μm.
m, and the aperture length 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 parallel resonator P ₁ -P ₃
The capacitance ratio (C _p / C _s ) of _P and the capacitance C _s of each series resonator S ₁ , S ₂ , and S ₃ is defined by the ratio of the product of the aperture length and the logarithm.

Therefore, FIG. 3 shows a graph of the pass characteristic of the filter of FIG. That is, FIG. 3A shows frequency characteristics when the logarithm of the electrode fingers 24a and 24b is set to the above-described FIG. 2A.

As described above, as the number of series resonators S ₁ increases, the capacitance ratio (C _p1 / C _s1 ) of the other stages (blocks) decreases. That is, in the structure shown in FIG. 15, impedance matching is performed with a constant capacitance ratio, but if impedance matching does not match for other stages, both shoulders of the pass band tend to be narrowed.

Therefore, the difference between the capacitance ratio (C _p1 / C _s1 ) and the capacitance ratio of the other stages due to the increase in the logarithm of the series resonator S ₁ becomes large, and impedance mismatching is not performed.
Up to about 200 pairs are acceptable, which is shown in Figure 2.
(B) and FIG. 3 (B) are shown.

From the above, by forming the series resonator S _{1 in} a larger number of logarithms than the other series resonators S ₂ and S ₃ ,
The current flowing around each of the electrode fingers 24a, 24b of the comb-shaped electrodes 25a, 25b of the first-stage series resonator S ₁ is reduced and the temperature rise is suppressed.

Further, since the logarithm is increased without increasing the difference with the capacitance ratio of the other series resonators S ₂ and S ₃ , the filter characteristics of the SAW filter (11) shown in FIGS. It is possible to prevent deterioration.

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 resistance is improved, and in particular, the power is applied near the resonance point of the resonator connected to the series arm. It is possible to improve the power resistance of the device when it is operated.

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, The number of pairs of electrode fingers of S ₃ is gradually reduced to 110 pairs and 100 pairs, and the others are the same as in FIG.

The capacity ratio at this time is 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 the difference between the capacitance ratio of the first stage and the capacitance ratio of the next stage and thereafter, the electrode fingers 24 as a result.
It is possible to reduce the current flowing through a and 24b and improve the power resistance without deteriorating the filter characteristics.

Similarly, in FIG. 4B, the series resonators S ₁ to
The logarithm of S ₃ is sequentially set to 140 pairs, 120 pairs, and 100 pairs, and FIG.
In (C), the graphs of the pass characteristics of the filter when the logarithm is sequentially set to 150 pairs, 125 pairs, and 100 pairs are shown in FIGS.
In FIG. 4, impedance matching is performed with the allowable range shown in FIGS. 4 (C) and 5 (C).

Next, FIG. 6 shows the structure of the third embodiment of the present invention.
The diagram is shown. SAW filter 21 of FIG._BParallel resonance
Bowl P₁~ P₃And series resonator S₁~ S₃Electrode finger 24
The series resonator S has the same logarithm of a and 24b.₁Opening
Long L_s1Only other series resonator S ₂, S₃Opening length L_s2，
L_s3(L_s2= L_s3) It is formed longer than the others, and the others are shown in Figure 1.
Is the same as.

The opening length and the like of FIG. 6 will be described with reference to FIG.
FIG. 8 shows a graph for showing the pass characteristics of the filter of FIG.
Show rough. As shown in FIG. 7A, the parallel resonator P₁
~ P ₃Of the logarithm of 50 pairs and the aperture length of 150 μm (L_p1= L_p2
= L_p3), And the series resonator S ₂, S₃The logarithm of 100 pairs,
Aperture length of 80 μm (L_s2= L_s3) And the series
Shaker S₁The logarithm of 100 pairs, opening length L_s1Was set to 60 μm
It is a thing. The pass characteristic of the filter in this case is shown in FIG.
It is shown in (A).

Further, in FIG. 7B, the series resonator S
The aperture characteristic L _{s1 of 1} is 40 μm, and the pass characteristic of the filter is shown in FIG. 8 (B).

Thus, the aperture length L of only the series resonator S ₁ is
By shortening _s1 , the resistance value of the comb-shaped electrodes 25a and 25b as a whole is reduced, the temperature rise is suppressed, and the power resistance can be improved.

The aperture length L _s1 of the series resonator S ₁ is 40
In the case of μm, the difference in the capacitance ratio from the capacitance ratio of the series resonators S ₂ and S _{3 in} the other stage becomes large, the insertion loss is slightly deteriorated, and the bandwidth is narrowed. Therefore, the aperture length L _s1 is allowed up to 60 μm in consideration of impedance matching.

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.

The fourth embodiment of the present invention shown in FIG. 9 is the third embodiment.
Together than the series resonators S ₁ of aperture length L _s1 other stages of the series resonators S _2, S ₃ is formed shorter in the embodiment, the series resonator S _2, aperture length of the S ₃ L _s2, L _{The s3} is formed to be long in sequence, and the improvement in power resistance is the same as that of the third embodiment.

Therefore, as shown in FIG. 9A, the number of parallel resonators P _{1 to} P ₃ is 50 and the aperture length is 150 μm (L
_{Let p1} = L _p2 = L _p3 ). In addition, the series resonators S _{1 to} S ₃
The logarithm of 100 is set as 100 pairs, and the aperture length is sequentially set to L _s1 = 70 μm,
It is formed by making L _s2 = 75 μm and L _s3 = 80 μm long, and the pass characteristic of the filter at this time is shown in FIG. 10 (A).

Similarly, in FIG. 9B, the opening length is set to L _s1 = 6.
Assuming that 0 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 aperture length is L _s1 = 4 in FIG. 9D.
0 μm, L _s2 = 60 μm, L _s3 = 80 μm, and the pass characteristics of each filter are shown in FIG.
It is shown in (D).

In this case, when the capacitance ratios of FIGS. 9A to 9D are sequentially increased and the capacitance ratio of the third stage is 0.938 in the conventional SAW filter, the larger the difference, the narrower the bandwidth becomes. , Insertion loss becomes large. In this case, FIG. 9 (b)
There is no deterioration of characteristics.

Next, FIG. 11 shows a diagram for explaining the fifth embodiment of the present invention. In the fifth embodiment of the present invention shown in FIG. 11, the number of logarithms of the first-stage series resonator S ₁ is larger than that of the other-stage series resonators S ₂ and S ₃ , and the opening length L _s1.
Is formed short.

As a result, the resistance value of the entire comb-shaped electrodes 25a, 25b of the series resonator S ₁ is reduced, and the current flowing through each electrode finger 24a, 24b is reduced, so that the temperature rise is suppressed and the power resistance is improved. Can be improved.

For example, as shown in FIG. 11A, the number of parallel resonators P _{1 to} P ₃ is 50 and the aperture length is 150 μm.
_{(L p1 = L p2 = L} p3) and then, the series resonator S _2, 100 pairs the logarithm of S _3, the aperture length obtained by a _{80μm (L s2 = L s3)} , the logarithm of the series resonators S ₁ 150 pairs, opening length L _s1 is 5
It is 0 μm. The pass characteristic of the filter in this case is shown in FIG.

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 within the range in which the difference is allowed.

Next, FIG. 12 shows a diagram for explaining the sixth embodiment of the present invention. In the sixth embodiment of the present invention shown in FIG. 12, the opening lengths of the first-stage series resonators S _{1 to} S ₃ are sequentially increased and the number of logarithms is sequentially decreased. Is the same as in the fifth embodiment.

For example, as shown in FIG. 12A, the number of parallel resonators P _{1 to} P ₃ is 50 and the aperture length is 150 μm.
(L _p1 = L _p2 = L _p3 ) In addition, the series resonator S ₁ ~
The logarithm of S ₃ is sequentially shortened to 150 pairs, 115 pairs, and 100 pairs, and the opening lengths are sequentially set to L _s1 = 70 μm, L _s2 = 75 μm,
It is formed by making L _s3 = 80 μm long, and the pass characteristic of the filter at this time is shown in FIG. 12 (B).

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, but the aperture length and the logarithm may be changed within the range in which the difference is allowed.

Although not shown, the first-stage series resonator S
The opening length L _{s1 of 1} is made 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 series resonators S ₁ -S ₃ are Even if the number of logarithms is sequentially decreased, the same effect can be obtained. On the contrary, the number of logarithms of the series resonator S ₁ is made larger than the logarithms of the series resonators S ₂ and S ₃ of the other stages (the logarithms of S ₂ and S ₃ are the same), and the opening lengths L _{s1 to} L s.
The same effect can be obtained by forming _s3 sequentially longer.

[0066]

As described above, according to the present invention, the number of logarithms of the first-stage second resonator is made larger than that of the first-stage resonator of the other stage, 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 resistance can be improved.

[Brief description of drawings]

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

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

FIG. 3 is a graph of pass characteristics of the filter of FIG.

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

5 is a graph of pass characteristics 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 the opening length and the like of FIG.

8 is a graph of pass characteristics of the filter of FIG.

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

10 is a graph of pass characteristics of the filter of FIG.

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

FIG. 12 is a diagram for explaining the 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 characteristic of FIG.

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

FIG. 17 is a graph of filter temperature rise.

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

[Explanation of symbols]

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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Motoharu Taniguchi 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Fujitsu Limited

Claims (6)

[Claims] 1. A one-terminal-pair surface acoustic wave resonator in which comb-shaped electrodes (25a, 25b) having a predetermined number of electrode fingers (24a, 24b) are meshed and aligned with each other, and a predetermined resonance frequency ( frp) first resonator (22)
In a parallel arm, and a second resonator (23) having a resonance frequency (frs) at least approximately matching the anti-resonance frequency (fap) of the first resonator is arranged in a plurality of stages in a ladder type arm. In the surface acoustic wave filter, the second resonator (S ₁ ) arranged in 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. The logarithm of the electrode fingers (24a, 24b) in the meshed state of the second resonator (S ₂ , S ₃ ) after the second resonator (S ₁ ) in the first stage and subsequent stages is defined as follows. 2. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter is formed in smaller numbers in sequence. 3. A one-terminal-pair surface acoustic wave resonator in which comb-shaped electrodes (25a, 25b) having a predetermined number of electrode fingers (24a, 24b) are meshed and aligned with each other, and a predetermined number of resonance frequencies are provided. The first resonator (2 having (frp))
2) as a parallel arm, and the anti-resonance frequency (fap
In a ladder type surface acoustic wave filter in which a plurality of second resonators (23) having a resonance frequency (frs) at least substantially equal to (1) are arranged in a series arm, the second resonator (23) arranged in the first stage from the input side. Resonator (S ₁ )
Of the overlapping electrode fingers (24a, 24a,
24b) the length of the second resonator (S ₂ , S ₃ ) of the other stage
The surface acoustic wave filter is 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
The surface acoustic wave filter according to claim 3, wherein the surface acoustic wave filter is formed so as to be sequentially long. 5. The logarithm of the electrode fingers (24a, 24b) in the engaged state in the second resonator (S ₁ ) of the _first stage is set to that of the second resonator (S ₂ , S ₃ ) of the other stage. 5. The surface acoustic wave filter according to claim 3, wherein the surface acoustic wave filter is formed in a number larger than a logarithm. 6. The electrode fingers (24a, 24a) in the engaged state after the second resonator (S ₁ ) in the first stage and subsequent stages.
6. The surface acoustic wave filter according to claim 5, wherein the logarithm of b) is sequentially reduced.